JP4696570B2 - Manufacturing method of high-tensile steel material with excellent hydrogen embrittlement resistance - Google Patents

Description

  The present invention relates to a method for producing a high-tensile steel material excellent in hydrogen embrittlement resistance, which is unlikely to cause hydrogen embrittlement such as delayed fracture, weld delayed cracking, and sulfide corrosion cracking. Is suitable.

In recent years, in the field of steel materials such as construction industry machinery, tanks, penstock, and line pipes, the strength of steel materials to be used has been increasing due to the increase in size of structures, and the environment for using steel materials has become increasingly severe. It is out.

However, it is known that such high strength of steel materials and severe usage environment generally increase the sensitivity of steel materials to hydrogen embrittlement. For example, in the field of high strength bolts, F11T class bolts in JIS B 1186 With regard to (tensile strength 1100 to 1300 N / mm ² ), the use of high-strength steel is limited, for example, it is described that it should not be used as much as possible.

Therefore, in Patent Document 1, Patent Document 2, Patent Document 3, Patent Document 4, etc., optimization of components, grain boundary strengthening, crystal grain refinement, utilization of hydrogen trap sites, organization morphology control, fine dispersion of carbides A method of manufacturing a steel sheet having excellent hydrogen embrittlement resistance using various techniques such as crystallization has been proposed.
JP-A-3-243745 JP 2003-73737 A Japanese Patent Laid-Open No. 2003-239041 JP 2003-321743 A

  However, even by the methods described in Patent Documents 1 to 4 and the like, it is difficult to obtain the hydrogen embrittlement resistance required when used in a severe corrosive environment when the strength level is high. There has been a demand for a method for producing a high-strength steel material having excellent hydrogen embrittlement characteristics.

  Therefore, an object of the present invention is to provide a method for producing a high-tensile steel material having a strength of 600 MPa or more and excellent in hydrogen embrittlement resistance than before.

In order to overcome the above-mentioned problems in the prior art, the present invention provides uniform cementite by defining the rate of temperature rise in the center part in the plate thickness direction of the steel material according to the component system, especially during the tempering treatment of the quenched / tempered material. The present invention provides a method for producing a high-strength steel material that achieves fine dispersion and is more excellent in hydrogen embrittlement resistance than conventional materials. The gist of the method is as follows.

1% by mass, C: 0.02 to 0.25%, Si: 0.01 to 0.8%, Mn: 0.5 to 2.0%, Al: 0.005 to 0.1%, N : 0.0005~0.008%, P: 0.03% or less, S: 0.03% or less, Si (mass%) of the billet and the balance being Fe and unavoidable impurities from the Ar ₃ transformation point or more × After quenching to a temperature of 700 + Cr (mass%) × 50 + 250 (° C.) or less, the average heating rate at the center of the steel material from the tempering start temperature to Si (mass%) × 700 + Cr (mass%) × 50 + 250 (° C.) is 1 ° C. / S and the average rate of temperature rise at the center of the steel material from Si (mass%) × 700 + Cr (mass%) × 50 + 250 (° C.) to Si (mass%) × 700 + Cr (mass%) × 50 + 370 (° C.) Tempering at 1 ° C / s or higher It characterized the door, the method of producing a high tensile steel having excellent hydrogen embrittlement resistance characteristics.

2 Further, the steel composition is characterized by containing, in mass%, one or more of Cu: 2% or less, Ni: 4% or less, Cr: 2% or less, Mo: 1% or less, A method for producing a high-tensile steel material having excellent hydrogen embrittlement resistance according to 1 .

3 Further, the steel composition is characterized by containing, by mass%, one or more of Nb: 0.05% or less, V: 0.5% or less, Ti: 0.03% or less, A method for producing a high-tensile steel material having excellent hydrogen embrittlement resistance as described in 1 or 2 .

4 Further, the steel composition is one or more of mass%, B: 0.003% or less, Ca: 0.01% or less, REM: 0.02% or less, Mg: 0.01% or less. The method for producing a high-tensile steel material having excellent hydrogen embrittlement resistance according to any one of 1 to 3 , wherein the high-tensile steel material is contained.

  According to the present invention, it is possible to produce a high-tensile steel material having a tensile strength of 600 MPa or more and extremely excellent hydrogen brittleness resistance.

The reasons for limiting the components in the present invention will be described. All the percentages indicating the chemical composition are mass%.
C: 0.02-0.25%
C is included to ensure strength, but if it is less than 0.02%, its effect is insufficient.On the other hand, if it exceeds 0.25%, the toughness of the base metal and the weld heat-affected zone deteriorates, and the weldability is remarkably high. to degrade. Therefore, the C content is limited to 0.02 to 0.25%.

Si: 0.01-0.8%
Si is contained as a deoxidizer and strength improving element in the steelmaking stage, but if it is less than 0.01%, its effect is insufficient, while if it exceeds 0.8%, the grain boundary becomes brittle and promotes the occurrence of delayed fracture. . Therefore, the Si content is limited to 0.01 to 0.8%.

Mn: 0.5-2.0%
Mn is contained to ensure the strength, but if it is less than 0.5%, its effect is insufficient. On the other hand, if it exceeds 2.0%, the toughness of the weld heat affected zone deteriorates and the weldability deteriorates remarkably. Therefore, the Mn content is limited to 0.5 to 2.0%.

Al: 0.005-0.1%
Al is added as a deoxidizing material, and at the same time, is effective in reducing the crystal grain size, but if it is less than 0.005%, the effect is not sufficient, while if it exceeds 0.1%, Surface flaws of the steel sheet are likely to occur. Therefore, the Al content is limited to 0.005 to 0.1%.

N: 0.0005-0.008%
N is added in order to refine the structure by forming a nitride with Ti or the like and to improve the toughness of the base material and the weld heat affected zone. If the addition is less than 0.0005%, the effect of refining the structure is not sufficiently brought about. On the other hand, the addition exceeding 0.008% impairs the toughness of the base metal and the weld heat-affected zone because the amount of solute N increases. Therefore, the N content is limited to 0.0005 to 0.008%.

P: 0.03% or less, S: 0.03% or less
P and S are both impurity elements, and when it exceeds 0.03%, it becomes impossible to obtain a sound base material and a welded joint. Therefore, the P and S contents are limited to 0.03% or less, respectively.

In the present invention, the following components can be further contained according to desired properties.
Cu: 2% or less
Cu has the effect of improving strength by solid solution strengthening and precipitation strengthening. However, if the Cu content exceeds 2%, hot cracking is likely to occur during steel piece heating or welding. Therefore, when adding Cu, the content is limited to 2% or less.

Ni: 4% or less
Ni has an effect of improving toughness and hardenability. However, if the Ni content exceeds 4%, the economy is inferior. Therefore, when Ni is added, its content is limited to 4% or less.

Cr: 2% or less
Cr has an effect of improving strength and toughness, and is excellent in high-temperature strength characteristics. However, when the Cr content exceeds 2%, the weldability deteriorates. Therefore, when adding Cr, the content is limited to 2% or less.

Mo: 1% or less
Mo has an effect of improving hardenability and strength, and is excellent in high-temperature strength characteristics. However, if the Mo content exceeds 1%, the economy is inferior. Therefore, when adding Mo, the content is limited to 1% or less.

Nb: 0.05% or less
Nb is added as a microalloying element to improve the strength. However, if it exceeds 0.05%, the toughness of the heat affected zone is deteriorated. Therefore, when Nb is added, its content is limited to 0.05% or less.

V: 0.5% or less
V is added to improve the strength as a microalloying element. However, if it exceeds 0.5%, the toughness of the heat affected zone is deteriorated. Therefore, when V is added, its content is limited to 0.5% or less.

Ti: 0.03% or less
Ti produces TiN during rolling and heating, suppresses the growth of austenite grains, and improves the toughness of the base metal and the weld heat affected zone. However, if the content exceeds 0.03%, the toughness of the heat affected zone is deteriorated. Therefore, when adding Ti, the content is limited to 0.03% or less.

B: 0.003% or less
B has the effect | action which improves hardenability. However, if it exceeds 0.003%, the toughness is deteriorated. Therefore, when adding B, the content is limited to 0.003% or less.

Ca: 0.01% or less
Ca is an element indispensable for the morphology control of sulfide inclusions. However, addition exceeding 0.01% causes a decrease in cleanliness. Therefore, when adding Ca, the content is limited to 0.01% or less.

REM: 0.02% or less
REM improves the SR cracking resistance by reducing the amount of solute S at the grain boundaries by producing sulfide as REM (O, S) in steel. However, addition exceeding 0.02% causes REM sulfide to accumulate significantly in the precipitated crystal zone, leading to deterioration of the material. Therefore, when adding REM, the addition amount is limited to 0.02% or less.

Mg: 0.01% or less
Mg may be used as a hot metal desulfurization material. However, addition exceeding 0.01% causes a decrease in cleanliness. Therefore, when adding Mg, the addition amount is limited to 0.01% or less.

Next, the reasons for limiting the manufacturing conditions in the present invention will be described.
In order to ensure the strength of the quenched base metal and the toughness of the base metal, quenching is performed from a temperature not lower than the Ar ₃ transformation point to a temperature not higher than Si (mass%) × 700 + Cr (mass%) × 50 + 250 (° C.). Quenching is performed at a rate of 0.5 ° C./s or higher, preferably 1 ° C./s or higher.

  It is specified to complete the transformation from austenite to martensite or bainite to strengthen the base material and to achieve uniform fine dispersion of cementite during tempering.

In the present invention, the formula for obtaining the Ar ₃ transformation point is not particularly specified. For example, Ar _{3 =} 910-310C (mass%)-80Mn (mass%)-20Cu (mass%)-15Cr (mass%)-55Ni (mass%) ) -80Mo (mass%).

Tempering conditions When tempering, the temperature range from Si (mass%) x 700 + Cr (mass%) x 50 + 250 (° C) to Si (mass%) x 700 + Cr (mass%) x 50 + 370 (° C) 1 ° C / s or more, preferably 2 ° C / s or more.

  The additive element has an effect on the formation / growth behavior of cementite produced during tempering. In particular, Si and Cr have the effect of delaying the formation / growth of cementite. The formation and growth of cementite is from Si (mass%) x 700 + Cr (mass%) x 50 + 250 (° C) to Si (mass%) x 700 + Cr (mass%) x 50 + 370 (° C) When the average rate of temperature rise at the center of the plate thickness in these temperature ranges is 1 ° C / s or higher, preferably 2 ° C / s or higher, cementite is not only grains of old austenite grain boundaries and lath boundaries. It is also formed inside and precipitates uniformly and finely.

  As a result, the agglomeration and coarsening of cementite at the prior austenite grain boundaries and lath boundaries, which are the main causes of deterioration of hydrogen embrittlement resistance, are suppressed, resulting in improved hydrogen embrittlement resistance and improved toughness. To do.

  Furthermore, if the average heating rate at the center of the plate thickness from the tempering start temperature to Si (mass%) x 700 + Cr (mass%) x 50 + 250 (° C) is defined as low, less than 1 ° C / s, hydrogen resistance It is preferable because the toughness is further improved together with the brittle properties.

  At the time of quenching, a small amount of cementite is generated by automatic tempering after the transformation is completed. From the start of tempering treatment to Si (mass%) × 700 + Cr (mass%) × 50 + 250 (° C), the dissolution of cementite generated by automatic tempering occurs, Si (mass%) × 700 + Cr (mass%) When it exceeds x50 + 250 (° C), cementite is produced.

  During quenching, the average temperature rise rate from the tempering start temperature to Si (mass%) x 700 + Cr (mass%) x 50 + 250 (° C) is reduced to less than 1 ° C / s. Gives enough time for the cementite produced by automatic tempering to dissolve.

  After that, the formation and growth of cementite occurs particularly Si (mass%) × 700 + Cr (mass%) × 50 + 250 (℃) to Si (mass%) × 700 + Cr (mass%) × 50 + 370 (℃ ) When the average temperature rise rate at the center of the plate thickness up to 1 ° C / s or higher, preferably 2 ° C / s or higher, from the tempering start temperature, Si (mass%) × 700 + Cr (mass%) × 50 Compared with the case where the average heating rate at the center of the plate thickness up to +250 (° C) is 1 ° C / s or more, a more uniform and fine cementite dispersion state is obtained, and the hydrogen brittleness resistance and toughness are improved. improves.

In the present invention, a steel slab may be manufactured so that quenching can be performed from the Ar ₃ transformation point or higher, and a method for manufacturing a slab from molten steel and a method for manufacturing a slab by rolling the slab are not particularly specified. . Steel melted by a converter method, an electric furnace method, or a slab manufactured by a continuous casting / ingot-making method can be used.

When rolled to form a steel slab to slab, without cooling below Ar ₃ transformation point, even if it starts to hot rolling, the slab was once cooled after re-heating above Ac ₃ transformation point Hot rolling may be started.

If the rolling is completed at the Ar ₃ transformation point or higher, there are no particular restrictions on other rolling conditions. As long as the rolling is performed at a temperature equal to or higher than the Ar ₃ transformation point, rolling may be performed in the non-recrystallized region even if rolling is performed in the recrystallized region.

In the present invention, if quenching is started from the state of an austenite single phase structure at or above the Ar ₃ transformation point, direct quenching may be performed after hot rolling, or quenching may be performed after reheating the hot rolled material.

  The heating method at the time of tempering may be a method that achieves a required temperature increase rate, such as induction heating, energization heating, infrared radiation heating, and atmosphere heating. Although the regulation of the average rate of temperature increase during tempering was performed at the center of the plate thickness, the temperature history near the center of the plate thickness is almost the same.

  Further, the temperature raising process during tempering is not particularly limited as long as a predetermined average temperature rising rate is obtained, and either a linear temperature history or a temperature history that stays at an intermediate temperature may be obtained. .

  The holding time at the tempering temperature is desirably 60 s or less in order to prevent deterioration of the hydrogen embrittlement resistance attributed to productivity and coarsening of precipitates. As for the cooling rate after tempering, it is desirable that the average cooling rate at the central part of the plate thickness from the tempering temperature to 200 ° C. is 0.05 ° C./s or more in order to prevent coarsening of precipitates during cooling. Note that the temperatures such as quenching temperature and tempering start temperature defined in the present invention are the temperatures at the center of the plate thickness.

  As mentioned above, although the case where this invention was applied to the steel plate was demonstrated, this invention is not limited to a steel plate, It is applicable to steel materials of various shapes, such as a shape steel and a bar steel.

  The effectiveness of the present invention will be described by way of examples. Steels A to L having chemical components shown in Table 1 were melted and cast into slabs, heated in a heating furnace, and then rolled into a steel plate. After rolling, it is directly quenched, and then from the start of tempering to Si (mass%) x 700 + Cr (mass%) x 50 + 250 (° C) using two solenoid type induction heating devices installed in series Is the first induction heating device. From the Si (mass%) × 700 + Cr (mass%) × 50 + 250 (℃) to the specified tempering temperature, the second induction heating device continuously A tempering treatment was performed.

  Moreover, the average temperature increase rate in the center portion of the plate thickness was controlled by the plate passing rate of the steel plate. In addition, when hold | maintaining at the tempering temperature, it hold | maintained within the range of +/- 5 degreeC by reciprocating and heating a steel plate. The cooling after heating was air cooling. The temperature at the center of the plate thickness, such as the tempering temperature and the quenching temperature, was calculated from the temperature measurement results at the surface sequentially.

  Table 2 shows the steel sheet production conditions, and Table 3 shows the tensile strength, fracture surface transition temperature (vTrs), and critical diffusible hydrogen content of the steel sheet obtained. Tensile strength was measured with a full thickness tensile test piece, and toughness was evaluated with vTrs obtained by a Charpy impact test using a test piece taken from the center of the plate thickness.

  The critical diffusible hydrogen amount is defined as the upper limit diffusible hydrogen amount that does not cause delayed fracture within 100h under a constant load of 90% of the tensile strength. The amount of diffusible hydrogen was measured by gas chromatography. The target of the material properties is vTrs: -40 ° C or less for steels A to F, limit diffusible hydrogen content: 0.3 mass ppm or more, and for steels G to L, vTrs: -40 ° C or less, limit diffusible hydrogen Amount: 0.2 mass ppm or more.

  As is apparent from Table 3, the toughness and critical diffusible hydrogen content of steel plates Nos. 1 to 14 (invention examples) produced by the method of the present invention satisfy the target values.

  On the other hand, in comparative steel plates No. 15 to 26 (comparative example), either or both of toughness and critical diffusible hydrogen amount are out of the target range. Hereinafter, these comparative examples will be described individually.

  Steel plates No. 15 and No. 16 whose direct quenching start temperatures are outside the scope of the present invention do not reach the target diffusible hydrogen content.

  Steel plate No. 17 whose direct quenching stop temperature is out of the range of the present invention has neither the toughness nor the limit diffusible hydrogen amount reaching the target value.

  Average rise in the center of the plate thickness from Si (mass%) × 700 + Cr (mass%) × 50 + 250 (℃) to Si (mass%) × 700 + Cr (mass%) × 50 + 370 (℃) In steel plates Nos. 18 to 26 whose temperature speed is out of the scope of the present invention, either or both of toughness and critical diffusible hydrogen amount do not reach the target value.

  In addition, No. 9 and No. 10 or No. 11 differ in the average rate of temperature rise at the center of the thickness from tempering start temperature to Si (mass%) × 700 + Cr (mass%) × 50 + 250 (° C) And No.12, the average heating rate at the center of the thickness from tempering start temperature to Si (mass%) x 700 + Cr (mass%) x 50 + 250 (° C) is less than 1 ° C / s. In contrast to Nos. 9 and 11, Nos. 10 and 12 of 1 ° C./s or more have improved toughness and a high critical diffusible hydrogen content.

Claims (4)

In mass%, C: 0.02 to 0.25%, Si: 0.01 to 0.8%, Mn: 0.5 to 2.0%, Al: 0.005 to 0.1%, N: 0.0005~0.008%, P: 0.03% or less, S: 0.03% or less, Si (mass%) of the billet and the balance being Fe and unavoidable impurities from the Ar ₃ transformation point or higher × 700 + Cr After quenching to a temperature of (mass%) × 50 + 250 (° C.) or less, the average rate of temperature rise at the center of the steel material from the tempering start temperature to Si (mass%) × 700 + Cr (mass%) × 50 + 250 (° C.) is 1 ° C. / less than s, and the average heating rate at the center of the steel material from Si (mass%) × 700 + Cr (mass%) × 50 + 250 (° C.) to Si (mass%) × 700 + Cr (mass%) × 50 + 370 (° C.) is 1 Tempering at over ℃ / s Wherein the method for producing a high tensile steel having excellent hydrogen embrittlement resistance characteristics. Furthermore, the steel composition, by mass%, Cu: 2% or less, Ni: 4% or less, Cr: 2% or less, Mo: characterized in that it contains one or more of 1% or less, wherein Item 2. A method for producing a high-tensile steel material excellent in hydrogen embrittlement resistance according to Item 1 . Furthermore, the steel composition, by mass%, Nb: 0.05% or less, V: 0.5% or less, Ti: characterized in that it contains one or more than 0.03% of claims Item 3. The method for producing a high-tensile steel material having excellent hydrogen embrittlement resistance according to Item 1 or 2 . Further, the steel composition contains one or more of mass%, B: 0.003% or less, Ca: 0.01% or less, REM: 0.02% or less, Mg: 0.01% or less. The method for producing a high-strength steel material excellent in hydrogen embrittlement resistance according to any one of claims 1 to 3 , wherein: