High-strength steel sheet and method for manufacturing the same

Abstract

The present application provides a high-strength steel sheet having a tensile strength of 1180 MPa or more and excellent part strength, tensile flange property, bendability, and delayed fracture resistance. The high-strength steel sheet has a steel sheet having a composition consisting of, in mass%, C: 0.090% to 0.390%, Si: 0.01% to 2.00%, Mn: 2.00% to 4.00%, P: 0.100% or less, S: 0.0200% or less, Al: 1.000% or less, N: 0.0100% or less, and O: 0.0100% or less, with the remainder consisting of Fe and inevitable impurities; and a microstructure in which the area ratio of martensite is 70% or more, the area ratio of ferrite is 10% or less, the area ratio of retained austenite is 10% or less, and the proportion of the number of martensite blocks in which metastable carbides exist to the number of martensite blocks is 2% or more.

Description

Technical Field

[0001] This invention relates to high-strength steel plates and their manufacturing methods. Background Technology

[0002] In order to balance reducing CO2 emissions through vehicle lightweighting and improving crashworthiness through vehicle body lightweighting, efforts are being made to increase the strength of steel sheets used in automobiles, and new laws and regulations are constantly being introduced.

[0003] Therefore, in order to improve the strength of the vehicle body, there are increasing instances of using high-strength steel plates with a tensile strength (TS) of 1180 MPa or higher (e.g., Patent Document 1) for the main structural components that form the frame of the car cab.

[0004] Existing technical documents

[0005] Patent documents

[0006] Patent Document 1: Japanese Patent Application Publication No. 2016-50343 Summary of the Invention

[0007] High-strength steel plates used in automotive reinforcing components and frame structures require excellent component strength (high impact energy absorption during collisions). This can be achieved by increasing the yield strength (YS) of the steel plate or by increasing the yield ratio (YR = yield strength YS / tensile strength TS).

[0008] Components such as collision boxes have punched end faces and bent sections. Therefore, the steel plates used for these components require good tensile flanges and bending properties.

[0009] Components made of high-strength steel plates with tensile strength of 1180 MPa or higher sometimes suffer from so-called delayed failure due to hydrogen intrusion.

[0010] For steel sheets used in automobiles, there is stress during stamping and assembly, and then there is the risk of hydrogen intrusion from the environment, so good resistance to delayed failure is required.

[0011] To increase the proportion of high-strength steel sheets used in automotive parts, the above-mentioned characteristics must be met in combination.

[0012] The present invention was made in view of the above problems, and its object is to provide a high-strength steel plate with a tensile strength of 1180 MPa or more and excellent component strength, tensile flange properties, bending properties and resistance to delayed failure.

[0013] The inventors conducted in-depth research and found that the above-mentioned objectives were achieved by adopting the following configuration, thus completing the present invention.

[0014] That is, the present invention provides the following [1] to

[14] .

[0015] [1] A high-strength steel plate comprising a steel plate having the following composition and microstructure, wherein the composition, by mass%, contains C: 0.090% to 0.390%, Si: 0.01% to 2.00%, Mn: 2.00% to 4.00%, P: less than 0.100%, S: less than 0.0200%, Al: less than 1.000%, N: less than 0.0100%, and O: less than 0.0100%, with the remainder consisting of Fe and unavoidable impurities; in the microstructure, the area fraction of martensite is 70% or more, the area fraction of ferrite is 10% or less, the area fraction of retained austenite is 10% or less, and the ratio of the number of martensite blocks containing metastable carbides to the number of martensite blocks is 2% or more.

[0016] [2] According to the high-strength steel plate described in [1] above, wherein the number density of metastable carbides in the martensite block containing metastable carbides is 1×10⁻⁶. 6 pcs / mm 2 above.

[0017] [3] The high-strength steel plate according to [1] or [2] above, wherein the above composition further contains, by mass%, at least one element selected from Ti: less than 0.200%, Nb: less than 0.200%, V: less than 0.200%, Ta: less than 0.10%, W: less than 0.10%, B: less than 0.0100%, Cr: less than 1.00%, Mo: less than 1.00%, Ni: less than 1.00%, Co: less than 0.010%, Cu: less than 1.00%, Sn: less than 0.200%, Sb: less than 0.200%, Ca: less than 0.0100%, Mg: less than 0.0100%, REM: less than 0.0100%, Zr: less than 0.100%, Te: less than 0.100%, Hf: less than 0.10%, and Bi: less than 0.200%.

[0018] [4] The high-strength steel plate according to any one of [1] to [3] above, wherein the steel plate has a soft layer as a surface layer, the upper surface layer is a portion extending from the surface of the steel plate along the thickness direction to 200 μm, and the soft layer is a portion in which the Vickers hardness at a position of 1 / 4 of the thickness of the steel plate is 85% or less.

[0019] [5] According to the high-strength steel plate described in [4] above, when the nanohardness at the position of 1 / 4 depth in the thickness direction of the soft layer is measured at more than 300 points in the range of 50μm×50μm, the ratio r of the measurement points with a nanohardness of 7.0GPa or above to the total number of measurement points is less than 0.10.

[0020] [6] According to the high-strength steel plate described in [4] or [5] above, wherein the standard deviation σ of the nanohardness at the position of 1 / 4 depth in the thickness direction of the soft layer is A The standard deviation σ of the nanohardness at the location at half the depth along the thickness direction of the aforementioned soft layer is below 1.8 GPa. B The pressure is below 2.2 GPa.

[0021] [7] The high-strength steel plate according to any one of [1] to [6] above, wherein the surface of the steel plate further has a metal coating as a pre-annealing coating.

[0022] [8] The high-strength steel plate according to any one of [1] to [6] above, wherein the surface of the steel plate is further provided with a coating.

[0023] [9] The high-strength steel plate according to [7] above, wherein the surface of the metal coating is further coated.

[0024]

[10] A method for manufacturing a high-strength steel plate, comprising the method of manufacturing a high-strength steel plate as described in any one of [1] to [3] above, wherein a cold-rolled plate is obtained by hot rolling, pickling and cold rolling of a steel billet having the composition described in [1] or [3] above, the cold-rolled plate is subjected to annealing and post-heating, wherein in the annealing, the cold-rolled plate is heated at a heating temperature of 800°C or higher, and then cooled to a cooling stop temperature of 150°C or lower, wherein in the cooling, the temperature exceeds Ms°C and is lower than 700°C. The residence time t1 of region T1 is less than 1000s, the first average cooling rate v1 of temperature region T2 above Ms-80℃ and below Ms℃ is 1.0℃ / s to 40.0℃ / s, and the second average cooling rate v2 of temperature region T3 above 150℃ and below Ms-80℃ is 0.3℃ / s or more and less than the first average cooling rate v1. The above-mentioned post-heating is carried out under the condition that the temperature X, which is the highest reached temperature, and the holding time Y above the temperature X-10℃ satisfy the following formula 1.

[0025] Equation 1: 8000 ≤ (273 + X) × (20 + Log) 10 (Y / 3600) ≤ 12000

[0026] The unit of temperature X is °C, and the unit of holding time Y is seconds.

[0027]

[11] In the method for manufacturing high-strength steel plate according to

[10] above, the temperature X satisfies the following formula 2.

[0028] Equation 2: 100≤X≤400

[0029]

[12] According to the manufacturing method of the high-strength steel sheet described in

[10] or

[11] above, in the annealing process, heating at the heating temperature is carried out in an atmosphere with a dew point of -30°C or higher, thereby forming a soft layer on the surface of the cold-rolled sheet. The upper surface layer is a portion extending from the surface of the cold-rolled sheet along the thickness direction to 200 μm. The soft layer is a portion in which the Vickers hardness at a position of 1 / 4 of the thickness of the cold-rolled sheet is 85% or less.

[0030]

[13] The method for manufacturing a high-strength steel plate according to any one of

[10] to

[12] above, wherein the cold-rolled plate is subjected to a metal plating treatment before the above-mentioned annealing is performed, and a metal plating layer as a pre-annealing plating layer is formed on the surface of the cold-rolled plate.

[0031]

[14] A method for manufacturing a high-strength steel sheet according to any one of

[10] to

[13] above, wherein, in the annealing process, the cold-rolled sheet is subjected to a plating treatment to form a coating.

[0032] According to the present invention, a high-strength steel sheet with a tensile strength of 1180 MPa or more and excellent component strength, tensile flange properties, bending properties and resistance to delayed failure can be provided. Attached Figure Description

[0033] Figure 1 This is an example of an electron diffraction pattern of martensite containing carbides.

[0034] Figure 2A It is a three-dimensional diagram showing the state of the test piece after being bent at 90°.

[0035] Figure 2B It is a three-dimensional diagram showing the state of the test piece under orthogonal bending processing.

[0036] Figure 3A This is the front view of the test subject.

[0037] Figure 3B It is a three-dimensional diagram of the test subject.

[0038] Figure 3C It is a three-dimensional diagram showing the state of the test specimen under axial crush test. Detailed Implementation

[0039] [High-strength steel plate]

[0040] The high-strength steel plate of the present invention has a steel plate (base steel plate) having a composition and microstructure described later.

[0041] The high-strength steel plate of the present invention has a tensile strength of 1180 MPa or more, and exhibits excellent component strength, tensile flange properties, bending properties, and resistance to delayed failure.

[0042] By applying the high-strength steel sheet of the present invention, for example, to structural components of automobiles, it is possible to achieve improved fuel efficiency due to vehicle body lightweighting, which has great industrial application value.

[0043] High strength refers to a tensile strength (TS) of 1180 MPa or higher, as determined by the tensile test described later.

[0044] Excellent component strength means that the yield ratio (YR) determined by the tensile test described later is 65% or higher.

[0045] Excellent tensile flange properties refer to a hole expansion rate (λ) of 30% or higher, as determined by the hole expansion test described later.

[0046] Excellent bending performance means that none of the five test pieces cracked in the bending test described later.

[0047] Excellent resistance to delayed failure means that in the delayed failure test described later, no cracks were detected after 48 hours when the TS was above 1180 MPa and below 1700 MPa, and no cracks were detected after 24 hours when the TS was above 1700 MPa.

[0048] <Steel Plate>

[0049] First, the steel plate (base steel plate) constituting the high-strength steel plate of the present invention will be described.

[0050] The steel sheet is, for example, a cold-rolled sheet that has undergone post-heating as described later.

[0051] There is no particular limitation on the thickness of the steel plate, which is usually 0.3mm to 2.8mm.

[0052] Composition

[0053] The composition of the steel plate is described (hereinafter also referred to as "the composition of the present invention" for convenience).

[0054] Unless otherwise specified, the "%" in the composition of this invention refers to "mass %".

[0055] (C: 0.090%~0.390%)

[0056] C is one of the important basic components of steel, especially in this invention, it affects the area ratio of martensite and ferrite.

[0057] If the carbon content is too low, the martensite area ratio decreases, making it difficult to achieve a total strength (TS) of 1180 MPa or higher. Therefore, the carbon content is 0.090% or higher, preferably 0.100% or higher, and more preferably 0.110% or higher.

[0058] On the other hand, if the carbon content is too high, the retained austenite increases excessively, and the hardness of the martensite formed from the retained austenite during punching increases significantly. As a result, cracking progresses during hole expansion, the hole expansion rate decreases, and the tensile flange properties decrease. In addition, the bending properties also decrease. Therefore, the carbon content is 0.390% or less, preferably 0.360% or less, and more preferably 0.350% or less.

[0059] (Si: 0.01%~2.00%)

[0060] Si enhances the strength of steel plates by suppressing the precipitation of cementite in martensite and through solid solution strengthening. To achieve this effect, the Si content is 0.01% or more, preferably 0.20% or more, and more preferably 0.30% or more.

[0061] On the other hand, if the Si content is too high, the area ratio of ferrite becomes excessive, and the formation of voids at the interface between ferrite and martensite during punching and reaming increases. As a result, the reaming rate decreases, and the tensile flange properties decrease. In addition, the strength and bending properties of the component deteriorate. Therefore, the Si content is 2.00% or less, preferably 1.50% or less, and more preferably 1.30% or less.

[0062] (Mn: 2.00%~4.00%)

[0063] Mn is one of the important basic components of steel, and especially in this invention, it affects the area ratio of martensite.

[0064] If the Mn content is too low, the martensite area ratio will decrease, making it difficult to achieve a total strength (TS) of 1180 MPa or higher. Therefore, the Mn content is 2.00% or higher, preferably 2.20% or higher, and more preferably 2.50% or higher.

[0065] On the other hand, if the Mn content is too high, the austenite is stabilized, the retained austenite increases excessively, and the hardness of the martensite formed from the retained austenite during punching is greatly increased. As a result, cracking progress is promoted during hole expansion, the hole expansion rate is reduced, and the tensile flange properties are reduced. Therefore, the Mn content is 4.00% or less, preferably 3.80% or less, and more preferably 3.60% or less.

[0066] (P: below 0.100%)

[0067] Phosphorus segregation at the original austenite grain boundaries causes grain boundary embrittlement, thus reducing the ultimate deformation capacity of the steel sheet. This results in a decrease in the porosity and tensile flangeability. Furthermore, the flexibility also decreases. Therefore, the phosphorus content is 0.100% or less, preferably 0.070% or less, more preferably 0.050% or less, and even more preferably 0.020% or less.

[0068] There is no particular lower limit for the phosphorus (P) content. However, P is a solid solution strengthening element that improves the strength of steel plates. Therefore, the P content is preferably 0.001% or more, and more preferably 0.002% or more.

[0069] (S: below 0.0200%)

[0070] Sulfide (S) exists in the form of sulfides, reducing the ultimate deformation capacity of the steel sheet. This results in a decrease in the hole expansion rate and tensile flange properties. Furthermore, the bendability also decreases. Therefore, the S content is 0.0200% or less, preferably 0.0100% or less, more preferably 0.0050% or less, and even more preferably 0.0020% or less.

[0071] There is no particular limit to the lower limit of sulfur content, but due to limitations in production technology, it is preferably 0.0001%.

[0072] (AI: below 1.000%)

[0073] Al is used to fully deoxidize and reduce inclusions in the steel.

[0074] However, if the Al content is too high, a large amount of ferrite will be generated, reducing the porosity and tensile flange properties. Therefore, the Al content is 1.000% or less, preferably 0.500% or less, and more preferably 0.100% or less.

[0075] On the other hand, in order to stably carry out deoxygenation, the Al content is preferably 0.010% or more, more preferably 0.015% or more, and even more preferably 0.020% or more.

[0076] (N: below 0.0100%)

[0077] Nitrogen (N) exists in the form of nitrides, reducing the ultimate deformation capacity of the steel sheet. This results in a decrease in the hole expansion ratio and tensile flange properties. Furthermore, the flexibility is also reduced. Therefore, the N content is 0.0100% or less, preferably 0.0070% or less, and more preferably 0.0050% or less.

[0078] There is no particular limit to the lower limit of nitrogen content, but due to limitations in production technology, it is preferably 0.0001%.

[0079] (O: below 0.0100%)

[0080] O, existing in the form of oxides, reduces the ultimate deformation capacity of the steel sheet. This results in a decrease in the hole expansion ratio and tensile flange properties. Furthermore, the flexibility also decreases. Therefore, the O content is 0.0100% or less, preferably 0.0050% or less, more preferably 0.0030% or less, and even more preferably 0.0020% or less.

[0081] There is no particular limit to the lower limit of O content, but due to limitations in production technology, it is preferably 0.0001%.

[0082] (Other elements)

[0083] The composition of the present invention may further contain, by mass%, at least one element selected from the elements described below.

[0084] (Ti, Nb, and V: each below 0.200%)

[0085] The contents of Ti, Nb, and V are preferably 0.200% or less, more preferably 0.100% or less, and even more preferably 0.080% or less. If the contents of each element are within this range, large amounts of coarse precipitates and inclusions will not be generated, the ultimate deformation capacity of the steel plate will not be reduced, and therefore the tensile flange properties (hole expansion ratio) are superior. In addition, the bending properties are also superior.

[0086] On the other hand, there is no particular limitation on the lower limit of the content of Ti, Nb, and V. However, Ti, Nb, and V improve the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing, as described later. Therefore, from the viewpoint of obtaining the effects of adding these elements, the content of Ti, Nb, and V is preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more, respectively.

[0087] (Ta and W: less than 0.10% each)

[0088] The contents of Ta and W are preferably 0.10% or less, more preferably 0.08% or less, and even more preferably 0.04% or less. If the contents of each element are within this range, large amounts of coarse precipitates and inclusions will not be generated, the ultimate deformation capacity of the steel plate will not be reduced, and therefore the tensile flange properties (hole expansion ratio) are superior. In addition, the bending properties are also superior.

[0089] On the other hand, there is no particular lower limit to the content of Ta and W. However, Ta and W improve the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing, as described later. Therefore, from the viewpoint of obtaining the effects of adding these elements, the content of Ta and W is preferably 0.01% or more, respectively.

[0090] (B: below 0.0100%)

[0091] The B content is preferably 0.0100% or less, more preferably 0.0080% or less, and even more preferably 0.0050% or less. If the B content is within this range, no cracks will be generated inside the steel sheet during hot rolling, as described later, and the ultimate deformation capacity of the steel sheet will not decrease, thus resulting in better tensile flange properties (expansion ratio). In addition, the bending properties are also better.

[0092] On the other hand, there is no particular lower limit to the B content. However, B segregates at the austenite grain boundaries during annealing, as described later, thus improving hardenability. Therefore, from the viewpoint of obtaining the desired effect of B addition, the B content is preferably 0.0003% or more, more preferably 0.0005% or more, and even more preferably 0.0010% or more.

[0093] (Cr, Mo, and Ni: less than 1.00% each)

[0094] The contents of Cr, Mo, and Ni are preferably 1.00% or less, more preferably 0.90% or less, and even more preferably 0.80% or less. If the contents of each element are within this range, the amount of coarse precipitates and inclusions does not increase, the ultimate deformation capacity of the steel plate does not decrease, and therefore the tensile flange properties (hole expansion ratio) are superior. In addition, the bending properties are also superior.

[0095] On the other hand, there is no particular limitation on the lower limit of the content of Cr, Mo, and Ni. However, Cr, Mo, and Ni improve hardenability. Therefore, from the viewpoint of obtaining the effect of adding these elements, the content of Cr, Mo, and Ni is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.03% or more.

[0096] (Co: less than 0.010%)

[0097] The Co content is preferably 0.010% or less, more preferably 0.008% or less, and even more preferably 0.006% or less. If the Co content is within this range, the amount of coarse precipitates and inclusions does not increase, and the ultimate deformation capacity of the steel plate does not decrease, thus resulting in better tensile flange properties (hole expansion ratio). In addition, the bending properties are also better.

[0098] On the other hand, there is no particular limit to the lower limit of the Co content. However, Co improves hardenability. Therefore, from the viewpoint of obtaining the effect of adding Co, the Co content is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.004% or more.

[0099] (Cu: less than 1.00%)

[0100] The Cu content is preferably 1.00% or less, more preferably 0.80% or less, and even more preferably 0.30% or less. If the Cu content is within this range, the amount of coarse precipitates and inclusions does not increase, and the ultimate deformation capacity of the steel plate does not decrease, thus resulting in better tensile flange properties (hole expansion ratio). In addition, the bending properties are also better.

[0101] On the other hand, there is no particular limit to the lower limit of the Cu content. However, Cu improves hardenability. Therefore, from the viewpoint of obtaining the effect of Cu addition, the Cu content is preferably 0.01% or more, more preferably 0.03% or more, and even more preferably 0.05% or more.

[0102] (Sn: below 0.200%)

[0103] The Sn content is preferably 0.200% or less, more preferably 0.100% or less, and even more preferably 0.050% or less. If the Sn content is within this range, no cracks will be generated inside the steel sheet during casting or hot rolling (described later), and the ultimate deformation capacity of the steel sheet will not be reduced, thus resulting in better tensile flange properties (expansion ratio). In addition, the bending properties are also better.

[0104] On the other hand, there is no particular limitation on the lower limit of Sn content. However, Sn improves hardenability. Therefore, from the viewpoint of obtaining the effect of Sn addition, the Sn content is preferably 0.001% or more, more preferably 0.004% or more, and even more preferably 0.008% or more.

[0105] (Sb: below 0.200%)

[0106] The Sb content is preferably 0.200% or less, more preferably 0.100% or less, and even more preferably 0.050% or less. If the Sb content is within this range, the amount of coarse precipitates and inclusions does not increase, and the ultimate deformation capacity of the steel plate does not decrease, thus resulting in better tensile flange properties (hole expansion ratio). In addition, the bending properties are also better.

[0107] On the other hand, there is no particular limitation on the lower limit of the Sb content. However, Sb controls the surface softening thickness and can adjust the strength. Therefore, from the viewpoint of obtaining the desired effect of Sb addition, the Sb content is preferably 0.001% or more, more preferably 0.003% or more, and even more preferably 0.007% or more.

[0108] (Ca, Mg and REM: less than 0.0100% each)

[0109] The contents of Ca, Mg, and REM (rare earth metals) are preferably 0.0100% or less, more preferably 0.0050% or less, and even more preferably 0.0030% or less. If the contents of each element are within this range, coarse precipitates and inclusions do not increase, and the ultimate deformation capacity of the steel plate does not decrease, thus resulting in superior tensile flange properties (hole expansion rate). Furthermore, the bending properties are also superior.

[0110] On the other hand, there is no particular limitation on the lower limit of the content of Ca, Mg, and REM. However, Ca, Mg, and REM make the nitrides and sulfides spherical, thereby improving the ultimate deformation capacity of the steel sheet. Therefore, from the viewpoint of obtaining the effects of adding these elements, the content of Ca, Mg, and REM is preferably 0.0002% or more, more preferably 0.0005% or more, and even more preferably 0.0010% or more, respectively.

[0111] (Zr and Te: less than 0.100% each)

[0112] The contents of Zr and Te are preferably 0.100% or less, more preferably 0.080% or less, and even more preferably 0.050% or less. If the contents of each element are within this range, the amount of coarse precipitates and inclusions does not increase, the ultimate deformation capacity of the steel plate does not decrease, and therefore the tensile flange properties (hole expansion ratio) are superior. In addition, the bending properties are also superior.

[0113] On the other hand, there is no particular lower limit to the content of Zr and Te. However, Zr and Te spherize the shape of nitrides and sulfides, thereby improving the ultimate deformation capacity of the steel sheet. Therefore, from the viewpoint of obtaining the effects of adding these elements, the content of Zr and Te is preferably 0.001% or more, more preferably 0.003% or more, and even more preferably 0.007% or more, respectively.

[0114] (Hf: below 0.10%)

[0115] The Hf content is preferably 0.10% or less, more preferably 0.08% or less, and even more preferably 0.05% or less. If the Hf content is within this range, the amount of coarse precipitates and inclusions does not increase, and the ultimate deformation capacity of the steel plate does not decrease, thus resulting in better tensile flange properties (hole expansion ratio). In addition, the bending properties are also better.

[0116] On the other hand, there is no particular lower limit to the Hf content. However, Hf causes nitrides and sulfides to become spherical, improving the ultimate deformation capacity of the steel sheet. Therefore, from the viewpoint of obtaining the desired effect of Hf addition, the Hf content is preferably 0.01% or more, and more preferably 0.02% or more.

[0117] (Bi: below 0.200%)

[0118] The Bi content is preferably 0.200% or less, more preferably 0.100% or less, and even more preferably 0.030% or less. If the Bi content is within this range, the amount of coarse precipitates and inclusions does not increase, the ultimate deformation capacity of the steel plate does not decrease, and the tensile flange properties (hole expansion ratio) are better. In addition, the bending properties are also better.

[0119] On the other hand, there is no particular limitation on the lower limit of the Bi content. However, Bi is an element that reduces segregation. Therefore, from the viewpoint of obtaining the effect of Bi addition, the Bi content is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.004% or more.

[0120] (The rest of the text)

[0121] The remaining portion of the composition of this invention consists of Fe and unavoidable impurities.

[0122] For the elements Ti, Nb, V, Ta, W, B, Cr, Mo, Ni, Co, Cu, Sn, Sb, Ca, Mg, REM, Zr, Te, Hf, and Bi, the effect of the present invention will not be impaired when the content of each element is less than the lower limit of the above preferred range. Therefore, these elements are treated as unavoidable impurities.

[0123] Micro-organization

[0124] Next, the microstructure of the steel plate will be described (hereinafter also referred to as "the microstructure of the present invention" for convenience).

[0125] (Martensite area ratio: 70% or more)

[0126] By using martensite as the main phase, a strength TS of 1180 MPa or higher can be achieved. Therefore, the martensite area fraction is 70% or more, preferably 75% or more, more preferably 80% or more, and even more preferably 85% or more.

[0127] There is no specific upper limit; the same effect can be achieved even if the martensite area ratio is 100%.

[0128] Martensite includes lower bainite, self-tempered martensite produced by cooling during annealing as described later, and martensite tempered by post-heating as described later.

[0129] As will be described later, the martensite is observed at 1 / 4 of the thickness of the steel plate.

[0130] (Ferrite area ratio: below 10%)

[0131] By reducing ferrite, good tensile flange properties and bending properties can be obtained. Therefore, the ferrite area fraction is 10% or less, preferably 8% or less, more preferably 5% or less, and even more preferably 3% or less.

[0132] There is no specific lower limit; the same effect can be achieved even if the ferrite area ratio is 0%.

[0133] Ferrite includes allotriomorph ferrite, idiomorph ferrite, Widmanstätten ferrite, and upper bainite, among others.

[0134] As will be described later, the ferrite is observed at a position one-quarter of the thickness of the steel plate.

[0135] The method for determining the area ratio of martensite and ferrite is as follows.

[0136] First, the sample was cut from the steel plate with the observation surface being a section of the plate thickness (L section at 1 / 4 of the plate thickness) parallel to its rolling direction. The observation surface of the sample was mirror-polished using diamond plaster, followed by final polishing using colloidal silica, and then etched using 1% nitric acid alcohol to expose the microstructure.

[0137] Next, under an accelerating voltage of 10 kV, the observation surface of the sample was observed using a scanning electron microscope (SEM) at a magnification of 3000x, resulting in SEM images of three fields of view.

[0138] The area ratio of each tissue was calculated from the obtained SEM images using Adobe Photoshop (Adobe Systems). Specifically, the area of ​​each tissue was divided by the measured area to obtain the area ratio of each tissue. The area ratio of each tissue was calculated for three fields of view, and their average value was taken as the area ratio of each tissue.

[0139] In SEM images, ferrite appears as a flat, gray structure, while martensite is a structure with a fine, layered internal structure, and the two can be distinguished from each other.

[0140] (Area ratio of retained austenite: less than 10%)

[0141] By reducing retained austenite, good component strength and tensile flange properties can be obtained. Therefore, the area ratio of retained austenite is 10% or less, preferably 8% or less, more preferably 5% or less, and even more preferably 4% or less.

[0142] There is no specific lower limit; the same effect can be achieved even if the area ratio of retained austenite is 0%.

[0143] The method for determining the area ratio of retained austenite is as follows.

[0144] First, the steel plate is ground with the measurement surface at 1 / 4 of its thickness (equivalent to 1 / 4 of the plate thickness in the depth direction from the surface of the steel plate). Then, it is further ground by 0.1 mm through chemical grinding to obtain the sample.

[0145] The integral reflection intensity of the (200), (220) and (311) planes of fcc iron (austenite) and the (200), (211) and (220) planes of bcc iron was measured using an X-ray diffraction device with a Co Kα ray source.

[0146] Calculate the intensity ratio of the integrated reflection intensity of each facet of fcc iron to that of each facet of bcc iron. Take the average of the nine intensity ratios as the volume fraction of retained austenite.

[0147] The volume fraction of retained austenite is considered as the area fraction of retained austenite.

[0148] (The ratio of martensite blocks containing metastable carbides to the total number of martensite blocks: 2% or more)

[0149] By using metastable carbides precipitated in the martensitic block, excellent component strength, flexural properties, and tensile flange properties are maintained, while resistance to delayed failure is improved.

[0150] To achieve this effect, the ratio of the number of martensite blocks containing metastable carbides to the total number of martensite blocks (also referred to as "ratio p") is 2% or more, preferably 5% or more, more preferably 10% or more, even more preferably 20% or more, and particularly preferably 30% or more.

[0151] There is no specific upper limit to the ratio p; it can be 100%.

[0152] Metastable carbides are metastable carbides that precipitate during the tempering of martensite.

[0153] Metastable carbides are, for example, Fe carbides (iron-based carbides) other than cementite, and can be exemplified by at least one carbide selected from epsilon (ε) carbides, eta (η) carbides and chi (χ) carbides.

[0154] The method for determining the ratio (p) of the number of martensite blocks containing metastable carbides to the total number of martensite blocks is as follows.

[0155] First, the steel plate was ground with its observation surface at 1 / 4 of its thickness (equivalent to 1 / 4 of the plate thickness in the depth direction from the surface), followed by electrolytic grinding to prepare a sample. The observation surface of the prepared sample was observed using a transmission electron microscope (TEM) at an accelerating voltage of 200 kV. The dislocation density of martensite is significantly higher than that of ferrite and retained austenite; therefore, by observing the strain contrast in the bright-field TEM image, the martensite within the field of view can be identified and distinguished.

[0156] Martensite blocks are one of the building blocks of martensite with a hierarchical structure. They are groups of laths with the same crystal orientation and crystal planarity. In bright-field TEM images, each block within a martensite exhibits different diffraction contrast, thus distinguishing it from other building blocks such as bales and laths, and enabling the identification of martensite blocks.

[0157] When an electron beam is incident from the

[100] direction of the martensite block, an electron diffraction pattern of the parent martensite phase is obtained. The crystal orientations of adjacent martensite blocks differ through the block boundaries, resulting in different contrasts in the bright-field image, thus distinguishing them from each other.

[0158] Figure 1 This is an example of an electron diffraction pattern of martensite containing carbides.

[0159] When carbides are observed in a single martensite block, such as Figure 1 As shown, in addition to the electron diffraction pattern of the parent phase martensite (α), the electron diffraction pattern of the carbides was also obtained.

[0160] Figure 1 In the diagram, black circles represent electron diffraction spots of the parent phase martensite when the electron beam is incident from the

[100] direction, and white circles represent electron diffraction spots of carbides.

[0161] Depend on Figure 1 The distance D1 between the electron diffraction spots of the parent phase martensite and the distance D2 between the electron diffraction spots of the carbide are calculated using the following formula 3 to determine the interplanar spacing d of the carbide. c The interplanar spacing d between the parent phase martensite and the crystal planes m The ratio d c / d m .

[0162] Equation 3: d c / d m =D1 / D2

[0163] For the observed martensitic blocks, the interplanar spacing d of the carbides c The interplanar spacing d between the parent phase martensite and the crystal planes m The ratio d c / d mWhen the value is 1.020 to 1.150, the martensite block is defined as a martensite block containing metastable carbides.

[0164] That is, "the ratio of the number of martensite blocks containing metastable carbides to the total number of martensite blocks" can be expressed as "ratio d c / d m The ratio of the number of martensite blocks to the total number of martensite blocks is 1.020 to 1.150.

[0165] The distance D1 between the electron diffraction spots of the parent phase martensite is a constant value, but the distance D2 between the electron diffraction spots of the carbide varies depending on the carbide.

[0166] For example, when the carbide is cementite, the distance D2 is equal to the distance D1. Therefore, compared to d... c / d m The value is 1 (D1 / D2 = d c / d m =1).

[0167] On the other hand, metastable carbides (ε-carbides, etc.) have a shorter distance D2 than cementite, and therefore a shorter distance D2 than d. c / d m The value of (=D1 / D2) is greater than 1. Therefore, the ratio d when metastable carbides are present will be greater than 1. c / d m The lower limit is set to "1.020".

[0168] It should be noted that, compared to d c / d m The upper limit is determined as "1.150" based on the distance D2 of the ε carbide.

[0169] Metastable carbides can exist inside the martensite block or at boundary regions such as block boundaries, but are preferably found inside the martensite block.

[0170] Observe 50 martensite blocks. Calculate the value obtained by dividing the number of martensite blocks containing metastable carbides by the number of observed martensite blocks and then multiplying by 100 (=(number of martensite blocks containing metastable carbides / 50) × 100). Use the calculated value as the proportion (p%) of the number of martensite blocks containing metastable carbides to the total number of martensite blocks.

[0171] (Number density of metastable carbides in martensite blocks containing metastable carbides: 1 × 10⁻⁶) 6 pcs / mm 2 above)

[0172] Based on the premise of superior resistance to delayed failure, a high number density of metastable carbides in the martensitic bulk is preferred. This is because a higher number density of metastable carbides results in greater hydrogen capture.

[0173] Specifically, the number density (also called "number density n") of metastable carbides in the martensite block containing metastable carbides is preferably 1 × 10⁻⁶. 6 pcs / mm 2 The above is preferred, and more preferably is 10×10. 6 pcs / mm 2 The above is further preferred to be 100×10 6 pcs / mm 2 above.

[0174] There is no specific upper limit to the number density n, for example, it can be 10,000,000 × 10 6 pcs / mm 2 The preferred value is 1000000×10 6 pcs / mm 2 More preferably, it is 100000×10 6 pcs / mm 2 Further preferred is 10000×10 6 pcs / mm 2 .

[0175] The number density (number density n) of metastable carbides in a martensitic block containing metastable carbides is calculated as follows.

[0176] When using the aforementioned TEM measurement scale p, a limited-field electron diffraction pattern was obtained in a single martensitic block containing metastable carbides. A dark-field image was then obtained using the electron diffraction spots from the metastable carbides. In the dark-field image, the metastable carbides exhibit white contrast.

[0177] The number of metastable carbides was calculated by photographing a 300 nm × 300 nm region within a single martensite block. It should be noted that adjacent martensite blocks passing through block boundaries can also exist within this 300 nm × 300 nm region.

[0178] The area of ​​a martensite block containing metastable carbides is defined as the area of ​​a single martensite block that yields a confined-field electron diffraction pattern. Adjacent martensite blocks have different crystal orientations across their block boundaries, resulting in varying contrast in bright-field images and thus distinguishing them from one another.

[0179] The above measurements were performed in three fields of view. The number of metastable carbides was divided by the area of ​​the martensite block containing metastable carbides in each field of view (= number of metastable carbides / area of ​​the martensite block containing metastable carbides). The average of these values ​​was taken as the number density (n) of metastable carbides in the martensite block containing metastable carbides.

[0180] (Equivalent circle diameter of metastable carbides)

[0181] The smaller the average equivalent circle diameter of metastable carbides in martensite, the higher the hydrogen trapping capacity, or the more it inhibits dislocation movement that contributes to delayed failure, thus exhibiting superior resistance to delayed failure.

[0182] Therefore, the average equivalent circle diameter of the metastable carbides in the martensitic block is preferably 20 nm or less, more preferably 5 nm or less.

[0183] The average equivalent circle diameter of metastable carbides in martensitic blocks is calculated as follows.

[0184] When using the aforementioned TEM measurement scale p, a limited-field electron diffraction pattern was obtained in a single martensitic block containing metastable carbides. A dark-field image was then obtained using the electron diffraction spots from the metastable carbides. In the dark-field image, the metastable carbides exhibit white contrast.

[0185] Dark-field images of a 300 nm × 300 nm region within a single martensite block were captured. Image processing was performed to obtain a binarized image that could distinguish metastable carbides. Particle analysis was then performed on the binarized image to determine the equivalent circle diameter for each metastable carbide particle. When metastable carbides overlapped in the dark-field image, the Watershed method was used to segment the binarized image.

[0186] Calculate the equivalent circle diameter (3 fields of view) for all metastable carbides existing in the 300 nm × 300 nm region. Calculate the average of the equivalent circle diameters from the 3 fields of view and use it as the average of the equivalent circle diameters of the metastable carbides in the martensite block.

[0187] (Remaining tissue)

[0188] The microstructure of the present invention may have a structure other than the martensite, ferrite and retained austenite described above (residual structure).

[0189] However, for the sake of not impairing the effect of the present invention, the area ratio of the remaining tissue is preferably 3% or less.

[0190] Examples of remaining microstructures include pearlite; carbides such as cementite precipitated in ferrite; alloy carbonitrides precipitated in ferrite; and microstructures known as microstructures of other steel plates.

[0191] It should be noted that metastable carbides precipitated in martensite and iron-based carbides such as cementite precipitated in martensite are not included in the remaining microstructure.

[0192] Soft Layer

[0193] The steel plate (base steel plate) constituting the high-strength steel plate of the present invention preferably has a soft layer on its surface.

[0194] During stamping and impact, the soft layer helps to suppress the propagation of bending cracks, thus providing excellent flexibility and excellent fracture resistance during impact.

[0195] The surface layer is the portion of the steel plate (cold-rolled plate) extending 200 μm along the thickness direction.

[0196] The soft layer is the decarburized layer; more specifically, it is the portion of the steel sheet (cold-rolled sheet) where the Vickers hardness is below 85% at a position one-quarter of the sheet thickness.

[0197] The Vickers hardness at 1 / 4 of the plate thickness refers to the Vickers hardness of a cross section (parallel to the surface of the steel plate) at 1 / 4 of the plate thickness (a point on the surface of the steel plate at a depth direction equivalent to 1 / 4 of the plate thickness).

[0198] Vickers hardness was measured according to JIS Z 2244-1:2020 with a load of 10 gf.

[0199] (thickness)

[0200] The thickness of the soft layer is preferably 3 μm or more, more preferably 7 μm or more, and even more preferably 11 μm or more.

[0201] On the other hand, the thickness of the soft layer is preferably 150 μm or less, more preferably 100 μm or less, and even more preferably 80 μm or less.

[0202] The thickness of the soft layer is calculated as follows.

[0203] First, using a Vickers hardness tester, with a load set to 10 gf, the Vickers hardness was measured at 1 μm intervals from a position 1 μm away from the surface of the steel plate along the thickness direction to a position 100 μm away along the thickness direction. Next, the Vickers hardness was measured at 20 μm intervals from the center of the plate thickness.

[0204] It should be noted that the Vickers hardness was determined after smoothing the section (L section) parallel to the rolling direction of the steel plate by wet grinding.

[0205] The portion (region) where the Vickers hardness at a position of 1 / 4 of the steel plate thickness is less than 85% is defined as the soft layer. The thickness of this portion (region) in the thickness direction is used as the thickness of the soft layer (unit: μm) to calculate the thickness.

[0206] (r)

[0207] The nanohardness at a location at 1 / 4 depth along the thickness direction of the soft layer is measured at more than 300 points (e.g., 512 points) within a range of 50 μm × 50 μm. The ratio of the number of measurement points with a nanohardness of 7.0 GPa or higher to the total number of measurement points is called the "ratio".

[0208] The ratio r is, for example, 0.20 or less, preferably 0.15 or less, and more preferably 0.10 or less.

[0209] A smaller ratio r indicates fewer hard microstructures (such as martensite) and inclusions in the microstructure. With fewer hard microstructures and inclusions in the microstructure, the generation, connection, and cracking progression of voids can be further suppressed during stamping and impact. That is, it results in better bending performance and better fracture resistance upon impact (SF, discussed later). max (The value is good).

[0210] (Standard deviation σ) A and standard deviation σ B )

[0211] The standard deviation of the nanohardness at a depth of 1 / 4 of the thickness in the soft layer is called the "standard deviation σ". A ".

[0212] The standard deviation of the nanohardness at the 1 / 2 depth position along the thickness direction of the soft layer is called the "standard deviation σ". B ".

[0213] Standard deviation σ A For example, it is 2.0 GPa or less, preferably 1.8 GPa or less, and more preferably 1.7 GPa or less.

[0214] Standard deviation σ B For example, it is 2.5 GPa or less, preferably 2.2 GPa or less, and more preferably 2.1 GPa or less.

[0215] Standard deviation σ A and standard deviation σ B Small size means smaller hardness differences among the components within the microstructure. In this case, the generation of voids, the connection of voids, and the progression of cracks can be further suppressed during stamping and impact. That is, it has better flexibility and better resistance to fracture upon impact.

[0216] Nanohardness is the hardness measured using the nanoindentation method.

[0217] More specifically, the nanohardness at the 1 / 4 and 1 / 2 depth positions along the thickness direction of the soft layer is calculated as follows.

[0218] First, if a coating and / or a metal coating (both described later) are formed on the surface of the steel plate, the coating and / or the metal coating are peeled off.

[0219] Then, mechanical grinding is performed on the surface of the steel plate to 1 / 4 (or 1 / 2) of the thickness depth of the soft layer, followed by polishing using diamond and alumina, and further grinding using colloidal silica. In this way, an L-section is obtained at 1 / 4 (or 1 / 2) of the thickness depth of the soft layer.

[0220] Next, using the Hysitron tribo-950 and a Berkovich-shaped diamond indenter, the nanohardness of the L-section of the soft layer was measured under the following conditions: load: 500 μN, measurement area: 50 μm × 50 μm, and dot spacing: 2 μm.

[0221] <Metallic coating (pre-annealing coating)>

[0222] The high-strength steel sheet of the present invention may also have a metallic coating (also known as a "pre-annealing coating") on its surface. The metallic coating is a layer different from the coatings described later.

[0223] As a metal coating, a metal electroplating layer is preferred.

[0224] During stamping and impact, the metal coating helps to suppress the formation of bending cracks, thus providing excellent flexibility and fracture resistance during impact.

[0225] Examples of metallic elements contained in a metallic coating include at least one element selected from Cr, Mn, Fe, Co, Ni, Cu, Ga, Ge, As, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Os, Ir, Rt, Au, Hg, Ti, Pb, and Bi. Fe or Ni is preferred.

[0226] That is, as a metal coating, it is preferable to have a metal coating composed of Fe and unavoidable impurities (hereinafter also referred to as "Fe-based metal coating").

[0227] Alternatively, as a metal coating, it is preferable to have a metal coating composed of Ni and unavoidable impurities (hereinafter also referred to as "Ni-based metal coating").

[0228] The composition of the Fe-based metal coating may further include at least one element selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co in an amount of no more than 10% by mass. When C is included, the C content is preferably no more than 0.08% by mass.

[0229] The amount of metal coating adhered (per single side) exceeds 0 g / m 2 Preferably 2.0 g / m 2 The above, more preferably 5.0 g / m 2 The above is further preferably 8.0 g / m 2 above.

[0230] On the other hand, there is no particular upper limit to the amount of metal coating applied (per side), but from a cost point of view, 60.0 g / m² is preferred. 2 The following is more preferably 50.0 g / m 2 The following is a further preferred value: 40.0 g / m 2 The following is particularly preferred: 30.0 g / m 2 The preferred value is 20.0 g / m³. 2 the following.

[0231] The amount of metal coating adhesion is calculated as follows.

[0232] First, a steel plate with a metallic coating is cut into 10mm × 15mm pieces and then embedded in resin to obtain an embedded sample. Using a scanning electron microscope (SEM) at an accelerating voltage of 15kV, the thickness of the metallic coating is measured at any three locations on the cross-section of the embedded sample at magnifications ranging from 2000 to 10000. The thickness of the metallic coating is then calculated by multiplying the average thickness at the three locations by the specific gravity of the metallic element (e.g., Fe) in the metallic coating.

[0233] <Coating>

[0234] The high-strength steel plate of the present invention may have a coating on the surface of the steel plate or metal coating. The coating is a layer different from the aforementioned metal coating and is formed by a plating process described later.

[0235] There are no particular limitations on the coating; examples include hot-dip galvanizing and electroplating. The coating can also be an alloyed coating (alloyed coating).

[0236] Examples of coatings include zinc plating (Zn plating) and Al plating, with zinc plating being preferred. Zinc plating may contain elements such as Al and Mg.

[0237] The composition of the coating is not particularly limited and can be a general composition.

[0238] For example, when the coating is a hot-dip galvanized layer or an alloyed hot-dip galvanized layer, the following composition can generally be cited: containing Fe: less than 20% by mass, Al: 0.001 to 1.0% by mass, further containing a total of 0 to 3.5% by mass of at least one of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi and REM, with the remainder consisting of Zn and unavoidable impurities.

[0239] When the coating is a hot-dip galvanized coating, the preferred adhesion amount per single side is 20-80 g / m². 2 Alternatively, alloyed hot-dip galvanized layers obtained by alloying such an amount of hot-dip galvanized layer are also preferred.

[0240] When the coating is a hot-dip galvanized coating, the Fe content in the coating is preferably less than 7% by mass. When the coating is an alloyed hot-dip galvanized coating, the Fe content in the coating is preferably 7 to 20% by mass, more preferably 7 to 15% by mass.

[0241] When the coating is a zinc coating (hot-dip galvanized coating or alloyed hot-dip galvanized coating), the coating adhesion amount is calculated as follows.

[0242] First, 0.6 g of a corrosion inhibitor (“IBIT700BK” (registered trademark), manufactured by Asahi Chemical Industry Co., Ltd.) was added to 1 L of a 10% hydrochloric acid aqueous solution to obtain a treatment solution. A steel sheet with a coating was then immersed in the obtained treatment solution as a test material to dissolve the coating.

[0243] The mass reduction of the test material is determined by measuring its mass before and after the coating dissolves. The coating adhesion weight (g / m²) is obtained by dividing the calculated mass reduction by the area of ​​the steel plate surface (the area covered by the coating). 2 ).

[0244] [Manufacturing method of high-strength steel plate]

[0245] Next, a method for manufacturing the high-strength steel plate of the present invention will be described (hereinafter also referred to as "the manufacturing method of the present invention" for convenience). The manufacturing method of the present invention is also a method for manufacturing the high-strength steel plate of the present invention described above.

[0246] <Hot rolling, pickling and cold rolling>

[0247] In the manufacturing method of the present invention, firstly, a cold-rolled sheet is obtained by hot rolling, pickling and cold rolling of a steel billet having the composition of the present invention as described above.

[0248] There are no particular limitations on the method of smelting molten steel into steel billets (steel billet materials), and well-known smelting methods such as using converters and electric furnaces can be adopted.

[0249] To prevent macroscopic segregation, steel billets are preferably manufactured by continuous casting, but they can also be manufactured by other methods such as ingot casting and slab casting.

[0250] The manufactured steel billet is temporarily cooled to room temperature, then reheated and hot-rolled (roughing and finishing), and then wound. This produces a hot-rolled sheet.

[0251] It should be noted that the manufactured steel billet can also be loaded into the heating furnace in a warm state without cooling to room temperature, or it can be rolled (rough rolling) immediately after being slightly heated.

[0252] Rough-rolled steel billets are used to obtain rough-rolled plates.

[0253] From the viewpoint of dissolving carbides and reducing rolling load, the temperature during rough rolling of steel billets (slab heating temperature) is preferably 1100°C or higher. On the other hand, to prevent increased oxide scale loss, the slab heating temperature is preferably 1300°C or lower.

[0254] The slab heating temperature is the surface temperature of the steel billet.

[0255] From the perspective of preventing malfunctions during hot rolling, it is preferable to heat the rough-rolled plate using a bar heater or similar device before finishing rolling, while reducing the slab heating temperature.

[0256] The finishing temperature is preferably above the Ar3 phase transformation point. This reduces the rolling load. Consequently, the reduction rate of austenite in its non-recrystallized state is reduced, suppressing the development of abnormal structures that elongate along the rolling direction, resulting in excellent machinability.

[0257] Finish rolling can be performed continuously by joining rough-rolled plates together. Rough-rolled plates can be temporarily wound up before finish rolling.

[0258] To reduce rolling load, part or all of the finishing rolling can be lubricated rolling. From the viewpoint of homogenizing the shape and material of the steel sheet, lubricated rolling is also preferred. The coefficient of friction during lubricated rolling is preferably in the range of 0.10 to 0.25.

[0259] From the viewpoint of ensuring good sheet flowability during cold rolling and annealing (described later), the preferred winding temperature after hot rolling is 300–700°C.

[0260] Next, the hot-rolled sheet obtained through hot rolling is pickled. Pickling removes oxides from the surface of the hot-rolled sheet, resulting in high-strength steel sheets with excellent chemical processing properties and coating quality in the final product. Pickling can be performed in one operation or in multiple operations.

[0261] After pickling, the hot-rolled sheet is subjected to any softening heat treatment, followed by cold rolling. This yields a cold-rolled sheet. The conditions for cold rolling are not particularly limited, but the cumulative reduction rate is preferably 20–75%. The number of rolling passes and the reduction rate per pass are not particularly limited.

[0262] The cold-rolled sheet thus obtained is subjected to the metal plating treatment described below (plating treatment before annealing), and then subjected to annealing and post-heating as described below.

[0263] It should be noted that the aforementioned hot-rolled plates (including pickled hot-rolled plates) and cold-rolled plates may also undergo a certain annealing process before metal plating treatment (plating treatment before annealing).

[0264] <Metal plating treatment (plating treatment before annealing)>

[0265] Metal plating treatment (pre-annealing plating treatment) is a treatment that forms the aforementioned metal plating layer (pre-annealing plating layer) on the surface (at least one side) of a cold-rolled sheet before the annealing process described later.

[0266] As a metal plating process, electroplating is preferred. This allows the formation of an electroplated metal layer as the metal coating.

[0267] As for the electroplating bath used in metal electroplating, sulfuric acid baths, hydrochloric acid baths, or mixed baths of both can be used. The amount of metal plating layer formed can be adjusted by factors such as the energizing time of the metal electroplating process.

[0268] Examples of metallic elements contained in the electroplating bath include at least one element selected from Cr, Mn, Fe, Co, Ni, Cu, Ga, Ge, As, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Os, Ir, Rt, Au, Hg, Ti, Pb, and Bi. Among these, Fe or Ni is preferred.

[0269] The electroplating bath may further contain at least one element selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co.

[0270] In the electroplating bath, the total content of these elements is preferably less than 10% by mass in the resulting metal coating.

[0271] In the electroplating bath before the energization begins, Fe ions (Fe 2＋) or Ni ions (Ni 2＋ The preferred content of ) is 0.5–2.0 mol / L.

[0272] Electroplating baths can contain metallic elements as metal ions, and non-metallic elements as part of boric acid, phosphoric acid, nitric acid, organic acids, etc.

[0273] Electroplating baths may further contain conductive additives such as sodium sulfate and potassium sulfate; chelating agents; pH buffers; etc.

[0274] From the perspective of maintaining a constant temperature, the preferred temperature for the electroplating bath is 30–85°C.

[0275] From the viewpoint of preventing the reduction in current efficiency caused by hydrogen generation, the pH of the electroplating bath is preferably 1.0 or higher, and considering conductivity, it is preferably 3.0 or lower.

[0276] From a productivity perspective, the preferred current density is 10 A / dm³. 2 From the viewpoint of easily controlling the amount of metal plating deposited, an adhesion of 150 A / dm is preferred. 2 the following.

[0277] From a productivity point of view, the speed at which the cold-rolled sheet passes through the electroplating bath (pass speed) is preferably 5 mpm or more, and from the point of view of stably controlling the amount of metal electroplating layer formed, it is preferably 150 mpm or less.

[0278] As a pretreatment for metal electroplating, degreasing and water washing can be performed to clean the surface of cold-rolled steel sheets, as well as pickling and water washing to activate the surface of cold-rolled steel sheets.

[0279] There are no particular limitations on the methods of degreasing and washing; any common methods can be used.

[0280] Pickling can be performed using various acids such as sulfuric acid, hydrochloric acid, nitric acid, and mixtures thereof. Sulfuric acid, hydrochloric acid, or mixtures thereof are preferred. Considering the ability to remove oxide films and to prevent surface defects caused by over-pickling, the concentration of the acid solution is preferably 1–20% by mass. Defoamers, pickling accelerators, pickling inhibitors, etc., may also be added to the acid solution.

[0281] <annealing>

[0282] Next, the obtained cold-rolled sheet is annealed.

[0283] In the annealing process, the cold-rolled sheet is roughly heated at the heating temperature described later, and then cooled to the cooling stop temperature described later.

[0284] Heating temperature: above 800℃

[0285] If the heating temperature is too low, the heating will occur within a two-phase region of ferrite and austenite, resulting in a lower martensite area ratio and failing to achieve the desired TS (trioxide). Therefore, the heating temperature is 800°C or higher, preferably 820°C or higher.

[0286] There is no particular upper limit to the heating temperature, but from the point of view of operability, 950°C is preferred.

[0287] There is no particular limitation on the heating time of the cold-rolled sheet at the heating temperature (heating time), but if it is too short, there is a risk that the reverse phase transformation to austenite cannot be fully carried out. Therefore, it is preferred to be 30 seconds or more, and more preferably 60 seconds or more.

[0288] There is no particular upper limit to the heating time, such as 6000s, but 3000s is preferred.

[0289] It should be noted that "s" represents seconds.

[0290] Dew point: Above -30℃

[0291] The dew point of the atmosphere at which the cold-rolled sheet is heated at the above-mentioned heating temperature is preferably -30°C or higher, more preferably -20°C or higher, even more preferably -15°C or higher, and particularly preferably -5°C or higher. This promotes the decarburization reaction of the cold-rolled sheet, facilitating the formation of the aforementioned soft layer.

[0292] On the other hand, from the viewpoint of properly preventing surface oxidation of the metal coating (pre-annealing coating) and ensuring good adhesion of the coating formed by the plating process described later, the dew point is preferably below 30°C.

[0293] Residence time t1 in the temperature range T1 above Ms℃ and below 700℃: less than 1000s

[0294] After being heated, the cold-rolled sheet is cooled to the cooling stop temperature and then passes through a temperature range of Ms℃ to 700℃, T1. If the cold-rolled sheet remains in the temperature range T1 for too long (dwell time t1), excessive ferrite phase transformation will occur, the ferrite area ratio will increase, and good tensile flange properties and bending properties cannot be obtained.

[0295] Therefore, the residence time t1 is less than 1000s, preferably less than 500s, and more preferably less than 300s.

[0296] There is no particular limitation on the lower limit of the residence time t1, but from the viewpoint of reducing the burden of equipment investment, it is preferably 1s, more preferably 5s, and even more preferably 10s.

[0297] Cooling in temperature region T1 can be continuous or intermittent. However, from the viewpoint of mitigating stress caused by microstructural changes during cooling, achieving a good steel sheet shape, and obtaining better tensile flange properties and bending performance, intermittent cooling is preferred.

[0298] The first average cooling rate v1 in the temperature range T2 (above Ms-80℃ and below Ms℃) is 1.0℃ / s to 40.0℃ / s.

[0299] The cold-rolled sheet, after passing through temperature region T1 (Ms℃~700℃), then passes through temperature region T2, which is above Ms-80℃ but below Ms℃. By cooling the cold-rolled sheet in temperature region T2 at a first average cooling rate v1 as described below, C is distributed from the martensite formed below Ms℃ to the untransformed austenite.

[0300] If the first average cooling rate v1 is too low, the C allocated to the untransformed austenite will precipitate coarsely as cementite, resulting in deterioration of flexibility.

[0301] Therefore, the first average cooling rate v1 is 1.0°C / s or more, preferably 2.0°C / s or more, and more preferably 3.0°C / s or more.

[0302] On the other hand, if the first average cooling rate v1 is too high, C is not easily distributed into the untransformed austenite. In the subsequent reheating, the driving force for the precipitation of metastable carbides is reduced, and sufficient resistance to delayed failure characteristics cannot be obtained.

[0303] Therefore, the first average cooling rate v1 is 40.0°C / s or less, preferably 25.0°C / s or less, and more preferably 20.0°C / s or less.

[0304] Continuous cooling is preferred for temperature range T2.

[0305] The second average cooling rate v2 for the temperature range T3 (above 150℃ and below Ms-80℃): above 0.3℃ / s and less than the first average cooling rate v1.

[0306] The cold-rolled sheet, after passing through temperature zone T2, then passes through temperature zone T3, which is above 150°C and below Ms-80°C. In temperature zone T3, the cold-rolled sheet is cooled at a second average cooling rate v2, as described below.

[0307] As a result, in the final microstructure, the ratio (p) of the number of martensite blocks containing metastable carbides to the total number of martensite blocks becomes higher.

[0308] The reasons for this are speculated as follows.

[0309] In the temperature range T3, a martensitic transformation occurs in C-enriched, untransformed austenite. At this point, C partially diffuses into the martensite mass, generating a precursor stage for metastable carbides. Consequently, subsequent post-heating significantly promotes the precipitation of metastable carbides.

[0310] If the second average cooling rate v2 is too low, carbon diffuses from the interior of the martensite block to the interface on the untransformed austenite side. As a result, there is more retained austenite, leading to poor component strength and tensile flange properties.

[0311] Therefore, the second average cooling rate v2 is 0.3°C / s or more, preferably 0.5°C / s or more, and more preferably 1.0°C / s or more.

[0312] On the other hand, the second average cooling rate v2 is less than the first average cooling rate v1.

[0313] If the second average cooling rate v2 is greater than the first average cooling rate v1, the diffusion of C in martensite becomes insufficient, and the precursor stage of metastable carbides cannot be fully generated. As a result, the precipitation of metastable carbides is suppressed in subsequent post-heating, the proportion p becomes lower, and the resistance to delayed degradation becomes insufficient.

[0314] Continuous cooling is preferred for temperature range T3.

[0315] It should be noted that Ms is the temperature at which the martensitic phase transformation begins (Ms point), and the value is determined by the Formastor test.

[0316] Bending and Rewinding Processing

[0317] The cold-rolled sheet can be bent and re-bent more than once during the annealing process.

[0318] The bending and re-bending process is preferably carried out in the temperature range of 150℃~Ms℃.

[0319] Bending and rebending refers to bending operations performed in a direction perpendicular to the rolling direction, followed by a rebending operation. Bending and rebending promote martensitic phase transformation. Consequently, metastable carbides precipitate efficiently during the subsequent reheating, as described later, increasing the ratio (p) of the number of martensite blocks containing metastable carbides to the total number of martensite blocks.

[0320] From the viewpoint of further increasing the ratio p, the number of bending and re-bending processes is preferably 1 or more, more preferably 2 or more, and even more preferably 3 or more.

[0321] On the other hand, if the number of bending and re-bending operations is too high, the El may decrease due to significant processing solidification. Therefore, the number of bending and re-bending operations is preferably 10 times or less, more preferably 8 times or less, and even more preferably 5 times or less.

[0322] There are no particular limitations on the method of bending and re-bending, but the use of rollers is commonly cited. The diameter of the rollers used for bending and re-bending is not particularly limited, but is preferably 300 to 2000 mm.

[0323] Cooling stop temperature: below 150℃

[0324] The cooling stop temperature is below 150℃. This ensures that the martensitic transformation is fully completed. When the cooling stop temperature exceeds 150℃, the untransformed austenite does not undergo martensitic transformation, resulting in an excessive amount of residual austenite and compromised component strength and tensile flange properties.

[0325] The cold-rolled sheet, which has been cooled to the cooling stop temperature, can be further cooled to room temperature.

[0326] There is no particular limitation on the cooling rate from the cooling stop temperature to room temperature. There are also no particular limitations on the cooling method; any method can be used, such as gas jet cooling, mist cooling, water cooling, or air cooling.

[0327] <Post-heating>

[0328] The cold-rolled sheet that has undergone annealing (heating and cooling) is then subjected to post-heating.

[0329] By performing post-heating, metastable carbides that enhance resistance to delayed failure precipitate in the martensite blocks formed during the cooling of the above-mentioned annealing. As a result, high-strength steel sheets with a strength (TS) of 1180 MPa or higher can be obtained, exhibiting good component strength, tensile flange properties, and flexural flexibility, as well as excellent resistance to delayed failure.

[0330] Formula 1

[0331] More specifically, annealed cold-rolled sheets are post-heated under conditions where the temperature X (in °C) representing the maximum reaching temperature and the holding time Y (in seconds) above X-10 °C satisfy Equation 1. As a result, the ratio (ratio p) of the number of martensite blocks containing metastable carbides to the total number of martensite blocks increases, resulting in excellent resistance to delayed failure.

[0332] Equation 1: 8000 ≤ (273 + X) × (20 + Log) 10 (Y / 3600) ≤ 12000

[0333] For convenience, the following will use the formula "(273 + X) × (20 + Log)" from Equation 1 above. 10(Y / 3600) is called the "variable part Z".

[0334] When the value of variable Z is too small, i.e., when the temperature X is too low and / or the holding time Y is too short, metastable carbides cannot be fully analyzed, and therefore the proportion p becomes lower.

[0335] Therefore, based on the reason that the ratio p increases, the value of the variable Z is 8000 or more, preferably 8500 or more, and more preferably 9000 or more.

[0336] On the other hand, when the value of variable Z is too high, i.e., when the temperature X is too high and / or the holding time Y is too long, metastable carbides transform into cementite, and the proportion p decreases.

[0337] Therefore, based on the reason that the ratio p increases, the value of the variable Z is 12000 or less, preferably 11500 or less, and more preferably 11000 or less.

[0338] Formula 2

[0339] In the post-heating process, the temperature X (unit: °C) preferably satisfies Equation 2. As a result, the number density (number density n) of metastable carbides in the martensite block containing metastable carbides becomes higher.

[0340] Equation 2: 100≤X≤400

[0341] That is, based on the reason that the number density n increases, the temperature X is preferably 100°C or higher, more preferably 120°C or higher, and even more preferably 150°C or higher.

[0342] For the same reason, the temperature X is preferably below 400°C, more preferably below 380°C, and even more preferably below 350°C.

[0343] The cold-rolled sheet that has undergone post-heating is then cooled to room temperature, for example.

[0344] Thus, the high-strength steel plate of the present invention is obtained through the manufacturing method of the present invention.

[0345] In the manufacturing method of the present invention, when the plating process described later is carried out, the high-strength steel sheet of the present invention obtained is a plated steel sheet with a coating.

[0346] The series of heat treatments in the manufacturing method of the present invention are not particularly limited in other conditions as long as the above-mentioned thermal process is met, and the equipment for performing the heat treatment is not particularly limited.

[0347] <Plating Treatment>

[0348] In the manufacturing method of the present invention, the cold-rolled sheet can be coated.

[0349] Examples of plating processes include hot-dip galvanizing (a process that forms a hot-dip galvanized layer) and alloying hot-dip galvanizing (a process that forms an alloyed hot-dip galvanized layer by performing an alloying process after hot-dip galvanizing). Electroplating can also be used to form an electroplated layer.

[0350] For example, plating treatment is performed on cold-rolled sheets that remain in the temperature range T1 (Ms℃~700℃).

[0351] When performing hot-dip galvanizing, it is preferable to immerse the cold-rolled sheet in a galvanizing bath at 440–500°C, and then adjust the coating adhesion by means of gas wiping or the like.

[0352] The preferred Al content in the zinc plating bath is 0.10–0.23% by mass.

[0353] Hot-dip galvanizing is preferably carried out during the annealing process in the temperature range T1 (Ms℃~700℃).

[0354] If the alloying temperature is too low, the Zn-Fe alloying rate becomes too slow, which can sometimes impair productivity. Therefore, a temperature of 470°C or higher is preferred.

[0355] On the other hand, if the alloying temperature is too high, the untransformed austenite will transform into pearlite, and sometimes the TS will decrease. Therefore, it is preferable to be below 600°C, and more preferably below 560°C.

[0356] The surface can be lightly spun after the plating process.

[0357] From the viewpoint of improving yield strength, the reduction rate of the surface sizing is preferably 0.05% or higher. There is no particular upper limit to the reduction rate, but from a productivity point of view, 1.50% is preferred.

[0358] Epidermal scalpel can be performed online or offline.

[0359] The target scalding rate can be achieved in one session or in multiple sessions.

[0360] From a productivity standpoint, the aforementioned annealing and plating processes are preferably carried out in a CAL (Continuous Annealing Line) or CGL (Continuous Galvanizing Line).

[0361] Example

[0362] The following examples illustrate the present invention. However, the present invention is not limited to the examples described below.

[0363] [Experimental Example 1]

[0364] <Steel Plate Manufacturing>

[0365] Molten steel with the composition shown in Tables 1 and 2 below (the remainder consists of Fe and unavoidable impurities) is melted in a converter and steel billets are obtained by continuous casting.

[0366] The obtained steel billet is hot-rolled to obtain a hot-rolled plate. Specifically, the steel billet is heated to 1250°C for rough rolling, then for finish rolling at a finishing temperature of 900°C, then for winding at 500°C, and finally cooled to room temperature to obtain a hot-rolled plate.

[0367] After pickling, the obtained hot-rolled sheet is subjected to softening heat treatment at 500°C, followed by cold rolling at a rolling rate of 50%. This yields a cold-rolled sheet with a thickness of 1.4 mm.

[0368] The obtained cold-rolled sheet was annealed and post-heated under the conditions shown in Table 3 below to obtain a steel sheet.

[0369] The heating time at the heating temperature is 200s.

[0370] The bending and re-bending process is carried out in temperature ranges T2 and T3.

[0371] Plating Treatment

[0372] A portion of cold-rolled steel sheets are subjected to hot-dip galvanizing while remaining in a temperature range T1 (Ms℃~700℃), forming a coating (hot-dip galvanized layer) on both sides. That is, hot-dip galvanized steel sheet (GI) is obtained.

[0373] The hot-dip galvanizing process uses a hot-dip galvanizing bath (bath temperature: 470°C) containing Al: 0.20% by mass and the remainder consisting of Zn and unavoidable impurities.

[0374] The adhesion weight of the hot-dip galvanized layer on each single side is 45g / m². 2 about.

[0375] The hot-dip galvanized layer consists of Fe: 0.1 to 1.0% by mass and Al: 0.2 to 1.0% by mass, with the remainder consisting of Fe and unavoidable impurities.

[0376] Another portion of the cold-rolled sheet undergoes alloyed hot-dip galvanizing treatment while remaining in the temperature range T1 (Ms℃~700℃), forming a coating (alloyed hot-dip galvanized layer) on both sides. This yields alloyed hot-dip galvanized steel sheet (GA).

[0377] The hot-dip galvanizing process uses a hot-dip galvanizing bath (bath temperature: 470°C) containing Al: 0.14% by mass and the remainder consisting of Zn and unavoidable impurities.

[0378] The alloying treatment is carried out at 550℃.

[0379] The adhesion weight of the alloyed hot-dip galvanized layer on each single side is 45 g / m². 2 about.

[0380] The resulting alloyed hot-dip galvanized layer consists of Fe: 7-15% by mass and Al: 0.1-1.0% by mass, with the remainder consisting of Fe and unavoidable impurities.

[0381] When a hot-dip galvanized layer is formed, “GI” will be recorded in the “Type” column of Table 3 below; when an alloyed hot-dip galvanized layer is formed, “GA” will be recorded in the “Type” column of Table 3 below; and when no coating is formed, “CR” will be recorded in the “Type” column of Table 3 below.

[0382] Hereinafter, hot-dip galvanized steel sheet (GI) and alloyed hot-dip galvanized steel sheet (GA) will also be referred to simply as "steel sheet".

[0383] <Observations on Microstructures>

[0384] For the obtained steel plates, the area ratios of martensite, ferrite, and retained austenite, the ratio of the number of martensite blocks containing metastable carbides to the total number of martensite blocks (ratio p), and the number density of metastable carbides in the martensite blocks containing metastable carbides (number density n) were determined according to the above method. The results are shown in Table 4 below.

[0385] The resulting steel plate has a microstructure consisting entirely of θ (cementite precipitated in ferrite) except for martensite, ferrite, and retained austenite.

[0386] <evaluate>

[0387] The obtained steel plates were subjected to the tests described below to evaluate various properties. The results are shown in Table 4 below.

[0388] Tensile Testing

[0389] The tensile test was performed in accordance with JIS Z 2241.

[0390] Specifically, JIS 5 test pieces are taken from the obtained steel sheet with the long side perpendicular to the rolling direction of the steel sheet. Using the taken test pieces, at a crosshead speed of 1.67 × 10⁻⁶... -1Tensile tests were conducted under conditions of mm / s to determine the yield strength (YS) [MPa], tensile strength (TS) [MPa], and total elongation (E1) [%]. The yield ratio (YR) (=100×YS / TS) [%] was then calculated.

[0391] A tensile strength (TS) of 1180 MPa or higher is considered high strength.

[0392] When the yield ratio (YR) is above 65%, the component is judged to have excellent strength.

[0393] Hole Enlargement Test

[0394] The hole enlargement test was conducted in accordance with JIS Z 2256.

[0395] Specifically, the obtained steel plate is cut to obtain a 100mm × 100mm test piece. A 10mm diameter hole is punched into the test piece with a 12.5% ​​gap. Then, using a die with an inner diameter of 75mm, a conical punch with a 60° apex angle is pressed into the hole under a creasing force of 9ton (88.26kN), and the hole diameter D at which cracking occurs is determined. f [mm]. Set the initial hole diameter to D0 [mm], and calculate the hole expansion rate λ [%) according to the following formula.

[0396] λ = {(D f -D0) / D0} × 100

[0397] When the expansion ratio (λ) is above 30%, it is judged to have excellent tensile flange properties.

[0398] Bending Test

[0399] The bending test was conducted in accordance with JIS Z 2248.

[0400] Specifically, a strip-shaped test piece with a width of 30 mm and a length of 100 mm is taken from the obtained steel plate, with the direction parallel to the rolling direction of the steel plate as the axis of the bending test. Using the taken test piece, a 90°V bending test is carried out under the conditions of a pressing load of 100 kN and a pressing holding time of 5 seconds.

[0401] Five test pieces were subjected to bending tests at the maximum bending radius R (R / t) with a value of 5 or less (e.g., when the plate thickness t is 1.4 mm, the bending radius R is 7.0 mm). Next, it was confirmed whether any cracks were formed at the edge of the bending apex.

[0402] The presence or absence of cracks was confirmed by observing the ridge of the curved apex using a digital microscope (RH-2000, manufactured by Hirox Co., Ltd.) at 40x magnification.

[0403] Only when no cracks were found in any of the 5 test pieces was the bending performance considered excellent, and "excellent" was recorded in Table 4 below.

[0404] On the other hand, if any one of the five test pieces develops a crack, it is judged as insufficient flexibility and recorded as "poor" in Table 4 below.

[0405] Delayed Destruction Test

[0406] A strip-shaped test piece, 70 mm wide and 16 mm long, was taken from the obtained steel plate by mechanically grinding the end face at a direction perpendicular to the rolling direction of the steel plate. The test piece was subjected to bending deformation by applying a stress equivalent to 800 MPa to the central part through four-point bending.

[0407] Next, the test piece subjected to bending deformation was immersed in a 0.1% by mass ammonium thiocyanate solution at 25°C with the pH adjusted to 5.0 using McIlvaine buffer.

[0408] Then, observe the test piece in the solution to confirm whether any cracks have formed.

[0409] For test pieces taken from steel plates with a TS value of 1180 MPa or higher but less than 1700 MPa, in Table 4 below, the case where a crack is confirmed after 48 hours is marked as "×", the case where no crack is confirmed after 48 hours but a crack is confirmed 60 hours ago is marked as "○", the case where no crack is confirmed after 60 hours but a crack is confirmed 72 hours ago is marked as "○○", the case where no crack is confirmed after 72 hours but a crack is confirmed 96 hours ago is marked as "○○○", and the case where no crack is confirmed after 96 hours is marked as "○○○○".

[0410] For test pieces taken from steel plates with a TS of 1700MPa or higher, in Table 4 below, the following cases are marked as follows: "×" if a crack is confirmed after 24 hours; "○" if no crack is confirmed after 24 hours but a crack is confirmed 36 hours ago; "○○" if no crack is confirmed after 36 hours but a crack is confirmed 48 hours ago; "○○○" if no crack is confirmed after 48 hours but a crack is confirmed 60 hours ago; and "○○○○" if no crack is confirmed after 60 hours.

[0411] Based on the excellent resistance to delayed failure characteristics, “○”, “○○”, “○○○” or “○○○○” are preferred, “○○”, “○○○” or “○○○○” are more preferred, “○○” or “○○○○” are even more preferred, “○○○” or “○○○○” are even more preferred, and “○○○○” is especially preferred.

[0412] The underlined lines in Tables 1 to 4 below indicate items outside the scope of this invention.

[0413]

[0414]

[0415]

[0416]

[0417]

[0418]

[0419] <Summary of Evaluation Results>

[0420] As shown in Table 4 above, the tensile strength of steel plates No.1, 7, 11-16 and 21-42 is above 1180 MPa, and the component strength, tensile flange properties, bending properties and resistance to delayed failure are all excellent.

[0421] It should be noted that when these steel plates are compared, the evaluation results of resistance to delayed failure characteristics are good in the following order (i), (ii), (iii) and (iv).

[0422] (i) The proportion p is greater than 2% and less than 20%, and the number density n is less than 1×10 6 pcs / mm

[0423] (ii) The proportion p is 2% or more but less than 20%, and the number density n is 1×10 6 More than one per mm

[0424] (iii) The proportion p is 20% or more but less than 30%.

[0425] (iv) The proportion p is 30% or more

[0426] In contrast, the steel plates No. 2 to 6, 8 to 10 and 17 to 20 have at least one of the following properties: tensile strength, component strength, tensile flange properties, bending properties and resistance to delayed failure.

[0427] [Experimental Example 2]

[0428] <Steel Plate Manufacturing>

[0429] Molten steel with the composition shown in Tables 1 and 2 above (the remainder consists of Fe and unavoidable impurities) is melted in a converter and steel billets are obtained by continuous casting.

[0430] The obtained steel billet is hot-rolled to obtain a hot-rolled plate. Specifically, the steel billet is heated to 1250°C for rough rolling, then for finish rolling at a finishing temperature of 900°C, then for winding at 500°C, and finally cooled to room temperature to obtain a hot-rolled plate.

[0431] After pickling, the obtained hot-rolled sheet is subjected to softening heat treatment at 500°C, followed by cold rolling at a rolling rate of 50%. This yields a cold-rolled sheet with a thickness of 1.2 mm.

[0432] The obtained cold-rolled steel was annealed and then reheated under the conditions shown in Table 5 below to obtain steel plates.

[0433] The heating time at the heating temperature is 200s.

[0434] The bending and re-bending process is carried out in temperature ranges T2 and T3.

[0435] In some cases, metal electroplating is performed before annealing as a metal plating treatment (pre-annealing plating treatment) to form an Fe-based metal coating or a Ni-based metal coating.

[0436] In the "Metal Plating Treatment" column of Table 5 below, the case of forming an Fe-based metal coating is marked as "Fe", the case of forming a Ni-based metal coating is marked as "Ni", and the case of not performing metal plating treatment is marked as "-".

[0437] As the electroplating bath, a solution containing 1.5 mol / L Fe was used. 2＋ ions or Ni 2＋ Ionized sulfuric acid bath (bath temperature: 50℃, pH: 2.0).

[0438] In the electroplating bath, cold-rolled steel sheet is used as the cathode, and iridium oxide electrode is used as the anode, with a flux of 45 A / dm. 2 Electrolysis is performed using a specific current density. The amount of metal coating formed is controlled by adjusting the energizing time.

[0439] Plating Treatment

[0440] A portion of cold-rolled sheets undergo hot-dip galvanizing while remaining in the temperature range T1 (Ms℃~700℃), forming a coating (hot-dip galvanized layer) on both sides. This results in hot-dip galvanized steel sheet (GI).

[0441] The hot-dip galvanizing process uses a hot-dip galvanizing bath (bath temperature: 470°C) containing Al: 0.20% by mass and the remainder consisting of Zn and unavoidable impurities.

[0442] The adhesion weight of the hot-dip galvanized layer on each single side is 45g / m². 2 about.

[0443] The hot-dip galvanized layer consists of Fe: 0.1 to 1.0% by mass and Al: 0.2 to 1.0% by mass, with the remainder consisting of Fe and unavoidable impurities.

[0444] Another portion of the cold-rolled sheets undergoes alloyed hot-dip galvanizing treatment while remaining in the temperature range T1 (Ms℃~700℃), forming a coating (alloyed hot-dip galvanized layer) on both sides. This yields alloyed hot-dip galvanized steel sheet (GA).

[0445] The hot-dip galvanizing process uses a hot-dip galvanizing bath (bath temperature: 470°C) containing Al: 0.14% by mass and the remainder consisting of Zn and unavoidable impurities.

[0446] The alloying treatment is carried out at 550℃.

[0447] The adhesion weight of the alloyed hot-dip galvanized layer on each single side is 45 g / m². 2 about.

[0448] The resulting alloyed hot-dip galvanized layer consists of Fe: 7-15% by mass and Al: 0.1-1.0% by mass, with the remainder consisting of Fe and unavoidable impurities.

[0449] In the “Type” column of Table 5 below, the case of forming a hot-dip galvanized layer is marked as “GI”, the case of forming an alloyed hot-dip galvanized layer is marked as “GA”, and the case of not forming a coating is marked as “CR”.

[0450] Hereinafter, hot-dip galvanized steel sheet (GI) and alloyed hot-dip galvanized steel sheet (GA) will also be referred to simply as "steel sheet".

[0451] <Observations on Microstructures>

[0452] For the obtained steel plates, the area ratios of martensite, ferrite, and retained austenite, the ratio of the number of martensite blocks containing metastable carbides to the total number of martensite blocks (ratio p), and the number density of metastable carbides in the martensite blocks containing metastable carbides (number density n) were determined according to the method described above. The results are shown in Table 6 below.

[0453] The resulting steel plate has a microstructure consisting entirely of θ (cementite precipitated in ferrite) except for martensite, ferrite, and retained austenite.

[0454] <Amount of metal coating>

[0455] The adhesion amount of the metal coating (Fe-based or Ni-based metal coating) was determined using the method described above. The results are shown in Table 6 below.

[0456] If no metal plating treatment was performed (i.e., no metal plating layer was formed), "-" will be recorded in the "Metal Plating Adhesion Amount" column of Table 6 below.

[0457] Properties of the soft layer

[0458] The thickness, proportion r (total number of measurement points: 512 points), and standard deviation σ of the soft layer were determined using the method described above. A and standard deviation σ B The results are shown in Table 6 below.

[0459] <evaluate>

[0460] The obtained steel plate was subjected to tensile and hole-expansion tests in the same manner as in Experiment 1, and the yield strength (YS), tensile strength (TS), yield ratio (YR), total elongation (E1), and hole expansion ratio (λ) were determined.

[0461] In addition, the obtained steel plate was subjected to bending and delayed failure tests in the same manner as in Test Example 1 to evaluate its bending performance and resistance to delayed failure.

[0462] The results are shown in Table 6 below.

[0463] The obtained steel plate was then subjected to the following tests to evaluate various properties. The results are shown in Table 6 below.

[0464] 90° Bending and Orthogonal Bending Machining

[0465] A 60mm × 65mm test piece T1 is taken from the obtained steel plate through shearing and end face grinding. The 60mm side of the test piece T1 is parallel to the rolling direction (L direction) of the steel plate, and the 65mm side is parallel to the width direction (C direction) of the steel plate.

[0466] The test piece T1 was subjected to the 90° bending and orthogonal bending processes described later.

[0467] Figure 2A It is a three-dimensional view showing the state of the test piece T1 after being bent at 90°.

[0468] Figure 2B It is a three-dimensional diagram showing the state of the test piece T1 under orthogonal bending processing.

[0469] exist Figure 2A and Figure 2B In the diagram, D1 represents the width direction (C direction), and D2 represents the rolling direction (L direction).

[0470] Specifically, the test piece T1 was first bent at 90° with a radius of curvature / thickness ratio of 4.2 (one-time bending). More specifically, such as... Figure 2A As shown, under the following conditions, punch B1 is pressed into test piece T1 placed on mold A1 with V groove.

[0471] Next, orthogonal bending (secondary bending) was performed on the test piece T1. More specifically, as follows: Figure 2B As shown, punch B2 is pressed into test piece T1 placed on support roller A2 under the following conditions.

[0472] (Conditions for 90° bending)

[0473] Test method: mold support, punch pressing

[0474] Molding load: 10 tons

[0475] Test speed: 30 mm / min

[0476] Duration: 5 seconds

[0477] Bending direction: Rolling direction (L direction)

[0478] (Conditions for orthogonal bending)

[0479] Test method: roller support, punch pressing

[0480] Roller diameter: φ30mm

[0481] Punch tip radius (R): 0.4mm

[0482] Roller spacing: (plate thickness × 2) + 0.5mm

[0483] Stroke speed: 20mm / min

[0484] Bending direction: Width direction (C direction)

[0485] The stroke at maximum load is determined based on the stroke-load curve obtained during orthogonal bending. Three 90° bends and three orthogonal bends are performed, and the average stroke at maximum load during these three trials is taken as SF. max (Unit: mm) Calculate.

[0486] In the calculated SF max Under the condition A below, it is judged to have excellent resistance to fracture during impact (resistance to bending fracture).

[0487] (Condition A)

[0488] When TS is above 1180MPa and below 1320MPa, SFmax When the thickness is above 24.5mm and the strength (TS) is above 1320MPa, the SF max It is 24.0mm or more.

[0489] Axial crush test

[0490] An axial crush test was performed as follows.

[0491] First, a 150mm × 100mm test piece is taken from the obtained steel plate by shearing. The 150mm edge of the test piece is parallel to the rolling direction (L direction) of the steel plate.

[0492] Next, using a die with a punch shoulder radius of 5.0 mm and a die shoulder radius of 5.0 mm, the test piece is formed (bent) to a depth of 40 mm. Figure 3A and Figure 3B As shown, make the hat-shaped part 10.

[0493] In addition to the test piece that becomes the hat-shaped part 10, an 80mm × 100mm test piece is cut from the obtained steel plate and used as the flat plate 20.

[0494] Spot weld the cap-shaped component 10 and the flat plate 20, as follows: Figure 3A and Figure 3B The test subject 30 is shown.

[0495] Figure 3A This is the front view of test subject 30. Figure 3B This is a three-dimensional view of test subject 30.

[0496] like Figure 3B As shown, the position of the spot weld 40 is 10 mm away from the end of the test body 30 and 20 mm away from each other.

[0497] Figure 3C This is a three-dimensional diagram showing the state of the test specimen 30 under axial crush test.

[0498] like Figure 3C As shown, one end of the test specimen 30 is joined to the base plate 50 by TIG welding. Next, an axial crush test is performed, in which the impactor 60 impacts the other end of the test specimen 30 at a constant velocity of 10 mm / min, crushing the test specimen 30 by 70 mm. Figure 3C In the diagram, D3 indicates the direction (crushing direction) in which the impactor 60 collides with the test object 30.

[0499] Observe the appearance of the test specimen 30 after the test to confirm whether there is axial crushing fracture (visual crack).

[0500] In Table 6 below, cases where no visible cracks are detected are marked with "◎", cases where one or fewer visible cracks are detected are marked with "○", and cases where two or more visible cracks are detected are marked with "×". Cases marked with "◎" or "○" are judged to have excellent resistance to fracture during impact (resistance to axial crushing fracture).

[0501]

[0502]

[0503]

[0504]

[0505] <Summary of Evaluation Results>

[0506] As shown in Table 6 above, the tensile strength of steel plates No. 43 to 94 is all above 1180 MPa, and the component strength, tensile flange properties, bending properties, and resistance to delayed failure are all excellent. Furthermore, the fracture resistance under impact (resistance to axial crushing fracture) is also excellent.

[0507] When the dew point is above -30°C and below -20°C, the thickness of the soft layer is less than 10 μm. In this case, the evaluation result of the fracture resistance (resistance to axial crushing fracture) under impact based on the axial crushing test is "○", but it is improved to "◎" when there is a metallic coating.

[0508] Symbol Explanation

[0509] A1: Mold

[0510] A2: Support roller

[0511] B1: Punch

[0512] B2: Punch

[0513] T1: Test piece

[0514] D1: Width direction (C direction)

[0515] D2: Rolling direction (L direction)

[0516] D3: Crushing direction

[0517] 10: Hat-shaped component

[0518] 20: Tablet

[0519] 30: Test subject

[0520] 40: Spot welding section

[0521] 50: Base plate

[0522] 60: Impactor

Claims

1. A high-strength steel plate, wherein the tensile strength of the high-strength steel plate is 1180 MPa or higher, and comprises a base steel plate. The base steel plate has the following composition and microstructure. The composition, by mass%, contains C: 0.090%–0.390%, Si: 0.01%–2.00%, Mn: 2.00%–4.00%, P: less than 0.100%, S: less than 0.0200%, Al: less than 1.000%, N: less than 0.0100%, and O: less than 0.0100%, with the remainder consisting of Fe and unavoidable impurities. In the aforementioned microstructure, The martensite area ratio is over 70%, of which... Martensite includes lower bainite and tempered martensite. The area fraction of ferrite is less than 10%, and the ferrite includes irregular ferrite, euhedral ferrite, Widmanstätten ferrite, and upper bainite. The area fraction of retained austenite is less than 10%. The proportion of martensite blocks containing metastable carbides to the total number of martensite blocks is more than 2%, wherein the metastable carbides are at least one carbide selected from ε carbides, η carbides and χ carbides.

2. The high-strength steel plate according to claim 1, wherein, The number density of metastable carbides in the martensite block containing metastable carbides is 1 × 10⁻⁶. 6 pcs / mm 2 above.

3. The high-strength steel plate according to claim 1, wherein, The composition, by mass%, further contains at least one element selected from Ti: less than 0.200%, Nb: less than 0.200%, V: less than 0.200%, Ta: less than 0.10%, W: less than 0.10%, B: less than 0.0100%, Cr: less than 1.00%, Mo: less than 1.00%, Ni: less than 1.00%, Co: less than 0.010%, Cu: less than 1.00%, Sn: less than 0.200%, Sb: less than 0.200%, Ca: less than 0.0100%, Mg: less than 0.0100%, REM: less than 0.0100%, Zr: less than 0.100%, Te: less than 0.100%, Hf: less than 0.10%, and Bi: less than 0.200%.

4. The high-strength steel plate according to claim 2, wherein, The composition, by mass%, further contains at least one element selected from Ti: less than 0.200%, Nb: less than 0.200%, V: less than 0.200%, Ta: less than 0.10%, W: less than 0.10%, B: less than 0.0100%, Cr: less than 1.00%, Mo: less than 1.00%, Ni: less than 1.00%, Co: less than 0.010%, Cu: less than 1.00%, Sn: less than 0.200%, Sb: less than 0.200%, Ca: less than 0.0100%, Mg: less than 0.0100%, REM: less than 0.0100%, Zr: less than 0.100%, Te: less than 0.100%, Hf: less than 0.10%, and Bi: less than 0.200%.

5. The high-strength steel plate according to any one of claims 1 to 4, wherein, The base steel plate has a soft layer on its surface. The surface layer is the portion extending from the surface of the base steel plate along the thickness direction to 200 μm. The soft layer is the portion where the Vickers hardness is below 85% at a position representing 1 / 4 of the thickness of the base steel plate.

6. The high-strength steel plate according to claim 5, wherein, When the nanohardness of the soft layer at a depth of 1 / 4 in the thickness direction is measured at more than 300 points within a range of 50 μm × 50 μm, the ratio r of the number of measurement points with a nanohardness of 7.0 GPa or higher to the total number of measurement points is less than 0.

10.

7. The high-strength steel plate according to claim 5, wherein, The standard deviation σ of the nanohardness at a depth of 1 / 4 along the thickness direction of the soft layer A Below 1.8 GPa The standard deviation σ of the nanohardness at the position at 1 / 2 depth along the thickness direction of the soft layer B The pressure is below 2.2 GPa.

8. The high-strength steel plate according to any one of claims 1 to 4, wherein, The surface of the base steel plate further has a metallic coating as a pre-annealing coating.

9. The high-strength steel plate according to any one of claims 1 to 4, wherein, The base steel plate further has a coating on its surface.

10. The high-strength steel plate according to claim 8, wherein, The surface of the metal coating has a further coating.

11. A method for manufacturing a high-strength steel plate, comprising the method for manufacturing the high-strength steel plate according to any one of claims 1 to 4. Cold-rolled sheets are obtained by hot rolling, pickling, and cold rolling a steel billet having the composition described in claim 1, 3, or 4. The cold-rolled sheet is subjected to annealing and post-heating. In the annealing process, the cold-rolled sheet is heated to a temperature above 800°C, and then cooled to a cooling stop temperature below 150°C. in, In the cooling process, The residence time t1 in the temperature range T1 above Ms℃ but below 700℃ is less than 1000s. The first average cooling rate v1 in the temperature range T2, above Ms-80℃ and below Ms℃, is 1.0℃ / s to 40.0℃ / s. The second average cooling rate v2 in the temperature range T3 (above 150℃ and below Ms-80℃) is above 0.3℃ / s and less than the first average cooling rate v1. The post-heating is performed under the condition that the temperature X, which is the highest temperature reached, and the holding time Y at the temperature X-10°C or higher satisfy the following equation 1. Formula 1: 8000≤(273+X)×(20+Log 10 (Y / 3600))≤12000 Wherein, the temperature X is in °C, and the holding time Y is in seconds.

12. The method for manufacturing high-strength steel plate according to claim 11, wherein, The temperature X satisfies the following equation 2. Equation 2: 100≤X≤400.

13. The method for manufacturing high-strength steel plate according to claim 11 or 12, wherein, In the annealing process, heating at the heating temperature is carried out in an atmosphere with a dew point of -30°C or higher, thereby forming a soft layer on the surface of the cold-rolled sheet. The surface layer is the portion extending from the surface of the cold-rolled sheet along the thickness direction to 200 μm. The soft layer is the portion where the Vickers hardness at a position of 1 / 4 of the thickness of the cold-rolled sheet is below 85%.

14. The method for manufacturing high-strength steel plate according to claim 11 or 12, wherein, Before the annealing is performed, the cold-rolled sheet is subjected to a metal plating treatment to form a metal plating layer on the surface of the cold-rolled sheet as a pre-annealing plating layer.

15. The method for manufacturing high-strength steel plate according to claim 11 or 12, wherein, In the annealing process, the cold-rolled sheet is plated to form a coating.

Images