JP6160574B2 - High-strength hot-rolled steel sheet excellent in strength-uniform elongation balance and method for producing the same - Google Patents

Description

  The present invention relates to a high-strength hot-rolled steel sheet suitable for use in welded structures, particularly welded structures such as building structures, marine structures, and pipelines that require excellent earthquake resistance, and particularly excellent welding. The present invention relates to a high-strength hot-rolled steel sheet having high properties and high deformability.

  In recent years, high strength and high toughness have been strongly demanded for steel materials for welded structures used for building structures, offshore structures, pipelines and the like. In recent years, in addition to improving strength and toughness, steel materials have been required to reduce yield ratio and improve uniform elongation from the viewpoint of improving earthquake resistance.

In response to such a requirement, for example, Patent Document 1 includes mass%, C: 0.03-0.08%, Si: 0.01-0.5%, Mn: 1.2-2.0%, Mo: 0.05-0.4%, Cu + Ni: 0.1. %, Ti: 0.005 to 0.04%, Nb: 0.005 to 0.07%, Al: Steel containing 0.08% or less is heated to a temperature of 1000 to 1300 ° C and hot at the rolling end temperature of Ar ₃ or higher. After rolling, accelerated cooling to 500 to 650 ° C at a cooling rate of 5 ° C / s or higher, and then reheating to 550 to 750 ° C at a heating rate of 0.5 ° C / s or higher, the metal structure becomes ferrite and bainite. Is a three-phase structure of island martensite, a structure containing island martensite with a volume fraction of 3-15% and residual austenite with a volume fraction of 2% or more, with a uniform elongation of 12% in the longitudinal direction. The manufacturing method of the low yield ratio high strength high toughness steel plate which is the above steel plate is described. According to the technique described in Patent Document 1, it is possible to obtain a low yield ratio high strength high toughness steel sheet having a high uniform elongation without deteriorating the weld heat affected zone toughness or adding a large amount of alloying elements. I can do it.

Patent Document 2 includes, in mass%, C: 0.02 to 0.15%, Si: 0.1 to 2.0%, Mn: 0.5 to 2.5%, Al: 0.01 to 0.1%, N: 0.01% or less, Nb: A slab having a composition containing one or more of 0.01% or less, V: 0.1% or less, Ti: 0.1% or less is heated to 1050 ° C. or higher, and then roughly rolled at a recrystallization temperature or higher, and then Ar ₃ Finish rolling with a cumulative reduction of 65% or more at a temperature of 900 ° C or less, cooling at a temperature of 5 ° C / s or more from a temperature of Ar ₃ or more, and Ts = 780−270 × C−90 × Mn−37 × Ni−70 In relation to Ts represented by × Cr−83 × Mo, hold for 30 to 300 s in the range of Ts-50 ° C to Ts + 100 ° C, then cool to 350 ° C to 450 ° C at 20 ° C / s or more and release. A cold steel sheet is cold formed into a hollow shape, and seam welded to form a microstructure composed of ferrite with an average crystal grain size of 10 μm or less and an area ratio of 70 to 90% and the balance of retained austenite, bainite, and martensite. Yes , The method of producing a high strength steel pipe retained austenite and high strength steel pipe is 5-15% by volume fraction is described. According to the technique described in Patent Document 2, a high-strength steel pipe excellent in buckling resistance due to compression and bending and excellent in low-temperature toughness is obtained.

  Non-Patent Document 1 describes a report on a TRIP (work-induced transformation plasticity) steel sheet utilizing retained austenite.

  In the technology described in Non-Patent Document 1, bainitic ferritic steel having a composition containing C: 0.2%, Si: 1.5%, Mn: 1.5%, Nb: 0.05% and further adding Ni, Cr, and Mo is used. After austenitizing, quenching and holding between Ms and Mf (austempering) increases the hardness up to HV430, and addition of Ni, Cr, Mo increases residual austenite and strain. It is said that it has sufficient stability to obtain the TRIP effect.

JP2008-248328 Japanese Patent No. 3740704

ISIJ International, Vol. 52 (2012), No. 10, pp. 1894-1901

  In the technique described in Patent Document 1, the metal structure is a three-phase structure in which a bainite phase and an island martensite phase are dispersed in a ferrite structure that is a soft phase, and the volume fraction of island martensite is determined. By setting the content to 3 to 15%, a low yield ratio and a high uniform elongation are achieved. However, as shown in the Examples of Patent Document 1, the obtained tensile strength-uniform elongation balance (TS × uEL) value is at most about 9000 MPa% or less.

In addition, in the technique described in Non-Patent Document 1, addition of Ni, Cr, and Mo in 0.2% C-1.5% Si-1.5% Mn-0.05% Nb-based steel remains without affecting the stability against strain. While a contributing to an increase in the amount of austenite, however, the steel described in non-Patent Document 1 is a C or Si is higher composition, weld cracking sensitivity index P _CM is above 0.25%, a problem in weldability Was leaving. Furthermore, in the technique described in Non-Patent Document 1, it is necessary to perform a process combining isothermal holding and rapid cooling, and in an actual operation such as in a hot rolling line, a sufficient isothermal holding time cannot be ensured on the line configuration. In addition, there is a problem that the retained austenite generated in the cooling process is easily decomposed into pearlite or bainite after being wound in a coil shape, and it is difficult to secure a desired metal structure.

  Moreover, in the technique described in Patent Document 2, it is necessary to set a process that combines isothermal holding and rapid cooling. Like the technique described in Non-Patent Document 1, in the actual operation as in the hot rolling line, Due to the line configuration, because sufficient isothermal holding time cannot be secured, the retained austenite generated during the cooling process is easily decomposed into pearlite and bainite after being wound in a coil shape, making it difficult to secure the desired metal structure. was there.

  It is an object of the present invention to provide a high-strength hot-rolled steel sheet excellent in strength-uniform elongation balance and a method for producing the same, which advantageously solves the problems of the prior art and can be applied as a steel material for welded structures with excellent weldability. And

  Here, “high strength” refers to a case where the tensile strength TS is 500 MPa or more. The term “high deformability” as used herein refers to a high ability to absorb deformation energy up to non-uniform deformation. Specifically, the product of tensile strength (TS) and uniform elongation (uEL) ( (Strength-uniform elongation balance) This is the case where TS x uEL is 12000 MPa% or more.

Also, "having excellent _{weldability", P CM (= C + Si} / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B) is The case where it is 0.25 or less.

  In order to achieve the above-mentioned object, the present inventors diligently studied the influence of the composition and production conditions on the strength-uniform elongation balance on the premise of application of a hot rolling line excellent in production efficiency. As a result, it has been found that it is necessary to adjust the retained austenite amount within a predetermined range in order to ensure a strength-uniform elongation balance as good as that of conventional TRIP steel.

  The relationship between the strength-uniform elongation balance and the amount of retained austenite is shown in FIG. FIG. 1 shows that the retained austenite amount is preferably adjusted to a range of 3 to 20% in order to ensure TS × uEL of 12000 MPa% or more.

  In order to secure such a retained austenite amount by using a hot rolling line and omitting the intermediate holding process in the steel plate cooling step, the base material structure (main area having an occupation ratio of 70% or more in area ratio) is obtained. As a phase), it has been thought that it is better to use polygonal ferrite. Polygonal ferrite can be produced even if the intermediate holding process in the steel sheet cooling step is omitted, and the aspect expressed by (average grain length in the rolling direction) / (average grain length in the plate thickness direction). It has also been found that residual austenite can be stably secured as shown in FIG.

  Such polygonal ferrite is high-temperature transformation ferrite that is produced with diffusion, and C is discharged into untransformed austenite in the process of transformation. In addition, new nucleation and further progress of transformation are possible due to the temperature decrease, and C concentration to untransformed austenite proceeds to stabilize untransformed austenite. On the other hand, bainitic ferrite and bainite are transformed at low temperatures, so if the intermediate holding process is omitted, the C emission accompanying transformation and C concentration to untransformed austenite become insufficient, and untransformed austenite is not stable, and after cooling In this winding process, it is transformed into pearlite or bainite, and a stable amount of retained austenite cannot be secured.

  In addition, from the viewpoint of promoting such polygonal ferrite transformation, the present inventors utilized Cr, which is a ferrite forming element, instead of a large amount of Si conventionally used, and further Mn and Mo It has been found effective to combine elements that enhance the hardenability of steel.

The present invention has been completed based on the above findings and further studies. That is, the gist of the present invention is as follows.
(1) By mass%, C: 0.04 to 0.15%, Si: 0.10 to 0.50%, Mn: 1.0 to 2.2%, P: 0.050% or less, S: 0.005% or less, Cr: 0.2 to 1.0%, Ti: 0.005 ~ 0.030%, Al: 0.010 ~ 0.050%, the following formula (1)
P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (1)
(Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, B: Content of each element (mass%))
In comprising Adjust so defined as weld crack sensitivity index P _CM satisfies 0.25% or less, and a composition comprising the balance Fe and unavoidable impurities, the polygonal ferrite phase as a main phase, a main phase, the area The main phase is composed of 3-20% residual austenite phase and the balance is 10% or less (including 0%) of martensite phase, bainite phase, or pearlite. and an average particle size including another phase 5μm or more, has a tissue aspect ratio of below 1.40 or less, the strength - high-strength hot-rolled steel sheet, characterized in that excellent uniform elongation balance.
(2) In (1), in addition to the above-mentioned composition, by mass%, Mo: 0.5% or less, Cu: 0.5% or less, Ni: 1.0% or less, Co: 1.0% or less Alternatively, a high-strength hot-rolled steel sheet comprising two or more types.
(3) In (1) or (2), in addition to the above-mentioned composition, the composition further comprises one or two selected from Nb: 0.10% or less and V: 0.10% or less in mass%. High strength hot rolled steel sheet.
(4) In any one of (1) to (3), a high-strength hot-rolled steel sheet containing Ca: 0.0005 to 0.0050% by mass% in addition to the above composition.
(5) A method for producing a hot-rolled steel sheet, in which a steel material is hot-rolled into a hot-rolled sheet and then cooled, and the steel material is, in mass%, C: 0.04 to 0.15%, Si: 0.10 to 0.50%, Mn: 1.0 to 2.2%, P: 0.050% or less, S: 0.005% or less, Cr: 0.2 to 1.0%, Ti: 0.005 to 0.030%, Al: 0.010 to 0.050%, (1) formula
P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (1)
(Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, B: Content of each element (mass%))
In comprising Adjust so defined as weld crack sensitivity index P _CM satisfies 0.25% or less, and a steel material having a composition the balance being Fe and unavoidable impurities, said hot rolling, heating the steel material Temperature: Rolling is performed after heating to a temperature in the range of 1100 to 1250 ° C., and the cooling is any time between 20 to 80 s from the starting point, starting from the time when the final pass of the hot rolling is taken (zero) The temperature of the hot-rolled sheet is in the range of 750 ° C. to 650 ° C. at the central part of the thickness, and the temperature of the hot-rolled sheet is 650 ° C. at the central part of the thickness before 80 seconds from the starting point. the below, the cooling stop temperature: 600 to 450 and cooled to cooling stop at a temperature of temperature range ° C., after the cooling, Ri taken up into a coil shape, and a polygonal ferrite phase as a main phase, a main phase, the area ratio 3-20% residual austenite phase and the balance is 10% or less (including 0%) A hot-rolled steel sheet comprising one or more of an incite phase, a bainite phase, and pearlite, and having a structure in which an average grain size including a main phase and other phases is 5 μm or more and an aspect ratio is 1.40 or less strength and said to Rukoto - the method of producing a high-strength hot-rolled steel sheet excellent in uniform elongation balance.
(6) In (5), in addition to the above composition, in addition to mass, Mo: 0.5% or less, Cu: 0.5% or less, Ni: 1.0% or less, Co: 1.0% or less Or the manufacturing method of the high intensity | strength hot-rolled steel plate characterized by including 2 or more types.
(7) In (5) or (6), in addition to the above-mentioned composition, the composition further includes one or two selected from Nb: 0.10% or less and V: 0.10% or less in mass%. A method for producing a high-strength hot-rolled steel sheet.
(8) The method for producing a high-strength hot-rolled steel sheet according to any one of (5) to (7), further including Ca: 0.0005 to 0.0050% by mass% in addition to the above composition.

  According to the present invention, it is excellent in weldability and can be applied as a steel material for welded structures. Tensile strength TS: 500 MPa or more and strength-uniform elongation balance TS × uEL is 12000 MPa% or more. A hot-rolled steel sheet can be manufactured easily and inexpensively, and has a remarkable industrial effect.

It is a graph which shows the influence of the amount of retained austenite on strength-uniform elongation balance TSxuEL. It is a graph which shows the influence of the aspect ratio which acts on the amount of retained austenite. It is explanatory drawing which shows typically the preferable cooling conditions after the hot rolling in the manufacturing method of this invention hot-rolled steel plate.

  First, the reasons for limiting the composition of the hot-rolled steel sheet of the present invention will be described. Hereinafter, the mass% in the composition is simply expressed as%.

The hot-rolled steel sheet of the present invention is C: 0.04-0.15%, Si: 0.10-0.50%, Mn: 1.0-2.2%, P: 0.050% or less, S: 0.005% or less, Cr: 0.2-1.0%, Ti: 0.005 ~ 0.030%, Al: 0.010 ~ 0.050%, the following formula (1)
P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (1)
(Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, B: Content of each element (mass%))
In weld crack sensitivity index P _CM defined includes adjusted so as to satisfy the following 0.25%, having a composition the balance being Fe and unavoidable impurities.

C: 0.04-0.15%
C is an element that contributes to the stabilization of austenite by forming a solid solution, and is an essential element for obtaining retained austenite. C is discharged from ferrite to untransformed austenite during the transformation from austenite to ferrite during cooling from a high temperature. The discharged C stabilizes the untransformed austenite and stably exists as retained austenite at least at room temperature. In order to obtain such an effect, a content of 0.04% or more is required. On the other hand, if it exceeds 0.15%, the weldability decreases. For this reason, C was limited to the range of 0.04 to 0.15%. In addition, Preferably it is 0.06 to 0.12%, More preferably, it is 0.08 to 0.12%.

Si: 0.10 to 0.50%
Si is an element that acts as a deoxidizer, suppresses scale formation during hot rolling, and contributes to a reduction in scale-off amount. In order to obtain such an effect, the content of 0.10% or more is required. On the other hand, the content exceeding 0.50% lowers toughness. For this reason, Si was limited to the range of 0.10 to 0.50%. In addition, Preferably it is 0.10 to 0.30%. Si is an element that suppresses cementite precipitation and greatly contributes to the formation of retained austenite. Conventionally, it is contained at 1.0% or more to ensure the desired amount of retained austenite and to achieve desired properties (work-induced transformation plasticity). ) Was secured. However, in the present invention, such an effect of Si is not utilized from the viewpoint of improving low temperature toughness and weldability.

Mn: 1.0-2.2%
Mn is an element that enhances the stability of austenite and suppresses the decomposition of austenite into pearlite or bainite. In order to secure such an effect, it is necessary to contain 1.0% or more of Mn. On the other hand, an excessive content exceeding 2.2% stabilizes austenite, suppresses the formation of high-temperature transformed ferrite, and prevents C from being discharged and concentrated into untransformed austenite. For these reasons, Mn is limited to a range of 1.0 to 2.2%. In addition, Preferably it is 1.2 to 1.6%.

P: 0.050% or less
P is an element that segregates at the grain boundary and lowers the toughness. In the present invention, P is preferably reduced as much as possible, but it is acceptable up to 0.050%. For this reason, P was limited to 0.050% or less. In addition, Preferably it is 0.030% or less. On the other hand, excessive P reduction leads to an increase in refining costs. For this reason, it is preferable to set it as 0.002% or more.

S: 0.005% or less
S is usually present as MnS in steel, but MnS is stretched thinly in the hot rolling process, which adversely affects ductility and toughness. Therefore, in the present invention, it is desirable to reduce S as much as possible, but up to 0.005% is acceptable. For this reason, S was limited to 0.005% or less. In addition, Preferably it is 0.003% or less. On the other hand, excessive S reduction leads to an increase in refining costs, so 0.0002% or more is more preferable.

Cr: 0.2-1.0%
Cr has a function of suppressing precipitation of cementite in untransformed austenite and is an element that greatly contributes to the formation of retained austenite and is an important element in the present invention. In order to acquire such an effect, it is necessary to contain 0.2% or more. On the other hand, excessive content exceeding 1.0% reduces weldability. For this reason, Cr was limited to the range of 0.2 to 1.0%. In addition, Preferably it is 0.2 to 0.8%, More preferably, it is 0.2 to 0.5%.

Ti: 0.005-0.030%
Ti is an element that combines with N to form TiN, which has the effect of fixing and detoxifying N, which significantly deteriorates the toughness of the steel. To obtain such an effect, it must contain 0.005% or more. . On the other hand, the content exceeding 0.030% increases the amount of Ti carbonitride precipitated along the cleaved surface of Fe, and decreases the toughness of the steel. For this reason, Ti was limited to 0.005 to 0.030% of range. In addition, Preferably it is 0.005-0.025%.

Al: 0.010 to 0.050%
Al is an element that acts as a strong deoxidizing agent. In order to obtain such an effect, it is necessary to contain 0.010% or more. Al is also an element that suppresses cementite precipitation and greatly contributes to the formation of retained austenite. However, if Al exceeds 0.050% and Al increases, Al oxide tends to remain as inclusions in the steel, and the cleanliness of the steel decreases. For this reason, Al was limited to the range of 0.010 to 0.050%. In addition, Preferably it is 0.010 to 0.040%.

In the present invention hot-rolled steel sheet, the above-mentioned components in the range described above, containing Adjust so P _CM satisfies 0.25% or less.

P _CM is the index representing the weld cracking sensitivity is a value defined by the following equation (1). Note that in the calculation of P _CM, shall be calculated as "zero" if it does not contain elemental described in (1).

P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (1)
(Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, B: Content of each element (mass%))
When P _CM value is increased, the welding crack sensitivity is increased. Therefore, in order to improve the weldability of the steel sheet, so that P _CM value is as low as possible, it is necessary to adjust the content of each component. In the present invention, shall be adjusted so that P _CM satisfies 0.25% or less. If P _CM is 0.25% or less, heat input: 5kJ / cm~20kJ / cm of about avoids cold cracking during welding.

When calculating the _PCM value using the equation (1), the elements not included in the elements described in the equation (1) are assumed to be zero.

  The above-described components are basic components. In the present invention, in addition to the basic composition, Mo: 0.5% or less, Cu: 0.5% or less, Ni: 1.0% or less, Co: 1.0 as necessary as a selective element. % Or less, and / or Nb: 0.10% or less, V: 0.10% or less, and / or Ca: 0.0005- 0.0050% can be contained.

One or more selected from Mo: 0.5% or less, Cu: 0.5% or less, Ni: 1.0% or less, Co: 1.0% or less
Mo, Cu, Ni, and Co are elements that have the effect of enhancing the stability of austenite and contribute effectively to the formation of retained austenite. Select one or more if necessary. it can. In order to obtain such effects, it is desirable to contain Mo: 0.05% or more, Cu: 0.05% or more, Ni: 0.05% or more, Co: 0.05% or more. On the other hand, if the content exceeds Mo: 0.5%, Cu: 0.5%, Ni: 1.0%, and Co: 1.0%, the above effects are saturated and weldability is reduced. For this reason, when it contained, it limited to Mo: 0.5% or less, Cu: 0.5% or less, Ni: 1.0% or less, Co: 1.0% or less.

One or two selected from Nb: 0.10% or less, V: 0.10% or less
Nb and V are elements that finely precipitate as carbonitrides and contribute to improving the strength of the steel through precipitation strengthening, and can be selected as necessary to contain one or two kinds. In order to obtain such an effect, it is desirable to contain Nb: 0.01% or more and V: 0.01% or more. On the other hand, when it contains exceeding Nb: 0.10% and V: 0.10% respectively, a coarse precipitate will be formed and base material toughness and weldability will fall. For this reason, when it contained, it limited to Nb: 0.10% or less and V: 0.10% or less.

Ca: 0.0005 to 0.0050%
Ca is an element that contributes to improving the toughness of the steel sheet through sulfide morphology control in which sulfides such as MnS that extend long in the rolling direction are spherical sulfides, and can be contained as necessary. In order to acquire such an effect, it is necessary to contain 0.0005% or more, but when it contains exceeding 0.0050%, Ca oxide cluster will be formed in steel and toughness will fall. For this reason, when it contained, Ca was limited to 0.0005 to 0.0050% of range.

  The balance other than the above components is Fe and inevitable impurities. Inevitable impurities include N: 0.005% or less and O (oxygen): 0.005% or less.

  Next, the reason for limiting the structure of the hot-rolled steel sheet of the present invention will be described.

  The hot-rolled steel sheet of the present invention comprises a polygonal ferrite phase as a main phase, a retained austenite phase with an area ratio of 3 to 20%, and a martensite phase with the balance being 10% or less (including 0%) in area ratio, It has a structure composed of one or more of bainite phase and pearlite. Here, the “main phase” refers to a phase that occupies 70% or more in area ratio.

  The polygonal ferrite phase, which is the main phase, is a high-temperature transformed ferrite phase that is produced while being diffused. During transformation, C is discharged to untransformed austenite, and the untransformed austenite is stabilized and easily retained as retained austenite. If the polygonal ferrite phase is less than 70% in area ratio, the amount of C discharged to untransformed austenite is insufficient, and it becomes difficult to secure a desired amount of retained austenite phase.

  The second phase other than the main phase is a retained austenite phase with an area ratio of 3 to 20%.

  The retained austenite phase is an indispensable phase for securing the strength-uniform elongation balance TS × uEL comparable to that of conventional TRIP steel. TS × uEL: In order to secure a high deformability exceeding 12000 MPa%, the retained austenite needs to contain 3% or more. On the other hand, if it exceeds 20%, the carbon concentration contained in the retained austenite decreases, and the retained austenite becomes unstable with respect to deformation, so that the uniform elongation is lowered and the strength-uniform elongation balance is significantly lowered. For this reason, the retained austenite was limited to a range of 3 to 20% by volume. In addition, Preferably it is 3 to 15%.

  In the hot-rolled steel sheet of the present invention, the balance other than the main phase and the second phase is one or more of martensite phase, bainite phase, and pearlite having an area ratio of 10% or less (including 0%). It is. If the phase other than the main phase and the retained austenite phase exceeds 10% in terms of area ratio, a desired strength-uniform elongation balance cannot be ensured.

  The hot-rolled steel sheet of the present invention has a structure in which the average particle size including the main phase and other phases is 5 μm or more. When the average particle size is less than 5 μm, a low-temperature transformation phase mainly without C diffusion occurs, and a desired retained austenite phase cannot be secured, resulting in a decrease in deformability.

  In the hot-rolled steel sheet of the present invention, the aspect ratio defined by (crystal grain diameter in the rolling direction) / (crystal grain diameter in the plate thickness direction) has the above-described average crystal grain size and is 1.40 or less. Have an organization. When the aspect ratio exceeds 1.40, as shown in FIG. 2, the amount of retained austenite phase is small, the desired strength-uniform elongation balance cannot be ensured, and the desired high deformability cannot be achieved.

  Below, the manufacturing method of this invention hot rolled sheet steel is demonstrated.

  In the present invention, a steel material such as a slab having the above composition is heated to a temperature in the range of heating temperature: 1100 to 1250 ° C.

  In addition, the manufacturing method of the steel material is not particularly limited. Any of the usual methods for manufacturing steel materials can be applied. For example, it is preferable to melt the molten steel having the above-described composition by a melting method such as a converter and to obtain a steel material such as a slab by a continuous casting method. Needless to say, the ingot-splitting rolling may be used as a steel material such as a steel piece.

  If the heating temperature of the steel material is less than 1100 ° C., the coarse carbonitride and segregation zone generated during casting cannot be lost, and desired precipitation strengthening and toughness improvement cannot be achieved. On the other hand, when heated to a high temperature exceeding 1250 ° C., the coarsening of crystal grains becomes remarkable, the toughness is remarkably lowered, and the material control advantage is lost. In addition, the scale-off amount increases, the yield decreases, the energy intensity increases, and this is economically disadvantageous. For this reason, the heating temperature of the steel material was limited to the range of 1100 to 1250 ° C. This temperature is the set temperature in the furnace.

  The heated steel material is then subjected to hot rolling.

  In the present invention, the rolling conditions for hot rolling are not particularly limited as long as the hot rolled sheet having a predetermined size and shape can be obtained. In addition, although the rolling completion temperature of hot rolling is not specifically limited, it is preferable to set it as the temperature of 750 degreeC or more from a viewpoint of the structure uniformity of a plate | board thickness direction.

  In the present invention, the cooling conditions after the end of hot rolling are adjusted as follows.

  In the present invention, the cooling after the end of hot rolling is performed at any time between 20 to 80 s from the starting point (zero s), starting from the time when the final pass of the hot rolling is started (zero s). Cooling conditions such as water cooling are set so that the temperature of the hot-rolled sheet falls below 650 ° C at the center of the plate thickness before the 80s elapses within the range of 750 ° C to 650 ° C at the center of the plate thickness. Adjust and cool.

  Polygonal ferrite is generated if the temperature at the center of the thickness of the hot-rolled sheet is within the above-mentioned range of 750 to 650 ° C. within the time period between 20 and 80 s from the starting point (zero s). And it can be set as the structure | tissue which has a polygonal ferrite phase as a main phase. On the other hand, when it is cooled below 650 ° C. in the time less than 20 s from the starting point (zero s), it becomes a structure mainly composed of bainitic ferrite phase or bainite phase.

  On the other hand, if the temperature at the center of the thickness of the hot-rolled sheet is within the above-mentioned temperature range of 750 to 650 ° C even after 80 s from the starting point, carbonitride and cementite are precipitated simultaneously with the ferrite transformation. This tends to occur, and as a result, C concentration to untransformed austenite is less likely to occur. Therefore, in the present invention, the temperature at the center of the thickness of the hot-rolled sheet is cooled to be lower than 650 ° C. before elapse of 80 s from the starting point, thereby facilitating C concentration to untransformed austenite. .

  Furthermore, in the present invention, cooling is performed so as to satisfy the above-described cooling conditions using cooling means such as water cooling on the run-out table, and cooling is stopped at a temperature range of 600 to 450 ° C. as a cooling stop temperature. And take up in a coil.

Cooling stop temperature: 600 ~ 450 ℃
When the cooling stop temperature is higher than 600 ° C., a part of untransformed austenite undergoes pearlite or bainite transformation after winding, and the desired retained austenite amount cannot be secured. On the other hand, when the cooling stop temperature is less than 450 ° C., a part of untransformed austenite undergoes martensitic transformation, and a desired retained austenite amount cannot be secured. For this reason, the cooling stop temperature was limited to a temperature in the range of 600 to 450 ° C.

  For reference, an example of the cooling curve of the above cooling is schematically shown in FIG. Both the cooling curves a and b are cooling curves that satisfy the scope of the present invention.

  Here, “water cooling” refers to a method of cooling by injecting cooling water onto the upper and lower surfaces of the hot-rolled plate from a water cooling zone continuously arranged on the run-out table. Here, there is no particular limitation on the arrangement interval of the water cooling zones, the water density, and the like. The steel plate temperature is the temperature at the center of the steel plate thickness obtained by heat transfer analysis based on the temperature measured with a surface thermometer.

  Molten steel having the composition shown in Table 1 was melted in a converter and made into a slab (steel material: wall thickness 220 mm) by a continuous casting method. After the obtained steel material is heated to the heating temperature shown in Table 2, the hot rolling is finished at the rolling end temperature shown in Table 2, cooled under the cooling conditions shown in Table 2, wound in a coil shape, Thickness: 16 mm hot-rolled steel sheet.

From the obtained hot-rolled steel sheet, a test piece was collected and subjected to a structure observation and a tensile test. The test method was as follows.
(1) Microstructure observation From the obtained hot-rolled steel sheet, a microstructural specimen was taken, polished and corroded (nitrite corrosion) so that the observation surface was a cross section in the rolling direction (L cross section), and an optical microscope ( Using a scanning electron microscope SEM (magnification: 2000 times) and the structure at the position of the plate thickness 1/2, the structure was photographed by taking 5 fields of view. About the obtained structure | tissue photograph, the identification of the structure | tissue and the tissue fraction were computed using the image-analysis apparatus. Furthermore, the average grain size including the main phase and other phases was determined by a cutting method in accordance with the provisions of JIS G 0551. In addition, the crystal grain size in the rolling direction and the crystal grain size in the plate thickness direction of polygonal ferrite are measured, and the aspect ratio defined by (crystal grain size in the rolling direction) / (crystal grain size in the plate thickness direction) is obtained. It was.
Since retained austenite is difficult to discriminate visually, the area fraction of the fcc phase was obtained by the SEM / EBSD method and used as the amount of retained austenite.

(2) Tensile test From the obtained hot-rolled steel sheet, a JIS No. 5 tensile test piece was collected so that the tensile direction would be the rolling direction, and a tensile test was performed in accordance with the provisions of JIS Z 2241. Tensile strength TS and uniform elongation uEl) were measured to obtain a strength-uniform elongation balance TS × uEl.
The obtained results are shown in Table 3.

Both Examples present invention, P _CM: terms of excellent weldability with 0.25% or less, tensile strength TS: and 500MPa or more high strength, strength - uniform elongation balance TS × UEL: having 12000 MPa% or more high deformability It is a hot-rolled steel sheet. On the other hand, in the comparative examples that are out of the scope of the present invention, the strength is reduced or the strength-uniform elongation balance TS × uEl: less than 12000 MPa%, and the desired strength-elongation balance cannot be secured.

Claims (8)

% By mass
C: 0.04 to 0.15%, Si: 0.10 to 0.50%,
Mn: 1.0-2.2%, P: 0.050% or less,
S: 0.005% or less, Cr: 0.2-1.0%,
Ti: 0.005-0.030%, Al: 0.010-0.050%
And a composition which comprises by adjusting as follows (1) weld crack sensitivity index P _CM defined by the equation satisfies 0.25% or less, the balance being Fe and unavoidable impurities,
The main phase is the polygonal ferrite phase, the retained austenite phase with an area ratio of 3 to 20%, and the remaining martensite phase, bainite phase, and pearlite with an area ratio of 10% or less (including 0%). It consists of a one or more of the average particle diameter including a main phase and other phases in the 5μm or more, the tissue aspect ratio of below 1.40 or less,
A high-strength hot-rolled steel sheet characterized by having an excellent balance between strength and uniform elongation.
Record
P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (1)
Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, B: Content of each element (mass%)   In addition to the above composition, the composition further contains one or more selected from Mo: 0.5% or less, Cu: 0.5% or less, Ni: 1.0% or less, and Co: 1.0% or less in mass%. The high-strength hot-rolled steel sheet according to claim 1, wherein   The high strength according to claim 1 or 2, further comprising one or two selected from Nb: 0.10% or less and V: 0.10% or less in mass% in addition to the composition. Hot rolled steel sheet.   The high-strength hot-rolled steel sheet according to any one of claims 1 to 3, further comprising Ca: 0.0005 to 0.0050% by mass% in addition to the composition. A method of manufacturing a hot-rolled steel sheet, which is subjected to hot rolling on a steel material to form a hot-rolled sheet and then cooled,
The steel material in mass%,
C: 0.04 to 0.15%, Si: 0.10 to 0.50%,
Mn: 1.0-2.2%, P: 0.050% or less,
S: 0.005% or less, Cr: 0.2-1.0%,
Ti: 0.005-0.030%, Al: 0.010-0.050%
And wherein by adjusting as follows (1) weld crack sensitivity index P _CM defined by the equation satisfies the following 0.25%, and a steel material having a composition the balance being Fe and unavoidable impurities,
The hot rolling is a rolling performed after heating the steel material to a temperature in the range of 1100 to 1250 ° C,
In the cooling, the temperature of the hot-rolled sheet is in the range of 750 ° C. to 650 ° C. at the central portion of the plate thickness for 20 to 80 seconds starting from the time when the final pass of the hot rolling is taken. Before 80 seconds from the time, the temperature of the hot-rolled sheet is lower than 650 ° C. at the center of the thickness, and the cooling stop temperature is cooling to stop at a temperature in the temperature range of 600 to 450 ° C.
After the cooling, Ri taken up into a coil shape, and a polygonal ferrite phase as a main phase, a main phase, and 3-20% residual austenite phase at an area ratio, the balance being at an area ratio of 10% or less (0% 1) or more of the martensite phase, bainite phase, and pearlite (including the main phase and other phases), and the average particle size including the main phase and other phases is 5 μm or more and the aspect ratio is 1.40 or less. intensity and wherein the hot-rolled steel sheet and to Rukoto - the method of producing a high-strength hot-rolled steel sheet excellent in uniform elongation balance.
Record
P _CM = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B (1)
Here, C, Si, Mn, Cu, Ni, Cr, Mo, V, B: Content of each element (mass%)   In addition to the above composition, the composition further contains one or more selected from Mo: 0.5% or less, Cu: 0.5% or less, Ni: 1.0% or less, and Co: 1.0% or less in mass%. The manufacturing method of the high intensity | strength hot-rolled steel plate of Claim 5 characterized by the above-mentioned.   The high strength according to claim 5 or 6, further comprising one or two selected from Nb: 0.10% or less and V: 0.10% or less in mass% in addition to the composition. A method for producing a hot-rolled steel sheet.   The method for producing a high-strength hot-rolled steel sheet according to any one of claims 5 to 7, further comprising Ca: 0.0005 to 0.0050% by mass% in addition to the composition.

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