JP6241274B2 - Manufacturing method of hot-rolled steel sheet - Google Patents

Description

  The present invention relates to a method of manufacturing a hot-rolled steel sheet, specifically, a hot-rolled steel sheet that is used by being formed into various shapes by press working or the like, and in particular, a high-tensile hot-rolled steel sheet that is excellent in ductility and stretch flangeability. It relates to a manufacturing method.

  In recent years, from the viewpoint of protecting the global environment, efforts have been made to reduce carbon dioxide emissions in many fields. Automakers are also actively developing technology to reduce vehicle weight for the purpose of reducing fuel consumption. However, it is not easy to reduce the weight of the vehicle body because the emphasis is also placed on improving the collision resistance for ensuring the safety of passengers.

  Therefore, in order to achieve both weight reduction of the vehicle body and improvement of impact resistance, it has been studied to reduce the thickness of the member using a high-strength steel plate. For this reason, a high-tensile steel sheet having both high strength and excellent formability is strongly desired, and several techniques have been proposed in order to meet these requirements. Among these, many studies have been made on steel sheets containing retained austenite because they exhibit excellent ductility due to the transformation induced plasticity (TRIP) phenomenon.

  For example, Patent Document 1 discloses a high-strength automobile having excellent impact safety and formability in which residual austenite having an average crystal grain size of 5 μm or less is dispersed in ferrite having an average crystal grain size of 10 μm or less. A steel sheet is disclosed. In a steel sheet containing retained austenite in the metal structure, austenite becomes martensite during processing and exhibits a large elongation due to transformation-induced plasticity, but the hole expandability is impaired due to the formation of hard martensite. In the hot-rolled steel sheet disclosed in Patent Document 1, ductility and hole expandability are improved by refining ferrite and retained austenite.

  Patent Document 2 discloses a high-strength steel plate having a tensile strength of 980 MPa or more excellent in elongation and stretch flangeability, in which a second phase composed of one or both of retained austenite and martensite is finely dispersed in crystal grains. Has been.

  The inventors of the present invention proposed a high-tensile hot-rolled steel sheet excellent in ductility and stretch flangeability and a method for producing the same according to Patent Document 3 and Patent Document 4. The manufacturing method disclosed in Patent Document 3 is cooled to a temperature range of 720 ° C. or less within 1 second after completion of hot rolling, and stays in a temperature range of more than 500 ° C. and 720 ° C. or less with a residence time of 1 to 20 seconds. After that, a high-strength hot-rolled steel sheet having good ductility and stretch flangeability is manufactured by winding in a temperature range of 350 to 500 ° C. Further, in Patent Document 4, grains mainly composed of bainite, containing appropriate amounts of polygonal ferrite and retained austenite, and surrounded by grain boundaries having a crystal orientation difference of 15 ° or more in a steel structure excluding retained austenite. A high-strength hot-rolled steel sheet having a good ductility and stretch flangeability is disclosed.

JP-A-11-61326 JP 2005-179703 A JP 2012-251200 A JP 2012-251201 A

  Since there are various types of processing for automobile parts, the required formability differs depending on the applied member, but ductility and stretch flangeability are positioned as important indicators of formability. It is desirable to have a high level. Furthermore, in recent years, it is desired to have higher strength than the conventional one, but the above-described conventional hot-rolled steel sheet and the manufacturing method thereof have the following problems.

  The steel sheet disclosed in Patent Document 1 is said to have improved ductility and hole expandability by refinement of ferrite and retained austenite, but the hole expansion ratio obtained is at most 1.5, and sufficient press formability is achieved. It is hard to say that it is prepared. In addition, in order to improve the work hardening index and improve the collision safety, it is necessary to make the main phase a soft ferrite phase, and it is difficult to obtain a high tensile strength.

  The steel sheet disclosed by Patent Document 2 contains a large amount of an expensive element such as Cu or Ni in order to refine the second phase to a nano size and disperse it in the crystal grains, or at a high temperature for a long time. It is necessary to perform a solution treatment, and the increase in manufacturing cost and the decrease in productivity are remarkable.

  In the method for manufacturing a steel sheet disclosed in Patent Document 3, it is difficult to control the sheet temperature because rapid cooling of several hundred degrees Celsius / s or more is continued to a temperature in the vicinity of 700 degrees Celsius.

  Furthermore, the steel sheet disclosed by Patent Document 4 has high strength and good ductility and stretch flangeability, but the strength-stretch flangeability balance (TS × λ) is less than 69000 MPa ·%. It is difficult to apply to members that require high stretch flangeability such as suspension parts.

  This invention is made | formed in view of such a subject of the prior art, and it aims at providing the manufacturing method of the hot-rolled steel plate which has the high ductility and stretch flangeability while having high intensity | strength. . Specifically, the present invention has a tensile strength of 780 MPa or more, a strength-ductility balance (TS × EL) of 20000 MPa ·% or more, and a strength-stretch flangeability balance (TS × λ) of 69000 MPa ·%. It aims at providing the manufacturing method of the hot-rolled steel plate which is the above.

  In light of the above situation, the present inventors have conducted extensive research on the chemical composition of hot-rolled steel sheets and the relationship between the steel structure and mechanical properties, and as a result, obtained the following knowledge a to e and completed the present invention. did.

  (A) The steel structure is preferably hard to obtain high strength, and the steel structure is preferably homogeneous to obtain excellent stretch flangeability. Therefore, in order to combine high strength and excellent stretch flangeability, bainite, which is a hard and homogeneous structure, is most suitable, and it is important to have a steel structure mainly composed of bainite.

  (B) However, bainite is a structure with poor ductility. For this reason, it is difficult to ensure excellent ductility simply by using a steel structure mainly composed of bainite.

  (C) In order to combine excellent ductility, it is effective to contain an appropriate amount of polygonal ferrite and retained austenite, but the surface layer of the steel sheet is more preferable than a uniform structure throughout the entire thickness direction. By increasing the amount of ferrite in the vicinity and making the inside a tilted structure mainly composed of bainite, stretch flangeability is maintained and ductility is further improved. Such a steel structure having an inclination in the thickness direction is a multi-pass hot rolling, and the surface layer of the steel sheet is improved by increasing the rolling reduction in the final rolling pass, the preceding rolling pass and the preceding rolling pass. By introducing shear strain in the vicinity, the ferrite transformation driving force in the vicinity of the surface layer is increased, and by stopping water cooling after rolling in an appropriate temperature range and retaining for a certain period of time, the ferrite transformation driving force in the vicinity of the surface layer is increased. It is realized by transforming after releasing the accumulated strain inside the steel plate while leaving it behind.

  (D) Moreover, ductility is improved by transformation-induced plasticity (TRIP) by containing a suitable amount of retained austenite.

  (E) By generating a larger amount of soft ferrite and retained austenite near the surface of the steel sheet than inside the steel sheet, the uniform elongation near the surface of the steel sheet is improved and the generation of microcracks during punching is suppressed. Is possible. And by making the inside of a steel plate into a bainite-based structure, it becomes possible to suppress the propagation of microcracks. Thereby, the formability in stretch flange forming, bending forming, or the like, in which the strain amount of the steel sheet surface layer portion is larger than that in the steel plate, can be improved.

The present invention made based on the above findings is as listed below.
(1) Hot-rolled steel sheet having a tensile strength of 780 MPa or more, a strength-ductility balance (TS × EL) of 20000 MPa ·% or more, and a strength-stretch flangeability balance (TS × λ) of 69000 MPa ·% or more. A manufacturing method of
In mass%, C: more than 0.08% and less than 0.30%, Si: 0.10% to 3.0%, Mn: 1.0% to 4.0%, P: 0.10% or less , S: 0.010% or less, sol. Al: 0.010% to 3.0%, N: 0.010% or less, and Si and sol. The total thickness of Al is 0.8% or more and 3.0% or less, and the slab having a chemical composition composed of Fe and impurities in the balance is subjected to multi-pass hot rolling, and the sheet thickness is more than 1.2 mm and less than 6 mm When making hot-rolled steel sheets,
After the rolling is completed, multi-pass hot rolling is performed in the temperature range of 860 ° C. or higher and 1050 ° C. or lower with the rolling reduction in the final rolling pass, the previous rolling pass and the previous rolling pass being 15% or more and 60% or less. After cooling within 0.3 seconds, after cooling to a temperature range of less than 850 ° C. Ar ₃ points or more at a cooling rate of 200 ° C./second or more and retaining in that temperature range for 1 second or more and less than 3 seconds After cooling at a cooling rate of 20 ° C./second or more to a temperature range of less than 750 ° C. to 600 ° C. or more and retaining in the temperature range for 1 second to less than 15 seconds, winding in a temperature range of 350 ° C. to 500 ° C. A method for producing a hot-rolled steel sheet, characterized in that residual austenite is contained by taking.

  (2) The method for producing a hot-rolled steel sheet according to (1), wherein multi-pass hot rolling that satisfies the following formula (1) is performed.

Here, the meaning of each symbol in the above formula (1) is as follows.
t: Time between passes (seconds) from the completion of rolling of the rolling pass immediately before the final rolling pass to the start of rolling of the final rolling pass
T: Rolling completion temperature (° C.) of the rolling pass immediately before the final rolling pass

  (3) The chemical composition is selected from the group consisting of Ti: 0.20% or less, Nb: 0.10% or less, and V: 0.50% or less in mass% instead of part of Fe The manufacturing method of the hot rolled sheet steel as described in (1) or (2) term which has a seed | species or 2 or more types.

  (4) Chemical composition is mass% instead of part of Fe, Cr: less than 1.0%, Mo: 0.5% or less, Ni: 1.0% or less, and B: 0.0050% or less The manufacturing method of the hot rolled sheet steel in any one of the (1)-(3) term which has 1 type or 2 types or more selected from the group which consists of.

  (5) The chemical composition is selected from the group consisting of Ca: 0.020% or less, Mg: 0.020% or less, and REM: 0.020% or less in mass% instead of part of Fe. The manufacturing method of the hot rolled sheet steel in any one of (1)-(4) term which has a seed | species or 2 or more types.

  (6) The hot-rolled steel sheet according to any one of (1) to (5), wherein the chemical composition has Cu: 1.0% by mass or less instead of part of Fe.

  (7) The method for producing a hot-rolled steel sheet according to any one of (1) to (6), wherein the chemical composition has Bi: 0.020% by mass or less instead of part of Fe.

  According to the present invention, it is possible to stably produce a hot-rolled steel sheet having high strength and excellent ductility and stretch flangeability, which is suitable as a material used for automobile members, machine structural members, or building members. .

  The method for producing a hot-rolled steel sheet according to the present invention will be specifically described below. In the following description, “%” regarding the chemical composition of steel means “% by mass” unless otherwise specified.

1. Chemical composition (1-1) C: more than 0.08% and less than 0.30% C has an action of promoting the formation of bainite and an action of stabilizing retained austenite. When the C content is 0.08% or less, it becomes difficult to secure the target bainite area ratio and the retained austenite area ratio. Therefore, the C content is more than 0.08%. Preferably it is 0.10% or more, more preferably 0.12% or more. On the other hand, when the C content is 0.30% or more, pearlite is preferentially generated, resulting in insufficient generation of bainite and retained austenite, and it is difficult to ensure the target bainite area ratio and residual austenite area ratio. Become. Therefore, the C content is less than 0.30%. Preferably it is 0.25% or less.

(1-2) Si: 0.10% or more and 3.0% or less Si, like Al, has a function of delaying the precipitation of cementite, whereby the amount of austenite remaining untransformed, that is, residual It is possible to increase the volume ratio of austenite. Further, Si can increase the strength of the steel sheet by solid solution strengthening and also has an effect of making the steel sound by deoxidation. If the Si content is less than 0.10%, it is difficult to obtain the effect by the above action. Therefore, the Si content is 0.10% or more. However, when the Si content exceeds 3.0%, the surface properties and chemical conversion properties of the steel sheet deteriorate, and further, the ductility and weldability deteriorate. Further, it leads to significant increase in A ₃ transformation point, which may make it difficult to stable hot rolling. Therefore, the Si content is 3.0% or less. Preferably it is 2.5% or less.
As will be described later, in the present invention, Si and sol. Although the total content of Al is important, Si is sol. Since the solution strengthening ability is higher than Al, when higher strength is required, the Si content is preferably 0.5% or more, more preferably 0.8% or more, and particularly preferably 1.0% or more.

(1-3) Mn: 1.0% or more and 4.0% or less Mn has the effect of suppressing the ferrite transformation and promoting the formation of bainite. If the Mn content is less than 1.0%, it is difficult to ensure the target bainite area ratio. Therefore, the Mn content is 1.0% or more. Preferably it is 1.5% or more, More preferably, it is 1.8% or more. On the other hand, if the Mn content exceeds 4.0%, ferrite transformation is excessively suppressed, and it becomes difficult to secure the area ratio of the desired polygonal ferrite. In addition, since the completion of the bainite transformation is delayed, carbon concentration to austenite is not promoted, and the generation of retained austenite becomes insufficient, making it difficult to secure the desired retained austenite area ratio. Furthermore, it becomes difficult to increase the carbon concentration in the retained austenite. Therefore, the Mn content is 4.0% or less. Preferably it is 3.6% or less, More preferably, it is 3.2% or less.

(1-4) P: 0.10% or less P is an element that is generally contained as an impurity, but is also an element that has an effect of increasing strength by solid solution strengthening. Therefore, P may be positively included. However, P is an element that is easily segregated. When the P content exceeds 0.10%, the formability and toughness due to grain boundary segregation are significantly reduced. Therefore, the P content is 0.10% or less. Preferably it is 0.030% or less, More preferably, it is 0.020% or less. The lower limit of the P content is not particularly required, but is preferably 0.001% or more from the viewpoint of refining costs.

(1-5) S: 0.010% or less S is an element contained as an impurity, and forms sulfide-based inclusions in the steel to lower the formability of the hot-rolled steel sheet. When the S content exceeds 0.010%, the moldability is significantly lowered. Therefore, the S content is 0.010% or less. Preferably it is 0.0050% or less, More preferably, it is 0.0030% or less, Most preferably, it is 0.0010% or less. The lower limit of the S content is not particularly required, but is preferably 0.0001% or more from the viewpoint of suppressing an increase in refining cost.

(1-6) sol. Al: 0.010% or more and 3.0% or less Al, like Si, has the effect of deoxidizing steel to make the steel plate sound and also suppressing the precipitation of cementite from austenite. Has the effect of promoting production. sol. If the Al content is less than 0.010%, it is difficult to obtain the effect of the above action. Therefore, sol. The Al content is 0.010% or more. Preferably it is 0.20% or more. On the other hand, sol. The Al content of 3.0 percent, inviting a significant increase in the A ₃ transformation point, which may make it difficult to stable hot rolling. Therefore, sol. The Al content is 3.0% or less. Preferably it is 2.5% or less, More preferably, it is 2.0% or less.

(1-7) N: 0.010% or less N is an element contained as an impurity and has an effect of reducing the formability of the steel sheet. If the N content exceeds 0.010%, the moldability is significantly reduced. Therefore, the N content is 0.010% or less. Preferably it is 0.0080% or less, More preferably, it is 0.0070% or less. The lower limit of the N content does not need to be specified, but considering the case where one or more of Ti, Nb, and V are included to refine the steel structure as described later, In order to promote precipitation, the N content is preferably 0.0010% or more, and more preferably 0.0020% or more.

(1-8) Si and sol. Total content of Al (Si + sol.Al): 0.8% or more and 3.0% or less As described above, since both Si and Al have an action of promoting the formation of retained austenite, the intended retained austenite area ratio From the viewpoint of securing Si and sol. The total content of Al (Si + sol.Al) is specified. If the total content (Si + sol.Al) is less than 0.8%, it is difficult to ensure the intended retained austenite area ratio. In addition, it is difficult to increase the carbon concentration in the retained austenite. Therefore, the total content (Si + sol.Al) is 0.8% or more. Preferably it is 1.0% or more, More preferably, it is 1.2% or more, Most preferably, it is 1.5% or more. On the other hand, the total content (Si + sol. Al) of 3.0 percent, inviting a significant increase in the _{A 3} transformation point, which may make it difficult to stable hot rolling. Therefore, the total content (Si + sol.Al) is set to 3.0% or less. Preferably it is 2.5% or less, More preferably, it is 2.2% or less.
In the method for producing a steel sheet according to the present invention, the elements listed below may be included as optional elements.

(1-9) One or more selected from the group consisting of Ti: 0.20% or less, Nb: 0.10% or less, and V: 0.50% or less Ti, Nb, and V are all , It precipitates as carbide or nitride in steel and has the effect of refining the steel structure by its pinning effect. Therefore, you may contain 1 type, or 2 or more types of these elements. However, even if it contains excessively, the effect by the said effect | action will be saturated and it will become uneconomical. Therefore, the Ti content is 0.20% or less, the Nb content is 0.10% or less, and the V content is 0.50% or less. In order to more surely obtain the effect of the above-described action of these elements, it is preferable to satisfy any of Ti: 0.005% or more, Nb: 0.002% or more, and V: 0.005% or more.

(1-10) Cr: Less than 1.0%, Mo: 0.5% or less, Ni: 1.0% or less, and B: 0.0050% or less , Mo, Ni and B all have the effect of improving hardenability. Moreover, Cr and Ni have the effect | action which stabilizes retained austenite, and Mo has the effect | action which precipitates a carbide | carbonized_material in steel and raises an intensity | strength. Moreover, Ni has the effect | action which suppresses effectively the grain boundary crack of the slab resulting from Cu, when containing Cu so that it may mention later. Therefore, you may contain 1 type, or 2 or more types of these elements.

  However, when the Cr content is 1.0% or more, the chemical conversion property is significantly lowered. Therefore, the Cr content is less than 1.0%. In order to more reliably obtain the effect of the above action, the Cr content is preferably 0.05% or more.

  Even if the Mo content exceeds 0.5%, the effect of the above action is saturated and disadvantageous in cost. Therefore, the Mo content is 0.5% or less. Preferably it is 0.2% or less. In order to more reliably obtain the effect of the above action, the Mo content is preferably set to 0.02% or more.

  Since Ni is an expensive element, a large amount is disadvantageous in terms of cost. Therefore, the Ni content is preferably 1.0% or less. In order to more reliably obtain the effect of the above action, the Ni content is preferably 0.05% or more.

  If the B content exceeds 0.0050%, the moldability is remarkably deteriorated. Therefore, the B content is 0.0050% or less. In order to more reliably obtain the effect of the above action, the B content is preferably set to 0.0002% or more.

(1-11) One or more selected from the group consisting of Ca: 0.020% or less, Mg: 0.020% or less, and REM: 0.020% or less Ca, Mg and REM are inclusions By adjusting the shape, the moldability is improved. Therefore, you may contain 1 type, or 2 or more types of these elements. However, if the content of these elements exceeds the above upper limit, the inclusions in the steel become excessive, and the formability may be lowered on the contrary. Therefore, the content of each element is as described above. Each element is preferably 0.010% or less, more preferably 0.005% or less. In order to more reliably obtain the effect of the above action, it is preferable to contain 0.0005% or more of any of the above elements. Here, REM refers to a total of 17 elements of Sc, Y and lanthanoid, and the content of REM refers to the total content of these elements. In the case of a lanthanoid, it is industrially added in the form of misch metal.

(1-12) Cu: 1.0% or less Since Cu has an action of precipitating at a low temperature and increasing the strength, it may be contained in steel. However, if the Cu content exceeds 1.0%, grain boundary cracking of the slab may occur. Therefore, the Cu content is 1.0% or less. Preferably it is less than 0.5%, more preferably less than 0.3%. In order to more reliably obtain the effect of the above action, the Cu content is preferably 0.05% or more.

(1-13) Bi: 0.020% or less Bi has the effect of improving the formability by refining the solidified structure, so it may be contained in the steel. However, even if the Bi content exceeds 0.020%, the effect of the above action is saturated, which is disadvantageous in terms of cost. Therefore, the Bi content is 0.020% or less. Preferably it is 0.010% or less. In order to more reliably obtain the effect of the above action, the Bi content is preferably set to 0.0005% or more.

  The balance other than the above is Fe and impurities.

2. Production Conditions A multi-pass hot rolling (also referred to as “multi-pass hot rolling” in this specification) is applied to a slab having the above chemical composition to produce a hot-rolled steel sheet. To obtain a hot-rolled steel sheet that has both high strength and excellent ductility and stretch flangeability, the shear strain introduced by hot rolling is used to reduce the accumulated strain between the vicinity of the steel sheet surface and the inside of the steel sheet. It is important to promote the ferrite transformation at a depth of 100 μm from the steel plate surface more efficiently than the inside of the steel plate by efficiently using the difference in driving force due to the difference in strain.

Specifically, in hot rolling, when a slab having the above-described chemical composition is subjected to multi-pass hot rolling to obtain a hot-rolled steel sheet having a sheet thickness of more than 1.2 mm and not more than 6 mm, one final rolling pass and one Multi-pass hot rolling is performed in the temperature range of 860 ° C. or more and 1050 ° C. or less with the reduction rate in the previous rolling pass and the previous rolling pass being 15% or more and 60% or less, and cooling is performed within 0.3 seconds after the completion of rolling. It was initiated at 200 ° C. / sec or more cooling rate was cooled to a temperature range of not lower than ₃ points lower than 850 ° C. Ar, cooled in 20 ° C. / sec or more after the residence time of less than 1 second for 3 seconds in this temperature range After cooling to a temperature range of less than 750 ° C. and 600 ° C. or more at a speed, and retaining in this temperature range for 1 second or more and less than 15 seconds, the residual austenite is taken up in a temperature range of 350 ° C. or more and 500 ° C. or less. Contains hot rolling Manufacture steel sheets.

  Furthermore, by performing multi-pass hot rolling that satisfies the following formula (1), recrystallization of austenite grains between passes is promoted and growth of the austenite grains is suppressed.

  Here, the symbol t in Equation (1) is the time (seconds) between passes from the completion of rolling of the rolling pass immediately before the final rolling pass to the start of rolling of the final rolling pass, and the symbol T is the final rolling pass. Is the rolling completion temperature (° C.) of the previous rolling pass.

The production method will be described in more detail below.
(2-1) Slab, slab temperature when used for hot rolling, hot rolling mode A slab used for hot rolling has the above-described chemical composition. The slab to be used for hot rolling can be a slab obtained by continuous casting or a slab obtained by casting / splitting, and if necessary, a slab obtained by adding hot working or cold working. be able to.

  The temperature of the slab to be subjected to hot rolling may be heated to a temperature that becomes an austenite single-phase region in order to perform hot rolling in the austenite region, and it is not necessary to specifically limit it, but a suitable rolling completion temperature described later is ensured. From the viewpoint of reducing the scale loss, it is preferable to set the temperature to 1050 ° C. or higher, and from the viewpoint of suppressing the scale loss. In addition, when the slab to be subjected to hot rolling is a slab obtained by continuous casting or a slab obtained by partial rolling and is in a high temperature state, it may be subjected to hot rolling without heating.

  In hot rolling, it is preferable to use a lever mill or a tandem mill as multi-pass rolling. In particular, from the viewpoint of industrial productivity, at least the final several stages are more preferably rolled using a tandem mill.

(2-2) Reduction ratio in the final rolling pass, the previous rolling pass and the previous rolling pass: 15% or more and 60% or less In the final rolling pass, the previous rolling pass and the previous rolling pass The rolling reduction is 15% or more and 60% or less. By reducing the rolling reduction ratio in the final rolling pass, the previous rolling pass, and the previous rolling pass to 15% or more, recrystallized austenite grains are mainly refined and introduced near the surface layer of the steel sheet. Due to the effect of shear strain, the recrystallized austenite grains near the surface layer of the steel sheet are further refined compared to the inside of the steel sheet. Furthermore, by setting the rolling reduction ratio of the final rolling pass to 15% or more, the strain introduced is used as a transformation driving force and a transformation nucleation site in combination with cooling conditions after hot rolling described later. Compared to the above, it is possible to promote the ferrite transformation in the vicinity of the surface layer of the steel sheet. When the rolling reduction in each rolling pass is less than 15%, the amount of shear strain introduced near the surface layer of the steel sheet becomes insufficient, and a hot-rolled steel sheet having both ductility and stretch flangeability cannot be obtained. Therefore, the rolling reduction in the final rolling pass, the previous rolling pass, and the previous rolling pass is 15% or more. It is preferably 20% or more, more preferably 30% or more, and further preferably 40% or more.

  On the other hand, the rolling reduction in each rolling pass is set to 60% or less from the viewpoint of suppressing the release of the flatness of the steel sheet and the introduced strain due to processing heat generation. Preferably it is 50% or less.

(2-3) Rolling completion temperature: 860 ° C. or higher and 1050 ° C. or lower The rolling completion temperature is 860 ° C. or higher and 1050 ° C. or lower. Thus, release of strain introduced by rolling is suppressed, and a hot-rolled steel sheet having both ductility and stretch flangeability can be obtained by appropriately performing subsequent cooling treatment.

  If rolling completion temperature is less than 860 degreeC, the deformation resistance at the time of rolling will become large, and it will become difficult to perform rolling by said rolling reduction. Accordingly, the rolling completion temperature is set to 860 ° C. or higher. Preferably it is 880 degreeC or more, More preferably, it is 900 degreeC or more.

  On the other hand, when the rolling completion temperature is higher than 1050 ° C., the strain introduced by rolling proceeds, and a hot-rolled steel sheet having both ductility and stretch flangeability cannot be obtained. Therefore, the rolling completion temperature is 1050 ° C. or lower. Preferably it is 1030 degrees C or less, More preferably, it is 1000 degrees C or less, Most preferably, it is 980 degrees C or less. In addition, these temperatures are the surface temperature of steel materials, and can be measured with a radiation thermometer or the like.

  (2-4) Time between passes from the completion of rolling of the rolling pass immediately before the final rolling pass to the start of rolling of the final rolling pass: Satisfying equation (1)

  Here, the meaning of each symbol is: t: time between passes (seconds) from the completion of rolling immediately before the final rolling pass to the start of rolling in the final rolling pass, T: the rolling pass immediately before the final rolling pass Rolling completion temperature (° C.).

  By satisfying the above formula (1), austenite recrystallization is promoted and grain growth of austenite between the passes from the completion of rolling of the rolling pass immediately before the final rolling pass to the start of rolling of the final rolling pass. Therefore, recrystallized austenite grains are reduced in size during rolling, which makes it easier to obtain a steel structure suitable for ductility and stretch flangeability.

(2-5) Primary cooling after completion of rolling: Start cooling within 0.3 seconds, and cool to a temperature range of less than 850 ° C. Ar ₃ points or more at a cooling rate of 200 ° C./second or more Due to strain introduced by rolling In order to efficiently utilize the driving force and transform it, the primary cooling of the completion of rolling starts cooling within 0.3 seconds, and a temperature range of less than 850 ° C. and Ar ₃ points or more at a cooling rate of 200 ° C./second or more. Allow to cool.

  The time from the completion of rolling to the start of cooling is within 0.3 seconds. Cooling to this temperature range, coupled with the residence time described later, it is possible to release the accumulated strain inside the steel sheet while leaving the ferrite transformation driving force in the vicinity of the surface layer of the steel sheet. Thereby, it is possible to obtain a steel structure suitable for ductility and stretch flangeability in which the amount of ferrite in the vicinity of the surface layer of the steel sheet is larger than that inside.

When the time until the start of cooling exceeds 0.3 seconds after completion of rolling or when the cooling rate is less than 200 ° C./second, the strain introduced on the surface of the steel sheet is released, and such a steel structure cannot be obtained. . When the primary cooling stop temperature is 850 ° C. or higher, the accumulated strain in the vicinity of the surface layer of the steel sheet is remarkably released, and a desired steel structure cannot be obtained. On the other hand, when the primary cooling stop temperature is lower than the Ar ₃ point, ferrite transformation inside the steel sheet becomes prominent and it does not become a bainite-based structure.

Therefore, the primary cooling after the completion of rolling starts cooling within 0.3 seconds, and cools to a temperature range of Ar less than 850 ° C. and ₃ or more points of Ar at a cooling rate of 200 ° C./second or more.

  The time from the completion of rolling to the start of cooling is preferably within 0.2 seconds, more preferably within 0.15 seconds. The cooling rate is preferably 250 ° C./second or more, more preferably 300 ° C./second or more. Although the upper limit value of the cooling rate is not particularly defined, if the cooling rate is increased, the cooling facility becomes large and the facility cost increases. Therefore, considering the equipment cost, 600 ° C./second or less is preferable.

(2-6) 850 ° C. lower than Ar ₃ point or more temperature region at a residence time: the residence time at 850 ° C. under Ar a temperature range above points ₃ to less than 1 second 3 seconds, and less than 1 second 3 seconds . This makes it possible to release the accumulated strain inside the steel sheet while leaving the ferrite transformation driving force in the vicinity of the surface layer of the steel sheet. Thereby, it is possible to obtain a steel structure suitable for ductility and stretch flangeability in which the amount of ferrite in the vicinity of the surface layer of the steel sheet is larger than that in the interior. If it is less than 1 second, the strain generation inside the steel sheet is insufficiently released, so the amount of ferrite produced increases and stretch flangeability decreases. On the other hand, if it is 3 seconds or more, the strain introduced into the surface of the steel sheet is released, and the ductility is lowered. Therefore, the residence time in the temperature range of less than 850 ° C. Ar ₃ points or more is 1 second or more and less than 3 seconds.

(2-7) Cooling rate to a temperature range of 600 ° C. or higher and lower than 750 ° C. and staying time in the temperature range: Cooling at 20 ° C./second or more and staying within 1 second to 15 seconds In the vicinity of the surface layer of the steel plate described above In order to secure the ferrite area ratio, cooling is performed at a cooling rate of 20 ° C./second or more to a temperature range of 600 ° C. or more and less than 750 ° C. at which ferrite transformation becomes active, and stays in this temperature range for 1 second or more and 15 seconds or less. . When the cooling rate is less than 20 ° C./second, ferrite transformation occurs during cooling inside the steel sheet, making it difficult to form a bainite-based structure. Therefore, the cooling rate to this temperature range shall be 20 degrees C / sec or more. Preferably it is 40 degreeC / second, More preferably, it is 60 degreeC / second, More preferably, it is 80 degreeC / second.

  Further, if the time for staying in this temperature range is less than 1 second, the ferrite transformation in the vicinity of the surface layer of the steel sheet does not sufficiently proceed and ductility is lowered. On the other hand, if the time for staying in this temperature range is longer than 15 seconds, the ferrite transformation inside the steel sheet may progress and the stretch flangeability may deteriorate. Furthermore, generation of cementite or pearlite becomes remarkable, and the ductility may be lowered. Therefore, the time for staying in this temperature range is within 15 seconds.

(2-8) Winding temperature: 350 to 500 ° C. The winding temperature is 350 to 500 ° C. When the coiling temperature is less than 350 ° C., hard martensite is generated, and stretch flangeability deteriorates. On the other hand, when the temperature exceeds 500 ° C., cementite and pearlite are prominently generated, and it is difficult to secure retained austenite, resulting in a decrease in ductility. Therefore, the coiling temperature is set to 350 ° C. or more and 500 ° C. or less.

(2-9) Thickness of hot-rolled steel sheet: more than 1.2 mm and 6 mm or less If the thickness of the hot-rolled steel sheet is 1.2 mm or less, it becomes difficult to ensure the rolling completion temperature and the rolling load becomes excessive, and heat Hot rolling may be difficult. Therefore, the thickness of the hot-rolled steel sheet of the present invention is more than 1.2 mm. Preferably it is 1.4 mm or more. On the other hand, if the thickness of the hot-rolled steel sheet exceeds 6 mm, it becomes difficult to refine the steel structure, and it becomes difficult to secure the above-described steel structure. In addition, it becomes difficult to obtain the above-described inclined structure. Therefore, the plate thickness is 6 mm or less. Preferably it is 5 mm or less.

(2-10) Others (2-10-1) Plating layer The surface of the hot rolled steel sheet according to the present invention having the above-described chemical composition and steel structure is provided with a plating layer for the purpose of improving corrosion resistance, etc. It is good also as a processed steel plate. The plating layer may be an electroplating layer or a hot dipping layer. Examples of the electroplating layer include electrogalvanizing and electro-Zn—Ni alloy plating. Examples of the hot dip plating layer include hot dip galvanizing, alloying hot dip galvanizing, hot dip aluminum plating, hot dip Zn-Al alloy plating, hot dip Zn-Al-Mg alloy plating, hot dip Zn-Al-Mg-Si alloy plating, etc. The The amount of plating adhesion is not particularly limited, and may be the same as the conventional one. Further, it is possible to further improve the corrosion resistance by performing an appropriate chemical conversion treatment (for example, application and drying of a silicate-based chromium-free chemical conversion treatment solution) after plating.

(2-10-2) Steel structure (Bainite area ratio at ¼ depth position of the plate thickness from the steel plate surface: 50% or more)
Since bainite is a hard and homogeneous structure and is the most suitable structure for combining high strength and excellent stretch flangeability, the bainite area at a 1/4 depth position of the plate thickness from the steel sheet surface. The rate is preferably 50% or more. More preferably, it is 60% or more.

(Polygonal ferrite area ratio at 1/4 depth position from the steel sheet surface: 2% or more and less than 30%)
The area ratio of polygonal ferrite at a position of ¼ depth of the plate thickness from the steel plate surface is preferably 2% or more and less than 30%. By including soft polygonal ferrite, the work hardening index at the initial deformation of the steel sheet is improved. Furthermore, since the carbon concentration to retained austenite is promoted as a reflective effect, the work hardening index in the later stage of deformation is also improved. As a result, the ductility and stretch flangeability of the steel sheet are improved. The area ratio of polygonal ferrite is more preferably 25% or less, and more preferably 20% or less.

(Residual austenite area ratio at ¼ depth position from the steel sheet surface: 3% or more)
Since retained austenite has the effect of increasing ductility by transformation induced plasticity (TRIP), the retained austenite area ratio at a 1/4 depth position of the sheet thickness from the steel sheet surface is preferably 3% or more. More preferably, it is 4% or more, more preferably 6% or more. The upper limit of the retained austenite area ratio need not be specified, but the retained austenite area ratio that can be secured in the above chemical composition is generally less than 40%.

  In addition, by setting the carbon concentration Cγ in the retained austenite to 0.4 mass% or more, the retained austenite is appropriately stabilized and a lot of transformation-induced plasticity (TRIP) is generated in the high strain region in the later stage of deformation. Ductility and stretch flangeability are further improved. Accordingly, the carbon concentration Cγ in the retained austenite is preferably 0.4% by mass or more. More preferably, it is 0.6 mass% or more, Most preferably, it is 0.8 mass% or more. Moreover, by making the carbon concentration Cγ in the retained austenite 2.0% by mass or less, excessive stabilization of the retained austenite can be suppressed, and transformation-induced plasticity (TRIP) can be expressed more reliably. Therefore, the carbon concentration Cγ in the retained austenite is preferably 2.0% by mass or less, and more preferably 1.8% by mass or less.

  The method for quantifying retained austenite includes X-ray diffraction, EBSP (Electron Back Scattering Diffraction Image, Electron Back Scattering Pattern) analysis, magnetic measurement, and the like, and the quantitative values may differ depending on the method. The area ratio of retained austenite specified in the present invention is a value measured by X-ray diffraction.

(Remaining area ratio excluding bainite, polygonal ferrite, and retained austenite at a depth of 1/4 of the sheet thickness from the steel sheet surface: 15% or less)
From the viewpoint of formability, the area ratio of the remainder excluding bainite, polygonal ferrite, and retained austenite at a position of ¼ depth from the steel sheet surface is preferably 15% or less, more preferably 10% or less. It is.

(Average grain size of grains surrounded by grain boundaries having a crystal orientation difference of 15 ° or more in the steel structure excluding the retained austenite at the 1/4 depth position of the plate thickness from the steel sheet surface: 15 μm or less)
Residual austenite is mainly formed between grains having a crystal orientation difference of 15 ° or more and between bainite laths. And since the former tends to be coarser than the latter, in order to finely disperse the retained austenite of the former, crystals of 15 ° or more from the steel sheet surface at a 1/4 depth position of the plate thickness. It is preferable to reduce the average grain size of grains having a misorientation and increase the generation site of residual austenite. More preferably, it is less than 12 micrometers, More preferably, it is less than 10 micrometers, Most preferably, it is less than 8 micrometers. Since the average particle size is preferably as small as possible, the lower limit of the average particle size need not be specified.

  The average particle diameter (D) is a value calculated by the following formula (2). In formula (2), N is the number of grains included in the average grain size evaluation area, Ai is the area of the i-th grain (i = 1, 2,..., N), and di is the i-th grain size. Indicates the equivalent circle diameter. These data are easily obtained by EBSP analysis. Specifically, phases are distinguished using the crystal structure definition of iron face-centered cubic lattice (FCC) and body-centered cubic lattice (BCC), and only those phases recognized as body-centered cubic lattice (BCC) It is calculated | required by analyzing.

  The grains having a crystal orientation difference of 15 ° or more are mainly ferrite grains and bainite blocks. In the method for measuring the ferrite grain size according to JIS G0552, the grain size is calculated even for grains having a crystal orientation difference of less than 15 °, and the bainite block is not calculated. It cannot be specified. Therefore, in the present invention, a value obtained by EBSP analysis is adopted.

(Relationship between the area ratio of ferrite and the area ratio of granular retained austenite at a depth position of 100 μm from the steel sheet surface and a quarter depth position from the steel sheet surface)
Forming methods with a large amount of strain in the steel sheet surface layer, such as stretch flange molding and bending, increase the deformability in the steel sheet surface layer and suppress the generation of microcracks during punching. This is very important. Therefore, it is preferable that the relationship between the steel structure at a depth position of 100 μm from the steel sheet surface of the hot-rolled steel sheet according to the present invention and the steel structure at a 1/4 depth position of the plate thickness is as follows. The surface layer from the surface to a depth of several tens of μm may be disturbed by the influence of oxide scale or cooling. Therefore, in order to avoid such disturbance, the surface of the steel plate is changed by the structure at a depth of 100 μm from the surface. Determine nearby tissues.

  The area ratio of ferrite at a depth of 100 μm from the steel sheet surface is more than 1.2 times the area ratio of ferrite at a depth of 1/4 of the sheet thickness from the steel sheet surface, and at a depth of 100 μm from the steel sheet surface. When the area ratio of granular retained austenite is larger than the area ratio of granular retained austenite at the 1/4 depth position of the sheet thickness from the steel sheet surface, the deformability near the steel sheet surface is higher than that inside the steel sheet, and punching It is possible to suppress the generation of minute cracks at the time. Furthermore, the hardness difference between the ferrite and the second phase is reduced, the uniform fine dispersion of retained austenite is promoted, the propagation of microcracks is suppressed, and as a result, the hole expandability is dramatically improved. Therefore, it is preferable that the following expressions (3) and (4) are satisfied.

In Formula (3) and Formula (4),
Vαs indicates the area ratio (%) of ferrite at a depth of 100 μm from the steel sheet surface.
Vγs represents the area ratio (%) of granular retained austenite at a depth of 100 μm from the steel sheet surface,
Vαq represents the area ratio (%) of the ferrite at the 1/4 depth position of the plate thickness from the steel plate surface,
Vγq represents the area ratio (%) of granular retained austenite at a position at a depth of ¼ of the plate thickness from the surface of the steel plate.

A 180 kg steel ingot having the chemical composition shown in Table 1 was melted in a high-frequency vacuum melting furnace and made into a 30 mm thick steel piece by hot forging. The steel slab was then heated to a temperature of 1250 ° C., and hot rolled under the conditions shown in Table 2 in a small test tandem mill to finish a steel plate having a thickness of 2 mm. After completion of rolling, water is cooled to a temperature range of Ar ₃ points or more and less than 850 ° C., and then stays for a predetermined time. Thereafter, water is cooled to a temperature range of 600 ° C. or more and less than 750 ° C. The steel sheet was cooled to the coiling temperature and charged in a furnace set at the coiling temperature, held for 30 minutes, and then cooled in the furnace to obtain a hot-rolled steel sheet. These conditions are also shown in Table 2.

The obtained hot-rolled steel sheet is mirror-polished on the thickness cross-section perpendicular to the rolling direction of the steel sheet, corroded with a nital or liquid repelling solution, and then observed with an optical microscope or a scanning electron microscope (SEM). Went. Furthermore, using a sample prepared by electropolishing after mirror polishing, the crystal orientation was measured and analyzed by the EBSP method. In optical microscopes and SEM observation images, it may be difficult to distinguish bainite, retained austenite, and martensite. Therefore, the area ratio of each phase and tissue was quantified by the following method.

  First, the total area ratio of bainite, martensite and retained austenite was measured by image analysis using the SEM observation image and the EBSP analysis result. Next, the total area ratio of retained austenite and martensite was measured from the repeller-corroded structure, and the value obtained by subtracting this total area ratio from the total area ratio of bainite, martensite and residual austenite previously measured was defined as did. The polygonal ferrite area ratio was measured by image analysis using SEM observation images and EBSP analysis results. The residual austenite area ratio was measured by X-ray diffraction, and at the same time, the carbon concentration in the residual austenite was also calculated. The total area ratio of bainite, polygonal ferrite and retained austenite measured above was subtracted from 100%, and the area ratio of the remaining structure was used.

  The average grain size of grains surrounded by grain boundaries having a crystal orientation difference of 15 ° or more in the steel structure excluding residual austenite was determined by EBSP analysis.

  As mechanical properties, tensile properties and stretch flangeability were evaluated. As for the tensile properties, a tensile test was performed in accordance with JIS Z2201 and JIS Z2241, and the tensile strength (TS) and total elongation (El) were measured. For stretch flangeability, a hole expansion test was performed in accordance with the Japan Iron and Steel Federation standard JFS T 1001, and the hole expansion ratio (λ) was determined.

  Table 3 summarizes the steel structure and mechanical properties of the obtained steel sheet.

  Test Nos. 1 to 8, 11, 14, 17, 19, 21, and 23, which are examples of the invention, have high tensile strength (TS), excellent strength-ductility balance (TS × El), and excellent strength-elongation. And flange balance (TS × λ).

  On the other hand, test numbers 9, 10, 12, 13, 15, 16, 18, 20, 22, 24-26, which are comparative examples outside the range defined in the present invention, are TS × E1 or TS × λ, or both. The characteristics of were unsatisfactory.

Claims (7)

A method for producing a hot-rolled steel sheet having a tensile strength of 780 MPa or more, a strength-ductility balance (TS × EL) of 20000 MPa ·% or more, and a strength-stretch flangeability balance (TS × λ) of 69000 MPa ·% or more. Because
In mass%, C: more than 0.08% and less than 0.30%, Si: 0.10% to 3.0%, Mn: 1.0% to 4.0%, P: 0.10% or less , S: 0.010% or less, sol. Al: 0.010% to 3.0%, N: 0.010% or less, and Si and sol. The total thickness of Al is 0.8% or more and 3.0% or less, and the slab having a chemical composition composed of Fe and impurities in the balance is subjected to multi-pass hot rolling, and the sheet thickness is more than 1.2 mm and less than 6 mm When making hot-rolled steel sheets,
After the rolling is completed, multi-pass hot rolling is performed in the temperature range of 860 ° C. or higher and 1050 ° C. or lower with the rolling reduction in the final rolling pass, the previous rolling pass and the previous rolling pass being 15% or more and 60% or less. After cooling within 0.3 seconds, after cooling to a temperature range of less than 850 ° C. Ar ₃ points or more at a cooling rate of 200 ° C./second or more and retaining in that temperature range for 1 second or more and less than 3 seconds After cooling at a cooling rate of 20 ° C./second or more to a temperature range of less than 750 ° C. to 600 ° C. or more and retaining in the temperature range for 1 second to less than 15 seconds, winding in a temperature range of 350 ° C. to 500 ° C. A method for producing a hot-rolled steel sheet, characterized in that residual austenite is contained by taking. The multipass hot rolling which satisfies following formula (1) is given, The manufacturing method of the hot rolled sheet steel of Claim 1 characterized by the above-mentioned.

Here, the meaning of each symbol in the above formula (1) is as follows.
t: Time between passes (seconds) from the completion of rolling of the rolling pass immediately before the final rolling pass to the start of rolling of the final rolling pass
T: Rolling completion temperature (° C.) of the rolling pass immediately before the final rolling pass   The chemical composition is one selected from the group consisting of Ti: 0.20% or less, Nb: 0.10% or less, and V: 0.50% or less in mass% instead of a part of the Fe Or the manufacturing method of the hot rolled sheet steel of Claim 1 or Claim 2 which has 2 or more types.   The chemical composition is mass% instead of a part of the Fe, Cr: less than 1.0%, Mo: 0.5% or less, Ni: 1.0% or less, and B: 0.0050% or less The manufacturing method of the hot rolled sheet steel of any one of Claim 1- Claim 3 which has 1 type, or 2 or more types selected from the group which consists of.   The chemical composition is one selected from the group consisting of Ca: 0.020% or less, Mg: 0.020% or less, and REM: 0.020% or less in mass% instead of a part of the Fe. Or the manufacturing method of the hot rolled sheet steel of any one of Claim 1- Claim 4 which has 2 or more types.   The method for producing a hot-rolled steel sheet according to any one of claims 1 to 5, wherein the chemical composition has Cu: 1.0 mass% or less instead of a part of the Fe.   The method for producing a hot-rolled steel sheet according to any one of claims 1 to 6, wherein the chemical composition has Bi: 0.020 mass% or less instead of a part of the Fe.