JP6690793B1 - High-strength steel sheet and method for manufacturing the same - Google Patents

Abstract

This high-strength steel sheet has a predetermined chemical composition and has a metal structure in area%, martensite: 70.0 to 95.0%, retained austenite: 5.0 to 30.0%, balance: 0. 10.0% and the average particle size of retained austenite: 0.2 to 2.0 μm, and the average Mn concentration [Mn] γ in the retained austenite was 7.0 ≦ [Mn] γ ≦ 20. 0 (i) is satisfied, and the C content [C] and the average C concentration in the martensite [C] M: satisfy 0.6 ≦ [C] M / [C] (ii). To do.

Description

The present invention relates to a high-strength steel plate and a method for manufacturing the same.
The present application claims priority based on Japanese Patent Application No. 2018-124974 filed in Japan on June 29, 2018, and the content thereof is incorporated herein.

  In recent years, from the viewpoint of protecting the global environment, efforts to reduce carbon dioxide emissions have started in many fields. Automobile manufacturers are also actively developing technologies for reducing the weight of vehicles to reduce fuel consumption. However, the weight reduction of the vehicle body is not easy because the emphasis is also placed on the improvement of the collision resistance property for ensuring the safety of the occupants.

Therefore, in order to achieve both the weight reduction of the vehicle body and the improvement of the collision resistance, it is considered to reduce the thickness of the member by using the high strength steel plate. Therefore, a steel sheet that has both high strength and excellent formability is strongly desired.
In recent years, it has been desired that steel sheets used for automobile parts such as structural members or reinforcing members have a high tensile strength of 1180 MPa or more and excellent formability. Furthermore, in addition to high tensile strength, in order to secure the rigidity of the member thinned using high strength steel plate for weight reduction of the vehicle body and to improve the collision energy absorption characteristics from the viewpoint of ensuring the safety of passengers, It is required to have a high yield stress.

  In general, a steel sheet containing a retained austenite in the metal structure exhibits a large elongation due to the effect of transformation-induced plasticity (TRIP) generated by the martensitic transformation of the retained austenite during working. Therefore, it is effective to use retained austenite in order to combine excellent formability in a high strength steel sheet.

  Heretofore, some techniques have been proposed which utilize such retained austenite to enhance strength and elongation.

  For example, in Patent Document 1, the space factor of retained austenite is 5% to 50%, the average crystal grain size of retained austenite is 5 μm or less, and tensile strength × total elongation is 20000 MPa. %, A high-strength steel sheet for automobiles having excellent collision resistance and formability is disclosed.

  In Patent Document 2, an elongation in which a retained austenite and / or martensite having an average particle diameter of 500 nm or less is contained as a second phase structure in the crystal grains at a space factor of 3 to 20% with respect to the entire structure, and elongation. A high-strength steel sheet having excellent flangeability is disclosed.

  Furthermore, in recent years, a medium Mn steel having a Mn content of about 3 to 10 mass% and containing a large amount of retained austenite has attracted attention as a material having an excellent balance between strength and ductility.

  For example, in Patent Document 3, C: 0.03% or more and 0.35% or less, Si: 0.5% or more and 3.0% or less, Mn: 3.5% or more and 10.0% or less in mass%, P: 0.1% or less, S: 0.01% or less, N: 0.008% or less, the balance being Fe and unavoidable impurities, and having an area ratio of 30.0% or more ferrite. The value obtained by dividing the Mn content (mass%) in the ferrite by the Mn content (mass%) in the steel plate is 0.80 or less, and the residual austenite content is 10.0% or more by volume ratio, Disclosed is a high-strength steel sheet with excellent workability, characterized in that the amount of Mn in the retained austenite is 6.0% by mass or more, and the average crystal grain size of the retained austenite is 2.0 μm or less. There is.

  Moreover, in patent document 4, C: 0.090% or more and less than 0.30% in mass%, Mn: 3.5% or more and less than 11.0%, Si: 0.01 to 2.5%, P: The composition including 0.05% or less, S: 0.05% or less, Al: 0.005 to 0.1%, N: 0.01% or less, and the balance Fe and unavoidable impurities, and a volume ratio of 80 % Or more of martensite phase and 3.0 to 20.0% by volume of retained austenite phase, and tensile strength TS: 1500 MPa or more and uniform elongation uEl: 6.0% or more. A high-strength hot-pressed member characterized by having the following tensile properties is disclosed.

  However, although the steel sheet described in Patent Document 1 is said to have improved ductility due to refinement of retained austenite, in order to increase the work hardening index and improve collision resistance, the main phase is a soft ferrite phase. Therefore, it is difficult to obtain high tensile strength.

  In addition, the steel sheet described in Patent Document 2 has an excellent strength-ductility balance obtained by dispersing fine retained austenite of submicron size in the crystal grains of tempered martensite or bainite. However, in the technique of Patent Document 2, it is necessary to contain a large amount of expensive elements such as Cu and Ni as austenite stabilizing elements, and to perform solution treatment for a long time at high temperature, which increases manufacturing cost and productivity. Is significantly reduced.

  Furthermore, according to Patent Document 3, by annealing a steel containing 3.5% or more and 10.0% or less of Mn in the two-phase region of austenite and ferrite, the retained austenite is stabilized by the concentration of Mn, It is said that a high-strength steel sheet having a tensile strength of 980 MPa or more and a strength-ductility balance of 24000 MPa ·% or more can be obtained. However, in the steel sheet of Patent Document 3, since it is necessary to contain 30.0% or more of soft ferrite in order to secure good ductility, it is difficult to further increase the strength.

  According to Patent Document 4, by containing martensite as a main phase and a retained austenite phase with a volume ratio of 3.0 to 20.0%, high tensile strength of 1500 MPa or more and uniform elongation of 6. An excellent balance of 0% or more and uniform elongation is obtained. However, Patent Document 4 does not take into consideration the improvement of the yield stress effective for ensuring the rigidity of the member and improving the collision energy absorption characteristics.

Japanese Patent Laid-Open No. 11-61326 Japanese Patent Laid-Open No. 2005-179703 Japanese Patent Laid-Open No. 2013-76162 International Publication No. 2016/063467

  The present invention has been made in order to solve the above-mentioned problems, and has a high tensile strength and a high yield stress, and an object is to provide a steel sheet having excellent ductility (elongation), and a manufacturing method thereof. To do. Specifically, it is an object to provide a high-strength steel sheet having a tensile strength of 1180 MPa or more, a yield stress of 800 MPa or more, and a strength-ductility balance of 18000 MPa ·% or more, and a manufacturing method thereof.

  The gist of the present invention is the following high-strength steel sheet and its manufacturing method.

(1) The chemical composition of the high-strength steel sheet according to one aspect of the present invention is% by mass, C: 0.08 to 0.45%, Si: 0.05 to 3.0%, Mn: 3.5. ~ 10.0%, P: 0.10% or less, S: 0.030% or less, sol. Al: 0.01 to 2.0%, N: 0.010% or less, Ti: 0 to 0.20%, Nb: 0 to 0.10%, V: 0 to 0.50%, Cr: 0% Above 1.0%, Mo: 0 ~ 0.50%, Ni: 0 ~ 1.0%, B: 0 ~ 0.0050%, Ca: 0 ~ 0.020%, Mg: 0 ~ 0.020. %, REM: 0 to 0.020%, Cu: 0 to 1.0%, Bi: 0 to 0.020%, balance: Fe and impurities, the metal structure is area%, martensite: 70. 0 to 95.0%, retained austenite: 5.0 to 30.0%, balance: 0 to 10.0%, and average particle size of retained austenite: 0.2 to 2.0 μm, The average Mn concentration in the retained austenite satisfies the following formula (i), and the C content and the average C concentration in the martensite are the formula (ii). Satisfactory.
7.0 ≦ [Mn] _γ ≦ 20.0 (i)
0.6 ≦ [C] _M / [C] (ii)
However, the symbols in the above formula have the following meanings.
[Mn] _γ : average Mn concentration (mass%) in the retained austenite
[C]: C content in the steel sheet (% by mass)
[C] _M : the average C concentration (mass%) in the martensite
(2) In the high-strength steel sheet according to (1) above, the chemical composition is mass% and Ti: 0.005 to 0.20%, Nb: 0.002 to 0.10%, and V: 0. One or more selected from 0.005 to 0.50% may be contained.
(3) In the high-strength steel sheet according to (1) or (2), the chemical composition is% by mass, Cr: 0.05% or more and less than 1.0%, Mo: 0.02 to 0.50. %, Ni: 0.05 to 1.0%, and B: 0.0002 to 0.0050%, may be contained.
(4) In the high-strength steel sheet according to any one of (1) to (3), the chemical composition is% by mass, Ca: 0.0005 to 0.020%, Mg: 0.0005 to 0. One or more selected from 020% and REM: 0.0005 to 0.020% may be contained.
(5) In the high-strength steel sheet according to any one of (1) to (4) above, the chemical composition may be mass% and may contain Cu: 0.05 to 1.0%.
(6) In the high-strength steel sheet according to any one of (1) to (5), the chemical composition may be mass% and Bi: 0.0005 to 0.020% may be contained.
(7) A method for producing a high-strength steel sheet according to another aspect of the present invention is the method for producing a high-strength steel sheet according to any one of (1) to (6) , including (1) to (6) For a slab having a chemical composition according to any of the above, a high-strength steel sheet that sequentially performs a hot rolling step, a cooling step, a winding step, a primary annealing step, an optional cold rolling step, and a secondary annealing step. In the hot rolling step, the rolling reduction in the final rolling pass and the rolling pass immediately before the final rolling pass is 15 to 60%, and the rolling ratio before the final rolling pass is The time between passes from the completion of rolling of the rolling pass to the start of rolling of the final rolling pass satisfies the following formula (v), and the rolling completion temperature of the final rolling pass is set to a temperature range of Ar _{3 to} 1100 ° C., In the cooling process, hot rolling after the hot rolling process The plate is air-cooled for 1 to 10 seconds and then cooled at an average cooling rate of 10 ° C./second or more, in the winding step, it is wound in a temperature range of 550 ° C. or less, and in the primary annealing step, The hot-rolled steel sheet is held so that the annealing temperature is in the temperature range of (Ac ₁ point-80 ° C) to (Ac ₃ point-55 ° C) and the holding time satisfies the following formula (vi), In the next annealing step, the hot-rolled steel sheet was held so that the annealing temperature was in the temperature range of (Ac ₃ points + 30 ° C) or higher and lower than (Ac ₃ points + 200 ° C) and the holding time was less than 150 seconds. Then, it is cooled to a temperature of 500 ° C. or lower so that the average cooling rate in the temperature range of Ac ₃ point to 500 ° C. is 15 ° C./sec or higher.
0.002 / exp (−6080 / (T ₁ +273)) ≦ t ₁ ≦ 2.0 (v)
2.3 × 10 ⁻⁸ × exp {23500 / (T ₂ +273)} ≦ t ₂ ≦ 4.0 × 10 ⁵ (vi)
However, the meaning of each symbol in the above formula is as follows.
t ₁ : time between passes (seconds) from completion of rolling immediately before the final rolling pass to start of rolling of the final rolling pass
T ₁ : Rolling completion temperature of the rolling pass immediately before the final rolling pass (° C.)
t ₂ : the holding time (seconds) at the annealing temperature of the primary annealing
T ₂ : the annealing temperature (° C.) of the primary annealing
(8) In the method for producing a high-strength steel sheet according to (7) above, the total rolling reduction may be 30% or more and less than 80% in the cold rolling step.

  According to the above aspect of the present invention, it is possible to obtain a steel sheet having high tensile strength and yield stress and excellent ductility.

It is a schematic diagram which shows the metallographic structure during secondary annealing maintenance.

  The present inventors have earnestly studied the relationship between the steel structure and the mechanical properties of medium Mn steel having a Mn content of about 3 to 10 mass%. As a result, the following findings have been obtained.

(A) In order to obtain retained austenite in medium Mn steel, it is general to perform heat treatment for a long time in the two-phase region of ferrite and austenite (the temperature region of A _{1 to} A ₃ ). In this case, the austenite stabilizing elements C and Mn are concentrated in the austenite, whereby a large amount of retained austenite is obtained. As a result, ductility is improved. On the other hand, since C, which is excellent in solid solution strengthening, is consumed for stabilizing austenite, it becomes difficult to increase the strength. Therefore, in order to have both high strength and excellent ductility, it is important to have a structure in which martensite strengthened by solid solution with C is the main component and residual austenite stabilized by Mn concentration is contained.

  (B) By including retained austenite in the steel structure mainly composed of martensite, the ductility can be improved by the TRIP effect even though the strength is high. However, when the retained austenite is coarse, coarse voids are formed by martensite generated by the work-induced transformation, so local deformation occurs early and the effect of improving ductility cannot be obtained. On the other hand, if the retained austenite is too fine, the stability of the retained austenite against deformation is excessively increased due to the three-dimensional restraint from the surrounding crystal grains. Therefore, in the high strength steel sheet, local deformation occurs before the TRIP effect is sufficiently exhibited. In some cases, the ductility may not be improved and the ductility may not be improved. Therefore, by controlling the average grain size of the retained austenite contained in the steel structure in an appropriate range, it becomes possible to have high strength and excellent ductility.

  (C) When the retained austenite stabilized by Mn enrichment is contained in the steel structure mainly composed of martensite strengthened by C, the Mn concentration in the retained austenite greatly affects the ductility and the yield stress. To do. If the Mn concentration in the retained austenite is excessively low, the stability of the retained austenite against deformation is low and the TRIP effect occurs during elastic deformation or in the early stage of plastic deformation, so that the yield stress decreases and the ductility cannot be improved. On the other hand, when the Mn concentration in the retained austenite is excessively high, not the TRIP effect but the twin induced plasticity (TWIP) effect becomes conspicuous and the yield stress decreases. Therefore, by controlling the Mn concentration in the retained austenite within an appropriate range, it is possible to obtain a steel sheet having both high tensile strength and yield stress and excellent ductility.

  (D) In order to produce a steel structure mainly containing martensite strengthened by C as a solid solution and containing retained austenite stabilized by Mn, the annealing process applied to the hot rolled steel sheet may be divided into two steps. It is valid. Specifically, first, the hot-rolled steel sheet is annealed at a temperature in the two-phase region of ferrite and austenite, or in the two-phase region of ferrite and cementite, or in the three-phase region of ferrite, austenite and cementite, and C is added to the austenite or cementite. And Mn are distributed. After that, annealing is performed in the austenite single-phase region to diffuse only C while suppressing the diffusion of Mn. By this annealing, C can be partially left in the austenite region having a high Mn concentration, and C can be diffused to the surroundings. As a result, by such annealing and cooling after annealing, the austenite region having a high Mn concentration becomes retained austenite, and the austenite region having a low Mn concentration becomes martensite having a constant C concentration, and a desired metallographic structure is obtained. .

  The present invention has been made based on the above findings. Hereinafter, each requirement of the high strength steel plate (steel plate according to this embodiment) according to one embodiment of the present invention will be described in detail.

(A) Chemical composition The reasons for limiting each element are as follows. In the following description, “%” regarding the content means “mass%”. Further, the numerical range represented by using "to" means a range including the numerical values before and after "to" as the lower limit value and the upper limit value, unless otherwise specified. That is, 0.08 to 0.45% means 0.08% or more and 0.45% or less.

C: 0.08 to 0.45%
C has a function of increasing the strength of the steel sheet by solid solution strengthening by forming a solid solution in martensite. If the C content is less than 0.08%, the amount of solid solution strengthening is small, and it becomes difficult to secure the desired steel plate strength. Therefore, the C content is 0.08% or more. The C content is preferably 0.10% or more, more preferably 0.12% or more.
On the other hand, when C is contained in excess of 0.45%, pearlite is preferentially generated, and it becomes difficult to secure a desired steel plate strength. Therefore, the C content is 0.45% or less. The C content is preferably 0.40% or less.

Si: 0.05-3.0%
Si has the function of increasing the strength of the steel sheet by solid solution strengthening and the function of soundening the steel by deoxidation. Further, Si is an element that delays the precipitation of cementite and increases the area ratio of retained austenite, and contributes to the improvement of ductility. If the Si content is less than 0.05%, it is difficult to obtain the effect of the above action. Therefore, the Si content is set to 0.05% or more.
On the other hand, when the Si content exceeds 3.0%, the surface properties of the steel sheet, the chemical conversion treatment property, and the weldability deteriorate significantly. Therefore, the Si content is 3.0% or less. The Si content is preferably 2.5% or less, more preferably 2.0% or less.

Mn: 3.5-10.0%
Mn has the function of enhancing the hardenability of steel and promoting the formation of martensite, and the function of forming a solid solution in austenite to stabilize the retained austenite. If the Mn content is less than 3.5%, it is difficult to secure the target martensite amount, retained austenite amount, and Mn concentration in the retained austenite. Therefore, the Mn content is set to 3.5% or more. The Mn content is preferably 4.0% or more, and more preferably 4.5% or more.
On the other hand, if the Mn content exceeds 10.0%, the amount of retained austenite produced is large, and it becomes difficult to secure the desired amount of martensite. Further, the Mn concentration in the austenite excessively increases, the TWIP effect becomes remarkable rather than the TRIP effect, and the yield stress decreases. Therefore, the Mn content is 10.0% or less. The Mn content is preferably 8.0% or less.

P: 0.10% or less P is an element generally contained as an impurity. P is an element that easily segregates, and if its content exceeds 0.10%, the formability and toughness significantly decrease due to grain boundary segregation. Therefore, the P content is 0.10% or less. The P content is preferably 0.050% or less, more preferably 0.030% or less, and further preferably 0.020% or less.
On the other hand, P is also an element having the effect of increasing strength by solid solution strengthening. Therefore, P may be positively contained. The lower limit of the P content does not have to be specified in particular, but it is preferably 0.001% or more in order to obtain the above effects.

S: 0.030% or less S is an element contained as an impurity and forms a sulfide-based inclusion in the steel to reduce the formability of the steel sheet. When the S content exceeds 0.030%, the moldability is significantly deteriorated. Therefore, the S content is 0.030% or less. The S content is preferably 0.010% or less, more preferably 0.005% or less, and further preferably 0.001% or less. The lower limit of the S content need not be specified in particular, but is preferably 0.0001% or more from the viewpoint of suppressing an increase in refining cost.

sol. Al: 0.01 to 2.0%
Similar to Si, Al has a function of deoxidizing steel to make the steel plate sound. Further, Al is an element that delays the precipitation of cementite and increases the area ratio of retained austenite, thereby contributing to the improvement of ductility. sol. When the content of Al (acid-soluble Al) is less than 0.01%, it is difficult to obtain the effect of the above action. Therefore, sol. The Al content is 0.01% or more. sol. The Al content is preferably 0.03% or more.
On the other hand, sol. If the Al content exceeds 2.0%, the A ₃ transformation point remarkably rises, making stable hot rolling difficult. Therefore, sol. The Al content is 2.0% or less. sol. The Al content is preferably 1.5% or less, more preferably 1.0% or less.

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. When the N content exceeds 0.010%, the formability is significantly reduced. Therefore, the N content is 0.010% or less. The N content is preferably 0.0080% or less, and more preferably 0.0070% or less. The lower limit of the N content does not have to be specified in particular, but considering the case where one or more kinds of Ti, Nb and V are contained to refine the steel structure as described later, the precipitation of carbonitride is promoted. Therefore, the N content is preferably 0.0010% or more, and more preferably 0.0020% or more.

  In the steel sheet according to the present embodiment, in addition to the above elements, the amount of Ti, Nb, V, Cr, Mo, Ni, B, Ca, Mg, REM, Cu and Bi shown below is further selected. You may contain an element of 1 or more types. Since these elements are not necessarily contained, the lower limit is 0%.

Ti: 0 to 0.20%
Nb: 0 to 0.10%
V: 0 to 0.50%
Ti, Nb and V precipitate in the steel as carbides or nitrides and have the effect of refining the steel structure by the pinning effect. Therefore, one or more selected from these elements may be contained. In order to more surely obtain the effects of the above-mentioned actions of these elements, it is preferable to contain at least one of Ti: 0.005% or more, Nb: 0.002% or more, and V: 0.005% or more. .
On the other hand, even if these elements are contained excessively, the effect due to the above-mentioned action is saturated and it becomes uneconomical. Therefore, even when it is contained, 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.

Cr: 0% or more and less than 1.0% Mo: 0 to 0.50%
Ni: 0 to 1.0%
B: 0 to 0.0050%
Cr, Mo, Ni and B have a function of enhancing hardenability. Further, Mo has the function of precipitating carbides in the steel to increase the strength. Further, Ni has an effect of effectively suppressing grain boundary cracking of the slab caused by Cu when Cu is contained as described later. Therefore, one or more selected from these elements may be contained.
In order to obtain the effect of the above action more reliably, the Ni content is 0.05% or more, the Cr content is 0.05% or more, the Mo content is 0.02% or more, and / or the B content is It is preferably 0.0002% or more.

On the other hand, when the Cr content is 1.0% or more, the chemical conversion treatability is significantly deteriorated. Therefore, even if it is contained, the Cr content is less than 1.0%.
Further, when the Mo content exceeds 0.50%, the effect due to the above-mentioned action is saturated, which is disadvantageous in cost. Therefore, even if it is contained, the Mo content is 0.50% or less. It is preferably 0.20% or less.
Further, since Ni is an expensive element, the inclusion of a large amount becomes disadvantageous in cost. Therefore, even if it is contained, the Ni content is 1.0% or less.
Further, when the B content exceeds 0.0050%, the moldability is remarkably deteriorated. Therefore, even if it is contained, the B content is 0.0050% or less.

Ca: 0 to 0.020%
Mg: 0 to 0.020%
REM: 0 to 0.020%
Ca, Mg and REM have the effect of enhancing the formability by adjusting the shape of the inclusions. Therefore, one or more selected from these elements may be contained. In order to obtain the effect of the above action more reliably, it is preferable to contain at least one of the above elements in an amount of 0.0005% or more.
On the other hand, if the contents of these elements exceed the above upper limits, the inclusions in the steel become excessive, which may rather reduce the formability. Therefore, even when it is contained, the Ca content is 0.020% or less, the Mg content is 0.020% or less, and the REM content is 0.020% or less. The content of each element is preferably 0.010% or less, and more preferably 0.005% or less.

  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 lanthanoid, it is industrially added in the form of misch metal.

Cu: 0 to 1.0%
Cu has the effect of precipitating at a low temperature and increasing the strength, so it may be contained in the steel. In order to obtain the effect of the above action more reliably, the Cu content is preferably 0.05% or more.
On the other hand, if the Cu content exceeds 1.0%, intergranular cracking of the slab may occur. Therefore, even if it is contained, the Cu content is 1.0% or less. The Cu content is preferably less than 0.5%, more preferably less than 0.3%.

Bi: 0 to 0.020%
Bi has the effect of enhancing the formability by refining the solidified structure, so it may be contained in the steel. In order to obtain the effect of the above action more reliably, the Bi content is preferably 0.0005% or more.
On the other hand, when the Bi content exceeds 0.020%, the effect of the above-mentioned action is saturated, which is disadvantageous in terms of cost. Therefore, even if it is contained, the Bi content is 0.020% or less. The Bi content is preferably 0.010% or less.

  In the chemical composition of the steel sheet according to this embodiment, the balance is Fe and impurities.

  The "impurity" is a component that is mixed by ores, raw materials such as scraps, and various factors of the manufacturing process when industrially manufacturing a steel material, and is acceptable within a range that does not adversely affect the present invention. Means

(B) Metal Structure of Steel Sheet The metal structure of the steel sheet according to the present embodiment will be described below. In the following description, “%” regarding the ratio (area ratio) of the tissue means “area%”.

Martensite: 70.0-95.0%
Martensite is a hard and homogeneous structure and is a structure suitable for obtaining high tensile strength. When the area ratio of martensite is less than 70.0%, it becomes difficult to obtain a desired tensile strength. Therefore, the area ratio of martensite is set to 70.0% or more. The area ratio of martensite is preferably 75.0% or more. On the other hand, if the area ratio of martensite exceeds 95.0%, the ductility is significantly reduced. Therefore, the area ratio of martensite is set to 95.0% or less. The martensite of the steel sheet according to the present embodiment is a general term including tempered martensite in addition to so-called fresh martensite that is non-diffusively generated by a shear mechanism.

Residual austenite: 5.0-30.0%
Retained austenite has a function of increasing ductility by the TRIP effect. If the area ratio of the retained austenite is less than 5.0%, it is difficult to obtain the effect of the above action. Therefore, the area ratio of retained austenite is set to 5.0% or more. The retained austenite area ratio is preferably 8.0% or more, more preferably 10.0% or more. On the other hand, when the area ratio of retained austenite exceeds 30.0%, the amount of martensite relatively decreases, and it becomes difficult to obtain a desired tensile strength. Therefore, the area ratio of retained austenite is 30.0% or less.

  Methods for quantifying residual austenite include X-ray diffraction, electron beam backscattering diffraction (EBSP) analysis, magnetic measurement, and the like, and the quantitative values may differ depending on the method. The area ratio of retained austenite defined by the steel sheet according to the present embodiment is a value measured by X-ray diffraction.

Remainder: 0 to 10.0%
The steel sheet according to the present embodiment may include, as a balance, polygonal ferrite, bainitic ferrite, bainite, cementite, pearlite, and the like, in addition to the above-described structure. When the total area ratio of the remaining structure excluding martensite and retained austenite exceeds 10.0%, it becomes difficult to obtain desired strength or ductility. Therefore, the area ratio of the balance is 10.0% or less. The area ratio of the balance is preferably 8.0% or less, and more preferably 6.0% or less. The area ratio of the balance may be 0%.

Average particle size of retained austenite: 0.2 to 2.0 μm
If the average particle size of the retained austenite exceeds 2.0 μm, coarse voids are formed by martensite generated by the work-induced transformation. In this case, local deformation occurs early and the effect of improving ductility cannot be obtained. Therefore, the average particle size of the retained austenite is set to 2.0 μm or less. The average particle size of the retained austenite is preferably 1.5 μm or less, more preferably 1.0 μm or less.
On the other hand, when the average grain size of the retained austenite is less than 0.2 μm, the stability of the retained austenite against deformation is excessively increased due to the three-dimensional constraint from the surrounding crystal grains, and the TRIP effect is sufficiently exhibited in the high strength steel sheet. In some cases, local deformation may occur before the deformation, and ductility may not be improved. Therefore, the average particle size of the retained austenite is 0.2 μm or more.

  The metal structure is identified and the area ratio is calculated by the following method. First, a cross section perpendicular to the rolling direction of the steel sheet was mirror-polished, and then a repeller-corroded sample was used to measure an area of ¼ depth from the steel sheet surface in a sheet width direction of 200 μm × a rolling surface normal direction of 50 μm. , And the total area ratio of retained austenite and martensite is calculated by binarization using a commercially available image processing software "Image-Pro". Here, the retained austenite and martensite form the bright part. A brightness histogram is created, and a binarization process is performed to separate the remaining austenite and martensite from the brightness range of the peaks appearing on the dark side up to 255 and to separate the remaining austenite and martensite.

  Next, the area ratio of retained austenite is calculated by X-ray diffraction measurement using a Co ray source. By using a sample whose surface thickness has been reduced from the surface of the steel sheet to a depth of 1/4 by chemical polishing, the (110) plane, (200) plane, and (211) plane of ferrite, and The integrated intensity of the (111) plane, the (200) plane, and the (220) plane of the retained austenite is obtained. Then, the volume ratio of the retained austenite is obtained by the strength averaging method, and the value is used as the area ratio of the retained austenite. A value obtained by subtracting the retained austenite area ratio obtained by X-ray diffraction measurement from the total area ratio of the retained austenite and martensite obtained above is defined as a martensite area ratio.

  The value obtained by subtracting the total area ratio of the martensite and the retained austenite obtained above from 100% is the area ratio of the residual structure.

The average particle size of the retained austenite is calculated by EBSP analysis and transmission electron microscope (TEM) observation of a thin film test piece taken from the surface of the steel sheet at a position of 1/4 depth of the sheet thickness. At that time, the observation magnification of TEM is set to 50,000 times, 10 austenite grain sizes are measured from the dark field image formed from the electron diffraction pattern of austenite, and the average value thereof is taken as the average grain size of the retained austenite.

In the steel sheet according to the present embodiment, after controlling the chemical composition and the metal structure as described above, the average Mn concentration [Mn] _γ in the retained austenite and the C concentration in the martensite and the C content in the steel sheet are further controlled. It is necessary to control the ratio [C] _M / [C] with the following.

7.0 ≦ [Mn] _γ ≦ 20.0 (i)
However, the symbols in the above formula have the following meanings.
[Mn] _γ : Average Mn concentration (% by mass) in retained austenite
The Mn concentration in retained austenite has a great influence on ductility and yield stress. In order to obtain desired yield stress and ductility, the average Mn concentration in the retained austenite needs to satisfy the above formula (i).

  When the average Mn concentration in the retained austenite is less than 7.0% by mass, the stability of the retained austenite against deformation is low, and the TRIP effect occurs during elastic deformation or in the early stage of plastic deformation. In this case, the yield stress decreases and the ductility cannot be improved. On the other hand, when the average Mn concentration in the retained austenite exceeds 20.0 mass%, the TWIP effect becomes remarkable rather than the TRIP effect, and the yield stress decreases. Therefore, the average Mn concentration in the retained austenite is set to 7.0 to 20.0% in mass%.

  The average Mn concentration in the retained austenite is obtained by the following method using an electron beam microanalyzer (FE-EPMA) equipped with a field emission electron gun. Mapping of Mn concentration by FE-EPMA at an interval of 0.1 μm for a 50 μm × 50 μm region of the sample used for the above X-ray diffraction measurement, that is, the sample that was chamfered from the surface of the steel plate to a depth of 1/4 of the plate thickness. Perform an analysis. Next, the retained austenite in the metal structure in the same region is identified by EBSP analysis. For the identified retained austenite, the average of the measured values of Mn concentration at arbitrary 10 points in one grain is set as the Mn concentration in the one grain. Then, the average value of the Mn concentration measured for each of the 10 grains is calculated, and this value is set as the average Mn concentration of the retained austenite.

0.6 ≦ [C] _M / [C] (ii)
However, the symbols in the above formula have the following meanings.
[C]: C content in the steel sheet (mass%)
[C] _M : average C concentration in martensite (mass%)
The C concentration in martensite greatly affects the tensile strength. In order to obtain a desired tensile strength, the C concentration in martensite needs to satisfy the above equation (ii) in relation to the C content in the steel sheet.

  When the ratio of the C concentration in martensite to the C content in the steel sheet is less than 0.6, the amount of solid solution strengthening by C is insufficient and the tensile strength of the steel sheet decreases. The higher the C concentration in martensite, the higher the tensile strength, so the upper limit is not specified.

In the steel sheet according to this embodiment, the C content in the steel sheet is measured by the high frequency combustion method.
The average C concentration in martensite is determined by the following method using a Vickers hardness meter. First, the Vickers hardness of martensite identified by the above method is measured at 10 points with a test force of 1 kgf, and the average value is obtained. Then, the average C concentration in the martensite is calculated based on the average value of the Vickers hardness of the martensite and the following equations (iii) and (iv).
^{_{[C] M = {1573- (}} 1573 2 -4096 × A) 0.5} / 2048 ··· (iii)
However, A in the above formula (iii) is a value calculated by the following formula (iv), and meanings of symbols in the formula (iv) are as follows.
A = HV-14 × [Mn] -27 × [Si] -200 (iv)
HV: average value of Vickers hardness of martensite [Mn]: Mn content in steel sheet (mass%)
[Si]: Si content (mass%) in the steel sheet

(C) Mechanical Properties In the steel sheet according to the present embodiment, it is preferable that the yield stress is 800 MPa or more, the tensile strength is 1180 MPa or more, and the value of TS × EL is 18000 MPa ·% or more. The above values have high yield stress and tensile strength, and are excellent in strength-ductility balance, so that weight reduction of the vehicle body and improvement of collision resistance can be achieved at the same time.

(D) Plating Layer A plating layer may be formed on the surface of the high-strength steel sheet having the above-described chemical composition and steel structure according to the present invention for the purpose of improving corrosion resistance and the like to obtain a surface-treated steel sheet. The plated layer may be an electroplated layer or a hot-dip plated layer.

  Examples of the electroplating layer include electrogalvanizing and electroplating Zn-Ni alloy. The hot-dip galvanizing layer includes hot-dip galvanizing, alloy hot-dip galvanizing, hot-dip aluminum coating, hot-dip Zn-Al alloy plating, hot-dip Zn-Al-Mg alloy plating, hot-dip Zn-Al-Mg-Si alloy plating, and the like. It is illustrated. The coating amount is not particularly limited and may be the same as the conventional one. Further, it is possible to further enhance the corrosion resistance by performing an appropriate chemical conversion treatment (for example, application and drying of a silicate-based chromium-free chemical conversion treatment liquid) after plating.

(E) Manufacturing conditions The high-strength steel sheet according to the embodiment of the present invention can be obtained by a manufacturing method including the following steps, for example.

<Casting process>
The steel having the above-described chemical composition is melted by a known means and then made into a steel ingot by a continuous casting method, or a steel obtained by making a steel ingot by an arbitrary casting method and then performing slab rolling. To be cut off. In the continuous casting process, in order to suppress the generation of surface defects due to inclusions, it is preferable to generate an external additional flow such as electromagnetic stirring in the molten steel in the mold.

  The steel ingot or billet may be cooled and then reheated to be subjected to hot rolling, and the ingot in the high temperature state after continuous casting or the billet in the high temperature state after slabbing as it is. Alternatively, it may be subjected to hot rolling while keeping it warm or by performing auxiliary heating. In the present embodiment, such a steel ingot and a steel slab are collectively referred to as a "slab" as a material for hot rolling.

<Hot rolling process>
Slab heating temperature: 1350 ° C or lower The heating temperature of the slab used for hot rolling is 1350 ° C or lower from the viewpoint of suppressing scale loss. The heating temperature is preferably 1280 ° C or lower. The lower limit of the heating temperature of the slab used for hot rolling is not particularly limited, and may be a temperature at which hot rolling can be completed at Ar ₃ points or more, as described later.

Rolling completion temperature: Ar ₃ points to 1100 ° C
The hot rolling is completed in a temperature range of Ar ₃ point or higher in order to refine the metal structure of the hot rolled steel sheet by transforming austenite after the rolling is completed. When the rolling completion temperature (temperature after completion of the final rolling pass) is less than Ar ₃ point, ferrite transformation occurs during hot rolling, and a coarse metallographic structure expanded in the rolling direction is formed in the hot rolled steel sheet. As a result, coarse retained austenite is generated in the metal structure after annealing, and ductility is likely to deteriorate. Therefore, the hot rolling completion temperature is set to Ar ₃ point or higher. A hot rolled steel sheet is obtained by hot rolling the slab.

Here, in the present embodiment, the Ar ₃ point is calculated by the following formula (I).
_{Ar 3 = 622.2 + 149.4 × [} C] -24.2 × [Si] -13.7 × [Mn] ··· (I)
However, each [element symbol] in the formula represents the content (mass%) of each element contained in the steel.

  On the other hand, when the rolling completion temperature exceeds 1100 ° C., the metal structure of the hot-rolled steel sheet becomes coarse, and the Mn concentration in the retained austenite becomes insufficient after annealing and the yield ratio decreases. Therefore, the completion temperature of hot rolling is set to 1100 ° C or lower. The completion temperature of hot rolling is preferably 1050 ° C or lower.

Reduction ratio in the final rolling pass and the rolling pass immediately before the final rolling pass: 15 to 60%
The rolling reductions in the final rolling pass and the immediately preceding rolling pass are preferably 15 to 60%, respectively. By setting the rolling reduction in each of the final rolling pass and the preceding rolling pass to 15% or more, the recrystallized austenite grains are mainly refined, and the subsequent cooling gives an equiaxed fine martensite structure. It is easy to be affected. This makes it possible to obtain fine retained austenite after annealing.

  If the rolling reduction in the final rolling pass and / or the rolling pass immediately before the final rolling pass is less than 15%, recrystallization of austenite may be insufficient and ductility may deteriorate after annealing. The rolling reductions in the final rolling pass and the immediately preceding rolling pass are each preferably 20% or more, and more preferably 25% or more. On the other hand, the rolling reduction in the final rolling pass and the rolling pass immediately before the final rolling pass is set to 60% or less, respectively, from the viewpoint of suppressing the flatness of the steel sheet and the release of the introduced strain due to working heat. The rolling reductions in the final rolling pass and the immediately preceding rolling pass are preferably 50% or less.

Time from completion of rolling of one rolling pass before the last rolling pass to start of rolling of the final rolling pass (interpass time): Formula (v) below is satisfied: 0.002 / exp (−6080 / (T ₁ +273) ) ≦ t ₁ ≦ 2.0 (v)
Here, the meaning of each symbol is as follows.
t _1: from the previous rolling the completion of the final rolling path, the path between the time until the rolling start of the final rolling path (s)
T ₁ : Rolling completion temperature of the rolling pass immediately before the final rolling pass (° C.)

  By satisfying the above equation (v), recrystallization of austenite is promoted and grain growth of austenite is promoted during the pass 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. Is suppressed. Therefore, the recrystallized austenite grains during rolling can be refined. This makes it easier to obtain a steel structure suitable for having both high tensile strength and yield stress after annealing and excellent ductility.

  When the hot rolling includes rough rolling and finish rolling, the rough rolled material may be heated between the rough rolling and the finish rolling in order to complete the finish rolling at the above temperature. At this time, it is desirable to suppress the temperature variation over the entire length of the rough rolled material at the start of finish rolling to 140 ° C. or less by heating so that the rear end of the rough rolled material becomes higher in temperature than the front end. This improves the uniformity of product properties within the coil.

  The heating method of the rough rolled material may be performed by using a known means. For example, a solenoid type induction heating device is provided between the rough rolling mill and the finishing rolling device, and the heating temperature rise amount is controlled based on the temperature distribution in the longitudinal direction of the rough rolling material on the upstream side of the induction heating device. May be.

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

<Cooling process>
After hot rolling is finished, air cooling is performed for 1 to 10 seconds and then cooled to a coiling temperature at an average cooling rate of 10 ° C./second or more. After hot rolling, the hot rolled steel sheet obtained by the hot rolling step is finished. Air-cool for 1 to 10 seconds. This promotes recrystallization of austenite grains processed by hot rolling and suppresses austenite grain growth. If the air-cooling time after hot rolling is less than 1 second, recrystallization of austenite will be insufficient, and a coarse metal structure expanded in the rolling direction will be formed. As a result, coarse retained austenite is formed after the secondary annealing and the workability of the steel sheet deteriorates. On the other hand, when air-cooled for more than 10 seconds, the metallographic structure of the hot-rolled steel sheet becomes coarse, the Mn concentration in the retained austenite becomes insufficient after annealing, and the yield ratio decreases.

  Further, when the average cooling rate up to the coiling temperature after air cooling is less than 10 ° C./sec, pearlite is easily generated, and Mn distribution progresses between ferrite and pearlite during coiling, resulting in high strength and ductility. It becomes difficult to obtain a suitable tissue. The upper limit of the average cooling rate is not particularly specified, but if the cooling rate is excessively high, it may be difficult to control the plate shape, so it is preferably set to 300 ° C./second or less.

<Winding process>
Winding temperature: 550 ° C. or less As will be described later, the annealing (primary annealing) performed after hot rolling promotes the distribution of Mn among ferrite, austenite, and cementite. Then, by the subsequent annealing (secondary annealing), it becomes possible to control the steel structure to contain mainly martensite solid-solution strengthened by C and contain residual austenite stabilized by Mn concentration.

  In order to obtain the effect, the winding temperature after hot rolling is set to 550 ° C or lower. When the winding temperature exceeds 550 ° C., pearlite is likely to be formed, Mn distribution proceeds between ferrite and pearlite during winding, and it becomes difficult to obtain a structure suitable for high strength and ductility. The winding temperature is preferably less than 400 ° C, more preferably less than 300 ° C.

<Primary annealing process>
The hot rolling step, the cooling step, and the hot-rolled steel sheet that has undergone the winding step described above are annealed at a temperature of a two-phase region of ferrite and austenite or ferrite and cementite or a three-phase region of ferrite, austenite and cementite. To do. This annealing is called "primary annealing". Prior to the primary annealing, the hot rolled steel sheet may be descaled by pickling or the like. By promoting the distribution of Mn between ferrite, austenite and cementite by the primary annealing, it becomes easy to obtain a metal structure suitable for high strength and ductility by the secondary annealing performed thereafter. The primary annealing condition is a condition that satisfies the following range.

Annealing temperature: (Ac ₁ point −80 ° C.) to (Ac ₃ points −55 ° C.)
The annealing temperature (primary annealing temperature) is (Ac ₁ point −80 ° C.) to (Ac ₃ point −55 ° C.). By annealing at a temperature in this range, Mn is effectively concentrated in austenite, cementite, or both, and by the subsequent secondary annealing, the area ratio and average particle size of the retained austenite, and the residual austenite It is possible to control the Mn concentration within a desired range.

When the primary annealing temperature is less than (Ac ₁ point−80 ° C.), it becomes difficult to secure a desired amount of retained austenite after the secondary annealing, and the ductility may deteriorate. On the other hand, when the primary annealing temperature exceeds (Ac ₃ point −55 ° C.), coarse austenite with a low Mn concentration is likely to be generated after the secondary annealing, and it becomes difficult to obtain desired yield stress and ductility. Further, when the primary annealing temperature is further increased to Ac ₃ or more, coarse retained austenite is not generated, but it is difficult to concentrate Mn into retained austenite.

Here, in the present invention, Ac ₁ point and Ac ₃ point are calculated by the following formulas (II) and (III), respectively.
Ac ₁ = 631.3 + 235.4 × [C] + 10.5 × [Si] −9.4 × [Mn] (II)
^{_{Ac 3 = 781.7 + 3.7 × [}} C] 0.5 -7.2 × [Si] -9.7 × [Mn] ··· (III)
However, each [element symbol] in the formula represents the content (mass%) of each element contained in the steel.

Holding time at the annealing temperature of primary annealing: Satisfying the following expression (vi) The holding time of the primary annealing preferably satisfies the following expression (vi) in relation to the above-mentioned primary annealing temperature.
2.3 × 10 ⁻⁸ × exp {23500 / (T ₂ +273)} ≦ t ₂ ≦ 4.0 × 10 ⁵ (vi)
However, the meaning of each symbol in the above formula (vi) is as follows.
t ₂ : holding time (seconds) at the primary annealing temperature
T _2: primary annealing temperature (℃)

Since the diffusion rate of Mn is very slow, the Mn distribution from ferrite to austenite or cementite is promoted by maintaining the above-mentioned primary annealing temperature for a predetermined time. By subjecting this annealed steel sheet to the secondary annealing as described later, it becomes possible to control the Mn concentration in the retained austenite within a desired range. When the retention time is less than the value calculated by [2.3 × 10 ⁻⁸ × exp {23500 / (T ₂ +273)}] on the left side of the formula (vi), the distribution of Mn becomes insufficient and the residual austenite The Mn concentration decreases, the yield strength decreases, and the ductility easily deteriorates. On the other hand, the Mn concentration ratio approaches an equilibrium state when it is held for a long time, so the effect is saturated even if annealing is performed for more than 4.0 × 10 ⁵ seconds, and the heat treatment cost only increases.

<Cold rolling process>
Cold rolling ratio: 30% or more and less than 80% The annealed steel sheet after the primary annealing step may be cold rolled according to a conventional method to obtain a cold rolled steel sheet. Further, the annealed steel sheet may be descaled by pickling or the like before cold rolling. When performing cold rolling, in order to promote recrystallization during secondary annealing, uniformize the metal structure after annealing, and improve ductility, the cold rolling reduction (total reduction in cold rolling (cumulative reduction)) ) Is preferably 30% or more. More preferably, the cold pressing rate is 40% or more. As a result, the metal structure after the secondary annealing is further refined, the texture is improved, and the ductility is improved. From this point of view, the cold pressing rate is more preferably more than 50%, and particularly preferably more than 60%. On the other hand, if the cold pressing rate is too high, the rolling load increases and rolling becomes difficult. Therefore, the cold pressing rate is preferably less than 80%, more preferably less than 70%.
When cold rolling is performed after the secondary annealing, cold rolling is performed on the steel sheet containing martensite that has been solid solution strengthened with C. In this case, the yield point is likely to appear. When the yield point appears, uneven deformation is likely to occur, and surface irregularities after press molding are likely to occur, which is not preferable.

<Secondary annealing process>
The hot rolled steel sheet (annealed steel sheet) after the above-described primary annealing step or the cold rolled steel sheet obtained in the cold rolling step is further annealed. This annealing is called "secondary annealing". By performing the secondary annealing, it becomes easy to obtain a metal structure mainly containing martensite strengthened by C as a solid solution and containing Mn-enriched retained austenite. This secondary annealing condition preferably satisfies the following range.

Annealing temperature: (Ac ₃ points + 30 ° C.) or higher and less than (Ac ₃ points + 200 ° C.) Annealing temperature (secondary annealing temperature) is (Ac ₃ points + 30 ° C.) or higher. This is because the desired volume fraction of retained austenite is generated, and therefore, as shown in FIG. 1, C existing in the cementite and / or the retained austenite produced by the primary annealing is added to the entire steel structure (especially Mn concentration). This is because the main phase is martensite that is solid solution strengthened by C and the second phase contains a retained austenite. However, if the secondary annealing temperature becomes too high, the austenite during heating becomes excessively coarse and the metal structure after cooling becomes coarse, and the Mn distribution promoted in the primary annealing decreases due to diffusion, and the ductility decreases. It easily deteriorates. Therefore, the secondary annealing temperature is preferably lower than (Ac ₃ point + 200 ° C). The secondary annealing temperature is more preferably less than (Ac ₃ points + 150 ° C.).

Holding time at the annealing temperature of the secondary annealing: less than 150 seconds If the holding time at the secondary annealing temperature is too long, Mn distributed in the primary annealing is diffused, yield stress decreases and ductility deteriorates. Easier to do. Therefore, the holding time is preferably less than 150 seconds and less than 120 seconds. From the viewpoint of suppressing the diffusion of Mn distributed in the primary annealing, the lower limit of the holding time at the secondary annealing temperature does not need to be particularly limited and may be more than 0 seconds.

  In the heating process in the secondary annealing, in order to suppress the diffusion of Mn distributed in the primary annealing, the average heating rate is preferably 5 ° C./sec or more, more preferably 20 ° C./sec or more, and 100 ° C. / Sec or more is more preferable.

In order to obtain a metal structure having martensite as a main phase, in the cooling process after soaking at the secondary annealing temperature, the average cooling rate in the temperature range of Ac ₃ point to 500 ° C. is 15 ° C./sec or more and 500 Cool to below ℃. When the average cooling rate in the temperature range of Ac ₃ points to 500 ° C. is less than 15 ° C./sec, ferrite is likely to be formed, and it becomes difficult to obtain a desired tensile strength, and precipitation of cementite causes the formation of martensite in martensite. The average C concentration [C] _{M of} [C] _M decreases, and [C] _M / [C] becomes less than 0.6. Since the area ratio of martensite increases as the cooling rate increases, the above average cooling rate is preferably higher than 30 ° C / sec, and more preferably higher than 50 ° C / sec.

On the other hand, if the cooling rate of Ac ₃ points to 500 ° C. is too fast, the shape of the steel sheet is impaired, so the average cooling rate in the temperature range of Ac ₃ points to 500 ° C. is preferably 200 ° C./second or less, It is more preferably less than 150 ° C / sec, and even more preferably less than 130 ° C / sec. Further, when the cooling stop temperature exceeds 500 ° C., cementite is formed, which makes it difficult to secure desired tensile strength and ductility, and the average C concentration [C] _{M in} martensite decreases, C] _M / [C] becomes less than 0.6. Therefore, the cooling stop temperature is set to 500 ° C. or lower.

<Plating process>
When manufacturing an electroplated steel sheet, the annealed steel sheet produced by the above-mentioned method is subjected to well-known pretreatment for surface cleaning and adjustment, if necessary, and then electroplated according to a conventional method. What is necessary is just that, and the chemical composition and the adhesion amount of the plating film are not limited. Examples of the type of electroplating include electrogalvanizing and electroplating Zn-Ni alloy.

  In the case of producing a hot-dip plated steel sheet, in the cooling process of the secondary annealing described above, after cooling to 450 ° C. or lower, the steel sheet is heated as necessary and immersed in a plating bath to perform hot dipping. Further, the alloying treatment may be performed by reheating after hot dipping. The chemical composition and the adhesion amount of the plating film are not limited. Examples of the type of hot dip galvanizing include hot dip galvanizing, hot dip galvanizing, hot dip aluminum coating, hot dip Zn-Al alloy plating, hot dip Zn-Al-Mg alloy plating, hot dip Zn-Al-Mg-Si alloy plating, and the like. To be done.

  The plated steel sheet may be subjected to an appropriate chemical conversion treatment after plating in order to further improve its corrosion resistance. The chemical conversion treatment is preferably carried out using a non-chromium type chemical conversion treatment liquid (for example, silicate series, phosphate series, etc.) instead of the conventional chromate treatment.

  The annealed steel plate and the plated steel plate thus obtained may be temper-rolled according to a conventional method. However, if the elongation percentage of the temper rolling is high, the ductility is deteriorated. Therefore, the elongation percentage of the temper rolling is preferably 1.0% or less, more preferably 0.5% or less.

  Hereinafter, the present invention will be described more specifically by way of examples, but the present invention is not limited to these examples.

  A 180 kg steel ingot having the chemical composition shown in Table 1 was melted in a high-frequency vacuum melting furnace and was hot forged into a steel piece having a thickness of 30 mm. This steel slab was hot-rolled in a small test tandem mill under the conditions shown in Table 2-1 to obtain a hot-rolled steel plate having a plate thickness of 2 to 4 mm. This hot-rolled steel sheet was heat-treated at various primary annealing temperatures shown in Table 2-2 for a predetermined time and then cooled to room temperature.

  Then, using the continuous annealing simulator, the obtained primary annealed steel sheets were heated to various secondary annealing temperatures shown in Table 2-2 and held for a predetermined time. Then, it cooled by various conditions shown in Table 2-2, and the annealed steel plate was obtained.

  Some of the above primary annealed steel sheets were subjected to pickling and cold rolling and then annealed.

With respect to the obtained steel sheet, a cross section perpendicular to the rolling direction of the steel sheet was mirror-polished, and a repeller-corroded sample was used. Was photographed with an optical microscope, and the total area ratio of retained austenite and martensite was calculated by binarization processing using commercially available image processing software "Image-Pro". In the binarization process, a luminance histogram was created, and regions of luminance exceeding the luminance of the peak appearing on the dark side up to 255 were defined as retained austenite and martensite, and separated from the other regions.

  Then, the area ratio of retained austenite was measured. First, a sample for measurement was prepared by reducing the thickness from the surface of the steel plate to 1/4 depth of the plate thickness by chamfering and chemical polishing. Subsequently, X-ray diffraction measurement (RINT2500HL, manufactured by Rigaku Corporation) was performed on the above sample using a Co ray source, and the (110) plane, (200) plane, and (211) plane of the ferrite and the retained austenite were obtained. The integrated intensity of the (111) plane, the (200) plane, and the (220) plane of was determined. Then, the volume ratio of the retained austenite was obtained by the strength averaging method, and the value was used as the area ratio of the retained austenite.

  A value obtained by subtracting the retained austenite area ratio obtained by X-ray diffraction measurement from the total area ratio of the retained austenite and martensite obtained above was defined as the martensite area ratio. The value obtained by subtracting the total area ratio of martensite and retained austenite obtained above from 100% was taken as the area ratio of the residual structure.

  Further, the presence of austenite was confirmed by measuring and analyzing the crystal orientation by the EBSP method from the surface of the steel sheet to a position at a depth of 1/4 of the plate thickness using a sample prepared by electrolytic polishing after mirror polishing. Then, a thin piece for TEM observation was taken from the position where the presence of austenite was confirmed, and the vicinity of the hole opened by the twin jet electrolysis method was observed by TEM. At that time, the observation magnification of TEM was set to 50,000 times, 10 austenite grain sizes were measured from the dark field image formed from the electron diffraction pattern of austenite, and the average value thereof was taken as the average grain size of the retained austenite. By these measurements, the average particle size of retained austenite was obtained.

  Then, with respect to the 50 μm × 50 μm region of the sample used for the X-ray diffraction measurement, that is, the sample which was chamfered from the surface of the steel plate to a depth of ¼ of the plate thickness, the Mn concentration was measured by FE-EPMA at intervals of 0.1 μm. Mapping analysis was performed. Next, residual austenite in the metal structure in the same region was identified by EBSP analysis. Then, with respect to the identified retained austenite, the average of the measured values of Mn concentration at any 10 points in one grain is defined as the Mn concentration in the one grain, and the average value of Mn concentration measured in each of 10 grains. Was calculated and the value was defined as the average Mn concentration of retained austenite.

  Furthermore, the Vickers hardness of the martensite identified by the above method was measured at 10 points with a test force of 1 kgf, the average value was calculated, and the martensite was calculated based on the equations (iii) and (iv). The average C concentration in the was calculated.

  Tensile properties were evaluated as mechanical properties. As for the tensile properties, a tensile test was carried out according to JIS Z 2241: 2011, and the yield stress (YS), tensile strength (TS) and total elongation (El) were measured. Specifically, the No. 5 test piece described in JIS Z 2241: 2011 was sampled from the central portion of the plate thickness of the steel sheet and subjected to a tensile test. At this time, the rolling direction of the steel sheet was set to be the longitudinal direction of the tensile test piece.

  The metallographic structure and mechanical properties of the obtained steel sheet are summarized in Table 3-1 and Table 3-2. In the present invention, a steel sheet having a yield stress of 800 MPa or more, a tensile strength of 1180 MPa or more, and a TS × EL value of 18000 MPa ·% or more is provided with high yield stress and tensile strength and strength. -It was judged that the ductility balance was excellent.

  With reference to Tables 1 to 3-2, Test Nos. 1 to 20 and 43, which are examples of the present invention, have high yield stress and tensile strength, and also have an excellent strength-ductility balance. On the other hand, in Test Nos. 21 to 42, which are comparative examples that do not satisfy the requirements of the present invention, at least one of YS, TS, and TS × El characteristics is inferior.

  According to the present invention, it is possible to obtain a steel sheet having high tensile strength and high yield stress and also having excellent ductility. Therefore, the high-strength steel sheet according to the present invention is suitable for use as a material for automobile members, machine structural members, building members and the like.

Claims (8)

The chemical composition is% by mass,
C: 0.08 to 0.45%,
Si: 0.05 to 3.0%,
Mn: 3.5-10.0%,
P: 0.10% or less,
S: 0.030% or less,
sol. Al: 0.01 to 2.0%,
N: 0.010% or less,
Ti: 0 to 0.20%,
Nb: 0 to 0.10%,
V: 0 to 0.50%,
Cr: 0% or more and less than 1.0%,
Mo: 0 to 0.50%,
Ni: 0 to 1.0%,
B: 0 to 0.0050%,
Ca: 0 to 0.020%,
Mg: 0 to 0.020%,
REM: 0 to 0.020%,
Cu: 0 to 1.0%,
Bi: 0 to 0.020%,
The balance: Fe and impurities,
The metal structure is the area%
Martensite: 70.0-95.0%,
Retained austenite: 5.0-30.0%,
Remainder: 0 to 10.0%, and
Average particle size of retained austenite: 0.2 to 2.0 μm,
The average Mn concentration in the retained austenite satisfies the following formula (i), and the C content and the average C concentration in the martensite satisfy the formula (ii).
High strength steel plate.
7.0 ≦ [Mn] _γ ≦ 20.0 (i)
0.6 ≦ [C] _M / [C] (ii)
However, the symbols in the above formula have the following meanings.
[Mn] _γ : average Mn concentration (mass%) in the retained austenite
[C]: C content in the steel sheet (% by mass)
[C] _M : the average C concentration (mass%) in the martensite The chemical composition is% by mass,
Ti: 0.005 to 0.20%,
Nb: 0.002-0.10%, and V: 0.005-0.50%,
Containing one or more selected from,
The high-strength steel plate according to claim 1. The chemical composition is% by mass,
Cr: 0.05% or more, less than 1.0%,
Mo: 0.02-0.50%,
Ni: 0.05 to 1.0%, and B: 0.0002 to 0.0050%,
Containing one or more selected from,
The high-strength steel plate according to claim 1 or 2. The chemical composition is% by mass,
Ca: 0.0005 to 0.020%,
Mg: 0.0005 to 0.020%, and REM: 0.0005 to 0.020%,
Containing one or more selected from,
The high-strength steel sheet according to any one of claims 1 to 3. The chemical composition is% by mass,
Cu: 0.05-1.0%
Containing,
The high-strength steel plate according to any one of claims 1 to 4. The chemical composition is% by mass,
Bi: 0.0005 to 0.020%
Containing,
The high-strength steel sheet according to any one of claims 1 to 5. A method for manufacturing the high-strength steel sheet according to any one of claims 1 to 6,
A hot rolling step, a cooling step, a winding step, a primary annealing step, an optional cold rolling step, and two for the slab having the chemical composition according to any one of claims 1 to 6. A method for manufacturing a high-strength steel sheet which sequentially performs the following annealing steps,
In the hot rolling step,
The rolling reductions in the final rolling pass and the rolling pass immediately before the final rolling pass are each set to 15 to 60%,
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 satisfies the following expression (v),
The rolling completion temperature of the final rolling pass is set to a temperature range of Ar ₃ points to 1100 ° C.,
In the cooling step, the hot rolled steel sheet after the hot rolling step is air-cooled for 1 to 10 seconds and then cooled at an average cooling rate of 10 ° C./second or more,
In the winding step,
Winding in the temperature range below 550 ℃,
In the primary annealing step,
The hot rolled steel sheet is held so that the annealing temperature is in the temperature range of (Ac ₁ point-80 ° C) to (Ac ₃ point -55 ° C) and the holding time satisfies the following formula (vi),
In the secondary annealing step,
After the annealing temperature of the hot-rolled steel sheet is maintained at a temperature range of (Ac ₃ points + 30 ° C.) or more and less than (Ac ₃ points + 200 ° C.) and a holding time of less than 150 seconds, Ac ₃ points Cooling to a temperature of 500 ° C. or lower so that the average cooling rate in the temperature range of ˜500 ° C. is 15 ° C./sec or higher,
Manufacturing method of high strength steel sheet.
0.002 / exp (−6080 / (T ₁ +273)) ≦ t ₁ ≦ 2.0 (v)
2.3 × 10 ⁻⁸ × exp {23500 / (T ₂ +273)} ≦ t ₂ ≦ 4.0 × 10 ⁵ (vi)
However, the meaning of each symbol in the above formula is as follows.
t ₁ : time between passes (seconds) from completion of rolling immediately before the final rolling pass to start of rolling of the final rolling pass
T ₁ : Rolling completion temperature of the rolling pass immediately before the final rolling pass (° C.)
t ₂ : the holding time (seconds) at the annealing temperature of the primary annealing
T ₂ : the annealing temperature (° C.) of the primary annealing In the cold rolling step, the total rolling reduction is 30% or more and less than 80%,
The method for manufacturing the high-strength steel sheet according to claim 7.