JP6390273B2 - Manufacturing method of hot-rolled steel sheet - Google Patents

Description

  The present invention relates to a method for producing a hot-rolled steel sheet that is formed into various shapes by press working or the like, particularly a high-tensile hot-rolled steel sheet having excellent stretch flangeability.

  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 anti-collision characteristics in order to ensure passenger safety.

  Therefore, in order to achieve both the weight reduction of the vehicle body and the anti-collision property, it has been studied to reduce the thickness of the member using a high-strength steel plate. However, increasing the strength of a steel sheet generally involves deterioration of formability. Therefore, in developing a high-strength steel sheet, it is important to increase the strength without degrading the formability. For example, steel plates used for automobile members such as structural members and suspension members that account for approximately 20% of the weight of the vehicle body are subjected to blanking and drilling by shearing, punching, etc., followed by stretch flange processing and burring processing. Etc. The press molding mainly made up is performed. As steel sheets excellent in formability suitable for such applications, for example, steel sheets mainly composed of ferrite or bainite strengthened by precipitation with fine precipitates such as Ti and Nb and methods for producing the same have been reported.

  In Patent Document 1, C: 0.01 to 0.10%, Si: 1.0% or less, Mn: 2.5% or less, P: 0.08% or less, S: 0.005% or less, Al: Stretch flangeability mainly composed of ferrite containing 0.015 to 0.050%, Ti: 0.10% to 0.30%, and having an average grain size of 5 μm or less surrounded by an orientation difference of 15 ° or more High-strength hot-rolled steel sheet and a method for producing the same are disclosed. Patent Document 2 discloses an ultra-high-strength hot-rolled steel sheet excellent in workability mainly composed of bainite containing Cr, Ti, and Nb and having an average particle size of less than 4 μm and a method for producing the same.

  On the other hand, 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 3 discloses a high-strength automobile having excellent impact safety and formability in which retained 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 undergoes martensitic transformation during processing and exhibits a large elongation due to transformation-induced plasticity, but the hole expandability is impaired by the formation of hard martensite. In the hot-rolled steel sheet disclosed in Patent Document 3, ductility and hole expandability are improved by refining ferrite and retained austenite. Patent Document 4 discloses a high-strength steel sheet having a tensile strength of 980 MPa or more excellent in elongation and stretch flangeability, in which a second phase composed of retained austenite and / or martensite is finely dispersed in crystal grains. Yes.

  In Patent Document 5 and Patent Document 6, the present inventors have proposed a high-tensile hot-rolled steel sheet excellent in ductility and stretch flangeability and a manufacturing method thereof. In Patent Document 5, after the hot rolling is completed in a temperature range of 860 ° C. or more and 1050 ° C. or less, it is cooled to a temperature range of 720 ° C. or less within 1 second, and in a temperature range of 500 ° C. or more and 720 ° C. or less for 1 second or more. A method for producing a high-strength hot-rolled steel sheet having good ductility and stretch flangeability is disclosed by staying for a stay time of 20 seconds or less and then winding in a temperature range of 350 ° C. to 500 ° C. In Patent Document 6, the average of grains surrounded by grain boundaries mainly composed of bainite, containing appropriate amounts of polygonal ferrite and retained austenite, and having a crystal orientation difference of 15 ° or more in the steel structure excluding retained austenite. A high-strength hot-rolled steel sheet having a particle size of 15 μm or less and good ductility and stretch flangeability is disclosed.

JP 2002-105595 A JP 2000-282175 A JP-A-11-61326 JP 2005-179703 A JP 2012-251200 A JP 2012-251201 A

  Since automobile members have various processing modes, the required formability varies depending on the applied member, but automobile underbody members such as a lower arm are desired to have high stretch flangeability as well as ductility. Furthermore, in recent years, it has been 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.

  In the hot-rolled steel sheet described in Patent Document 1, a large amount of Ti addition of 0.1% or more is indispensable in order to make it finer, so that the cost is increased, and furthermore, the manufacturing method avoids the consistent precipitation of TiC. There is a problem that the winding process cannot be performed at a temperature higher than 500 ° C. and lower than 600 ° C.

  In the hot-rolled steel sheet described in Patent Document 2, the alloy cost is increased because a composite addition of Ti, Nb, and Cr is necessary, and the manufacturing method is a so-called transition boiling region where the coiling temperature is 300 to 500 ° C. There is concern about material fluctuations due to temperature unevenness during winding.

  The steel sheet described in Patent Document 3 is said to have improved ductility and hole expandability due to the refinement of ferrite and retained austenite, but the hole expansion ratio is 1.5 at most and has sufficient stretch flangeability. It's hard to say. Further, in order to improve the work hardening index and improve the collision resistance safety, the main phase needs to be a soft ferrite phase, and it is difficult to obtain a high tensile strength.

  The steel sheet described in Patent Document 4 contains a large amount of expensive elements such as Cu and Ni in order to refine the second phase to the nano size and disperse it in the crystal grains, It is necessary to carry out the conversion treatment, and the increase in manufacturing cost and the decrease in productivity are remarkable.

  The steel sheet manufacturing method described in Patent Document 5 has a problem that it is difficult to control the sheet temperature because rapid cooling of several hundred degrees C / s or more is continued to a temperature in the vicinity of 700 degrees Centigrade. Moreover, since it is a so-called transition boiling region where the winding temperature is 300 to 500 ° C., there is a concern about material fluctuation due to temperature unevenness during winding.

  Although the steel sheet described in Patent Document 6 has high strength and good ductility and stretch flangeability, the strength-stretch flangeability balance (TS × λ) is 69000 MPa ·% or more and the strength-ductility balance (TS XEL) does not satisfy 16000 MPa ·% or more, and further improvement is required for application to members that require high stretch flangeability such as automobile undercarriage.

  This invention is made | formed in view of the prior art mentioned above, and it aims at providing the manufacturing method of the hot-rolled steel plate which has high intensity | strength and excellent stretch flangeability. Specifically, a method for producing a hot-rolled steel sheet having a tensile strength of 780 MPa or more, a strength-stretch flangeability balance (TS × λ) of 69000 MPa ·% or more, and a strength-ductility balance (TS × EL) of 16000 MPa ·% or more is provided. For the purpose.

  In view of the above situation, the present inventors have earnestly studied the chemical composition of the hot-rolled steel sheet, the relationship between the steel structure and the mechanical properties, and the manufacturing method, and as a result, obtained the following knowledge and completed the present invention. .

  (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 ductility simply by using a steel structure mainly composed of bainite. It is effective to contain an appropriate amount of polygonal ferrite to improve the ductility, but the amount of ferrite in the vicinity of the steel sheet surface layer is increased and the inside is a bainite-based structure rather than a uniform structure over the entire plate thickness. The structure maintains stretch flangeability and improves ductility.

  (C) In such a steel structure having an inclination in the thickness direction, the hot rolling is multipass rolling, and the final rolling pass, the rolling pass one before the final rolling pass, and the two previous rolling passes. By increasing the rolling reduction in the rolling pass and introducing shear strain in the vicinity of the steel sheet surface layer, the ferrite transformation driving force in the vicinity of the steel sheet surface layer is increased, and water cooling after rolling is stopped in an appropriate temperature range and retained for a certain period of time. It is realized by transforming after releasing the accumulated strain in the steel sheet while leaving the ferrite transformation driving force in the vicinity of the steel sheet surface layer.

  (D) By generating more soft ferrite in the vicinity of the steel sheet surface layer than in the steel sheet, it is possible to improve the uniform elongation in the vicinity of the steel sheet surface layer and to suppress the generation of microcracks during the punching process. 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 plate surface layer portion is larger than that in the steel plate, can be improved.

  (E) In order to further improve stretch flangeability, it is necessary to suppress the formation of coarse pearlite and cementite, which are the starting points of cracks during punching, and this includes Si and Al, which have the effect of delaying the precipitation of cementite. It is effective to add a certain amount or more. However, when a certain amount or more of Si and Al is added, retained austenite is easily generated in the steel sheet. A steel sheet containing retained austenite provides high ductility due to the TRIP effect, but stretch martensite is deteriorated because hard martensite is formed by the processing-induced transformation. In order to achieve high stretch flangeability in high-strength steel sheets, it is necessary to achieve both the suppression of coarse pearlite and cementite generation and the suppression of residual austenite generation. For this purpose, the coiling temperature should be over 500 ° C and below 600 ° C. Furthermore, the knowledge that it is realized by controlling the average cooling rate of the steel sheet after winding was obtained.

The gist of the present invention based on the above findings is as follows.
(1) By mass%, C: more than 0.03% and less than 0.30%, Si: 0.01% to 3.0%, Mn: 1.0% to 4.0%, P: 0.00. 10% or less, S: 0.010% or less, sol. Al: 0.01% to 3.0%, N: 0.010% or less, and Si and sol. Multi-pass hot rolling is applied to a slab having a total composition of Al (Si + sol.Al) of 0.5% or more and 3.0% or less and the balance of Fe and impurities, and the thickness is 1. A method for producing a hot-rolled steel sheet that is a hot-rolled steel sheet having a thickness of more than 2 mm and not more than 6 mm, wherein the rolling reduction ratio is 15 in the final rolling pass, the rolling pass immediately before the final rolling pass, and the rolling pass two times before the final rolling pass. % To 60%, multi-pass hot rolling is completed in a temperature range of 860 ° C. to 1050 ° C., and cooling is started within 0.3 seconds after the rolling is completed, and an average cooling rate of 200 ° C./second or more is achieved. Less than 850 ° C., Ar is cooled to a temperature range of ₃ points or more, and is retained in the temperature range for 1 second or more and less than 3 seconds, and then at an average cooling rate of 20 ° C./second or more and less than 750 ° C. Cool to a temperature range, and for 1 to 15 seconds in that temperature range Winding is started in a temperature range of more than 500 ° C. and less than 600 ° C., and the average cooling rate up to 450 ° C. is 0.007 ° C./second or more and 1.0 ° C./second or less. A method for producing a hot-rolled steel sheet.

(2) The method for producing a hot-rolled steel sheet as described in (1) above, wherein multi-pass hot rolling that satisfies the following formula (1) is performed.
0.002 / exp (−6080 / (T + 273)) ≦ t ≦ 2.0 (1)
Here, the meaning of each symbol is as follows.
t: Time between passes (seconds) from the completion of rolling 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 a part of Fe. The manufacturing method of the hot-rolled steel sheet as described in said (1) or (2) containing 1 type or 2 types or more.

  (4) The chemical composition is mass% instead of part of Fe, Cr: less than 1.0%, Mo: 0.50% or less, Ni: 1.0% or less, and B: 0.0050% The manufacturing method of the hot-rolled steel plate in any one of said (1)-(3) containing 1 type (s) or 2 or more types selected from the group which consists of the following.

  (5) The chemical composition is selected from the group consisting of Ca: 0.020% or less, Mg: 0.020% or less, 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 said (1)-(4) containing 1 type or 2 types or more.

  (6) The method for producing a hot-rolled steel sheet according to any one of (1) to (5), wherein the chemical composition contains 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 contains Bi: 0.020% by mass or less instead of a part of Fe.

  According to the present invention, it is possible to stably manufacture a hot-rolled steel sheet having high strength and excellent stretch flangeability and ductility, which is suitable as a material used for an automobile underbody member.

  The method for producing a hot-rolled steel sheet according to the present invention will be specifically described below. In the following description, all percentages relating to the chemical composition of steel are mass% unless otherwise specified.

1. Chemical composition (1-1) C: more than 0.03% and less than 0.30% C has an action of promoting the formation of bainite. When the C content is 0.03% or less, it is difficult to secure the target bainite area ratio. Therefore, the C content is more than 0.03%. Preferably it is 0.06% or more, More preferably, it is 0.10% or more. On the other hand, when the C content is 0.30% or more, pearlite is preferentially generated. As a result, the generation of bainite becomes insufficient, and it becomes difficult to secure the target bainite area ratio. Therefore, the C content is less than 0.30%. Preferably it is 0.25% or less.

(1-2) Si: 0.01% or more and 3.0% or less Si has an action of increasing the strength of the steel sheet by solid solution strengthening and an action of making the steel sound by deoxidation. Furthermore, it contributes to the improvement of stretch flangeability by suppressing the precipitation of iron-based carbides such as cementite. If the Si content is less than 0.01%, it is difficult to obtain the effect by the above action. Therefore, the Si content is 0.01% or more. As will be described later, in the present invention, Si and sol. The total content of Al (Si + sol.Al) is important, but Si is sol. Since the solution strengthening ability is higher than that of Al, when higher strength is required, the Si content is preferably 0.5% or more, more preferably 0.8% or more, and particularly preferably. Is 1.0% or more. However, if the Si content exceeds 3.0%, the surface properties of the steel sheet, the chemical conversion treatment, the ductility and the weldability deteriorate significantly. The cause a significant increase in the 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.

(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 ensure the target polygonal ferrite area ratio in the vicinity of the steel sheet surface layer. 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, and if its content exceeds 0.10%, the decrease in formability and toughness due to grain boundary segregation becomes significant. Therefore, the P content is 0.10% or less. Preferably it is 0.050% or less, More 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: 0.01% or more and 3.0% or less Al, like Si, has the action of deoxidizing steel and making the steel plate sound. Furthermore, it contributes to the improvement of stretch flangeability by suppressing the precipitation of iron-based carbides such as cementite. sol. If the Al content is less than 0.01%, it is difficult to obtain the effect of the above action. Therefore, sol. Al content shall be 0.01% or more. Preferably it is 0.03% 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): 0.5% or more and 3.0% or less As described above, both Si and Al have an action of suppressing precipitation of iron-based carbides such as cementite and have stretch flangeability. In the present invention, Si and sol. The total content of Al (Si + sol.Al) is specified. If the total content (Si + sol.Al) is less than 0.5%, the above-described effects are insufficient, and coarse pearlite and cementite are produced, and stretch flangeability may deteriorate. Therefore, the total content (Si + sol.Al) is 0.5% or more, preferably 1.0% or more, and more preferably 1.2% 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 3.0% or less, preferably 2.5% or less, and more preferably 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 steel It precipitates as carbide or nitride inside 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: 0.50% or less, Ni: 1.0% or less, and B: one or more selected from the group consisting of 0.0050% or less Cr , Mo, Ni, and B have the effect of improving hardenability. Mo has the effect of increasing the strength by precipitating carbides in the steel. 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 0.50%, the effect by the above action is saturated and disadvantageous in cost. Therefore, the Mo content is 0.50% or less. Preferably it is 0.20% 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 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.

2. Production conditions A hot rolling steel sheet is produced by subjecting a slab having the above chemical composition to multi-pass hot rolling. In order 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 make a difference in 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 sheet surface more efficiently than the inside of the steel sheet by efficiently using the difference in transformation driving force due to the difference in strain.

Specifically, in the hot rolling, the rolling reduction in the final rolling pass, the rolling pass immediately before the final rolling pass, and the rolling pass two steps before the final rolling pass is 15% to 60%, respectively, and 860 ° C. above 1050 ° C. subjected to multi-pass hot rolling at a temperature region below the start of the cooling within 0.3 seconds after rolling completion, 200 ° C. / sec or more average cooling rate at 850 ° C. under Ar for _three or more After cooling to a temperature range and staying in the temperature range for 1 second or more and less than 3 seconds, it is cooled to a temperature range of less than 750 ° C. and 600 ° C. or more at an average cooling rate of 20 ° C./second or more. At a temperature range of more than 500 ° C. and less than 600 ° C., and then the average cooling rate up to 450 ° C. is 0.007 ° C./sec or more and 1.0 ° C. A hot-rolled steel sheet is obtained by setting it to / sec or less. 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.

0.002 / exp (−6080 / (T + 273)) ≦ t ≦ 2.0 (1)
Here, the meaning of each symbol is as follows.

t: Time between passes (seconds) from the completion of rolling 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 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 is not particularly limited, but is a viewpoint for securing a rolling completion temperature described later. Is preferably 1050 ° C. or higher, and is preferably 1350 ° C. or lower from the viewpoint of suppressing 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) Rolling ratio in the final rolling pass, the rolling pass immediately before the final rolling pass and the rolling pass two steps before the final rolling pass: 15% or more and 60% or less The final rolling pass and the previous rolling pass And the rolling reduction in the 2nd previous rolling pass shall be 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.

  When the rolling completion temperature is less than 860 ° C., the recrystallization of austenite grains between passes is difficult to promote, so the anisotropy of the hot-rolled steel sheet becomes strong and the stretch flangeability deteriorates. Further, the deformation resistance during rolling becomes large, and it becomes difficult to perform rolling at the above 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 exceeds 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, More 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. Further, in order to more reliably suppress the release of strain introduced by rolling, the rolling completion temperature in the rolling pass one step before the final rolling pass and the rolling pass two steps before the final rolling pass may be in the same range. preferable.

(2-4) The time between passes from the completion of rolling immediately before the final rolling pass to the start of rolling of the final rolling pass satisfies equation (1) 0.002 / exp (−6080 / (T + 273)) ≦ t ≦ 2.0 (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: Cooling is started within 0.3 seconds after completion of rolling, and cooling is performed to an average cooling rate of 200 ° C./second or more to a temperature range of less than 850 ° C., Ar ₃ points or more. In order to efficiently utilize the driving force due to the strain introduced by the transformation, the primary cooling after the completion of rolling starts cooling within 0.3 seconds and is less than 850 ° C. at an average cooling rate of 200 ° C./second or more. Ar Cool down to a temperature range of ₃ points or more. Cooling to this temperature range, coupled with the residence time described later, it is possible to release accumulated strain inside the steel sheet while leaving the ferrite transformation driving force in the vicinity of the steel sheet surface layer. This makes it possible to obtain a steel structure suitable for ductility and stretch flangeability in which the amount of ferrite in the vicinity of the steel sheet surface layer is greater than that in the interior.
When the time until the start of cooling exceeds 0.3 seconds after completion of rolling or when the average cooling rate is less than 200 ° C./second, the strain introduced in the vicinity of the steel sheet surface layer is released, and such a steel structure is obtained. Absent. When the primary cooling stop temperature is 850 ° C. or higher, the accumulated strain in the vicinity of the steel sheet surface layer is remarkably released, and a desired steel structure cannot be obtained. On the other hand, if the primary cooling stop temperature is lower than the Ar ₃ point, ferrite transformation inside the steel sheet becomes prominent and a bainite-based structure is not obtained. Therefore, the primary cooling after the completion of rolling starts cooling within 0.3 seconds, and cools to a temperature range of less than 850 ° C. and ₃ or more Ar points at an average 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 average cooling rate is preferably 300 ° C./second, more preferably 400 ° C./second.

(2-6) Less than 850 ° C. Ar residence time in temperature range of ₃ points or more: 1 second or more, less than 3 seconds Less than 850 ° C. Ar residence time in temperature range of ₃ points or more is 1 second or more and less than 3 seconds To do. 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 steel sheet surface layer. This makes it possible to obtain a steel structure suitable for ductility and stretch flangeability in which the amount of ferrite in the vicinity of the steel sheet surface layer is greater than that in the interior. If it is less than 1 second, the strain release inside the steel sheet is insufficient, so the amount of ferrite produced inside the steel sheet increases, and the stretch flangeability decreases. On the other hand, at 3 seconds or more, the strain introduced in the vicinity of the steel sheet surface layer is released, the amount of ferrite produced is reduced, and the ductility is lowered. Therefore, the residence time in the temperature range of 850 ° C. or lower Ar ₃ point or higher is set to 1 second or more and less than 3 seconds.

(2-7) Average cooling rate to a temperature range of 600 ° C. or more and less than 750 ° C. and residence time in the temperature range: Cooling at 20 ° C./second or more and staying within 1 second to less than 15 seconds Ferrite in the vicinity of the steel sheet surface layer In order to ensure the area ratio, cooling is performed at an average 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 the temperature range for 1 second or more and less than 15 seconds. Let When the average cooling rate is less than 20 ° C./second, ferrite transformation occurs during cooling inside the steel sheet, and it is difficult to form a bainite-based structure. Therefore, the average 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. If the time for staying in the temperature range is less than 1 second, the ferrite transformation in the vicinity of the steel sheet surface layer does not proceed sufficiently, and ductility is lowered. On the other hand, when the time for staying in the above temperature range is 15 seconds or more, the ferrite transformation inside the steel sheet may progress and the stretch flangeability may deteriorate. Further, the formation of pearlite and cementite becomes remarkable, and the stretch flangeability and ductility may be lowered. Therefore, the time for staying in the temperature range is 1 second or more and less than 15 seconds.

(2-8) Winding step: Winding is started in a temperature range of more than 500 ° C. and less than 600 ° C., and then the average cooling rate up to 450 ° C. is 0.007 ° C./sec or more and 1.0 ° C./sec or less. In order to obtain high stretch flangeability in the steel sheet, in the present invention, it is necessary to suppress the formation of coarse pearlite and cementite and the formation of retained austenite. For this reason, the winding process starts in a temperature range of more than 500 ° C. and less than 600 ° C., and then the average cooling rate up to 450 ° C. is set to 0.007 ° C./second or more and 1.0 ° C./second or less. When winding the steel sheet having the above chemical composition is started at 500 ° C. or less, or after winding at over 500 ° C. and less than 600 ° C., when cooling to 450 ° C. at an average cooling rate of over 1.0 ° C./second, Residual austenite is generated in the steel sheet, and stretch flangeability deteriorates. Further, in the transition boiling region where the winding start temperature is 500 ° C. or less, temperature unevenness is likely to occur in the longitudinal direction and width direction of the steel sheet, and characteristic fluctuations are likely to occur. On the other hand, when the winding is started at 600 ° C. or higher, ferrite transformation proceeds inside the steel sheet, and it is difficult to form a bainite-based structure. Furthermore, when starting at 600 ° C. or higher, or after winding at over 500 ° C. and less than 600 ° C., and then cooling to 450 ° C. at an average cooling rate of less than 0.007 ° C./sec, formation of coarse pearlite and cementite is remarkable. And stretch flangeability deteriorates. A preferable range of the winding start temperature is 520 ° C. or higher and 580 ° C. or lower. A preferable average cooling rate from the start of winding to 450 ° C. is 0.008 ° C./second or more and 0.3 ° C./second 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 is difficult to perform primary cooling under the above-described conditions after completion of rolling, and it becomes difficult to obtain a gradient structure suitable for ductility and stretch flangeability. 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 obtained by the above-described manufacturing method may be provided with a plating layer for the purpose of improving corrosion resistance, etc., and may be a surface-treated steel sheet. . 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: 60% 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 area ratio of bainite at the 1/4 depth position of the plate thickness from the steel plate surface. Is preferably 60% or more. More preferably, it is 70% or more.

(Polygonal ferrite area ratio at ¼ depth position from the steel sheet surface: 5% or more and less than 30%)
The polygonal ferrite area ratio at a 1/4 depth position from the steel sheet surface is preferably 5% or more and less than 30%. By containing 5% or more of soft polygonal ferrite, ductility is improved. On the other hand, when it contains excessively, stretch flangeability will fall. The polygonal ferrite area ratio is more preferably 8% to 25%, and more preferably 8% to 20%.

(Residual austenite area ratio at the 1/4 depth position of the plate thickness from the steel plate surface: less than 3%)
Residual austenite has the effect of increasing ductility by transformation-induced plasticity (TRIP), while hard martensite generated by transformation reduces stretch flangeability. In order to obtain high stretch flangeability, the area ratio of retained austenite is preferably less than 3%.

(Remaining area ratio excluding bainite, retained austenite, and polygonal ferrite at a quarter depth 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, retained austenite and polygonal ferrite at a position of ¼ depth from the steel sheet surface is preferably 15% or less. More preferably, it is 10% or less.

(Relationship between the area ratio of ferrite at a depth position of 100 μm from the steel plate surface and a quarter depth position from the steel plate 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.

Vαs> 1.5Vαq (2)
here,
Vαs is the area ratio (%) of polygonal ferrite at a depth of 100 μm from the steel sheet surface.
Vαq is the area ratio (%) of polygonal ferrite at the 1/4 depth position of the plate thickness from the steel plate surface,
Respectively.

  In addition, since the surface layer from the surface to several tens of μm deep may be disturbed by the influence of oxide scale or cooling, in order to avoid such disturbance, the structure at a depth of 100 μm from the surface is near the steel sheet surface layer. Determine the organization.

  When the area ratio of polygonal ferrite at a position of 100 μm depth from the steel sheet surface is more than 1.5 times the area ratio of polygonal ferrite at a position of ¼ depth of the sheet thickness from the steel sheet surface, compared to the inside of the steel sheet As a result, the deformability in the vicinity of the steel sheet surface layer can be increased, and generation of microcracks during punching can be suppressed, resulting in improved stretch flangeability. Therefore, it is preferable that the above formula (2) is satisfied.

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 with a test small tandem mill to finish a steel plate having a thickness of 1.6 to 3.4 mm. After the completion of rolling, the water was cooled to a temperature range of less than 850 ° C. Ar ₃ points or more and stayed for a predetermined time. Thereafter, it is cooled with water to a temperature range of 600 ° C. or more and less than 750 ° C., and stays for a predetermined time, further cooled to a predetermined winding temperature, charged in a furnace set at the winding temperature, and predetermined average cooling Cooled to 450 ° C. at a rate. Then, the furnace was cooled to obtain a hot rolled steel sheet. These conditions are shown in Table 2. In Table 2, passes 1, 2 and 3 in the final hot-rolling three-pass rolling condition mean two steps before the final pass, one step before the final pass, and the final pass, respectively. The time between final rolling passes is the 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.

  The “cooling rate” in Table 2 represents the “average cooling rate” in the present invention. Since the average cooling rate cannot be measured directly, it was calculated by heat transfer calculation from the plate thickness, surface temperature, and cooling conditions.

The obtained hot-rolled steel sheet was mirror-polished in the rolling direction vertical cross section of the steel sheet and corroded with a nital corrosion liquid or a repelling corrosion liquid, and then the structure was observed using an optical microscope or a scanning electron microscope (SEM). 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 and martensite and the area ratio of polygonal ferrite at a position at a depth of ¼ from the steel sheet surface were calculated from EBSP analysis.

Specifically, an EBSP analysis was performed on a region having a rolling direction of 200 μm × rolling surface normal direction of 50 μm with respect to a rolling direction vertical cross section at a position of ¼ depth of the plate thickness from the surface of the steel plate, and surrounded by a boundary having an orientation difference of 5 ° or more Regions with an average grain orientation difference of 0.5 ° or more are bainite and martensite, and regions with an grain orientation average orientation difference of less than 0.5 ° surrounded by boundaries with orientation differences of 5 ° or more are polygonal ferrites. The area ratio was determined using software “OIM Analysis ^™ ” attached to the EBSP analyzer.

  Moreover, the residual austenite area ratio was calculated | required by the X-ray-diffraction measurement using the sample face-cut from the rolling surface normal direction to 1/4 depth of plate | board thickness. Furthermore, using a sample that has undergone repeller corrosion, an image of a rolling direction vertical cross section at a quarter depth of the plate thickness from the surface of the steel plate is taken in a rolling direction of 200 μm × a rolling surface normal direction of 50 μm. The total area ratio of retained austenite and martensite was calculated by binarization using “ The value obtained by subtracting the total area ratio of retained austenite and martensite obtained by repeller corrosion from the sum of the total area ratio of bainite and martensite calculated by EBSP analysis and the residual austenite area ratio determined by X-ray diffraction measurement. Rate.

  The total area ratio of bainite, polygonal ferrite, and retained austenite obtained above was subtracted from 100% to obtain the area ratio of the remaining structure. Further, the area ratio of polygonal ferrite at a position of 100 μm depth from the steel sheet surface was also obtained from the EBSP analysis result in the same manner as described above.

  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 Japan Iron and Steel Federation standard JFS T 1001, and a hole expansion ratio (λ) was obtained.

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

  Test Nos. 1 to 4, 7 to 9, 12 to 18, 22 to 24, 28, 29, 33, 34, 36, 37, 39, 40, 42, and 43, which are invention examples, have high tensile strength (TS). And having an excellent strength-ductility balance (TS × El) and an excellent strength-stretch flange balance (TS × λ). On the other hand, the comparative example outside the range defined by the present invention is inferior in the characteristics of TS × E1 or TS × λ or both.

Claims (7)

At a position of 1/4 depth of the plate thickness from the surface of the steel sheet, in area%, bainite: 60% or more, polygonal ferrite: 5% or more but less than 30%, residual austenite: less than 3%, bainite, residual austenite or polygonal ferrite The balance except for: having a steel structure of 15% or less, tensile strength of 780 MPa or more, strength-stretch flangeability balance (TS × λ) of 69000 MPa ·% or more, and strength-ductility balance (TS × EL) A method for producing a steel sheet having mechanical properties of 16000 MPa ·% or more,
In mass%, C: more than 0.03% and less than 0.30%, Si: 0.01% to 3.0%, Mn: 1.0% to 4.0%, P: 0.10% or less , S: 0.010% or less, sol. Al: 0.01% to 3.0%, N: 0.010% or less, and Si and sol. Multi-pass hot rolling is applied to a slab having a total composition of Al (Si + sol.Al) of 0.5% or more and 3.0% or less and the balance of Fe and impurities, and the thickness is 1. A method for producing a hot-rolled steel sheet that is a hot-rolled steel sheet having a thickness of more than 2 mm and not more than 6 mm, wherein the rolling reduction ratio is 15 in the final rolling pass, the rolling pass immediately before the final rolling pass, and the rolling pass two times before the final rolling pass. % To 60%, multi-pass hot rolling is completed in a temperature range of 860 ° C. to 1050 ° C., and cooling is started within 0.3 seconds after the rolling is completed, and an average cooling rate of 200 ° C./second or more is achieved. Less than 850 ° C., Ar is cooled to a temperature range of ₃ points or more, and is retained in the temperature range for 1 second or more and less than 3 seconds, and then at an average cooling rate of 20 ° C./second or more and less than 750 ° C. Cool to a temperature range, and for 1 to 15 seconds in that temperature range Winding is started in a temperature range of more than 500 ° C. and less than 600 ° C., and the average cooling rate up to 450 ° C. is 0.007 ° C./second or more and 1.0 ° C./second or less. A method for producing a hot-rolled steel sheet. 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.
0.002 / exp (−6080 / (T + 273)) ≦ t ≦ 2.0 (1)
Here, the meaning of each symbol is as follows.
t: Time between passes (seconds) from the completion of rolling 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 containing 2 or more types.   Instead of a part of the Fe, the chemical composition is in mass%, Cr: less than 1.0%, Mo: 0.50% 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 Claims 1-3 containing 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 steel plate in any one of Claims 1-4 containing 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 contains Cu: 1.0% by 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 contains Bi: 0.020% by mass or less instead of a part of the Fe.