JP5787492B2 - Steel pipe manufacturing method - Google Patents

Description

  Aspects of the present disclosure relate to seamless pipes formed from steel containing micro-alloying additions of boron and titanium, which have a yield strength of at least 100 ksi (690 MPa), excellent toughness and Good weldability. Such tubes are suitable for use in high strength line pipes, such as X100 in the API 5L standard and other possible applications.

  It is preferred to add boron to the steel in the amount of microalloying because such addition may improve the mechanical properties of the steel. For example, the addition of boron may improve the curability, i.e. the ability of the steel to harden by heat treatment. When boron is added, it may move to the grain boundary and suppress the phase transformation from austenite to ferrite at the grain boundary, which may improve the likelihood of martensite. In addition, boron is effective at very low concentrations, thus significantly improving curability at a relatively low cost.

  To achieve such an advantage, boron should remain in the free element state. However, boron readily reacts with impurities present in the steel, such as nitrogen. When boron nitride occurs, the positive effect on curability that boron provides can be reduced because of the reduction of free boron.

  In order to deal with such problems, a strong nitride forming agent such as titanium can be added to the steel composition to prevent the formation of boron nitride. However, concomitantly, relatively coarse titanium nitride particles can form during solidification. Such particles may grow further during reheating prior to hot rolling, which may reduce the toughness exhibited by the steel and reduce the degree of property improvement provided by boron addition. There is.

SUMMARY In one aspect, a method is provided for producing a boron-titanium steel that exhibits a yield strength of at least 100 ksi (690 MPa), excellent toughness, and good weldability. The method comprises providing a composition comprising carbon, titanium and boron. The method may additionally include adding one or more of manganese, silicon, nickel, chromium, molybdenum, vanadium and niobium to the composition. Also in this method, cooling of the composition is sufficient to suppress grain coarsening of the titanium nitride (TiN) precipitate that occurs within the composition from casting and to limit the size of the TiN precipitate to less than about 50 nm. It may also include performing at such a high cooling rate. This method may further include performing hot rolling of the composition such that the microstructure is fine-tuned and a particle size of about 20-50 μm is achieved prior to transformation. The method further includes cooling the composition in air after hot rolling and subjecting the composition to austenization and quenching, or cooling the composition in air after hot rolling. And subjecting the composition to austenitization, quenching and tempering, or about 5 to 50 ° C./second immediately after hot rolling without any subsequent heat treatment with forced cooling of the composition. It may also include performing at a cooling rate. In certain embodiments, the steel composition may be formed into a steel pipe, such as a seamless pipe.

As an additional aspect, a method for manufacturing a steel pipe is provided. This method
The carbon (C) content is about 0.04 to 0.12% by weight;
The titanium (Ti) content is about 0.01 to 0.03% by weight;
The boron (B) content is about 0.0005 to 0.003% by weight and the nitrogen (N) content is less than or equal to about 0.008% by weight;
Providing a steel composition, wherein:
The concentration of each element is a concentration based on the total weight of the steel composition. In one embodiment, about 0.0005 to 0.002 wt% boron may remain in solid solution to improve curability. In a further aspect, substantially all of the nitrogen may be present in TiN particle form such that the formation of boron nitride is avoided and the aforementioned solid solution content of boron is provided. In this method, the rod cast from the steel composition is further cooled by selecting the cooling rate around the center of the rod so that the TiN particles produced in the rod have an average size of less than about 50 nm. Also included. This method may additionally include forming a tube from the rod. As an additional aspect, the yield strength exhibited by the shaped steel can be greater than or equal to about 100 ksi (about 690 MPa) as measured according to ASTM E8. In certain embodiments, the steel composition may be formed to produce a seamless tube.

As a further aspect, a method for producing a steel composition is provided. This method
The carbon (C) content is about 0.04 to 0.12% by weight;
Manganese (Mn) content is about 0.6 to 1.6 wt%,
The silicon (Si) content is about 0.05 to 0.3% by weight;
The nickel (Ni) content is less than or equal to about 0.5% by weight;
The chromium (Cr) content is less than or equal to about 0.5% by weight;
The molybdenum (Mo) content is equal to or less than about 0.5% by weight;
The vanadium (V) content is less than or equal to about 0.15% by weight;
The niobium (Nb) content is less than or equal to about 0.05% by weight;
The titanium (Ti) content is about 0.01 to 0.03% by weight;
The boron (B) content is about 0.0005-0.0030% by weight and the nitrogen (N) content is less than or equal to 0.008% by weight;
Providing a steel composition, wherein:
The concentration of each element is a concentration based on the total weight of the steel composition. In one embodiment, about 0.0005 to 0.002 wt% boron remains in solid solution to improve curability. The method further includes casting the steel composition to a size of less than about 50 nm such that substantially all of the nitrogen in the cast steel composition is avoided in the formation of boron nitride and results in a solid solution content of boron. It also includes performing to exist in the form of TiN particles. The method further includes hot rolling the shaped steel composition and cooling the shaped steel composition immediately after hot rolling at a cooling rate in the range of about 5 to 50 ° C./second. In certain embodiments, the formed steel composition is cooled immediately after hot rolling at a cooling rate in the range of about 10 to 30 ° C./second.

  The final microstructure of the steel composition after cooling without any tempering after cooling may contain a mixture of bainite and martensite, and the amount of martensite is about 30% or less. . In a particular embodiment, the amount of martensite contained in the microstructure can be about 5% or less.

As an additional aspect, a method for producing a steel composition is provided. This method
The carbon (C) content is about 0.04-0.08% by weight;
Manganese (Mn) content is about 0.8-1.6 wt%,
The silicon (Si) content is about 0.05 to 0.3% by weight;
Molybdenum (Mo) content is about 0.3 wt% or less,
The titanium (Ti) content is about 0.01 to 0.03% by weight;
The boron (B) content is about 0.0005-0.003% by weight and the nitrogen (N) content is less than or equal to 0.008% by weight;
Providing a steel composition, wherein the concentration of each element is a concentration based on the total weight of the steel composition. The method further includes casting the steel composition such that substantially all of the nitrogen in the cast steel composition is present in the form of TiN particles less than about 50 nm in size so as to avoid the formation of boron nitride. Include. The method may further include hot rolling the formed steel composition and air cooling immediately after hot rolling at a cooling rate of less than about 1 ° C./second to austenitize and quench the composition. include.

  The final microstructure of the steel composition without any tempering after quenching may contain a mixture of bainite and martensite. In certain embodiments, the amount of martensite contained in the microstructure is about 30% or less. In a further embodiment, the amount of martensite contained in the microstructure is about 20% or less.

As a further aspect, a method for producing a steel composition is provided. This method
The carbon (C) content is about 0.04-0.12 wt%;
Manganese (Mn) content is about 0.8 to 1.6 wt%,
The silicon (Si) content is about 0.05-0.3 wt%;
The nickel (Ni) content is less than or equal to 0.5% by weight;
The chromium (Cr) content is less than or equal to about 0.5% by weight;
The molybdenum (Mo) content is equal to or less than about 0.5% by weight;
The vanadium (V) content is less than or equal to about 0.15% by weight;
The niobium (Nb) content is less than or equal to about 0.05% by weight;
The titanium (Ti) content is about 0.01 to 0.03% by weight;
The boron (B) content is about 0.0005-0.0030% by weight and the nitrogen (N) content is less than or equal to 0.008% by weight;
Providing a steel composition, wherein the concentration of each element is a concentration based on the total weight of the steel composition, and from about 0.0005 to about 0.005 to improve the hardenability. 002% by weight boron remains a solid solution. The method further includes casting the steel composition to a size of less than about 50 nm such that substantially all of the nitrogen in the cast steel composition is avoided in the formation of boron nitride and results in a solid solution content of boron. It also includes performing to exist in the form of TiN particles. The method also includes hot rolling the cast steel composition and air cooling the formed steel composition immediately after hot rolling at a cooling rate of less than about 1 ° C./second. The method further includes austenitizing and quenching the composition. Optionally, the method may further include tempering the composition at a temperature in the range of about 400 to 700 ° C.

  In the tempering embodiment, the final microstructure of the tempered air cooling composition may include a mixture of tempered bainite and martensite, the amount of martensite being about 30. % Or more. In certain embodiments, the amount of martensite contained in the air cooling composition can be about 50% or more.

FIG. 1 is a schematic flow diagram of one embodiment of a method for manufacturing a boron-titanium (B / Ti) steel pipe. FIG. 2 is a continuous cooling transformation (CCT) plot of one embodiment of steel composition 1. FIG. 3 is a scanning electron micrograph of the microstructure of the steel composition 1 cooled at a cooling rate of about 2 ° C./second, 5 ° C./second, 10 ° C./second and 20 ° C./second from the austenite region. Show. 4A and 4B are plots of impact energy (CVN) shown by an embodiment of steel composition 1 subjected to accelerated cooling, where (A) shows impact energy as a function of cooling rate, and (B). The impact energy is shown as a function of temperature. FIG. 5 is a diagram in which the hardness shown by the embodiment of the steel composition 1 in the state of being tempered after quenching is plotted as a function of the tempering temperature. FIG. 6 shows a scanning electron micrograph of the microstructure of an embodiment of steel composition 1 that was tempered at about 410 ° C. after quenching. FIG. 7 is a continuous cooling transformation (CCT) plot of one embodiment of steel composition 2. FIG. 8 shows a steel composition cooled from the austenite region at cooling rates of about 0.2 ° C./second, 0.5 ° C./second, 1 ° C./second, 10 ° C./second, 30 ° C./second, and 50 ° C./second. The scanning electron micrograph which took the fine structure of 2 aspects is shown. FIG. 9 is a continuous cooling transformation (CCT) plot of one embodiment of steel composition 3. FIG. 10 shows a steel composition cooled from the austenite region at cooling rates of about 0.2 ° C./second, 0.5 ° C./second, 1 ° C./second, 10 ° C./second, 30 ° C./second, and 50 ° C./second. The scanning electron micrograph which took the fine structure of 3 aspects is shown. 11A-11B are diagrams plotting the hardness shown by embodiments of steel compositions 2 and 3 as a function of cooling rate from hot rolling: (A) Composition 2 and (B) Composition 3. FIG. FIGS. 12A-12B show scanning electron micrographs of microstructures of embodiments of steel compositions 2 and 3 in the as-quenched state: (A) composition 2 and (B) composition 3. FIG. . FIGS. 13A-13B show scanning electron micrographs of the microstructures of embodiments of steel compositions 2 and 3 that have been tempered after quenching: (A) composition 2 and (B) composition 3. Show. FIG. 14 is a plot of the hardness shown by embodiments of steel compositions 2 (black squares) and 3 (white squares) as a function of tempering temperature. FIG. 15 is a plot of the hardness exhibited by embodiments of steel composition 2 and reference Nb-V steel as a function of average cooling rate in the range of 800 ° C. to 500 ° C.

DETAILED DESCRIPTION Aspects of the present disclosure provide compositions and manufacturing methods for alloyed low carbon steel with low boron content. In particular, a boron / titanium (B / Ti) steel with improved toughness associated with controlled amount of titanium nitride (TiN) particles will be discussed in detail. The addition of titanium and boron improves the curability during austenite decomposition because free boron can remain substantially in solid solution.

  The cooling rate during casting can be used to control the size of the TiN precipitate. The size in a particular embodiment can include the diameter of the precipitate. The size in other embodiments may include the maximum dimension of the precipitate. For example, as discussed in detail below, using a cooling rate of about 10 to 30 ° C./min or more during casting can produce fine TiN precipitates with an average size of less than about 50 nm. Such TiN precipitates are not harmful to toughness because of their small size. In addition, such precipitates may inhibit excessive grain growth that occurs during processing operations such as reheating prior to hot rolling. Smaller austenite grain sizes can improve toughness after accelerated cooling or quenching because of the smaller martensite / bainite packet size.

The mechanical properties and microstructure exhibited by the steel composition are further affected by heat treatment after hot rolling. In one embodiment, the hot-rolled steel composition is cooled in air at a cooling rate of less than about 1 ° C./second and then reheated to the austenitic zone and quenched. In another embodiment, the hot-rolled steel composition is cooled in air and then reheated to the austenitic zone and tempered following quenching. In a further aspect, the steel composition may be subjected to accelerated cooling at a cooling rate in the range of about 5 to 50 ° C./second immediately after hot rolling.

  For compositions that have been treated in such a manner, especially compositions that have been tempered following quenching, excellent mechanical property combinations can be obtained. For example, a sample that has been tempered followed by tempering at about 500 ° C. can exhibit a yield and tensile strength of about 118 and 127 ksi, respectively, in addition to a range of impact energy measured at about −60 ° C. 143-173J.

  As another example, a sample that has been subjected to accelerated cooling may exhibit good impact energy, particularly when the cooling rate is about 10-20 ° C./second. For example, the impact energy observed when the temperature is set to −20 ° C. or higher is about 220 J or higher. These and other advantages of the disclosed aspects are discussed in detail below.

  FIG. 1 illustrates one embodiment of a boron-titanium (B / Ti) steel manufacturing method 100. In certain embodiments, the composition may be formed into a tube form. The method 100 shown in FIG. 1 includes a steel casting operation (collectively referred to as a steel casting operation 102) shown in blocks 110, 112, and 114, and a steel forming operation (steel forming operation 104) shown in blocks 116, 120, 122, and 124. And steel heat treatment operations (collectively referred to as heat treatment operations 106) shown in blocks 126 and 128. It will be appreciated that in some embodiments, one or more of the heat treatment operations may be omitted to some extent or completely if desired.

  During the steel casting operation 102, B / Ti steel is cast from the molten state. In certain aspects, the steel casting operation 102 may include a continuous casting operation. For example, the steel casting operation 102 may include iron melting / refining 110, ladle processing 112, and continuous casting 114 as known in the art.

  In one embodiment, such steel may contain the elements shown in Table 1 below in concentrations in the ranges indicated therein, unless otherwise specified herein, the concentrations are based on the total weight of the steel composition. In weight percent (% by weight).

The concentration of the element is further selected based on the carbon equivalent of the composition CE _Pcm [where CE _Pcm is calculated as _follows :

And the concentration of each element is expressed in weight%]
It is also possible to carry out such that is less than about 0.22.

  As shown in Table 1, such cast steel may include boron-titanium steel alloys, which include carbon (C), boron (B) and titanium (Ti) as well as manganese (Mn), silicon (Si ), Nickel (Ni), chromium (Cr) molybdenum (Mo), vanadium (V) and niobium (Nb). There may also be sulfur (S), phosphorus (P), copper (Cu) and nitrogen (N) impurities, however, in one embodiment, the concentration of such impurities can be suitably as low as possible. Reduce it to a small amount.

  C is an element that increases the strength of steel at a low cost by addition. If the C content is less than about 0.04%, in some embodiments, it may be difficult to obtain the desired strength for the composition. On the other hand, in other embodiments, the toughness and weldability can be adversely affected if the C content of the steel is greater than about 0.12% by weight. Thus, in one embodiment, the C content may range from about 0.04 to 0.12% by weight. In other embodiments, the C content may range from about 0.04 to 0.08% by weight. Thus, if the range of C is lowered, the composition can be processed even without tempering (that is, as-quenched), and good toughness can still be achieved.

  B is an element whose addition is effective for improving the hardenability of steel. For example, B can improve curability by suppressing the formation of ferrite. If the B content is less than about 0.0005% by weight, in some embodiments it may be difficult to obtain the desired hardenability of the steel. However, as another aspect, if the B content is too high, coarse boron carbide is generated at the grain boundaries, and the toughness may be adversely affected. Thus, in one embodiment, the B concentration in the composition may range from about 0.0005 to 0.003% by weight. In other embodiments, the B concentration in the composition may range from about 0.0005 to 0.002% by weight. At least a portion of B in such a composition may be in the free element state as a solid solution.

  Si is an element that, when added, provides a deoxygenating effect during the steel manufacturing process and also increases the strength of the steel. If the Si content is too low, in some embodiments, the steel may be susceptible to oxidation and the concentration of trace contents may increase. However, on the other hand, if the Si content of the steel is too high, in some embodiments both the toughness and formability of the steel may be reduced. Thus, in a particular embodiment of the composition, the Si concentration may be in the range of about 0.05 to 0.3% by weight.

  Mn and Cr are elements that can be used in combination with B, Mo, and Ni for the purpose of improving curability. For example, adding them in alloying may help to prevent the formation of ferrite and pearlite from austenite during cooling. They may further improve fine-tuning of the microstructure by lowering the bainite starting temperature. In addition, Mn can cause solid solution hardening. In certain embodiments, the Mn concentration may range from about 0.6 to 1.6% by weight. In a further aspect, Cr may not be included in the composition. In other embodiments, the Cr concentration may be in the range of about 0.5 wt% or less.

Mo is an element used for the purpose of improving the curability of the steel composition. When Mo is alloyed, the degree to which phosphorus is sequestered by the grain boundaries is lowered, and the intergranular fracture resistance may be improved. Mo may further improve the curability effect brought about by B. In certain embodiments, Mo may not be included in the composition. In other embodiments, the Mo concentration may be in the range of about 0.5 wt% or less.

  Ni is an alloying additive that can improve curability and improve toughness. In certain embodiments, Ni may not be included in the composition. In other embodiments, the Ni concentration may be in the range of about 0.5 wt% or less.

  Ti is an element effective in improving the effect of B in steel by fixing nitrogen impurities such as TiN and suppressing the formation of boron nitride by addition. If the Ti content is too low, in some embodiments it may be difficult to obtain the positive effect that boron has on curability. As one aspect, when the Ti content is greater than about 0.03% by weight, hot ductility and toughness can be adversely affected by the formation of coarse TiN and TiC. Thus, in certain embodiments, the Ti concentration may range from about 0.01 to 0.03% by weight.

  In an alternative embodiment, the Ti concentration may be specified such that the ratio of Ti to N relative to the concentration of N is maintained above about 3.4 (in terms of concentration expressed in weight percent).

  In certain embodiments, substantially all of the N present in the composition may be in the form of TiN. In certain embodiments, about 90% or more, about 92% or more, about 94% or more, about 96% or more, about 98% or more and about 99% or more of the N content of the composition is present in the form of TiN. It may be. Possible forms of TiN include, but are not limited to, particles.

  Nb is an alloying additive that can be used for the purpose of finely adjusting the austenite grain size of the composition. Nb may further improve the effect of boron on curability and cause precipitation hardening. In certain embodiments, Nb may not be included in the composition. In other embodiments, the concentration of Nb may be in the range of about 0.05% by weight or less.

  V is an alloying additive that can be used for the purpose of causing precipitation hardening. In certain embodiments, V may not be included in the composition. In other embodiments, the concentration of V may be in the range of about 0.15 wt% or less.

  O is an impurity that may be present in the steel composition in the form of an oxide, for example. If the oxygen content is high, impact characteristics may be deteriorated. Therefore, it is preferable to reduce the oxygen content. In one embodiment, the upper limit of oxygen content may be about 0.0050% by weight. In another embodiment, the upper limit of oxygen content is about 0.0015% by weight or less.

  Cu is not required for aspects of the steel composition, but may be present. In some aspects, depending on the manufacturing process, it may be impossible to avoid the presence of Cu. Thus, in one embodiment, the maximum Cu content may be about 0.10 wt% or less.

  S, P, Ca, N, etc. are impurities, and their concentrations are preferably kept as low as possible. In certain embodiments, the concentration of each of S, P, Ca and N may independently be as follows: S is about 0.005 wt% or less, P is about 0.015 wt% or less, Ca is About 0.003 wt% or less, and N about 0.008 wt% or less. In an alternative embodiment, the concentration of each of S, P, Ca and N may independently be as follows: S is about 0.003% or less, P is about 0.015% or less, Ca is about 0.002 or less and N is about 0.006% by weight or less.

  Liquid steel may be continuously cast in the steel casting operation 114. In a particular embodiment, the liquid steel is cast into a rod, but it can be understood that it can be cast into other shapes. In particular, the cooling rate of the cast bar may be selected such that the size of the TiN precipitates produced during solidification is controlled. In certain embodiments, the cooling rate during casting may be maintained at a selected rate so that grain coarsening of TiN precipitates is suppressed. In certain embodiments, the cooling rate may be selected such that the TiN precipitate size is less than about 50 nm. In one aspect, the cooling rate from casting may be maintained such that the cooling rate around the center of the rod is greater than about 5 ° C./min. In a further aspect, the cooling rate from casting may be maintained such that the cooling rate around the center of the rod is greater than about 10 ° C / min. In another aspect, the cooling rate from casting may be maintained such that the cooling rate around the center of the rod is greater than about 20 ° C./min. In an additional aspect, the cooling rate from casting may be maintained such that the cooling rate around the center of the rod is greater than about 30 ° C./min.

  In one aspect, the rod so processed may then be formed into a tubular rod, or tube, in a steel forming operation 104, and more particularly, a seamless tube. The substantially cylindrical solid steel rod may be subjected to a first reheating operation (block 116) to bring the temperature to the austenite region, i.e. about 1200 ° C to 1300 ° C, preferably about 1250 ° C. In blocks 120 and 122, the rods are further drilled, and in certain preferred embodiments, the Mannesmann method is used to drill holes at temperatures in the range of about 1100 to 1200 ° C., followed by hot rolling for about 900 to 1100. The temperature is in the range of ℃.

  The hot rolled seamless steel pipe preferably has a substantially uniform wall thickness both on the circumference of the pipe and on the longitudinal axis along the pipe axis. As an example, a tube molded in such a manner may have an outer diameter in the range of about 60 to 273 mm and a wall thickness in the range of about 6 to 25 mm. As another example, a solid bar having an outer diameter of about 290 mm can be hot rolled in such a manner to produce a tube having an outer diameter of about 244.5 mm and a wall thickness of about 16 mm. It is.

  The microstructure may be fine-tuned by the reduced cross-sectional area experienced by the tube during hot rolling. Fine adjustment of the microstructure advantageously makes it possible to obtain the desired mechanical properties of the work tube. Next, the hot-rolled seamless steel pipe thus manufactured may be cooled to room temperature. In certain embodiments, the austenite grain size exhibited by the steel after hot rolling but before transformation can range from about 10 to 50 μm. In other embodiments, the austenite grain size exhibited by the steel after hot rolling but before transformation can be about 20 to 50 μm.

  When austenite fine-tuning occurs to such a degree, it is beneficial to have a good balance of strength and toughness following subsequent heat treatments such as quenching or quenching after the selected composition has accelerated cooling from the final rolling temperature. This can be achieved without the need for tempering. As another aspect, when austenite fine-tuning occurs to such a degree, a composition with a high carbon concentration can achieve a good balance of strength and toughness when subjected to heat treatments such as quenching and tempering. It becomes possible.

  Embodiments of the composition may be cooled at block 124 from hot rolling to air cooling or accelerated cooling. When cooling with air, a cooling rate of less than about 1 ° C./second may be achieved for tubes having a wall thickness greater than about 8 mm. Moreover, the intensity | strength and toughness which the said steel composition shows can be improved also by using subsequent heat processing.

In the case of accelerated cooling, cooling may be performed directly from hot rolling without an intermediate cooling step to room temperature (block 124). Several devices can be used to achieve a cooling rate faster than that corresponding to natural air cooling, including but not limited to forced air flow, water spray and air-water. Mixture spray is included. The refrigerant flow may be directed to the outer wall of the tube, or may be directed to the inner and outer walls of the tube to improve the microstructure uniformity. In certain embodiments, using the cooling alternatives described above, cooling rates in the range of about 5 to 50 ° C./second can be achieved by accelerated cooling. In other embodiments, cooling rates in the range of about 10 to 50 ° C./second may be used. In further embodiments, cooling rates in the range of about 10 to 20 ° C./second may be used. As an additional aspect, such cooling rates can be used for tubes with wall thicknesses in the range of about 8 mm and 25 mm.

  Hot-rolled tube forming can be achieved using multiple finishing steps. Non-limiting examples of such finishing steps include cutting the tube to a length, for example, about 8 to 15 m, cutting off unwanted portions at the end of the tube, straightening the tube and non- Performing destructive inspection (eg, electromagnetic inspection, ultrasonic inspection) may be included. In such a manner, a metallic tubular rod having a substantially straight profile with a composition within the range illustrated in Table 1 can be produced.

  Optionally, one or more heat treatment operations 106 may be performed on the tube after undergoing the forming operation 104. In one aspect, quenching may be performed at block 126. For example, the composition may be reheated a second time before being quenched to a temperature above the austenite range, ie about Ae3 (eg, about 870-950 ° C.). The soak time at the maximum temperature may range from about 5 to 30 minutes. Furthermore, it is possible to cool the composition from about maximum temperature to about room temperature by quenching with water spray.

  In other embodiments, the tempered composition may further be tempered at block 128. Tempering can be performed by heating to a temperature in the range of about 400 to 700 ° C., holding the tempering temperature for a selected time, and performing air cooling from the tempering temperature to about room temperature. is there. The composition may be held at the tempering temperature for about 10 to 60 minutes.

  The manufacture, microstructure and mechanical properties of embodiments of the three steel compositions of the present disclosure (referred to as compositions 1, 2 and 3) are discussed in the following examples. In addition, the performance benefits achieved by such compositions are also considered. It can be understood that this example is considered for purposes of illustration and should not be construed to limit the scope of the disclosed embodiments.

  The concentrations of alloying elements present in compositions 1, 2 and 3 are shown in Table 2 below.

  Composition 1 is a composition devised for the purpose of producing a fine bainite structure after accelerated cooling from the austenite region. In contrast, composition 2 with the highest carbon content is a composition designed for use in quenching and tempering as discussed below after air cooling from hot rolling. Composition 2 is substantially free of Cr and Nb compared to the other compositions because of the higher carbon content. Moreover, the composition 3 having the lowest carbon content is a composition devised for the purpose of obtaining high strength and good toughness in a state of being quenched without being tempered.

  The steel having compositions 1, 2 and 3 was melted in an about 20 kg vacuum induction furnace, and then the electroslag was remelted to lower the sulfur content. Thereafter, compositions 1, 2 and 3 were cast to produce a slab having a thickness of about 140 mm and then hot rolled to a final thickness of about 16 mm. During hot rolling, reheating and finishing temperatures of about 1200-1250 ° C. and 950-1000 ° C., respectively, were used. Thereafter, all the hot-rolled plates were air cooled to room temperature.

The hot-rolled composition 1 was subjected to one of the following post-rolling processing procedures: a) quenching after reheating to an austenite region at a temperature of about 900-950 ° C. I do,
b) Reheating to make the austenite zone at a temperature of about 900-950 ° C. followed by quenching and tempering.
c) Accelerated cooling is performed after reheating to the austenite region at a temperature of about 920-950 ° C.

  Quenching was performed in water with gentle agitation to bring the temperature from about 900-950 ° C. to about room temperature. When tempering was also performed, the composition was heated in the range of about 300 ° C. to 450 ° C., soaking time was about 1 hour.

  In accelerated cooling, the composition is cooled in a mixture of air and water at a cooling rate in the range of about 5 to 45 ° C./second from a reheating temperature of about 900 to 950 ° C. to about room temperature. Carried out. In selecting the reheating temperature prior to accelerated cooling, the temperature was selected so as to obtain a typical austenite microstructure that is achieved industrially just at the exit of the hot rolling mill. A complete heat treatment was performed using a Gleeble 3500 thermo-mechanical simulator.

Hot-rolled compositions 2 and 3 were subjected to one of the following two post-rolling processing procedures:
a) Quenching after reheating to the austenite region.
b) After reheating to the austenite region, quenching and tempering are performed.

  Quenching was performed by heating the composition to a temperature of about 925 ° C. (Composition 2) or about 930 ° C. (Composition 3) for a soak time of about 10 minutes. Cooling was performed in water using mild agitation from quenching temperature to near room temperature. When tempering was performed, the composition was heated to a temperature in the range of about 400 to 700 ° C. so that the soak time at the maximum temperature was about 30 minutes.

The continuous cooling transformation (CCT) behavior was evaluated by carrying out an expansion test using a Gleeble 3500 thermomechanical simulation apparatus for each composition. Compositions 1, 2 and 3 were reheated at about 5 ° C./s to about 920 ° C., 925 ° C. and 930 ° C., respectively, and the soak time at maximum temperature was about 10 minutes. In selecting the austenitizing temperature, a temperature about 20-30 ° C. higher than the Ac3 temperature corresponding to the individual composition was selected. In composition 1, about 0.5 to 50
Cooling rates in the range of ° C / second were investigated, and for compositions 2 and 3, cooling rates in the range of about 0.2 to 50 ° C / second were investigated. In addition, the resulting microstructure characterization was performed using optical and scanning electron microscopy.

In addition, the mechanical properties of the composition so processed were evaluated by mechanical tests, which included one or more of a tensile test, a hardness test and a Charpy test. In each case, tensile samples and full-size Charpy samples were taken in the transverse direction. Tensile testing is done in accordance with ASTM E8, “Standard Test Methods for
Performed according to “Tension Testing of Metallic Materials”, which is incorporated herein by reference in its entirety, and the results reported are the average of two samples.

The Charpy test is performed according to ASTM E23, “Standard Test Methods.
for Notched Bar Impact Testing of Metallic Materials ", which is incorporated herein by reference in its entirety, and the results reported are the average of 2 or 3 samples. 1 was performed at temperatures of about −20 ° C., 0 ° C. and room temperature, while impact tests were performed for compositions 2 and 3 at temperatures of about −60 ° C., −40 ° C., −20 ° C., 0 ° C. and room temperature. did.

  Hardness testing was performed according to ASTM E92, “Standard Test Methods for Vickers Hardness of Metallic Materials”, which is incorporated herein by reference in its entirety.

Example 1 Evaluation of Continuous Cooling Transformation (CCT) Behavior and Microstructure when Composition 1 is Cooled at Cooling Rates in the Range of about 0.5 and 50 ° C./sec Composition 1 is subjected to expansion measurements The obtained CCT chart is shown in FIG. FIG. 2 shows the temperature trajectory as a function of cooling rate when transformation occurs at about 5%, 20%, 50%, 80% and 95%. Due to reheating conditions at about 920 ° C. for about 10 minutes, the austenite grain size before transformation was estimated to be about 10-20 μm based on a sample cooled at about 50 ° C./second.

  Two transformation regions can be observed in the CCT diagram shown in FIG. 2, which corresponds to a cooling rate slower than about 5 ° C./second and a faster cooling rate. When the cooling rate is less than about 5 ° / second, it is observed that the phase transformation begins at about 550-600 ° C. The microstructure produced under such conditions is mainly austenite in addition to being bainite, as shown in the micrographs corresponding to cooling rates of about 2 ° C./second and 5 ° C./second shown in FIG. Was holding some. When the cooling rate is about 5 ° C./second or more, the transformation start temperature drops to about 450 ° C., which is close to about 452 ° C., which is the martensitic transformation temperature calculated according to the Andrews equation. The microstructure observed under such conditions was also mainly bainite as shown in the micrographs corresponding to cooling rates of about 10 ° C./second and 20 ° C./second shown in FIG. However, it should be noted that the bainite structure is finer and does not retain an austenite region having substantially uneven density.

  Moreover, the hardness measured value after cooling the composition 1 with various cooling rates (0.5-50 degreeC / sec) is also shown in the CCT chart of FIG. It can be observed that the hardness range extends from about 262 Hv when the cooling rate is about 2 ° C./second to about 340 Hv or more when the cooling rate is about 50 ° C.

Furthermore, based on the high measured hardness when the cooling rate is about 50 ° C./second, it is predicted that a small martensite region may appear when the cooling rate is about 50 ° C./second and above. Is done. However, it should be noted that substantially no large martensite region was observed in the microstructure corresponding to the sample cooled at about 50 ° C.

Example 2-Toughness Evaluation of Composition 1-Accelerated Cooling Conditions For the purpose of examining the impact properties that Composition 1 exhibits under accelerated cooling conditions, several Charpy were used using the thermal cycle discussed above with respect to the CCT diagram. The test was conducted. Samples prepared with cooling rates of about 5 ° C./second, 10 ° C./second, 30 ° C./second and 45 ° C./second were tested. Charpy tests were performed at temperatures of about 25 ° C, 0 ° C and -20 ° C. The results of this impact test are shown in Table 3 and FIGS. 4A and 4B, in which the impact energy (Charpy V-notch, CVN) was complementarily plotted as a function of cooling rate and test temperature, respectively.

  By examining Table 3 and FIGS. 4A and 4B, it can be observed that the impact energy value decreases as the cooling rate is increased. Moreover, it can be observed that excellent impact properties were obtained over the entire range of cooling rates tested. For example, the impact energy measured for samples tested at temperatures in the range of about 25 ° C. and −20 ° C. was in the range of about 335 to 240 J. Moreover, it can be seen from FIGS. 4A and 4B that the highest impact energy values correspond to samples cooled at cooling rates in the range of about 5 to 10 ° C./second. Nevertheless, even when the cooling rate was about 30 ° C./second, the impact energy value obtained at about −20 ° C. was about 220 J or more.

Example 3 Mechanical Evaluation of Composition 1-as-quenched Tables 4 and 5 show the tensile and impact properties exhibited by composition 1 in the as-quenched state.

  In general, as-quenched compositions showed improved strength and impact energy compared to as-rolled samples (from 25 ° C to -20 ° C, YS to 69 ksi, UTS to 99 ksi, CVN to 6-6 8J). Such an improvement can be attributed to the fact that the fine structure is generally fine-tuned and the large austenite region with uneven shading is substantially lost.

Example 4 Mechanical Evaluation of Composition 1-Tempering after Quenching a) Hardness Samples were quenched for the purpose of examining the tempering behavior of Composition 1 under tempering conditions following quenching. And then tempered at a temperature in the range of about 350 ° C. to 440 ° C. for about 1 hour. The measured hardness values are shown in FIG. In general, the hardness in the as-quenched state is about 362 Hv, but it can be observed that when tempering is performed from about 300 to 400 ° C., the hardness is somewhat lowered to a range of about 350 to 335 Hv. Further, the sample that had been tempered at about 440 ° C. showed a significant decrease in hardness, down to about 280 +/− 20 Hv.

b) Tensile and impact properties Two types of tempering conditions of more than about 400 ° C, namely 410 ° C and 440 ° C, were selected for use with a quench plate large enough to measure tensile and impact properties. Tables 6 and 7 below summarize the comparable measurements taken on the as-quenched samples in addition to summarizing the experimental results.

  It can be observed that a good combination of strength and toughness could be achieved in the tempered state after quenching. For example, the yield and tensile strength measured in the tempered state after quenching was about 129 ksi and about 138-141 ksi, respectively. In contrast, the yield strength measured for the as-quenched material was lower, about 121 ksi, while the tensile strength was higher, about 156 ksi.

  At the same time, it was also confirmed that the impact energy exhibited by the sample tempered after quenching was higher than that measured at a comparable temperature to the as-quenched sample. For example, samples subjected to tempering at 410 and 440 ° C. had an impact energy of about 215 and 170 J, respectively, at about 24 ° C., while the as-quenched material had an impact energy of about 150 J. . The difference in impact energy at about −20 ° C. was even greater, and the samples subjected to tempering at about 410 and 440 ° C. showed impact energy of about 136 and 113 J, respectively, but were quenched. The impact energy exhibited by the raw material was about 42 J.

  Although not limited in scope by theory, we believe that such differences in properties can be explained theoretically by the microstructure of the composition. As shown in FIG. 6, in addition to the presence of bainite and martensite in the microstructure shown in composition 1 in the tempered state after quenching, carbides are finely dispersed, thereby quenching The yield strength exhibited by the later tempered material is higher than that exhibited by the as-quenched material.

Summary of Composition 1 By examining the hardness, toughness, and tensile properties in the as-quenched state and in the tempered state after quenching, the hardness decreases as the tempering temperature increases, and is noticeable at around 400 ° C. It can be observed that a slow decline begins. Moreover, as a result of testing toughness in the as-quenched state and after tempering (410 ° C. and 440 ° C.), the toughness in the state of tempering after quenching at 410 ° C. It can be seen that it is generally higher than the as-quenched state and the state after tempering at 440 ° C. after quenching. In addition, when the tempering is performed from about 410 ° C. to 440 ° C., the yield strength is slightly improved, while when the tempering is performed from about 410 ° C. to 440 ° C., the maximum tensile strength is slightly decreased. . Such results indicate that the embodiment of Composition 1 in the tempered state after quenching provides a beneficial combination of toughness and strength as compared to simply as-quenched when in the range of about 410-440 ° C. It is shown that.

  It was also observed that excellent impact energy and hardness values were obtained despite accelerated cooling. Most notably, the impact energy values achieved when in the range of about 10 to 20 ° C./s were higher than about 220 J at about −20 ° C. and the ductile region was about 80% or more. In addition, the hardness value was in the range of about 300-320 Hv.

Example 5-Evaluation of continuous cooling transformation (CCT) behavior and microstructure exhibited by composition 2-3 before heat treatment Cooling rates were about 0.2, 0.5, 5, 10, 30 and 50 ° C / sec. The CCT charts created using the measured expansion measurements of compositions 2 and 3 are shown in FIGS. 7 and 9, respectively. The transformation start temperatures shown in these figures are the temperatures determined as the first deviation from the linear behavior of both expansion curves. The austenite particle size of Compositions 2 and 3 was estimated to be in the range of about 20 to 30 μm from values measured for samples cooled at about 50 ° C./second. Further, FIGS. 8 and 10 show optical micrographs taken of compositions 2 and 3 cooled at cooling rates of about 0.2, 0.5, 1, 10, 30, and 50 ° C./second.

  Depending on the measured CCT diagram and the observed microstructure, the transformation behavior exhibited by compositions 2 and 3 may be the same. The main transformation product is bainite when cooling is performed in the range of about 5 ° C./sec to 30 ° C./sec. When the cooling rate is made slower, polygonal ferrite becomes a major component. When composition 2 is cooled at a cooling rate of about 10 ° C./second, martensite appears and when composition 3 is cooled at about 30 ° C./second, martensite appears and cooling is about 50 ° C./second. When done in seconds, martensite is the major phase for both compositions.

  It can also be observed that there are many notable differences between the two types of compositions. As one aspect, in composition 2, when the cooling rate is less than about 5 ° C./second, a large amount of pearlite appears in addition to bainite. However, in composition 3, a more complex microstructure was observed, with bainite occupying a larger portion in addition to pearlite and retaining some austenite. While not theoretically limiting the range, such differences may be attributed to the lower carbon content of Composition 3, thereby reducing the total fraction of pearlite in addition to Cr. The addition of Nb and Nb is promoted to form bainite.

  Another aspect is that compositions 2 and 3 differ in the ratio of bainite. Despite the similarity in transformation temperature and austenite grain size, the bainite structure of composition 3 is generally finer than that of composition 2. Although not theoretically limiting the range, such observations are believed to be the result of adding Cr and Nb by alloying.

As a further aspect, composition 2 is more prone to martensite. As discussed above, the lowest cooling rate when martensite is observed in composition 2 is about 10
While the lowest cooling rate when martensite was observed in Composition 3 was about 30 ° C./second, while it was ° C./second. Further noteworthy is that composition 3 has only a few martensite spots observed at about 30 ° C./second, while the concentration of martensite present in composition 2 is similar to that of bainite. It is in the point which observed that it is some or more.

  From such observation, it can be understood that the composition 3 having a low carbon content is more advantageous for the bainite structure over a wider range of cooling rates than the composition 2. Again, although not theoretically limiting the range, such observations can also be the result of adding Cr and Nb in alloying.

  The hardness values shown by compositions 2 and 3 are also shown in FIGS. 11A and 11B as a function of cooling rate. Creusot-Loire modeling (Ph.Maynier, B.Jungmann and J.Dollet, "Creusot-Loire system for the prediction of the mechanical properties of low alloy steel products", Hardenability concepts with applications to steels, D.V.Doane and J S. Kirkaldy, The Metallurgical Society of AIME (1978) (see page 518), the values of the calculations performed using the same graph are shown as a comparison (dashed line). It should be noted that composition 3 exhibits a slightly higher hardness value than that of composition 2 at a cooling rate of less than about 30 ° C./s despite the lower carbon content.

  Although not limiting in scope by theory, such an increase in hardness may be due to the fine adjustment of the microstructure of composition 3 already described. Moreover, in the composition 3, when the cooling rate is further reduced, there is a possibility that the hardness can be improved by reprecipitation of Nb dissolved in the austenitizing stage as fine carbides.

Example 6 Mechanical Evaluation of Composition 2-3-as-quenched The tensile and impact properties measured for composition 3 as-quenched are shown in Tables 8 and 9 below. Table 9 also shows the hardness characteristics of the composition 2. Corresponding SEM micrographs of these compositions are shown in FIGS. 12A and 12B. These results indicate that carbon affects the microstructure and mechanical properties of the composition.

  The microstructure of the as-quenched composition 2 was mainly martensite and some bainite regions were present (FIG. 12A). In addition, composition 2 exhibited a relatively high hardness of about 350 Hv. Based on experience with other systems, it was predicted that the toughness of such systems would be poor, so neither a tensile test nor an impact test was performed.

  In contrast, the microstructure of as-quenched composition 3 was predominantly bainite and there was little martensite area present (FIG. 12B). In this case, tensile and impact tests were performed to obtain a beneficial combination of properties. The measured yield strength was about 121 ksi, and the ratio of yield strength to tensile strength was low, about 0.82. Furthermore, it was also confirmed that the temperature at which transition from ductility to brittleness (measured as that corresponding to a shear area of about 50%) was about −40 ° C. In addition, the measured impact energy was substantially constant and about 160 J over a range of about -20 ° C to 20 ° C.

Example 7-Mechanical evaluation of composition 2-3-tempered after quenching Example 6 confirms that in the case of composition 3 beneficial properties are obtained when in the as-quenched state did. In order to further explore the effect of tempering on compositions 2 and 3, similar tests and evaluations were performed on samples of compositions 2 and 3 that were tempered after quenching.

  FIGS. 13A and 13B show scanning electron micrographs of the microstructures of compositions 2 and 3 in the tempered state after quenching. In both compositions, the microstructure consisted mainly of bainite that was slightly tempered. There were also some tempered small martensite regions, especially in the composition 2 with the higher carbon content.

  The hardness results obtained from small samples of compositions 2 and 3 that were tempered at about 400 to 700 ° C. are shown in FIG. It can be observed that both compositions show similar gradual hardness changes in response to increased tempering temperatures. As expected, it was confirmed that the composition 2 showed higher hardness than the composition 3 when the as-quenched state and the tempering temperature were low because the carbon content was higher. However, conversely, it was confirmed that when the tempering temperature was about 550 ° C. or higher, the hardness exhibited by the composition 3 was higher than that exhibited by the composition 2.

  Without being bound by theory, it is believed that such a result could be the result of alloying Nb and Cr into composition 3. While the former induces some precipitation hardening, the latter may delay cementite grain coarsening.

  In view of such observations, tensile and impact energy tests were further performed on samples of Compositions 2 and 3, which had been heat treated at a temperature of about 500 ° C. The results of the tensile test are shown in Table 10, while the impact energy is shown in Table 11.

  The mechanical properties after tempering at about 500 ° C. for about 30 minutes confirmed that both compositions 2 and 3 showed a good combination of strength and toughness. Compositions 2 and 3 exhibited a yield strength of about 118 ksi and a maximum tensile strength of about 126-127 ksi.

  In addition, compositions 2 and 3 exhibited a ductile to brittle transition temperature of about −60 ° C. or less and a shear area of approximately 100% over the entire temperature range tested. The upper shelf energy corresponding to 100% shear area is about 180 J for both alloys, which is a good value considering the level of strength exhibited by these compositions. In addition, the impact energy differences exhibited by these compositions over a temperature range of about −40 ° C. to 20 ° C. are only slight, whereas composition 2 showed that it is about 177 to 185 J. Composition 3 showed that it was about 175 to 189J.

  It should be noted that the tensile and impact energy properties exhibited by compositions 2 and 3 are approximately the same despite differences in alloying content. Although not limited in scope by theory, when comparing the chemical properties exhibited by the two compositions, Cr and Nb are added by alloying even if the carbon content in composition 3 is low. It seems that this is almost offset.

Summary of Compositions 2 and 3 Inspecting Compositions 2 and 3 which were hot-rolled and then reheated and quenched following air cooling showed that the carbon content was maintained below about 0.07% (composition Product 3), which showed good toughness. Moreover, when tempering was performed at a temperature of about 500 ° C. after quenching, a good combination of strength and toughness was obtained. In this case, excellent mechanical properties were obtained with both compositions 2 and 3, the yield strength was about 118 ksi and the impact energy at about −40 ° C. was about 175-179 J. In addition, it was also confirmed that the surface undergoes almost complete ductile fracture over the entire test temperature range examined, with both materials having a ductile to brittle transition temperature well below about −60 ° C.

Example 7-Thermal cycle simulation in the heat affected zone (HAZ) for composition 2 A thermal cycle simulation in HAZ was performed on a sample of composition 2. Using the Hannerz model (NE Hannerz, “Effect of Cb on HAZ ductility in construction HT Steels”, Welding Journal, May 1975), which is incorporated herein by reference in its entirety. The heat generation in the HAZ was estimated by changing the conditions and the shape of the tube. The computational thermal cycle was reproduced using a thermo-mechanical simulator Gleeble. In addition, the microstructure and hardness exhibited by the heat treated sample was also analyzed.

The results were compared with the results corresponding to a commercial low carbon Nb-V microalloyed steel used in the manufacture of X65 thick-walled seamless tubes. Nb-V steel shows approximately the same carbon content and P _cm value as composition 2, but no boron is added as shown in Table 12.

  Various preheat temperature and heat input simulations were performed on tubes with wall thickness of about 16 mm and / or 25 mm (Table 13). In all cases, the maximum reheat temperature was about 1350 ° C. in order to obtain the maximum possible austenite grain size. Such conditions are known to be disadvantageous for toughness because of their high curability. Simulations were performed with values in the range of about 450 to 1210 J / mm for heat input (HI) and the preheat temperature was in the range of no preheat to about 250 ° C.

  The hardness results for all tests are shown in the last column of Table 13. Assuming a maximum hardness of about 300 HV, this corresponds to the maximum HAZ hardness specified by the standard API 5L for X65-X80 PSL2 tubes (offshore applications). It is clear from Table 13 that the minimum heat input should be about 500 J / mm when using a preheat of about 150 ° C. in the range of wall thicknesses analyzed. Without preheating, a tube with a wall thickness of about 25 mm would increase the minimum input to about 950 J / mm.

  By comparing the hardness measured as a function of the average cooling rate with the results obtained for Nb-V steel using the same welding conditions (FIG. 15), it is clear that both steels show almost the same hardenability. Such a result indicates that the steel of composition 2 has no severe welding restrictions, because the hardness behavior it exhibits as a function of cooling rate is substantially similar to that exhibited by commercial X65 steel. Because it is the same.

  At present, there is no standard specification for grades better than X100 regarding maximum hardness in HAZ. However, considering the maximum value specified in API 5L for X80 PSL 2 (offshore) as a reference, the steel of composition 2 uses a preheat of about 150 ° C. and a minimum heat input of about 500 J / mm, It will meet such requirements.

In summary, a low carbon steel with alloying additions of boron and titanium is provided. Free nitrogen impurities are substantially consumed by reacting with titanium to form TiN precipitates. Furthermore, casting parameters are also selected so that such precipitates do not cause grain coarsening. For example, by using a cooling rate of about 10 to 40 ° C./min or more during casting, fine TiN precipitates having an average diameter of less than about 50 nm can be achieved. Substantial removal of free nitrogen impurities further allows free boron to remain in solid solution, thereby improving curability during austenite decomposition. Such compositions are quenched from hot rolling in air and then quenched, tempered following tempering, or accelerated cooling to about 5 to 50 ° C. immediately after hot rolling. Subjecting to a range of cooling rates can provide an excellent balance of strength and toughness.

  The term “about” as used throughout this specification should be understood to include its ordinary meaning as understood by those skilled in the art. When the term “about” is used herein with respect to an individual value or range of values, the exact value or range of values indicated is also intended to be part of this disclosure.

  While the above description has shown, described and pointed out novel basic features of the present teachings, those skilled in the art will not be limited to the details of the apparatus shown, various omissions in terms of the mode of use thereof, It will be understood that substitutions, changes and / or additions may be made without departing from the scope of the present teachings. Accordingly, the scope of the present teachings should not be limited to the above considerations, but should be limited by the appended claims.

Claims (28)

A method of manufacturing a steel pipe,
The carbon (C) content is 0.04-0.12 wt%,
Manganese (Mn) content is 0.6 to 1.6 wt%,
The silicon (Si) content is 0.05 to 0.3% by weight;
The nickel (Ni) content is less than or equal to 0.5% by weight;
The chromium (Cr) content is less than or equal to 0.5% by weight;
The molybdenum (Mo) content is less than or equal to 0.5% by weight;
The vanadium (V) content is less than or equal to 0.15% by weight;
The niobium (Nb) content is less than or equal to 0.05% by weight;
Titanium (Ti) content is 0.01 to 0.03 wt%,
The boron (B) content is 0.0005 to 0.003% by weight ;
The nitrogen (N) content is less than or equal to 0.008% by weight and the balance consists of iron (Fe) and inevitable impurities, where
The concentration of each element is a concentration based on the total weight of the steel composition, and 0.0005 to 0.002% by weight of boron is present as a solid solution for improving the hardenability.
A steel rod is cast from the steel composition, and the casting is cooled so that the cooling rate at the center of the rod is 10 ° C./min or more. At this time, the cooling rate at the center of the rod is The resulting TiN particles are selected to exhibit an average diameter of less than 50 nm, and more than 90% of the nitrogen is present in the form of TiN particles, thereby avoiding the formation of boron nitride and present as said solid solution of boron The rod is then hot rolled to form a steel pipe, and the steel pipe from the hot rolling is cooled from austenitic to room temperature at a rate of 5 to 50 ° C./second, provided that The steel pipe has a final microstructure of bainite containing 30% or less martensite, and the yield strength of the steel pipe is 100 ksi (690 MPa) measured according to ASTM E8. large,
A method characterized by that.   The method according to claim 1, wherein the steel bar is cooled so that a cooling rate around a center part of the bar is greater than 30 ° C./min from casting. The elemental concentration of the steel composition is the carbon equivalent (CEpcm) of the composition [where CEpcm is calculated.

And the concentration of each element is expressed in weight%]
The method of claim 1, wherein is selected to be less than 0.22. A steel pipe having a final microstructure of bainite containing 30% or more martensite is obtained by tempering the quenched steel pipe of claim 1 at a temperature in the range of 400 to 700 ° C. over 10 to 60 minutes. How to get . A method for producing a cast steel composition comprising:
The carbon (C) content is from 0.04 to 0.12% by weight;
Manganese (Mn) content is 0.8 to 1.6 wt%,
The silicon (Si) content is 0.05 to 0.3% by weight;
The nickel (Ni) content is less than or equal to 0.5% by weight;
The chromium (Cr) content is less than or equal to 0.5% by weight;
The molybdenum (Mo) content is less than or equal to 0.5% by weight;
The vanadium (V) content is less than or equal to 0.15% by weight;
The niobium (Nb) content is less than or equal to 0.05% by weight;
Titanium (Ti) content is 0.01 to 0.03 wt%,
The boron (B) content is from 0.0005 to 0.0003% by weight ;
The nitrogen (N) content is less than or equal to 0.008% by weight and the balance consists of iron (Fe) and inevitable impurities, where
The concentration of each element is a concentration based on the total weight of the steel composition, and 0.0005 to 0.002% by weight of boron exists as a solid solution for improving the hardenability.
Prepare the steel composition,
The steel composition is cast, wherein the steel composition is present at a rate of 10 ° C./min or more from the casting, and all the nitrogen in the steel composition is present in the form of TiN particles having a size of less than 50 nm. So that the formation of boron nitride is avoided and results in the amount of boron present as the solid solution, then cooling
The cast steel composition is hot rolled, and the cast steel composition is immediately moved from the austenitic temperature to room temperature at a rate of 5 to 50 ° C./second without subsequent heat treatment immediately after the hot rolling. To obtain a final microstructure of bainite containing 30% or less martensite.
A method characterized by that. Furthermore,
Reheating the cast steel composition to 1200 to 1300 ° C;
Drilling the cast steel composition at a temperature in the range of 1100 to 1200 ° C, and hot rolling the cast steel composition at a temperature in the range of 900-1100 ° C.
The method of claim 5, further comprising:   The method according to claim 5, wherein the austenite grain size of the steel composition before cooling from hot rolling is in the range of 20 to 50 µm.   The method of claim 7, wherein the cooling of the composition is performed immediately after hot rolling at a cooling rate in the range of 10 to 50 ° C / sec.   The method of claim 7, wherein the composition is cooled immediately after hot rolling at a cooling rate in the range of 10 to 20 ° C / second.   The method of claim 8 wherein the yield strength of the cast steel composition after hot rolling and cooling is at least 100 ksi (690 MPa) as measured according to ASTM E8. 9. The method of claim 8, wherein the Charpy V-notch impact energy is 220 J or greater when a full size sample of the cast steel composition is measured according to ASTM E23 at a temperature equal to or higher than −20 ° C. The carbon (C) content of the composition is 0.05-0.10 wt%,
Manganese (Mn) content is 0.8 to 1.6 wt%,
The silicon (Si) content is 0.05 to 0.30% by weight;
Nickel (Ni) content is 0.4 wt% or less,
Chromium (Cr) content is 0.3 wt% or less,
Molybdenum (Mo) content is 0.3 wt% or less,
Vanadium (V) content is 0.1 wt% or less,
Niobium (Nb) content is 0.04% by weight or less,
The titanium (Ti) content is 0.015 to 0.025 wt% or less,
The boron (B) content is 0.0005-0.015% by weight and the nitrogen (N) content is less than or equal to 0.007% by weight;
The method of claim 5.   A method of forming a tube from a cast steel composition according to claim 5. The formed cast steel composition is air-cooled immediately after hot rolling at a cooling rate of less than 1 ° C./second, and the final microstructure of the steel composition consists of bainite containing less than 30% martensite 6. The method of claim 5, wherein the austenitizing and quenching is performed. Furthermore,
Reheating the cast steel composition to 1200 to 1300 ° C;
Drilling the cast steel composition at a temperature in the range of 1100 to 1200 ° C, and hot rolling the cast steel composition at a temperature in the range of 900-1100 ° C.
15. The method of claim 14, further comprising: The carbon (C) content of the steel composition is 0.07 to 0.10% by weight,
Manganese (Mn) content is 1.0 to 1.4% by weight,
The silicon (Si) content is 0.05 to 0.15% by weight;
Nickel (Ni) content is 0.4 wt% or less,
Chromium (Cr) content is 0.35 wt% or less,
Molybdenum (Mo) content is 0.3 wt% or less,
Vanadium (V) content is 0.1 wt% or less,
Niobium (Nb) content is 0.04% by weight or less,
The titanium (Ti) content is 0.02 to 0.03 wt% and the boron (B) content is 0.001 to 0.002 wt%,
The method of claim 15. The Rukoto subjected to tempering the cast steel composition was subjected to quenching of claim 16 at a temperature in the range of 400 to 600 ° C., the final microstructure of bainite containing 30% or more of martensite A method of obtaining a cast steel composition having   The yield strength of the composition after hot rolling, cooling, austenitizing and quenching is greater than 100 ksi as measured according to ASTM E8, and a full size sample of the composition is equal to −40 ° C. according to ASTM E23 or The method according to claim 17, wherein Charpy V-notch impact energy when measured at a temperature higher than that is 170 J or more. The carbon content of the steel composition is 0.04 to 0.08% by weight;
The manganese content is 1.0 to 1.4% by weight,
The silicon content is 0.05 to 0.15% by weight,
The chromium content is less than 0.35% by weight,
The molybdenum content is 0.2 to 0.3% by weight,
The niobium content is 0.03 to 0.04% by weight,
The titanium content is 0.02 to 0.03 wt% and the boron content is 0.001 to 0.002 wt%,
The method of claim 15.   20. A method according to claim 19 wherein the steel is reheated to the austenitic zone and quenching is performed without subsequent tempering.   The yield strength of the composition after hot rolling, cooling, austenitizing and quenching is greater than 100 ksi as measured according to ASTM E8, and a full size sample of the composition is equal to −40 ° C. according to ASTM E23 or 21. The method of claim 20, wherein the Charpy V-notch impact energy is greater than 90J when measured at higher temperatures.   15. A method of forming a tube from a cast steel composition according to claim 14.   The method of claim 1, further comprising heating the steel pipe to an austenite temperature range and quenching.   The process of claim 1 wherein more than 96% of the nitrogen is present in the form of TiN particles.   The method of claim 1, wherein the final microstructure of the steel composition comprises bainite containing no more than 30% martensite.   6. A process according to claim 5, wherein more than 90% of the nitrogen is present in the form of TiN particles.   15. A process according to claim 14 wherein more than 90% of the nitrogen is present in the form of TiN particles.   6. The method of claim 5, wherein the bainite is upper bainite.