Title of invention : Steel plate and manufacturing method thereof
Technical field
[0001]
The present invention relates to a steel sheet and a method for manufacturing the same.
This application claims priority based on Japanese Patent Application No. 2019-229401 filed in Japan on December 19, 2019, the content of which is incorporated herein.
Background technology
[0002]
In recent years, from the perspective of regulating greenhouse gas emissions as a countermeasure against global warming, there is a demand for improved fuel efficiency of automobiles. Therefore, the application of high-strength steel sheets is expanding more and more in order to reduce the weight of the vehicle body and ensure collision safety. For example, Patent Document 1 below discloses a high-strength steel sheet having a tensile strength of 950 MPa or more.
[0003]
In addition, hot-dip galvanized ultra-high-strength steel sheets are required for areas that require rust resistance. For example, Patent Document 2 below discloses a hot-dip galvanized steel sheet having a tensile strength of 1300 MPa or more.
prior art documents
patent literature
[0004]
Patent document 1: Japanese republication WO2018/020660
Patent document 2: Japanese republication WO2018/011978
Non-patent literature
[0005]
Non-Patent Document 1: Yasuhiro Sekimoto, Morimichi Tanaka, Ryozo Sawada, Masayoshi Koga: Tetsu to Hagane, vol. 61 (1975), No. 10, pp. 2337-2349
SUMMARY OF THE INVENTION
Problems to be Solved by the Invention
[0006]
However, when spot welding is performed on a hot-dip galvanized steel sheet having a tensile strength of 1300 MPa or more, liquid metal embrittlement cracking (LME) may occur during spot welding. The reason for this is thought to be that molten zinc penetrates into prior austenite grain boundaries during spot welding, embrittlement of the steel, and tensile stress is applied to that portion, causing LME.
[0007]
Therefore, an object of the present invention is to provide a steel sheet having high strength, excellent LME resistance and excellent bendability, and a method for producing the same.
Means to solve problems
[0008]
The present inventors have diligently studied methods for suppressing the occurrence of LME as described above. As a result, it was thought that the generation of LME could be suppressed by segregating N at the former austenite grain boundaries and suppressing the penetration of molten zinc during spot welding.
[0009]
The gist of the present invention obtained as described above is as follows.
[0010]
[1] A steel sheet according to one aspect of the present embodiment has a chemical composition, in mass%,
C: 0.15% or more and 0.45% or less,
Si: 0.01% or more and 2.50% or less,
　Mn+Cr: 1.20% or more and 4.00% or less,
Al: 0.10% or more and 2.00% or less,
P: 0.040% or less,
S: 0.010% or less,
N: 0.0010% or more and 0.010% or less,
O: 0.006% or less,
Mo: 0% or more, 0.50% or less,
Ti: 0% or more and 0.20% or less,
Nb: 0% or more and 0.20% or less,
B: 0% or more, 0.010% or less,
V: 0% or more, 0.50% or less,
Cu: 0% or more and 1.00% or less,
W: 0% or more, 0.10% or less,
Ta: 0% or more, 0.10% or less,
Ni: 0% or more and 1.00% or less,
Sn: 0% or more, 0.050% or less,
Co: 0% or more, 0.50% or less,
Sb: 0% or more and 0.050% or less,
As: 0% or more, 0.050% or less,
Mg: 0% or more, 0.050% or less,
Ca: 0% or more and 0.040% or less,
Y: 0% or more, 0.050% or less,
Zr: 0% or more and 0.050% or less,
La: 0% or more and 0.050% or less,
Ce: 0% or more, 0.050% or less,
with the remainder consisting of Fe and impurities,
The tensile strength is 1300 MPa or more,
The ratio (R/t) of the limit bending radius to the plate thickness is less than 3.5,
When the depth position of 30 μm from the surface in the plate thickness direction is position A, and the depth position of 1/4 of the plate thickness in the plate thickness direction from the surface is position B,
At the position A, AlN exists at a number density of 3000/mm2 or more and 6000/mm2 or less;
　　The metal structure at the position B contains 90% or more of martensite by volume;
The hardness of the position A is 1.20 times or more the hardness of the position B.
[2] In the steel sheet according to [1], the chemical composition is, in mass%,
Mo: 0.01% or more and 0.50% or less,
Ti: 0.001% or more and 0.20% or less,
Nb: 0.0001% or more and 0.20% or less,
B: 0.0001% or more and 0.010% or less,
V: 0.001% or more and 0.50% or less,
Cu: 0.001% or more and 1.00% or less,
W: 0.001% or more and 0.10% or less,
Ta: 0.001% or more and 0.10% or less,
Ni: 0.001% or more and 1.00% or less,
Sn: 0.001% or more and 0.050% or less,
Co: 0.001% or more and 0.50% or less,
Sb: 0.001% or more and 0.050% or less,
As: 0.001% or more and 0.050% or less,
Mg: 0.0001% or more and 0.050% or less,
Ca: 0.001% or more and 0.040% or less,
Y: 0.001% or more and 0.050% or less,
Zr: 0.001% or more and 0.050% or less,
La: 0.001% or more and 0.050% or less
Ce: 0.001% or more and 0.050% or less,
It may contain one or more selected from the group consisting of.
[3] The steel sheet according to [1] or [2] may have a hot-dip galvanized layer on the surface.
[4] In the steel sheet according to [3], the hot-dip galvanized layer may be an alloyed hot-dip galvanized layer.
[5] A method for manufacturing a steel sheet according to another aspect of the present invention includes heating a slab having the chemical composition according to [1] or [2] to 1050° C. or higher, followed by A hot rolling step of performing rough rolling at a rolling reduction of 10% or more using rolls having a temperature of ° C. or less and then performing finish rolling;
a winding step of cooling and winding the slab after the hot rolling step to form a steel strip;
a heating step of heating the steel strip after the winding step to a temperature range of Ac3 or more and less than 900°C in an atmosphere with an N concentration of 80% or more, and holding the steel strip in the temperature range for 5 seconds or more;
a cooling step of cooling the steel strip after the heating step to a temperature of less than 550°C at an average cooling rate of 20°C/s or more;
have
[6] In the method for manufacturing a steel sheet according to [5], a hot dip galvanized layer may be formed on the surface of the steel strip by hot dip galvanizing the steel strip after the cooling step.
[7] In the steel sheet manufacturing method described in [6], a heat alloying treatment may be performed after the hot-dip galvanization is performed.
Effect of the invention
[0011]
According to the present invention, it is possible to provide a steel sheet having high strength, excellent LME resistance and excellent bendability, and a method for producing the same.
Brief description of the drawing
[0012]
[Fig. 1] Fig. 1 is a schematic diagram showing a state of a test for spot-welding two steel plates and evaluating resistance to molten metal embrittlement cracking (LME resistance).
MODE FOR CARRYING OUT THE INVENTION
[0013]
Embodiments of the present invention will be described below. In addition, the embodiment illustrated below is for facilitating understanding of the present invention, and is not for limiting and interpreting the present invention. The present invention can be modified and improved from the following embodiments without departing from its gist.
[0014]
[steel plate]
The steel sheet according to this embodiment has a chemical composition of, in mass%,
C: 0.15% or more and 0.45% or less,
Si: 0.01% or more and 2.50% or less,
　Mn+Cr: 1.20% or more and 4.00% or less,
Al: 0.10% or more and 2.00% or less,
N: 0.0010% or more and 0.010% or less,
P: 0.040% or less,
S: 0.010% or less,
O: 0.006% or less,
Mo: 0% or more, 0.50% or less,
Ti: 0% or more and 0.20% or less,
Nb: 0% or more and 0.20% or less,
B: 0% or more, 0.010% or less,
V: 0% or more, 0.50% or less,
Cu: 0% or more and 1.00% or less,
W: 0% or more, 0.10% or less,
Ta: 0% or more, 0.10% or less,
Ni: 0% or more and 1.00% or less,
Sn: 0% or more, 0.050% or less,
Co: 0% or more, 0.50% or less,
Sb: 0% or more and 0.050% or less,
As: 0% or more, 0.050% or less,
Mg: 0% or more, 0.050% or less,
Ca: 0% or more and 0.040% or less,
Y: 0% or more, 0.050% or less,
Zr: 0% or more and 0.050% or less,
La: 0% or more and 0.050% or less,
Ce: 0% or more, 0.050% or less,
with the remainder consisting of Fe and impurities,
The tensile strength is 1300 MPa or more,
The ratio (R/t) of the limit bending radius to the plate thickness is less than 3.5,
When the depth position of 30 μm from the surface in the plate thickness direction is position A, and the depth position of 1/4 of the plate thickness in the plate thickness direction from the surface is position B,
AlN is present at the position A at a number density of 3000/mm2 or more and 6000/mm2 or less;
　　The metal structure at the position B consists of 90% or more by volume of martensite and the remaining structure;
The hardness of the position A is 1.20 times or more the hardness of the position B.
The steel plate according to this embodiment will be described below.
[0015]

Next, the chemical composition of the steel sheet that is desirable for obtaining the effects of the present invention will be described. The chemical composition of the steel sheet means the chemical composition of the central portion and the surface layer of the steel sheet, and the chemical composition of the surface layer means the chemical composition of the matrix excluding the Al oxide particles in the surface layer. The chemical composition of the central portion of the steel sheet and the chemical composition of the matrix of the surface layer portion may be the same, or may be different from each other and within the range of the chemical composition of the steel sheet described below. In addition, "%" regarding the content of an element means "mass %" unless otherwise specified.
[0016]
"C: 0.15% or more and 0.45% or less"
C is an element that increases the strength of the steel sheet and is added to increase the strength of the steel sheet. A C content of 0.15% or more can sufficiently increase the strength of the steel sheet. Further, when the C content is 0.45% or less, breakage in the elastic region of the steel sheet can be suppressed. In order to effectively suppress breakage in the elastic region of the steel sheet, the C content is preferably 0.40% or less, more preferably 0.35% or less.
[0017]
"Si: 0.01% or more and 2.50% or less"
　Si is added as a solid-solution strengthening element to contribute to increasing the strength of the steel sheet. From this point of view, the lower limit of the Si content is 0.01% or more, preferably 0.02% or more. If the Si content increases, the central portion of the steel sheet becomes embrittled and the formability of the steel sheet deteriorates. Therefore, the Si content is 2.50% or less, preferably 2.20% or less.
[0018]
"Mn + Cr: 1.20% or more and 4.00% or less"
　Mn and Cr are elements added to improve the hardenability and strength of the steel sheet. In order to obtain such effects, the total content of Mn and Cr should be 1.20% or more. The total content of Mn and Cr is preferably 1.50% or more, preferably 2.00% or more. If the total content of Mn and Cr is too large, the hardness distribution in the surface layer of the steel sheet becomes too large due to the segregation of Mn and Cr. , preferably 3.50% or less, more preferably 3.00% or less.
[0019]
"Al: 0.10% or more and 2.00% or less"
In the steel sheet according to the present embodiment, AlN is segregated at the prior austenite grain boundaries in order to increase the solid solution N concentration at the prior austenite grain boundaries. Therefore, the Al content of the steel sheet is important.
The Al content is set to 0.10% or more in order to favorably segregate AlN at the former austenite grain boundaries. When the Al content is less than 0.10%, the segregation amount of AlN at the prior austenite grain boundary is insufficient, resulting in Since the concentration of solid solution N becomes insufficient, it is not possible to suitably prevent the inflow of molten zinc during spot welding (no suitable LME resistance can be obtained). The Al content is preferably 0.20% or more, more preferably 0.30% or more.
On the other hand, if the Al content exceeds 2.00%, it is not preferable because it increases the risk of slab cracking during continuous casting. Therefore, the Al content is set to 2.00% or less, preferably 1.7% or less, and more preferably 1.4% or less.
[0020]
"P: 0.040% or less"
P tends to segregate in the central part of the steel plate and may embrittle the weld zone. By setting the P content to 0.040% or less, embrittlement of the weld zone can be suppressed. Since it is preferable not to contain P, the lower limit of the P content is 0%, but it is economically disadvantageous to make the P content less than 0.001%. The lower limit may be set at 0.001%.
[0021]
"S: 0.010% or less"
S is an element that may adversely affect the weldability of steel sheets and the manufacturability during casting and hot rolling. For this reason, the S content is set to 0.010% or less. Since it is preferable not to contain S, the lower limit of the S content is 0%, but it is economically disadvantageous to make the S content less than 0.001%, so may be defined as 0.001%.
[0022]
"N: 0.0010% or more and 0.010% or less"
N is an element that can suppress the inflow of molten zinc during spot welding by allowing it to exist in the prior austenite grain boundary in a solid solution state. Therefore, the N content in this embodiment is 0.0010% or more, preferably 0.0030% or more, and more preferably 0.0040% or more. On the other hand, if the N content is excessive, the slab may crack during continuous casting. Therefore, the N content in the present embodiment is 0.010% or less, preferably 0.010% or less, and more preferably 0.0070% or less.
[0023]
"O: 0.006% or less"
O is an element that forms coarse oxides, impairs bendability and hole expandability, and causes blowholes during welding. If the O content exceeds 0.006%, the deterioration of the hole expansibility and the occurrence of blowholes become remarkable. Therefore, O is made 0.006% or less. Since it is preferable not to contain O, the lower limit of the O content is 0%.
[0024]
The rest of the chemical composition of the steel sheet is Fe and impurities. Impurities in this embodiment are components that do not affect the effects. However, the following elements may be contained instead of part of Fe. Since the following elements are not essential elements for obtaining the effect of this embodiment, the lower limit of the content is 0%.
[0025]
"Mo: 0% or more and 0.50% or less, B: 0% or more and 0.010% or less"
Mo and B are elements that improve the hardenability and contribute to the improvement of the strength of the steel sheet. Although the effect of these elements can be obtained even if they are added in small amounts, it is preferable to set the Mo content to 0.01% or more and the B content to 0.0001% or more in order to sufficiently obtain the effect. On the other hand, from the viewpoint of suppressing the deterioration of the pickling property, weldability, hot workability, etc. of the steel sheet, the upper limit of the Mo content is 0.50% or less, and the upper limit of the B content is 0.010% or less. preferably.
[0026]
"Ti: 0% or more and 0.20% or less, Nb: 0% or more and 0.20% or less, V: 0% or more and 0.50% or less"
　Ti, Nb and V are elements that contribute to improving the strength of the steel sheet. These elements contribute to increasing the strength of the steel sheet by strengthening precipitates, strengthening fine grains by suppressing the growth of ferrite grains, and strengthening dislocations by suppressing recrystallization. Although the effect of these elements can be obtained even by adding a small amount, in order to sufficiently obtain the effect, it is preferable to add 0.001% or more of Ti, 0.0001% or more of Nb, and 0.001% or more of V. . However, from the viewpoint of suppressing the deterioration of the formability of the steel sheet due to an increase in the precipitation of carbonitrides, the Ti and Nb contents are 0.20% or less, and the V content is 0.50% or less. Preferably.
[0027]
"Cu: 0% or more and 1.00% or less, Ni: 0% or more and 1.00% or less"
Cu and Ni are elements that contribute to improving the strength of the steel sheet. Although the effect of these elements can be obtained even by adding a small amount, the contents of Cu and Ni are preferably 0.001% or more in order to sufficiently obtain the effect. On the other hand, from the viewpoint of suppressing deterioration of pickling properties, weldability, hot workability, etc. of the steel sheet, the contents of Cu and Ni are preferably 1.00% or less.
[0028]
Furthermore, the following elements may be intentionally or unavoidably added to the steel plate center and surface layer portions in place of part of Fe within the range in which the effects of the present invention can be obtained. That is, W: 0% or more and 0.10% or less or 0.001% or more and 0.10% or less, Ta: 0% or more and 0.10% or less or 0.001% or more and 0.10% or less, Sn: 0% or more and 0.050% or less or 0.001% or more and 0.050% or less Sb: 0% or more and 0.050% or less or 0.001% or more and 0.050% or less As: 0% or more and 0.050% or less or 0.001% or more and 0.050% or less Mg: 0% or more and 0.050% or less or 0.0001% or more and 0.050% or less Ca: 0% 0.040% or less or 0.001% or more and 0.040% or less, Zr: 0% or more and 0.050% or less or 0.001% or more and 0.050% or less, Co: 0% or more and 0 .50% or less or 0.001% or more and 0.050% or less, and Y: 0% or more and 0.050% or less or 0.001% or more and 0.050% or less, La: 0% or more and 0.050% or less. 050% or less or 0.001% or more and 0.050% or less, and Ce: 0% or more and 0.050% or less or 0.001% or more and 0.050% or less REM (rare earth metal: ) may be added to one or both of the center portion and the surface layer portion of the steel sheet.
[0029]

Next, the metal structure of the steel sheet according to this embodiment will be described. The percentage of metallographic structure is expressed in terms of volume fraction. When the area ratio is measured by image processing, the area ratio is regarded as the volume ratio. In the following description of the procedure for measuring the volume ratio, the terms "volume ratio" and "area ratio" may be used together.
In the steel sheet according to the present embodiment, the metal structure at the position (position B) ¼ of the thickness from the surface of the steel sheet contains martensite with a volume fraction of 90% or more.
[0030]
(Martensite)
Martensite is a hard structure with a high dislocation density, so it contributes to the improvement of tensile strength. From the viewpoint of achieving a tensile strength of 1300 MPa or more, the volume fraction of martensite at the position of 1/4 of the sheet thickness is set to 90% or more, preferably 95% or more. Moreover, the upper limit of the volume fraction of martensite is not particularly limited, and may be set to 100%.
[0031]
(Remaining organization)
The residual structure other than martensite is not particularly limited, and examples include ferrite, retained austenite, pearlite, and bainite.
[0032]
Next, the method for measuring the volume fraction of martensite will be explained.
[0033]
The volume fraction of martensite is obtained by the following procedure. Etch the observation surface of the sample with a repeller liquid, and FE-SEM an area of ​​100 μm × 100 μm within the range of 1/8 to 3/8 of the plate thickness centered on 1/4 of the plate thickness at a magnification of 3000 times. to observe. In repellent corrosion, martensite and retained austenite are not corroded, so the area ratio of the uncorroded region is the total area ratio of martensite and retained austenite. The volume ratio of martensite is calculated by subtracting the volume ratio of retained austenite measured by X-rays from the area ratio of the uncorroded region.
The volume fraction of retained austenite can be calculated by measurement using an X-ray diffractometer. In the measurement using an X-ray diffractometer, first, a region from the plate surface (rolled surface) of the sample to the surface at a depth of 1/4 of the plate thickness is removed by mechanical polishing and chemical polishing. Next, on a surface at a depth of 1/4 of the plate thickness t, using MoKα rays as characteristic X-rays, (200), (211) of the bcc phase and (200), (220), (220), (200), (220) of the fcc phase 311), and the volume fraction of retained austenite can be calculated based on these integrated intensity ratios.
[0034]
In addition, martensite can be distinguished from other tissues in an electron channeling contrast image obtained by a scanning electron microscope. In the above image, regions with a high dislocation density and substructures such as blocks and packets within crystal grains are martensite.
[0035]

In this embodiment, solid solution N is segregated in the former austenite grain boundaries in the surface layer of the steel sheet, thereby suppressing the penetration of molten zinc into the former austenite grain boundaries during spot welding and suppressing the occurrence of LME. Since N has a high affinity with Al, solid solution N can be effectively segregated at the prior austenite grain boundaries by precipitating a certain amount of AlN at the prior austenite grain boundaries.
In the steel plate according to the present embodiment, the number density of AlN at position A is 3000 pieces/mm 2 or more and 6000 pieces/mm 2 or less. By setting the number density of AlN in the steel sheet surface layer represented by position A to 3000 pieces/mm 2 or more, solid solution N can be sufficiently segregated at the prior austenite grain boundaries. The solute N segregated at the former austenite grain boundaries suppresses the penetration of molten zinc into the former austenite grain boundaries during spot welding. Preferably, the AlN number density at position A is 3500/mm 2 or more. If a large number of AlN are present inside the steel sheet, the toughness is lowered. Therefore, it is preferable that the number density of AlN is 2000/mm 2 or less at the center of the steel sheet in the thickness direction. In addition, in the steel sheet according to the present embodiment, since the Al concentration is high, weak deoxidation products such as SiMn composite oxides are not generated. Also, since dissolved oxygen is low, secondary deoxidation products are also reduced, resulting in less oxides than normal. On the other hand, by setting the number density of AlN at the position A to 6000/mm 2 or less, it is possible to prevent AlN from becoming a starting point of fracture, fracture at a low strain, and the desired strength not being obtained. Preferably, the AlN number density at position A is 5000/mm 2 or less.
[0036]
Next, a method for measuring the number density of AlN at position A will be described.
[0037]
First, the steel plate is cut perpendicular to the surface along the rolling direction. Next, from the surface of the steel sheet at a depth position A of 30 μm, a sample that can be observed in an area of ​​10 μm×10 μm is taken by FIB processing to prepare a thin film sample with a thickness of 100 nm or more and 300 nm or less. After that, the sample at the depth position A is subjected to a field emission transmission electron microscope and EDS (energy dispersive X-ray analysis) therein, and the elemental mapping of Al and N of the thin film sample is made in the range of 10 μm × 10 μm. Create 20 fields of view at 9000x magnification. In places where AlN is precipitated, the number of detected Al and N is significantly higher than in places where it is not precipitated. By counting and dividing this number by the observation area, the number density of AlN at position A can be obtained.
Here, the surface of the steel sheet at the position A is the depth position from the plating and steel plate interface in the case of a plated steel plate, the position from the steel plate surface in the case of a cold-rolled steel plate, and the position from the steel plate surface in the case of a hot-rolled steel plate. refers to the depth position from the interface between the steel plate and the scale.
Moreover, the sampling position of the position A shall be the center position of the width direction of a steel plate.
[0038]

The steel plate according to the present embodiment has a tensile strength (TS) of 1300 MPa or more as a strength that contributes to weight reduction of automobile bodies.
　In addition, the tensile strength is JIS Z 220 in the direction perpendicular to the rolling direction from the steel plate 1: 1998, a JIS No. 5 tensile test piece is taken, and a tensile test is performed according to JIS Z 2241: 2011 to measure.
[0039]

In the steel plate according to this embodiment, the hardness at position A is 1.20 times or more the hardness at position B. That is, the steel plate according to the present embodiment has a structure in which the surface layer portion is harder than the inner portion. This is because, as will be described later, a large amount of solid solution N is present in the surface layer portion due to annealing in an N2 atmosphere.
[0040]
Next, the method of measuring hardness at positions A and B will be described.
Hardness is measured in accordance with the Vickers hardness test JISZ2244:2009. The load is set so that the indentation becomes several μm, and the area of ​​400 μm×400 μm is measured at a pitch of 0.2 μm. Then, the average hardness at the position A and the average hardness at the position B are calculated.
Position B is in the range of 1/8 to 3/8 of the plate thickness centered at 1/4 of the plate thickness from the surface layer of the steel plate, and the center in the width direction.
[0041]

In the steel sheet according to the present embodiment, the ratio (R/t) of limit bending radius to plate thickness is less than 3.5 as bendability that contributes to formability of automobile parts. The limit bending radius R is measured by performing a bending test according to JIS Z 2248:2006.
[0042]

The plate thickness of the steel plate according to this embodiment is not particularly limited, but can be 0.5 mm to 4.0 mm.
[0043]
The steel sheet of the present embodiment may have a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, or an electro-galvanized layer on the surface of the steel sheet. Even when the plating layer is formed in this way, the steel sheet of the present embodiment exhibits desired properties.
[0044]
[Manufacturing method of steel plate]
Next, an example of a manufacturing method for obtaining the steel plate of this embodiment will be described.
In the method for manufacturing a steel sheet according to the present embodiment, after heating a slab having the above-described chemical composition to 1050 ° C. or higher, a roll having a diameter of 100 mm or more and a temperature of 300 ° C. or lower is used at a rolling reduction of 10% or more. A hot rolling step in which finish rolling is performed after rough rolling;
a winding step of cooling and winding the slab after the hot rolling step to form a steel strip;
a heating step of heating the steel strip after the winding step to a temperature range of Ac3 or more and less than 900°C in an atmosphere with an N concentration of 80% or more, and holding the steel strip in the temperature range for 5 seconds or more;
a cooling step of cooling the steel strip after the heating step to a temperature of less than 550°C at an average cooling rate of 20°C/s or more;
have
[0045]
(Hot rolling process)
In the hot rolling process, a slab having the above chemical composition is heated to 1050°C or higher, and a roll having a diameter (hereinafter sometimes simply referred to as "diameter") of 100 mm or more and a temperature of 300°C or lower is rolled. After performing rough rolling at a rolling reduction of 10% or more using the steel sheet, finish rolling is performed.
[0046]
　Slab heating temperature: 1050°C or higher
In the steel sheet manufacturing method according to the present embodiment, the slab heating temperature in the hot rolling process is set to 1050°C or higher. By setting the slab heating temperature to 1050° C. or higher, the AlN present in the slab can be sufficiently solutionized, and sufficient AlN can be present at the prior austenite grain boundaries in the final product. The slab heating temperature is preferably 1100° C. or higher. Although the upper limit of the heating temperature is not particularly specified, it is generally 1300° C. or less.
[0047]
Roll diameter in rough rolling: 100 mm or more, roll temperature: 300°C or less, rolling reduction: 10% or more
In the steel sheet manufacturing method according to the present embodiment, in each pass, the roll diameter in rough rolling is set to 100 mm or more, the roll temperature is set to 300°C or less, and the rolling reduction is set to 10% or more.
By setting the roll diameter to 100 mm or more, the steel sheet can be suitably cooled by heat removal by the rolls during rough rolling, and AlN can be sufficiently precipitated. Although the upper limit of the roll diameter is not particularly defined, it may be set to 500 mm or less from the viewpoint of equipment costs.
By setting the roll temperature to 300°C or less, the steel sheet can be suitably cooled by removing heat from the rolls during rough rolling, and AlN can be sufficiently precipitated. The roll temperature is the surface temperature of the roll, and the center of the width of the roll at the position where the rotation angle is 90 degrees when the roll is rotated around the axis of the roll from the contact surface between the roll and the steel plate on the delivery side of the roll. The surface temperature of the part is measured using a radiation thermometer, and the surface temperature of the roll is calculated from the temperature at the measurement position to be the average during the contact between the roll and the steel plate. Non-Patent Document 1 was referred to for the calculation method. The surface temperature of a normal roll is 400° C. or higher. Therefore, while measuring the roll temperature, the roll temperature is controlled to be 300° C. or lower by, for example, adjusting the amount of water sprayed from the roll entrance side to the roll. Although the lower limit of the roll temperature is not particularly defined, it may be set to 100° C. or higher from the viewpoint of productivity.
By setting the rolling reduction in rough rolling to 10% or more, sufficient strain can be applied and the number of AlN precipitation sites can be increased. The draft is preferably 15% or more. Although the upper limit of the rolling reduction is not particularly defined, it may be set to 50% or less from the viewpoint of manufacturability.
[0048]
In the steel sheet manufacturing method according to the present embodiment, the conditions for finish rolling are not particularly defined, and may be performed according to a conventional method.
[0049]
(Winding process)
In the steel sheet manufacturing method according to the present embodiment, after the hot rolling process, the slab is cooled and coiled to form a steel strip. Conditions for the winding process are not particularly defined, and may be carried out according to a conventional method.
[0050]
(cold rolling process)
After winding, further cold rolling may be performed as necessary. Although the cumulative rolling reduction in cold rolling is not particularly limited, it is preferably 30 to 70% from the viewpoint of the shape stability of the steel sheet.
[0051]
(heating process)
Next, in the heating process, the steel strip after the winding process is heated to the austenite single phase region. In the heating process, the AlN precipitated in the hot rolling process functions as pinning particles against austenite grain growth and exists at the austenite grain boundaries. Therefore, AlN present at the austenite grain boundaries is arranged at the prior austenite grain boundaries in the steel sheet after cooling. Further, solid solution N penetrates into the surface layer of the steel sheet by heating in an N2 atmosphere. Since the affinity between solute N and Al present in AlN is high, solute N segregates at prior austenite grain boundaries.
In the steel sheet manufacturing method according to the present embodiment, the steel strip after the winding process is heated to a temperature range of Ac3 or more and less than 900 ° C. in an atmosphere with an N concentration of 80% or more, and Hold for at least a second (heating step).
By setting the N 2 concentration in the atmosphere of the heating process to 80% or more, the number density of AlN in the surface layer and the solid solution N concentration existing in the prior austenite grain boundaries become sufficient values. The N2 concentration in the heating step is preferably 85% or more. Although the upper limit of the N 2 concentration in the atmosphere of the heating process is not particularly set, it may be set to 95% or less from the viewpoint of manufacturing cost.
By setting the dew point to -30°C or lower in the heating process, it is possible to suppress the growth of internal oxides on the surface layer of the steel sheet. If the dew point exceeds −30° C., the high Al steel sheet (containing 0.10% or more Al) having a high Ac3 point tends to coarsen oxides during the heating process, resulting in a decrease in bendability. The dew point in the heating step is preferably -40°C or lower. Although the lower limit of the dew point in the heating step is not specified, it may be −50° C. or higher from the viewpoint of production cost.
[0052]
A desired metal structure (90% or more of martensite) can be obtained by setting the heating temperature in the heating process to Ac3 or higher.
When the heating temperature in the heating process is 900°C or higher, it is not preferable because the manufacturing cost increases. Therefore, the heating temperature in the heating step is set to less than 900°C.
[0053]
A desired metal structure can be obtained by setting the holding time in the temperature range of Ac3 or more and less than 900°C to 5 seconds or more in the heating step. The holding time in the temperature range is preferably 10 seconds or longer.
Although the upper limit of the holding time in this temperature range is not specified, it may be set to 500 seconds or less from the viewpoint of productivity. In the temperature maintenance, the temperature of the steel strip need not be constant.
[0054]
(cooling process)
Next, the steel strip after the heating process is cooled to a temperature of less than 550°C at an average cooling rate of 20°C/s or more (cooling step). In the cooling process, it is built into a metal structure of 90% or more martensite.
A sufficient amount of martensite structure can be obtained by setting the average cooling rate to a temperature of less than 550°C at 20°C/s or more.
[0055]
In this embodiment, by controlling the N2 atmosphere during annealing, solid solution N is introduced into the surface layer of the steel sheet, and the subsequent cooling conditions are controlled to generate a hard structure. This is different from the nitriding treatment in which N is introduced into the surface layer of the steel sheet after controlling the metallographic structure. If the nitriding treatment is performed after the formation of the hard structure, martensite is excessively tempered, and sufficient tensile strength cannot be ensured.
[0056]
The surface of the steel sheet after the cooling process may be hot-dip galvanized. As a result, a hot-dip galvanized steel sheet having a hot-dip galvanized layer formed on the surface of the steel sheet is obtained. When hot-dip galvanizing is performed, the temperature of the hot-dip galvanizing bath in which the steel sheet is immersed may be the condition that has been conventionally applied. That is, the temperature of the hot-dip galvanizing bath is, for example, 440° C. or higher and 550° C. or lower.
[0057]
Also, after the hot-dip galvanization is applied as described above, a heat alloying treatment may be applied. As a result, a galvannealed steel sheet having a galvannealed layer formed on the surface of the steel sheet is obtained. The heating temperature for alloying in the heat alloying treatment may be the conditions that have been conventionally applied. That is, the heating temperature for alloying is, for example, 400° C. or higher and 600° C. or lower. The heating method for alloying is not particularly limited, and a heating method suitable for conventional hot-dip plating equipment, such as direct heating by combustion gas, induction heating, or direct electric heating, can be used. After the alloying treatment, the steel sheet is cooled to 200° C. or less and subjected to temper rolling if necessary.
[0058]
In addition, the following examples are given as methods for manufacturing electrogalvanized steel sheets. For example, alkali degreasing, washing with water, pickling, and washing with water are performed as pretreatments for plating on the above steel sheet. After that, the pretreated steel sheet is coated with, for example, a liquid circulation type electroplating apparatus, a plating bath containing zinc sulfate, sodium sulfate, and sulfuric acid is used, and a current density of about 100 A/dm 2 is used to obtain a predetermined plating thickness. Electrolytically treat until
Example
[0059]
The present invention will be described more specifically with reference to examples.
[0060]

A slab having the chemical composition shown in Tables 1-1 and 1-2 was cast. The remainder of the chemical compositions shown in Tables 1-1 and 1-2 are iron and impurities. The slab after casting was subjected to a hot rolling process under the conditions shown in Table 2-1. Some of the hot-rolled steel sheets were cold-rolled at the cold-rolling rates shown in Table 2-1 after the hot-rolling process. In the cold rolling process column in Table 2-1, "-" indicates that the cold rolling process was not performed. The values ​​shown in the table represent the minimum roll diameter, maximum roll temperature, and minimum rolling reduction. Next, the slab after the hot rolling process was cooled and wound up to form a steel strip (winding process). The steel strip after the winding process was subjected to a heating process and a cooling process under the conditions shown in Table 2-2.
For some examples, hot-dip galvanizing and alloying were performed after the cooling process.
[0061]
[Table 1-1]

[0062]
[Table 1-2]

[0063]
[Table 2-1]

[0064]
[Table 2-2]

[0065]

A JIS No. 5 tensile test piece described in JIS Z 2201: 1998 was taken from the steel plate in the direction perpendicular to the rolling direction, and subjected to JIS Z 2241: 2011.Tensile strength was measured by performing a tensile test. Table 3 shows the results.
[0066]

Using the obtained steel plate as a sample, the observed surface of the sample was etched with a repeller solution. A region of 100 μm × 100 μm within the range of 1/8 to 3/8 of the plate thickness centered on 1/4 of the plate thickness is observed at a magnification of 3000 times using FE-SEM, and the area of ​​the uncorroded region asked for a rate. This area ratio is the total area ratio of martensite and retained austenite, and this area ratio is regarded as the volume ratio and designated as A.
In addition, the volume fraction of retained austenite was obtained as follows using an X-ray diffraction device. First, the region from the plate surface (rolled surface) of the sample to the surface at a depth of 1/4 of the plate thickness was removed by mechanical polishing and chemical polishing. Next, using MoKα rays as characteristic X-rays, (200), (211) of the bcc phase and (200), (220), (311) of the fcc phase ) was obtained for the integrated intensity ratio of the diffraction peak. The volume ratio of retained austenite was calculated based on these integrated intensity ratios, and this volume ratio was defined as B.
The difference between the volume fractions A and B obtained by the above two methods, that is, (AB) was taken as the volume fraction of martensite.
Table 3 shows the volume fraction of martensite obtained in this way.
[0067]

　Samples were taken from the central part of the plate width. Cut perpendicular to the surface of the steel sheet along the rolling direction, then from a depth of 30 μm from the surface of the steel sheet, a sample that can be observed in an area of ​​10 μm × 10 μm by FIB processing. A thin film sample having a thickness of 100 nm or more and 300 nm or less was prepared.
After that, the sample at the depth position A is subjected to elemental mapping of Al and N of the thin film sample in the range of 10 μm × 10 μm using a field emission transmission electron microscope and EDS (energy dispersive X-ray analysis) therein. Created at 9000x magnification. In places where AlN is precipitated, the number of detected Al and N is significantly higher than in places where it is not precipitated. The number density of AlN at the position A was obtained by counting and dividing the number by the observed area.
The results are shown in Table 3.
[0068]

The hardness was measured according to the Vickers hardness test JISZ2244:2009. The load was set so that the indentation was several μm, and the area of ​​400 μm×400 μm was measured at a pitch of 0.2 μm. Then, the average hardness at position A (surface layer hardness) and the average hardness at position B (center hardness) were calculated.
The results are shown in Table 3.
[0069]

　Example No. A test piece of 50 mm × 80 mm was taken from the central part of the plate width of the steel plates of 1 to 34, 36 and 38 to 46. Moreover, Example Nos. After producing steel sheets of 1 to 34, 36 and 38 to 46, they were immersed in a hot dip galvanizing bath to produce hot dip galvanized steel sheets, and test pieces of 50 mm×80 mm were taken. A cold-rolled steel sheet or hot-rolled steel sheet having the same example number and a test piece taken from a hot-dip galvanized steel sheet are overlapped, and the following spot welding is performed to evaluate the resistance to molten metal embrittlement cracking (LME resistance). did
Figure 1 shows the state of the test. A hot-dip galvanized steel sheet was used as the steel sheet 1d in FIG. 1, and a steel sheet 1e was used as the steel sheet to be evaluated. Welding conditions are as follows.
Using a servo motor pressurized single-phase AC spot welder (power supply frequency 50 Hz), while pressurizing at a pressure of 450 kgf (4413 kg m / s 2), the current value is 6.5 kA, the inclination angle θ of the electrode (line 5 and the line 6) was 3°, no upslope, energization time was 0.4 seconds, and the holding time after energization was 0.1 seconds, and the plated steel sheets were welded. After that, the nugget center region of the steel plate was observed using an optical microscope to evaluate the presence or absence of LME cracks.
[0070]

A bending test piece of 50 mm × 100 mm was taken from the steel plate, and a bending test was performed in accordance with JIS Z 2248:2006 to evaluate the bendability by "minimum bending R/plate thickness t where cracks do not occur". This time, steel sheets with R/t of less than 3.5 were accepted. Table 3 shows the results.
[0071]
[Table 3]

[0072]
As shown in Tables 1-1 to 3, the desired properties were obtained in the examples that satisfied the requirements of the present invention. On the other hand, the desired properties were not obtained in the comparative examples that did not satisfy at least one of the requirements of the present invention. Specifically, it was as follows.
[0073]
　No. In No. 31, since the amount of C was small, the tensile strength was 1070 MPa and did not reach 1300 MPa.
　No. No. 32 broke in the elastic region in the tensile test because the C content was excessive.
　No. In No. 33, since the total amount of Mn and Cr was small, the martensite fraction was low and the tensile strength was 1230 MPa, which did not reach 1300 MPa.
　No. In No. 34, since the amount of Al was small, the number density of AlN was as low as 1900 pieces / mm 2, and the surface layer hardness (position A) / center hardness (position B) was as small as 1.05, so LME cracks occurred. occured.
　No. In No. 35, since the amount of Al was excessive, the embrittlement due to Al was remarkable and the slab cracked, and the subsequent tests were stopped.
　No. In No. 36, since the amount of N was small, the number density of AlN was as small as 1800 pieces / mm 2, and the surface layer hardness (position A) / center hardness (position B) was as small as 1.04, so LME cracks occurred. occured.
　No. In No. 37, since the amount of N was excessive, embrittlement due to AlN was remarkable, and the slab cracked, and subsequent tests were stopped.
[0074]
　No. In No. 38, the slab heating temperature was insufficient, the number density of AlN was as low as 2900/mm 2 , and the surface layer hardness (position A)/center hardness (position B) was as small as 1.18. , LME cracking occurred.
　No. In No. 39, the roll diameter used in the hot rolling process is less than 100 mm, the number density of AlN is as low as 2200 pieces / mm 2, and the surface layer hardness (position A) / center hardness (position B) is 1 Since it was as small as 0.11, LME cracking occurred.
　No. 40, the roll temperature in the hot rolling process is over 300 ° C., the number density of AlN is as low as 2200 pieces / mm 2, and the surface layer hardness (position A) / center hardness (position B) is 1 Since it was as small as 0.15, LME cracking occurred.
　No. In No. 41, the reduction ratio in the hot rolling process was small, and the number density of AlN was as low as 2100 pieces/mm 2 , so LME cracks occurred.
　No. In No. 42, the heating temperature in the heating step was less than Ac3, so the martensite fraction was low and the tensile strength was 1190 MPa, which did not reach 1300 MPa.
　No. In No. 43, since the N 2 concentration in the atmosphere in the heating process was low, the surface layer hardness/center hardness was as small as 1.16, and LME cracking occurred.
　No. No. 44 had a short holding time in the temperature range of Ac3 or more and less than 900° C. in the heating process, so the martensite fraction was low and the tensile strength was 1250 MPa, which did not reach 1300 MPa.
　No. In No. 45, the cooling rate in the cooling process was low, so the martensite fraction was low and the tensile strength was 1270 MPa, which did not reach 1300 MPa.
　No. 46 had a high dew point, so R/t was not less than 3.5.
The scope of the claims
[Claim 1]
The chemical composition is mass %,
C: 0.15% or more and 0.45% or less,
Si: 0.01% or more and 2.50% or less,
　Mn+Cr: 1.20% or more and 4.00% or less,
Al: 0.10% or more and 2.00% or less,
P: 0.040% or less,
S: 0.010% or less,
N: 0.0010% or more and 0.010% or less,
O: 0.006% or less,
Mo: 0% or more, 0.50% or less,
Ti: 0% or more and 0.20% or less,
Nb: 0% or more and 0.20% or less,
B: 0% or more, 0.010% or less,
V: 0% or more, 0.50% or less,
Cu: 0% or more and 1.00% or less,
W: 0% or more, 0.10% or less,
Ta: 0% or more, 0.10% or less,
Ni: 0% or more and 1.00% or less,
Sn: 0% or more, 0.050% or less,
Co: 0% or more, 0.50% or less,
Sb: 0% or more and 0.050% or less,
As: 0% or more, 0.050% or less,
Mg: 0% or more, 0.050% or less,
Ca: 0% or more and 0.040% or less,
Y: 0% or more, 0.050% or less,
Zr: 0% or more and 0.050% or less,
La: 0% or more and 0.050% or less,
Ce: 0% or more, 0.050% or less,
with the remainder consisting of Fe and impurities,
The tensile strength is 1300 MPa or more,
The ratio (R/t) of the limit bending radius to the plate thickness is less than 3.5,
When the depth position of 30 μm from the surface in the plate thickness direction is position A, and the depth position of 1/4 of the plate thickness in the plate thickness direction from the surface is position B,
AlN is present at the position A at a number density of 3000/mm2 or more and 6000/mm2 or less;
　　The metal structure at the position B contains 90% or more of martensite by volume;
　The hardness at the position A is 1.20 times or more the hardness at the position B;
A steel plate characterized by:
[Claim 2]
The chemical composition, in % by mass,
Mo: 0.01% or more and 0.50% or less,
Ti: 0.001% or more and 0.20% or less,
Nb: 0.0001% or more and 0.20% or less,
B: 0.0001% or more and 0.010% or less,
V: 0.001% or more and 0.50% or less,
Cu: 0.001% or more and 1.00% or less,
W: 0.001% or more and 0.10% or less,
Ta: 0.001% or more and 0.10% or less,
Ni: 0.001% or more and 1.00% or less,
Sn: 0.001% or more and 0.050% or less,
Co: 0.001% or more and 0.50% or less,
Sb: 0.001% or more and 0.050% or less,
As: 0.001% or more and 0.050% or less,
Mg: 0.0001% or more and 0.050% or less,
Ca: 0.001% or more and 0.040% or less,
Y: 0.001% or more and 0.050% or less,
Zr: 0.001% or more and 0.050% or less,
La: 0.001% or more and 0.050% or less
Ce: 0.001% or more and 0.050% or less,
The steel sheet according to claim 1, comprising one or more selected from the group consisting of:
[Claim 3]
The steel sheet according to claim 1 or 2, characterized by having a hot-dip galvanized layer on the surface.
[Claim 4]
The steel sheet according to claim 3, wherein the hot-dip galvanized layer is an alloyed hot-dip galvanized layer.
[Claim 5]
After heating the slab having the chemical composition according to claim 1 or 2 to 1050°C or higher, it was subjected to rough rolling at a rolling reduction of 10% or higher using rolls having a diameter of 100 mm or higher and a temperature of 300°C or lower. After that, a hot rolling step of performing finish rolling;
a winding step of cooling and winding the slab after the hot rolling step to form a steel strip;
a heating step of heating the steel strip after the winding step to a temperature range of Ac3 or more and less than 900°C in an atmosphere with an N concentration of 80% or more, and holding the steel strip in the temperature range for 5 seconds or more;
a cooling step of cooling the steel strip after the heating step to a temperature of less than 550°C at an average cooling rate of 20°C/s or more;
A method for producing a steel plate having
[Claim 6]
A hot-dip galvanized layer is formed on the surface of the steel strip by hot-dip galvanizing the steel strip after the cooling process
The method for manufacturing a steel plate according to claim 5, characterized in that:
[Claim 7]
After applying the hot-dip galvanizing, heat alloying treatment is applied
The method for manufacturing a steel plate according to claim 6, characterized by: