TWI664300B - Steel sheet and manufacturing method thereof - Google Patents

Abstract

The steel sheet according to one aspect of the present invention has a predetermined chemical composition, and the steel structure in the steel sheet contains, as a volume fraction, soft fertilized iron: 0% to 30%, and residual Vostian iron: 3% to 40. %, Fresh Asada loose iron: 0% ~ 30%, total of bolai iron and cis carbon iron: 0% ~ 10%, the remaining part contains hard ferrous iron; in the interior of the steel plate, the residual Voss of aspect ratio of 2.0 or more Tian Tie, whose ratio of the total amount of residual Vostian Tie is more than 50%; there is a soft layer with a thickness of 1 to 100 μm in the thickness direction from the surface; among the ferrous iron contained in the soft layer, The volume fraction of crystal grains with a ratio less than 3.0 is more than 50%; the volume fraction of residual Vosstian iron in the soft layer is: more than 50% of the volume fraction of residual Vosstian iron in the steel plate; In the range of 0.2 μm or more and 5 μm or less from the surface, a peak indicating the emission intensity of the wavelength of Si appears.

Description

Steel plate and manufacturing method thereof

FIELD OF THE INVENTION The present invention relates to a steel plate and a method for manufacturing the same.

BACKGROUND OF THE INVENTION In recent years, from the viewpoint of regulating the emission of greenhouse gases accompanying global warming measures, it is required to further increase the fuel cost of automobiles. Then, in order to reduce the weight of the vehicle body and ensure collision safety, the use of high-strength steel plates in automotive components is gradually expanding. Of course, the steel sheet to be supplied to a component for an automobile requires various workability not only for strength but also press workability, weldability, and the like when forming a component. Specifically, from the viewpoint of press workability, an excellent elongation ratio (total elongation ratio in a tensile test; El) and a stretch flangeability (hole expansion ratio; λ) are often required for a steel sheet. .

As a method for improving press workability in a high-strength steel sheet, a DP steel (Dual Phase steel) having a ferrous iron phase and a Mata loose iron phase is known (for example, refer to Patent Document 1). DP steel has excellent ductility. However, DP steel has poor hole expandability because the hard phase becomes the starting point of void formation.

In addition, as a technique for improving the ductility of a high-strength steel sheet, there is a TRIP steel in which a Vostian iron phase remains in a steel structure and uses a TRIP (metastatically induced plasticity) effect (for example, refer to Patent Document 2). TRIP steel is more ductile than DP steel. However, TRIP steel has poor hole expansion properties. Furthermore, in the TRIP steel, a large amount of an alloy such as Si must be added in order to leave Vostian iron. Therefore, the chemical conversion treatment and plating adhesion of TRIP steel are also poor.

In addition, Patent Document 3 describes a high-strength steel sheet having a tensile strength of 800 MPa or more and excellent hole expandability, the microstructure of which contains 70% or more of toughened iron or toughened ferrous iron in terms of area ratio. Patent Document 4 describes a high-strength steel sheet with a tensile strength of 800 MPa or more and excellent hole expandability and ductility. The microstructure is such that the main phase is toughened iron or toughened ferrous iron, and the second phase is Voss. Tin iron, let the remainder be fat iron or loose iron.

As a technique for improving the bending workability of a high-strength steel sheet, for example, Patent Document 5 describes a high-strength cold-rolled steel sheet which is manufactured by decarburizing a steel sheet, and the surface portion is made of a main body of ferrous iron. Make up. In addition, Patent Document 6 describes an ultra-high-strength cold-rolled steel sheet produced by subjecting a steel sheet to decarburization annealing and having a soft layer on a surface layer portion. However, the technologies described in Patent Documents 5 and 6 have insufficient fatigue resistance characteristics.

In addition, Non-Patent Document 1 discloses that a secondary annealing method in which a steel sheet is subjected to secondary annealing is used to improve the elongation and hole expandability of the steel sheet. However, in the technique described in Non-Patent Document 1, the flexibility is insufficient. Advance technical literature

Patent Literature Patent Literature 1: Japanese Patent Application Laid-Open No. 6-128688 Patent Literature 2: Japanese Patent Application Laid-Open No. 2006-274418 Patent Literature 3: Japanese Patent Application Laid-Open No. 2003-193194 Patent Literature 4: Japanese Patent Application Laid-Open No. 2003 -193193 Patent Document 5: Japanese Patent Application Publication No. 10-130782 Patent Document 6: Japanese Patent Application Publication No. 5-195149

Non-Patent Literature Non-Patent Literature 1: K. Sugimoto et al .: ISIJ int., (1993), 775.

SUMMARY OF THE INVENTION Problems to be Solved by the Invention As far as conventional high-strength steel sheets are concerned, no one having excellent bending properties and excellent fatigue resistance has appeared. The present invention has been made in view of the above-mentioned circumstances, and an object thereof is to provide a steel sheet, a hot-dip galvanized steel sheet, and a method for manufacturing the same, which have good ductility and hole expandability, and have excellent fatigue resistance, bendability, and plating adhesion.

Means for Solving the Problems In order to solve the above problems, the inventors of the present case have conducted careful discussions repeatedly. As a result, it was found that it is sufficient if the hot-rolled steel sheet or cold-rolled steel sheet having a predetermined chemical composition is subjected to secondary heat treatment (annealing) with different conditions, thereby forming the steel sheet into a predetermined steel structure and forming a predetermined thickness. At the same time as the surface layer of the steel structure, an internal oxide layer containing Si oxide is formed at a predetermined depth.

Specifically, by the first heat treatment, the inside of the steel plate is made into a steel structure mainly composed of a lath-like structure such as Asada loose iron, and the surface layer is made into a steel structure mainly composed of soft ferrous iron. 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). As a result, the steel plate obtained after the second heat treatment and random galvanizing will become a steel structure in which needle-shaped residual Vostian iron is dispersed, and the surface layer will become a kind of soft ferrous iron as the main body and dispersed in a small amount. Composite structure of Asada loose iron and residual Vostian iron with predetermined thickness. Such a steel sheet and a hot-dip galvanized steel sheet have excellent ductility and hole expandability, and the balance between bendability and fatigue resistance is good.

What's more, during the first and second heat treatments mentioned above, the alloy elements such as Si contained in the steel are inhibited from oxidizing outside the steel sheet, and an internal oxide layer containing Si oxide is formed at a predetermined depth. Thereby, excellent chemical conversion treatment properties and plating adhesion can be obtained. The present invention has been completed based on these findings. The gist of this invention is as follows.

(1) A steel sheet related 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%, 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% ~ 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%, and REM: 0% ~ 0.0100% , The remaining part is composed of Fe and impurities; the steel structure in the range of 1 / 8th to 3 / 8th of the thickness centered on the position of 1 / 4th of the thickness from the surface contains, by volume fraction, soft fertilized iron: 0% ~ 30%, Residual Vostian Iron: 3% ~ 40%, Fresh Asada Iron: 0% ~ 30%, Total of Polite Iron and Xueming Carbon Iron: 0% ~ 10%, the rest contains hard Fertilized iron; 1/4 thickness from the aforementioned surface, the aforementioned position as the center, 1/8 thickness to 3/8 thickness, In the above-mentioned residual Vosstian iron having an aspect ratio of 2.0 or more, the ratio of the total number of the remaining Vosstian irons is 50% or more; a region having the following hardness is defined as a soft layer: the hardness is 1 / 8 thickness to 3/8 thickness in the aforementioned range of less than 80% of the hardness; at this time, there is a soft layer with a thickness of 1 to 100 μm in the thickness direction from the aforementioned surface; the ferrous iron contained in the soft layer Among the crystal grains, the volume fraction of the crystal grains having an aspect ratio of less than 3.0 is 50% or more; the volume fraction of the residual Vostian iron in the soft layer is in the aforementioned range of 1/8 to 3/8 thick. 50% or more of the above-mentioned volume fraction of the residual Vostian iron; from the aforementioned surface, the luminous intensity of the wavelength of Si is analyzed by the high-frequency glow discharge analysis method in the thickness direction of the aforementioned board, and at this time, it is calculated from the aforementioned surface When it is larger than 0.2 μm and within a range of 5 μm or less from the aforementioned surface, a peak representing the aforementioned emission intensity of the aforementioned wavelength of Si appears. (2) The steel sheet according to the above (1), wherein the chemical composition may contain one or two or more members selected from the group consisting of: Ti: 0.001% to 0.300%, and V: 0.001% to 1.00 % And Nb: 0.001% ~ 0.100%. (3) The steel sheet according to the above (1) or (2), wherein the chemical composition may contain one or two or more members selected from the group consisting of: Cr: 0.001% to 2.00%, and Ni: 0.001% to 2.00%, Cu: 0.001% to 2.00%, Co: 0.001% to 2.00%, Mo: 0.001% to 1.00%, W: 0.001% to 1.00%, and B: 0.0001% to 0.0100%. (4) The steel sheet according to any one of the above (1) to (3), wherein the chemical composition may further contain one or two selected from the group consisting of: Sn: 0.001% to 1.00% , And Sb: 0.001% ~ 1.00%. (5) The steel sheet according to any one of the above (1) to (4), wherein the chemical composition may contain one or two or more members selected from the group consisting of: Ca: 0.0001% to 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%. (6) The steel sheet according to any one of (1) to (5), wherein the chemical group may also satisfy the following formula (1): Si + 0.1 × Mn + 0.6 × Al ≧ 0.35 ... 1) Si, Mn, and Al in the formula (1) are the contents of the respective elements in terms of mass%. (7) The steel sheet according to any one of the above (1) to (6), in the aforementioned range from 1/8 thickness to 3/8 thickness in which the aforementioned position is 1/4 thickness from the aforementioned surface is the center, The volume fraction of tempered Asada loose iron can also be 0% ~ 50%. (8) The steel sheet according to any one of (1) to (7) above, which may have a hot-dip galvanized layer on the surface. (9) The steel sheet according to any one of (1) to (7) above, which may have a galvanized layer on the surface. (10) A method for manufacturing a steel sheet according to another aspect of the present invention is a method for manufacturing a steel sheet according to any one of the above (1) to (9), and is performed on a hot-rolled steel sheet or a cold-rolled steel sheet. After the first heat treatment satisfying the following (a) to (e), a second heat treatment satisfying the following (A) to (E) is performed; the hot-rolled steel sheet has any one of the above (1) to (6) The steel slab of the chemical composition is hot rolled and pickled, and the cold rolled steel sheet is cold rolled from the aforementioned hot rolled steel sheet; (a) from 650 ° C to the highest heating The gas environment around the hot-rolled steel sheet or the cold-rolled steel sheet is set to a gas environment containing H _{2 of} 0.1% by volume or more and satisfying the following formula (2); (b) A _c3 -30 The highest heating temperature of ℃ ~ 1000 ℃, hold for 1 second to 1000 seconds; (c) from 650 ℃ to the maximum heating temperature, heating at an average heating rate of 0.5 ℃ / second ~ 500 ℃ / second; (d) at the highest After the heating temperature is maintained, cooling is performed at an average cooling rate of 5 ° C / sec or more from 700 ° C to Ms; (e) The aforementioned cooling is performed at an average cooling rate of 5 ° C / sec or more until M (A) From 650 ° C to the maximum heating temperature, set the gas environment around the hot-rolled steel sheet or the cold-rolled steel sheet to contain H _{2 of} 0.1% by volume or more And satisfy the gas environment of the following formula (3); (B) at the highest heating temperature of A _c1 + 25 ℃ ~ A _c3 -10 ℃, hold for 1 second to 1000 seconds; (C) from 650 ℃ to the maximum heating temperature So far, heating at an average heating rate of 0.5 ° C / sec to 500 ° C / sec; (D) cooling from the maximum heating temperature to below 480 ° C so that the average cooling rate between 600 and 700 ° C is 3 ° C / sec or more (E) After cooling at an average cooling rate of 3 ° C / sec or more, hold it at 300 ° C to 480 ° C for more than 10 seconds; -1.1 ≦ log (PH ₂ O / PH ₂ ) ≦ -0.07 ... 2) In formula (2), PH ₂ O represents the partial pressure of water vapor, and PH ₂ represents the partial pressure of hydrogen; log (PH ₂ O / PH ₂ ) <-1.1 ・ ・ (3) In formula (3) , PH ₂ O represents the partial pressure of water vapor, and PH ₂ represents the partial pressure of hydrogen. (11) The method for manufacturing a steel sheet according to the above (10), which may be the following method: a method for manufacturing the steel sheet according to the above (8); and in the second heat treatment, 650 ° C. to a maximum heating cooled in the second heat treatment, the more the (D); a temperature between far, the constant atmosphere containing 0.1 vol% of H _2, O ₂ 0.020% by volume or less, and satisfies the formula (3) The stage after the process is performed by hot-dip galvanizing.

Effects of the Invention According to the present invention, it is possible to provide a steel sheet, a hot-dip galvanized steel sheet, and a method for manufacturing the same, which are excellent in ductility and hole expansion, and have excellent fatigue resistance, bendability, and excellent plating adhesion. The high-strength steel sheet and high-strength hot-dip galvanized steel sheet of the present invention are suitable for forming into various kinds by press working, because they have good ductility and hole expandability, and have excellent fatigue resistance, bendability, and plating adhesion. Shaped automotive steel sheet.

The form "steel sheet" used to implement the invention The steel sheet 1 of this embodiment shown in Fig. 1 has a steel sheet interior 11 having a range of 1/8 to 3/8 thickness centered at a position of 1/4 thickness from the surface, and The soft layer 12 disposed on the surface of the steel plate. The 1 / 4-thickness position is a 1 / 4-depth portion of the thickness t of the steel plate from the surface of the steel plate. In FIG. 1, the area corresponding to the symbol 1 / 4t is marked. The so-called 1 / 8th to 3 / 8th thickness range is the range between the 1 / 8th depth region and the 3 / 8th depth region of the steel plate thickness t from the surface of the steel sheet, the 1 / 8th thickness position, and the 3 / 8th thickness region. The thick positions in FIG. 1 correspond to positions marked with the symbol 1 / 8t and the symbol 3 / 8t. The soft layer 12 is a region having a hardness of 80% or less of the hardness of the inner part 11 of the steel sheet, as described later. The steel sheet 1 may be further provided with a hot-dip galvanized layer, an electro-galvanized layer, and the like on its surface (that is, the surface of the soft layer 12). Hereinafter, the steel plate of this embodiment will be described in detail. First, the chemical composition of the steel sheet will be described. In the following description, [%] indicating an element content means [mass%].

"C: 0.050% ~ 0.500%" C is an element that greatly increases strength. C stabilizes Vosstian iron to obtain residual Vosstian iron, thereby effectively combining strength and formability. However, once the C content is more than 0.500%, the weldability will be significantly deteriorated. Therefore, the C content is set to 0.500% or less. From the point of point weldability, the C content is preferably 0.350% or less, and more preferably 0.300% or less, 0.250% or less, or 0.200% or less. On the other hand, if the C content is less than 0.050%, sufficient residual Vosstian iron cannot be obtained, and it becomes difficult to ensure sufficient strength and formability. Therefore, the C content is set to be 0.050% or more. In order to further improve the strength and formability, the C content should be more than 0.075%, and more preferably 0.100% or 0.200%.

"Si: 0.01% to 3.00%" Si is an element that stabilizes residual Vostian iron by suppressing the generation of iron-based carbides in a steel sheet, and improves the strength and formability. However, Si is an element that embrittles steel. When the Si content exceeds 3.00%, the hole expandability of the steel sheet becomes insufficient. Moreover, once the Si content is more than 3.00%, problems such as cracks in the steel billet after casting tend to occur. Therefore, the Si content is set to 3.00%. Further, since Si can impair the impact resistance characteristics of the steel sheet, the Si content should preferably be 2.50% or less, more preferably: 2.00% or less and 1.80% or less. On the other hand, when the Si content is less than 0.01%, coarse iron-based carbides are generated in large amounts, and the strength and formability are deteriorated. Therefore, the Si content is set to 0.01% or more. From this point of view, the lower limit of Si is preferably 0.10%, more preferably: 0.25%, 0.30%, or 0.50%.

"Mn: 0.50% to 5.00%" Mn is added to improve the hardenability of the steel sheet and increase the strength. However, once the Mn content exceeds 5.00%, the elongation and hole expandability of the steel sheet become insufficient. In addition, if the Mn content is more than 5.00%, coarse Mn thickened portions are formed in the central portion of the plate thickness of the steel sheet, and embrittlement tends to occur easily, and problems such as cracks in the billet after casting tend to occur. Therefore, the Mn content is set to 5.00% or less. In addition, as the Mn content increases, the spot weldability also deteriorates. From this point of view, the Mn content is preferably 3.50% or less, and more preferably 3.00% or less and 2.80% or less. On the other hand, if the Mn content is less than 0.50%, a soft structure is formed in a large amount during cooling after annealing, and it becomes difficult to ensure a sufficiently high maximum tensile strength. Accordingly, the Mn content must be 0.50% or more. In order to further increase the strength, the Mn content should be more than 0.80%, and more preferably: more than 1.00% or more than 1.50%.

"P: 0.0001% to 0.1000%" P is an element that embrittles steel. When the P content is more than 0.1000%, the elongation and hole expandability of the steel sheet become insufficient. In addition, once the P content is more than 0.1000%, problems such as cracks in the steel billet after casting become easy to occur. Therefore, the P content is set to 0.1000% or less. In addition, P is an element that causes embrittlement of a molten portion formed by spot welding. In order to obtain sufficient welded joint strength, the P content should be set to 0.0400% or less, and more preferably set to 0.3000% or less or 0.0200% or less. On the other hand, setting the P content to less than 0.0001% leads to a significant increase in manufacturing costs. From this point of view, the P content is set to 0.0001% or more. The P content is preferably set to be 0.0010% or more, 0.0012%, or 0.0015% or more.

"S: 0.0001% to 0.0100%" S combines with Mn to form coarse MnS, and reduces formability such as ductility, hole expansion (stretch flange), and bendability. Therefore, the upper limit value of S is set to 0.0100% or less. In addition, S deteriorates the spot weldability. Therefore, it is preferably set to 0.0070% or less, and more preferably 0.0050% or 0.0030% or less. On the other hand, setting the S content to less than 0.0001% leads to a significant increase in manufacturing costs. Therefore, the S content is set to 0.0001% or more. The S content should be set to 0.0003% or more, and more preferably set to: 0.0006% or more, or 0.0010% or more.

"Al: 0.001% to 2.500%" Al is an element that embrittles steel. Once the Al content is more than 2.500%, it becomes easy to have problems such as cracks in the steel billet after casting. Therefore, the Al content is set to 2.500% or less. In addition, as the Al content increases, the spot weldability deteriorates. Therefore, the Al content should be set to 2.000% or less, and more preferably 1.500% or 1.000% or less. On the other hand, even if the lower limit of the Al content is not particularly limited, the steel sheet of the present embodiment will have an effect, but Al is an impurity that is present in a small amount in the raw material. Setting the content to less than 0.001% causes a significant increase in manufacturing costs. Accordingly, the Al content is set to 0.001% or more. In addition, Al is also an effective element as a deoxidizing material. In order to obtain a sufficient deoxidizing effect, the Al content should be set to 0.010% or more. Furthermore, Al is an element that suppresses the formation of coarse carbides, and it may be added for the purpose of stabilizing the residual Vosstian iron. In order to maintain the stability of the Wastfield iron, the Al content should preferably be set to 0.100% or more, and more preferably 0.250% or more.

"N: 0.0001% ~ 0.0100%" N forms coarse nitrides and deteriorates the ductility, hole expandability (stretch flange), and bendability. Therefore, it is necessary to suppress the addition of N. the amount. When the N content is more than 0.0100%, the deterioration of formability becomes significant. From this viewpoint, the upper limit of the N content is set to 0.0100%. In addition, N is a cause of blowholes during welding, and therefore, the smaller the content, the better. The N content is preferably 0.0075% or less, and more preferably: 0.0060% or less and 0.0050% or less. Even if the lower limit of the N content is not particularly limited, the steel sheet of the present embodiment will exert an effect. However, if the N content is set to less than 0.0001%, the manufacturing cost will increase significantly. From this point of view, the lower limit of the N content is set to 0.0001% or more. The N content should be more than 0.0003%, more preferably: more than 0.0005% or more than 0.0010%.

"O: 0.0001% to 0.0100%" O forms oxides and deteriorates moldability such as ductility, hole expandability (stretched flangeability), and bendability. Therefore, it is necessary to suppress the content. When the O content is more than 0.0100%, the deterioration of formability becomes significant. From this viewpoint, the upper limit of the O content is set to 0.0100%. Further, the O content is preferably 0.0050% or less, more preferably 0.0030% or less and 0.0020% or less. Even if the lower limit of the O content is not particularly limited, the steel sheet of the present embodiment will have an effect, but if the O content is set to less than 0.0001%, the manufacturing cost will increase significantly, so 0.0001% is used as the lower limit. The O content can also be set to: 0.0005% or more, 0.0010% or more, or 0.0012% or more.

“Si + 0.1 × Mn + 0.6 × Al ≧ 0.35” Residual Vosstian iron will have the worry of decomposing into toughened iron, boron iron, or coarse cis-carbon iron during heat treatment. Si, Mn, and Al are elements which are particularly important to suppress the decomposition of the residual Vosted iron to improve the formability, and it is preferable to satisfy the following formula (1). The value on the left side of formula (1) is more preferably 0.60 or more, and more preferably 0.80 or more or 1.00% or more. Si + 0.1 × Mn + 0.6 × Al ≧ 0.35 (1) (Si, Mn, and Al in the formula (1) are the contents of the respective elements in mass%.)

The steel sheet according to this embodiment may further contain any one or more of the following elements according to requirements. However, the steel sheet of this embodiment can solve the problem because it does not contain any of the following elements, so the content of any of the following elements may be 0%.

"Ti: 0% ~ 0.300%" Ti is an element that enhances the strength of the steel sheet by strengthening by precipitation, suppressing fine grain strengthening by growth of ferrous grains and iron crystal grains, and suppressing re-crystallization to improve differential strength. . However, if the Ti content is more than 0.300%, the precipitation of carbonitrides will increase and the formability will deteriorate. Therefore, the Ti content is preferably 0.300% or less. From the viewpoint of formability, the Ti content is preferably 0.150% or less. Even if the lower limit of the Ti content is not particularly limited, the steel sheet of this embodiment will exhibit effects, but in order to fully obtain the effect of improving the strength by Ti, the Ti content is preferably 0.001% or more. In order to increase the strength of the steel sheet, the Ti content is preferably 0.010% or more.

"V: 0% to 1.00%" V is an element that enhances the strength of the steel sheet by strengthening the precipitates, suppressing the fine grain strengthening caused by the growth of iron crystal grains in the fertilized grains, and suppressing the re-crystallization to improve the strength of the steel. However, when the V content is more than 1.00%, carbonitrides are excessively precipitated and the formability is deteriorated. Therefore, the V content should be 1.00% or less, and more preferably 0.50% or less. Even if the lower limit of the V content is not particularly limited, the steel sheet of this embodiment will exhibit effects, but in order to fully obtain the effect of increasing the strength brought by V, the V content is preferably 0.001% or more, and more preferably 0.010% or more.

"Nb: 0% to 0.100%" Nb is an element that enhances the strength of the steel sheet by strengthening the precipitates, inhibiting the fine grain strengthening caused by the growth of iron crystal grains in the fertile grains, and suppressing the differential discharge strengthening obtained by recrystallization. However, if the Nb content is more than 0.100%, the precipitation of carbonitrides will increase and the formability will deteriorate. Therefore, the Nb content should be 0.100% or less. From the viewpoint of formability, the Nb content is more preferably 0.060% or less. Even if the lower limit of the Nb content is not particularly limited, the steel sheet of the present embodiment will exhibit effects, but in order to fully obtain the effect of improving the strength brought by Nb, the Nb content is preferably 0.001% or more. In order to increase the strength of the steel sheet, the Nb content is preferably 0.005% or more.

"Cr: 0% to 2.00%" Cr is an element that improves hardenability and is effective for high strength. It can also be added in place of some C and / or Mn. If the Cr content is more than 2.00%, the workability at high temperatures is impaired and the productivity is reduced. From this point of view, the Cr content should preferably be 2.00% or less, and more preferably 1.20% or less. Even if the lower limit of the Cr content is not particularly limited, the steel sheet of this embodiment will exhibit effects, but in order to fully obtain the high strength effect brought by 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 and inhibits the transformation at high temperatures. It can also be added in place of part of C and / or Mn. Once the Ni content is more than 2.00%, the weldability is impaired. From this point of view, the Ni content should preferably be 2.00% or less, and more preferably 1.20% or less. Even if the lower limit of the Ni content is not particularly limited, the steel sheet of the present embodiment will exhibit effects, but in order to fully obtain the high-strength effect brought by the addition of 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 exists in steel as fine particles to improve strength, and can be added instead of part of C and / or Mn. Once the Cu content is greater than 2.00%, the weldability is impaired. From this point of view, the Cu content should be set to 2.00% or less, and more preferably 1.20% or less. Even if the lower limit of the Cu content is not particularly limited, the steel sheet of this embodiment will exhibit effects, but in order to fully obtain the high-strength effect brought by the addition of 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 hardenability and is effective for high strength. It can also be added in place of part of C and / or Mn. If the Co content is more than 2.00%, the processability at high temperatures is impaired and the productivity is reduced. From this point of view, the Co content should preferably be 2.00% or less, and more preferably 1.20% or less. Even if the lower limit of the Co content is not particularly limited, the steel sheet of the present embodiment will exhibit effects, but in order to fully obtain the high-strength effect brought by the addition of 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 which is effective for increasing the strength and inhibits the transformation of the phase at a high temperature, and may be added instead of part of C and / or Mn. If the Mo content is more than 1.00%, workability at high temperatures is impaired and productivity is reduced. From this point of view, the Mo content should preferably be 1.00% or less, and more preferably 0.50% or less. Although the lower limit of the Mo content is not particularly limited, the steel sheet of the present embodiment will exhibit effects, but in order to fully obtain the high-strength effect brought by the addition of 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 and inhibits the transformation at high temperatures. It can also be added in place of some C and / or Mn. When the W content is more than 1.00%, the processability at high temperatures is impaired and the productivity is decreased. From this point of view, the W content is preferably 1.00% or less, and more preferably 0.50% or less. The lower limit of the W content is not particularly limited, and the steel sheet of this embodiment will exert an effect, but in order to fully obtain the high strength brought by W, the W content is preferably 0.001% or more, and more preferably 0.010% or more.

"B: 0% to 0.0100%" B is an element that is effective for high strength by suppressing the phase transition at high temperature, and can be added instead of part of C and / or Mn. If the B content is more than 0.0100%, the processability at high temperatures is impaired and the productivity is reduced. From this point of view, the B content should preferably be 0.0100% or less. From the viewpoint of productivity, the B content is more preferably 0.0050% or less. Even if the lower limit of the B content is not particularly limited, the steel sheet of this embodiment will exhibit effects, but in order to fully obtain the high-strength effect brought by the addition of B, the B content should preferably be 0.0001% or more. For higher strength, the B content is preferably 0.0005% or more.

"Sn: 0% to 1.00%" Sn is an element that is effective for high-strengthening by suppressing the coarsening of the structure. It is not necessary to add 1.00% as the upper limit. Once the amount of Sn added is more than 1.00%, the steel sheet will be excessively brittle, and the steel sheet may break during rolling. Therefore, the Sn content should be 1.00% or less. The lower limit of the Sn content is not particularly limited, and the steel sheet of this embodiment will exert an effect, but in order to fully obtain the high strength effect 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 that is effective for high-strengthening by suppressing the coarsening of the structure. It may be added at the upper limit of 1.00%. If the amount of Sb added is more than 1.00%, the steel sheet will be excessively brittle, and the steel sheet may break during rolling. Therefore, the Sb content should be 1.00% or less. The lower limit of the Sb content is not particularly limited, and the steel sheet of this embodiment will exert an effect, but in order to fully obtain the high strength effect brought by Sb, the Sb content is preferably 0.001% or more, and more preferably 0.005% or more.

"Choose from Ca, Mg, Ce, Zr, La, Hf, Bi, and REM one or two or more types: 0% to 0.0100%, respectively." The so-called REM is the abbreviation of Rare Earth Metal, usually Refers to the element to which the lanthanide series belongs. However, in this embodiment, REM means excluding Ce and La. In this embodiment, La and / or Ce are often added as a rare earth metal alloy (mischmetall), and may be a compound containing a lanthanoid series element other than La and / or Ce. Even if the steel sheet according to the present embodiment contains lanthanoid elements other than La and / or Ce as impurities, the steel sheet still has an effect. In addition, even if the metal La and / or Ce is added, the steel sheet of the present embodiment still has an effect. In this embodiment, the REM content is a total value of the content of the elements to which the lanthanoid series belongs.

The effects of these elements are described below. Ca, Mg, Ce, Zr, La, Hf, Bi, and REM are elements effective for improving formability, and they may contain one or two or more of them at 0.0001% to 0.0100%, respectively. 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 be impaired. Therefore, the content of each of the above elements should be 0.0100% or less, and more preferably 0.0070% or less. When two or more of the above-mentioned elements are contained, the content of Ca, Mg, Ce, Zr, La, Hf, Bi, and REM should preferably be 0.0100% or less in total. Even if the lower limit of the content of each element is not particularly limited, the steel sheet of this embodiment will exhibit effects, but in order to fully obtain the effect of improving the formability of the steel sheet, the content of each element should be 0.0001% or more. From the viewpoint of formability, the content of one or more of Ca, Mg, Ce, Zr, La, Hf, Bi, and REM is more preferably 0.0010% or more.

The remaining parts of the above elements are Fe and impurities. In addition, the Ti, V, Nb, Cr, Ni, Cu, Co, Mo, W, B, Sn, and Sb are all contained as impurities and in a trace amount smaller than the appropriate lower limit. This is permissible. In addition, it is acceptable that Ca, Mg, Ce, Zr, La, Hf, Bi, and REM are contained as impurities in an extremely small amount that is smaller than the aforementioned appropriate lower limit. In addition, it contains H, Na, Cl, Sc, Zn, Ga, Ge, As, Se, Y, Tc, Ru, Rh, Pd, Ag, Cd, In, Te, Cs, Ta, Re, Os, Ir, Pt, Au, Pb are allowed as impurities.

Next, the steel structure (microstructure) of the steel plate inner part 11 of the steel plate of this embodiment is demonstrated. [%] In the description of the content of each tissue is [vol%].

(Microstructure inside steel plate 11) In the steel plate according to this embodiment, a steel structure in a range of 1/8 to 3/8 thickness centered on a position of 1/4 thickness from the surface (hereinafter, sometimes referred to as " The steel structure inside the steel plate ") contains: soft ferrous iron: 30% or less, residual Vostian iron: 3% to 40%, fresh Asada iron: 30% or less, the total of bolai iron and cis-carbon iron: 10% or less; and the ratio of the residual Vosstian iron with an aspect ratio of 2.0 or more to the total remaining Vosstian iron is 50% or more.

"Soft Fertilizer: 0% ~ 30%" Fertilizer is a type of tissue with excellent ductility. However, due to its low strength, ferrous iron is a structure that is difficult to apply to high-strength steel plates. In the steel sheet of this embodiment, the steel structure inside the steel sheet contains 0% to 30% of soft fertile iron. The "soft fertilized iron" in the present embodiment means ferrous iron that does not contain residual Vostian iron in the crystal grains. Soft ferritic iron has low strength, is more likely to generate strain concentration, and is more likely to be damaged than peripheral parts. Once the volume fraction of soft fertilized iron is greater than 30%, the strength-formability balance will be significantly deteriorated. Therefore, the iron content of soft fertilizers is limited to 30% or less. The soft fertilized iron should be limited to 15% or less, or 0%.

"Residual Vosstian Iron: 3% ~ 40%" Residual Vosstian Iron is an organization that improves the balance of strength and ductility. In the steel sheet according to 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 inside the steel plate is set to 3% or more, preferably 5% or more, and more preferably 7% or more. On the other hand, to set the volume fraction of the residual Vosstian iron to be greater than 40%, a large amount of C, Mn, and / or Ni needs to be added, and the weldability is significantly impaired. Therefore, the volume fraction of the residual Vosted iron is set to 40% or less. In order to improve the weldability and convenience of the steel sheet, the volume fraction of the residual Vosted iron should be set to 30% or less, and more preferably 20% or less.

"New born Asada loose iron: 0% ~ 30%" New born Asada loose iron will greatly increase the tensile strength. On the other hand, fresh Asada loose iron can be the starting point of destruction and significantly impair the impact resistance. Therefore, the volume fraction of the fresh-born Asada iron is set to 30% or less. In particular, in order to improve the shock resistance, the volume fraction of the fresh Asada loose iron should be set to 15% or less, and more preferably 7% or less. Although the amount of fresh iron in Asada may be 0%, it is preferably 2% or more in order to ensure the strength of the steel plate.

"Total of Boraite and Schering carbon iron: 0% ~ 10%" The microstructure inside the steel sheet of the steel sheet may also contain Boreir and / or Schering carbon iron. However, if the volume fraction of boron iron and / or skeletal carbon iron is large, the ductility is deteriorated. Therefore, the volume fraction of Plei iron and citronite is limited to a total of 10% or less. The volume fraction of bolai iron and schmidt carbon iron should be less than 5%, or 0%.

"The ratio of the number of residual Vosstian iron having 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 sheet is important. The aspect ratio is large, that is, the residual Vostian iron after elongation is very stable in the initial stage of deformation of the steel plate caused by processing. However, in the residual Vostian iron with a large aspect ratio, strain concentration occurs at the apex portion as the processing progresses, and a modest metamorphosis causes a TRIP (metamorphism-induced plasticity) effect. Therefore, if the steel structure inside the steel sheet contains residual Vosstian iron with a large aspect ratio, the ductility can be improved without compromising toughness, hydrogen embrittlement resistance, hole expansion, and the like. From the above point of view, in the present embodiment, the ratio of the number of residual Vosstian irons having an aspect ratio of 2.0 or more to the total number of remaining Vosstian irons is 50% or more. The number ratio of the residual Vosstian iron with an aspect ratio of 2.0 or more is preferably 60% or more, more preferably 70% or more, and more preferably 80% or more.

"Tempered Asada loose iron" Tempered Asada loose iron is a structure that will greatly increase the tensile strength of the steel plate without impairing the impact resistance. It may also be contained in the steel structure inside the steel plate. However, once a large amount of tempered Asada iron is generated inside the steel sheet, there may be cases where sufficient residual Vosda iron cannot be obtained. Therefore, the volume fraction of tempered Asada loose iron should be limited to less than 50% or less than 30%. In addition, the content of the tempered Asada loose iron is not necessary for the steel plate of this embodiment, so the lower limit of the tempered Asada loose iron is 0%.

In the steel sheet of the present embodiment, the remaining part of the steel structure inside the steel sheet is mainly composed of "hard ferritic iron" which surrounds residual Vostian iron in the crystal grains. Hard ferritic iron is formed by performing a second heat treatment on a steel sheet for heat treatment having a steel structure described below; the steel structure contains: toughened iron, toughened ferrous iron, tempered Asada loose iron, A slat-like structure composed of one or two or more types of fresh Asada loose iron. The hard ferritic iron has high strength because it surrounds residual Vostian iron in the crystal grains. In addition, compared with the case where the residual Vostian iron exists in the grain boundary of the fertile iron, the hard ferrous iron has a good formability because the interface between the ferrous iron and the residual Vostian iron does not easily peel off.

In addition, the remaining part of the steel structure in the steel plate may contain toughened iron. In addition, the toughened iron in this embodiment contains granular toughened iron composed of fine BCC crystals and coarse iron-based carbides, and upper toughened iron composed of lath-shaped BCC crystals and coarse iron-based carbides. And lower toughened iron composed of plate-like BCC crystals and fine iron-based carbides arranged side by side in parallel inside. In the steel sheet of this embodiment, the remaining part of the steel structure in the steel sheet is mainly composed of hard ferrous iron. That is, the remaining part of the steel structure inside the steel sheet contains hard fertilized iron more than toughened iron.

(Microstructure of the surface layer) "A region having the following hardness is defined as a soft layer: the hardness is 80% or less of the hardness in the range of 1/8 thickness to 3/8 thickness; at this time, there is 1 to 100 μm in the surface layer. "Soft layer" In order to improve the flexibility of the steel sheet, softening the surface layer of the steel sheet is one of the necessary requirements. In the steel sheet of this embodiment, when a region having a hardness of 80% or less of the internal hardness (average hardness) of the steel sheet is defined as a soft layer, a soft layer having a thickness of 1 to 100 μm exists in the thickness direction from the surface of the steel sheet. In other words, on the surface layer portion of the steel plate, there is a soft layer having a hardness of 80% or less of the average hardness inside the steel plate, 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, sufficient flexibility cannot be obtained. The thickness of the soft layer (the depth range from the surface) is preferably 5 μm or more, and more preferably 10 μm or more.

On the other hand, when 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.

"The volume fraction of the crystal grains of ferrous iron contained in the soft layer with an aspect ratio of less than 3.0 is 50% or more" The volume of the crystal grains of ferrous iron contained in the soft layer with an aspect ratio of less than 3.0 When the fraction (the ratio of the ferrous iron crystal grains to the volume fraction of the ferrous iron grains in the soft layer, the ferrous iron crystal grains having an aspect ratio of less than 3.0) is less than 50%, the bendability is deteriorated. Therefore, the volume fraction of the crystal grains having an aspect ratio of less than 3.0 among the ferrous grain iron contained in the soft layer is set to 50% or more. The volume fraction should be more than 60%, and more preferably 70%. The ferrous iron contained in the soft layer includes both the hard ferrous iron and the soft ferrous iron.

"The volume fraction of the residual Vosstian iron in the soft layer is: 50% or more of the volume fraction of the residual Vosstian iron in the interior of the steel sheet." The residual Vosstian iron contained in the soft layer suppresses fatigue cracking. Progress to improve the fatigue strength of steel plates. Accordingly, the volume fraction of the residual Vosstian iron contained in the soft layer is set to 50% or more of the volume fraction of the residual Vosstian iron in the interior of the steel sheet. Preferably, the volume fraction of the residual Vosstian iron contained in the soft layer is 60% or more, 70% or more, or 80% or more of the volume fraction of the residual Vosstian iron in the steel plate. The area ratio of the residual Vosstian iron in the steel plate refers to the residual Vosstian iron contained in the range of 1 / 8th to 3 / 8th of the thickness of the steel plate from the surface. The area ratio.

"Internal oxide layer containing Si oxide" In the steel sheet of this embodiment, when the analysis is performed by the high-frequency glow discharge (high-frequency GDS) analysis method in the depth direction (plate thickness direction) from the surface, the calculation starts from the surface. When it is larger than 0.2 μm and within a range of 5 μm or less from the surface, a peak indicating a luminous intensity at a wavelength of Si appears. This means that the steel sheet is internally oxidized, and has an internal oxide layer containing Si oxide in a range of more than 0.2 μm from the surface of the steel sheet and 5 μm or less from the surface. A steel sheet having such an internal oxide layer can suppress the formation of an oxide film such as Si oxide on the surface of the steel sheet due to the heat treatment during manufacturing. Therefore, a steel sheet having such an internal oxide layer has excellent chemical conversion treatment properties and plating adhesion.

When the steel plate of this embodiment is analyzed by the high-frequency glow discharge analysis method in the depth direction from the surface, the range from 0.2 μm to 5 μm from the surface and from 0 μm to 0.2 μm from the surface (depth 0.2) The region where the μm is shallower) may have a peak indicating the luminous intensity of the wavelength of Si. The presence of a peak in these two ranges indicates that the steel sheet has an internal oxide layer and an external oxide layer containing Si oxide on the surface.

FIG. 2 is a graph showing the difference between the depth from the surface and the luminous intensity (Intensity) of the wavelength of Si when the steel plate of this embodiment is analyzed by the high-frequency glow discharge analysis method in the depth direction from the surface. relationship. In the steel sheet of the present embodiment shown in FIG. 2, a peak (from the internal oxide layer) showing a luminous intensity of the wavelength of Si appears in a range of 0.2 μm and 5 μm or less from the surface. Further, a peak (from an external oxide layer (I _MAX )) indicating a luminous intensity of a wavelength of Si appeared in a range of 0 (outermost surface) to 0.2 μm from the surface. From this, it can be seen that the steel plate shown in FIG. 2 has an internal oxide layer and an external oxide layer.

FIG. 3 is a graph showing the depth from the surface of the steel plate different from the present embodiment when the analysis is performed by the high-frequency glow discharge analysis method in the depth direction (plate thickness direction) from the surface. The relationship between the emission intensity (Intensity) of the wavelength. In the steel sheet shown in FIG. 3, the peak showing the luminous intensity of the wavelength of Si appears in the range of 0 (outermost surface) to 0.2 μm from the surface, but not in the range of more than 0.2 μm and less than 5 μm. appear. This means that the steel sheet does not have an internal oxide layer, but only an external oxide layer.

"Hot-dip galvanized layer" The steel sheet of this embodiment may have a hot-dip galvanized layer formed on the surface (double-sided or single-sided) (hereinafter, the steel sheet of this embodiment having hot-dip galvanizing is referred to as "the steel sheet of this embodiment" Hot-dip galvanized steel sheet "). The hot-dip galvanized layer may also be an alloyed hot-dip galvanized layer in which the hot-dip galvanized layer is alloyed. In the case where the hot-dip galvanized layer is not alloyed, the iron content in the hot-dip galvanized layer is preferably less than 7.0% by mass. In the case of an alloyed hot-dip galvanized layer whose hot-dip galvanized layer has been alloyed, the iron 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.

Although the coating adhesion amount of the hot-dip galvanized layer is not particularly limited, from the viewpoint of corrosion resistance, it is preferably 5 g / m ² or more per side, more preferably: 20 to 120 g / m ² , and further 25 to 75 g / Within the range of m ² .

In the hot-dip galvanized steel sheet of this embodiment, an upper-layer plating layer may be further provided on the hot-dip galvanized layer for the purpose of improving paintability and weldability. In addition, in the hot-dip galvanized steel sheet of this embodiment, various treatments may be applied to the hot-dip galvanized layer, such as: chromate treatment, phosphate treatment, treatment for improving lubricity, and treatment for improving weldability.

"Electro-galvanized layer" An electro-galvanized layer may be formed on the surface of the steel sheet of this embodiment. The electro-galvanized layer can be formed by a conventional method.

"Heat treatment steel plate" The heat treatment steel plate (referred to as "heat treatment steel plate of this embodiment") used as the material of the steel plate of this embodiment is explained below. Specifically, the steel sheet for heat treatment of this embodiment has a chemical composition of any of the above-mentioned steel sheets, and has a steel structure (microstructure) shown below. [%] In the description of the content of each tissue is [vol%].

(Microstructure inside the steel sheet for heat treatment) "The lath-like structure has a volume fraction of 70% or more in total." In the steel sheet for heat treatment of this embodiment, the 1/4 thickness from the surface is centered at 1 / 8th thickness ~ The steel structure (steel structure inside the steel plate for heat treatment) in the range of 3/8 thickness contains a lath-like structure with a volume fraction of 70% or more; the lath-like structure is made of toughened iron and toughened fertilizer. One or two types of granulated iron, tempered Asada iron, and newly-born Asada iron.

Since the steel sheet for heat treatment contains the above-mentioned lath-like structure in a volume fraction of 70% or more in total, the steel structure inside the steel sheet for heat treatment will be made of hard fertilizer when the steel sheet for heat treatment is subjected to the second heat treatment described later. Grain iron is the main body. Once the total volume fraction of the lath-like structure in the steel sheet for heat treatment is less than 70%, a steel sheet obtained by subjecting the steel sheet for heat treatment to a second heat treatment will have a steel structure rich in soft fertile iron. As a result, the steel sheet of this embodiment cannot be obtained. The steel structure inside the heat treatment steel plate among the heat treatment steel plates preferably contains the above-mentioned lath structure in a volume fraction of 80% or more, more preferably 90% or more in total, or 100%.

"Number density of residual Vostian iron particles in the steel sheet for heat treatment having an aspect ratio of less than 1.3 and a major diameter of more than 2.5 µm" The steel structure inside the steel sheet for heat treatment in this embodiment contains a residue in addition to the above-mentioned strip-like structure Vosstian iron, and the number density of the remaining Vosstian iron particles whose aspect ratio is less than 1.3 and the major diameter is more than 2.5 μm is limited to 1.0 × 10 ^-2 pieces / μm ² or less.

Once the residual Vosstian iron existing in the steel structure inside the heat treatment steel plate is coarse and massive, the steel plate obtained by performing the second heat treatment on the steel heat treatment steel plate has a coarse and lumpy residual Vossian iron inside. Grains, and there is a case where residual Vosstian iron having an aspect ratio of 2.0 or more cannot be sufficiently ensured. Therefore, in the steel sheet for heat treatment, the number density of the coarse massive residual Vostian iron particles having an aspect ratio of less than 1.3 and a major diameter of more than 2.5 μm is set to 1.0 × 10 ^-2 pieces / μm ² or less. The lower the number density of the coarse and residual Vostian iron particles in the steel sheet for heat treatment, the better, and it is preferably 0.5 × 10 ^-2 pieces / μm ² or less.

In addition, once the residual Vosstian iron is excessively present inside the steel sheet for heat treatment, a part of the residual Vosstian iron is isotropic by subjecting the steel sheet for heat treatment to a second heat treatment described later. As a result, in the steel sheet obtained after the second heat treatment, there may be a case where a residual Vosstian iron having an aspect ratio of 2.0 or more cannot be sufficiently secured. Therefore, the volume fraction of the residual Vosstian iron contained in the steel structure inside the steel sheet for heat treatment should be 10% or less.

(Microstructure of the surface layer of the steel sheet for heat treatment) "A soft layer containing soft fertilized iron with a volume fraction of 80% or more" The steel sheet for heat treatment of the steel sheet material of this embodiment will be a surface layer composed of a soft layer The soft layer contains 80% or more soft fertilized iron in volume fraction. The thickness of the soft layer in the steel sheet for heat treatment is set to 1 μm to 50 μm. Once the thickness of the soft layer in the steel sheet for heat treatment is less than 1 μm in the depth direction from the surface, the second heat treatment is performed on the steel sheet for heat treatment, and the thickness of the soft layer (the depth range from the surface) is formed. Will be insufficient. On the other hand, if the thickness of the soft layer in the steel sheet for heat treatment is greater than 50 μm in the depth direction from the surface, the second heat treatment is performed on the steel sheet for heat treatment. (Depth range) becomes too high, and therefore, a decrease in the strength of the steel sheet by having a soft layer appears. Therefore, the thickness of the soft layer in the steel sheet for heat treatment is 50 μm or less, and preferably 10 μm or less.

"Internal oxide layer containing Si oxide" In the steel sheet for heat treatment of this embodiment, when analyzed by the high-frequency glow discharge (high-frequency GDS) analysis method in the depth direction from the surface, it is larger than 0.2 μm from the surface. Further, in a range of 5 μm or less, a peak indicating a light emission intensity at a wavelength of Si appears. This means that the steel sheet for heat treatment is internally oxidized and has an internal oxide layer containing Si oxide within a range of 0.2 μm from the surface and 5 μm or less. A steel sheet for heat treatment having an internal oxide layer in the above range can suppress the formation of an oxide film such as a Si oxide on the surface of the steel sheet with the heat treatment during production.

When the steel sheet for heat treatment of this embodiment is analyzed by high-frequency glow discharge analysis in the depth direction from the surface, the range from 0.2 μm to 5 μm from the surface and from 0 μm to 0.2 μm from the surface (depth A region that is shallower than 0.2 μm) may have a peak indicating the luminous intensity of the wavelength of Si. This means that the steel sheet for heat treatment has an internal oxide layer and an external oxide layer containing Si oxide on the surface.

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

In the method for manufacturing a steel sheet according to this embodiment, as shown in FIG. 4, a hot-rolled steel sheet or a cold-rolled steel sheet is subjected to a first heat treatment shown below to manufacture a steel sheet for heat treatment. The hot-rolled steel sheet has the above-mentioned chemical composition. The steel billet is hot rolled and pickled, and the cold rolled steel sheet is cold rolled. 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 may be performed using a dedicated heat treatment line, or may be performed using an existing annealing line.

(Casting step) In the production of the steel sheet according to this embodiment, first, a steel blank having the above-mentioned chemical composition (composition) is cast. The hot-rolled steel slabs can be produced using continuous casting steel slabs or thin steel slab casting machines. After casting, the steel billet can be temporarily cooled to normal temperature and then hot rolled, or it can be directly hot rolled at the high temperature as it is. The method of directly supplying the cast steel billet to the hot rolling at a high temperature as it is, is more suitable because the energy required for the hot rolling heating can be omitted.

(Heating steel billet) Before hot rolling, heat the steel billet. In the case of manufacturing the steel sheet of this embodiment, it is preferable to select a steel blank heating condition satisfying the formula (4) shown below.

[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 as follows A _c3 is a value represented by the following formula (9), and ts (T) is the residence time (sec) of the billet at the billet heating temperature T.)

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

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

[Mathematical formula 4] (In formula (7), T is the heating temperature (° C) of the steel billet, and R is the 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 molecule of formula (4) indicates the distribution degree of the Mn content from α to γ in the two-phase region retention of α (fertilizer 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) is a term corresponding to the diffusion distance of Mn atoms 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 make the Mn concentration distribution in the steel sufficiently homogeneous, it is desirable to select and set the heating conditions of the steel billet so that the value of the formula (4) is 1.0 or less. The smaller the value of the formula (4), the smaller the number density of the coarse massive Vostian iron particles in the steel sheet for heat treatment and the steel sheet.

(Hot rolling) After heating the steel billet, hot rolling is performed. When the completion temperature (completion temperature) of the hot rolling is less than 850 ° C, the rolling reaction force will increase, and it will become difficult to obtain a predetermined sheet thickness stably. Therefore, the completion temperature of the hot rolling is preferably set to 850 ° C or higher. From the viewpoint of the rolling reaction force, the completion temperature of the hot rolling is preferably set to 870 ° C or higher. On the other hand, in order to set the completion temperature of the hot rolling to more than 1050 ° C., it is necessary to use a heating device or the like to heat the steel sheet in the steps from the end of the heating of the steel billet to the completion of the hot rolling, and high cost is required. Therefore, it is desirable to set the completion temperature of hot rolling to 1050 ° C or lower. In order to easily ensure the temperature of the steel sheet during hot rolling, the completion temperature of the hot rolling should be set below 1000 ° C, and more preferably below 980 ° C.

(Pickling) Next, the hot-rolled steel sheet thus manufactured is pickled. Pickling is a step to remove oxides on the surface of hot-rolled steel sheet, and it is important to improve the chemical conversion treatment of the steel sheet. The pickling of the hot-rolled steel sheet may be performed once or divided into several times.

(Cold-rolled) The hot-rolled steel sheet after pickling can also be used as 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, once the total reduction ratio is greater than 85%, the steel sheet will lose ductility, and the risk of steel sheet fracture during cold-rolling will increase. 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 does not matter if cold rolling is not performed. In order to improve the uniformity of the shape of the steel sheet to obtain a good appearance, and at the same time to make the temperature of the steel sheet in the first heat treatment and the second heat treatment equal to obtain good ductility, the rolling reduction rate of cold rolling should be set to a total of 0.5% or more, more preferably Set it to 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 after 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) From 650 ° C to the maximum heating temperature, set the gas environment around the hot-rolled steel sheet or cold-rolled steel sheet to a gas containing 0.1% by volume or more of H ₂ and satisfying the following formula (2) surroundings. -1.1 ≦ log (PH ₂ O / PH ₂ ) ≦ -0.07 (2) (In the formula (2), PH ₂ O represents the partial pressure of water vapor, and PH ₂ represents the partial pressure of hydrogen.) 1 In the heat treatment, by satisfying the above (a), the oxidation reaction outside the steel sheet is suppressed, and the decarburization reaction is promoted. In the first heat treatment, in the temperature range from 650 ° C to the maximum heating temperature, the surroundings of the steel plate must be set to the gas environment described in (a) above; from 650 ° C to the maximum heating In all temperature ranges up to the temperature, it is preferable to set the surroundings of the steel plate to the gaseous environment described in (a) above.

Once the H ₂ in the gas environment is less than 0.1% by volume, the oxide film existing on the surface of the steel plate cannot be fully restored, and an oxide film is formed on the steel plate. Therefore, the steel sheet obtained after the second heat treatment has reduced chemical conversion treatability and plating adhesion. On the other hand, once the H ₂ content in the gas environment is greater than 20% by volume, the effect is saturated. In addition, once the H ₂ content in the gas environment is greater than 20% by volume, the risk of hydrogen explosion on mechanical operations will increase. Therefore, the content of H ₂ in the gas environment is preferably set to 20% by volume or less.

In addition, when log (PH ₂ O / PH ₂ ) is less than -1.1, in addition to the external oxidation of Si and Mn in the surface layer portion of the steel sheet, the decarburization reaction becomes insufficient, and a soft layer on the surface layer of the steel sheet for heat treatment is formed. The thickness will become thin. 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 after the second heat treatment is insufficient.

(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. Once the maximum heating temperature is lower than (A _c3 -30) ° C, massive, coarse ferrous iron remains in the steel sheet structure inside the steel sheet in the steel sheet for heat treatment. As a result, the steel sheet obtained after the second heat treatment of the heat-treated steel sheet has too many soft ferrous iron phases, and at the same time, the ratio of the number of residual Vostian irons having an aspect ratio of 2.0 or more is insufficient, and characteristics are deteriorated. The maximum heating temperature should be (A _c3 -15) ° C or more, and more preferably (A _c3 +5) ° C or more. On the other hand, once it is excessively heated to a high temperature, decarburization of the surface layer may proceed excessively, and fatigue resistance may become insufficient, and the cost of fuel required for heating may increase, and the furnace body may be damaged. Therefore, the maximum heating temperature should be set below 1000 ° C.

In the first heat treatment, the holding time at the maximum heating temperature is set to 1 second to 1000 seconds. Once the holding time is less than 1 second, massive iron grains remain in the steel sheet structure inside the steel sheet in the steel sheet for heat treatment. As a result, the steel sheet obtained after the second heat treatment has an excessive soft ferrite grain volume ratio and deteriorates its characteristics. 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, not only the effect brought about by heating to the maximum heating temperature is saturated, but productivity is also impaired. 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 first heat treatment, once the average heating rate from 650 ° C to the maximum heating temperature is less than 0.5 ° C / sec, Mn segregation occurs during the heat treatment and a coarse lump Mn thickened region is formed. After the second heat treatment, The properties of the obtained steel sheet are deteriorated. In order to suppress the formation of massive Vosstian iron, the average heating rate should be set to 1.5 ° C / sec or more. On the other hand, if the average heating rate is more than 500 ° C / second, the decarburization reaction does not proceed sufficiently. Therefore, the average heating rate is set to 500 ° C./second or less. The average heating rate from 650 ° C to the maximum heating temperature is the difference between 650 ° C and the maximum heating temperature, divided by the elapsed time from the surface temperature of the steel plate from 650 ° C to the maximum heating temperature. The value obtained.

(d) After the maximum heating temperature is maintained, cooling is performed so that an 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 plate structure inside the steel plate of the steel plate for heat treatment into a lath-like structure main body, the temperature range up to 700 ° C to Ms shown by the following formula (10) is maintained at the maximum heating temperature. Cooling was performed such that the average cooling rate was 5 ° C / second or more. Once the average cooling rate is less than 5 ° C / sec, lump iron may be formed. The average cooling rate should preferably be 10 ° C / second or more, and more preferably 30 ° C / second or more. In addition, although the upper limit of the average cooling rate is not particularly limited, special equipment is required to perform cooling at an average cooling rate of more than 500 ° C / second. Therefore, the average cooling rate should be 500 ° C / sec or less. The average cooling rate in the temperature range from 700 ° C to Ms is the value obtained by dividing the difference between 700 ° C and Ms by the time elapsed from the surface temperature of the steel sheet from 700 ° C to Ms. . Ms is calculated by the following formula.

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) Cooling is performed at an average cooling rate of 5 ° C./second or more until the cooling stop temperature is lower than Ms. In the first heat treatment, cooling is performed at an average cooling rate in a temperature range from 700 ° C. to Ms of 5 ° C./sec or more, and to a cooling stop temperature of Ms or less. The cooling stop temperature may be room temperature (25 ° C). By setting the cooling stop temperature to be equal to or lower than Ms, the steel sheet structure inside the steel sheet in the steel sheet for heat treatment obtained after the first heat treatment becomes a lath-like structure main body. The cooling stop temperature refers to the surface temperature of the steel sheet at the moment after the spraying of the cooling medium (cooling water, atmosphere, etc.) for reducing the temperature of the steel sheet.

In the manufacturing method of this embodiment, after the steel sheet is cooled in the first heat treatment to a temperature of Ms or less and a cooling stop temperature of room temperature or more, the steel sheet may be continuously subjected to the second heat treatment shown below. Alternatively, after cooling to room temperature in the first heat treatment and winding up, a second heat treatment described below may be performed.

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, and becomes a steel sheet of this embodiment. In addition, by performing hot-dip galvanizing (and further performing alloying treatment if necessary), the hot-dip galvanized steel sheet of this embodiment is obtained. 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 quenching and tempering treatment. In addition, in order to remove oxides existing on the surface of the steel sheet for heat treatment, the steel sheet for heat treatment may be subjected to a pickling treatment.

(Second heat treatment) The second heat treatment is performed on the steel plate (steel plate for heat treatment) to which the first heat treatment has been performed. The second heat treatment satisfies the following (A) to (E). (A) From 650 ° C to heating to the maximum heating temperature, the gas environment around the steel plate is set to a gas environment containing H _{2 of} 0.1% by volume or more and satisfying the following formula (3). log (PH ₂ O / PH ₂ ) <-1.1 ... (3) (In formula (3), PH ₂ O represents the partial pressure of water vapor, and PH ₂ represents the partial pressure of hydrogen.)

In the second heat treatment, the gas environment around the hot-rolled steel sheet or cold-rolled steel sheet must be set to the gaseous environment described in (A) in the temperature range from 650 ° C to the maximum heating temperature; In all temperature ranges from 650 ° C to the maximum heating temperature, it is preferable to set the surroundings of the steel plate to the gaseous environment described in (A) above. In the case where the steel sheet is subjected to hot-dip galvanizing, in the second heat treatment, in the entire temperature range from 650 ° C to the maximum heating temperature, it is necessary to set the periphery of the steel sheet as described in (A) above. Documented gas environment. Furthermore, when the steel sheet is subjected to hot-dip galvanizing, in the second heat treatment, the gas environment around the steel sheet must be set to contain 0.1 vol. Or more of H ₂ and O ₂ to 0.020 vol.% Or less, and satisfy the above formula ( 3). In the second heat treatment, since the above (A) is satisfied, the decarburization reaction on the surface of the steel sheet is suppressed, and the surface layer portion after decarburization during the first heat treatment is subjected to carbon atoms supplied from the inside of the steel sheet. . As a result, on the surface of the steel sheet after the second heat treatment, a composite structure having a predetermined thickness in which a small amount of loose Asada iron and residual Vostian iron are dispersed is formed.

Once the 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 fully restored, and an oxide film may be formed on the steel sheet. Therefore, the chemical conversion treatability of the steel sheet obtained after the second heat treatment is reduced. Furthermore, once the steel sheet is hot-dip galvanized and the H ₂ in the gas environment is less than 0.1% by volume or the O ₂ in the gas environment is greater than 0.020% by volume, the plating adhesion of the steel sheet is reduced. In addition, once the H ₂ content in the gas environment is greater than 20% by volume, the effect is saturated. In addition, once the H ₂ content in the gas environment is greater than 20% by volume, the risk of hydrogen explosion on mechanical operations will increase. Therefore, the content of H ₂ in the gas environment is preferably set to 20% by volume or less. A suitable H ₂ range is 2.0% by volume or more, and more preferably 3.0% by volume or more. A suitable O ₂ range is 0.010% by volume or less, and more preferably 0.005% by volume or less.

In addition, once log (PH ₂ O / PH ₂ ) is -1.1 or more, the decarburization reaction at the surface of the steel sheet will proceed excessively, so the soft layer obtained after the second heat treatment and forming the surface layer of the steel sheet will have a thicker thickness, and The intensity will be insufficient. The lower the value of log (PH ₂ O / PH ₂ ), the better, so there is no need to set a lower limit for this value. However, to set the value of log (PH ₂ O / PH ₂ ) to less than -2.2, special equipment is needed, so it is appropriate to set the lower limit of the value of log (PH ₂ O / PH ₂ ) to -2.2.

(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 is set to (A _c1 +25) ° C to (A _c3 -10) ° C. Once the maximum heating temperature is lower than (A _c1 +25) ° C, the skimmer carbon iron in the steel will be incompletely melted and remain, and the residual Vossian iron fraction in the internal structure of the steel plate obtained after the second heat treatment will be insufficient, and the characteristics will be deterioration. In order to increase the hard structure fraction in the steel sheet obtained after the second heat treatment to obtain a steel sheet with higher strength, the maximum heating temperature should be set to (A _c1 +40) ° C or higher.

On the other hand, once the maximum heating temperature is higher than (A _c3 -10) ° C, almost or all of the microstructure becomes Vosstian iron, and the lath-like structure in the steel plate (steel plate for heat treatment) before the second heat treatment disappears. Become unable to survive in the steel plate. As a result, the residual Vosstian iron fraction in the internal structure of the steel sheet obtained after the second heat treatment may be insufficient, and at the same time, the number of residual Vosstian irons having an aspect ratio of 2.0 or more may be insufficient, and the characteristics may be significantly deteriorated. From this point of view, the maximum heating temperature is set to A _c3 -10 ° C or lower. In order to allow the lath-like structure in the steel sheet before the second heat treatment to fully exist in the steel sheet to further improve the characteristics of the steel sheet, the maximum heating temperature should be set to (A _c3 -20) ° C or lower, and more preferably (A _c3 -30) Below ℃.

In the second heat treatment, the holding time at the maximum heating temperature is set to 1 second to 1000 seconds. Once the holding time is less than 1 second, the diffusion of carbon atoms from the inside of the steel plate to the surface layer becomes insufficient, and at the same time, the Xueming carbon iron in the steel will be incompletely melted and remain, and there is a concern that the characteristics of the steel plate will deteriorate. The holding time should be more than 30 seconds. On the other hand, if the holding time is too long, the diffusion of carbon atoms from the inside of the steel sheet to the surface layer proceeds excessively, and the effect of decarburizing the surface layer in the first heat treatment disappears. Therefore, the hold time is set to an upper limit of 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. Once the average heating rate from 650 ° C to the maximum heating temperature in the second heat treatment is less than 0.5 ° C / sec, the lath-like structure carefully prepared by the first heat treatment will be restored, so it does not have in the crystal grains. The volume fraction of soft fertile iron in Vostian iron granules will increase. On the other hand, if the average heating rate is more than 500 ° C / second, the decarburization reaction does not proceed sufficiently. The average heating rate from 650 ° C to the maximum heating rate is the difference between 650 ° C and the maximum heating rate divided by the time elapsed from the surface temperature of the steel plate from 650 ° C to the maximum heating rate. value.

(D) In the second heat treatment, the average cooling rate between 600 and 700 ° C is 3 ° C / sec or more, and the maximum cooling rate is between 600 and 700 ° C in the second heat treatment. When the temperature is 3 ° C / sec or more, the maximum heating temperature is cooled to 480 ° C. When the average cooling rate in this temperature range is less than 3 ° C / sec, coarse carbides are generated and the characteristics of the steel sheet are impaired. The average cooling rate in this temperature range should preferably be 10 ° C / second or more. The upper limit of the average cooling rate in this temperature range may be set even if it is not particularly set. However, since a special cooling device is required to be set to more than 200 ° C / sec, it should be set to 200 ° C / sec or less. The average cooling rate in this temperature range is a value obtained by dividing the temperature difference between 600 and 700 ° C (that is, 100 ° C) by the time required for cooling from 700 ° C to 600 ° C.

(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. In the second heat treatment, once the holding time between 300 ° C and 480 ° C is less than 10 seconds, the carbon will not be sufficiently thickened in the unaltered Vostian iron, so the lath-shaped fertilizer grains will not grow sufficiently, and C The thickening of the North Vostian Iron will not progress. As a result, fresh Asada loose iron is generated, and the characteristics of the steel plate are greatly deteriorated. In order to make the carbon thickening in the Vostian iron fully progress and reduce the amount of loose iron in Asada to improve the characteristics of the steel plate, the holding time should be set to 100 seconds or more. The term “holding at 300 ° C. to 480 ° C. for more than N seconds” means that the period during which the temperature of the steel sheet is within a temperature range of 300 ° C. to 480 ° C. is set to N seconds or more.

By performing the second heat treatment described above, the steel sheet according to this embodiment can be obtained. In this embodiment, the steel sheet may be cold-rolled for shape correction. Cold rolling may be performed after the first heat treatment, and may be performed after the second heat treatment. It is also acceptable to perform both after the first heat treatment and after the second heat treatment. Regarding the reduction ratio of cold rolling, the reduction ratio should preferably be 3.0% or less, and more preferably 1.2% or less. Once the cold rolling reduction ratio is greater than 3.0%, some Vostian irons will disappear due to processing-induced metamorphosis, and there is a concern that the characteristics will be damaged. On the other hand, the lower limit value of the rolling rate of the cold rolling is not particularly limited, and even if cold rolling is not performed, the characteristics of the steel sheet according to this embodiment will be exhibited.

(Hot Galvanizing) In the method for manufacturing a steel sheet according to this embodiment, a single hot-dip galvanizing step may be performed to form a hot-dip galvanizing layer on the surface of the base material steel sheet after the second heat treatment. After the formation of the hot-dip galvanized layer, an alloying treatment of the plated layer may be performed.

For the hot-dip galvanizing and alloying treatment, as long as the conditions specified in the manufacturing method of this embodiment are satisfied, the cooling step (D) in the second heat treatment may be performed at any time after the cooling step (D) is completed. For example, as shown in mode [1] in FIG. 5, after the cooling step (D) and the isothermal holding step (E), a plating treatment may be performed (an alloying treatment may be further performed as required); as shown in FIG. 6. As shown in Mode [2], after the cooling step (D), a plating treatment (an alloying treatment is further performed as required) may be performed, and then isothermal maintenance (E) may be performed. Alternatively, as shown in mode [3] in FIG. 7, after the cooling step (D) and the isothermal holding step (E), it may be cooled to room temperature for a time, and then subjected to a plating treatment (further depending on requirements) (Alloyed).

The plating conditions such as the temperature of the galvanizing bath and the composition of the galvanizing bath in the hot-dip galvanizing step can be general conditions and are not particularly limited. For example, the temperature of the plating bath may be 420 to 500 ° C, the immersion plate temperature of the steel sheet is 420 to 500 ° C, and the immersion time is 5 seconds or less. Although the plating bath is preferably a plating bath containing Al: 0.08 to 0.2%, it may also contain Fe, Si, Mg, Mn, Cr, Ti, Pb, etc. of other impurities. The weight per unit area of the hot-dip galvanizing is preferably controlled by conventional methods such as gas wiping. The weight per unit area is usually 5 g / m ² or more per side, but it is preferably 25 to 75 g / m ² , and more preferably 20 to 120 g / m ² .

For the high-strength hot-dip galvanized steel sheet on which the hot-dip galvanized layer is formed, as described above, alloying treatment can also be performed as required. Although the alloying treatment may be performed in accordance with a general method, the temperature of the alloying treatment should preferably be 460 to 600 ° C. Once the alloying treatment is less than 460 ° C, not only the alloying speed will be slowed down and productivity will be impaired, but the alloying treatment unevenness will also occur. Therefore, the alloying treatment temperature should be set to 460 ° C or higher. 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. Therefore, the alloying treatment temperature is preferably set to 600 ° C or lower. The alloying temperature is preferably set to 480 to 580 ° C or lower. In addition, the heating time of the alloying treatment is preferably set to 5 to 60 seconds. The alloying treatment is preferably performed on the condition that the iron concentration in the hot-dip galvanized layer is 6.0% by mass or more.

In addition, an electro-galvanized layer may be formed on the surface of the steel sheet according to this embodiment. The galvanized layer can be formed by a conventional method.

Next, a description will be given of a method for measuring each configuration of the steel sheet and the steel sheet for heat treatment of this embodiment. "Measurement of the steel structure" Among steel plates and steel plates for heat treatment, the ferrous iron (soft ferrous iron and hard ferrous iron), tempered iron, tempered Asada loose iron, and fresh Asada contained in the steel structure inside and on the surface of the steel plate The volume fractions of loose iron, boron iron, cis-carbon iron, upper toughened iron, and toughened fertile iron can be measured by the methods shown below.

Samples were taken with a section parallel to the rolling direction and the thickness direction of the steel plate as the observation surface, and the observation surface was ground and then etched with nitric acid. Next, a field emission scanning electron microscope (FE-SEM: Field Emission) is used in one to several observation fields in a range from one-eight to three-thick in a range of 1/4 thickness from the surface to the center of the observation surface. Scanning Electron Microscope) observes 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 loose iron, the newly-born Asada loose iron, the bolai iron, and the citronite were measured as volume fractions. In the present case, a region having a lower structure in the crystal grains and carbides having several variants precipitated was judged as tempered Asada scattered iron. In addition, the area where the precipitous carbon iron was deposited in layers was judged to be boron iron. The area where the brightness is small and the lower tissue cannot be recognized is judged to be fertile iron (soft iron or hard iron). Areas with high brightness and no underlying structure due to etching were judged to be fresh Asada loose iron or residual Vostian iron. The respective volume fractions were calculated by the point counting method, and this was used as the volume fraction of each tissue. As for the volume ratio of the fresh Asada loose iron, it can be obtained by subtracting the volume ratio of the residual Vostian iron obtained by the X-ray diffraction method. The volume fractions of the hard fertilized iron and the soft fertilized iron are determined together with the volume fraction of the fertilized iron measured in the method described below to determine the respective volume fractions. As for the volume fraction of the fresh Asada loose iron, the volume fraction of the remaining Vostian iron obtained by the X-ray diffraction method described below can be subtracted from the volume fraction of the fresh Asada loose iron or residual Vostian iron. To get it.

Among the steel sheets and steel sheets for heat treatment, the volume fraction of the residual Vostian iron contained in the steel sheet was evaluated by the X-ray diffraction method. In the range of 1 / 8th to 3 / 8th of the thickness from the 1 / 4th-thick position from the surface of the plate, the surface parallel to the plate is finished into a mirror surface, and the FCC iron is measured by X-ray diffraction method. The area fraction is used as the volume fraction of the residual Vostian 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 Vostian iron inside the steel sheet was evaluated by performing high-resolution crystal structure analysis by 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 and finished into a mirror surface. In addition, in order to remove the processed layer of the surface layer, electrolytic polishing or mechanical polishing using colloidal silicon dioxide is performed. Next, regarding the surface layer portion of the steel plate containing the soft layer and the inside of the steel plate (the range from 1/8 to 3/8 thick centered on the 1/4 thickness from the surface), the total area of the observation field of view is 2.0 × 10 ^{The method of -9} m ² or more (several fields of view to the same field of view) can be used to analyze the crystal structure by the EBSD method. For the analysis of the data obtained by the EBSD method during the measurement, "OIM Analysys 6.0" produced by TSL was used. The distance between the evaluation points (step) is 0.01 to 0.20 μm. Based on the observation results, the regions that would be judged as FCC iron were determined as residual Vosstian iron, and the residual Vosstian iron volume fractions in the soft layer and the steel plate were calculated respectively.

"Measurement of the aspect ratio and length of the residual Vostian iron grains" In the steel sheet and the steel sheet for heat treatment, the aspect ratio and length of the residual Vostian iron grains contained in the steel structure inside the steel sheet were observed by FE-SEM Grains, and evaluated by high-resolution crystal orientation analysis by the EBSD method (electron backscatter diffraction method).

First, a cross section parallel to the rolling direction and the thickness direction of the steel plate is used as an observation surface and a sample is taken. Then, the observation surface is ground and finished into a mirror surface. Next, in the observation surface, a range of 1/4 thickness to 3/8 thickness from the center of the 1 / 4-thick position from the surface is one to several observation fields of view, for a total of 2.0 × 10 ^-9 m ² or more (several fields of view and Any area of the same field of view) was analyzed by EBSD method. Next, in order to avoid measurement errors, only the Vosstian iron with a major axis length of 0.1 μm or more was selected from the residual Vosstian iron grain crystal orientation measured by the above method to draw a crystal orientation distribution map. The boundary where the crystal orientation difference of 10 ° or more occurs is regarded as the crystal grain boundary of the residual Vostian iron particles. The aspect ratio is set to the value obtained by dividing the major axis length of the residual Vostian iron particles by the minor axis length. The long diameter is set as: the length of the long axis of the residual Vostian iron particles. For the analysis of the data obtained by the EBSD method during the measurement, "OIM Analysys 6.0" produced by TSL was used. The distance between the evaluation points (step) is 0.01 to 0.20 μm. Based on the observation results, the region that would be judged as FCC iron was judged as remaining Vosstian iron. From this result, the ratio of the number of residual Vostian irons with an aspect ratio of 2.0 or more to the total number of remaining Vostian irons was obtained. The aspect ratio of the iron in the fertile grains was evaluated by observing the crystal grains using FE-SEM, and performing high-resolution crystal orientation analysis by the EBSD method (electron backscatter diffraction method). For the analysis of the data obtained by the EBSD method, "OIM Analysys 6.0" produced by TSL was used. The distance between the evaluation points (step) is 0.01 to 0.20 μm. According to the observation results, the area that will be judged as BCC iron is regarded as fat iron, and the crystal orientation distribution map is drawn. Then, a boundary where a crystal orientation difference of 15 ° or more occurs is regarded as a crystal grain boundary. The aspect ratio is a value obtained by dividing the length of the major axis of each of the ferrite grains by the length of the minor axis. In addition, as for the fertile iron with a large aspect ratio, there is unrecrystallized ferrous iron that is elongated in the rolling direction due to cold rolling, but this is similar to the ferrous iron with a large aspect ratio in the steel sheet of the embodiment Can be clearly distinguished. The azimuthal gradient of the crystalline grains of the unrecrystallized ferrous iron is larger than that of the ferrous iron in the steel sheet of this embodiment. Specifically, the two can be distinguished through the GAM value (Grain Average Misorientation) obtained by the Electron Back Scatter Diffraction Patterns (EBSD) method. Generally, the GAM value of the unrecrystallized ferrous iron is 0.5 ° or more, and the ferrous iron with a large aspect ratio in the steel sheet of this embodiment has a GAM value of 0.5 ° or less.

"Fatfield iron grains containing hard iron grains (hard iron grains) / Fatfield iron grains without soft iron grains (soft fat grain irons)" The method of distinguishing between those who do not and those who do not will be explained. First, FE-SEM was used to observe the crystal grains, and high-resolution crystal orientation analysis was performed by the EBSD method. Specifically, a plate thickness section parallel to the rolling direction of the steel plate is used as an observation surface and a sample is taken. The observation surface is polished and finished into a mirror surface. In addition, in order to remove the processed layer of the surface layer, electrolytic polishing or mechanical polishing using colloidal silicon dioxide is performed. Next, for the inside of the steel plate (the range from 1 / 8th thickness to 3 / 8th thickness centered on the 1 / 4th thickness from the surface), a total of 2.0 × 10 ^-9 m ² or more (from several fields to the same field of view) (Available) The crystal structure was analyzed by the EBSD method. Next, for the data obtained from the BCC iron, the boundary where the crystal orientation difference of 15 ° or more occurs is regarded as the crystal grain boundary, and the distribution map of the crystal grain boundary of the ferrous iron is drawn. Next, in order to avoid measurement errors, from the data obtained by the FCC iron, only the Vostian iron particles with a major axis length of 0.1 μm or more are used to describe the distribution of the crystal grains, and the distribution diagrams of the grain boundaries with the ferrite grains For overlap. If there is more than one Vostian iron granule completely contained in one ferritic iron granule, it is regarded as "fertilized iron granule containing Vostian iron granule". In addition, those who are not adjacent to Vosstian iron granules, or is adjacent to Vosstian iron granules only on the boundary with another grain, are regarded as "fertilized iron granules without Vostian iron granules."

"Thickness of the soft layer" The hardness distribution from the surface layer to the inside of the steel sheet can be obtained by, for example, the following method. A sample having a plate thickness section parallel to the rolling direction of the steel plate was taken as an observation surface, and the observation surface was polished and finished into a mirror surface. In addition, in order to remove the processed layer of the surface layer, colloidal silicon dioxide was used for chemical polishing. Using the micro-hardness measuring device on the observation surface of the obtained sample, a position with a depth of 5 μm from the outermost layer was used as a starting point, and from the surface to a position of 1/8 of the thickness of the plate, the top was spaced at a distance of 10 μm in the thickness direction of the steel plate. A square pyramid-shaped Vickers indenter with an angle of 136 ° was pressed in with a load of 2 g. At this time, the indentation load is set so that the Vickers indentations do not interfere with each other. After that, the diagonal length of the indentation is measured using an optical microscope, a scanning electron microscope, or the like, and converted into a Vickers hardness (Hv). Next, the measurement position was moved by 10 μm or more in the rolling direction, the starting point was set to a depth position of 10 μm from the outermost layer, and the same measurement was performed up to a position where the plate thickness was 1/8 of the thickness. Thereby, in fact, hardness measurement data with a pitch of 5 μm in the depth direction can be obtained. The reason why the measurement interval is not simply set to a pitch of 5 μm is to prevent the indentations from interfering with each other. As for the Vickers hardness, five points were measured for each thickness position, and the average value was used as the hardness at the thickness position. Each data is interpolated with a straight line to obtain a hardness distribution map in the depth direction. Read the hardness position at a depth of 80% or less of the hardness of the base material from the hardness distribution chart to determine the thickness of the soft layer.

"High-frequency glow discharge (high-frequency GDS) analysis" When analyzing steel plates and heat-treated steel plates by a high-frequency glow discharge analysis method, a conventional high-frequency GDS analysis method can be used. Specifically, the following method was used: The surface of the steel sheet was placed in an Ar gas environment, and a voltage was applied to generate a glow plasma. In this state, the surface of the steel sheet was analyzed in the depth direction while being sputtered. Then, in the glow plasma, atoms are excited to emit a specific emission spectral wavelength of the element. From the characteristic emission spectral wavelength of the element, the element contained in the material (steel plate) is identified, and the emission intensity of the identified element is used to estimate the material. The amount of elements. Data in the depth direction can be estimated from the sputtering time. Specifically, the relationship between the sputtering time and the sputtering depth is obtained in advance using a standard sample, whereby the sputtering time can be converted into the sputtering depth. According to this, the sputtering depth converted from the sputtering time can be defined as the depth of the material from the surface. In addition, in the high-frequency GDS analysis of the steel plate and the heat treatment steel plate of this embodiment, a commercially available analysis device can be used. In this embodiment, a high-frequency glow discharge emission analysis device GD-Profiler2 manufactured by HORIBA, Ltd. is used.

[Examples] Next, examples of the present invention will be described. The conditions in the examples are examples of conditions adopted to confirm the feasibility and effect of the present invention. The present invention is not limited by such a condition example. The present invention can adopt various conditions as long as it does not depart from the gist of the present invention and achieve the purpose of the present invention.

(Example 1) Steel having a chemical composition shown in Table 1 was melted, and a steel billet was produced. The steel slab heating temperatures shown in Tables 2 and 3 and the values of formula (4) shown in Tables 2 and 3 were used as the steel slab heating conditions. The steel slab heating conditions were used to heat the steel slabs and the rolling completion temperature Hot rolling was performed at the temperatures shown in Tables 2 and 3, and hot-rolled steel sheets were produced. After that, the hot-rolled steel sheet was pickled and the surface rust was removed. Thereafter, a portion of the hot-rolled steel sheet was cold-rolled to prepare a cold-rolled steel sheet.

[Table 1]

[Table 2]

[table 3]

The thus obtained hot-rolled steel sheet having a thickness of 1.2 mm or the cold-rolled steel sheet having a thickness of 1.2 mm is subjected to the first heat treatment and / or the second heat treatment shown below. For some steel sheets, after the cold-rolled steel sheet is cooled to the cooling stop temperatures shown in Tables 4 and 5 in the first heat treatment, the second heat treatment is continuously performed without cooling the cold-rolled steel sheet 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 then to room temperature. In addition, the first heat treatment is not performed on some steel plates.

(First heat treatment) The average heating rate from 650 ° C to the maximum heating temperature shown in Tables 4 and 5 was heated to the maximum heating temperature shown in Tables 4 and 5, and Table 4 and Table were maintained at the maximum heating temperature. 5 shows the hold time. Thereafter, cooling was performed at 700 ° C. to Ms at the average cooling rates shown in Tables 4 and 5, and then cooled to the cooling stop temperatures shown in Tables 4 and 5. In the first heat treatment, a gas environment containing H _{2 at a} concentration shown in Tables 4 and 5 and log (PH ₂ O / PH ₂ ) having the values shown in Tables 4 and 5 was reached at 650 ° C. to a maximum. Heating is performed up to the heating temperature.

A _c3 shown in Tables 4 and 5 is obtained by the following formula (9), and Ms is obtained by the following formula (10). 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.) 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 the mass of the element in the steel %.)

[Table 4]

[table 5]

(Second heat treatment) The average heating rate from 650 ° C to the maximum heating temperature shown in Tables 6 and 7 was heated to the maximum heating temperature shown in Tables 6 and 7, and Table 6 and Table were maintained at the maximum heating temperature. 7 shows the hold time. Then, it cooled at the average cooling rate shown in Table 6 and Table 7, and cooled to the cooling stop temperature shown in Table 6 and Table 7. Thereafter, the steel sheets were obtained by maintaining the holding times shown in Tables 6 and 7 between 300 ° C and 480 ° C, and cooling to room temperature. In the second heat treatment, a gas environment containing H _{2 at a} concentration shown in Tables 6 and 7 and log (PH ₂ O / PH ₂ ) having the values shown in Tables 6 and 7 was reached at 650 ° C. to a maximum. Heating is performed up to the heating temperature. Next, the steel sheet after the second heat treatment is subjected to an electrogalvanization step to form electrogalvanized layers on both surfaces of the steel sheet, and an electrogalvanized steel sheet (EG) is obtained.

A _c1 shown in Tables 6 and 7 is obtained by the following formula (8). 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 6]

[TABLE 7]

Next, for each of the steel plates thus obtained, the steel structure (steel structure inside the steel plate) in the range of 1/8 to 3/8 thickness centered on the position of 1/4 thickness from the surface was measured by the above method, and Investigate volume fractions of soft fertilized iron, residual Vostian iron, tempered Asada iron, fresh Asada loose iron, bolai iron and ciming carbon iron (poly iron + ciming carbon iron) . The volume fractions of the toughened iron and the hard ferrous iron were also investigated.

In addition, the inside of each steel plate was investigated by the above-mentioned method, and the ratio of the number of residual Vostian irons with respect to the total number of remaining Vostian irons having an aspect ratio of 2.0 or more was investigated. These results are shown in Tables 8 and 9.

[TABLE 8]

[TABLE 9]

Next, for each steel plate, the steel structure was measured by the method described above, and the thickness of the soft layer (the depth range from the surface) and the crystal grains of the iron particles contained in the soft layer with an aspect ratio of less than 3.0 were investigated. Number ratio. In addition, for each steel plate, the steel structure was measured by the method described above, and the volume fraction of the residual Vostian iron in the soft layer and the residual Vostian iron in the range of 1/8 to 3/8 thick were investigated. Volume ratio (Residual γ volume ratio in the soft layer / Residual γ volume ratio inside the steel sheet). These results are shown in Tables 10 and 11.

In addition, for each steel plate, the above method was used to analyze the high-frequency glow discharge analysis method in the depth direction from the surface to investigate the peak of the luminous intensity of the wavelength of Si that was greater than 0.2 μm and less than 5 μm in depth. (This peak indicates whether it has the following: an internal oxide layer containing a Si oxide). Then, in each steel plate, when the depth from the surface is greater than 0.2 μm and the depth is less than 5 μm, a peak indicating the luminous intensity of the wavelength of Si appears, and it is evaluated as “with” an internal oxidation peak; the peak does not appear Otherwise, it was evaluated as "no" internal oxidation peak. The results are shown in Tables 10 and 11.

"EG" described on the surfaces in Tables 10 and 11 indicates galvanized steel sheets.

[TABLE 10]

[TABLE 11]

In addition, the maximum tensile stress (TS), elongation (El), hole expandability (hole expandability), bendability (minimum bending radius), and fatigue resistance ( Fatigue limit / TS). The results are shown in Tables 12 and 13.

A direction perpendicular to the rolling direction was taken as a tensile direction, and a JIS No. 5 tensile test piece was taken. 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 results of the maximum tensile stress (TS), elongation (El), and hole expandability (hole enlargement ratio) measured by the above method were used. The value represented by the following formula (11) is calculated. When the value represented by formula (11) is 80 × 10 ^-7 or more, it is evaluated that the balance of strength, elongation, and hole expandability is good. 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 12 and 13.

According to JIS Z 2248, a steel plate was cut out in a direction perpendicular to the rolling direction, and the end surface was mechanically polished to produce a test piece of 35 mm × 100 mm. Then, for the produced test piece, a 90-degree V bending test was performed using a die and a punch having a 90 ° R at the tip of 0.5 to 6 mm. The bending edge of the test piece after the bending test is observed with a magnifying glass, and the minimum bending radius without cracks is taken as the critical bending radius. A steel sheet having a critical bending radius of less than 3.0 mm was evaluated as having good bendability.

The fatigue resistance is evaluated by a plane bending fatigue test. The test piece was a JIS No. 1 test piece, and the stress ratio was set to -1. Repetition frequency is set to 25Hz, the number of repetitions ¹⁰⁷ times the maximum stress as the fatigue fracture limit yet. Then, a steel sheet having a ratio of fatigue limit to maximum tensile stress (TS) (fatigue limit / TS) of 0.45 or more was evaluated as having good fatigue resistance.

For each steel sheet, the chemical conversion treatability was measured by the method shown below. Each steel plate was cut into 70 mm × 150 mm, and sprayed at 40 ° C. to apply 18 g / l of a degreasing agent (trade name: Fine cleaner E2083) manufactured by Nihon Parkerizing Co., Ltd. Aqueous solution for 120 seconds. Next, the steel plate to which the degreasing agent has been applied is washed with water and degreased, and then immersed in a 0.5 g / l aqueous solution of a surface conditioner (trade name: Prepalene XG) manufactured by Nippon Sease Co., Ltd. at room temperature for 60 seconds. After that, the steel sheet to which the surface conditioner has been applied is immersed in a zinc phosphate treating agent (trade name: Palbond L3065) manufactured by Nippon Sease Co., Ltd. for 120 seconds, and then washed with water and dried. Thereby, a chemical conversion treatment film composed of a zinc phosphate coating film is formed on the surface of the steel sheet.

For steel plates that have been formed with chemical conversion treatment films, test pieces with a width of 70 mm and a length of 150 mm were taken. Then, using a scanning electron microscope (SEM) at a magnification of 1000 times, three places (the central portion and both end portions) along the length direction of the test piece were observed. Then, for each test piece, the degree of crystal particle adhesion of the chemical conversion treatment film was evaluated by the following criteria.

"G" (GOOD) did not clearly see on the surface the part which was not covered by the chemical conversion treatment film. "B" (BAD) on the surface can clearly see the parts not covered by the chemical conversion treatment film.

[TABLE 12]

[TABLE 13]

The steel sheet of the example of the present invention has high strength, good balance of strength, elongation, and hole expandability, and fatigue resistance, bendability, and chemical conversion treatment.

For the steel plates of Experimental Nos. 11, 16, 27, 45, and 46, since the first heat treatment was not performed, the metal structure did not contain hard ferrous iron, and the strength, elongation, and hole expansion ratio were balanced. difference. In the steel plate of Experimental Example No. 2, since the maximum heating temperature in the first heat treatment was low, the number ratio of the residual Vostian irons with an aspect ratio of 2.0 or more was insufficient, and the strength, elongation, and hole expansion ratio The balance becomes worse. In the steel sheet of Experimental Example No. 3, since the maximum heating temperature in the first heat treatment was high, the thickness of the soft layer in the steel sheet for heat treatment and the steel sheet became thicker, and the fatigue resistance became lower.

In the steel plate of Experimental Example No. 5, because the average heating rate up to 650 ° C to the maximum heating temperature in the first heat treatment was slow, the number ratio of the residual Vostian irons with an aspect ratio of 2.0 or more was insufficient, and the strength, The balance between elongation and hole expansion becomes worse. In the steel plates of Experimental Nos. 6, 15, and 23, since the log (PH ₂ O / PH ₂ ) in the first heat treatment was low, the thickness of the soft layer in the heat treatment steel plate and the steel plate was insufficient, and the bendability was insufficient. Worse.

In the steel sheet of Experimental Example No. 8, since the cooling rate in the first heat treatment was slow, the lath-like structure of the steel sheet for heat treatment was insufficient, and the soft fertilizer grain iron fraction was increased in the internal structure of the steel sheet. Therefore, in the steel plate of Experimental Example No. 8, the balance of strength, elongation, and hole expansion ratio deteriorates. In the steel plates of Experimental Nos. 9, 10, 19, 22, and 48, since the log (PH ₂ O / PH ₂ ) in the second heat treatment is high, the volume ratio of residual γ in the soft layer and the residual γ in the steel plate The volume ratio is insufficient, and the fatigue resistance is deteriorated.

For the steel plates of Experimental Nos. 6, 15, and 23, since the first heat treatment and the second heat treatment have low log (PH ₂ O / PH ₂ ), the internal oxide layer has not been formed, and the chemical conversion treatment properties The evaluation is "B". For the steel plates of Experimental Nos. 11, 16, and 46, because the first heat treatment was not performed and the log (PH ₂ O / PH ₂ ) of the second heat treatment was low, the internal oxide layer was not formed and chemical conversion was performed. The evaluation of handling property was "B".

In the steel plate of Experimental Example No. 24, since the highest reaching temperature in the second heat treatment was high, the metal structure did not contain hard ferrous iron, and the balance of strength, elongation, and hole expansion ratio deteriorated. For the steel plate of Experimental Example No. 33, since the holding time between 300 ° C and 480 ° C was insufficient during the second heat treatment, the fresh Asada loose iron fraction of the internal structure increased, and the strength, elongation, and expansion Poor porosity becomes worse.

For the steel plate of Experimental Example No. 36, since the cooling stop temperature during the first heat treatment was high, the number ratio of the residual Vostian irons with an aspect ratio of 2.0 or more was insufficient, and the strength, elongation, and hole expansion ratio were insufficient. The balance becomes worse. In the steel plate of Experimental Example No. 41, the cooling rate during the second heat treatment is slow, so the total score of the boron iron and cis-carbon iron in the internal structure of the steel plate increases, and the strength, elongation, The balance of the hole expansion ratio becomes worse.

For the steel plate of Experimental Example No. 62, since the maximum heating temperature in the second heat treatment was low, the residual Vostian iron fraction in the internal structure of the steel plate was insufficient, and the strength, elongation, and hole expansion ratios were not sufficient. The balance becomes worse.

The chemical composition of the steel plates of Experimental Examples Nos. 68 to 72 is outside the scope of the present invention. In the steel plate of Experimental Example No. 68, the maximum tensile stress (TS) was insufficient because the C content was insufficient. In the steel plate of Experimental Example No. 69, the Nb content was large, and the bendability was deteriorated. In the steel sheet of Experimental Example No. 70, the maximum tensile stress (TS) was insufficient because the Mn content was insufficient. In the steel plate of Experimental Example No. 71, since the Si content was large, the hole expandability was deteriorated. In the steel plate of Experimental Example No. 72, since the Mn content and the P content were large, the elongation and hole expandability were deteriorated.

(Example 2) Steel having a chemical composition shown in Table 14 was melted, and a steel blank was produced. The steel slab heating temperature shown in Table 15 and Table 16 and the value of formula (4) shown in Table 15 and Table 16 were used as the steel slab heating condition. The steel slab heating condition was used to heat the steel slab and the rolling completion temperature Hot rolling was performed at the temperatures shown in Tables 15 and 16, and a hot-rolled steel sheet was produced. After that, the hot-rolled steel sheet was pickled and the surface rust was removed. Thereafter, a portion of the hot-rolled steel sheet was cold-rolled to prepare a cold-rolled steel sheet.

[TABLE 14]

[Table 15]

[TABLE 16]

The thus obtained hot-rolled steel sheet having a thickness of 1.2 mm or the cold-rolled steel sheet having a thickness of 1.2 mm is subjected to the first heat treatment and / or the second heat treatment shown below. For some experimental examples, after the cold-rolled steel sheet was cooled to the cooling stop temperatures shown in Tables 17 and 18 in the first heat treatment, the second heat treatment was continuously performed without cooling the cold-rolled steel sheet 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 then to room temperature.

(First heat treatment) The average heating rate from 650 ° C to the maximum heating temperature shown in Tables 17 and 18 was heated to the maximum heating temperature shown in Tables 17 and 18, and Tables 17 and Tables were maintained at the maximum heating temperature. 18 shows the hold time. Thereafter, cooling was performed at 700 ° C. to Ms at the average cooling rates shown in Tables 17 and 18, and then cooled to the cooling stop temperatures shown in Tables 17 and 18. In the first heat treatment, a gas environment containing H _{2 at a} concentration shown in Tables 17 and 18 and log (PH ₂ O / PH ₂ ) having the values shown in Tables 17 and 18 was reached at 650 ° C. to a maximum. Heating is performed up to the heating temperature.

A _c3 shown in Tables 17 and 18 is obtained by the following formula (9), and Ms is obtained by the following formula (10). 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.)

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 17]

[TABLE 18]

(Second heat treatment to hot-dip galvanizing) In each of the experimental examples, the experimental examples No. 1 'to 76' were heated under the conditions shown in Tables 19 and 20, and shown in Tables 19 and 20 The cooling rate was cooled to the cooling stop temperature, and after isothermal holding was performed under the conditions shown in Tables 19 and 20, the molten zinc plating bath was immersed and alloyed. That is, the hot-dip galvanizing treatment is performed at the timing shown in the pattern [1] in FIG. 5. However, in Experimental Example 76 ', no alloying treatment was performed.

For Experimental Example Nos. 77 'to 84', 86 ', and 87', after heating under the conditions shown in Table 20, cooling to the temperature of the molten galvanizing bath at the cooling rate shown in Table 20, and then immersing them in the melt Galvanizing bath and alloying treatment. After further cooling to the cooling stop temperature shown in Table 20, it was maintained isothermally under the conditions shown in Table 20. That is, the hot-dip galvanizing treatment is performed at the timing shown in the pattern [2] in FIG. 6. However, in Experimental Example 82 ', no alloying treatment was performed.

For Experimental Example No. 85 ', heating was performed under the conditions shown in Table 20, and cooling was performed to the cooling stop temperature at the cooling rate shown in Table 20, and then isothermal holding was performed under the conditions shown in Table 20. First, temporarily cool to room temperature. After that, the steel sheet was heated again to the temperature of the hot-dip galvanizing bath, and then immersed in the hot-dip galvanizing bath to perform an alloying treatment. That is, the hot-dip galvanizing treatment is performed in accordance with the pattern [3] shown in FIG. 7.

In addition, each example of hot-dip galvanizing was performed by immersing in a molten zinc bath at 460 ° C., whereby the weight of each side of the steel sheet was 50 g / m ² . In addition, in the second heat treatment, a gas environment containing H _{2 at a} concentration shown in Tables 19 and 20 and a log (PH ₂ O / PH ₂ ) having the values shown in Tables 19 and 20 was reached at 650 ° C to Heat up to the maximum heating temperature.

A _c1 shown in Tables 6 and 7 is obtained by the following formula (8). 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 19]

[TABLE 20]

Next, for each of the thus obtained galvanized steel sheets, a steel structure (a steel structure inside the steel sheet) in a range of 1/8 to 3/8 thickness centered at a position of 1/4 thickness from the surface was measured by the above method. ), And the volume of soft fertilized iron, residual Vostian iron, tempered Asada iron, newborn Asada iron, Polite iron and Xueming carbon iron (Poly iron + Xueming carbon iron) respectively Rate survey. The volume fractions of the toughened iron and the hard ferrous iron were also investigated.

In addition, the inside of the steel sheet of each of the hot-dip galvanized steel sheets was examined by the above-mentioned method for the ratio of the number of residual Vosstian irons with respect to the total number of remaining Vosstian irons having an aspect ratio of 2.0 or more. These results are shown in Table 21 and Table 22.

[TABLE 21]

[TABLE 22]

Next, for each hot-dip galvanized steel sheet, the steel structure was measured by the above method, and the thickness of the soft layer (the depth range from the surface) and the aspect ratio of the crystal grains of the soft ferrous iron contained in the soft layer were less than 3.0. The number of crystal particles.

In addition, for each hot-dip galvanized steel sheet, the steel structure was measured by the method described above, and the ratio of the residual γ volume ratio in the soft layer to the residual γ volume ratio inside the steel sheet (residual γ volume ratio in the soft layer / inside the steel sheet) was investigated. Residual γ volume fraction). These results are shown in Tables 23 and 24.

In addition, for each hot-dip galvanized steel sheet, the above method was used to analyze the high-frequency glow discharge analysis method in the depth direction from the surface to investigate the luminous intensity of the wavelength of Si that was greater than 0.2 μm and less than 5 μm in depth. The peak (this peak indicates whether it has the following: an internal oxide layer containing a Si oxide) appears.

Then, in each hot-dip galvanized steel sheet, from the surface, the depth in the depth direction is greater than 0.2 μm and the depth is less than 5 μm, and the peak indicating the luminous intensity of the wavelength of Si appears, and it is evaluated as “with” the internal oxidation peak; Those peaks that did not appear were evaluated as "no" internal oxidation peaks. The results are shown in Tables 23 and 24.

[TABLE 23]

[TABLE 24]

In addition, for each hot-dip galvanized steel sheet, the maximum tensile stress (TS), elongation (El), hole expandability (hole expansion rate), bendability (minimum bending radius), Fatigue characteristics (fatigue limit / TS). The results are shown in Tables 25 and 26. A direction perpendicular to the rolling direction was taken as a stretching direction, and a JIS No. 5 tensile test piece was taken. 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 results of the maximum tensile stress (TS), elongation (El), and hole expandability (hole enlargement ratio) measured by the above method were used. The value represented by the following formula (11) is calculated. When the value represented by formula (11) is 80 × 10 ^-7 or more, it is evaluated that the balance of strength, elongation, and hole expandability is good.

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 25 and 26.

According to JIS Z 2248, a steel plate was cut out in a direction perpendicular to the rolling direction, and the end surface was mechanically polished to produce a test piece of 35 mm × 100 mm. Then, for the produced test piece, a 90-degree V bending test was performed using a die and a punch having a 90 ° R at the tip of 0.5 to 6 mm. The bending edge of the test piece after the bending test is observed with a magnifying glass, and the minimum bending radius without cracks is taken as the critical bending radius. A steel sheet having a critical bending radius of less than 3.0 mm was evaluated as having good bendability.

The fatigue resistance is evaluated by a plane bending fatigue test. The test piece was a JIS No. 1 test piece, and the stress ratio was set to -1. Repetition frequency is set to 25Hz, the number of repetitions ¹⁰⁷ times the maximum stress as the fatigue fracture limit yet. Then, a steel sheet having a ratio of fatigue limit to maximum tensile stress (TS) (fatigue limit / TS) of 0.45 or more was evaluated as having good fatigue resistance.

In addition, for each hot-dip galvanized steel sheet, the plating adhesion was measured by the following method.

A test piece of 30 mm × 100 mm was taken from each hot-dip galvanized steel sheet, and a 90 ° V bending test was performed. Thereafter, a commercially available transparent tape Cellotape (registered trademark) was stuck along the curved ridge line, and the width of the plating to which the tape was attached was measured as the peeling width. The evaluation is as follows.

"G" (GOOD): The peeling of the plating is small or does not affect the actual use (the peeling width is less than 0 ~ 10mm). "B" (BAD): The peeling is very strong (peeling width is more than 10mm). In terms of adhesion, a sample evaluated as G was judged to be acceptable.

[TABLE 25]

[TABLE 26]

The evaluation results of each experimental example are described below.

The hot-dip galvanized steel sheet of the example of the present invention is high-strength, the balance of strength, elongation, and hole expandability is good, and its fatigue resistance, bendability, and plating adhesion are good.

For the steel plates of Experimental Nos. 14 ', 19', 30 ', 48', and 49 ', since the first heat treatment was not performed, the metal structure did not contain hard ferrous iron, so the strength, elongation, The balance of the hole expansion ratio becomes worse.

For the steel plate of Experimental Example No. 2 ', since the maximum heating temperature in the first heat treatment was low, the ratio of the number of residual Vostian irons with an aspect ratio of 2.0 or more was insufficient, and the strength, elongation, and hole expansion The balance of the rates becomes worse.

In the steel sheet of Experimental Example No. 3 ', since the maximum heating temperature in the first heat treatment was high, the thickness of the soft layer in the steel sheet for heat treatment and the hot-dip galvanized steel sheet became thicker, and the fatigue resistance became lower.

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 ratio of the number of residual Vostian irons with an aspect ratio of 2.0 or more was insufficient, and the strength The balance of elongation, elongation and hole expansion becomes worse.

In the steel plates of Experimental Nos. 6 ', 18', and 26 ', since the log (PH ₂ O / PH ₂ ) in the first heat treatment was low, the bendability and plating adhesion were deteriorated.

For the steel plate of Experimental Example No. 8 ', the cooling rate in the first heat treatment was slow, so the lath-like structure of the steel plate for heat treatment was insufficient, and the soft fertilizer galvanized steel had an increased iron fraction in the internal structure. . Therefore, the steel sheet of Experimental Example No. 8 'has a poor balance of strength, elongation, and hole expansion ratio.

For the steel plates of Experimental Nos. 9 ', 10', 22 ', 25', 30 ', 48', and 51 ', since the log (PH ₂ O / PH ₂ ) in the second heat treatment is high, they are soft. The ratio of the residual γ volume ratio in the layer to the residual γ volume ratio inside the steel sheet is insufficient, and the fatigue resistance is deteriorated.

In the steel plate of Experimental Example No. 27 ', since the highest reached temperature in the second heat treatment was high, the metal structure did not contain hard ferrous iron, and thus the balance of strength, elongation, and hole expansion ratio deteriorated.

For the steel plate of Experimental Example No. 36 ', since the holding time between 300 ° C and 480 ° C in the second heat treatment was insufficient, the fresh Asada loose iron fraction of the internal structure increased, and the strength, elongation, The balance of the hole expansion ratio becomes worse.

For the steel plate of Experimental Example No. 39 ', the cooling stop temperature during the first heat treatment was high, so the ratio of the number of residual Vostian irons with an aspect ratio of 2.0 or more was insufficient, and the strength, elongation, and hole expansion The balance of the rates becomes worse.

For the steel plate of Experimental Example No. 44 ', the cooling rate during the second heat treatment was slow, so the total score of the boron iron and cis-carbon iron in the internal structure of the hot-dip galvanized steel plate increased, and the strength, The balance between elongation and hole expansion becomes worse.

In the steel sheet of Experimental Example No. 65 ', since the maximum temperature reached during the second heat treatment was low, the residual Vostian iron fraction in the internal structure of the hot-dip galvanized steel sheet was insufficient, and the strength, elongation, The balance of the hole expansion ratio becomes worse.

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

Although suitable embodiments and examples of the present invention have been described above, these embodiments and examples are at best only examples within the scope of the present invention, and can be constituted without departing from the scope of the present invention. Make additions, omissions, replacements, and other changes. In other words, the present invention is not limited to the foregoing description, but is limited only by the scope of the attached patent application. Of course, the scope of the present invention can be appropriately changed.

1‧‧‧ steel plate

11‧‧‧ 1/4 thickness from the surface of the steel plate is in the range of 1/8 thickness to 3/8 thickness in the center (inside the steel plate)

12‧‧‧ soft layer

FIG. 1 is a cross-sectional view of a steel plate according to this embodiment in a parallel rolling direction and a plate thickness direction. Fig. 2 is a graph showing the depth from the surface and the light emission of the wavelength of Si when the steel plate of this embodiment is analyzed by the high-frequency glow discharge analysis method in the depth direction (plate thickness direction) from the surface. Relationship of Intensity. FIG. 3 is a graph showing the depth from the surface of the steel plate different from the present embodiment when the analysis is performed by the high-frequency glow discharge analysis method in the depth direction (plate thickness direction) from the surface. The relationship between wavelength emission intensity (Intensity). FIG. 4 is a flowchart of a method for manufacturing a steel sheet according to the present embodiment. 5 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 process in the method for manufacturing a hot-dip galvanized steel sheet according to the present embodiment. FIG. 6 is a diagram showing a second example of the temperature / time pattern of the second heat treatment to the hot-dip galvanizing and alloying process in the method for manufacturing a hot-dip galvanized steel sheet according to this embodiment. FIG. 7 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 process in the method for manufacturing a hot-dip galvanized steel sheet according to the present embodiment.

Claims (11)

A steel plate characterized in that it 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% 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% 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%, the remaining part is composed of Fe and It is made of impurities; the steel structure in the range of 1 / 8th to 3 / 8th of the thickness centered on the 1 / 4th thickness from the surface contains, in volume fraction, soft fertilized iron: 0% to 30%, Residual Vostian iron: 3% ~ 40%, fresh Asada loose iron: 0% ~ 30%, total of bolai iron and Xueming carbon iron: 0% ~ 10%, and the remaining part contains hard fertilized iron; since The above-mentioned position from the above-mentioned surface to 1 / 4-thick is in the aforementioned range from 1 / 8-thick to 3 / 8-thick. The aforementioned residual Vosstian iron having an aspect ratio of 2.0 or more accounts for 50% or more of the total number of the aforementioned residual Vosstian irons; a region having the following hardness is defined as a soft layer: the hardness is 1/8 of the aforementioned Thickness ~ 3 / 8th of the thickness in the aforementioned range is less than 80%; at this time, there is a soft layer with a thickness of 1 to 100 μm in the thickness direction from the aforementioned surface; crystal grains of ferrous iron contained in the soft layer The volume fraction of the crystal grains with an aspect ratio of less than 3.0 is 50% or more; the volume fraction of the residual Vostian iron in the soft layer is: the foregoing in the foregoing range of 1/8 to 3/8 thick The above-mentioned volume fraction of the residual Vostian iron is more than 50%; the luminous intensity of the wavelength of Si is analyzed by the high-frequency glow discharge analysis method in the thickness direction of the plate from the surface, and at this time, it is greater than 0.2 from the surface. In the range of μm and 5 μm or less from the surface, a peak indicating the emission intensity of the aforementioned wavelength of the Si appears. The steel sheet according to claim 1, wherein the aforementioned chemical composition contains one or two or more members selected from the group consisting of: Ti: 0.001% to 0.300%, V: 0.001% to 1.00%, and Nb: 0.001% ~ 0.100%. The steel sheet of claim 1 or 2, wherein the aforementioned chemical composition contains one or two or more selected from the group consisting of: Cr: 0.001% to 2.00%, Ni: 0.001% to 2.00%, and Cu: 0.001 % To 2.00%, Co: 0.001% to 2.00%, Mo: 0.001% to 1.00%, W: 0.001% to 1.00%, and B: 0.0001% to 0.0100%. The steel sheet according to claim 1 or 2, wherein the aforementioned chemical composition contains one or two selected from the group consisting of: Sn: 0.001% to 1.00%, and Sb: 0.001% to 1.00%. The steel sheet according to claim 1 or 2, wherein the aforementioned chemical composition contains one or two or more members selected from the group consisting of: Ca: 0.0001% to 0.0100%, Mg: 0.0001% to 0.0100%, and 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%. For example, the steel sheet of claim 1 or 2, wherein the aforementioned chemical composition satisfies the following formula (1): Si + 0.1 × Mn + 0.6 × Al ≧ 0.35‧‧‧ (1) Si, Mn, and Al in formula (1) It is set as the content of each element in mass%. For the steel plate of claim 1 or 2, in which the volume fraction of tempered Asada loose iron is 0 in the aforementioned range of 1/8 to 3/8 thickness centered at the aforementioned position of 1/4 thickness from the aforementioned surface. % ~ 50%. The steel sheet of claim 1 or 2 has a hot-dip galvanized layer on the surface. The steel plate of claim 1 or 2 has a galvanized layer on the surface. A method for manufacturing a steel sheet, which is a method for manufacturing a steel sheet according to any one of claims 1 to 9, wherein a hot rolled steel sheet or a cold rolled steel sheet is subjected to the first step satisfying the following (a) to (e) After the heat treatment, a second heat treatment that satisfies the following (A) to (E) is performed; the hot-rolled steel sheet is obtained by hot-rolling and pickling a steel slab having a chemical composition according to any one of claims 1 to 6. The cold-rolled steel sheet is obtained by cold-rolling the hot-rolled steel sheet; (a) from 650 ° C to the maximum heating temperature, the hot-rolled steel sheet or the surroundings of the cold-rolled steel sheet are heated; The gas environment is set to: a gas environment containing H _{2 of} 0.1% by volume or more and satisfying the following formula (2); (b) the highest heating temperature at A _c3 -30 ° C to 1000 ° C, maintained for 1 second to 1000 seconds; c) From 650 ° C to the maximum heating temperature, heating at an average heating rate of 0.5 ° C / sec to 500 ° C / s; (d) After holding at the maximum heating temperature, average cooling rate from 700 ° C to Ms 5 ° C / s or more for cooling; (e) the aforementioned cooling at an average cooling rate of 5 ° C / s or more until the cooling stop temperature below Ms is ; (A) self-heated to between 650 ℃ until until the maximum heating temperature, the aforementioned hot rolled steel sheet or the cold rolled steel sheet is set to the surrounding atmosphere: 0.1 vol% of H ₂ and satisfies the following formula (3 ) Gas environment; (B) at the highest heating temperature of A _c1 + 25 ℃ ~ A _c3 -10 ℃, hold for 1 second to 1000 seconds; (C) from 650 ℃ to the maximum heating temperature, at an average heating rate of 0.5 ℃ / s ~ 500 ℃ / s for heating; (D) the average cooling rate between 600 ~ 700 ℃ is 3 ℃ / s or more, from the maximum heating temperature to below 480 ℃; (E) average After cooling at a cooling rate of 3 ° C / sec or more, hold it at 300 ° C to 480 ° C for more than 10 seconds; -1.1 ≦ log (PH ₂ O / PH ₂ ) ≦ -0.07‧‧‧ (2) In formula (2) , PH ₂ O represents the partial pressure of water vapor, PH ₂ represents the partial pressure of hydrogen; log (PH ₂ O / PH ₂ ) <-1.1‧‧‧ (3) In formula (3), PH ₂ O represents water vapor Partial pressure, PH ₂ represents the partial pressure of hydrogen. The method for manufacturing a steel sheet as described in claim 10 is a method for manufacturing a steel sheet as described in claim 8. In the second heat treatment, between 650 ° C and the maximum heating temperature, the aforementioned gas environment is constant to contain 0.1% by volume. The above H ₂ and O ₂ are 0.020% by volume or less and satisfy the aforementioned formula (3); in the aforementioned second heat treatment, a hot-dip galvanizing treatment is performed at a stage subsequent to the cooling process of the aforementioned (D).