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. 2020-003677, which was filed in Japan on January 14, 2020, and the contents of which are incorporated herein.
Background technology
[0002]
　In recent years, from the perspective of improving fuel efficiency, which leads to environmental conservation, efforts have been made to reduce the weight of the car body using high-strength steel sheets. In general, it is difficult to apply forming methods such as drawing and stretch forming, which are applied to mild steel plates, to the processing of steel plates with extremely high strength, and bending forming is the main forming method.
[0003]
For example, Patent Document 1 below discloses a technique for forming parts by bending a high-strength steel plate.
prior art documents
patent literature
[0004]
Patent Document 1: International Publication No. 2017/195795
SUMMARY OF THE INVENTION
Problems to be Solved by the Invention
[0005]
However, when forming a part by bending a high-strength steel plate, the rate of strain on the outer side of the bent steel plate is high, so the thickness reduction rate in that region is large, and the member rigidity is suitable. was difficult to obtain.
[0006]
Therefore, an object of the present invention is to provide a steel sheet having a tensile strength of 980 MPa or more and having suitable member rigidity even when bending is performed, and a method for manufacturing the same.
Means to solve problems
[0007]
The present inventors have diligently studied how to prevent thickness reduction on the outer side of a bent steel plate. As a result, by making the steel plate such that the yield strength (YP) is high at the portion where the strain rate is high, even if the steel plate has a tensile strength of 980 MPa or more, it is a suitable member in the region outside the bent steel plate. I thought it would give me strength.
[0008]
The gist of the present invention obtained as described above is as follows.
[0009]
[1] A steel sheet according to an aspect of the present invention has a chemical composition, in mass%,
C: 0.050% or more and 0.500% 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% or more, 0.100% or less,
S: 0% or more, 0.050% or less,
N: 0% or more, 0.010% or less,
O: 0% or more and 0.006% or less,
Mo: 0% or more, 1.000% or less,
Ti: 0% or more and 0.200% or less,
Nb: 0% or more and 0.200% or less,
B: 0% or more, 0.010% or less,
V: 0% or more, 0.200% or less,
Cu: 0% or more and 1.000% or less,
W: 0% or more, 0.100% or less,
Ta: 0% or more, 0.100% or less,
Ni: 0% or more and 1.000% or less,
Sn: 0% or more, 0.050% or less,
Co: 0% or more, 0.500% 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 0.050% 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 balance consisting of iron and impurities,
　The metal structure at the position of 1/4 of the plate thickness from the surface is, in terms of volume ratio,
　　Ferrite and epitaxial ferrite: 10% or more and less than 50%,
　　Ratio of epitaxial ferrite to the total volume ratio of ferrite and epitaxial ferrite: 5% or more and 30% or less,
　　Martensite: 20% or more, 70% or less, and
Bainite: 50% or less,
　　Retained austenite: 15% or less,
　　Remaining organization: 5% or less,
and the total volume fraction of the bainite, the retained austenite and the residual structure is 50% or less,
In a cross section along the rolling direction and perpendicular to the surface at a position 1/4 of the plate thickness from the surface, the length A of the interface between the epitaxial ferrite and the ferrite and the epitaxial ferrite and the A/B, which is the ratio of the interface length B with martensite, is greater than 1.5,
The ratio of the major axis to the minor axis of the martensite is 5.0 or more, and
　The tensile strength is 980 MPa or more.
[0010]
[2] In the steel sheet according to [1], the chemical composition is, in mass%,
Mo: 0.010 to 1.000%,
B: 0.0001 to 0.010%,
Ti: 0.010 to 0.200%,
Nb: 0.010 to 0.200%,
　V: 0.010 to 0.200%
Cu: 0.001 to 1.000%,
Ni: 0.001-1.000%
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 of the steel sheet.
[4] The steel sheet according to [1] or [2] may have a galvannealed layer on the surface of the steel sheet.
[5] The steel sheet according to [1] or [2] may have an electrogalvanized layer on the surface of the steel sheet.
[0011]
[6] A method for manufacturing a steel sheet according to another aspect of the present invention comprises:
in % by mass,
C: 0.050% or more and 0.500% 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% or more, 0.100% or less,
S: 0% or more, 0.050% or less,
N: 0% or more, 0.010% or less,
O: 0% or more and 0.006% or less,
Mo: 0% or more, 1.000% or less,
Ti: 0% or more and 0.200% or less,
Nb: 0% or more and 0.200% or less,
B: 0% or more, 0.010% or less,
V: 0% or more, 0.200% or less,
Cu: 0% or more and 1.000% or less,
W: 0% or more, 0.100% or less,
Ta: 0% or more, 0.100% or less,
Ni: 0% or more and 1.000% or less,
Sn: 0% or more, 0.050% or less,
Co: 0% or more, 0.500% 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 0.050% 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
hot-rolling a slab having a chemical composition with the balance being iron and impurities to obtain a hot-rolled steel sheet having a prior austenite grain size of less than 30 μm;
a cooling step of cooling the hot-rolled steel sheet to 500°C or less at an average cooling rate of 20°C/sec or more;
a winding step of winding the hot-rolled steel sheet after the cooling step at 500°C or less;
a cold-rolling step of pickling the hot-rolled steel sheet after the winding step and cold-rolling it at a rolling reduction of 30% or less to obtain a cold-rolled steel sheet;
An annealing step of heating the cold-rolled steel sheet to a first temperature range of (Ac3 point -100) ° C to 900 ° C and soaking in the first temperature range for 5 seconds or more;
an annealing cooling step of cooling the cold-rolled steel sheet after the annealing step at an average cooling rate of 2.5°C/sec to 50°C/sec in a second temperature range of 750°C to 550°C;
have
[0012]
[7] In the method for manufacturing a steel sheet according to [6], the hot rolling step includes a finish rolling step in which the slab is continuously passed through a plurality of rolling stands for rolling,
The finish rolling process is:
the rolling start temperature in the third rolling stand from the final rolling stand is 800°C to 1000°C;
Rolling at a rolling reduction of more than 10% in each of the three rolling stands in the latter stage of the finish rolling process;
The time between passes between each rolling stand in the three rolling stands in the latter stage of the finishing rolling process is within 3.0 seconds;
It is the difference between the temperature Tn on the delivery side of the n-th rolling stand in the three-stage rolling stand of the latter stage in the finish rolling process and the temperature Tn+1 on the entry side of the rolling stand of the (n+1) stage. Some (T n - T n+1) may be greater than 10°C.
[8] In the steel sheet manufacturing method described in [6] or [7], hot-dip galvanization may be formed by immersing the cold-rolled steel sheet after the annealing and cooling step in a hot-dip galvanizing bath.
[9] In the steel sheet manufacturing method described in [8], the hot-dip galvanizing may be alloyed in a temperature range of 300°C to 600°C.
Effect of the invention
[0013]
According to the present invention, it is possible to provide a steel sheet having a tensile strength of 980 MPa or more and having suitable member rigidity even when bending is performed, and a method for manufacturing the same.
Brief description of the drawing
[0014]
1 is a schematic diagram showing an example of a length A of an interface between epitaxial ferrite and ferrite and a length B of an interface between epitaxial ferrite and martensite; FIG.
MODE FOR CARRYING OUT THE INVENTION
[0015]
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.
[0016]
[steel plate]
The steel sheet according to this embodiment has a chemical composition of, in mass%,
C: 0.050% or more and 0.500% 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% or more, 0.100% or less,
S: 0% or more, 0.050% or less,
N: 0% or more, 0.010% or less,
O: 0% or more and 0.006% or less,
Mo: 0% or more, 1.000% or less,
Ti: 0% or more and 0.200% or less,
Nb: 0% or more and 0.200% or less,
B: 0% or more, 0.010% or less,
V: 0% or more, 0.200% or less,
Cu: 0% or more and 1.000% or less,
W: 0% or more, 0.100% or less,
Ta: 0% or more, 0.100% or less,
Ni: 0% or more and 1.000% or less,
Sn: 0% or more, 0.050% or less,
Co: 0% or more, 0.500% 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 0.050% 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, and
The balance consists of Fe and impurities,
The volume ratio of the metal structure in 1/4 part of the plate thickness is
　Ferrite and epitaxial ferrite: 10% or more and less than 50%,
The proportion of epitaxial ferrite in the sum of the structural proportions of ferrite and epitaxial ferrite: 5% or more and 30% or less,
Martensite: 20% or more, 70% or less,
Bainite: 50% or less,
Retained austenite: 15% or less, and
Remaining organization: 5% or less,
and the total volume fraction of bainite, retained austenite and residual structure is 50% or less,
The ratio of the length A of the interface between the epitaxial ferrite and the ferrite to the length B of the interface between the epitaxial ferrite and the martensite, that is, A/B is more than 1.5 can be,
The ratio of the major axis to the minor axis of martensite is 5.0 or more, and
　The tensile strength is 980 MPa or more.
The steel plate according to this embodiment will be described below.
[0017]

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, the element "%" regarding the content means "% by mass" unless otherwise specified.
[0018]
"C: 0.050% or more and 0.500% 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 0.050% or more can sufficiently increase the strength of the steel sheet. It is preferably 0.100% or more, more preferably 0.150% or more. On the other hand, if the C content is more than 0.500%, the martensite becomes very hard, so it is likely to crack even in the elastically deformable region, and the steel sheet breaks from the cracks that occur, failing to obtain the desired strength. From that point of view, the C content is 0.500% or less, preferably 0.400% or less.
[0019]
"Si: 0.01% or more and 2.50% or less"
　Si is an element that stabilizes ferrite. That is, since Si increases the Ac3 transformation point, it is possible to form a large amount of ferrite in a wide annealing temperature range, and is added from the viewpoint of improving the structure controllability of the steel sheet. In order to obtain such effects, the steel sheet according to the present embodiment has a Si content of 0.01% or more. In addition, Si is an element necessary for suppressing coarsening of iron-based carbides in the center of the steel sheet and increasing the strength and formability of the steel sheet. Moreover, Si is added as a solid-solution strengthening element in order to contribute to increasing the strength of the steel sheet. From these points of view, the lower limit of the Si content is preferably 0.10% or more, more preferably 0.30% or more. If the Si content increases, the steel sheet becomes brittle and the formability of the steel sheet deteriorates, so the Si content is set to 2.50% or less, preferably 1.80% or less.
[0020]
"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 steel sheet according to the present embodiment has a Mn+Cr content of 1.20% or more. If the amounts of Mn and Cr are too large, the hardenability is excessively increased, and sufficient epitaxial ferrite cannot be obtained. is preferred. Moreover, in order to ensure sufficient hardenability, the content of Mn is preferably 1.20% or more. Cr may not be contained, and its lower limit is 0%.
[0021]
"Al: 0.10% or more and 2.00% or less"
Al is an element that promotes the formation of epitaxial ferrite. Therefore, the Al content is 0.10% or more. The Al content is preferably 0.30% or more, more preferably 0.40% or more. On the other hand, by setting the Al content to 2.00% or less, slab cracking during continuous casting can be suppressed.
[0022]
"P: 0% or more, 0.100% 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 0.100% 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%. However, from an economical point of view, the lower limit of the P content may be 0.001%.
[0023]
"S: 0% or more, 0.050% or less"
S is an element that may adversely affect the weldability of the steel sheet and the manufacturability during casting and hot rolling. For this reason, the S content is set to 0.050% or less. Since it is preferable not to contain S, the lower limit of the S content is 0%. However, from an economical point of view, the lower limit of the S content may be 0.001%.
[0024]
"N: 0% or more, 0.010% or less"
　N forms coarse nitrides and may deteriorate the bendability of the steel sheet, so it is necessary to reduce the amount of addition. By setting the N content to 0.010% or less, the deterioration of the bendability of the steel sheet can be suppressed. In addition, since N may cause blowholes during welding, the N content is preferably as low as possible, ideally at 0%. However, from an economical point of view, the lower limit of the N content may be 0.0005%.
[0025]
"O: 0% or more, 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%.
[0026]
The rest of the chemical composition of the steel sheet is Fe and impurities. However, the following elements may be contained in place of part of Fe. However, the lower limit is set to 0 because it does not have to be contained.
Mo: 0% or more, 1.000% or less,
Ti: 0% or more and 0.200% or less,
Nb: 0% or more and 0.200% or less,
B: 0% or more, 0.010% or less,
V: 0% or more, 0.200% or less,
Cu: 0% or more and 1.000% or less,
W: 0% or more, 0.100% or less,
Ta: 0% or more, 0.100% or less,
Ni: 0% or more and 1.000% or less,
Sn: 0% or more, 0.050% or less,
Co: 0% or more, 0.500% 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 0.050% 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 and 0.050% or less.
[0027]
"Mo: 0% or more and 1.000% 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 0.010% 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 deterioration of the pickling property, weldability, hot workability, etc. of the steel sheet, the upper limit of the Mo content is 1.000% or less, and the upper limit of the B content is 0.010% or less. preferably.
[0028]
"Ti: 0% or more and 0.200% or less, Nb: 0% or more and 0.200% or less, V: 0% or more and 0.200% 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, it is preferable to add Ti, Nb, and V in an amount of 0.010% or more in order to sufficiently obtain the effect. However, from the viewpoint of suppressing the deterioration of the formability of the steel sheet due to an increase in precipitation of carbonitrides, the contents of Ti, Nb, and V are preferably 0.200% or less.
[0029]
"Cu: 0% or more and 1.000% or less, Ni: 0% or more and 1.000% 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 each preferably 1.000% or less.
[0030]
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 0.100% or less, Ta: 0% or more and 0.100% or less, Sn: 0% or more and 0.050% or less, Co: 0% or more and 0.500% or less, Sb: 0% or more and 0.050% or less As: 0% or more and 0.050% or less Mg: 0% or more and 0.050% or less Ca: 0% or more and 0.050% or less Zr: REM such as 0% or more and 0.050% or less, Y: 0% or more and 0.050% or less, La: 0% or more and 0.050% or less, and Ce: 0% or more and 0.050% or less (Rare-Earth Metal) may be added.
[0031]

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 this embodiment, the metal structure at the position of 1/4 of the thickness from the surface of the steel sheet has a volume ratio of
　Ferrite and epitaxial ferrite: 10% or more and less than 50%,
The ratio of epitaxial ferrite to the total volume ratio of ferrite and epitaxial ferrite: 5% or more and 30% or less,
Martensite: 20% or more, 70% or less,
Bainite: 50% or less,
Retained austenite: 15% or less, and
Remaining organization: 5% or less,
including.
However, the total volume fraction of bainite, retained austenite and residual structure is 50% or less.
[0032]
(ferrite)
Ferrite is a soft phase obtained by two-phase annealing in which the steel is heated and held at a temperature of Ac1 point or more and Ac3 point or less, or by slow cooling after annealing. In addition, in the steel sheet according to the present embodiment, ferrite and epitaxial ferrite are included in the metal structure at least at the position of 1/4 of the thickness from the surface of the steel sheet.
The lower limit of the volume fraction is not particularly limited as long as ferrite is included in the metal structure at a position 1/4 of the plate thickness from the surface of the steel plate. However, in order to suitably improve the ductility of the steel sheet, it is preferable that the ferrite content is 10% or more.
In addition, although the upper limit of the ferrite volume ratio is not particularly limited, it is necessary to limit the ferrite volume ratio in order to achieve a strength of 980 MPa or more, and the ferrite volume ratio is preferably less than 50%.
[0033]
(Epitaxial ferrite: the total volume ratio with ferrite is 10% or more and less than 50%, and the ratio of epitaxial ferrite to the total volume ratio with ferrite is 5% or more and 30% or less)
Epitaxial ferrite is obtained by growing the ferrite obtained during the two-phase region annealing during the subsequent slow cooling. The present inventors have found that a steel sheet containing a large amount of Al, which is a ferrite-stabilizing element, has a sufficiently high growth rate of ferrite during the slow cooling described above, so that desired epitaxial ferrite can be obtained. In the structure according to the present embodiment, ferrite that grows from the interface between ferrite and austenite to the austenite side during slow cooling after dual-phase annealing is referred to as epitaxial ferrite. That is, epitaxial ferrite is formed between martensite and ferrite. The dislocation density of epitaxial ferrite is lower than that of martensite and higher than that of ferrite. Therefore, epitaxial ferrite is more deformable than martensite, but has a higher yield strength (YP) than ferrite. Thus, by arranging a structure having a yield strength intermediate between a hard structure such as martensite and a soft structure such as ferrite, the yield strength in a high strain rate region can be increased. . However, as explained below, the dislocation density in the epitaxial ferrite is low if the epitaxial ferrite is not formed with a suitable thickness. Dislocations in epitaxial ferrite are introduced to alleviate plastic deformation due to martensite transformation, but if the epitaxial ferrite does not have an appropriate thickness, the dislocations move beyond the epitaxial ferrite into the ferrite. , the dislocation density in the epitaxial ferrite decreases. If the dislocation density of the epitaxial ferrite is low, the yield strength is insufficient and the above effect cannot be obtained. As in this embodiment, the hard structure inside the steel sheet coiled after hot rolling is made into an acicular structure, and the austenite generated by the subsequent annealing is also made into an acicular structure. Therefore, martensite generated by cooling after annealing can also have an acicular structure. As a result, the width (thickness) of epitaxial ferrite present around martensite can be controlled within an appropriate range. As a result, the dislocation density in the epitaxial ferrite is preferably controlled without decreasing, and the yield strength in the region where the strain rate is high when working the steel sheet can be increased. A method for distinguishing between epitaxial ferrite and ferrite will be described later, but since the degree of progress of corrosion in etching differs depending on the difference in dislocation density, they can be clearly distinguished from each other by micrographs.
[0034]
The sum of the volume fractions of ferrite and epitaxial ferrite at a position 1/4 of the plate thickness from the surface of the steel plate is 10% or more and less than 50%. Also, the ratio of epitaxial ferrite to the total volume fraction of ferrite and epitaxial ferrite is 5% or more and 30% or less. By controlling the volume fraction of epitaxial ferrite as described above, a high yield strength can be exhibited even in a region where the strain rate is high.
[0035]
(Martensite: 20% or more, 70% or less)
Martensite is a hard structure with a high dislocation density, so it is a structure that contributes to the improvement of tensile strength. The volume fraction of martensite is set to 20% or more and 70% or less in consideration of the balance between strength and workability. Martensite in this embodiment includes fresh martensite and tempered martensite. From the viewpoint of achieving a tensile strength of 980 MPa or more, the volume fraction of martensite is preferably 30% or more. Moreover, the volume fraction of martensite is preferably 55% or less from the viewpoint of ensuring suitable bendability.
[0036]
(Bainite: 50% or less)
Although bainite has a high dislocation density and is a hard structure, it has a lower dislocation density and is softer than martensite, so it has the effect of improving ductility. Therefore, up to 50% bainite may be contained in order to obtain desired properties. On the other hand, the proportion of bainite may be 0% because it is not an essential metal structure for obtaining the effects of this embodiment.
[0037]
(Residual austenite: 15% or less)
Retained austenite improves ductility through the TRIP effect and contributes to uniform elongation. Therefore, it may contain retained austenite up to 15%. On the other hand, the percentage of retained austenite may be 0% because it is not an essential metal structure for obtaining the effects of the present embodiment.
[0038]
(Remaining organization: 5% or less)
Perlite and the like are examples of residual tissue. These structures are made 5% or less in order to reduce workability.
[0039]
The total volume fraction of bainite, retained austenite and residual structure shall be 50% or less. By setting the volume ratio to 50% or less, the effect of the present embodiment can be ensured.
[0040]
Next, a method for distinguishing between ferrite, epitaxial ferrite, and martensite and a method for calculating the composition ratio will be described. Organizations that do not fall under this category are referred to as remaining organizations. In calculating the structure ratio, the area ratio obtained from the structure photograph is regarded as the volume ratio.
[0041]
The identification of each metal structure and the calculation of the volume ratio are performed by EBSD (Electron Back Scattering Diffraction), X-ray measurement, corrosion using a nital reagent or Repeller solution, and a scanning electron microscope. The observation area is located at the center of the width of the steel sheet, and is a 100 μm×100 μm area of ​​a cross section along the rolling direction of the steel sheet and perpendicular to the surface of the steel sheet. Observation is performed at a magnification of 3000 times. In the plate thickness direction, the microstructures (constituent elements) in the vicinity of the steel plate surface and in the vicinity of the steel plate center may differ greatly from those of other portions. Therefore, in the present embodiment, the microstructure is observed on the basis of the 1/4 plate thickness position.
[0042]
The outline of the measurement procedure is as follows.
First, the X-ray diffraction intensity of the polished sample is measured to determine the volume fraction of retained austenite. Subsequently, after etching using a nital reagent, the secondary electron image obtained by FE-SEM was observed, (i) pearlite, (ii) epitaxial ferrite and ferrite, (iii) martensite, bainite and The area ratio of pearlite is determined by distinguishing between three types of retained austenite. (ii) Epitaxial ferrite and ferrite are distinguished from each other by the brightness of the image under controlled observation conditions, and the area ratio of each is obtained.
After that, etching is performed using a Repeller reagent, and a secondary electron image obtained by FE-SEM is observed. In this observation, bainite is distinguished from martensite and retained austenite, and the area ratio of bainite is determined. Finally, the area ratio of martensite is obtained by subtracting the volume ratio of retained austenite measured using X-rays from the area ratio of martensite and retained austenite.
[0043]
The specific steps are explained below.
The volume fraction of retained austenite can be calculated by measuring diffraction intensity using X-rays.
[0044]
In the measurement using X-rays, mechanical and chemical polishing are used to remove the sample from the plate surface to the position of 1/4 of the depth. From the integrated intensity ratio of the diffraction peaks of (200), (211) of the bcc phase, (200), (220), and (311) of the fcc phase using MoKα rays at the position of 1/4 plate thickness , it is possible to calculate the structure fraction of retained austenite. A 5-peak method is used as a general calculation method.
[0045]
The identification of perlite is performed according to the following procedure. The observation surface of the sample is corroded with a nital reagent, and a 100 μm × 100 μm area in the range of 1/8 to 3/8 of the plate thickness centered on the plate thickness of 1/4 is scanned with an FE-SEM at a magnification of 3000 times. Observe. A region in which ferrite and cementite are arranged in a lamellar shape is discriminated as pearlite from the position of cementite contained in the tissue and the arrangement of the cementite. The area ratio is determined by performing image analysis using image analysis software ImageJ.
[0046]
　Ferrite and epitaxial ferrite are identified by the following procedure. The observation surface of the sample is corroded with a mixed solution of 3% nitric acid and ethanol as a nital reagent, and a 100 μm × 100 μm area is etched within a range of 1/8 to 3/8 of the plate thickness centering on 1/4 of the plate thickness. , FE-SEM at a magnification of 3000 times. Portions with uniform contrast (portions that do not contain substructures such as blocks or packets, cementite, or retained austenite in grains and appear in a single uniform contrast) are ferrite and epitaxial ferrite. The area ratio calculated by image analysis using image analysis software Image J is regarded as the area ratio of ferrite and epitaxial ferrite.
[0047]
　In order to distinguish between ferrite and epitaxial ferrite, the observation conditions are set to an acceleration voltage of 15 kV and a WD of 10 mm. Among the observed images analyzed using the image analysis software Image J, the structure with a brightness peak at 85% or more of the whole is martensite, the structure with a peak at 60% or more and less than 85% is ferrite, and 45% or more. A structure with a peak below 60% is epitaxial ferrite. Thereby, ferrite and epitaxial ferrite can be distinguished, and the ratio of epitaxial ferrite in ferrite and epitaxial ferrite can be calculated.
[0048]
The identification of bainite is performed according to the following procedure. The observation surface of the sample is etched with a repeller liquid, and an area of ​​100 μm × 100 μm within the range of 1/8 to 3/8 of the plate thickness centered on 1/4 is scanned with an FE-SEM at a magnification of 3000 times. Observe at magnification. Bainite can be identified from the position of cementite contained within the structure and the arrangement of cementite. Specifically, cementite having a plurality of variants is discriminated as bainite, and its area ratio is determined using image analysis software Image J.
[0049]
The identification of martensite is performed according to the following procedure. The observation surface of the sample is etched with a repeller liquid, and an area of ​​100 μm × 100 μm within the range of 1/8 to 3/8 of the plate thickness centered on 1/4 is scanned with an FE-SEM at a magnification of 3000 times. Observe at magnification. In repellent corrosion, martensite and retained austenite are difficult to corrode, and the area ratio of these structures is the total area ratio of martensite and retained austenite. The area ratio of this uncorroded region can be obtained using image analysis software Image J, and the area ratio of martensite can be calculated by subtracting the volume ratio of retained austenite measured by X-rays.
[0050]

In the steel sheet according to this embodiment, the ratio of the major axis to the minor axis of martensite is 5.0 or more. This indicates that the martensite has a large aspect ratio and is a so-called acicular structure. If the ratio is less than 5.0, it means that the needle-like structure of martensite in the present embodiment has collapsed. By setting the ratio to 5.0 or more, the width (thickness) of epitaxial ferrite formed between ferrite and martensite is reduced, and the dislocation density in the epitaxial ferrite can be increased. As a result, epitaxial ferrite having a suitable yield strength can be formed between ferrite and martensite, and high yield strength can be obtained even in a region where the strain rate is high when working the steel sheet. The above ratio is preferably 6.0 or more, more preferably 7.0 or more.
[0051]
Next, the method for measuring the ratio of the long diameter to the short diameter of martensite will be explained.
First, the area of ​​each martensite identified by the above image processing is measured. Next, the length of each martensite is measured. Here, the major axis is the maximum length of a line segment connecting two points on the circumference of martensite. Subsequently, for each martensite, a value obtained by dividing the area by the major axis is calculated as the minor axis. Finally, for each martensite, the ratio of the major axis to the minor axis is calculated, and the average value is obtained.
[0052]

In the steel sheet according to the present embodiment, in the cross section perpendicular to the surface of the steel sheet along the rolling direction, the ratio of the interface length A between epitaxial ferrite and ferrite to the interface length B between epitaxial ferrite and martensite A/B is greater than 1.5. If the ratio is more than 1.5, sufficient epitaxial ferrite will be present at the interface between martensite and ferrite, and the yield strength will preferably increase in a region where the strain rate is high due to bending or the like. As a result, it is possible to suppress the reduction in plate thickness at the relevant portion, so that suitable member rigidity can be obtained.
A/B is preferably 1.7 or more, more preferably 1.8 or more.
Although the upper limit of A/B is not particularly limited, it may be set to 3.0 in consideration of the ratio of epitaxial ferrite and the ratio of the major axis to the minor axis of martensite.
[0053]
Next, the method for measuring A/B mentioned above will be explained.
FIG. 1 is a schematic diagram showing an example of the length A of the interface between epitaxial ferrite and ferrite and the length B of the interface between epitaxial ferrite and martensite. Each tissue region can be identified with image processing software. The ratio of the interface length A between epitaxial ferrite and ferrite to the interface length B between epitaxial ferrite and martensite in the entire visual field obtained by image processing software is A/B.
[0054]

A test piece for SEM observation was taken from the obtained annealed steel sheet, and after polishing a cross section parallel to the rolling direction and the plate thickness direction, each structure was identified by the following method and the volume ratio was measured. The volume ratio of each tissue is shown in Tables 3-1 to 3-3.
[0081]
The volume fraction of retained austenite was calculated by measuring the diffraction intensity using X-rays.
[0082]
In the measurement using X-rays, mechanical polishing and chemical polishing were used to remove from the plate surface of the sample to the depth of 1/4. From the integrated intensity ratio of the diffraction peaks of (200), (211) of the bcc phase, (200), (220), and (311) of the fcc phase using MoKα rays at the position of 1/4 plate thickness , the structure fraction of retained austenite was calculated. A 5-peak method was used as a general calculation method.
[0083]
The identification of perlite was performed according to the following procedure. The observation surface of the sample is corroded with a nital reagent, and a 100 μm × 100 μm area in the range of 1/8 to 3/8 of the plate thickness centered on the plate thickness of 1/4 is scanned with an FE-SEM at a magnification of 3000 times. Observed. A region in which ferrite and cementite are arranged in a lamellar shape was identified as pearlite from the position of cementite contained in the tissue and the arrangement of the cementite. The area ratio was determined by performing image analysis using image analysis software ImageJ.
[0084]
　Ferrite and epitaxial ferrite were identified by the following procedure. The observation surface of the sample is corroded with a mixed solution of 3% nitric acid and ethanol as a nital reagent, and a 100 μm × 100 μm area is etched within a range of 1/8 to 3/8 of the plate thickness centering on 1/4 of the plate thickness. , using FE-SEM at a magnification of 3000 times. Portions with uniform contrast (portions that do not contain substructures such as blocks or packets, cementite, or retained austenite within crystal grains and appear in a single uniform contrast) were determined to be ferrite and epitaxial ferrite. The area ratio calculated by image analysis using image analysis software Image J was regarded as the area ratio of ferrite and epitaxial ferrite.
[0085]
In order to distinguish between ferrite and epitaxial ferrite, the observation conditions were set to an acceleration voltage of 15 kV and a WD of 10 mm. Among the observed images analyzed using the image analysis software Image J, the structure with a brightness peak at 85% or more of the whole is martensite, the structure with a peak at 60% or more and less than 85% is ferrite, and 45% or more. A structure having a peak at less than 60% was judged to be epitaxial ferrite. As a result, ferrite and epitaxial ferrite could be distinguished, and the ratio of epitaxial ferrite in ferrite and epitaxial ferrite was calculated.
[0086]
The identification of bainite was performed according to the following procedure. The observation surface of the sample is etched with a repeller liquid, and an area of ​​100 μm × 100 μm within the range of 1/8 to 3/8 of the plate thickness centered on 1/4 is scanned with an FE-SEM at a magnification of 3000 times. Observed under magnification. Bainite was determined from the position of cementite contained in the structure and the arrangement of cementite. Specifically, cementite having multiple variants was discriminated as bainite, and its area ratio was determined using image analysis software Image J.
[0087]
The identification of martensite was performed according to the following procedure. The observation surface of the sample is etched with a repeller liquid, and an area of ​​100 μm × 100 μm within the range of 1/8 to 3/8 of the plate thickness centered on 1/4 is scanned with an FE-SEM at a magnification of 3000 times. Observed under magnification. Martensite and retained austenite are difficult to corrode in repellent corrosion, and the area ratio of these structures was taken as the total area ratio of martensite and retained austenite. The area ratio of this uncorroded region was determined using image analysis software Image J, and the area ratio of martensite was calculated by subtracting the volume ratio of retained austenite measured by X-rays.
[0088]

A/B, which is the ratio of the interface length A between epitaxial ferrite and ferrite to the interface length B between epitaxial ferrite and martensite, was measured by the following method.
As described above, the region of each structure is identified by image processing software, and the ratio of the length A of the interface between epitaxial ferrite and ferrite and the length B of the interface between epitaxial ferrite and martensite obtained by the image processing software was set as A/B.
The results are shown in Table 3.
[0089]

[0095]
As shown in Tables 1-1 to 3-3, the desired characteristics were obtained in the examples that satisfied the requirements of this embodiment. On the other hand, the desired characteristics were not obtained in the comparative examples that did not satisfy at least one of the requirements of the present embodiment. Specifically, it is as follows.
[0096]
　No. In No. 44, the desired metal structure could not be obtained and the desired strength could not be obtained due to the small amount of C.
　No. No. 45 broke in the elastic region in the tensile test due to the large amount of C.
　No. In No. 46, the desired metal structure was not obtained and the desired strength was not obtained because the total amount of Mn and Cr was small.
　No. In No. 47, since the total amount of Mn and Cr was large, epitaxial ferrite was not sufficiently formed, and the desired increase in yield strength was not obtained.
　No. In No. 48, since the amount of Al was small, epitaxial ferrite was not sufficiently formed, and the desired increase in yield strength was not obtained.
　No. In No. 49, since the amount of Al was large, embrittlement due to Al was remarkable and the slab cracked, so the subsequent tests were stopped.
[0097]
　No. In No. 50, the rolling start temperature at the third rolling stand from the final rolling stand was low, and the rolling load was increased, so rolling was not possible, so the subsequent tests were stopped.
　No. In 50', since the rolling start temperature in the third rolling stand from the final rolling stand was high, the aspect ratio of martensite was not 5.0 or more, and suitable epitaxial ferrite around martensite could not be obtained. , the desired increase in yield strength was not obtained.
　No. In No. 51, since the rolling reduction at the third rolling stand after the finish rolling was low, acicular martensite and suitable epitaxial ferrite around it could not be obtained, and the desired increase in yield strength could not be obtained. rice field.
　No. In No. 52, needle-shaped martensite and suitable epitaxial ferrite around it were not obtained because the rolling reduction at the second stage of the finishing rolling and the first stage of the latter (that is, the final rolling stand) was low. , the desired increase in yield strength was not obtained.
　No. In No. 53, since the rolling reduction in the first stage after the finish rolling (that is, the final rolling stand) was low, acicular martensite and suitable epitaxial ferrite around it could not be obtained, and the desired increase in yield strength was reduced. I didn't get it.
　No. In No. 54, the maximum value of the interpass time between rolling stands was more than 3.0 seconds, so acicular martensite and suitable epitaxial ferrite around it could not be obtained, and the desired increase in yield strength was obtained. I couldn't.
　No. 55 is the difference between the temperature Tn on the delivery side of the n-stage rolling stand and the temperature Tn+1 on the entry side of the (n+1)-stage rolling stand in the third stage of the finishing rolling. Since the maximum value of a certain (T n−T n+1) was 10° C. or less, acicular martensite and suitable epitaxial ferrite around it could not be obtained, and the desired amount of increase in yield strength could not be obtained.
[0098]
　No. In No. 56, the average cooling rate in the cooling process was less than 20° C./sec, so acicular martensite and suitable epitaxial ferrite around it could not be obtained, and the desired increase in yield strength could not be obtained.
　No. In No. 57, the cooling stop temperature in the cooling step was higher than 500°C, so acicular martensite and suitable epitaxial ferrite around it could not be obtained, and the desired increase in yield strength could not be obtained.
　No. In No. 58, since the coiling temperature was over 500° C., acicular martensite and suitable epitaxial ferrite around it could not be obtained, and the desired increase in yield strength could not be obtained.
　No. In No. 59, since the rolling reduction in the cold rolling process was more than 30%, the acicular structure formed in the hot-rolled steel sheet could not be maintained, and the desired increase in yield strength could not be obtained.
　No. In 60, the soaking temperature in the annealing process was less than (Ac3 point - 100) ° C., so the desired metal structure was not obtained, the tensile strength was insufficient, and the desired increase in yield strength was not obtained. rice field.
[0099]
　No. In No. 61, the soaking temperature in the annealing process was over 900 ° C., so the desired metal structure could not be obtained, and the needle-like structure generated in the hot-rolled steel sheet could not be maintained, resulting in the desired yield strength. No increase was obtained.
　No. 62 has an average cooling rate of less than 2.5 ° C./sec in the annealing cooling process Therefore, the desired metal structure was not obtained, and the tensile strength was insufficient.
　No. In No. 63, since the average cooling rate in the annealing cooling step was more than 50° C./sec, sufficient epitaxial ferrite could not be obtained, and the desired increase in yield strength could not be obtained.
Industrial applicability
[0100]
According to the present invention, it is possible to provide a steel sheet having a tensile strength of 980 MPa or more and having suitable member rigidity even when bending is performed, and a manufacturing method thereof, which is extremely useful industrially.
The scope of the claims
[Claim 1]
The chemical composition is mass %,
C: 0.050% or more and 0.500% 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% or more, 0.100% or less,
S: 0% or more, 0.050% or less,
N: 0% or more, 0.010% or less,
O: 0% or more and 0.006% or less,
Mo: 0% or more, 1.000% or less,
Ti: 0% or more and 0.200% or less,
Nb: 0% or more and 0.200% or less,
B: 0% or more, 0.010% or less,
V: 0% or more, 0.200% or less,
Cu: 0% or more and 1.000% or less,
W: 0% or more, 0.100% or less,
Ta: 0% or more, 0.100% or less,
Ni: 0% or more and 1.000% or less,
Sn: 0% or more, 0.050% or less,
Co: 0% or more, 0.500% 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 0.050% 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 balance consisting of iron and impurities,
　The metal structure at the position of 1/4 of the plate thickness from the surface is, in terms of volume ratio,
　　Ferrite and epitaxial ferrite: 10% or more and less than 50%,
　　Ratio of epitaxial ferrite to the total volume ratio of ferrite and epitaxial ferrite: 5% or more and 30% or less,
　　Martensite: 20% or more, 70% or less,
Bainite: 50% or less,
　　Retained austenite: 15% or less, and
　　Remaining organization: 5% or less,
and the total volume fraction of the bainite, the retained austenite and the residual structure is 50% or less,
In a cross section along the rolling direction and perpendicular to the surface at a position 1/4 of the plate thickness from the surface, the length A of the interface between the epitaxial ferrite and the ferrite and the epitaxial ferrite and the A/B, which is the ratio of the interface length B with martensite, is greater than 1.5,
The ratio of the major axis to the minor axis of the martensite is 5.0 or more, and
A tensile strength of 980 MPa or more,
steel plate.
[Claim 2]
The chemical composition, in % by mass,
Mo: 0.010 to 1.000%,
B: 0.0001 to 0.010%,
Ti: 0.010 to 0.200%,
Nb: 0.010 to 0.200%,
　V: 0.010 to 0.200%,
Cu: 0.001 to 1.000%, and
Ni: 0.001-1.000%
The steel sheet according to claim 1, containing one or more selected from the group consisting of:
[Claim 3]
The steel sheet according to claim 1 or 2, having a hot-dip galvanized layer on the surface of the steel sheet.
[Claim 4]
The steel sheet according to claim 1 or 2, having an alloyed hot-dip galvanized layer on the surface of the steel sheet.
[Claim 5]
The steel sheet according to claim 1 or 2, having an electrogalvanized layer on the surface of the steel sheet.
[Claim 6]
in % by mass,
C: 0.050% or more and 0.500% 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% or more, 0.100% or less,
S: 0% or more, 0.050% or less,
N: 0% or more, 0.010% or less,
O: 0% or more and 0.006% or less,
Mo: 0% or more, 1.000% or less,
Ti: 0% or more and 0.200% or less,
Nb: 0% or more and 0.200% or less,
B: 0% or more, 0.010% or less,
V: 0% or more, 0.200% or less,
Cu: 0% or more and 1.000% or less,
W: 0% or more, 0.100% or less,
Ta: 0% or more, 0.100% or less,
Ni: 0% or more and 1.000% or less,
Sn: 0% or more, 0.050% or less,
Co: 0% or more, 0.500% 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 0.050% 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
hot-rolling a slab having a chemical composition with the balance being iron and impurities to obtain a hot-rolled steel sheet having a prior austenite grain size of less than 30 μm;
a cooling step of cooling the hot-rolled steel sheet to 500°C or less at an average cooling rate of 20°C/sec or more;
a winding step of winding the hot-rolled steel sheet after the cooling step at 500°C or less;
a cold-rolling step of pickling the hot-rolled steel sheet after the winding step and cold-rolling it at a rolling reduction of 30% or less to obtain a cold-rolled steel sheet;
An annealing step of heating the cold-rolled steel sheet to a first temperature range of (Ac3 point -100) ° C to 900 ° C and soaking in the first temperature range for 5 seconds or more;
an annealing cooling step of cooling the cold-rolled steel sheet after the annealing step in a second temperature range of 750°C to 550°C at an average cooling rate of 2.5°C/sec to 50°C/sec;
A method for manufacturing a steel plate.
[Claim 7]
The hot rolling process includes a finish rolling process in which the slab is continuously passed through a plurality of rolling stands and rolled,
The finish rolling process is:
the rolling start temperature in the third rolling stand from the final rolling stand is 800°C to 1000°C;
Rolling at a rolling reduction of more than 10% in each of the three rolling stands in the latter stage of the finish rolling process;
The time between passes between each rolling stand in the three rolling stands in the latter stage of the finishing rolling process is within 3.0 seconds;
It is the difference between the temperature Tn on the delivery side of the n-th rolling stand in the three-stage rolling stand of the latter stage in the finish rolling process and the temperature Tn+1 on the entry side of the rolling stand of the (n+1) stage. a (T n−T n+1) is greater than 10° C.
The method for manufacturing the steel sheet according to claim 6.
[Claim 8]
The steel sheet manufacturing method according to claim 6 or 7, wherein the cold-rolled steel sheet after the annealing and cooling step is immersed in a hot-dip galvanizing bath to form a hot-dip galvanized steel sheet.
[Claim 9]
The steel sheet manufacturing method according to claim 8, wherein the hot-dip galvanizing is alloyed in a temperature range of 300°C to 600°C.