TWI671410B - Steel sheet and manufacturing method thereof - Google Patents

Abstract

This steel plate has a predetermined chemical composition, and the steel structure inside the steel plate contains, by volume fraction, soft fertilized iron: 0% to 30%, residual Vostian iron: 3% to 40%, and fresh Asada loose iron: 0% ~ 30%, the total of bolai iron and ciming carbon iron: 0% ~ 10%, and the remaining part contains hard fertilized iron; the residual Vostian iron with an aspect ratio of 2.0 or more accounts for one of the total remaining Vostian irons The number ratio is 50% or more. There is a soft layer with a thickness of 1 to 100 μm in the thickness direction from the surface. The volume fraction of the grains with an aspect ratio of 3.0 or more in the ferrous iron contained in the soft layer is 50. % Or more, the volume fraction of the residual Vosstian iron in the soft layer is 80% or less of the volume fraction of the residual Vosstian iron in the steel plate, and is greater than 0.2 μm and less than 10.0 μm from the surface The peak of the luminous intensity showing the wavelength of Si mentioned above appears in the range.

Description

Steel plate and manufacturing method thereof

The invention relates to a steel plate and a manufacturing method thereof.

Awareness of environmental issues has increased in recent years. Therefore, it is important to reduce the weight of vehicle bodies in the automotive industry for the purpose of improving fuel consumption. On the other hand, there is also a need to increase the strength of the car body to ensure collision safety. In order to balance the weight reduction and collision safety of the car body, a high-strength steel plate may be used as the car body material. However, the higher the strength of the steel sheet, the more difficult the press forming is. This is because generally, the higher the strength of the steel sheet, the lower the formability such as ductility, bendability, and hole expandability.

Moreover, like an ultra-high-strength steel sheet with a tensile strength greater than 980 MPa, not only the formability, but also the problem of hydrogen embrittlement and cracking of the steel sheet must be solved. Hydrogen embrittlement cracking is a phenomenon in which a steel component that exhibits high stress under use is suddenly damaged due to the intrusion of hydrogen into the steel from the environment. This phenomenon is also called delayed damage from the aspect of the damage. It is generally known that as the tensile strength of a steel sheet is increased, hydrogen embrittlement cracking of the steel sheet is more likely to occur. This is because the higher the tensile strength of the steel sheet, the more the stress remaining on the steel sheet after the part is formed. The sensitivity to this hydrogen embrittlement cracking (delayed failure) is called hydrogen embrittlement resistance. When an automobile steel sheet is used, hydrogen embrittlement cracking is particularly likely to occur in a bent portion where a large plastic strain is applied. Therefore, when a high-strength steel sheet is used for an automobile component, it is required not only to improve the formability such as ductility, bendability, and hole expandability, but also to improve the hydrogen embrittlement resistance of the bent portion.

Conventionally, a high-strength steel sheet with high press workability has been known as a DP steel (Dual Phase steel) having a ferrous iron phase and a Mata loose iron phase (for example, refer to Patent Document 1). DP steel has excellent ductility. However, the hard phase of DP steel will be the starting point of void formation, so the hole expandability and bendability are poor.

Further, a high-strength steel sheet having good ductility is known as a TRIP steel in which a Vostian iron phase remains in a steel structure to utilize a TRIP (transformation-induced plasticity) effect (for example, refer to Patent Documents 2 and 3). The ductility of TRIP steel is higher than that of DP steel. However, TRIP steel has poor hole expandability. In addition, TRIP steel must contain a large amount of alloys such as Si because it retains Vosstian iron. Therefore, the TRIP steel has poor plating adhesion and chemical conversion treatment.

In addition, Patent Document 4 describes a high-strength steel sheet having excellent hole expandability, the microstructure of which contains 70% or more of toughened iron or toughened iron in terms of area ratio, and the tensile strength is 800 MPa or more. Patent Document 5 describes a high-strength steel sheet having a tensile strength of 800 MPa or more and excellent hole expandability and ductility. The microstructure of the high-strength steel sheet is based on toughened iron or toughened iron, and the second phase is Vostian iron, and the remainder is fat iron or Asada loose iron.

Non-patent document 1 discloses that by using a two-annealing method in which a steel sheet is twice-annealed, the elongation and hole expandability of the steel sheet can be improved. However, it has been difficult to improve the ductility and hole expandability of conventional high-strength steel plates and the hydrogen embrittlement resistance of the bent portion at the same time.

Prior Art Literature Patent Literature Patent Literature 1: Japanese Patent Laid-Open No. 6-128688 Patent Literature 2: Japanese Patent Laid-Open No. 2006-274418 Patent Literature 3: Japanese Patent Laid-Open No. 2008-56993 Patent Literature 4: Japanese Patent Japanese Patent Application Laid-Open No. 2003-193194 Patent Literature 5: Non-Patent Literature of Japanese Patent Laid-Open No. 2003-193193

Non-Patent Document 1: K. Sugimoto et al ,, ISIJ International, Vol. 33 (1993), No. 7, pp775-782

SUMMARY OF THE INVENTION Problems to be Solved by the Invention The present invention has been made in view of the foregoing circumstances. An object of the present invention is to provide a high-strength steel sheet having excellent formability, fatigue characteristics, and hydrogen embrittlement resistance in a bent portion, and a method for manufacturing the same.

Means for Solving the Problems The present inventors have been actively reviewing the problems repeatedly to solve the problems. It was found that the hot-rolled steel sheet or cold-rolled steel sheet with a predetermined chemical composition was subjected to two different heat treatments (annealing) under different conditions, thereby effectively forming the steel sheet into a predetermined steel structure and effectively forming a steel structure having a predetermined thickness and steel structure. Of the surface. Furthermore, it was found that by forming an internal oxide layer containing a Si oxide at a predetermined depth, it is possible to ensure the plating adhesion and chemical conversion treatability sought for automotive steel sheets.

Specifically, the metal structure in the steel plate and the surface layer of the steel plate is made into a structure mainly composed of a lath-like structure such as Asada loose iron by the first heat treatment. Then, in the second heat treatment, the maximum heating temperature was set to a two-phase region of α (fertilized iron) and γ (vostian iron), and decarburization treatment was performed at the same time. As a result, the steel plate obtained after the two heat treatments has a steel structure with needle-shaped residual Vostian iron dispersed therein, and the steel structure of the surface layer is mainly composed of lath-shaped ferrous iron with a large aspect ratio and has a predetermined thickness. Furthermore, it was found that the steel sheet described above is excellent in all of the characteristics including excellent formability, fatigue characteristics, and hydrogen embrittlement resistance of the bent portion.

In the first and second heat treatments described above, the alloy elements such as Si contained in the steel can be prevented from oxidizing outside the steel plate, and an internal oxide layer containing a Si oxide can be formed at a predetermined depth, thereby obtaining an excellent chemical conversion treatment. Sex. In addition, when a plating layer is formed on the surface of a steel sheet, excellent plating adhesion can be obtained. The present invention has been completed based on the above findings. The gist of the present invention is as follows.

(1) A steel sheet according to one aspect of the present invention has the following chemical composition: contained in mass%: C: 0.050% to 0.500%, Si: 0.01% to 3.00%, Mn: 0.50% to 5.00%, and P: 0.0001 % ~ 0.1000%, S: 0.0001% ~ 0.0100%, Al: 0.001% ~ 2.500%, N: 0.0001% ~ 0.0100%, O: 0.0001% ~ 0.0100%, Ti: 0% ~ 0.300%, V: 0% ~ 1.00%, Nb: 0% to 0.100%, Cr: 0% to 2.00%, Ni: 0% to 2.00%, Cu: 0% to 2.00%, Co: 0% to 2.00%, Mo: 0% to 1.00% , W: 0% ~ 1.00%, B: 0% ~ 0.0100%, Sn: 0% ~ 1.00%, Sb: 0% ~ 1.00%, Ca: 0% ~ 0.0100%, Mg: 0% ~ 0.0100%, Ce : 0% ~ 0.0100%, Zr: 0% ~ 0.0100%, La: 0% ~ 0.0100%, Hf: 0% ~ 0.0100%, Bi: 0% ~ 0.0100%, and REM: 0% ~ 0.0100%, and the rest The part is composed of Fe and impurities; the steel structure in the range of 1/8 thickness to 3/8 thickness centered on the 1/4 thickness position from the surface contains, as a volume fraction, soft iron grains: 0% ~ 30%, residual Vostian iron: 3% to 40%, fresh Asada loose iron: 0% to 30%, the total of bolai iron and ciming carbon iron: 0% to 10%, and the remaining part contains hard fertilizer Iron; Residual Voss of aspect ratio of 2.0 or more in the aforementioned range of 1/8 thickness to 3/8 thickness The ratio of the iron to the total residual Vostian iron is 50% or more. When a region having a hardness of 80% or less of the hardness in the aforementioned range of 1/8 thickness to 3/8 thickness is defined as a soft layer, From the surface, a soft layer with a thickness of 1 to 100 μm exists in the thickness direction. Among the ferrous iron contained in the soft layer, the volume fraction of the crystal grains with an aspect ratio of 3.0 or more is 50% or more, and the remaining in the soft layer The volume fraction of Vosstian iron is less than 80% of the volume fraction of residual Vosstian iron in the aforementioned range of 1/8 thickness to 3/8 thickness, and it is higher from the surface in the direction of the plate thickness. When the frequency glow photodischarge analysis method shows the emission intensity of the wavelength of Si, a peak of the emission intensity of the wavelength of the above-mentioned Si appears in the range from 0.2 μm to 10.0 μm from the surface.

(2) The steel sheet according to (1) above, wherein the aforementioned chemical composition contains one or more of the following elements: Ti: 0.001% to 0.300%, V: 0.001% to 1.00%, and Nb: 0.001% to 0.100% .

(3) The steel sheet according to (1) or (2) above, wherein the aforementioned chemical composition contains one or more of the following elements: Cr: 0.001% to 2.00%, Ni: 0.001% to 2.00%, and Cu: 0.001% ~ 2.00%, Co: 0.001% ~ 2.00%, Mo: 0.001% ~ 1.00%, W: 0.001% ~ 1.00%, and B: 0.0001% ~ 0.0100%.

(4) The steel sheet according to any one of (1) to (3) above, wherein the aforementioned chemical composition contains one or two of the following elements: Sn: 0.001% to 1.00%, and Sb: 0.001% to 1.00%.

(5) The steel sheet according to any one of (1) to (4) above, wherein the aforementioned chemical composition contains one or more of the following elements: Ca: 0.0001% to 0.0100%, Mg: 0.0001% to 0.0100%, Ce: 0.0001% to 0.0100%, Zr: 0.0001% to 0.0100%, La: 0.0001% to 0.0100%, Hf: 0.0001% to 0.0100%, Bi: 0.0001% to 0.0100%, and REM: 0.0001% to 0.0100%.

(6) The steel sheet according to any one of (1) to (5), wherein the chemical composition satisfies the following formula (i). Si + 0.1 × Mn + 0.6 × Al ≧ 0.35 (i) (Si, Mn, and Al in the formula (i) represent the content of each element by mass%).

(7) The steel sheet according to any one of (1) to (6) above, which has a hot-dip galvanized layer or an electro-galvanized layer on the surface.

(8) A method for manufacturing a steel sheet according to another aspect of the present invention is a method for manufacturing a steel sheet as described in any one of (1) to (6) above. After the first heat treatment described in (a) to (e), a second heat treatment satisfying the following (A) to (E) is performed, and the hot-rolled steel sheet system will have any one of the above (1) to (6). A steel billet having a chemical composition is prepared by hot rolling and pickling, and the cold-rolled steel sheet is obtained by cold-rolling the hot-rolled steel sheet. (a) A gas environment containing H2 of 0.1% by volume or more and satisfying the following formula (ii) during the period from 650 ° C to the maximum heating temperature. (b) Hold at the highest heating temperature of Ac3-30 ℃ ~ 1000 ℃ for 1 second to 1000 seconds. (c) Heating is performed such that the average heating rate in the temperature range from 650 ° C to the maximum heating temperature is 0.5 ° C / second to 500 ° C / second. (d) After maintaining at the maximum heating temperature, cooling is performed so that the average cooling rate in a temperature range from 700 ° C to Ms is 5 ° C / second or more. (e) The cooling at an average cooling rate of 5 ° C / sec or more is performed until the cooling stop temperature is lower than Ms. (A) A gas environment where H2 is 0.1 vol% or more and O2 is 0.020 vol% or less and the log (PH2O / PH2) satisfies the following formula (iii) during the period up to the maximum heating temperature of 650 ° C. (B) Hold at the highest heating temperature of Ac1 + 25 ℃ ~ Ac3-10 ℃ for 1 second to 1000 seconds. (C) Heating is performed such that the average heating rate from 650 ° C to the maximum heating temperature is 0.5 ° C / second to 500 ° C / second. (D) Cooling is performed so that the average cooling rate in a temperature range from 700 to 600 ° C is 3 ° C / second or more. (E) After cooling at an average cooling rate of 3 ° C / sec or more, it is held at 300 ° C to 480 ° C for 10 seconds or more. log (PH2O / PH2) <-1.1 ・ ・ ((ii) -1.1 ≦ log (PH2O / PH2) ≦ -0.07 ... (iii) (In formula (ii) and formula (iii), PH2O represents water vapor content Pressure, PH2 stands for hydrogen partial pressure).

(9) The method for manufacturing a steel sheet as described in (8) above, in which a galvanizing treatment is performed in a stage subsequent to the cooling process in (D).

ADVANTAGE OF THE INVENTION According to the said aspect of this invention, it is possible to provide the outstanding high ductility and hole expansion property, the chemical conversion treatment property, the plating adhesion, and the fatigue characteristic and hydrogen embrittlement resistance of a bending process part are excellent. Strength steel plate and manufacturing method thereof.

"Steel Plate" for Implementing the Invention A steel plate (a steel plate of the present embodiment) according to an embodiment of the present invention will be described in detail below. First, the chemical composition of the steel sheet of this embodiment will be described. In the following description, [%] indicating an element content means [mass%].

"C: 0.050 ~ 0.500%" C is an element that can significantly increase the strength of the steel sheet. In addition, C stabilizes Vosstian Iron, so it is necessary to obtain the remaining Vosstian Iron which contributes to the improvement of ductility. Therefore, C can effectively balance strength and formability. If the C content is less than 0.050%, it is impossible to obtain the residual Vossian iron sufficiently, and it is difficult to ensure sufficient strength and formability. Therefore, the C content is set to be above 0.050%. In order to further improve the strength and formability, the C content should be more than 0.075%, and more preferably 0.100% or more. On the other hand, if the C content is more than 0.500%, the weldability is significantly deteriorated. Therefore, the C content is set to 0.500% or less. From the viewpoint of point fusion properties, the C content is preferably 0.350% or less, and more preferably 0.250% or less.

"Si: 0.01 to 3.00%" Si is an element that suppresses the formation of iron-based carbides in the steel sheet, stabilizes the residual Vostian iron, and improves the strength and formability. When the Si content is less than 0.01%, a large amount of coarse iron-based carbides are generated, which deteriorates the strength and formability. Therefore, the Si content is set to 0.01% or more. From this viewpoint, the lower limit of Si is preferably 0.10% or more, and more preferably 0.25% or more. On the other hand, Si is an element that embrittles steel. If the Si content is more than 3.00%, the hole expandability of the steel sheet will be insufficient. In addition, if the Si content is more than 3.00%, problems such as cracking of the cast steel billet may easily occur. Therefore, the Si content is set to 3.00% or less. In addition, Si impairs the impact resistance of the steel sheet. Therefore, the Si content should be 2.50% or less, and preferably 2.00% or less.

"Mn: 0.50 to 5.00%" Mn is included because it can improve the hardenability of the steel sheet and increase the strength. If the Mn content is less than 0.50%, a soft structure is formed in a large amount during cooling after annealing, and it is difficult to ensure a sufficiently high tensile strength. Therefore, the Mn content must be set to 0.50% or more. In order to further increase the strength, the Mn content is preferably 0.80% or more, and more preferably 1.00% or more. On the other hand, if the Mn content is more than 5.00%, the elongation and hole expandability of the steel sheet will be insufficient. In addition, if the Mn content is more than 5.00%, a thick Mn-concentrated portion will be formed in the central portion of the steel plate thickness, which will easily cause embrittlement, and easily cause problems such as cracking of the cast steel embryo. Therefore, the Mn content is set to 5.00% or less. In addition, if the Mn content is increased, the spot weldability is also deteriorated, so the Mn content is preferably 3.50% or less, and more preferably 3.00% or less.

"P: 0.0001 ~ 0.1000%" P is an element that makes the steel brittle. If the P content is more than 0.1000%, the elongation and hole expandability of the steel sheet are insufficient. In addition, if the P content is more than 0.1000%, problems such as cracking of the cast steel billet may easily occur. Therefore, the P content is set to 0.1000% or less. In addition, P is an element that embrittles the molten portion generated by spot welding. In order to obtain sufficient welded joint strength, the P content should preferably be 0.0400% or less, and more preferably 0.0200% or less. On the other hand, setting the P content below 0.0001% will accompany a significant increase in manufacturing costs. Therefore, the P content is set to be 0.0001% or more. And the P content should be set to 0.0010% or more.

"S: 0.0001 to 0.0100%" S is an element that forms a large MnS by connecting with Mn, and reduces formability such as ductility, hole expandability (elongation and flangeability), and bendability. Therefore, the S content is set to 0.0100% or less. In addition, S deteriorates the spot weldability. Therefore, the S content should be set to 0.0070% or less, and more preferably 0.0050% or less. On the other hand, when the S content is set to less than 0.0001%, a significant increase in manufacturing costs is accompanied. Therefore, let the S content be 0.0001% or more. The S content should preferably be set to 0.0003% or more, and more preferably 0.0006% or more.

"Al: 0.001 to 2.500%" Al is an element that embrittles steel. If the Al content is more than 2.500%, problems such as cracking of the cast steel billet will easily occur. Therefore, the Al content is set to 2.500% or less. In addition, if the Al content is increased, the spot weldability is deteriorated. Therefore, the Al content should preferably be 2.000% or less, and more preferably 1.500% or less. On the other hand, although the lower limit of the Al content is not particularly limited, the effect can be obtained, but Al is an impurity present in a trace amount in the raw material, and if the content is less than 0.001%, the manufacturing cost will increase significantly. Therefore, the Al content is set to 0.001% or more. Al is also an element that can be effectively used as a deoxidizing material, so in order to fully obtain the deoxidizing effect, it is desirable to set the Al content to be above 0.010%. In addition, since Al is an element that suppresses the generation of coarse carbides, it may also be contained for the purpose of stabilizing the residual Vosted iron. In order to stabilize the residual Wastfield iron, the Al content should preferably be set to 0.100% or more, and more preferably 0.250% or more.

"N: 0.0001 to 0.0100%" N forms coarse nitrides and deteriorates ductility, hole expandability (elongation, flangeability), and bendability. Therefore, its content must be suppressed. If the N content is more than 0.0100%, the formability is significantly deteriorated. Therefore, the N content is set to 0.0100% or less. In addition, N may cause porosity during welding, so its content should be small. The N content is preferably 0.0075% or less, and more preferably 0.0060% or less. Although the lower limit of the N content is not particularly limited, although an effect can be obtained, if the N content is less than 0.0001%, a significant increase in manufacturing costs will be caused. Therefore, the N content is set to 0.0001% or more. The N content is preferably more than 0.0003%, and more preferably more than 0.0005%.

"O: 0.0001 to 0.0100%" O forms oxides and deteriorates ductility, hole expandability (elongation, flangeability), and bendability. Therefore, its content must be suppressed. If the O content is more than 0.0100%, the formability will be significantly deteriorated, so it is desirable to set the upper limit of the O content to 0.0100%. The O content is preferably 0.0050% or less, and more preferably 0.0030% or less. Although the lower limit of the O content is not particularly limited, the effect can be obtained, but if the O content is less than 0.0001%, it will accompany a large increase in manufacturing costs, so it is appropriate to set the lower limit to 0.0001%.

“Si + 0.1 × Mn + 0.6 × Al ≧ 0.35” Residual Vostian iron may be decomposed into toughened iron, boron iron, or coarse cis-carbon iron during heat treatment. In order to suppress the decomposition of residual Vostian iron and improve the formability, Si, Mn and Al are particularly important elements. In order to suppress the decomposition of the residual Vostian iron, the following formula (1) should be satisfied. The value on the left side of formula (1) is preferably 0.60 or more, and more preferably 0.80 or more. Si + 0.1 × Mn + 0.6 × Al ≧ 0.35 (1) (Si, Mn, and Al in the formula (1) represent the content of each element in terms of mass%).

The steel sheet of this embodiment basically contains the above-mentioned elements, but may also contain a member selected from Ti, V, Nb, Cr, Ni, Cu, Co, Mo, W, B, Sn, Sb, Ca, Mg, Ce, and One or more elements of Zr, La, Hf, Bi, and REM. These elements are arbitrary and are not required to be contained, so the lower limit is 0%.

"Ti: 0 ~ 0.300%" Ti is an element that helps increase the strength of the steel sheet by precipitation strengthening, inhibiting the growth of ferrous iron grains to strengthen the fine grains, and suppressing the differential strengthening performed by recrystallization. However, if the Ti content is greater than 0.300%, the precipitation of carbonitrides will increase and the formability will be deteriorated. Therefore, when it is contained, the Ti content is preferably 0.300% or less. From the viewpoint of formability, the Ti content is preferably 0.150% or less. Although the lower limit of the Ti content is not particularly limited, the effect can be obtained, but in order to fully obtain the effect of improving the strength by containing Ti, the Ti content is preferably 0.001% or more. In order to increase the strength of the steel sheet, the Ti content is more preferably 0.010% or more.

"V: 0 to 1.00%" V is an element that helps increase the strength of the steel sheet by precipitation strengthening, inhibiting the growth of ferrous iron grains to strengthen the fine grains, and suppressing the repulsive strengthening of differential rows. However, if the V content is more than 1.00%, carbonitrides will be excessively precipitated and the formability will be deteriorated. Therefore, when it is contained, the V content is preferably 1.00% or less, and more preferably 0.50% or less. Although the lower limit of the V content is not particularly limited, the effect can be obtained, but in order to fully obtain the effect of increasing the strength by containing V, the V content is preferably 0.001% or more, and more preferably 0.010% or more.

"Nb: 0 ~ 0.100%" Nb is an element that helps increase the strength of the steel sheet by precipitation strengthening, suppressing the growth of ferrous iron grains to strengthen the fine grains, and suppressing the repulsive strengthening of the differential row. However, if the Nb content is more than 0.100%, the precipitation of carbonitrides will increase and the formability will deteriorate. Therefore, when it is contained, the Nb content should be 0.100% or less. From the viewpoint of moldability, the Nb content is preferably 0.060% or less. Although the lower limit of the Nb content is not particularly limited, the effect can be obtained, but in order to fully obtain the effect of increasing the strength by containing Nb, the Nb content is preferably 0.001% or more. In order to increase the strength of the steel sheet, the Nb content is more preferably 0.005% or more.

"Cr: 0 to 2.00%" Cr is an element that improves the hardenability of the steel sheet and effectively increases its strength. However, if the Cr content is more than 2.00%, the workability during heat treatment will be impaired and the productivity will be reduced. Based on this, the content of Cr should preferably be 2.00% or less, and more preferably 1.20% or less. Although the lower limit of the Cr content is not particularly limited, the effect can be obtained, but in order to fully obtain the effect of increasing the strength by containing Cr, the Cr content is preferably 0.001% or more, and more preferably 0.010% or more.

"Ni: 0 to 2.00%" Ni is an element that is effective for increasing the strength of a steel sheet by suppressing the transformation at high temperatures. However, if the Ni content is more than 2.00%, the weldability will be impaired. Based on this, the Ni content should preferably be 2.00% or less, and more preferably 1.20% or less. Although the lower limit of the Ni content is not particularly limited, the effect can be obtained, but in order to fully obtain the effect of increasing strength by containing Ni, the Ni content is preferably 0.001% or more, and more preferably 0.010% or more.

"Cu: 0 to 2.00%" Cu is an element that can exist as fine particles in steel to improve the strength of steel plates. However, if the Cu content is more than 2.00%, the weldability will be impaired. Therefore, when it is contained, the Cu content should preferably be 2.00% or less, and more preferably 1.20% or less. Although the lower limit of the Cu content is not particularly limited, the effect can be obtained, but in order to fully obtain the effect of increasing strength by containing Cu, the Cu content is preferably 0.001% or more, and more preferably 0.010% or more.

"Co: 0 to 2.00%" Co is an element that improves the hardenability and is effective for increasing the strength of steel plates. However, if the Co content is more than 2.00%, the workability during heat treatment will be impaired and the productivity will be reduced. Based on this, the Co content should be set to 2.00% or less, and more preferably 1.20% or less. Although the lower limit of the Co content is not particularly limited, the effect can be obtained, but in order to fully obtain the effect of high strength by containing Co, the Co content is preferably 0.001% or more, and more preferably 0.010% or more.

"Mo: 0 to 1.00%" Mo is an element that is effective for increasing the strength of a steel sheet by suppressing phase transformation at high temperatures. However, if the Mo content is more than 1.00%, the workability during heat treatment will be impaired and the productivity will be reduced. Based on this, the Mo content is preferably set to 1.00% or less, and more preferably 0.50% or less. Although the lower limit of the Mo content is not particularly limited, the effect can be obtained, but in order to fully obtain the effect of increasing strength by containing Mo, the Mo content is preferably 0.001% or more, and more preferably 0.005% or more.

"W: 0 to 1.00%" W is an element that is effective for increasing the strength of a steel sheet by suppressing phase transformation at high temperatures. However, if the W content is more than 1.00%, the workability during heat treatment will be impaired and the productivity will be reduced. Based on this, when contained, the W content is 1.00% or less, and more preferably 0.50% or less. Although the lower limit of the W content is not particularly limited, the effect can be obtained, but in order to fully obtain the high strength effect brought by W, the W content is preferably 0.001% or more, and more preferably 0.010% or more.

"B: 0 ~ 0.0100%" B is an element that is effective for increasing the strength of a steel sheet by suppressing phase transformation at high temperatures. However, if the B content is more than 0.0100%, the workability during heat treatment will be impaired and the productivity will be reduced. Based on this, the B content should preferably be 0.0100% or less when contained. From the viewpoint of productivity, the B content is preferably 0.0050% or less. Although the lower limit of the B content is not particularly limited, the effect can be obtained, but in order to fully obtain the effect of increasing strength by containing B, the B content should preferably be 0.0001% or more. For higher strength, the B content is more preferably 0.0005% or more.

"Sn: 0 to 1.00%" Sn is an element effective in suppressing coarsening of the structure and increasing the strength of the steel sheet. However, if the Sn content is greater than 1.00%, the steel sheet will be excessively brittle and the steel sheet may be fractured during rolling delay. Therefore, when it is contained, the Sn content should be 1.00% or less. Although the lower limit of the Sn content is not particularly limited, the effect can be obtained, but in order to fully obtain the effect of high strength brought by Sn, the Sn content is preferably 0.001% or more, and more preferably 0.010% or more.

"Sb: 0 to 1.00%" Sb is an element effective for suppressing the coarsening of the structure and increasing the strength of the steel sheet. However, if the Sb content is greater than 1.00%, the steel sheet will be excessively brittle and the steel sheet may be fractured during rolling delay. Therefore, when it is contained, the Sb content should be 1.00% or less. Although the lower limit of the Sb content is not particularly limited, the effect can be obtained, but in order to fully obtain the effect of high strength brought by Sb, the Sb content is preferably 0.001% or more, and more preferably 0.005% or more.

"One or two of Ca, Mg, Ce, Zr, La, Hf, Bi, and REM are 0 to 0.0100%, respectively." REM is the abbreviation of Rare Earth Metal. In this embodiment, it refers to a genus other than Ce and La. Elements of the lanthanum series. In this embodiment, REM, Ce, and La are mostly added in the form of a rare earth metal, and may contain a lanthanoid element in combination. The effect can be obtained even if lanthanoid elements other than La and / or Ce are contained as impurities. Moreover, the effect can be obtained even by adding metal La and / or Ce. In this embodiment, the content of REM is a total value of the content of lanthanoid elements other than Ce and La.

The reason why these elements are contained is as follows. Ca, Mg, Ce, Zr, La, Hf, Bi, and REM are elements that can effectively improve the formability, and each of them can contain one or two or more kinds at 0.0001% to 0.0100%. If the content of one or more of Ca, Mg, Ce, Zr, La, Hf, Bi, and REM is more than 0.0100%, the ductility may decrease. Therefore, when it is contained, the content of each element is preferably 0.0100% or less, and more preferably 0.0070% or less. When two or more of the above elements are contained, the total content of Ca, Mg, Ce, Zr, La, Hf, Bi, and REM should preferably be 0.0100% or less. Although the lower limit of the content of each element is not particularly limited, the effect can be obtained, but in order to fully obtain the effect of improving the formability of the steel sheet, the content of each element is preferably 0.0001% or more. From the viewpoint of moldability, the content of one or more of Ca, Mg, Ce, Zr, La, Hf, Bi, and REM is more preferably 0.0010% or more.

The steel sheet of this embodiment contains the above-mentioned elements, and the remainder is Fe and impurities. The aforementioned Ti, V, Nb, Cr, Ni, Cu, Co, Mo, W, B, Sn, and Sb may be allowed to be contained in a trace amount below the aforementioned lower limit as an impurity. In addition, Ca, Mg, Ce, Zr, La, Hf, Bi, and REM may be allowed to be contained as an impurity in an extremely small amount below the aforementioned lower limit. Furthermore, H, Na, Cl, Sc, Zn, Ga, Ge, As, Se, Y, Tc, Ru, Rh, Pd, Ag, Cd, In, Te, Cs, Ta, and Re, Os, Ir, Pt, Au, and Pb were considered as impurities.

Next, the steel structure (microstructure) of the steel plate of this embodiment is demonstrated. [%] In each tissue content description is [vol%]. (Steel Structure Inside Steel Plate) As shown in FIG. 1, in the steel plate 1 of this embodiment, the thickness of the steel plate 1 from the surface of the steel plate is 1/4 of the thickness (the thickness of the steel plate is 1/4 from the surface in the thickness direction). Position) is a steel structure with a thickness ranging from 1/8 to 3/8 in the center (hereinafter referred to as "steel structure inside the steel plate"), containing 0-30% soft fertile iron, 3% -40% The total amount of residual Vostian iron, 0 ~ 30% of fresh Asada loose iron, bolai iron, and Xueming carbon iron weaving is 0-10%, and the residual Vostian iron with an aspect ratio of 2.0 or more accounts for the total residual Vo The number of Stalin irons is more than 50%.

"Soft ferrous iron: 0 ~ 30%" Ferrous iron is a structure with excellent ductility. However, due to the low strength of ferrous iron, it is a difficult structure to use for high-strength steel plates. In the steel sheet according to this embodiment, the steel structure (microstructure inside the steel sheet) inside the steel sheet contains 0% to 30% of soft fertile iron. The "soft fertilized iron" in the present embodiment means the ferrous iron that does not contain residual Vostian iron in the crystal grains. The strength of soft fertilized iron is low, and compared with the peripheral part, strain is easy to concentrate and easy to break. If the volume fraction of soft fertilized iron is more than 30%, the balance between strength and formability will be significantly deteriorated. Therefore, the iron content of soft fertilizers is limited to 30% or less. It is better to limit the iron content of soft fertilizers to 15% or less.

"Residual Vosstian Iron: 3% ~ 40%" Residual Vosstian Iron is an organization that can improve the strength-ductility balance. In the steel sheet of this embodiment, the steel structure inside the steel sheet contains 3% to 40% of residual Vostian iron. From the viewpoint of formability, the volume fraction of the residual Vostian iron in the steel sheet is 3% or more, and preferably 5% or more, and more preferably 7% or more. On the other hand, if the volume fraction of the residual Wastfield iron is set to be greater than 40%, a large amount of C, Mn and / or Ni should be contained. At this time, the weldability is significantly deteriorated. Therefore, it is assumed that the volume fraction of the residual Vosted iron is 40% or less. In order to improve the weldability and convenience of the steel sheet, it is preferable to set the volume fraction of the residual Vosted iron to 30% or less, and more preferably 20% or less.

"New born Asada loose iron: 0 ~ 30%" New born Asada loose iron can greatly improve tensile strength. On the other hand, fresh Asada loose iron can be the starting point of destruction and significantly impair the impact resistance. Therefore, it is assumed that the volume fraction of the fresh Asada scattered iron is 30% or less. In particular, in order to improve the shock resistance characteristics, it is preferable to set the volume fraction of fresh Mata loose iron to 15% or less, and more preferably 7% or less. Fresh Asada loose iron can also be 0%, but to ensure the strength of the steel plate, it should be more than 2%.

"Total of Plei iron and Schering carbon iron: 0 ~ 10%" The steel structure inside the steel sheet may also contain Plei iron and / or Schering carbon iron. However, as the volume fraction of bolai iron and / or skeletal carbon iron increases, the ductility deteriorates. Therefore, it is advisable to limit the volume fraction of Plei iron and / or Xueming carbon iron to 10% or less in total. The volume fraction of boron iron and / or skeletal carbon iron is preferably 5% or less in total, and 0% may be used.

"The ratio of the number of residual Vosstian iron with an aspect ratio of 2.0 or more is 50% or more of the total remaining Vosstian iron" In this embodiment, the aspect ratio of the residual Vosstian iron particles in the steel structure inside the steel plate Very important. The large aspect ratio, that is, the extended residual Vostian iron, is stable during the initial deformation of the steel plate after processing. However, with the progress of the residual Vostian iron with a large aspect ratio, the strain will be concentrated on the front end, and it will deform properly to produce a TRIP effect. Therefore, the steel structure inside the steel sheet can improve the ductility without impairing the toughness, the hydrogen embrittlement resistance, the hole expandability, and the like by containing residual Vosstian iron with a large aspect ratio. From the above point of view, in the steel plate of this embodiment, the ratio of the number of residual Vosstian iron to the total remaining Vostian iron is set to 50% or more. The ratio of the number of residual Vostian irons with an aspect ratio of 2.0 or more is preferably 70% or more, and more preferably 80% or more.

"Tempered Asada loose iron" Tempered Asada loose iron is a structure that can significantly increase the tensile strength of the steel plate without impairing the impact resistance, and it is also possible to include it in the steel structure inside the steel plate. However, if a large amount of tempered Asada scattered iron is generated inside the steel sheet, there is a case where the residual Vostian iron cannot be obtained sufficiently. Therefore, the volume fraction of tempered Asada loose iron should be limited to 50% or less, and more preferably 30% or less.

In the steel plate of this embodiment, the remaining part of the steel structure inside the steel plate is mainly "hard fertilized iron" in which residual Vostian iron is enclosed in grains. The so-called mainly refers to the largest volume fraction of hard fertilized iron in the remaining part of the tissue. The hard ferritic iron is a steel sheet for heat treatment having a steel structure including a lath-like structure composed of one or two or more types of toughened iron, tempered Asada iron, and fresh Asada iron. Formed by heat treatment. Hard ferritic iron has high strength because it contains residual Vostian iron in grains. In addition, compared to the case where there is residual Vostian iron in the grain boundary of the ferrous iron, the hard ferrous iron is less likely to peel off the interface between the ferrous iron and the residual Vostian iron, so it has good formability.

In addition, the remaining part of the steel structure in the steel plate may contain toughened iron in addition to the above-mentioned hard fat iron. The toughened iron according to this embodiment includes: granular toughened iron composed of fine BCC crystals and coarse iron-based carbides; upper toughened iron composed of slab-like BCC crystals and coarse iron-based carbides; The plate-like BCC crystals are composed of toughened iron under fine iron-based carbides arranged in parallel with the inside thereof, and toughened iron granules not containing iron-based carbides.

(Microstructure of Surface Layer) Next, the steel structure (microstructure) of the surface layer of the steel sheet will be described.

"When a region with a hardness of 80% or less (inside of the steel plate) with a hardness in the range of 1/8 thickness to 3/8 thickness (inside the steel plate) is defined as a soft layer, the surface layer has a soft layer with a thickness of 1 to 100 μm. One of the necessary requirements for flexibility is to soften the surface of the steel sheet. In the steel sheet of this embodiment, when a region having a hardness of 80% or less (average hardness) inside the steel sheet is defined as the soft layer, the soft layer has a thickness of 1 to 100 μm from the surface of the steel sheet in the thickness direction. In other words, a soft layer having a hardness of 80% or less of the average hardness inside the steel sheet exists in the surface layer portion of the steel sheet, and the thickness of the soft layer is 1 to 100 μm.

If the thickness of the soft layer is less than 1 μm in the depth direction (plate thickness direction) from the surface, the flexibility after processing cannot be sufficiently obtained. The thickness (depth range from the surface) of the soft layer is preferably 5 μm or more, and more preferably 10 μm or more. On the other hand, if the thickness of the soft layer is more than 100 μm, the strength of the steel sheet is significantly reduced. Therefore, the thickness of the soft layer is set to 100 μm or less. The thickness of the soft layer is preferably 70 μm or less.

[Volume fraction of grains with a length-to-width ratio of 3.0 or more in the ferrous grain iron contained in the soft layer] Volume fraction of grains with a length-to-width ratio of 3.0 or more in the ferrous grain iron contained in the soft layer (relative to If the total grain size of the ferrous iron accounts for the volume fraction of the soft layer, and the ratio of the ferrous iron grains having an aspect ratio of less than 3.0) is less than 50%, the hydrogen embrittlement resistance of the bent portion will be deteriorated. Therefore, the volume fraction of crystal grains having an aspect ratio of 3.0 or more in the ferrous iron contained in the soft layer is 50% or more. It should be above 60%, more preferably above 70%. Here, the desired target iron includes soft iron and hard iron. The reason why the aspect ratio of the ferrous iron in the soft layer affects the hydrogen embrittlement resistance of the bent portion is not clear, but we speculate as follows. That is, in the steel sheet of the present embodiment, the steel structure of the soft layer is greatly different from the steel structure (internal structure) inside the steel sheet. However, in the steel sheet of this embodiment, the ratio of the number of crystal grains having an aspect ratio of 3.0 or more in the surface layer is 50% or more, so the shape similarity between the surface layer and the internal structure is high. Therefore, we speculate that the local concentration of stress and strain caused by bending the boundary between the surface layer and the interior can be suppressed, thereby improving the hydrogen embrittlement resistance.

[The volume fraction of residual Vosstian iron in the soft layer is 80% or less of the volume fraction of residual Vosstian iron in the interior of the steel plate] The volume fraction of residual Vosstian iron contained in the soft layer is limited to from The surface is centered on the position of 1/4 thickness of the steel plate, and the volume fraction of the residual Vostian iron contained in the range of 1/8 thickness to 3/8 thickness is less than 80%, which can improve the hydrogen resistance of the bent portion. Embrittlement characteristics. Relative to the volume fraction of residual Vosstian iron contained in the range of 1/8 thickness to 3/8 thickness, the volume fraction of residual Vosstian iron contained in the soft layer should be 50% or less, and more preferably 30% or less . The mechanism for improving the hydrogen embrittlement resistance of the bent portion by the volume fraction of the residual Vosstian iron in the soft layer is not clear, but we speculate as follows. That is, in the bent portion, a larger plastic strain occurs toward the curved outer surface from the center of the plate thickness. Therefore, most of the residual Vostian iron existing near the outer surface of the bend will be transformed into Asada loose iron by processing-induced metamorphosis. And this kind of residual Vostian iron processing-induced metamorphosis of Asada loose iron is extremely hard and brittle, so it will adversely affect the resistance to hydrogen embrittlement and cracking. Therefore, we believe that the smaller the volume fraction of residual Vosstian iron contained in the soft layer compared to the range of 1 / 8th to 3 / 8th thickness of steel plates, the higher the volume fraction Hydrogen embrittlement resistance of the bent portion.

"Internal oxide layer containing Si oxide" When the steel sheet of this embodiment uses a high-frequency glow discharge (high-frequency GDS) analysis method to analyze the luminous intensity of the wavelength of Si from the surface in the depth direction (thickness direction of the plate), From the range of more than 0.2 μm to 10.0 μm or less, a peak showing a luminous intensity at a wavelength of Si appears. A peak system showing a luminous intensity showing a wavelength of Si in a range from 0.2 μm to 10.0 μm from the surface indicates internal oxidation of the steel sheet, and a range from 0.2 μm to 10.0 μm from the surface of the steel sheet includes Internal oxide layer of Si oxide. A steel sheet having an internal oxide layer in the aforementioned depth range is inhibited from forming an oxide film such as Si oxide on the surface of the steel sheet accompanying heat treatment during manufacturing, and therefore has excellent chemical conversion treatment properties and plating adhesion.

When the steel plate of this embodiment is analyzed in the depth direction from the surface by the high-frequency glow discharge analysis method, it may be in a range from 0.2 μm to 10.0 μm from the surface, and from 0 μm to 0.2 μm from the surface. In both ranges (area shallower than 0.2 μm in depth), there are peaks showing the luminous intensity of the wavelength of Si. Having a peak in both ranges indicates that not only the steel plate has an internal oxide layer, but also an external oxide layer containing a Si-containing oxide on the surface.

FIG. 2 is a graph showing the intensity of the luminous intensity of the wavelength of Si and the luminous intensity of the wavelength of Si when the luminous intensity of the wavelength of Si is analyzed from the surface in the depth direction from the surface using the high-frequency glow discharge analysis method. Relationship chart. In the steel sheet of this embodiment shown in FIG. 2, a peak (from an internal oxide layer) showing a luminous intensity at a wavelength of Si appears in a range from 0.2 μm to 10.0 μm from the surface. In addition, a peak (from an external oxide layer (I _MAX )) showing a light intensity of a wavelength of Si appears in a range from 0 (outermost surface) to 0.2 μm from the surface. Therefore, it can be seen that the steel sheet shown in FIG. 2 has not only an internal oxide layer but also an external oxide layer.

FIG. 3 is a graph showing the relationship between the depth from the surface and the luminous intensity (Intensity) showing the wavelength of Si when a steel plate different from this embodiment is analyzed from the surface in the depth direction by the high-frequency glow discharge analysis method. In the steel sheet shown in FIG. 3, a peak system showing a light intensity of a wavelength of Si appears in a range from 0 (outermost surface) to 0.2 μm from the surface, but does not appear in a range from 0.2 μm to 10.0 μm. This case indicates that the steel sheet does not have an internal oxide layer but only an external oxide layer.

"Galvanized layer" A galvanized layer (a hot-dip galvanized layer or an electrogalvanized layer) may be formed on the surface (two or one side) of the steel sheet of this embodiment. The hot-dip galvanized layer may also be an alloyed hot-dip galvanized layer formed by alloying the hot-dip galvanized layer. When the hot-dip galvanized layer is unalloyed, the Fe content in the hot-dip galvanized layer is preferably less than 7.0% by mass. When the hot-dip galvanized layer is an alloyed hot-dip galvanized layer, the Fe content is preferably 6.0% by mass or more. The alloyed hot-dip galvanized steel sheet has better weldability than the hot-dip galvanized steel sheet.

There is no particular limitation on the amount of galvanized coating on the galvanized layer, but from the viewpoint of corrosion resistance, it is preferably 5 g / m ² or more per side, in the range of 20 to 120 g / m ² , and preferably 25 to 75 g / m Within the range of ² .

The steel sheet of this embodiment may be provided with a galvanized layer, and an upper plating layer may be further provided on the galvanized layer in order to improve paintability and weldability. In addition, various treatments can be performed on the galvanized steel sheet, such as chromate treatment, phosphate treatment, lubricity improvement treatment, and weldability improvement treatment.

The steel sheet of this embodiment is formed by subjecting the following steel sheet (the material before the second heat treatment; the "steel sheet for heat treatment") described below to the second steel sheet produced in the step including the first heat treatment.

"Steel plate for heat treatment" The steel plate for heat treatment of this embodiment can be used as a material for the steel plate of this embodiment. Specifically, the steel sheet for heat treatment, which is the material of the steel sheet according to this embodiment, preferably has the same chemical composition as the steel sheet according to this embodiment, and has a steel structure (microstructure) shown below. In addition, [%] in each tissue content description means [volume%] as long as there is no particular limitation.

That is, the steel structure (the steel structure inside the steel plate) in the range of 1/8 thickness to 3/8 thickness centered on the position of 1/4 thickness from the surface should contain more than 70% of the total by volume fraction. A lath-like structure composed of one or more types of toughened iron, tempered Asada iron, and newly-born Asada iron, and contains residual Vostian iron with an aspect ratio of less than 1.3 and an aspect diameter of more than 2.5 μm The number density of the residual Vostian iron particles is 1.0 × 10 ^-2 pieces / μm ² or less. In the steel structure of the surface layer portion in the range of 0 to 20 μm in the depth direction from the surface, the volume fraction of the ferrous iron is less than 50%. In addition, the toughened iron includes granular toughened iron composed of fine BCC crystals and coarse iron-based carbides, upper toughened iron composed of slab-like BCC crystals and coarse iron-based carbides, and plate-like BCC crystals and fine iron-based carbides arranged in parallel with the interior thereof are made of toughened iron and toughened iron particles that do not contain iron-based carbides.

Hereinafter, a preferred steel structure (microstructure) of the steel sheet for heat treatment, which is the material of the steel sheet of the present embodiment, will be described. [%] In each tissue content description is [vol%].

(Steel structure inside steel plate for heat treatment) "Contains a lath-like structure with a total volume fraction of 70% or more" The steel plate for heat treatment of this embodiment is centered on a position of 1/4 thickness of the steel plate from the surface. The steel structure (steel structure inside the steel plate) in the range of 1/8 thickness to 3/8 thickness should preferably contain more than 70% of the total volume of toughened iron, tempered Asada iron, and newly-born Asada iron. A lath-like structure consisting of one or two or more.

A steel sheet produced by subjecting the steel sheet for heat treatment to a second heat treatment to be described later, which contains the above-mentioned lath-like structure in a volume fraction of 70% or more, is mainly composed of hard ferrous iron. When the total volume fraction of the lath-like structure is less than 70%, the steel structure inside the steel plate obtained by performing the second heat treatment on the steel plate for heat treatment will become a steel containing a large amount of soft fertilized iron, and the cost cannot be obtained. Steel plate of the embodiment. The steel structure inside the steel plate of the heat treatment steel plate should preferably contain more than 80% of the above-mentioned lath-like structure in volume fraction, and more preferably 90% or more, and 100% is not a problem.

"Number density of residual Vosstian iron particles having an aspect ratio of less than 1.3 and an aspect diameter of more than 2.5 µm" The steel structure inside the steel sheet for the heat-treated steel sheet may contain the residual Vosstian iron in addition to the above-mentioned lath-like structure. However, when the residual Vosstian iron is contained, the number density of the residual Vosstian iron particles with an aspect ratio of less than 1.3 and an aspect diameter of more than 2.5 μm should be limited to 1.0 × 10 ^-2 pieces / μm ² or less.

If the residual Vostian iron in the steel structure existing in the steel plate is coarse, there will be coarse residual Vastian iron particles in the steel plate produced by performing the second heat treatment on the steel plate for heat treatment, and it is difficult to fully ensure it. In the case of the ratio of the number of residual Vostian irons with an aspect ratio of 2.0 or more. Therefore, it is assumed that the number density of the coarse massive residual Vostian iron particles having an aspect ratio of less than 1.3 and an aspect diameter of more than 2.5 μm is 1.0 × 10 ^-2 pieces / μm ² or less. The number density of the coarse lumped Vostian iron particles should be as low as possible, and preferably 0.5 × 10 ^-2 pieces / μm ² or less.

In addition, if there is excessive residual Vastian iron in the steel sheet of the heat treatment steel sheet, the second heat treatment described later is applied to the steel sheet for heat treatment to make a part of the residual Vastian iron isotropic. As a result, there may be a case where the inside of the steel sheet obtained after the second heat treatment cannot sufficiently ensure the residual Vostian iron with an aspect ratio of 2.0 or more. Therefore, the volume fraction of the residual Vosstian iron contained in the steel structure inside the steel sheet for the heat treatment steel sheet is preferably 10% or less.

(Microstructure of the surface layer of the steel plate for heat treatment) "The volume fraction of the ferrous iron in the surface layer portion ranging from the surface of the steel plate in the depth direction to 20 µm is less than 20%." The steel plate for heat treatment as the material for the steel plate of this embodiment It is preferable that the ferrous iron is in a range of 20 μm from the surface of the steel plate in a depth direction to less than 20% by volume fraction. If the volume fraction of ferrous iron is 20% or more, it is formed in a soft layer of a steel plate obtained by performing a second heat treatment on a steel plate for heat treatment. The volume fraction of ferrous iron particles with an aspect ratio greater than 3.0 cannot be satisfied. Predetermined range. The smaller the volume fraction of the ferrous iron, the better, and more preferably below 10%, and 0% is not a problem.

"Manufacturing method of the steel plate of this embodiment" Next, the manufacturing method of the steel plate of this embodiment is demonstrated.

The method for manufacturing a steel sheet according to this embodiment is to perform hot rolling on a steel billet having the above-mentioned chemical composition by hot rolling and pickling, or cold rolling on a hot rolled steel sheet by cold rolling. The first heat treatment is shown to produce a steel sheet for heat treatment. After that, the steel sheet for heat treatment is subjected to the second heat treatment shown below. The first heat treatment and / or the second heat treatment can be carried out using a dedicated heat treatment line, or a conventional annealing line may be used.

(Casting step) When manufacturing the steel sheet of this embodiment, first, a steel blank having the above-mentioned chemical composition (composition) is cast. The steel billet for hot rolling can be produced by continuous casting billet or using a thin billet continuous casting machine. After casting, the billet can be temporarily cooled to normal temperature and then hot rolled, or hot rolled directly at high temperature. It is better to directly supply the hot-rolled steel billet to the hot rolling at a high temperature because the energy required for the hot rolling can be reduced.

(Steel billet heating) The steel billet is heated before hot rolling. When manufacturing the steel sheet according to this embodiment, it is desirable to select a heating condition of the steel billet that satisfies the following formula (4).

[Mathematical formula 1] (In formula (4), fγ is a value represented by the following formula (5), WMnγ is a value represented by the following formula (6), D is a value represented by the following formula (7), and A _c1 is The value represented by the formula (8), A _c3 is the value represented by the following formula (9), and ts (T) is the billet residence time (sec) of the billet heating temperature T).

[Mathematical formula 2] (In formula (5), the heating temperature (° C) of the T-based steel billet, the C content (% by mass) in the WC-based steel, A _c1 is a value represented by the following formula (8), and A _c3 is the following formula ( 9) The value shown).

[Mathematical formula 3] (In formula (6), the heating temperature (° C) of the T-based steel billet, the Mn content (% by mass) in the WMn-based steel, A _c1 is a value represented by the following formula (8), and A _c3 is the following formula ( 9) The value shown).

[Mathematical formula 4] (In the formula (7), the heating temperature (° C) of the T-based steel billet and the R-constant gas constant; 8.314 J / mol).

A _c1 = 723-10.7 × Mn-16.9 × Ni + 29.1 × Si + 16.9 × Cr ... (8) (The symbol of the element in the formula (8) is the mass% of the element in the steel). A _c3 = 879-346 × C + 65 × Si-18 × Mn + 54 × Al (9) (The symbol of the element in the formula (9) is the mass% of the element in the steel).

The molecular formula (4) shows the degree to which α is distributed to the Mn content of γ in the two-phase region retention of α (fertilized iron) and γ (vostian iron). The larger the formula (4), the more heterogeneous the Mn concentration distribution in the steel. The denominator of formula (4) corresponds to the term of the distance of the Mn atoms diffused in γ in the γ single-phase region retention. The larger the denominator of the formula (4), the more homogeneous the Mn concentration distribution. In order to fully homogenize the Mn concentration distribution in the steel, the heating conditions of the steel billet should be selected so that the value of formula (4) is 1.0 or less. The smaller the value of the formula (4), the lower the number density of coarse and massive Vostian iron particles in the steel plate inside the steel plate for heat treatment and the steel plate obtained by subjecting the heat treatment steel plate to the second heat treatment.

(Hot rolling) Hot rolling is performed after heating the steel billet. If the completion temperature (completion temperature) of the hot rolling is lower than 850 ° C, the rolling reaction force will increase, and it will be difficult to stably produce the specified sheet thickness. Therefore, the completion temperature of hot rolling should be set to 850 ° C or higher. From the viewpoint of rolling reaction force, the completion temperature of hot rolling should be set to 870 ° C or higher. On the other hand, if the completion temperature of the hot rolling is set to be higher than 1050 ° C., it is necessary to use a heating device or the like to heat the steel plate in the steps from the end of the heating of the steel billet to the completion of the hot rolling, resulting in high costs. Therefore, the completion temperature of hot rolling should be set below 1050 ° C. In order to easily ensure the temperature of the steel sheet during hot rolling, the completion temperature of hot rolling should be set below 1000 ° C, and more preferably below 980 ° C.

(Pickling step) Next, the hot-rolled steel sheet produced as described above is pickled. The step of removing the oxide on the surface of the hot-rolled steel sheet by pickling is very important to improve the chemical conversion treatment and plating adhesion of the steel sheet. The pickling of the hot-rolled steel sheet may be performed once or in several times.

(Cold-rolled rolling) It is also possible to cold-roll a hot-rolled steel sheet after pickling to produce a cold-rolled steel sheet. By cold-rolling a hot-rolled steel sheet, a steel sheet having a predetermined thickness can be manufactured with high accuracy. In cold rolling, if the total rolling reduction rate (cumulative rolling reduction in cold rolling) is greater than 85%, the steel sheet will lose ductility, thereby increasing the risk of the steel sheet breaking during cold rolling. Therefore, the total reduction ratio should preferably be 85% or less, and more preferably 75% or less. The lower limit of the total reduction ratio in the cold rolling step is not particularly limited, and it is not necessary to perform cold rolling. In order to improve the shape homogeneity of the steel sheet to obtain a good appearance, and to uniform the temperature of the steel sheet in the first heat treatment and the second heat treatment to obtain good ductility, the rolling reduction rate of cold rolling should be set to 0.5% in total It is more preferably 1.0% or more.

(First heat treatment) Next, a first heat treatment is performed on the hot-rolled steel sheet after pickling or the cold-rolled steel sheet obtained by cold-rolling the hot-rolled steel sheet to produce a steel sheet for heat treatment. The first heat treatment is performed to satisfy the following conditions (a) to (e). (a) During the period from 650 ° C. to the maximum heating temperature, a gas environment containing H _{2 of} 0.1% by volume or more and satisfying the following formula (3) is set. log (PH ₂ O / PH ₂ ) <-1.1 (3) (In formula (3), log represents a common logarithm, PH ₂ O represents a partial pressure of water vapor, and PH ₂ represents a partial pressure of hydrogen).

In the first heat treatment, by satisfying the above (a), the decarburization reaction at the surface layer portion of the steel sheet can be suppressed, and the formation of ferrous iron can be suppressed.

If H ₂ in the gas environment is less than 0.1% by volume, the oxide film existing on the surface of the steel sheet cannot be sufficiently reduced, and an oxide film is formed on the steel sheet. Therefore, the chemical conversion treatability and plating adhesion of the steel sheet obtained after the second heat treatment are reduced. On the other hand, if the content of H ₂ in the gas environment is more than 20% by volume, the effect is saturated. In addition, if the H ₂ content in the gas environment is greater than 20% by volume, the risk of hydrogen explosion during operation will increase. Therefore, it is preferable to set the H ₂ content in the gas environment below 20% by volume. In addition, when log (PH ₂ O / PH ₂ ) is -1.1 or more, a decarburization reaction is promoted at the surface layer portion of the steel sheet, and ferrous iron is formed at the surface layer portion. As a result, the ratio of the ferrite grains to iron grains having an aspect ratio of less than 3.0 in the steel sheet after the second heat treatment was increased.

(b) Hold at the highest heating temperature of A _c3 -30 ℃ ~ 1000 ℃ for 1 second to 1000 seconds. In the first heat treatment, the maximum heating temperature is set to A _c3 -30 ° C or higher. When the maximum heating temperature is lower than A _c3 -30 ° C, massive coarse iron grains remain in the steel structure inside the steel plate of the heat treatment steel plate. As a result, the volume fraction of the soft fertile iron phase of the steel sheet obtained after the second heat treatment of the steel sheet for heat treatment will be excessive, and the ratio of the number of residual Vostian irons with an aspect ratio of 2.0 or more will be insufficient, resulting in poor characteristics . The maximum heating temperature is preferably A _c3 -15 ° C or higher, and more preferably A _c3 + 5 ° C or higher. On the other hand, excessive heating to high temperatures increases the fuel cost of heating and damages the furnace body. Therefore, the maximum heating temperature is set to 1000 ° C or lower.

In the first heat treatment, the holding time at the highest heating temperature is set to 1 second to 1000 seconds. If the holding time is less than 1 second, massive coarse iron grains remain in the steel structure inside the steel sheet of the heat treatment steel sheet. As a result, the volume fraction of the soft fertilized iron in the steel sheet obtained after the second heat treatment is too large, and the characteristics are deteriorated. The holding time should be more than 10 seconds, and more preferably more than 50 seconds. On the other hand, if the holding time is too long, the effect of heating to the maximum heating temperature will be saturated, and productivity will be impaired. Therefore, the holding time is set to 1000 seconds or less.

(c) Heating is performed such that the average heating rate in the temperature range from 650 ° C to the maximum heating temperature is 0.5 ° C / second to 500 ° C / second. In the first heat treatment, when heating, in the temperature range from 650 ° C to the maximum heating temperature, if the average heating rate is less than 0.5 ° C / sec, Mn segregation is promoted during the heat treatment, and a coarse lump Mn concentration region is formed. . In this case, the properties of the steel sheet obtained after the second heat treatment are deteriorated. In order to suppress the formation of massive Vostian iron, the average heating rate from 650 ° C to the maximum heating temperature is set to be 0.5 ° C / sec or more. It should preferably be 1.5 ° C / sec or more. On the other hand, setting the upper limit of the average heating rate to be greater than 500 ° C / second is difficult in actual operation and the temperature control is not easy. Therefore, the upper limit of the average heating rate is set to 500 ° C / second. The average heating rate from 650 ° C to the maximum heating temperature is obtained by dividing the elapsed time from the surface temperature of the steel sheet from 650 ° C to the maximum heating temperature by the difference between 650 ° C and the maximum heating temperature.

(d) After maintaining at the maximum heating temperature, cooling is performed so that the average cooling rate in a temperature range from 700 ° C to Ms is 5 ° C / second or more. In the first heat treatment, in order to make the steel structure inside the steel plate of the heat treatment steel plate-like structure as the main body, and after maintaining it at the maximum heating temperature, cooling in a temperature range from 700 ° C to Ms shown in the following formula (10) The cooling rate is such that the average cooling rate is 5 ° C / second or more. If the average cooling rate is lower than 5 ° C / sec, lump-shaped ferrous iron may be formed in the steel sheet for heat treatment. At this time, the volume fraction of the soft fertilized iron of the steel sheet obtained after the second heat treatment is too large, which deteriorates properties such as tensile strength. The average cooling rate should preferably be 10 ° C / second or more, and more preferably 30 ° C / second or more. Although the upper limit of the average cooling rate does not need to be specifically limited, if it is cooled at an average cooling rate of more than 500 ° C / sec, special equipment is required. Therefore, the average cooling rate should be set below 500 ° C / sec. The average cooling rate in the temperature range from 700 ° C to Ms or less is obtained by dividing the elapsed time from the surface temperature of the steel sheet from 700 ° C to Ms by the difference between 700 ° C and Ms.

Ms = 561-407 × C-7.3 × Si-37.8 × Mn-20.5 × Cu-19.5 × Ni-19.8 × Cr-4.5 × Mo ・ (10) (The symbol of the element in the formula (10) is that the element is in Mass% in steel).

(e) The cooling at the above-mentioned average cooling rate of 5 ° C./second or more is performed until the cooling stop temperature of Ms or less. In the first heat treatment, a cooling system having an average cooling rate in a temperature range of 700 ° C. to Ms of 5 ° C./sec or more is performed until a cooling stop temperature of Ms or less shown in formula (10). The cooling stop temperature may be room temperature (25 ° C). By setting the cooling stop temperature to Ms or less, it is possible to obtain a steel structure in which the steel structure inside the steel sheet for the heat treatment steel sheet produced after the first heat treatment is mainly a lath-like structure.

In the manufacturing method of this embodiment, in the first heat treatment, the second heat treatment shown below can also be performed on the steel sheet cooled to a cooling stop temperature below Ms and above room temperature. In the first heat treatment, the second heat treatment described below may be performed after cooling to room temperature and winding.

The steel sheet cooled to room temperature in the first heat treatment is the steel sheet for heat treatment of the present embodiment described above. The steel sheet for heat treatment is subjected to the second heat treatment shown below to produce a steel sheet according to this embodiment. In this embodiment, various treatments may be performed on the steel sheet for heat treatment before the second heat treatment. For example, in order to correct the shape of the steel sheet for heat treatment, the steel sheet for heat treatment may be subjected to temper rolling. In addition, in order to remove the oxide existing on the surface of the steel sheet for heat treatment, it is also possible to perform a pickling treatment on the steel sheet for heat treatment.

(Second heat treatment) A second heat treatment is performed on the steel plate (steel plate for heat treatment) after the first heat treatment. The second heat treatment is performed under conditions satisfying the following (A) to (E). (A) During the period from 650 ° C to the maximum heating temperature, set H ₂ to 0.1 vol% or more, O ₂ to 0.020 vol% or less, and log (PH ₂ O / PH ₂ ) that satisfies the following formula (4) surroundings. -1.1 ≦ log (PH ₂ O / PH ₂ ) ≦ -0.07 (4) (In the formula (3), log represents a common logarithm, PH ₂ O represents a partial pressure of water vapor, and PH ₂ represents a partial pressure of hydrogen). By satisfying the above (A) in the second heat treatment, it is possible to suppress the oxidation reaction outside the steel sheet and promote the decarburization reaction in the surface layer portion.

If H ₂ in the gas environment is less than 0.1% by volume or O _{2 is} greater than 0.020% by volume, the oxide film existing on the surface of the steel sheet cannot be sufficiently reduced, and an oxide film is formed on the steel sheet. As a result, the chemical conversion treatability and plating adhesion of the steel sheet obtained after the second heat treatment are reduced. The range of H ₂ is preferably 1.0% by volume or more, and more preferably 2.0% by volume or more. The range of O ₂ is preferably 0.010% by volume or less, and more preferably 0.005% by volume or less. If the content of H ₂ in the gas environment is greater than 20% by volume, the effect will be saturated. In addition, if the H ₂ content in the gas environment is greater than 20% by volume, the risk of hydrogen explosion during operation will increase. Therefore, it is preferable to set the H ₂ content in the gas environment below 20% by volume.

When log (PH ₂ O / PH ₂ ) is less than -1.1, external oxidation of Si and Mn on the surface layer of the steel plate occurs, which makes the decarburization reaction insufficient, and the surface structure of the steel plate obtained after the second heat treatment cannot meet the expected range. Therefore, log (PH ₂ O / PH ₂ ) is set to be -1.1 or more. The log (PH ₂ O / PH ₂ ) is preferably -0.8 or more. On the other hand, if log (PH ₂ O / PH ₂ ) is greater than -0.07, the decarburization reaction proceeds excessively, and the strength of the steel sheet obtained after the second heat treatment is insufficient. Therefore, log (PH ₂ O / PH ₂ ) is set to -0.07 or less.

(B) Hold at the highest heating temperature of (A _c1 +25) ℃ ~ (A _c3 -10) ℃ for 1 second to 1000 seconds. In the second heat treatment, the maximum heating temperature was set to (A _c1 +25) ° C to (A _c3 -10) ° C. If the maximum heating temperature is lower than (A _c1 +25) ° C, citronite in the steel will melt and remain, so that the residual Vostian iron fraction in the internal structure of the steel plate obtained after the second heat treatment is insufficient, resulting in characteristics Worse. In order to increase the hard structure fraction in the steel plate obtained after the second heat treatment to obtain a steel plate with higher strength, the maximum heating temperature should be set to (A _c1 +40) ° C or higher.

On the other hand, if the maximum heating temperature is higher than (A _c3 -10) ° C, almost or all of the internal steel structure will become Vostian iron, and the lath-like structure in the steel plate (steel plate for heat treatment) before the second heat treatment It disappears, so that the lath-like structure of the steel sheet before the second heat treatment cannot be stored in the steel sheet after the second heat treatment. As a result, the residual Vosstian iron fraction in the internal structure of the steel plate obtained after the second heat treatment will be insufficient, and the number of residual Vosstian iron ratios with an aspect ratio of 2.0 or more will also be insufficient, resulting in significant deterioration in characteristics. . Therefore, the maximum heating temperature is set to (A _c3 -10) ° C or lower. In order that the lath-like structure in the steel sheet before the second heat treatment can fully exist, and the characteristics of the steel sheet are further improved, the maximum heating temperature should be set below (A _c3 -20) ° C and (A _c3 -30) ° C The following is better.

In the second heat treatment, the holding time at the highest heating temperature is set to 1 second to 1000 seconds. If the holding time is less than 1 second, the citronite in the steel will melt and remain, and the properties of the steel sheet may deteriorate. The holding time should be set to more than 30 seconds. On the other hand, if the holding time is too long, the effect of heating to the maximum heating temperature will not only be saturated, but productivity will also be reduced. Therefore, the holding time is set to 1000 seconds or less.

(C) Heating is performed such that the average heating rate from 650 ° C to the maximum heating temperature is 0.5 ° C / second to 500 ° C / second. In the second heat treatment, if the average heating rate from 650 ° C to the maximum heating temperature is less than 0.5 ° C / s, the lath-like structure produced in the first heat treatment will be restored, so that the grains do not have soft fertilizers of Vostian iron particles. The volume fraction of granular iron increases. On the other hand, if the average heating rate is more than 500 ° C / second, the decarburization reaction cannot proceed sufficiently.

(D) Cooling from the maximum heating temperature to 480 ° C or lower so that the average cooling rate from 700 to 600 ° C is 3 ° C / second or more. In the second heat treatment, the temperature was cooled from the maximum heating temperature to 480 ° C or lower. At this time, the average cooling rate between 700 and 600 ° C is set to 3 ° C / second or more. If the average cooling rate is less than 3 ° C / sec. In the above range, coarse carbides are generated and the properties of the steel sheet are impaired. The average cooling rate should be set to 10 ° C / sec or more. The upper limit of the average cooling rate is not particularly limited and may not be limited, but if it is higher than 200 ° C / sec, a special cooling device is required, so it is set to 200 ° C / sec or less.

(E) Hold at 300 ° C to 480 ° C for 10 seconds or more. Next, the steel plate is held in a temperature region between 300 ° C and 480 ° C for 10 seconds or more. If the holding time is less than 10 seconds, the carbon cannot be sufficiently concentrated in the undistorted Vossfield iron. At this time, the slat-shaped fertilized iron was not able to grow sufficiently and it was not possible to enrich the C-Wotsfield iron. As a result, after the above-mentioned holding, fresh Mata loose iron is generated at the time of final cooling, resulting in greatly deteriorated steel plate characteristics. In order to fully enrich the carbon in the Vosstian iron and reduce the amount of loose iron in Asada to improve the characteristics of the steel plate, it is desirable to set the holding time to be more than 100 seconds. Although there is no need to limit the upper limit of the holding time, too long will also reduce productivity, so the holding time can also be set below 1000 seconds. When the cooling stop temperature is lower than 300 ° C, it can be heated to 300 ~ 480 ° C and then maintained.

<Galvanizing step> The steel sheet after the second heat treatment may also be subjected to hot-dip galvanizing to form a hot-dip galvanized layer on the surface. In addition, after the formation of the hot-dip galvanized layer, an alloying treatment of the plating layer may be performed. Alternatively, the steel sheet after the second heat treatment may be subjected to electrogalvanization for forming an electrogalvanized layer on the surface.

The hot-dip galvanizing and alloying treatment and electro-galvanizing can be performed at any time after the cooling step (D) of the second heat treatment is completed as long as the conditions specified in the present invention are satisfied. For example, as shown in the mode [1] shown in FIG. 4, after the cooling step (D) and the isothermal holding step (E), a plating treatment can be performed (an alloying treatment can be further performed if necessary), and the mode shown in FIG. 5 [2] After the cooling step (D), a plating treatment may be performed (an alloying treatment may be further performed as required), and then isothermal maintenance (E) may be performed. Alternatively, as shown in the mode [3] shown in FIG. 6, after the cooling step (D) and the isothermal holding step (E), the temperature may be temporarily cooled to room temperature, and then a plating treatment may be performed (alloying may be further performed as required). deal with).

General conditions can be used for the plating conditions such as the temperature of the galvanizing bath and the composition of the galvanizing bath in the hot-dip galvanizing step, and there is no particular limitation. For example, the temperature of the plating bath may be 420 to 500 ° C, the temperature of the plate entering the plating bath may be 420 to 500 ° C, and the immersion time may be 5 seconds or less. The plating bath is preferably a plating bath containing 0.08 to 0.2% Al, and the other may also contain Fe, Si, Mg, Mn, Cr, Ti, and Pb which are unavoidable impurities. In addition, the unit area amount of the hot-dip galvanizing is preferably controlled by a known method such as gas wiping. The unit surface weight is generally only required to be 5 g / m ² or more per one surface, but is preferably 20 to 120 g / m ² , and more preferably 25 to 75 g / m ² .

The high-strength hot-dip galvanized steel sheet on which the hot-dip galvanized layer is formed may be subjected to alloying treatment as necessary as described above. In the alloying treatment, it is preferable to set the alloying treatment temperature to 460 ~ 600 ° C. If the alloying treatment is lower than 460 ° C, the alloying speed will be slowed, so that productivity will be reduced and uneven alloying treatment will occur. On the other hand, if the alloying treatment temperature is higher than 600 ° C, the alloying progresses excessively and the plating adhesion of the steel sheet is deteriorated. The alloying temperature should be 480 ~ 580 ℃. The heating time for alloying treatment should be set to 5 ~ 60 seconds. The alloying treatment is preferably performed under conditions such that the iron concentration in the hot-dip galvanized layer is 6.0% by mass or more.

The conditions for performing galvanizing are not particularly limited.

By performing the second heat treatment described above, the steel sheet according to this embodiment can be obtained. In this embodiment, cold-rolling can be performed on the steel sheet in order to correct the shape. The cold rolling may be performed after the first heat treatment or after the second heat treatment. It may be performed after both the first heat treatment and after the second heat treatment. In terms of the cold rolling reduction rate, the rolling reduction rate should be set to 3.0% or less, and more preferably 1.2% or less. If the rolling reduction rate of cold rolling is greater than 3.0%, a part of the residual Vostian iron will be transformed into Asada loose iron due to processing-induced metamorphosis, resulting in a reduction in the volume fraction of the residual Vostian iron, which will damage the characteristics. Yu. On the other hand, the lower limit value of the rolling reduction rate of cold rolling is not particularly limited, and the characteristics of the steel sheet of this embodiment can be obtained without performing cold rolling.

Next, a method for measuring each configuration of the steel sheet of the present embodiment and the heat treatment steel sheet of the present embodiment will be described. `` Measurement of the steel structure '' Ferritic iron (soft fertile iron, hard ferrous iron), toughened iron, tempered Asada loose iron, newborn Asada loose iron, Pola iron, The volume fractions of Xueming carbon iron and residual Vostian iron can be measured by the following methods.

Samples were taken with a plate thickness section parallel to the rolling direction of the steel plate as the observation surface, and the observation surface was polished, and then nitrate etching was performed. Next, when observing the steel structure inside the steel plate, it is in one or more observation fields in the range of 1/8 thickness to 3/8 thickness centered on the observation surface at a position of 1/4 thickness from the surface. When observing the steel structure of the soft layer, the field emission scanning electron microscope (FE-SEM: Field Emission Scanning Electron) is used in one or more observation fields in the region including the depth range of the soft layer from the outermost layer of the steel plate. Microscope) observe a total area of 2.0 × 10 ^-9 m ² or more. Then, the area fractions of the ferrous iron, the toughened iron, the tempered Asada iron, the newly-born Asada iron, the boron iron, the citronite, and the residual Vosda iron were measured and used as volume fractions. Here, a region having a lower structure in the crystal grains and carbides precipitated in a plurality of deformed bodies was judged to be tempered Asada iron. In addition, the area where the citronite precipitated into layers was judged to be boron iron or citronite. The area where the brightness is small and the lower tissue cannot be observed is judged to be fat iron (soft iron fat iron or hard iron iron). The area with high brightness and no underlying structure due to etching was judged to be fresh Asada loose iron or residual Vosda iron. The remainder was judged to be toughened iron. The volume fraction of each tissue was calculated by using the point counting method for each volume fraction. The volume fractions of the hard fertilized iron and the soft fertilized iron are based on the measured volume fraction of the ferrous iron, and the individual volume fractions are determined by the method described later. The volume fraction of the fresh Asada loose iron is calculated from the volume fraction of the fresh Asada loose iron or the residual Vosted iron by subtracting the volume fraction of the remaining Vosted iron obtained by the X-ray diffraction method described later.

In the steel sheet of the present embodiment and the heat-treatment steel sheet used as a material thereof, the volume fraction of the residual Vostian iron contained in the steel sheet is evaluated by the X-ray diffraction method. Specifically, in the range from 1/8 thickness to 3/8 thickness centered on the position of 1/4 thickness from the thickness surface of the plate, the surface parallel to the plate surface is made into a mirror surface and diffracted by X-rays The area fraction of FCC iron was determined by the method, and this was used as the volume fraction of residual Vosstian iron.

"Ratio of volume fraction of residual Vosstian iron contained in soft layer and volume fraction of residual Vosstian iron contained in steel plate" In the steel sheet of this embodiment, volume fraction of residual Vosstian iron contained in soft layer The ratio to the volume fraction of the residual Vosstian iron inside the steel sheet was evaluated by performing high-resolution crystal structure analysis using the EBSD method (electron backscatter diffraction method). Specifically, a sample was taken with a plate thickness section parallel to the rolling direction of the steel plate as an observation surface, and the observation surface was polished to a mirror surface. In order to remove the processed layer of the surface layer, electrolytic polishing or mechanical polishing using a silicic acid glue is performed. Next, the surface area of the steel plate including the soft layer and the inside of the steel plate (range from 1/8 thickness to 3/8 thickness centered at a position of 1/4 thickness from the surface), use the EBSD method to observe the total area of the visual field. A total of 2.0 × 10 ^-9 m ² or more (multiple fields or the same field of view) can be used to analyze the crystal structure. For the analysis of the data measured by the EBSD method during the measurement, "OIM Analysys 6.0" manufactured by TSL was used. In addition, the distance between steps (step) is 0.01 to 0.20 μm. From the observation results, the area determined as the FCC iron was determined to be the residual Vosstian iron, and the volume fraction of the residual Vosstian iron in the soft layer and the steel plate was calculated individually.

`` Measurement of the aspect ratio and length of the residual Vostian iron particles '' The aspect ratio and length of the residual Vostian iron particles contained in the steel structure inside the steel plate were evaluated by EBSD method for high-resolution crystal orientation analysis . Specifically, a sample was taken with a plate thickness section parallel to the rolling direction of the steel plate as an observation surface, and the observation surface was polished to a mirror surface. In order to remove the processed layer of the surface layer, electrolytic polishing or mechanical polishing using a silicic acid glue is performed. Next, for the inside of the steel plate (range from 1/8 thickness to 3/8 thickness centered on the position of 1/4 thickness from the surface), use the EBSD method to measure a total of 2.0 × 10 ^-9 m ² or more (multiple fields or the same The crystal structure can be analyzed in the area of any field of view). From the observation results, the area judged to be FCC iron was regarded as residual Vosstian iron. Then, in order to avoid measurement errors, since the crystal orientation of the residual Vosstian iron grains measured by the above method is used, only the crystal orientation distribution map of the Vosstian iron grains having a major axis length of 0.1 μm or more is plotted. The boundary that produces a crystal orientation difference of 10 ° or more is regarded as the grain boundary of the residual Vostian iron particles. The aspect ratio is the value obtained by dividing the length of the major axis of the residual Wastfield iron particles by the minor axis length. The major axis is the major axis length of the residual Vostian iron particles. From this result, the ratio of the number of residual Vostian irons to the total remaining Vostian irons with an aspect ratio of 2.0 or more was obtained. The data analysis by the EBSD method uses "OIM Analysys 6.0" made by TSL. In addition, the distance between steps (step) is 0.01 to 0.20 μm.

"Fertilized iron granules (hard ferrous iron) containing Vostian iron granules / Fertilized iron granules (soft ferrous iron) not containing Vostian iron granules" Included (inclusive) The method of the grains of Vostian iron grains and the grains not containing Vostian iron grains will be explained. First, the crystal grains were observed by FE-SEM, and high-resolution crystal orientation analysis was performed by the EBSD method. Specifically, a sample was taken with a plate thickness section parallel to the rolling direction of the steel plate as an observation surface, and the observation surface was polished to a mirror surface. In order to remove the processed layer of the surface layer, electrolytic polishing or mechanical polishing using a silicic acid glue is performed. Next, for the inside of the steel plate (range from 1/8 thickness to 3/8 thickness centered on the position of 1/4 thickness from the surface), use the EBSD method to measure a total of 2.0 × 10 ^-9 m ² or more (multiple fields or the same The crystal structure can be analyzed in the area of any field of view). Then, based on the data measured from BCC iron, the boundary where the azimuth difference of 15 ° or more is generated is taken as the grain boundary, and the grain boundary distribution of the ferrous iron particles is depicted. Next, in order to avoid measurement errors, the data measured from the FCC iron only depicts the grain distribution diagram of Vostian iron grains with a major axis length of 0.1 μm or more, and overlaps with the grain boundary distribution diagram of the ferrous grain iron grains. As long as there is more than one Vostian iron granule in one fat granule, it is regarded as "ferrous granule containing Vostian iron granule". However, when it is not adjacent to the Vostfield iron grains, or only when the boundary with other grains is adjacent to the Vostfield iron grains, it is regarded as "fat grain iron grains that do not contain the Vostfield iron grains".

"Hardness between the surface layer and the inside of the steel plate" The hardness distribution between the surface layer and the inside of the steel plate for determining the thickness of the soft layer can be obtained by the following method, for example. Samples were taken with a plate thickness section parallel to the rolling direction of the steel plate as the observation surface, and the observation surface was polished to be a mirror surface, and chemical polishing was performed using a silicic acid glue to remove the processed layer of the surface layer. The micro hardness measuring device for the observation surface of the prepared sample starts from a position with a depth of 5 μm from the outermost layer and starts from the surface to a position where the thickness of the plate is 1/8, and presses into the vertex angle at a pitch of 10 μm along the thickness of the steel plate ° Quadrangular pyramid Vickers indenter. At this time, the indentation load is set so that each Vickers indentation does not affect each other. For example, 2gf. Then, the diagonal length of the indentation is measured with an optical microscope or a scanning electron microscope, and converted into a Vickers hardness (Hv). Next, the measurement position was moved by 10 μm or more in the rolling direction, and the same measurement was performed from a position at a depth of 10 μm from the outermost layer to a position where the plate thickness was 1/8 of the thickness. Then, the measurement position was moved by 10 μm or more in the rolling direction, and the same measurement was performed from the surface to a position of 1/8 of the plate thickness starting from a position with a depth of 5 μm from the outermost layer. Next, the measurement position was moved by 10 μm or more in the rolling direction, and the same measurement was performed from a position at a depth of 10 μm from the outermost layer to a position where the plate thickness was 1/8 of the thickness. As shown in FIG. 7, the Vickers hardness of each of five points was measured for each thickness position by repeating the process. In this way, in fact, hardness measurement data with a pitch of 5 μm in the depth direction can be obtained. In order to prevent indentations from affecting each other, the measurement interval is not limited to a 5 μm pitch. The average value of 5 points was made into the hardness at the thickness position. The hardness distribution in the depth direction is obtained by linear interpolation between the data. The thickness of the soft layer is obtained by reading the depth position where the hardness is 80% or less of the hardness of the base metal from the hardness distribution chart. On the other hand, the hardness inside the steel plate ranges from 1/8 thickness to 3/8 thickness centered on the 1/4 thickness position. The hardness is measured at least 5 points with a micro hardness measuring device according to the same emphasis as above, and the value is measured. Find the average. As the micro hardness measuring device, for example, FISCHERSCOPE (registered trademark) HM2000 XYp can be used.

"Ratio of the aspect ratio of ferrous iron contained in the soft layer to grains having an aspect ratio of 3.0 or more" The aspect ratio of ferrous iron in the soft layer was observed by FE-SEM, and the EBSD method (electron Backscatter diffraction method) was evaluated by high-resolution crystal orientation analysis. The data analysis by the EBSD method uses "OIM Analysys 6.0" made by TSL. In addition, the distance between steps (step) is 0.01 to 0.20 μm. From the observation results, the area judged as BCC iron was regarded as fertile iron and the crystal orientation distribution map was drawn. Then, the boundary where a crystal orientation difference of 15 ° or more occurs is regarded as a grain boundary. The aspect ratio is the value obtained by dividing the length of the long axis of each iron grain by the length of the short axis.

"High-frequency glow discharge (high-frequency GDS) analysis" When analyzing the steel plate of this embodiment and the steel plate for heat treatment by the high-frequency glow discharge analysis method, a known high-frequency GDS analysis method can be used. Specifically, the following method is used: A method in which the surface of a steel plate is placed in an Ar gas environment, and the surface of the steel plate is sputter-plated and analyzed in a depth direction while a voltage is applied to generate a glow paste. Then, the elements in the material (steel plate) are identified by the emission spectrum wavelength unique to the element in which the atoms can be excited in the plasma, and the amount of the elements contained in the material is estimated from the luminous intensity of the identified elements. Data in the depth direction can be estimated from the sputtering time. Specifically, the relationship between the sputtering time and the sputtering depth can be obtained in advance using a standard sample to convert the sputtering time into the sputtering depth. Therefore, the sputtering depth obtained by changing the sputtering time can be defined as the depth from the surface of the material. A commercially available analysis device can be used for high-frequency GDS analysis. In this embodiment, a high-frequency glow discharge light emission analysis device GD-Profiler2 manufactured by Horiba, Ltd. is used. Examples

Next, an embodiment of the present invention will be described. The condition in the embodiment is an example of the condition adopted for confirming the feasibility and effect of the present invention. The present invention is not limited to this one condition example. As long as the object of the present invention can be achieved without departing from the gist of the present invention, the present invention can adopt various conditions.

A steel billet made of steel having a chemical composition shown in Table 1 was melted. The steel billet was subjected to the following hot rolling to produce a hot-rolled steel sheet: The steel billet heating temperatures shown in Tables 2 to 5 were performed under the steel billet heating conditions set to the values of the formula (4) shown in Tables 2 to 5. The heating and rolling completion temperature were set to the temperatures shown in Tables 2 to 5. After that, the surface scale was removed by pickling the hot-rolled steel sheet. Then, a part of the hot-rolled steel sheet is cold-rolled into a cold-rolled steel sheet.

[Table 1]

[Table 2]

[table 3]

[Table 4]

[table 5]

The hot-rolled steel sheet having a thickness of 1.2 mm or the cold-rolled steel sheet with a thickness of 1.2 mm obtained as described above is subjected to the first heat treatment and / or the second heat treatment shown below. In addition, in some examples, the cold-rolled steel sheet that has been cooled in the first heat treatment to the cooling stop temperature shown in Tables 6 to 9 is subjected to the second heat treatment without being cooled to room temperature. In other embodiments, the second heat treatment is performed after cooling to the cooling stop temperature in the first heat treatment, and after cooling to room temperature. On the other hand, a part of the examples is performed without the first heat treatment.

(First heat treatment) The conditions shown in Tables 6 to 9 were heated to the maximum heating temperature and maintained at the maximum heating temperature. Then, it cooled to the cooling stop temperature. In the first heat treatment, in a gas environment containing H ₂ and log (PH ₂ O / PH ₂ ) at the concentrations shown in Tables 6 to 9 as the values shown in Tables 6 to 9, heating from 650 ° C to the maximum Heating temperature so far.

A _c3 is obtained by the following formula (9), and Ms is obtained by the following formula (10). A _c3 = 879-346C + 65Si-18Mn + 54Al (9) (The symbol of the element in the formula (9) is the mass% of the element in the steel). Ms = 561-407 × C-7.3 × Si-37.8 × Mn-20.5 × Cu-19.5 × Ni-19.8 × Cr-4.5 × Mo ・ (10) (The symbol of the element in the formula (10) is that the element is in Mass% in steel).

[TABLE 6]

[TABLE 7]

[TABLE 8]

[TABLE 9]

(Second heat treatment) The average heating rate from 650 ° C to the maximum heating temperature was set to the conditions shown in Tables 10 to 13, and heated to the maximum heating temperature and maintained at the maximum heating temperature. Then, an average cooling rate of 700 to 600 ° C. is used as the average cooling rate shown in Tables 10 to 13, and it is cooled to the cooling stop temperature. In the second heat treatment, heating was performed from 650 ° C to the maximum heating temperature in the gas environment shown in Tables 10 to 13.

Next, a part of the high-strength steel sheet (Experimental Examples Nos. 54 and 69) after the second heat treatment was electro-galvanized, and electro-galvanized layers were formed on both surfaces of the high-strength steel sheet to obtain an electro-galvanized steel sheet (EG). In each experimental example, the experimental examples No. 1 'to 80' are alloyed at a time point after cooling and isothermal holding under the conditions shown in the table (that is, at the time point shown in the pattern [1] in FIG. 4). Hot-dip galvanized. In addition, among these experimental examples No. 1 'to 80', experimental examples 1 'to 16', 18 'to 58', 60 'to 73', and 75 'to 80' are alloyed after hot-dip galvanizing. The experimental examples 17 ′, 59 ′, and 74 ′ were not alloyed after hot-dip galvanizing.

Experimental example No. 81 '~ 88' is based on the pattern shown in FIG. 5 [2]. After heating to the maximum heating temperature as shown in the table, after cooling at an average cooling rate, alloying melting is performed in addition to experimental example No. 86. Galvanized and alloyed, and cooled and maintained isothermally under the conditions shown in Tables 10-13.

The experimental example No. 89 'is based on the mode [3] shown in FIG. 6, and is heated to the maximum heating temperature under the conditions shown in the table, and then cooled and isothermally maintained, and then temporarily cooled to room temperature, and then alloyed. Melt galvanizing and alloying.

Each example of the hot-dip galvanizing system was immersed in a molten zinc bath at 460 ° C., and the surface area of each side of the steel sheet was 50 g / m ² .

A _c1 is obtained by the following formula (8), and A _c3 is obtained by the above formula (9). A _c1 = 723-10.7 × Mn-16.9 × Ni + 29.1 × Si + 16.9 × Cr (8) (The symbol of the element in the formula (8) is the mass% of the element in the steel).

[TABLE 10]

[TABLE 11]

[TABLE 12]

[TABLE 13]

Next, the steel plates of the experimental examples No. 1 to No. 78 and the experimental examples No. 1 'to No. 89' obtained as described above were measured by the method described above at the center of the 1/4 thickness position from the surface. For the steel structure (steel structure inside the steel plate) in the range of / 8 thickness to 3/8 thickness, the soft fertile iron, residual Vostian iron, tempered Asada iron, fresh Asada iron, Pola and Total volume of Xueming carbon iron, volume fraction of hard fat iron and toughened iron.

The steel plates of the steel plates of Experimental Examples No. 1 to No. 78 and Experimental Examples No. 1 'to No. 89' used the above method to investigate the residual Vastian iron accounted for the total residual Vastian iron with an aspect ratio of 2.0 or more. The number ratio. These results are shown in Tables 14-17.

[TABLE 14]

[Table 15]

[TABLE 16]

[TABLE 17]

Next, the steel plates of Experimental Examples No. 1 to No. 78 and Experimental Examples No. 1 'to No. 89' used the above methods to measure the steel structure and hardness, and investigated the thickness of the soft layer and the ferrous iron with an aspect ratio of 3.0 or more It accounts for the volume fraction of ferrous iron in the soft layer, the ratio of the residual Vostian iron in the soft layer to the volume fraction of the residual Vostian iron in the interior of the steel sheet. The results are shown in Tables 18 to 21.

In addition, the steel plates of Experimental Nos. 1 to 78 and Experimental Nos. 1 'to 89' were analyzed by the above-mentioned method from the surface in the depth direction and analyzed by high-frequency glow discharge analysis to show the light emission at the wavelength of Si. The peaks of the intensity were investigated in a range of more than 0.2 μm and a depth of 10.0 μm or less. There was a peak showing a luminous intensity at a wavelength of Si (a peak showing an internal oxide layer containing a Si-containing oxide). In the steel plates of Experimental Examples No. 1 to No. 78 and Experimental Examples No. 1 'to No. 89', Si appeared in the depth direction from the surface to a depth greater than 0.2 μm and less than 10.0 μm. The peak of the luminous intensity of the wavelength is evaluated as "yes" by internal peaks, and the peak of no light emission is evaluated as "none" by internal peaks of oxidation. The results are shown in Tables 18 to 21.

[TABLE 18]

[TABLE 19]

[TABLE 20]

[TABLE 21]

In addition, the steel plates of Experimental Examples No. 1 to 78 and Experimental Examples No. 1 'to No. 89' were used to investigate the maximum tensile stress (TS), elongation (El), and expandability (expanded Porosity), Hydrogen embrittlement resistance of the bent portion, chemical conversion treatment, or plating adhesion. The results are shown in Tables 22 to 25.

A JIS No. 5 tensile test piece was taken with the rolling direction orthogonal to the rolling direction, the maximum tensile stress and elongation were measured according to JIS Z2241, and the hole expandability was measured according to JIS Z2256. Then, those having a maximum tensile stress of 700 MPa or more were evaluated as good.

In addition, in order to evaluate the balance between strength, elongation, and hole expandability, the following formula was calculated using the results of the maximum tensile stress (TS), elongation (El), and hole expandability (hole enlargement ratio) measured by the methods described above. (11). The larger the value represented by formula (11), the better the balance between strength, elongation and hole expandability. It was evaluated that the value of the formula (11) was 80 × 10 ^-7 or more. TS ² × El × λ (11) (In formula (11), TS represents the maximum tensile stress (MPa), El represents the elongation (%), and λ represents the hole expandability (%)). The results are shown in Tables 22 to 25.

The hydrogen embrittlement resistance of the bent portion was evaluated by the following method. First, a short strip-shaped test piece of 30 mm × 120 mm was taken from the steel plate so that the long-side direction of the test piece was perpendicular to the rolling direction of the steel plate, and the two ends of the test piece were subjected to hole processing for bolt fastening. Next, the test piece was bent 180 ° with a punch having a radius of 5 mm. Then, the rebounded U-bend test piece was fastened with a bolt and a nut to load the stress. At this time, a GL5 mm strain gauge was attached to the top of the U-bend test piece, and a strain of 0.8 times the tensile strength was loaded under the control of the strain amount. At this time, the stress is set by converting the strain into a stress from a stress-strain curve obtained in advance in a tensile test. The end face of the U-bend test piece is in a state after shearing. The U-bend test piece subjected to stress was subjected to cathode hydrogenation until the test piece was broken. As the electrolyte solution, 3 g / L ammonium thiocyanate was added to a 3% NaCl aqueous solution, and the charge current density was set to -0.05 mA / cm ² . The test piece after the fracture was immediately stored in liquid nitrogen, and the amount of hydrogen in the steel was measured by a temperature-increased hydrogen analysis method using a gas chromatograph (temperature increase rate: 100 ° C./hour, measured up to 300 ° C.). The amount of hydrogen released from the steel material from room temperature to 200 ° C was taken as the amount of diffusible hydrogen. The same test was performed three times, and the average value was defined as the critical diffusible hydrogen amount. For materials with a tensile strength of 1100 MPa or less, those whose critical diffusible hydrogen content is 1.0 ppm or more are judged to be "Ex", those who are 0.6 to 1.0 ppm are judged to be "G", and those who are less than 0.6 ppm are judged to be "B". For materials with a tensile strength of more than 1100 MPa and less than 1350 MPa, the critical diffusible hydrogen amount is 0.8 ppm or more and is judged to be "Ex", the 0.5 to 0.8 ppm is judged to be "G", and the less than 0.5 ppm is judged to be "B". ". For materials with a tensile strength greater than 1350 MPa, those with a critical diffusible hydrogen content of 0.6 ppm or more were judged as "Ex", those with 0.3 to 0.6 ppm were judged as "G", and those with less than 0.3 ppm were judged as "B".

In addition, the steel plates of No. 1 to No. 78 except for Experimental Examples No. 54 and No. 69 were used to measure chemical conversion treatability by the following method. The steel plate was cut into 70 mm × 150 mm, and an 18 g / l aqueous solution of a degreaser (trade name: Fine Cleaner E2083) made by Nihon Parkerizing Co., Ltd. was sprayed at 40 ° C. for 120 seconds. Then, the steel plate coated with the degreasing agent was washed with water and degreased, and the steel plate was immersed in a 0.5 g / l aqueous solution of a surface conditioner (trade name: PREPALENE XG) manufactured by Nihon Parkerizing Co., Ltd. at room temperature for 60 seconds. Thereafter, the steel sheet after the surface conditioner was applied was immersed in a zinc phosphate treatment agent (trade name: PALBOND L3065) manufactured by Nihon Parkerizing Co., Ltd. for 120 seconds, washed with water, and dried. As a result, a chemical conversion treatment film composed of a zinc phosphate film is formed on the surface of the steel sheet.

A test piece having a width of 70 mm and a length of 150 mm was taken from the steel plate on which the chemical conversion treatment film was formed. Then, using a scanning electron microscope (SEM), three places (the central portion and both end portions) in the longitudinal direction of the test piece were observed at a magnification of 1000 times. For each test piece, the degree of grain adhesion of the chemical conversion treatment film was evaluated according to the following criteria.

On the "Ex" surface, zinc phosphate crystals with a chemical conversion treatment film are closely adhered. "G" Zinc phosphate crystals were scattered, and slight gaps were found between adjacent crystals (the zinc phosphate film was not attached, and the part was generally called "exposed"). "B" on the surface was clearly found to be covered with a film that had not been chemically treated.

"EG" on the surface of Tables 21 to 25 indicates an electrogalvanized steel sheet, "GI" indicates a galvanized steel sheet, and "GA" indicates an alloyed galvanized steel sheet.

In addition, the steel plates of Experimental Examples No. 54, No. 69, and No. 1 'to No. 89' were used to measure the plating adhesion by the methods described below.

A test piece of 30 mm × 100 mm was taken from these steel plates and subjected to a 90 ° V bending test. Then, a commercially available SELLOTAPE (registered trademark) was attached along a curved ridge line, and the plating width attached to the tape was measured as the peeling width. The evaluation is as follows. Ex: Small plating peeling (peeling width less than 5mm) G: Peeling that does not affect actual use (peeling width 5mm or more and less than 10mm) B: severe peeling (peeling width 10mm or more) Plating adhesion is based on Ex , G is qualified.

The evaluation results of each experimental example are described below.

[TABLE 22]

[TABLE 23]

[TABLE 24]

[TABLE 25]

Experimental examples No. 1, 3, 4, 7, 10, 12 to 14, 18, 19, 21 to 23, 27, 28, 30 to 34, 36, 37, 39 to 42, 44 to 46 are examples of the present invention , 49, 50, 52 ~ 63, 66 ~ 70, 76 ~ 78, 1'3 ', 4', 7 ', 10' ~ 14 ', 16' ~ 19 ', 23', 24'26 '~ 28' , 32 ', 33', 35 '~ 39', 41 ', 42', 44 '~ 47', 49 '~ 51', 54 ', 55', 57 '~ 68', 71 '~ 75', 81 '~ 89' has high strength, good ductility and hole expansion, good bendability after processing, chemical conversion treatment, or good plating adhesion.

Since the steel plates of Experimental Nos. 11, 17, 29, 47, and 48 were not subjected to the first heat treatment, the metal structure did not contain hard ferrous iron, and the balance of strength, elongation, and hole expansion ratio was poor. In the steel plate of Experimental Example No. 2, the highest heating temperature in the first heat treatment was low, so the soft fertilized iron was large and the number of residual Vostian irons with an aspect ratio of 2.0 or more was insufficient, resulting in strength, elongation, and hole expansion. Poor balance of rates.

The steel plate of Experimental Example No. 5 had a slow average heating rate from 650 ° C to the maximum heating temperature in the first heat treatment, so the number ratio of the residual Vostian iron with an aspect ratio of 2.0 or more was insufficient, and the strength, elongation, Poor balance of hole expansion. The steel sheets of Experimental Examples No. 6, 15, ₁₆ , and 24 had high log (PH ₂ O / PH ₂ ) during the first heat treatment, and the desired surface layer structure could not be obtained, so the hydrogen embrittlement resistance of the bent portion was poor.

Since the cooling rate in the first heat treatment was slow in the steel plate of Experimental Example No. 8, the fraction of the soft fertile iron in the internal structure of the steel plate increased. Therefore, the balance of strength, elongation, and hole expansion ratio of the steel plate of Experimental Example No. 8 was poor. In the steel plates of Experimental Nos. 9, 15, 20, 25, 48, and 51, because the log (PH ₂ O / PH ₂ ) in the second heat treatment was low, the desired surface layer structure could not be obtained, so the hydrogen embrittlement resistance of the bent portion was Chemical properties are poor.

Regarding the steel plates of Experimental Examples No. 9, 20, 25, 48, and 51, the evaluation of the chemical conversion handleability was "B" because there were no internal oxidation peaks.

Because the highest heating temperature in the second heat treatment was high for the steel plate of Experimental Example No. 26, the metal structure did not contain hard ferrous iron, and the desired surface structure could not be obtained, so the balance of strength, elongation, and hole expansion ratio was poor. In addition, the hydrogen embrittlement resistance of the bent portion is poor. In the steel plate of Experimental Example No. 35, the holding time between 300 ° C and 480 ° C in the second heat treatment was insufficient. Therefore, the loose iron fraction of the newly grown Asada in the internal structure increased, and the strength, elongation, and hole expansion ratio were balanced. difference.

In the steel plate of Experiment No. 38, the cooling stop temperature during the first heat treatment was high, so the number ratio of the residual Vostian iron with an aspect ratio of 2.0 or more was insufficient, and the balance of strength, elongation, and hole expansion ratio was poor. In the steel plate of Experimental Example No. 43, the cooling rate during the second heat treatment was slow, so the total scores of the boron iron and Schiff carbon iron in the internal structure of the steel plate increased, and the strength, elongation, and hole expansion ratio were balanced. difference.

In the steel plate of Experimental Example No. 64, the highest heating temperature in the second heat treatment was low, so the residual Vossian iron fraction in the internal structure of the steel plate was insufficient, and the balance of strength, elongation, and hole expansion ratio was poor. Since the log (PH ₂ O / PH ₂ ) in the second heat treatment of the steel sheet of Experimental Example No. 65 was large, the thickness of the soft layer in the surface structure of the steel sheet became thick, and the maximum tensile stress (TS) was insufficient.

The chemical composition of the steel plates of Experimental Examples Nos. 71 to 75 is outside the scope of the present invention. In the steel plate of Experimental Example No. 71, the maximum tensile stress (TS) was insufficient because the C content was insufficient. Since the steel plate of Experimental Example No. 72 had a large Nb content, the bendability after processing was poor. Since the steel sheet of Experimental Example No. 73 had insufficient Mn content, the maximum tensile stress (TS) was insufficient. The steel plate of Experimental Example No. 74 had a large Si content, and therefore had poor hole expandability. Since the steel plate of Experimental Example No. 75 had a large Mn content and a P content, the elongation and hole expandability were poor.

The steel plates of Experiment No. 15 ', 22', 34 ', 52', and 53 'were not subjected to the first heat treatment, so the metal structure did not contain hard ferrous iron, and the balance of strength, elongation, and hole expansion ratio was poor. .

Since the maximum heating temperature in the first heat treatment was low in the steel plate of Experimental Example No. 2 ', the number ratio of the residual Vostian iron with an aspect ratio of 2.0 or more was insufficient, and the balance of strength, elongation, and hole expansion ratio was poor.

In the steel plate of Experimental Example No. 5 ', the average heating rate from 650 ° C to the maximum heating temperature in the first heat treatment was slow, so the number ratio of the residual Vostian iron with an aspect ratio of 2.0 or more was insufficient, and the strength and elongation The balance of hole expansion rate is poor.

The steel sheets of Experimental Nos. 6 ', 20', 21 ', and 29' had high log (PH ₂ O / PH ₂ ) during the first heat treatment, and the desired surface structure could not be obtained, so the hydrogen embrittlement resistance of the bent portion difference.

In the steel plate of Experimental Example No. 8 ', the cooling rate during the first heat treatment was slow, so that the iron fraction of the soft fertilizer particles increased. Therefore, the balance of strength, elongation, and hole expansion ratio is poor.

Experimental example No. 9 ', 20', 22 ', 25', 29 ', 30', 53 ', 56' The steel sheet in the second heat treatment has a low log (PH ₂ O / PH ₂ ) and cannot obtain the desired The surface layer structure is poor in hydrogen embrittlement resistance of the bent portion.

The steel plates of Experiment No. 9 ', 22', 25 ', 30', 53 ', and 56' had no internal oxidation peaks because no soft layer was formed in the surface structure of the steel plate, so the evaluation of the plating adhesion was " B ".

In the steel plate of Experimental Example No. 31 ', the highest reaching temperature in the second heat treatment was high, so the hard structure did not contain iron and the desired surface structure could not be obtained. Therefore, the balance of strength, elongation, and hole expansion ratio was poor. , The hydrogen embrittlement resistance of the bent portion is poor.

In the steel plate of Experimental Example No. 40 ', the holding time between 300 ° C and 480 ° C in the second heat treatment was insufficient. Therefore, the loose iron fraction of the fresh Asada in the internal structure increased, and the strength, elongation, and expansion Poor balance.

The steel plate of Experimental Example No. 43 'had a high cooling stop temperature during the first heat treatment, so the number ratio of the residual Vostian iron with an aspect ratio of 2.0 or more was insufficient, and the balance of strength, elongation, and hole expansion ratio was poor.

In the steel plate of Experimental Example No. 48 ', the cooling rate during the second heat treatment was slow, so the total scores of the boron iron and cis-carbon iron in the internal structure of the steel plate increased, and the strength, elongation, and hole expansion ratios were increased. Poor balance.

In the steel plate of Experimental Example No. 69 ', the highest reaching temperature in the second heat treatment was low, so the residual Vostian iron fraction in the internal structure of the steel plate was insufficient, and the balance of strength, elongation, and hole expansion ratio was poor.

Since the log (PH ₂ O / PH ₂ ) in the second heat treatment was large for the steel plate of Experimental Example No. 70 ′, the thickness of the soft layer in the surface structure of the steel plate became thicker, and the maximum tensile stress (TS) was insufficient.

The chemical composition of the steel plates of Experimental Examples No. 76 'to 80' is outside the scope of the present invention. Among these, the steel plate of Experimental Example No. 76 'had insufficient C content, so that the maximum tensile stress (TS) was insufficient. Since the steel plate of Experimental Example No. 77 'had a large Nb content, the bendability after processing was poor. Since the steel sheet of Experimental Example No. 78 'had insufficient Mn content, the maximum tensile stress (TS) was insufficient. Since the steel sheet of Experimental Example No. 79 'had a large Si content, the hole expandability was poor. Since the steel plate of Experimental Example No. 80 'had a large Mn content and a high P content, the elongation and hole expandability were poor.

The preferred embodiments and examples of the present invention have been described above, but these embodiments and examples are only examples within the scope of the present invention, and the structures can be added as long as they do not depart from the scope of the present invention. , Omit, replace other changes. That is, the present invention is not limited to the foregoing description, but is limited only by the scope of the attached patent application, and may be appropriately changed within its scope.

Industrial Applicability According to the present invention, it is possible to provide a high-strength steel sheet excellent in ductility and hole expansion, chemical conversion treatment, plating adhesion, and good bending property after processing, and a method for manufacturing the same. The steel sheet of the present invention is excellent in ductility and hole expandability and has good bendability after processing. Therefore, it can be used as a steel sheet for automobiles that can be formed into various shapes by press working or the like. In addition, the steel sheet of the present invention is suitable for a steel sheet having a chemical conversion treatment film or a plating layer on the surface due to its excellent chemical conversion treatment properties and plating adhesion.

1‧‧‧ steel plate

11‧‧‧From the surface of the steel plate, the range is from 1/8 thickness to 3/8 thickness centered on the position of 1/4 thickness (inside the steel plate)

12‧‧‧ soft layer

FIG. 1 is a cross-sectional view of a steel plate according to this embodiment, which is parallel to the rolling direction and the thickness direction. FIG. 2 shows the relationship between the depth from the surface and the luminous intensity (Intensity) showing the wavelength of Si when the steel plate of this embodiment is analyzed in the depth direction (plate thickness direction) from the surface by the high-frequency glow discharge analysis method. chart. Fig. 3 shows the light emission from the surface and the wavelength of Si when a steel plate (comparative steel plate) different from this embodiment is analyzed from the surface in the depth direction (plate thickness direction) by a high-frequency glow discharge analysis method. A graph of the relationship between Intensity. FIG. 4 is a line diagram showing a first example of a temperature / time pattern of the second heat treatment to the hot-dip galvanizing and alloying treatment in the method for manufacturing a steel sheet according to this embodiment. FIG. 5 is a line diagram showing a second example of the temperature / time pattern of the second heat treatment to the hot-dip galvanizing and alloying treatment in the method for manufacturing a steel sheet according to this embodiment. Fig. 6 is a line diagram showing a third example of the temperature / time pattern of the second heat treatment to the hot-dip galvanizing and alloying treatment in the method for manufacturing a steel sheet according to this embodiment. FIG. 7 is a schematic diagram showing an example of hardness measurement of the steel plate according to the embodiment.

Claims (10)

A steel plate characterized by having the following chemical composition: contained in mass%: C: 0.050% to 0.500%, Si: 0.01% to 3.00%, Mn: 0.50% to 5.00%, P: 0.0001% to 0.1000%, S: 0.0001% to 0.0100%, Al: 0.001% to 2.500%, N: 0.0001% to 0.0100%, O: 0.0001% to 0.0100%, Ti: 0% to 0.300%, V: 0% to 1.00%, Nb: 0% ~ 0.100%, Cr: 0% ~ 2.00%, Ni: 0% ~ 2.00%, Cu: 0% ~ 2.00%, Co: 0% ~ 2.00%, Mo: 0% ~ 1.00%, W: 0% ~ 1.00%, B: 0% ~ 0.0100%, Sn: 0% ~ 1.00%, Sb: 0% ~ 1.00%, Ca: 0% ~ 0.0100%, Mg: 0% ~ 0.0100%, Ce: 0% ~ 0.0100 %, Zr: 0% ~ 0.0100%, La: 0% ~ 0.0100%, Hf: 0% ~ 0.0100%, Bi: 0% ~ 0.0100%, REM: 0% ~ 0.0100%, and the remainder is composed of Fe and impurities ; From the surface, the steel structure in the range of 1/8 thickness to 3/8 thickness centered on the 1/4 thickness position is contained in volume fraction: soft ferrous iron: 0% ~ 30%, residual Voss Tiantie: 3% ~ 40%, fresh Asada loose iron: 0% ~ 30%, the total of bolai iron and Xueming carbon iron: 0% ~ 10%, and the remaining part contains hard fertilized iron; the aforementioned 1/8 Thickness ~ 3/8 thickness in the aforementioned range, with a residual aspect ratio of 2.0 or more The ratio of the total residual Vostian iron is 50% or more. When a region having a hardness of 80% or less of the hardness in the aforementioned range of 1/8 thickness to 3/8 thickness is defined as a soft layer, the surface starts from the aforementioned surface. There is a soft layer with a thickness of 1 to 100 μm in the thickness direction. Among the ferrous iron contained in the soft layer, the volume fraction of the grains with an aspect ratio of 3.0 or more is 50% or more. The residual Voss in the soft layer is The volume fraction of Tiantie is 80% or less of the volume fraction of the residual Vostian iron in the aforementioned range of 1 / 8th to 3 / 8th of the thickness. When the glow discharge analysis method shows the emission intensity of the wavelength of Si, a peak of the emission intensity of the wavelength of the above-mentioned Si appears in the range from 0.2 μm to 10.0 μm from the surface. For example, the steel sheet of claim 1, wherein the aforementioned chemical composition contains one or more of the following elements: Ti: 0.001% to 0.300%, V: 0.001% to 1.00%, and Nb: 0.001% to 0.100%. For example, the steel sheet of claim 1, wherein the aforementioned chemical composition contains one or more of the following elements: Cr: 0.001% to 2.00%, Ni: 0.001% to 2.00%, Cu: 0.001% to 2.00%, and Co: 0.001% ~ 2.00%, Mo: 0.001% ~ 1.00%, W: 0.001% ~ 1.00%, and B: 0.0001% ~ 0.0100%. For example, the steel sheet of claim 1, wherein the aforementioned chemical composition contains one or two of the following elements: Sn: 0.001% to 1.00%, Sb: 0.001% to 1.00%. For example, the steel sheet of claim 1, wherein the aforementioned chemical composition contains one or more of the following elements: Ca: 0.0001% ~ 0.0100%, Mg: 0.0001% ~ 0.0100%, Ce: 0.0001% ~ 0.0100%, Zr: 0.0001% ~ 0.0100%, La: 0.0001% ~ 0.0100%, Hf: 0.0001% ~ 0.0100%, Bi: 0.0001% ~ 0.0100%, and REM: 0.0001% ~ 0.0100%. The steel sheet according to any one of claims 1 to 5, wherein the aforementioned chemical composition satisfies the following formula (i): Si + 0.1 × Mn + 0.6 × Al ≧ 0.35‧‧‧ (i) (Si in formula (i) , Mn, and Al represent the content of each element in terms of mass%). The steel sheet according to any one of claims 1 to 5, which has a hot-dip galvanized layer or an electro-galvanized layer on the surface. The steel sheet according to claim 6, which has a hot-dip galvanized layer or an electro-galvanized layer on the surface. A method for manufacturing a steel sheet is a method for manufacturing a steel sheet according to any one of claims 1 to 6, and the method is characterized in that the hot-rolled steel sheet or the cold-rolled steel sheet is subjected to the following items (a) to (e): 1 After the heat treatment, a second heat treatment that satisfies the following (A) to (E) is performed. The aforementioned hot-rolled steel sheet is obtained by hot-rolling and pickling a steel billet having a chemical composition according to any one of claims 1 to 6. It is obtained that the cold-rolled steel sheet is obtained by cold-rolling the hot-rolled steel sheet; (a) during the period from 650 ° C to the maximum heating temperature, it is set to contain 0.1% by volume of H ₂ and satisfy the following formula (ii) the gas environment; (b) hold at the highest heating temperature of A _c3 -30 ℃ ~ 1000 ℃ for 1 second to 1000 seconds; (c) the average heating speed in the temperature range from 650 ℃ to the maximum heating temperature is Heating from 0.5 ° C / second to 500 ° C / second; (d) after maintaining at the maximum heating temperature, cooling with an average cooling rate in a temperature range from 700 ° C to Ms of 5 ° C / second or more; and (e) cooling at an average cooling rate of 5 ° C / sec or more to a cooling stop temperature of Ms or less; (A) the maximum heating at 650 ° C During the period up to the temperature, a gas environment in which H ₂ is 0.1 vol% or more and O ₂ is 0.020 vol% or less and log (PH ₂ O / PH ₂ ) satisfies the following formula (iii); (B) is in A _c1 + Hold at the highest heating temperature of 25 ℃ ~ A _c3 -10 ℃ for 1 second ~ 1000 seconds; (C) heating with the average heating speed from 650 ℃ to the maximum heating temperature of 0.5 ℃ / second ~ 500 ℃ / second; (D) cooling such that the average cooling rate in the temperature range from 700 to 600 ° C is 3 ° C / sec or more; and (E) cooling at an average cooling rate of 3 ° C / sec or more, at 300 ° C to 480 ° C Hold for more than 10 seconds; log (PH ₂ O / PH ₂ ) <-1.1‧‧‧ (ii) -1.1 ≦ log (PH ₂ O / PH ₂ ) ≦ -0.07‧‧‧ (iii) (formula (ii ) And formula (iii), PH ₂ O represents a partial pressure of water vapor, and PH ₂ represents a partial pressure of hydrogen). If the method of manufacturing a steel sheet according to claim 9 is performed, it is subjected to a hot-dip galvanizing treatment in a stage subsequent to the cooling process in the aforementioned (D).