JP2010275587A - Hot-rolled steel sheet and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength hot-rolled steel sheet with which since this steel sheet has the high yield ratio and good durability and high difference between the stational-state and the moving-state, this steel sheet is suitable for applying to a member for automobile, etc. <P>SOLUTION: The steel micro-structure on the surface layer part having 100 μm depth in the sheet thickness direction from the steel sheet surface is composed by volume ratio of 2-15% retained austenite and 3% to <19% martensite and the balance ferrite, and also, the average granular diameter of the ferrite is <1.5 μm, and steel micro-structure at the center part of the sheet thickness having >1/4 of the sheet thickness to the depth in the sheet thickness direction from the steel sheet surface is composed of 10% to <45% martensite and ≤2% retained austenite and the balance ferrite and also, the average granular diameter of the martensite is ≤1.5 μm and the same of the ferrite is ≤2.0 μm, and this hot-rolled steel sheet has the mechanical characteristics of ≥780 MPa tensile strength, ≥0.70 yield ratio and TS×El value which is the product of the tensile strength and the breaking elongation is ≥20,000 MPa×(%). <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a hot-rolled steel sheet and a manufacturing method thereof. More specifically, the present invention relates to a high-strength hot-rolled steel sheet having a high yield ratio and good ductility suitable for, for example, a material for structural members of automobiles and industrial equipment, and a method for producing the same.

  A so-called hot-rolled steel sheet produced by continuous hot rolling is widely used as a relatively inexpensive structural material, for example, as a material for structural members of automobiles and industrial equipment. In recent years, for the purpose of reducing the weight of automobiles and the like, the strength of steel sheets used for various structural members has been increased, and the strength of these hot-rolled steel sheets has also been increased.

  By the way, in general, since the workability such as ductility is lowered when the tensile strength of the steel sheet is increased, as a method for securing good workability while increasing the tensile strength, the retained austenite phase and the martensite phase are added to the main phase ferrite. Several multi-phase steel sheets in which second phases such as a bainite phase and a precipitation phase are mixed have been proposed.

For example, in Patent Document 1, as a retained austenite steel sheet in which the second phase is mainly a retained austenite phase, C: less than 0.16 to 0.30% (“%” relating to the chemical composition unless otherwise specified in this specification) Means “mass%”), Si: 0.5 to 3.0%, Mn: 0.5 to 3.0%, Si + Mn: more than 1.5 to 6.0%, P: 0 0.02% or less, S: 0.01% or less, Al: 0.005 to 0.10%, and Fe as main components, and if necessary, Ca: 0.0005 to 0.01% or REM: 0 0.005 to 0.05%, which is composed of three phases of ferrite, bainite, and retained austenite as a microstructure, and the ratio of the ferrite space factor (V _F ) to the ferrite particle size (d _F ) (V _F / d _F ) is 7 or more and 2 μm or less Stenite space factor is 5% or more, yield ratio (YR): 60% or more, strength-ductility balance (tensile strength x total elongation): 2000 kgf / mm ² % or more, hole expansion ratio (d / d0) An invention relating to a high yield ratio hot rolled high strength steel sheet having excellent formability and a method for producing the same having characteristics of: 1.1 or more, uniform elongation: 10% or more is disclosed.

  Patent Document 2 includes C: 0.03 to 0.2%, Mn: 0.5 to 2.0% as a steel plate whose second phase is a martensite phase, and further includes Si and Al. The total amount of one or two of them is at least one of 0.02 to 4.0%, P: 0.02 to 0.2%, Cr: 0.02 to 1.0%, and the balance Is composed of Fe and inevitable components, has a martensite space factor of 3 to 30%, an average crystal grain size of the martensite is 5 μm or less, and has a work hardening index of 0.13 or more as a property of the steel sheet. An invention relating to an automotive high-strength steel sheet having a ratio of 75% or less, tensile strength × total elongation of 18000 or more, and hole expansibility of 1.2 or more is disclosed.

  In addition, as a method of ensuring good workability while increasing the tensile strength of the steel sheet, it is considered effective to refine the steel structure of the steel sheet. Such an invention is also disclosed.

JP-A-5-171345 JP-A-11-61327

  As described above, in recent years, the strength of steel sheets used as materials for various structural members has been increased to reduce the weight of automobiles and the like. Here, in addition to high tensile strength, hot-rolled steel sheets used for automobile undercarriage parts, bumper parts, shock absorbing members, etc., from the viewpoint of lightness, excellent durability, and shock absorbing performance, etc. It is desirable to have a high yield ratio. There is also a need to have good ductility that can withstand molding into complex shapes.

From such a background, as described above, in the prior art, increasing the strength of the steel sheet by various methods has been studied, but these conventional techniques have the following problems.
If the second phase is mainly the retained austenite phase as in the invention disclosed in Patent Document 1, it is difficult to efficiently increase the tensile strength of the steel sheet. If attention is focused only on increasing the strength, it is conceivable to increase the content of C, Si, etc. However, since the weldability deteriorates significantly and the risk of delayed fracture increases, the content of these elements is practical. Restricted above. Therefore, it is practically difficult to secure a tensile strength of 780 MPa or more in the invention disclosed in Patent Document 1.

  Moreover, when the second phase is a martensite phase as in the invention disclosed in Patent Document 2, the yield ratio of the steel sheet is lowered. For this reason, in the invention disclosed in Patent Document 2, it is difficult to ensure a high yield ratio exceeding 0.70 while securing a tensile strength of 780 MPa or more. As described above, hot-rolled steel sheets used for automobile undercarriage parts, bumper parts, shock absorbing members, etc. must have a high yield ratio from the viewpoints of weight reduction, excellent durability and shock absorption capacity, etc. Since it is required, it is not preferable to apply the invention disclosed in Patent Document 2 to a steel sheet used for these applications.

  The present invention has been made in view of the problems of these conventional techniques, and has a high tensile strength of 780 MPa or more, a high yield ratio of 0.70 or more, and the product of tensile strength and total elongation. A first object is to provide a hot-rolled steel sheet having a good TS × El value of 20000 MPa ·% or more and a method for manufacturing the hot-rolled steel sheet. It is a second object to provide without including an excessive alloy element.

The present inventors diligently studied a method of providing a high yield hot rolled steel sheet having a high yield ratio and good ductility for a high strength hot rolled steel sheet having a multiphase structure. As a result, the knowledge listed below was obtained.
(A) According to Hall-Petch's law, the yield stress and the yield ratio increase with the refinement of crystal grains.
(B) When a hard secondary phase such as a martensite phase or a retained austenite phase is contained in a steel structure in which the average grain size of ferrite as a main phase is refined to 2.0 μm or less, the ductility is dramatically improved.
(C) In this case, ductility is further improved by refining the hard second phase of the martensite phase or the retained austenite phase.
(D) The duplex steel sheet having the steel structure can be manufactured by controlling the structure in the hot rolling process. That is, a ferrite transformation from hot-worked austenite can form a fine ferrite phase, and the remainder can be martensite transformed at a low temperature to become a martensite phase or remain austenite without transformation. It can be left to form a retained austenite phase.
(D) In the prior art, a method for producing a dual-phase hot-rolled steel sheet composed of a ferrite phase and a martensite phase usually has a predetermined cooling after a part of the steel structure is ferrite transformed in the cooling process after hot rolling. The remainder is martensitic transformed by quenching at a speed. The multi-phase hot-rolled steel sheet manufactured in this way has a homogeneous multi-phase structure in the sheet thickness direction and a low yield ratio.
(E) However, when a steel plate having a predetermined chemical composition is subjected to multi-pass rolling in a stable austenite region and a shear strain is applied to the surface layer portion, the rolled substructure of the processed austenite becomes finer, and further abruptly immediately after rolling. When cooled and maintained in a predetermined temperature range, the fine rolled substructure becomes nucleation sites of ferrite to form fine polygonal ferrite grains, and the remainder is enriched with a large amount of carbon to form austenite. Stabilize. Thereby, a fine steel structure containing retained austenite in addition to martensite is formed in the surface layer portion of the steel sheet.
(F) The retained austenite sandwiched between such fine polygonal ferrites is extremely stable, and the transformation to martensite is remarkably suppressed. This is presumably because the Ms point was lowered to room temperature or lower due to the influence of the three-dimensional restraining force due to the fine particles.
(G) The retained austenite phase formed in the steel sheet surface layer portion has an effect of increasing the yield ratio of the steel plate and improving ductility by the TRIP effect.
(H) That is, by subjecting a steel sheet having a predetermined chemical composition to hot rolling under a predetermined condition, the steel sheet thickness center portion is formed as a steel structure mainly composed of a ferrite phase and a martensite phase, and has high tensile strength. In addition to ensuring a good ductility, the steel layer has a steel structure containing a retained austenite phase, which compensates for the low yield ratio due to the structure at the center of the plate thickness and achieves a high yield ratio. Furthermore, the ductility can be further increased.
(I) A steel sheet having such a steel structure has a strength-ductility characteristic that surpasses that of conventional retained austenitic steels, while the concentration of strain in the tensile deformation process is dispersed and is based on a multiphase structure.

  The present invention has been made on the basis of these findings. The surface layer portion of the steel sheet has a fine ferrite phase as a main phase and a steel structure containing a retained austenite phase. It is based on the technical idea that a high-strength hot-rolled steel sheet having a high yield ratio and good ductility can be obtained by making a steel structure composed of a phase and a martensite phase. is there.

  In the present invention, the steel structure of the surface layer part having a depth of 100 μm in the thickness direction from the steel sheet surface contains, by volume ratio, residual austenite: 2% to 15% and martensite: 3% to less than 19%. The steel structure of the center portion of the plate thickness, in which the balance is made of ferrite and the average grain size of the ferrite is less than 1.5 μm, and the depth in the plate thickness direction from the steel plate surface is 1/4 or more of the plate thickness, The martensite content is 10% or more and 45% or less and the retained austenite content is 2% or less. The balance is made of ferrite, the martensite has an average particle size of 1.5 μm or less, and the ferrite has an average particle size of 2. The mechanical properties are 0 μm or less, tensile strength: 780 MPa or more, yield ratio: 0.70 or more, and TS × El value, which is the product of tensile strength and breaking elongation, is 20000 MPa ·% or more. This is a hot-rolled steel sheet.

  The hot-rolled steel sheet according to the present invention has C: 0.1% to 0.2%, Mn: 1% to 3%, Si + Al: 0.3% to less than 1.0%, Cr: 0.1 %: 0.6% or less and N: 0.001% or more and 0.008% or less, and further selected from the group consisting of Ti: 0.05% or less and Nb: 0.05% or less Or it is preferable to have a chemical composition which contains 2 types and the remainder consists of Fe and an impurity.

From another viewpoint, the present invention is a multi-pass hot rolling that completes rolling in a temperature range of [Ae ₃ points−50 (° C.)] to [Ae ₃ points + 50 (° C.)] on the slab having the above chemical composition. The steel sheet is rolled to form a hot-rolled steel sheet, and the hot-rolled steel sheet is cooled to a temperature range of 700 ° C. or lower at an average cooling rate of 600 ° C./second or higher within 0.4 seconds after completion of rolling, and then 600 ° C. or higher and 700 ° C. A method for producing a hot-rolled steel sheet, wherein the steel sheet is maintained in a temperature range of 0.4 ° C. or less for 0.4 seconds to 5 seconds and then cooled to a temperature range of 450 ° C. or less at an average cooling rate of 100 ° C./second or more. .

  According to the present invention, it has a high tensile strength of 780 MPa or more without including an excessive amount of alloying elements that impairs various properties, and has a high yield ratio of 0.70 or more, tensile strength, and total elongation. A high-strength hot-rolled steel sheet having a good ductility of a product TS × El value of 20000 MPa ·% or more is obtained. If the hot-rolled steel sheet according to the present invention is applied to, for example, automobile members, the collision safety of those members can be further improved.

FIG. 1 is a drawing showing the shape of a rapidly changing tensile tensile test piece. FIG. 2 is a graph showing the relationship between the average particle size of the martensite phase at the center of the plate thickness in the example and the TS × El value. FIG. 3 is a graph showing the relationship between the volume fraction of retained austenite in the surface layer portion and the TS × El value in the examples.

First, the steel structure and chemical composition of the hot rolled steel sheet according to the present invention will be described.
1. Steel structure (1) Steel structure in the surface layer part having a depth of 100 μm from the surface of the steel sheet The steel structure in the surface layer part is a volume ratio of retained austenite: 2% to 15% and martensite: 3% or more The content is less than 19%, the balance is made of ferrite, and the average particle size of ferrite is less than 1.5 μm.

  The steel structure in the surface layer is determined so that the steel sheet has a high yield ratio and good ductility. That is, the center portion of the plate thickness, which will be described later, has a multiphase structure for the purpose of ensuring a high tensile strength, but such a steel structure reduces the yield ratio. For this reason, by controlling the steel structure of a surface layer part, while improving a yield ratio, ductility is improved further.

(A) Volume ratio If the volume ratio of the retained austenite phase is less than 2%, the intended yield ratio and ductility may not be obtained. Therefore, the volume ratio of the retained austenite phase is 2% or more. On the other hand, if the volume fraction of the retained austenite phase exceeds 15%, the stability of the retained austenite phase is lowered, and the target ductility may not be obtained. Therefore, the volume ratio of the retained austenite phase is 15% or less.

  If the volume ratio of the martensite phase is less than 3%, the intended strength and ductility may not be obtained. Therefore, the volume ratio of the martensite phase is 3% or more. On the other hand, if the volume ratio of the martensite phase is 19% or more, the intended ductility may not be obtained. Therefore, the volume ratio of the martensite phase is less than 19%.

The balance is the ferrite phase.
(B) Average particle size When the average particle size of the ferrite phase is 1.5 μm or more, strain concentrates on the ferrite phase during deformation, and the desired good ductility may not be obtained. Therefore, the average particle size of the ferrite phase is less than 1.5 μm.

The hot-rolled steel sheet according to the present invention has a steel structure in which a surface layer part having a depth of 100 μm in the thickness direction from the steel sheet surface has the above volume ratio and average particle diameter.
(2) Steel structure in the center of the plate thickness where the depth in the plate thickness direction is 1/4 or more of the plate thickness from the surface of the steel plate. The steel structure in the center of the plate thickness is by volume ratio, martensite: 10% or more and 45%. And retained austenite: 0% or more and 2% or less, the balance being made of ferrite, the average particle size of martensite being 1.5 μm or less, and the average particle size of ferrite being 2.0 μm or less.

  The steel structure in the central portion of the plate thickness is determined in order to make the steel plate have high tensile strength and good ductility. That is, the martensite that can effectively improve the tensile strength and ensure good ductility is the second phase, and the ferrite phase and the martensite phase are made fine.

(A) Volume ratio If the volume ratio of the martensite phase is less than 10%, the intended tensile strength may not be obtained. Therefore, the volume ratio of martensite is 10% or more. On the other hand, when the volume fraction of the martensite phase exceeds 45%, the ductility is significantly reduced. Therefore, the volume ratio of martensite is 45% or less.

  If the volume ratio of the retained austenite phase exceeds 2%, the intended tensile strength may not be obtained. Therefore, the volume ratio of the retained austenite phase is 2% or less. Since the retained austenite phase in the central portion of the plate thickness is not essential, the lower limit of the volume ratio of the retained austenite phase is not particularly specified. That is, it may be 0%.

The balance is the ferrite phase.
(B) Average particle diameter If the average particle diameter of the martensite phase exceeds 1.5 μm, the intended tensile strength and ductility may not be obtained. Therefore, the average particle size of the martensite phase is 1.5 μm or less.

  If the average particle size of the ferrite phase exceeds 2.0 μm, the intended tensile strength may not be obtained. Therefore, the average grain size of the ferrite phase in the center portion of the plate thickness is 2.0 μm or less.

  The volume fractions of the martensite phase and the ferrite phase are obtained by converting into a three-dimensional volume fraction from a two-dimensional image obtained by SEM image or EBSD analysis. Further, the volume fraction of the retained austenite phase is determined by an X-ray diffraction test. Moreover, the average particle diameter of a martensite phase and a ferrite phase is a nominal particle diameter calculated | required from the two-dimensional image by a SEM image or EBSD analysis.

The hot-rolled steel sheet according to the present invention has a steel structure in which a thickness center portion whose depth in the sheet thickness direction is 1/4 or more of the sheet thickness from the steel sheet surface has the above volume ratio and average particle diameter.
2. Chemical composition [C: 0.1% or more and 0.2% or less]
If the C content is less than 0.1%, a predetermined amount of martensite phase cannot be secured in the surface layer portion and the center of the plate thickness, and a predetermined amount of retained austenite phase can be secured in the surface layer portion. In some cases, the intended tensile strength, ductility, or yield ratio cannot be obtained. Therefore, the C content is 0.1% or more. On the other hand, if the C content exceeds 0.2%, the volume ratio of the martensite phase or the retained austenite phase becomes excessive, and the target ductility may not be obtained. Therefore, the C content is 0.2% or less.

[Mn: 1% to 3%]
Mn is an element having an effect of improving hardenability, and is an important element for securing the target martensite and retained austenite volume fraction. If the Mn content is less than 1%, the effect of improving the hardenability becomes insufficient, it becomes difficult to secure the volume ratio of the target martensite and retained austenite, and it is difficult to ensure the target strength and ductility. There is a case. Therefore, the Mn content is 1% or more. Preferably, it is 1.5% or more. On the other hand, if the Mn content exceeds 3%, the second phase fraction becomes excessive, and ductility is lowered or weldability is impaired. Therefore, the Mn content is 3% or less. Preferably, it is 2.5% or less.

[Total content of Si and Al: 0.3% or more and less than 1.0%]
Si and Al are useful elements for generating martensite. By promoting the formation of ferrite and suppressing the formation of carbides, it has the effect of securing martensite. Furthermore, it has the effect | action which raises the intensity | strength of steel by solid solution strengthening, and the effect | action which makes steel healthy by deoxidation. If the total content of Si and Al is less than 0.3%, it may be difficult to obtain the effect of the above action. Therefore, the total content of Si and Al is 0.3% or more. Since Si and Al have the same kind of properties and can substitute each other, the lower limit of the respective contents of Si and Al is not specified. However, since the strengthening ability of steel is higher than that of Al, it is preferable to use Si more actively than Al. Therefore, the Si content is preferably 0.3% or more. On the other hand, Al has the effect | action which suppresses the coarsening of a crystal grain by forming nitride and an oxide in steel. Therefore, the Al content is preferably 0.001% or more. On the other hand, when the total content of Si and Al is 1.0% or more, the embrittlement of the steel may be remarkable. Therefore, the total content of Si and Al is less than 1.0%. Preferably, the Si content is 0.7% or less and the Al content is 0.1% or less.

[Cr: 0.1% to 0.6%]
Cr is an element that has the effect of stabilizing austenite and ensuring martensite and strengthening steel. If the Cr content is less than 0.1%, the desired strength may not be obtained. Therefore, the Cr content is 0.1% or more. On the other hand, if the Cr content exceeds 0.6%, the ferrite transformation is excessively suppressed, and it becomes difficult to ensure the target ferrite volume fraction. Therefore, the Cr content is 0.6% or less.

[N: 0.001% to 0.008%]
N is an element that has an action of binding to Ti and / or Nb to form nitrides in the steel and suppressing coarsening of crystal grains. If the N content is less than 0.001%, the amount of the nitride formed in the steel is too small, so that the effect of the above action is insufficient and the crystal grains may be coarsened. Therefore, the N content is 0.001% or more. On the other hand, if the N content exceeds 0.008%, the nitride formed in the steel becomes coarse, which adversely affects the ductility of the steel sheet. Therefore, the N content is 0.008% or less.

[Ti: 0.05% or less and Nb: 0.05% or less, one or two]
Ti and Nb are elements having an effect of suppressing the coarsening of crystal grains by forming nitrides in the steel, so that one or two of them are contained. However, when the content of Ti or Nb exceeds 0.05%, coarse nitrides are formed and the ductility of the steel sheet is significantly reduced, or ferrite transformation is suppressed to ensure the target ferrite volume fraction. I can't. Therefore, one or two of Ti: 0.05% or less and Nb: 0.05% or less are contained. In order to surely obtain the effect by the above action, it is preferable to contain 0.002% or more of Ti or 0.01% or more of Nb.

The balance other than those described above is Fe and impurities.
3. Manufacturing Method Next, an example of a method for manufacturing a hot-rolled steel sheet according to the present invention will be described.

A slab having the above chemical composition is subjected to multi-pass hot rolling to complete rolling in a temperature range of [Ae ₃ points−50 (° C.)] or more and [Ae ₃ points + 50 (° C.)] or less to form a hot rolled steel sheet, This hot-rolled steel sheet is cooled to a temperature range of 700 ° C. or lower at an average cooling rate of 600 ° C./second or higher within 0.4 seconds after completion of rolling, and then is cooled to a temperature range of 600 ° C. or higher and 700 ° C. or lower for 0.4 seconds. Hold for 5 seconds or more, and then cool to a temperature range of 450 ° C. or less at an average cooling rate of 100 ° C./second or more.

  That is, the steel structure is subjected to multi-pass hot rolling that completes rolling in a predetermined temperature range, thereby surely adding work strain in the austenite and performing rapid cooling immediately after rolling to recrystallize austenite. Is preferably obtained by promoting ferrite transformation by maintaining the temperature in a predetermined temperature range.

If the rolling completion temperature of multi-pass hot rolling is less than [Ae ₃ points-50 (° C.)], ferrite transformation may occur in the rolling process, and the intended steel structure may not be obtained. Therefore, the rolling completion temperature of the multi-pass hot rolling is preferably [Ae ₃ points−50 (° C.)] or higher. On the other hand, if the rolling completion temperature of multi-pass hot rolling exceeds [Ae ₃ points + 50 (° C.)], the processing strain applied to the steel sheet by rolling is easily released, and the steel structure may not be obtained. . Therefore, the rolling completion temperature of multi-pass hot rolling is preferably set to [Ae ₃ points + 50 (° C.)] or less.

If the time required for rapid cooling to a temperature range of 700 ° C. or less after the completion of rolling exceeds 0.4 seconds or the average cooling rate is less than 600 ° C./second, the processing strain applied to the steel sheet by rolling May be released and the steel structure may not be obtained. Accordingly, it is preferable to cool to a temperature range of 700 ° C. or less at an average cooling rate of 600 ° C./second or more within 0.4 seconds after completion of rolling. After the rapid cooling, 600 ° C. or more and 700 ° C. or less at which ferrite transformation proceeds. However, in order to make the ferrite transformation proceed reliably, it is preferable that the time for holding in the temperature range is 0.4 seconds or more. On the other hand, since the carbide | carbonized_material which has a bad influence on ductility will precipitate when the time hold | maintained in the said temperature range exceeds 5 second, it is preferable that the time hold | maintained in the said temperature range shall be 5 second or less.

  After the above holding, in order to make a martensitic transformation excluding the stable retained austenite phase formed in the steel sheet surface layer portion of the remainder that has not undergone ferrite transformation, the average cooling rate of 100 ° C./second or more is 450 ° C. or less. It is preferable to cool to a temperature range.

  Thus, according to the present invention, the steel structure has a chemical composition that does not involve an excessive amount of alloy elements that impairs various properties, and has a high tensile strength of 780 MPa or more and a high value of 0.70 or more. It is possible to provide a high-strength hot-rolled steel sheet having both a yield ratio and a good ductility of TS × El value, which is a product of tensile strength and total elongation, of 20000 MPa ·% or more.

The present invention will be described more specifically with reference to examples.
Using a test hot rolling mill as a test material, slabs (thickness: 35 mm, width: 170 mm, length: 75 mm) composed of steels A, B and C having the chemical composition shown in Table 1 were tested. It was. Steels A and B are steels that satisfy the chemical composition defined in the present invention according to claim 2, and Steel C is steel that does not satisfy the chemical composition defined in the present invention according to claim 2.

  Each slab was held in a heating furnace having an in-furnace temperature of 1250 ° C. for 1 hour, then subjected to 3 to 4 passes of rough hot rolling, and further subjected to 3 passes of finish rolling to obtain steel plate samples. The finished thickness of each sample was 2 mm. Table 2 shows the rolling conditions.

The obtained steel sheet was examined for mechanical properties by observing the steel structure and conducting a tensile test. Each evaluation method is listed below.
(A) Volume ratio and grain size of ferrite and martensite About the plate thickness section parallel to the rolling direction of the steel plate, the surface layer portion and the plate thickness center portion were photographed by SEM at a magnification of 3000 times, and from the obtained SEM two-dimensional image The volume ratio and the particle size were measured, and the average value was obtained for the particle size.

(B) Volume ratio of residual austenite phase A surface having a depth of 100 μm in the thickness direction from the surface of the steel plate, which is revealed by mechanical polishing and removed by chemical polishing, and is (1/4) the thickness of the plate An X-ray diffraction test was performed on the center plane of the plate thickness, and the austenite amount was quantitatively analyzed.

(C) Mechanical properties A tensile test was performed using a JIS No. 5 tensile test piece to obtain 0.2% proof stress, tensile strength, and elongation at break. The ratio of 0.2% proof stress and tensile strength was determined as the yield ratio.

(D) Static motion difference For test numbers 3 to 6, 13, and 14, using a rapid change tensile test apparatus using a rapidly changing tensile tensile test piece having a parallel portion width of 10 mm and a gauge length of 20 mm shown in FIG. The tests were performed at strain rates of 0.0025 / sec and 1000 / sec, and the difference in tensile strength was defined as a static difference. In the case of the strength member of an automobile, the strain rate received when it collides during high-speed traveling may reach 1000 / second, and an improvement in dynamic strength under high-speed deformation is required, but this difference in static motion is large. This means that high strength is ensured in the event of a collision, which indicates that collision safety is high.

  The results are summarized in Table 3. FIG. 2 is a graph showing the relationship between the average particle size of the martensite phase at the center of the plate thickness in the example and the TS × El value, and FIG. 3 shows the amount of retained austenite in the surface layer in the example. It is a graph which shows the relationship with TS * El value.

  Test numbers 1, 5, 6, 8, 9, 13 to 15 in Table 3 are examples of the invention satisfying the conditions of the present invention, and test numbers 2 to 4, 7, 10 to 12, and 16 are those of the present invention. This is a comparative example that does not satisfy the conditions.

  Test Nos. 1, 5, 6, 8, 9, 13 to 15 have a high tensile strength of 780 MPa or more, a high yield ratio of 0.70 or more, and a good ductility of TS × El value of 20000 MPa ·% or more. Have both. Furthermore, test numbers 5, 6, 13 and 14 for which the static difference was evaluated showed very high static difference characteristics as compared with test numbers 3 and 4.

  On the other hand, the test numbers 2 to 4, 7, and 10 to 12 had a low yield ratio and / or TS × El value. Test No. 16 had a tensile strength of less than 780 MPa and a low TS × El value.

Claims (3)

The steel structure of the surface layer part having a depth of 100 μm in the thickness direction from the surface of the steel sheet contains, by volume ratio, residual austenite: 2% to 15% and martensite: 3% to less than 19%, with the balance being ferrite And the average particle size of the ferrite is less than 1.5 μm,
The steel structure at the center of the plate thickness whose depth in the plate thickness direction is ¼ or more of the plate thickness from the surface of the steel plate contains, by volume, martensite: 10% to 45% and residual austenite: 2% or less. And the balance is made of ferrite, the average particle size of the martensite is 1.5 μm or less, the average particle size of the ferrite is 2.0 μm or less,
A hot-rolled steel sheet having mechanical properties such that a tensile strength: 780 MPa or more, a yield ratio: 0.70 or more, and a TS × El value that is a product of tensile strength and elongation at break is 20000 MPa ·% or more.   In mass%, C: 0.1% to 0.2%, Mn: 1% to 3%, Si + Al: 0.3% to less than 1.0%, Cr: 0.1% to 0.6% And N: 0.001% or more and 0.008% or less, and further containing one or two selected from the group consisting of Ti: 0.05% or less and Nb: 0.05% or less The hot-rolled steel sheet according to claim 1, wherein the balance has a chemical composition comprising Fe and impurities. The slab having the chemical composition according to claim 2 is subjected to multi-pass hot rolling to complete rolling in a temperature range of [Ae ₃ points−50 (° C.)] or more and [Ae ₃ points + 50 (° C.)] or less. The hot-rolled steel sheet is cooled to a temperature range of 700 ° C. or lower at an average cooling rate of 600 ° C./second or higher within 0.4 seconds after completion of the rolling, and then a temperature of 600 ° C. or higher and 700 ° C. or lower. A method for producing a hot-rolled steel sheet, wherein the steel sheet is held in a region for 0.4 seconds or more and within 5 seconds and then cooled to a temperature region of 450 ° C. or less at an average cooling rate of 100 ° C./second or more.

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