JP5880235B2 - Steel plate manufacturing method - Google Patents

Description

  The present invention relates to a method for manufacturing a steel sheet, and specifically relates to a method for manufacturing a steel sheet suitable for a material for an impact-absorbing member having a high effective flow stress, in which the occurrence of cracks when an impact load is applied is suppressed.

In recent years, from the viewpoint of global environmental protection, as part of reducing CO ₂ emissions from automobiles, there has been a demand for weight reduction of automobile bodies, and high strength steel sheets for automobiles have been directed. This is because it is possible to reduce the thickness of the automobile steel sheet by improving the strength of the steel sheet. On the other hand, social demands for improving the collision safety of automobiles are also increasing, and it is desired not only to increase the strength of steel sheets, but also to develop steel sheets that have excellent impact resistance in the event of a collision while traveling. Yes.

Here, since each part of the member for automobiles is deformed at a high strain rate of several tens to 10 ³ / s at the time of a collision, a high strength steel plate excellent in dynamic strength characteristics is required.
As such a high-strength steel sheet, a high-strength composite steel such as a low alloy TRIP steel having a high static difference (difference between static strength and dynamic strength) and a dual-phase structure steel having a second phase mainly composed of martensite. Phase structure steel plates are known.

As for the low alloy TRIP steel, for example, Patent Document 1 discloses a work-induced transformation type high strength steel plate (TRIP steel plate) for absorbing automobile collision energy that has excellent dynamic deformation characteristics.
Moreover, the following invention is disclosed regarding the multiphase-structure steel plate which has the 2nd phase which has martensite as a main body.

  Patent Document 2 discloses an average grain size ds of nanocrystal grains made of fine ferrite grains and having a crystal grain size of 1.2 μm or less, and an average crystal grain size dL of microcrystal grains having a crystal grain size exceeding 1.2 μm. Discloses a high-strength steel sheet that satisfies the relationship dL / ds ≧ 3, is excellent in strength and ductility balance, and has a static difference of 170 MPa or more.

Patent Document 3 discloses a steel sheet having a high static ratio, which is composed of a two-phase structure of martensite having an average particle diameter of 3 μm or less and martensite having an average particle diameter of 5 μm or less.
Patent Document 4 discloses a cold-rolled steel sheet that contains 75% or more of a ferrite phase having an average particle size of 3.5 μm or less and the balance of which is made of tempered martensite and has excellent impact absorption characteristics.

In Patent Document 5, a pre-strained two-phase structure composed of ferrite and martensite is formed, and a cold rolling in which a static difference at a strain rate of 5 × 10 ^{2 to} 5 × 10 ³ / s satisfies 60 MPa or more. A steel sheet is disclosed.

  Furthermore, Patent Document 6 discloses a high-strength hot-rolled steel sheet having excellent impact resistance properties composed of only a hard phase such as 85% or more of bainite and martensite.

Japanese Patent Laid-Open No. 11-80879 JP 2006-161077 A JP 2004-84074 A JP 2004-277858 A JP 2000-17385 A JP-A-11-269606

  The steel sheet which is the material of the conventional shock absorbing member has the following problems. That is, in order to improve the impact absorption energy of an impact absorbing member (hereinafter also simply referred to as “member”), the strength of a steel plate (hereinafter also simply referred to as “steel plate”) that is a material of the impact absorbing member is increased. It is essential.

However, in "Plasticity and processing" Vol. 46, No. 534, pages 641-645, the average load (F _ave ) that determines the impact absorption energy is
F _ave ∝ (σY · t ² ) / 4
As disclosed that σY: effective flow stress t: given as plate thickness, the impact absorption energy greatly depends on the plate thickness of the steel plate. Therefore, simply increasing the strength of the steel plate has a limit in achieving both a reduction in the thickness of the shock absorbing member and high shock absorbing performance.

  By the way, as disclosed in, for example, International Publication No. 2005/010396 pamphlet, International Publication No. 2005/01097 pamphlet, and further International Publication No. 2005/010398 pamphlet, the impact absorbing energy of the impact absorbing member is in its shape. Also depends heavily on.

  In other words, by optimizing the shape of the shock absorbing member so as to increase the work of plastic deformation, the shock absorbing energy of the shock absorbing member can be dramatically reduced to a level that cannot be achieved simply by increasing the strength of the steel plate. There is a possibility that can be increased.

  However, even if the shape of the shock absorbing member is optimized so as to increase the plastic deformation work, if the steel sheet does not have a deformability capable of withstanding the plastic deformation work, the assumed plastic deformation is completed. Before this, the impact absorbing member is cracked at an early stage. As a result, the amount of plastic deformation work cannot be increased, and the impact absorbing energy cannot be dramatically increased. Further, if the impact absorbing member is cracked at an early stage, an unexpected situation such as damage to other members disposed adjacent to the impact absorbing member may occur.

  Conventionally, based on the technical idea that the impact absorption energy of the shock absorbing member depends on the dynamic strength of the steel plate, it has been directed to increase the dynamic strength of the steel plate, but simply increasing the dynamic strength of the steel plate. If it is oriented, there may be a significant decrease in deformability. For this reason, even if the shape of the shock absorbing member is optimized so as to increase the work of plastic deformation, the shock absorbing energy of the shock absorbing member cannot always be dramatically increased.

  In addition, since the shape of the shock absorbing member has been studied on the premise that the steel plate manufactured based on the above technical idea is used, the optimization of the shape of the shock absorbing member is based on the deformability of the existing steel plate from the beginning. Until now, the study itself of improving the deformability of the steel sheet and optimizing the shape of the shock absorbing member so as to increase the work of plastic deformation has not been sufficiently performed.

  As described above, in order to increase the shock absorbing energy of the shock absorbing member, it is important not only to increase the strength of the steel sheet but also to optimize the shape of the shock absorbing member so as to increase the work of plastic deformation. is there.

  As for steel sheets, the work of plastic deformation is increased while suppressing the occurrence of cracks when an impact load is applied so as to enable optimization of the shape of the impact absorbing member that can increase the work of plastic deformation. It is important to increase the effective flow stress.

  In order to make it possible to increase the shock absorption energy of the shock absorbing member, the present inventors have intensively studied a method for suppressing the occurrence of cracking at the time of impact load loading and further increasing the effective flow stress, The new knowledge listed below was obtained.

  (A) In order to increase the impact absorption energy of the impact absorbing member, it is effective to improve the effective flow stress (hereinafter referred to as “5% flow stress”) when 5% true strain is applied to the steel sheet. It is.

(B) It is effective to improve the uniform elongation and the local ductility in order to suppress the occurrence of cracks when an impact load is applied.
(C) In order to increase the 5% flow stress of the steel sheet, it is effective to improve the yield strength and the work hardening coefficient in the low strain region.

  (D) In order to improve the yield strength and the work hardening coefficient in the low strain region, the steel structure of the steel sheet is a composite containing bainite as the main phase and martensite and residual austenite that are harder than bainite in the second phase. It is necessary to organize phases.

  (E) Martensite and retained austenite contained in the second phase contribute to improvement of work hardening coefficient and improvement of uniform elongation in a low strain region. Therefore, it is necessary to limit the lower limit of the martensite area ratio and retained austenite.

  (F) On the other hand, when the martensite area ratio and the retained austenite area ratio are excessive, the local ductility is lowered. Therefore, it is necessary to limit the upper limits of the martensite area ratio and the retained austenite area ratio.

  (G) When the ferrite which is the remaining structure is coarse, strain is easily concentrated on soft ferrite, and the yield strength is lowered. In addition, local ductility is reduced. Therefore, it is necessary to define the upper limit of the average grain size of ferrite.

(H) As described above, in order to increase the shock absorption energy of the shock absorbing member, it is effective to improve the flow stress by 5% for the steel sheet. It is effective to improve the uniform elongation and the local ductility. However, in order to meet the severe needs in recent years, the product of the uniform elongation and the hole expansion ratio is 300% ² or more and 5% It is necessary that the effective flow stress when the true strain is applied is 900 MPa or more.

  (I) When the hardness ratio between bainite as the main phase and martensite contained in the second phase is moderately suppressed, generation of movable dislocations due to plastic deformation is suppressed, and it is easy to ensure higher yield strength. Become. Therefore, it is preferable to limit the upper limit of the hardness ratio between bainite and martensite as the main phase.

  (J) On the other hand, when the hardness ratio between the bainite as the main phase and the martensite contained in the second phase is appropriately increased, the work hardening coefficient is improved and the uniform elongation is increased in the low strain region due to the inclusion of martensite. It is easy to improve. Therefore, it is preferable to limit the lower limit of the hardness ratio between bainite and martensite as the main phase.

  (K) In a multiphase steel sheet with bainite as the main phase, if the strain is concentrated only on bainite due to plastic deformation and suppresses work hardening, the occurrence of cracks along the shear bands and grain boundaries in bainite is suppressed. Therefore, it becomes easy to improve the local ductility. On the other hand, if excessive hardening of the second phase is suppressed by plastic deformation, an increase in the hardness difference between the main phase and the second phase can be avoided, the occurrence of cracks from both interfaces is suppressed, and local ductility is improved. It becomes easy to plan.

  Therefore, in order to obtain higher local ductility in a multiphase steel sheet having bainite as the main phase, it is preferable to appropriately distribute strain between the bainite as the main phase and the second phase. That is, it is preferable to work harden the bainite, which is the main phase, and the second phase to the same extent during plastic deformation. As an index for this purpose, it is preferable to use the ratio of work hardening rate after 10% tensile deformation. In a multiphase steel sheet containing bainite as the main phase and martensite in the second phase, 10% tensile is used. It is preferable to limit the upper and lower limits of the ratio between the work hardening rate of bainite after deformation and the work hardening rate of martensite after 10% tensile deformation.

(L) The microstructure described above can be obtained by combining a specific chemical composition, hot rolling conditions, cold rolling conditions, and annealing conditions. Specifically, the hot rolling conditions include multi-pass hot rolling that completes rolling at _three or more points of Ar, starts cooling within 0.4 seconds after completion of rolling, and an average cooling rate of 600 ° C. / Cooling to a temperature range of 620 ° C. or more and 720 ° C. or less under a cooling condition in which the time required for cooling to 720 ° C. after completion of rolling in the rolling pass two seconds before the final rolling pass is 4 seconds or less, The temperature is maintained for 1 second to 10 seconds in the temperature range, and then cooled to a temperature range of 300 ° C. to 610 ° C. at an average cooling rate of 10 ° C./second to 100 ° C./second, and wound. The rolling condition is to perform cold rolling at a rolling reduction of 40% or more and 70% or less, and the annealing condition is 10 seconds in a temperature range of (Ac ₃ points−30 ° C.) or more (Ac ₃ points + 100 ° C.). Hold for 300 seconds or less, then 50 A temperature range of 0 ° C. or higher and 650 ° C. or lower is cooled at an average cooling rate of 15 ° C./second or higher, and heat treatment is performed in a temperature range of 300 ° C. or higher and 500 ° C. or lower and held for 30 seconds or longer and 3000 seconds or shorter.

The present invention has been made on the basis of the above new findings, and the gist thereof is as follows.
(1) Microstructure comprising area%, bainite: more than 50%, martensite: 3% or more and 30% or less and residual austenite: 3% or more and 15% or less, with the balance being ferrite having an average particle size of less than 5 μm And a product of uniform elongation and hole expansion ratio is 300% ² or more, and a method for producing a steel sheet having mechanical properties such that an effective flow stress when a true strain of 5% is applied is 900 MPa or more. ,
The manufacturing method of the steel plate characterized by having the following process (A)-(C):
(A) By mass%, C: 0.08% to 0.30%, Mn: 1.5% to 3.5%, Si + Al: 0.50% to 3.0%, P: 0.00. Multi-pass hot rolling that completes rolling at ₃ or more points of Ar in a slab containing 10% or less, S: 0.010% or less and N: 0.010% or less, and having a chemical composition consisting of the remaining Fe and impurities The cooling is started within 0.4 seconds after the completion of rolling, the average cooling rate is 600 ° C./second or more, and the cooling is completed to 720 ° C. from the completion of rolling in the rolling pass two steps before the final rolling pass. Is cooled to a temperature range of 620 ° C. or more and 720 ° C. or less under a cooling condition that requires 4 seconds or less, and held in the temperature range for 1 second or more and 10 seconds or less, and then 10 ° C./second or more and 100 ° C./second or less. Temperature at an average cooling rate of 300 ° C or higher and 610 ° C or lower A hot rolling process to form a hot-rolled steel sheet by cooling to a temperature range and winding;
(B) a cold rolling step for forming a cold rolled steel sheet by subjecting the hot rolled steel sheet obtained by the hot rolling process to cold rolling at a rolling reduction of 40% to 70%; and (C) the cold rolling The cold-rolled steel sheet obtained by the hot rolling process is held for 10 seconds to 300 seconds in a temperature range of (Ac ₃ points−30 ° C.) to (Ac ₃ points + 100 ° C.), and then 500 ° C. to 650 ° C. An annealing process in which a temperature range is cooled at an average cooling rate of 15 ° C./second or more, and heat treatment is performed in a temperature range of 300 ° C. to 500 ° C. for 30 seconds to 3000 seconds.

  (2) The chemical composition is selected from the group consisting of Cr: 0.5% or less, Mo: 0.5% or less, and B: 0.010% or less in mass% instead of a part of the Fe. The manufacturing method of the steel plate as described in said (1) containing 1 type or 2 types or more.

  (3) The chemical composition is selected from the group consisting of Ti: less than 0.04%, Nb: less than 0.030%, and V: less than 0.5% in mass%, instead of a part of the Fe. The manufacturing method of the steel plate as described in said (1) or (2) containing 1 type (s) or 2 or more types.

  (4) The chemical composition is replaced by a part of the Fe, in terms of mass%, Ca: 0.010% or less, Mg: 0.010% or less, REM: 0.050% or less, and Bi: 0.050 % The manufacturing method of the steel plate in any one of said (1)-(3) containing 1 type, or 2 or more types selected from the group which consists of below.

The steel sheet produced by the method according to the present invention contains area%, bainite: more than 50%, martensite: 3% to 30% and residual austenite: 3% to 15%, with the balance being the average particle size Mechanical properties of having a microstructure composed of ferrite of less than 5 μm, a product of uniform elongation and hole expansion ratio of 300% ² or more, and effective flow stress of 900 MPa or more when 5% true strain is applied Have The microstructure preferably satisfies the following formulas (1) and (2).

1.2 ≦ H _M0 / H _B0 ≦ 1.6 (1)
0.9 ≦ {(H _M10 / H _M0 ) / (H _B10 / H _B0 )} ≦ 1.3 (2)
Here, the symbol in a formula represents the following values.

H _M0 : initial average nano hardness of the martensite,
H _B0 : initial average nano hardness of the bainite,
H _M10 : Average nano hardness of the martensite after 10% tensile deformation,
H _B10 : Average nano hardness of the bainite after 10% tensile deformation.

  The steel sheet having the above microstructure and mechanical properties manufactured according to the present invention is a material for the shock absorbing portion in the shock absorbing member having a shock absorbing portion that absorbs shock energy by axially crushing and plastically deforming into a bellows shape. It is suitable as. Particularly suitable as a material for a shock absorbing member for automobiles, for example, as a shock absorbing member for automobiles, a crash box having a cylindrical main body having a closed cross section (for example, a side member while supporting a bumper reinforcement) It is preferably used as a material that is attached to a body shell and is axially crushed by an impact load applied from a bumper reinforcement and is plastically deformed into a bellows shape. It can also be advantageously used as a material for automobile side members, front upper rails, side sills, and cross members.

  According to the present invention, it is possible to produce a steel plate that can suppress or eliminate the occurrence of cracking when an impact load is applied and also has a high effective flow stress, and the impact absorbing energy of the impact absorbing member can be dramatically increased. Can be increased. By applying such a shock absorbing member to a product such as an automobile, it becomes possible to further improve the crash safety of the product, so that the present invention is extremely useful industrially.

It is explanatory drawing which shows the example of the application site | part of an impact-absorbing member. It is a double view which shows an example of the shape of an impact-absorbing part. It is a double view which shows an example of the shape of an impact-absorbing part.

  Hereinafter, the manufacturing method of the steel sheet according to the present invention and the microstructure, mechanical properties, usage, etc. of the manufactured steel sheet will be described more specifically. In the following description, all percentages relating to the chemical composition of steel are mass%.

1. Chemical composition (1) C: 0.08% or more and 0.30% or less C has an action of promoting the generation of martensite and residual austenite contained in the main phase, bainite and the second phase. Moreover, it has the effect | action which improves the tensile strength by raising the intensity | strength of a martensite. Moreover, it has the effect | action which strengthens steel by solid solution strengthening and improves yield strength and tensile strength. However, if the C content is less than 0.08%, it may be difficult to obtain the effect of the above action. Therefore, the C content is set to 0.08% or more. Preferably it is more than 0.12%, more preferably more than 0.14%. On the other hand, if the C content exceeds 0.30%, martensite and austenite may be generated excessively, leading to a significant decrease in local ductility. In addition, the weldability is significantly deteriorated. Therefore, the C content is 0.30% or less. Preferably it is less than 0.20%, more preferably less than 0.19%.

(2) Mn: 1.5% or more and 3.5% or less Mn has an action of promoting the generation of martensite and retained austenite contained in the main phase of bainite and the second phase. Moreover, it has the effect | action which strengthens steel by solid solution strengthening and improves yield strength and tensile strength. Moreover, since the strength of bainite is increased by solid solution strengthening, it has the effect of improving local ductility by increasing the hardness of bainite under high strain load conditions. If the Mn content is less than 1.5%, it may be difficult to obtain the effect by the above action. Therefore, the Mn content is 1.5% or more. It is preferably more than 1.8%, more preferably more than 2.0%, particularly preferably more than 2.2%. On the other hand, if the Mn content exceeds 3.5%, the bainite transformation is excessively delayed, and as a result, it is difficult to stabilize the retained austenite and it is difficult to ensure the predetermined retained austenite. Become. Therefore, the Mn content is 3.5% or less. Preferably it is less than 3.1%, more preferably less than 2.8%, particularly preferably less than 2.5%.

(3) Si + Al: 0.50% or more and 3.0% or less Si and Al promote the generation of retained austenite by suppressing the formation of carbides in bainite, and have the effect of improving uniform ductility and local ductility. Moreover, it has the effect | action which strengthens steel by solid solution strengthening and improves yield strength and tensile strength. Furthermore, since the strength of bainite is increased by solid solution strengthening, it has the effect of improving local ductility by increasing the hardness of bainite under high strain load conditions. When the total content of Si and Al (hereinafter, also referred to as “(Si + Al) amount”) is less than 0.50%, it is difficult to obtain the effect by the above-described action. Therefore, the amount of (Si + Al) is 0.50% or more. Preferably it is 1.0% or more, More preferably, it is 1.3% or more. On the other hand, even if the amount of (Si + Al) is set to 3.0% or more, the effect by the above action is saturated and disadvantageous in cost. In addition, the transformation point is raised and the productivity is hindered. Therefore, the amount of (Si + Al) is 3.0% or less. Preferably it is 2.5% or less, More preferably, it is less than 2.2%, Most preferably, it is less than 2.0%.

  Since Si has an excellent solid solution strengthening capability, the Si content is preferably 0.50% or more, and more preferably 1.0% or more. On the other hand, since Si has the effect of reducing chemical conversion properties and weldability, the Si content is preferably less than 1.9%, more preferably less than 1.7%, and more preferably 1.5%. It is particularly preferable to make it less than.

(4) P: 0.10% or less P is generally contained as an impurity, and has an action of segregating at grain boundaries to embrittle the steel and promoting the occurrence of cracks when an impact load is applied. If the P content exceeds 0.10%, the steel becomes brittle due to the above action, and it becomes difficult to suppress the occurrence of cracks when an impact load is applied. Therefore, the P content is 0.10% or less. Preferably it is less than 0.020%, more preferably less than 0.015%.

(5) S: 0.010% or less S is generally contained as an impurity, and has the effect of forming sulfide inclusions in the steel and degrading formability. If the S content exceeds 0.010%, the effect of the above action becomes obvious. Therefore, the S content is set to 0.010% or less. Preferably it is 0.005% or less, More preferably, it is less than 0.003%, Most preferably, it is 0.001% or less.

(6) N: 0.010% or less N is generally contained in steel as an impurity and has the effect of deteriorating ductility. When the N content exceeds 0.010%, the ductility is remarkably reduced. Therefore, the N content is set to 0.010% or less. Preferably it is 0.0006% or less, More preferably, it is 0.0050% or less.

The elements described below are optional elements that can be contained in steel as necessary.
(7) One or more selected from the group consisting of Cr: 0.5% or less and Mo: 0.5% or less and B: 0.01% or less Cr, Mo, and B have a hardenability. Enhances and promotes the formation of bainite. Moreover, it has the effect | action which accelerates | stimulates the production | generation of a martensite and a retained austenite. Furthermore, it has the effect | action which strengthens steel by solid solution strengthening and improves yield strength and tensile strength. Therefore, you may contain 1 type or 2 types selected from the group which consists of Cr, Mo, and B. FIG. However, if the Cr content exceeds 0.5%, the Mo content exceeds 0.5%, or the B content exceeds 0.01%, the uniform elongation and local ductility are significantly reduced. There is a case. Therefore, the Cr content is 0.5% or less, the Mo content is 0.5% or less, and the B content is 0.01% or less. In order to obtain the above-described effect more reliably, it is preferable to satisfy any of Cr: 0.1% or more, Mo: 0.1% or more, and B: 0.0010% or more.

(8) One or more selected from the group consisting of Ti: less than 0.04%, Nb: less than 0.030%, and V: less than 0.5%. It has the effect of suppressing the growth of austenite grains during annealing by forming carbonitrides and reducing cracking susceptibility. Moreover, it has the effect | action which improves yield strength by the effect | action of precipitation strengthening by depositing in bainite. Therefore, one or more of Ti, Nb and V may be contained. However, even if the Ti content is 0.04% or more, the Nb content is 0.030% or more, and the V content is 0.5% or more, the effect of the above action is saturated and disadvantageous in cost. Therefore, the Ti content is less than 0.04%, the Nb content is less than 0.030%, and the V content is less than 0.5%. The Ti content is preferably less than 0.020%. The Nb content is preferably less than 0.020%, and more preferably 0.015% or less. The V content is preferably 0.30% or less. In order to more reliably obtain the effect of the above action, it is preferable to satisfy any of Ti: 0.01% or more, Nb: 0.005% or more, and V: 0.010% or more. When Nb is contained, the Nb content is more preferably 0.010% or more.

(9) One or more selected from the group consisting of Ca: 0.010% or less, Mg: 0.010% or less, REM: 0.050% or less, and Bi: 0.050% or less Ca, Mg And REM has the effect | action which improves local ductility by controlling the shape of an inclusion and Bi refines | miniaturizes a solidification structure | tissue. Therefore, you may contain 1 type, or 2 or more types of these elements. However, if Ca and Mg are contained in excess of 0.010%, and REM is contained in excess of 0.050%, many coarse oxides are generated in the steel, and formability is increased. Is damaged. About Bi, when it contains exceeding 0.050%, it segregates at a grain boundary and inhibits weldability. Therefore, the content of each element is specified as described above. The Ca, Mg and REM contents are each preferably 0.0009% or less, and the Bi content is preferably 0.010% or less. In order to obtain the effect of the above operation more surely, any of the conditions of Ca: 0.0008% or more, Mg: 0.0008% or more, REM: 0.0008% or more, and Bi: 0.0010% is required. It is preferable to satisfy.

2. Microstructure (1) Multiphase structure The steel structure of a steel sheet produced by the method according to the present invention is composed of bainite as a main phase in order to increase the effective flow stress by obtaining a high yield strength and a work hardening coefficient in a low strain region. And having a multiphase structure containing martensite and retained austenite in the second phase. The balance of the second phase is ferrite.

(2) Area ratio of bainite: more than 50% In a multiphase steel sheet having bainite as a main phase, the bainite area ratio affects the yield strength. That is, the yield strength is improved by increasing the area ratio of bainite. If the area ratio of bainite is less than 50%, it becomes difficult to obtain an impact-absorbing member having good impact-absorbing ability due to insufficient yield strength. Therefore, the area ratio of bainite is set to more than 50%.

(3) Martensite area ratio: 3% or more and 30% or less In a multiphase steel sheet having bainite as a main phase, martensite improves yield strength and work hardening rate in a low strain region, and increases flow stress by 5%. Has an effect. It also has the effect of increasing uniform elongation. If the martensite area ratio is less than 3%, it becomes difficult to obtain an impact-absorbing member having good impact-absorbing ability due to lack of 5% flow stress and uniform elongation. Therefore, the martensite area ratio is set to 3% or more. On the other hand, if the martensite area ratio exceeds 30%, the local ductility is lowered, and cracks due to unstable buckling tend to occur. Therefore, the area ratio of martensite is 30% or less. The area ratio of martensite is preferably 25% or less, more preferably 15% or less.

(4) Residual austenite area ratio: 3% or more and 15% or less In a multiphase steel sheet having bainite as a main phase, retained austenite improves yield strength and work hardening rate in a low strain region, and increases flow stress by 5%. Has an effect. It also has the effect of increasing uniform elongation. If the retained austenite area ratio is less than 3%, it becomes difficult to obtain an impact-absorbing member having good impact-absorbing ability due to lack of 5% flow stress and uniform elongation. Therefore, the retained austenite area ratio is set to 3% or more. On the other hand, if the retained austenite area ratio exceeds 15%, the local ductility is lowered and cracking due to unstable buckling is likely to occur. Therefore, the area ratio of retained austenite is set to 15% or less.

(5) Average grain size of ferrite as the remaining structure: less than 5 μm When the average grain size of ferrite as the remaining structure is 5 μm or more, strain tends to concentrate on the soft ferrite, and the yield strength is reduced. It becomes difficult to increase the% flow stress. Moreover, local ductility falls and it becomes difficult to suppress generation | occurrence | production of the crack at the time of impact load loading. Therefore, the average grain size of ferrite is 5 μm or less. Preferably it is less than 4.0 μm, more preferably less than 3.0 μm. The lower limit of the average grain size of ferrite need not be specified.

  The area ratio of ferrite need not be specified, but the lower limit is preferably 1% or more, and more preferably 5% or more. On the other hand, the upper limit is preferably 20% or less, more preferably 15% or more, and particularly preferably 10% or less.

(6) Hardness ratio between bainite and martensite: 1.2 ≦ H _M0 / H _B0 ≦ 1.6
Hardness ratio (H _M0 / H _B0 ) of the initial average nano hardness (H _M0 ) of martensite contained in the second phase to the initial average nano hardness (H _B0 ) of bainite as the main phase is 1.2 or more Thus, it becomes easy to improve the work hardening coefficient and the uniform elongation in the low strain region by containing martensite, and the generation of cracks is effectively suppressed. Therefore, the hardness ratio (H _M0 / H _B0 ) is preferably 1.2 or more.

On the other hand, by setting the hardness ratio (H _M0 / H _B0 ) to 1.6 or less, the hardness ratio between the bainite main phase and the hard second phase is moderately suppressed, and the occurrence of movable dislocations is suppressed by plastic deformation. Therefore, it is easy to improve the yield strength. Thereby, it becomes easy to improve the shock absorption energy and to obtain a shock absorbing member having a good shock absorbing ability. Therefore, the hardness ratio (H _M0 / H _B0 ) is preferably 1.6 or less.

(7) Work hardening rate ratio of martensite to bainite: 0.9 ≦ {(H _M10 / H _M0 ) / (H _B10 / H _B0 )} ≦ 1.3
In a multiphase steel sheet with bainite as the main phase, suppressing strain hardening and work hardening in bainite due to plastic deformation suppresses the occurrence of cracks along the shear bands and grain boundaries in bainite, resulting in local ductility. It becomes easy to improve. On the other hand, if excessive hardening of the second phase due to plastic deformation is suppressed, the difference in hardness between the main phase and the second phase is suppressed, cracking from the interface between the two is suppressed, and local ductility is improved. It becomes easy to make. Therefore, in order to obtain higher local ductility in a multiphase steel sheet having bainite as the main phase, it is preferable to appropriately distribute the strain between the main phase bainite and the second phase. That is, it is preferable to work harden the bainite, which is the main phase, and the second phase to the same extent during plastic deformation. As an index for this purpose, it is preferable to use the ratio of work hardening rate after 10% tensile deformation. In a multiphase steel sheet containing bainite as the main phase and martensite in the second phase, 10% tensile is used. It is preferable to limit the upper limit and the lower limit of the ratio of the work hardening rate after 10% tensile deformation of martensite which is the hardest phase to the work hardening rate of bainite after deformation.

Specifically, the marten relative to the work hardening rate (H _B10 / H _B0 ) of bainite obtained from the initial average nano hardness (H _B0 ) of bainite and the average nano hardness (H _B10 ) of bainite after 10% tensile deformation. Work hardening, which is the ratio of the work hardening rate (H _M10 / H _M0 ) of martensite obtained from the initial average nano hardness (H _M0 ) of the site and the average nano hardness (H _M10 ) of martensite after 10% tensile deformation It is preferable to limit an upper limit and a lower limit for the rate ratio {(H _M10 / H _M0 ) / (H _B10 / H _B0 )}.

  When the work hardening rate ratio is 0.9 or more, strain is concentrated on bainite due to plastic deformation and work hardening is suppressed, and cracks are prevented from occurring along shear bands and grain boundaries in bainite. , Local ductility is improved. Therefore, the work hardening rate ratio is preferably 0.9 or more. On the other hand, if the work hardening rate ratio is 1.3 or less, excessive hardening of martensite is suppressed, and local ductility is improved. Therefore, the work hardening rate ratio is preferably 1.3 or less.

3. Mechanical properties The steel sheet produced by the method according to the present invention has a mechanical property that the product of the uniform elongation and the hole expansion ratio is 300% ² or more, and the effective flow stress when the true strain of 5% is applied is 900 MPa or more. Have.

As described above, in order to increase the impact absorption energy of the impact absorbing member, it is effective to improve the flow stress by 5% for the steel sheet, and to suppress the occurrence of cracking when an impact load is applied, uniform elongation is required. It is effective to improve the local ductility. However, in order to meet these severe needs in recent years, the product of the uniform elongation and the hole expansion ratio is 300% ² or more and the true strain is 5%. It is necessary that the effective flow stress at the time of imparting is 900 MPa or more. Therefore, it shall have the said mechanical characteristic. The product of the uniform elongation and the hole expansion rate is preferably 400% ² or more, and the effective flow stress when a true strain of 5% is applied is preferably 930 MPa or more.

As other mechanical properties of the steel sheet produced by the method according to the present invention, it is desirable that YS is 600 MPa or more and TS is 900 MPa or more.
4). Applications The steel plate described above is preferably applied to the impact absorbing portion in an impact absorbing member having an impact absorbing portion that absorbs impact energy by axially crushing and plastically deforming into a bellows shape.

  When the steel plate is applied to the impact absorbing portion, the occurrence of cracks in the impact absorbing member when an impact load is applied is suppressed or eliminated, and the effective flow stress is high. Can be dramatically improved. This is proved by showing an appropriate average crushing load and a high stable buckling rate (ratio of specimens in which cracks do not occur) in the crushing test of the impact absorbing member.

  FIG. 1 is an explanatory diagram illustrating an example of an application site of an impact absorbing member in an automobile. As an impact absorbing member having an impact absorbing portion that absorbs impact energy by plastic deformation in a bellows shape by crushing the shaft, for example, in an automobile member, a member (front and rear crashes) shown by hatching in FIG. Examples of the members include boxes 2 and 3, front and rear side members 4 and 5, front upper rail 6, side sill 7, and cross member 8. Furthermore, it can be applied to the bumper reinforcement 11 and the center pillar 12.

2 and 3 are two views showing an example of the shape of the shock absorbing portion.
As the shape of the shock absorbing portion, a cylindrical body having a closed cross section is suitable. For example, a cylindrical body having a quadrangular closed cross section as shown in FIG. 2 or an octagonal closed cross section as shown in FIG. can do. 2 and 3 show an example in which the cross-sectional shape in the axial direction is constant, the present invention is not limited to this, and a continuously changing shape may be used. 2 and 3 show examples in which the cross-sectional shape is a square or an octagon, the cross-sectional shape is not limited to this, and may be an arbitrary polygon.

  Generally, such an automobile impact absorbing member is formed from a steel plate by, for example, bending and welding to form a cylindrical body with a closed cross-section, and if necessary, further bending the two-dimensional or three-dimensional to the obtained cylindrical body. And so on.

5). Plating layer A surface-treated steel sheet may be provided by providing a plating layer on the surface of the steel sheet described above for the purpose of improving corrosion resistance. 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.

6). Manufacturing Method According to the present invention, a steel plate is manufactured by a method having the following steps (A) to (C):
(A) C: 0.08% or more and 0.30% or less, Mn: 1.5% or more and 3.5% or less, Si + Al: 0.50% or more and 3.0% or less, P: 0.10% or less, A slab containing S: 0.010% or less and N: 0.010% or less and having a chemical composition consisting of the balance Fe and impurities is subjected to multi-pass hot rolling to complete rolling at Ar ₃ points or more, and rolled. Cooling is started within 0.4 seconds after completion, the average cooling rate is 600 ° C./second or more, and the time required for cooling to 720 ° C. after completion of rolling in the rolling pass two steps before the final rolling pass After cooling to a temperature range of 620 ° C. or more and 720 ° C. or less under a cooling condition of 4 seconds or less, and holding in the temperature range for 1 second or more and 10 seconds or less, at an average cooling rate of 10 ° C./second or more and 100 ° C./second or less. Cooling to a temperature range of 300 ° C to 610 ° C and winding Hot rolling step of hot-rolled steel sheet by;
(B) a cold rolling step for forming a cold rolled steel sheet by subjecting the hot rolled steel sheet obtained by the hot rolling process to cold rolling at a rolling reduction of 40% to 70%; and (C) the cold rolling The cold-rolled steel sheet obtained by the hot rolling process is held for 10 seconds to 300 seconds in a temperature range of (Ac ₃ points−30 ° C.) to (Ac ₃ points + 100 ° C.), and then 500 ° C. to 650 ° C. An annealing process in which a temperature range is cooled at an average cooling rate of 15 ° C./second or more, and heat treatment is performed in a temperature range of 300 ° C. to 500 ° C. for 30 seconds to 3000 seconds.

The microstructure described above can be easily obtained by applying the above manufacturing conditions. The reason is not clear, but qualitatively, it can be considered as follows.
That is, by applying the above hot rolling conditions, a microstructure in which ferrite and other hard phases are finely and uniformly dispersed is formed, and by performing the cold rolling on this, further homogenization of the structure can be achieved. In addition to being promoted, recrystallization in the subsequent annealing step is promoted, so that the microstructure after annealing is made finer and uniform. By applying the above annealing conditions, the ferrite is finely and uniformly dispersed from the beginning, and the other hard phase finely and uniformly dispersed becomes a preferential nucleation site for austenite transformation. Combined with the suppression of ferrite grain growth, the ferrite grain growth is drastically suppressed, and the other hard phases finely and uniformly dispersed become preferential nucleation sites for austenite transformation. As a result, the transformed austenite is also finely and uniformly dispersed from the beginning. Furthermore, it is considered that the above microstructure can be achieved by the above-mentioned finely dispersed ferrite, which significantly suppresses the grain growth of austenite after transformation, and obtains a fine and uniform structure by their synergistic action.

(A) Hot rolling step The slab having the above chemical composition is subjected to multi-pass hot rolling to complete rolling at Ar ₃ points or more, and cooling is started within 0.4 seconds after completion of rolling, and the average cooling rate Is a temperature range of 620 ° C. or more and 720 ° C. or less under a cooling condition in which the time required for cooling to 720 ° C. after completion of rolling in the rolling pass two steps before the final rolling pass is 4 seconds or less. By cooling to a temperature range of 300 ° C. to 610 ° C. at an average cooling rate of 10 ° C./second to 100 ° C./second, and winding up. A hot-rolled steel sheet is used.

In order to obtain the microstructure after cold rolling and annealing described later, as described above, it is important to control the microstructure of the hot-rolled steel sheet as the base material.
The rolling is multipass rolling. The reduction amount per pass is preferably 15% or more and 60% or less. Since a larger amount of reduction per pass can introduce more strain into austenite, the ferrite crystal grains generated by the subsequent transformation are refined, and the structure of the hot-rolled steel sheet is refined. The For this reason, it is preferable that the amount of reduction per pass is 20% or more, particularly for three passes from the last rolling pass to the final rolling pass. More preferably, it is 22% or more, and particularly preferably 30% or more. On the other hand, from the viewpoint of securing controllability of rolling load and plate shape, the rolling reduction per pass is preferably less than 50%. In particular, when it is desired to easily control the plate shape, the rolling reduction is preferably 45% or less.

The rolling completion temperature needs to be transformed from austenite to ferrite after completion of rolling in order to make the microstructure of the hot rolled steel sheet fine and uniform. Therefore, the rolling completion temperature is set to Ar ₃ point or more. From the viewpoint of avoiding an increase in rolling load, the temperature is preferably 780 ° C. or higher. From the viewpoint of refinement of the structure of hot-rolled steel sheet, rolling completion temperature is as low as long as the temperature range of more than Ar ₃ point or 780 ° C. preferred. This is because the work strain introduced into the austenite by rolling is efficiently accumulated, and the refinement of the microstructure of the hot-rolled steel sheet is promoted. From the viewpoint of homogenizing the structure of the hot-rolled steel sheet, the rolling completion temperature is preferably set to 850 ° C. or higher. More preferably, it is 900 degreeC or more. This is because by appropriately raising the rolling completion temperature, the ferrite and other hard phases can be uniformly dispersed together with the ferrite, thereby further improving the formability of the steel sheet after cold rolling and annealing. Note that the rolling completion temperature is preferably 980 ° C. or less from the viewpoint of suppressing the release of processing strain introduced into the austenite by rolling and efficiently promoting the refinement of the microstructure of the hot-rolled steel sheet. More preferably, it is 930 degrees C or less.

  After the completion of rolling, the release of the working strain introduced into the austenite is suppressed, and this is used as a driving force to transform from austenite to ferrite at a stretch to produce a structure having fine ferrite crystal grains. Cooling is started within 4 seconds, the average cooling rate is 600 ° C./second or more, and the time required for cooling from the completion of rolling in the rolling pass two steps before the final rolling pass to 720 ° C. is 4 seconds or less. It is necessary to perform rolling and cooling under the following cooling conditions. The time required for cooling to 720 ° C. after completion of rolling in the rolling pass two steps before the final rolling pass is preferably 3.5 seconds or less. The average cooling rate at this time is preferably 900 ° C./second or more, more preferably more than 1000 ° C./second.

  The temperature range of 620 ° C. or more and 720 ° C. or less is a temperature range where the ferrite transformation is activated. When the above-described cooling conditions are applied and the strain introduced into the austenite is released and kept in this temperature range, ferrite precipitates not only from the austenite grain boundaries but also from within the grains, and nucleation of ferrite transformation occurs. Since it occurs at a high density, a structure in which fine ferrite crystal grains are uniformly dispersed can be generated. Therefore, after the cooling, it is cooled to a temperature range of 620 ° C. or more and 720 ° C. or less, and is kept in the temperature range for 1 second or more. On the other hand, if the holding time in the temperature range exceeds 10 seconds, ferrite grain growth may be promoted. Therefore, the holding time in the temperature range is set to 10 seconds or less.

  Following the above holding, the film is cooled and wound up to a temperature range of 300 ° C. to 610 ° C. at an average cooling rate of 10 ° C./second to 100 ° C./second. Subsequent to the rolling and cooling described above, by performing such cooling and winding, the microstructure of the hot-rolled steel sheet is composed of fine proeutectoid ferrite and bainite or bainitic ferrite, and finely and uniformly dispersed. The microstructure described above can be achieved after cold rolling and annealing.

  When the average cooling rate is less than 10 ° C./second, coarse pearlite precipitates along segregation to form a band-like structure, and there is a region where ferrite coarsening is likely to proceed in the annealing process after cold rolling. In some cases, the ferrite is not refined and the structure is not uniform after annealing. In addition, the coarsening of the iron carbide is caused, and in the annealing process after cold rolling, it becomes difficult to suppress the grain growth of the ferrite, and it may be impossible to refine the ferrite. Therefore, the average cooling rate is 10 ° C./second or more. Preferably it is 15 degreeC / second or more, More preferably, it is 20 degreeC / second or more. On the other hand, when the average cooling rate exceeds 100 ° C./second, the flatness of the steel sheet may be impaired. Therefore, the average cooling rate is set to 100 ° C./second or less. Preferably it is 80 degrees C / sec or less.

  When the coiling temperature is less than 300 ° C., the hot-rolled steel sheet is significantly hardened, and the cold rolling property is hindered. Therefore, the coiling temperature is set to 300 ° C. or higher. Preferably it is 350 degreeC or more. On the other hand, when the coiling temperature exceeds 610 ° C., coarse pearlite precipitates along the segregation to form a band-like structure, resulting in a region where ferrite coarsening is likely to proceed in the annealing process after cold rolling. As a result, the ferrite may not be refined or the structure may not be uniform after annealing. In addition, the coarsening of the iron carbide is caused, and in the annealing process after cold rolling, it becomes difficult to suppress the grain growth of the ferrite, and it may be impossible to refine the ferrite. Therefore, the coiling temperature is 610 ° C. or lower. Preferably it is 500 degrees C or less.

  The slab to be subjected to hot rolling is made into a slab by continuous casting or casting and partial rolling after melting the steel having the above chemical composition. From the viewpoint of productivity, it is preferable to use continuous casting. When continuous casting is used, it is preferable to perform molten steel flow by external magnetic field or mechanical stirring in the mold in order to improve crack resistance by inclusion control. The slab thus obtained may be directly subjected to rolling, or may be subjected to hot rolling after being kept warm or reheated.

The temperature of the slab used for hot rolling is preferably less than 1280 ° C. in order to prevent coarsening of austenite. More preferably, it is 1250 ° C. or less, and particularly preferably 1200 ° C. or less. The lower limit of the temperature of the slab to be subjected to hot rolling is not particularly limited as long as the rolling can be completed at an Ar ₃ point temperature or higher described later.

  Hot rolling consists of rough hot rolling and finish hot rolling, and the slab is made into a rough bar by rough hot rolling, and the obtained rough bar is made into a hot-rolled steel sheet by finish hot rolling. It is normal. In this case, it is preferable that the rough bar obtained by rough hot rolling is reheated to 1000 ° C. or higher before being subjected to finish hot rolling. It is more preferable to reheat to 1050 ° C. or higher.

The rough bar may be heated by, for example, installing a heating device in front of the finishing hot rolling stand group and performing induction heating, energization heating, or heating using a gas or an infrared heater as a heat source. When such a coarse bar is heated, secondary scaling can be effectively removed by subsequent descaling with high-pressure water, and cooling fluctuations and surface flaws caused by the scale can be generated. Suppression is possible. Preferably, the reheating temperature of the slab is set to 1050 ° C. or higher.
(B) Cold rolling step The hot rolled steel plate obtained by the hot rolling step is cold rolled at a rolling reduction of 40% or more and 70% or less to obtain a cold rolled steel plate.

The hot-rolled steel sheet is descaled by pickling or the like and then cold-rolled. In order to promote recrystallization in the subsequent annealing process by cold rolling and to make the microstructure after annealing fine and uniform, the rolling reduction is made 40% or more. On the other hand, if the rolling reduction is too high, the rolling load increases and rolling becomes difficult, so the rolling reduction is set to 70% or less. Preferably it is less than 60%. The steel sheet after cold rolling is subjected to a degreasing treatment as necessary and subjected to annealing.
(C) Annealing Step The cold rolled steel sheet obtained by the cold rolling step is held in a temperature range of (Ac ₃ points−30 ° C.) to (Ac ₃ points + 100 ° C.) for 10 seconds to 300 seconds, and then Annealing is performed by cooling a temperature range of 500 ° C. or more and 650 ° C. or less at an average cooling rate of 15 ° C./second or more, and performing a heat treatment that maintains a temperature range of 300 ° C. or more and 500 ° C. or less for 30 seconds or more and 3000 seconds or less.

By subjecting the cold-rolled steel sheet to the heat treatment, a target microstructure can be obtained.
First, it is held for 10 seconds or more and 300 seconds or less in a temperature range of (Ac ₃ points −30 ° C.) or more (Ac ₃ points + 100 ° C.), but the holding temperature at this time is less than (Ac ₃ points −30 ° C.) Then, austenitization becomes insufficient, and the target microstructure may not be achieved after annealing. Therefore, the holding temperature is set to (Ac ₃ points-30 ° C.) or higher. It is preferably more than (Ac ₃ points−20 ° C.), more preferably more than (Ac ₃ points−10 ° C.), and particularly preferably more than (Ac ₃ points + 20 ° C.).

  If the holding time at this time is less than 10 seconds, uniform structure control becomes difficult, and the target microstructure may not be achieved. Therefore, the holding time is 10 seconds or longer. It is preferable to set it for 60 seconds or more.

On the other hand, if the holding temperature exceeds (Ac ₃ points + 100 ° C.) or the holding time exceeds 300 seconds, austenite grain growth is suppressed even when the hot rolling process and the cold rolling process described above are applied. In some cases, it is difficult to obtain the desired structure after annealing. Therefore, the holding temperature is (Ac ₃ points + 100 ° C.) or less, and the holding time is 300 seconds or less. The holding temperature is preferably (Ac ₃ points + 50 ° C.) or less, and more preferably (Ac ₃ points + 20 ° C.) or less. Moreover, it is preferable to set it as 950 degrees C or less. The holding time is preferably less than 200 seconds.

  Subsequently, the temperature range of 500 ° C. or more and 650 ° C. or less is cooled at an average cooling rate of 15 ° C./second or more, and the target microstructure is maintained at a temperature range of 300 ° C. or more and 500 ° C. or less for 30 seconds or more and 3000 seconds or less. Build in. When the average cooling rate is less than 15 ° C./second, ferrite is excessively formed, and the target structure may not be obtained after annealing. Therefore, the average cooling rate is set to 15 ° C./second or more. Preferably it is 30 ° C./second or more, more preferably 40 ° C./second or more. From the viewpoint of tissue control, the upper limit of the average cooling rate need not be specified. However, if the average cooling rate is extremely high, uneven cooling may occur and the plate shape may be impaired. Therefore, the average cooling rate is preferably 150 ° C./second or less. More preferably, it is less than 130 ° C./second.

  Holding for 30 seconds or more in the temperature range of 300 ° C. or more and 500 ° C. or less is to generate a predetermined retained austenite. If the holding temperature at this time exceeds 500 ° C., it becomes difficult to obtain a target structure. The temperature range is preferably 330 ° C. or higher and 450 ° C. or lower, and more preferably 350 ° C. or higher and 430 ° C. or lower. The holding time in the temperature range is preferably 200 seconds or longer. The upper limit of the holding time in the temperature range is set to 3000 seconds or less from the viewpoint of productivity.

Heating to a temperature range of (Ac ₃ points-30 ° C.) or more and (Ac ₃ points + 100 ° C.) or less is performed at a temperature range of 500 ° C. or more at a heating rate of less than 20 ° C./second from the viewpoint of stability of structure control. Preferably it is done.

In addition, when aiming to improve ductility by increasing the volume fraction of ferrite, it is maintained for 10 seconds to 300 seconds in a temperature range of (Ac ₃ points −30 ° C.) to (Ac ₃ points + 100 ° C.). After that, it is preferable to cool at 50 ° C. or more at a cooling rate of less than 10 ° C./second. The cooling rate is preferably less than 5.0 ° C./second, more preferably less than 3.0 ° C./second, and particularly preferably less than 2.0 ° C./second. In order to further increase the ferrite volume fraction, cooling at 80 ° C. or higher is preferable, and cooling at 100 ° C. or higher is more preferable. On the other hand, in order to obtain the target structure, since it is preferable to suppress the ferrite volume fraction to some extent, the cooling amount is preferably 200 ° C. or less. More preferably, it is 160 degrees C or less.

  In order to improve the balance between high impact absorption characteristics and crack resistance, cooling to room temperature after holding for 30 seconds to 3000 seconds in a temperature range of 300 ° C to 500 ° C is performed at a temperature of 170 ° C to 300 ° C. It is preferable to cool the region at a cooling rate of 2 ° C./second or more and less than 30 ° C./second. Preferably, it is 5 ° C./second or more and 20 ° C./second or less.

  When manufacturing a plated steel sheet, the cold-rolled steel sheet manufactured by the above-described method may be electroplated or hot-plated according to a conventional method. The plating method, the chemical composition of the plating film, and the alloying treatment after plating are performed. It is not limited to the presence or absence. In the case of hot dipping, the hot dipping may be carried out in the steel sheet production line after the heat treatment is carried out in the annealing step by the heat treatment and held in the temperature range of 300 ° C. to 500 ° C. for 30 seconds to 3000 seconds. Examples of the plating type are as described above.

  The cold-rolled steel sheet and the plated steel sheet thus obtained may be subjected to temper rolling according to a conventional method. The elongation of temper rolling is preferably 1.0% or less from the viewpoint of securing good ductility. More preferably, it is 0.5% or less.

  Experiments were performed using slabs (thickness: 30 mm, width: 160 to 250 mm, length: 70 to 90 mm) having the chemical composition shown in Table 1. In either case, 180 kg of molten steel was vacuum-melted and cast, and then heated at a furnace temperature of 1250 ° C. and hot forged at a temperature of 950 ° C. or higher to form a slab.

  The slab was reheated at 1200 ° C. within 1 hour, then subjected to 4 passes of rough hot rolling using a hot rolling tester, and further subjected to 3 passes of finish hot rolling to obtain a plate thickness: A 3 mm hot rolled steel sheet was used. Table 2 shows the hot rolling conditions.

  Further, after cold rolling to a plate thickness of 1.6 mm (reduction rate: 47%), heat treatment shown in Table 3 was performed using a continuous annealing simulator. These conditions are shown in Table 3.

The following investigation was conducted on the cold-rolled steel sheet thus obtained.
Yield strength (YS: 0.2% yield strength), tensile strength (TS), and uniform elongation (u-El) were determined by collecting JIS No. 5 tensile test pieces and conducting a tensile test.

Moreover, the hole expansion rate according to Japan Iron and Steel Federation standard JFS T 1001-1996 was conducted, and the hole expansion rate was obtained.
After mirror-polishing the cross section parallel to the rolling direction of the steel plate, the strain is removed by electrolytic polishing, EBSD analysis is performed for the 1/4 depth position of the plate thickness, and martensite is obtained from the grain interface orientation difference map and image quality map. The site area ratio was calculated. Martensite has a relatively high internal dislocation density, and the image quality of EBSD is clearly lower than others, so separation and determination are easy.

  Residual austenite is obtained by exposing the 1/4 depth position of the plate thickness by mechanical polishing and chemical polishing of 100 μm, and measuring the diffraction intensity of the γ (111), (200), (220) planes with an X-ray diffractometer. The area ratio was determined.

  The cross section parallel to the rolling direction of the steel sheet was mirror-polished and then subjected to nital corrosion, and observed with a scanning electron microscope, whereby the ferrite average particle diameter was determined by the intercept method from 1000 and 2000 times secondary electron images.

The area ratio of bainite was calculated by subtracting martensite, residual austenite, and ferrite measured by the above method from the entire structure.
The nanohardness of bainite and martensite was determined by the nanoindentation method. A 1/4 depth position of the plate thickness was polished with emery paper, and then mechanochemical polishing was performed with colloidal silica. Further, the processed layer was removed by electrolytic polishing and used for the test. Nanoindentation was performed using a Barkovic indenter with an indentation load of 500 μN. The indentation size at this time is 0.1 μm or less in diameter. Twenty points were randomly measured for each of bainite and martensite, and the average nanohardness of each was determined. Even after 10% tensile deformation, the average nano hardness of bainite and martensite was determined by the above method.

  Furthermore, a rectangular tube member was produced by bending and welding using the steel plate, and an axial crush test was performed with an axial collision speed of 64 km / h to evaluate the impact absorption performance. The shape of the cross section perpendicular to the axial direction of the rectangular tube member was a regular octagon, the axial length of the rectangular tube member was 200 mm, and the axial direction was perpendicular to the rolling direction.

  For the square tube member, the cross-sectional shape factor (Wp /) using the length of one side of the regular octagon (the length of the straight portion excluding the curved portion of the corner) (Wp) and the plate thickness (t) of the steel plate. For t) of 20 and 16, the average crush load and the stable buckling rate were investigated.

Here, the stable buckling rate is the ratio of the number of test specimens that were not cracked by the axial crush test to the total number of test specimens.
In general, the smaller the cross-sectional shape factor (Wp / t), the higher the impact absorption energy. However, as the cross-sectional shape factor (Wp / t) decreases, the amount of plastic deformation work per unit crushing amount increases. For this reason, the possibility that a crack will occur during crushing increases, and as a result, the plastic deformation work cannot be increased, and the shock absorption energy may not be increased.

Under the present crush test conditions, when Wp / t = 20, the average crushing load is preferably 0.30 kN / mm ² or more, and the stable buckling rate is preferably 80% or more. In the case of Wp / t = 16, the average crushing load is preferably 0.35 kN / mm ² or more, and the stable buckling rate is preferably 30% or more.

  Table 4 shows data related to microstructure and nano hardness, and Table 5 shows data related to mechanical properties.

The rectangular tube member using the steel plate manufactured by the method according to the present invention has the microstructure and mechanical characteristics described above, and the average load due to axial crushing in the cross-sectional shape factor Wp / t = 20 is 0. It is as high as 34 kN / mm ² or more. Furthermore, the stable buckling rate at the cross-sectional shape factor Wp / t = 20 is 80% or more, and the stable buckling rate at the cross-sectional shape factor Wp / t = 16 is 30% or more, so that cracking due to axial crushing hardly occurs.

  1: car body, 2: front crash box, 3: rear crash box, 4: front side member (front frame), 5: rear side member (rear frame), 6: front upper rail, 7: side sill (rocker), 8 : Cross member, 9, 10: Shock absorber, 11: Bumper reinforcement, 12: Center pillar (B post)

Claims (4)

Bainite: more than 50%, martensite: 3% or more and 30% or less and retained austenite: 3% or more and 15% or less, with the balance being a microstructure made of ferrite with an average particle size of less than 5 μm, And the product of the uniform elongation and the hole expansion ratio is 300% ² or more, and the manufacturing method of the steel sheet having mechanical properties that the effective flow stress when applying 5% true strain is 900 MPa or more,
The manufacturing method of the steel plate characterized by having the following process (A)-(C):
(A) By mass%, C: 0.08% to 0.30%, Mn: 1.5% to 3.5%, Si + Al: 0.50% to 3.0%, P: 0.00. Multi-pass hot rolling that completes rolling at ₃ or more points of Ar in a slab containing 10% or less, S: 0.010% or less and N: 0.010% or less, and having a chemical composition consisting of the remaining Fe and impurities The cooling is started within 0.4 seconds after the completion of rolling, the average cooling rate is 600 ° C./second or more, and the cooling is completed to 720 ° C. from the completion of rolling in the rolling pass two steps before the final rolling pass. Is cooled to a temperature range of 620 ° C. or more and 720 ° C. or less under a cooling condition that requires 4 seconds or less, and held in the temperature range for 1 second or more and 10 seconds or less, and then 10 ° C./second or more and 100 ° C./second or less. Temperature at an average cooling rate of 300 ° C or higher and 610 ° C or lower A hot rolling process to form a hot-rolled steel sheet by cooling to a temperature range and winding;
(B) a cold rolling step for forming a cold rolled steel sheet by subjecting the hot rolled steel sheet obtained by the hot rolling process to cold rolling at a rolling reduction of 40% to 70%; and (C) the cold rolling The cold-rolled steel sheet obtained by the hot rolling process is held for 10 seconds to 300 seconds in a temperature range of (Ac ₃ points−30 ° C.) to (Ac ₃ points + 100 ° C.), and then 500 ° C. to 650 ° C. An annealing process in which a temperature range is cooled at an average cooling rate of 15 ° C./second or more, and heat treatment is performed in a temperature range of 300 ° C. to 500 ° C. for 30 seconds to 3000 seconds.   The chemical composition is one type selected from the group consisting of Cr: 0.5% or less, Mo: 0.5% or less, and B: 0.010% or less in mass%, instead of a part of the Fe Or the manufacturing method of the steel plate of Claim 1 containing 2 or more types.   The chemical composition is one selected from the group consisting of Ti: less than 0.04%, Nb: less than 0.030%, and V: less than 0.5% by mass% instead of part of the Fe. Or the manufacturing method of the steel plate of Claim 1 or Claim 2 containing 2 or more types.   Instead of a part of the Fe, the chemical composition is, in mass%, Ca: 0.010% or less, Mg: 0.010% or less, REM: 0.050% or less, and Bi: 0.050% or less. The manufacturing method of the steel plate in any one of Claims 1-3 containing 1 type, or 2 or more types selected from the group which consists of.

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