JP2022086737A - Steel sheet and method for producing the same - Google Patents

Abstract

To provide a steel sheet that has high strength and also has high resistance to delayed fracture.SOLUTION: A steel sheet contains, in mass%, C: 0.20% or more and 0.90% or less, Si: 0.50% or less, Mn: 1.50% or less, P: 0.050% or less, S: 0.020% or less, Al: 0.10% or less, N: 0.010% or less, with the balance being Fe and inevitable impurities. In an area ratio to the whole steel structure, rolled perlite is 95% or more. The maximum hardness/minimum hardness, a ratio between maximum hardness and minimum hardness measured in plate thickness direction, is 1.0 or more and 2.0 or less. The tensile strength is 1500 MPa or more.SELECTED DRAWING: None

Description

The present invention relates to a steel sheet used for parts for automobiles and the like and a method for manufacturing the same.

In recent years, from the viewpoint of global environmental conservation, further improvement of fuel efficiency of automobiles has been required for the purpose of suppressing CO ₂ emissions. To improve the fuel efficiency of automobiles, it is effective to reduce the weight of automobiles by thinning the parts. Therefore, in recent years, the amount of high-strength steel sheets used for automobile parts has been increasing.

However, in parts using high-strength steel sheets, there is generally concern about deterioration of delayed fracture resistance. Here, delayed fracture means that when a component is placed in a hydrogen intrusion environment under stress, hydrogen invades the steel plate constituting the component, reducing the interatomic bonding force or locally. It is a phenomenon in which micro-cracks are generated by causing such deformation, and the micro-cracks grow to break.

Since microcracks are likely to occur from the former austenite grain boundaries, the existence of the former austenite grain boundaries is one of the factors that cause delayed fracture.

As a technique for suppressing delayed fracture of such a high-strength steel sheet, for example, Patent Document 1 states that the component composition is mass%, C: 0.3 to 1.0%, Si: 2.0% or less. Mn: 2.0% or less, P: 0.005 to 0.1%, S: 0.05% or less, Al: 0.005 to 0.1%, N: 0.01% or less, Cr: Contains one or more of 0.2% or more and 4.0% or less, Mo: 0.2% or more and 4.0% or less, Ni: 0.2% or more and 4.0% or less, and the balance. Is composed of Fe and unavoidable impurities, and the main phase structure is a layered structure in which ferrite and carbide are layered, the aspect ratio of the carbide is 10 or more, and the interval between layers is 50 nm or less. A high-strength steel plate having a tensile strength of 1500 MPa or more, which is characterized by having a volume ratio of 65% or more with respect to water, is disclosed.

Further, in Patent Document 2, the contents of C, Si and Mn are mass%, and the formula (1): "C ≧ 0.1" and the formula (2): "17.53C + 13.75Si + 6.25Mn <24" are described. The unfilled portion has a chemical composition consisting of Fe and unavoidable impurities, exhibits a cold-processed metal structure of either a ferrite + spherical cementite structure, a ferrite + pearlite structure, or a pearlite structure, and has a spring limit value of 250 MPa or more. A steel sheet for a spring having an excellent conductivity having a conductivity of 7% IACS or more is disclosed.

JP-A-2010-138488 JP-A-2004-156120

However, although the technique of Patent Document 1 has an effect of suppressing delayed fracture, the effect is not always sufficient, and even when looking at the figure presented in the examples, pearlite is sufficiently rolled. The structure and pearlite that has not been sufficiently rolled are mixed, and the current situation is that further improvement in delayed fracture resistance is required.

Further, although the technique of Patent Document 2 utilizes the pearlite structure, it seems that the delayed fracture resistance is not sufficient because the cold rolling ratio is insufficient.

The present invention has been developed in view of the above-mentioned current situation, and an object of the present invention is to provide a steel sheet having high strength and excellent delayed fracture resistance and a method for manufacturing the same.

By the way, the inventors have conducted diligent studies in order to achieve the above object, and have obtained the following findings.
(A) By strongly processing the pearlite structure in various directions, the old austenite grain boundaries are reduced, the strength is increased, and the delayed fracture resistance is excellent.
(B) By reducing the hardness ratio of each pearlite phase in the plate thickness direction in the steel sheet, the generation of microcracks can be suppressed and excellent delayed fracture resistance can be obtained.
(C) High strength can be obtained by making the lamellar spacing of the pearlite phase before strong machining fine, and in order to make the lamellar spacing fine, the holding temperature during heat treatment before strong machining is set according to the amount of C. It is preferable to control.

The present invention has been completed with further studies based on the above findings.
That is, the gist structure of the present invention is as follows.
[1] In terms of mass%, C: 0.20% or more and 0.90% or less, Si: 0.50% or less, Mn: 1.50% or less, P: 0.050% or less, S: 0.020%. Hereinafter, Al: 0.10% or less, N: 0.010% or less, and the balance has a component composition consisting of Fe and unavoidable impurities, and the area ratio with respect to the entire steel structure is 95% or more for rolled pearlite. A steel plate having a maximum hardness / minimum hardness of 1.0 or more and 2.0 or less, which is the ratio of the maximum hardness to the minimum hardness measured in the plate thickness direction, and a tensile strength of 1500 MPa or more.
[2] The component composition is further selected from Cr: 4.0% or less, Mo: 4.0% or less, V: 0.5% or less, and Ni: 0.10% or less in mass%. The steel sheet according to [1], which contains at least one of the above.
[3] Further, as the component composition, in mass%, Nb: 0.020% or less, Ti: 0.020% or less, Cu: 0.20% or less, B: 0.0020% or less, Sb: 0. The steel sheet according to [1] or [2], which contains at least one selected from 10% or less and Sn: 0.10% or less.
[4] It has the component composition according to any one of [1] to [3], has pearlite of 95% or more in terms of area ratio with respect to the entire steel structure, and has an average lamella spacing of 300 nm or less of the pearlite structure. A method for manufacturing a steel sheet, which comprises a cold rolling step of cold rolling a certain steel material under cold rolling conditions in a plurality of directions satisfying all of the following conditions 1) to 4).
1) Number n in the rolling direction: n is an integer, n ≧ 2
2) Minimum total rolling reduction in one rolling direction r: r ≧ 10%
3) Total reduction rate in all directions R: R ≧ 90%
4) When reading the angle of each rolling direction after the first cold rolling with respect to the first cold rolling direction, which is more than 0 ° and 90 ° or less, it is the largest angle in each rolling direction. Angle Xmax: 60 ° ≤ Xmax ≤ 90 °
[5] The steel material having the component composition according to any one of [1] to [3] is held at a heating temperature of 900 ° C. or higher for 1 minute or longer, and then cooled at an average cooling rate of 10 ° C./sec or higher. , (850-380 [C] ^1/2 ) ° C or higher (950-380 [C] ^1/2 ) ° C or lower ([C] is the C content contained in the steel material) for 20 minutes or longer. Manufacture of high-strength steel sheets including a heat treatment step of cooling to room temperature and a cold rolling step of performing cold rolling under cold rolling conditions in multiple directions satisfying all of the following conditions 1) to 4). Method.
1) Number n in the rolling direction: n is an integer, n ≧ 2
2) Minimum total rolling reduction in one rolling direction r: r ≧ 10%
3) Total reduction rate in all directions R: R ≧ 90%
4) When reading the angle of each rolling direction after the first cold rolling with respect to the first cold rolling direction, which is more than 0 ° and 90 ° or less, it is the largest angle in each rolling direction. Angle Xmax: 60 ° ≤ Xmax ≤ 90 °

According to the present invention, a steel sheet having high strength and excellent delayed fracture resistance can be obtained. In particular, by applying the steel plate of the present invention and a high-strength steel plate using the steel plate to parts for automobiles, it is possible to improve the performance of the automobile body through the weight reduction of the automobile body.

The present invention will be described based on the following embodiments.
The steel plate of the present invention has C: 0.20% or more and 0.90% or less, Si: 0.50% or less, Mn: 1.50% or less, P: 0.050% or less, S: 0 in mass%. .020% or less, Al: 0.10% or less, N: 0.010% or less, and the balance has a component composition consisting of Fe and unavoidable impurities, and the area ratio to the entire steel structure is 95 for rolled pearlite. % Or more, the maximum hardness / minimum hardness which is the ratio of the maximum hardness and the minimum hardness measured in the plate thickness direction is 1.0 or more and 2.0 or less, and the tensile strength is 1500 MPa or more.

First, the composition of the high-strength steel sheet of the present invention will be described. In the description of the component composition below, "%", which is a unit of the content of the component, means "mass%".

C: 0.20% or more and 0.90% or less C is necessary for forming carbides and obtaining high strength. Further, C is necessary from the viewpoint of making the lamellar spacing of pearlite finer, increasing the intensity of pearlite, and ensuring that the intensity (TS) intended in the present invention is TS ≧ 1500 MPa. If the C content is less than 0.20%, the above-mentioned predetermined strength cannot be obtained. Therefore, the C content is 0.20% or more. It is preferably 0.30% or more, and more preferably 0.40% or more. On the other hand, when the C content exceeds 0.90%, not only the cold rollability is deteriorated, but also the ratio of the maximum hardness to the minimum hardness of each pearlite is increased, so that the delayed fracture resistance is deteriorated. Therefore, the C content is set to 0.90% or less. The C content is preferably 0.88% or less, more preferably 0.86% or less.

Si: 0.50% or less Si is a strengthening element by solid solution strengthening and is added to increase the strength. However, if Si is added too much, an oxide is formed, so that the oxide becomes a starting point of delayed fracture and deteriorates the delayed fracture resistance. In addition, since Si is an element that suppresses the formation of carbides, it not only increases the lamella spacing of pearlite and lowers its strength, but also increases the ratio of the maximum hardness to the minimum hardness of each pearlite to withstand delayed fracture. Deteriorates properties. Therefore, the Si content is 0.50% or less. The Si content is preferably 0.40% or less, more preferably 0.30% or less. The lower limit of the Si content is not particularly limited, but is preferably 0.001% or more.

Mn: 1.50% or less Mn is contained in order to improve the hardenability of steel and secure a predetermined strength. However, if the amount of Mn is too large, a martensite structure is likely to be formed due to microsegregation, a predetermined area ratio of pearlite cannot be obtained, and the delayed fracture resistance is deteriorated. Therefore, the Mn content is 1.50% or less. The Mn content is preferably 1.40% or less, more preferably 1.30% or less. The lower limit of the Mn content is not particularly limited, but is preferably 0.10% or more.

P: 0.050% or less P is an element that reinforces steel, but if its content is high, microsegregation will deteriorate the delayed fracture resistance. Therefore, the P content is set to 0.050% or less. The P content is preferably 0.030% or less. The lower limit of the P content is not particularly limited, but at present, the lower limit industrially feasible is about 0.003%. Therefore, the P content is preferably 0.003% or more. The P content is more preferably 0.002% or more.

S: 0.020% or less S deteriorates the delayed fracture resistance through the formation of MnS and the like. Therefore, the S content is 0.020% or less. The S content is preferably 0.010% or less. The lower limit of the S content is not particularly limited, but at present, the lower limit that can be industrially implemented is about 0.0002%. Therefore, the S content is preferably 0.0002% or more. The S content is more preferably 0.0005% or more.

Al: 0.10% or less Al is added to sufficiently deoxidize, reduce coarse inclusions in steel, and improve delayed fracture resistance. However, when the Al content exceeds 0.10%, nitride-based precipitates such as AlN are coarsely formed, so that the delayed fracture resistance is deteriorated. Therefore, the Al content is set to 0.10% or less. It is preferably 0.08% or less.

N: 0.010% or less N is an element that produces a nitride-based precipitate such as AlN that can pin the grain boundaries, and is added to improve the delayed fracture resistance. However, when the content exceeds 0.10%, nitride-based precipitates such as AlN are coarsely formed, so that the delayed fracture resistance is deteriorated. Therefore, the N content is 0.010% or less. It is preferably 0.005% or less. The lower limit of the N content is not particularly limited, but at present, the lower limit industrially feasible is about 0.0006%. Therefore, it is preferably 0.0006% or more. More preferably, it is 0.0010% or more.

The rest other than the above are Fe and unavoidable impurities.

In addition to the above components, the steel sheet of the present invention may contain the following components as optional components. In the present invention, when the following optional components are contained below the lower limit of each component, the components are included as unavoidable impurities described later.

Cr: 4.0% or less, Mo: 4.0% or less, V: 0.5% or less, Ni: 0.10% or less, Nb: 0.020% or less, Ti: 0.020% or less, Cu: One or more selected from 0.20% or less, B: 0.0020% or less, Sb: 0.10% or less, Sn: 0.10% or less Cr, Mo, V, Ni It can form charcoal and contain it to increase the strength of the steel after cold rolling. However, if the amount of any of the elements is too large, the amount of precipitates such as carbides becomes excessive and coarsens, so that the delayed fracture resistance is deteriorated. Therefore, when Cr is contained, the Cr content is preferably 4.0% or less, more preferably 3.5% or less. When Mo is contained, the Mo content is preferably 4.0% or less, more preferably 3.5% or less. When V is contained, the V content is preferably 0.5% or less, more preferably 0.4% or less. When Ni is contained, the Ni content is preferably 0.10% or less, more preferably 0.05% or less. On the other hand, the lower limit of Cr, Mo, V and Ni is not particularly limited, but 0.005% or more is preferable for all of them for the purpose of increasing the strength of the steel.

Nb and Ti contribute to high strength through the formation of fine precipitates. However, when a large amount of Nb or Ti is contained, the amount of precipitates in the carbonitride system becomes excessive and coarsens, so that the delayed fracture resistance characteristics deteriorate. Therefore, when one or two selected from Nb and Ti are contained, the Nb content is preferably 0.020% or less, and the Ti content is preferably 0.020% or less. More preferably, the Nb content is 0.015% or less, and the Ti content is 0.015% or less. On the other hand, the lower limit of Nb and Ti is not particularly limited, but 0.002% or more is preferable for the purpose of increasing the strength of the steel.

B is an element that improves the hardenability of steel. By containing B, the effect of obtaining a predetermined strength can be obtained even when the Mn content is small. However, when the B content exceeds 0.0020%, the amount of nitride-based precipitates such as BN becomes excessive and coarsens, so that the delayed fracture resistance characteristics deteriorate. Therefore, the B content is preferably 0.0020% or less. More preferably, it is 0.0015% or less. On the other hand, the lower limit of B is not particularly limited, but 0.0002% or more is preferable for the purpose of increasing the strength of the steel.

Cu has the effect of improving the corrosion resistance in the usage environment of the automobile and suppressing the invasion of hydrogen into the steel sheet by the corrosion product covering the surface of the steel sheet. However, when the Cu content becomes excessive, the amount of precipitates such as CuS becomes excessive and becomes coarse, so that the delayed fracture resistance characteristics deteriorate. Therefore, the Cu content is preferably 0.20% or less. The Cu content is more preferably 0.15% or less. On the other hand, the lower limit of Cu is not particularly limited, but 0.005% or more is preferable for the purpose of obtaining the minimum corrosion resistance of automobiles.

Sb and Sn suppress oxidation and nitriding on the surface layer of the steel sheet, and thus suppress reduction of C and B in the steel due to oxidation and nitriding on the surface of the steel sheet. Further, Sb suppresses the formation of ferrite on the surface layer of the steel sheet and contributes to high strength. However, when the Sb and Sn contents exceed 0.10%, microsegregation causes deterioration of the delayed fracture resistance. Therefore, the Sb content is preferably 0.10% or less. The Sb content is more preferably 0.08% or less. The Sn content is preferably 0.10% or less. The Sn content is more preferably 0.08% or less. On the other hand, the lower limit of Sb and Sn is not particularly limited, but 0.005% or more is preferable for the purpose of suppressing the reduction of C and B in the steel and increasing the strength of the steel.

Next, the steel structure of the high-strength steel sheet of the present invention will be described.

The steel structure of the high-strength steel plate of the present invention has a rolled pearlite content of 95% or more. In the following description, the area ratio refers to the area ratio with respect to the entire steel structure.

Area ratio of rolled pearlite: 95% or more Rolled pearlite is necessary from the viewpoint of obtaining high strength, eliminating old austenite grain boundaries, and obtaining good delayed fracture resistance. Therefore, the area ratio of rolled pearlite is 95% or more. It is preferably 97% or more, and more preferably 99% or more. The upper limit is not particularly limited and may be 100%.

The pearlite referred to here is a structure composed of ferrite and needle-like cementite, and the rolled pearlite is a structure in which pearlite is rolled. The hardness of the rolled pearlite is 300 or more in terms of HV. Regarding the conversion method, the Vickers hardness HV and the hardness measured in the examples of the present invention were measured using pearlite samples having different hardnesses, and when the hardness of pearlite was 300 or more in terms of HV, it was rolled. Define.

Area ratio of other metal phases: less than 5% The structure of the steel sheet according to one embodiment of the present invention may contain other metal phases other than rolled pearlite. Here, if the area ratio of the other metal phase is less than 5%, it is acceptable.
Examples of other metal phases include ferrite, martensite, and bainite.

The ferrite referred to here is a structure composed of crystal grains of the BCC lattice, and is formed by transformation from austenite at a relatively high temperature. Martensite refers to a hard structure generated from austenite below the martensitic transformation point (also simply referred to as the Ms point), so-called fresh martensite as hardened and so-called fresh martensite reheated and tempered. It shall contain both tempered martensite. Bainite is a hard structure in which fine carbides are dispersed in needle-shaped or plate-shaped ferrite, and is produced from austenite at a relatively low temperature (above the martensitic transformation point).

Here, the area ratio of each phase is measured as follows.

That is, a test piece is collected from the base material region of the steel sheet so that the L cross section parallel to the rolling direction is the test surface. Then, the test surface of the test piece is mirror-polished and the structure is exposed with a nital solution. The test surface of the test piece showing the structure is observed by SEM at a magnification of 1500 times, and the area ratio other than the rolled pearlite at the plate thickness 1/4 position is measured by the point counting method.

In the SEM image, martensite exhibits a white tissue. Further, among the martensites, the tempered martensite has fine carbides precipitated inside. Ferrites have a black structure. In bainite, white carbides are precipitated in the black structure. From these points, each phase is identified in the SEM image. However, depending on the surface orientation of the block grains and the degree of etching, it may be difficult for carbides inside to appear. In that case, sufficient etching shall be performed to confirm.

The area ratio of rolled pearlite is calculated by subtracting the area ratio other than rolled pearlite from 100%.

Maximum hardness / minimum hardness, which is the ratio of maximum hardness to minimum hardness measured in the plate thickness direction, is 1.0 or more and 2.0 or less. In order to improve the delayed fracture resistance, the initial crack of delayed fracture is suppressed. There is a need. Since the initial crack of delayed fracture often occurs when the hardness difference between the phases is large, it is preferable that the hardness ratio of each rolled pearlite is small. In order to measure the hardness of each phase, the indentation size is preferably 500 nm or less in diagonal length. Further, in order to secure sufficient hardness data, the indentation interval is preferably 5.0 μm or less. However, in order to prevent the indentations from interfering with each other, the indentation interval is preferably 1.5 μm or more. In the present invention, the maximum hardness / minimum hardness, which is the ratio of the maximum hardness to the minimum hardness measured in the plate thickness direction, is 2.0 or less. It is preferably 1.8 or less, and more preferably 1.7 or less. The lower the hardness ratio is, the more preferable it is, and the lowest is 1.0, so the lower limit is 1.0.

Here, the hardness is measured as follows.

That is, a test piece for observing the metallographic structure is collected from the central portion of the plate width of the base material region of the steel plate. Then, the test piece for observing the metallographic structure was polished, and under a load of 500 μN (loading 10s, unloading 10s), a measurement area of 50 μm × 50 μm in the cross section parallel to the rolling direction and in the plate thickness direction was 3 μm. Measure at intervals. This measurement is carried out from the front surface to the back surface of the steel sheet. From this result, the maximum hardness / minimum hardness was obtained with the largest value among the measured values as the maximum hardness and the smallest value as the minimum hardness. The hardness was measured using a TriboScope / TriboIndenter manufactured by Hysiron. The load may be smaller or larger than 500 μN as long as the indentation size is smaller than the length of each rolled pearlite in the plate thickness direction. The indentation interval may be adjusted according to the indentation size so that the indentation does not become too close, and the indentation interval does not have to be 3 μm. Normally, the indentation interval is 3 times or more the indentation size. If the hardness can be measured from the front surface to the back surface of the steel sheet, the measurement area at one time is not limited to 50 μm × 50 μm.

The steel sheet according to the embodiment of the present invention may have a plating layer. Examples of the plating layer include Zn-based plating and Al-based plating.

Next, the characteristics (mechanical characteristics) of the steel sheet of the present invention will be described.

The tensile strength of the steel sheet according to one embodiment of the present invention is 1500 MPa or more. The tensile strength of the steel sheet according to one embodiment of the present invention is preferably 1600 MPa or more, more preferably 1700 MPa or more, still more preferably 1800 MPa or more. The upper limit of the tensile strength of the steel sheet according to the embodiment of the present invention is not particularly limited, but 2500 MPa or less is used from the viewpoint of easy balancing with other characteristics and from the viewpoint of preventing damage to the blade during shearing. preferable.

Further, "excellent in delayed fracture resistance" means that the critical load stress obtained by the method described later is equal to or higher than the yield strength (hereinafter, also simply referred to as YS). The critical load stress is preferably (YS + 100) MPa or more, more preferably (YS + 200) MPa or more.

Here, the tensile strength (TS) and the yield strength (YS) are measured as follows.
That is, a JIS No. 5 test piece having a distance between gauge points of 50 mm and a width between gauge points of 25 mm is collected from the central portion of the plate width of the base metal region of the steel plate so that the rolling direction is the longitudinal direction. Then, using the collected JIS No. 5 test piece, a tensile test is performed in accordance with the provisions of JIS Z 2241 (2011), and the tensile strength (TS) and the yield strength (YS) are measured. The tensile speed is 10 mm / min.

In addition, the delayed fracture resistance can be evaluated by the following method.
That is, a 110 mm × 30 mm test piece is collected from the steel sheet so that the shear end face is in the longitudinal direction, and the test piece is subjected to V-shaped bending in the longitudinal direction. Then, using bolts, nuts and taper washers, the V-shaped bent test piece is bolted from both sides of the plate surface, and various load stresses at 10 MPa intervals from 1200 MPa to 2400 MPa are applied to the V-shaped bent portion. A molded member test piece is prepared so as to cover the above. Here, the load stress is adjusted by using the correlation between the load stress and the bolt tightening amount. The correlation between the load stress and the bolt tightening amount is obtained by CAE analysis using the YU model. In the CAE analysis, the stress-strain curve obtained by the tensile test is used.

Then, the various molded member test pieces produced were immersed in a hydrochloric acid aqueous solution having a pH of 3 (25 ° C.) for 96 hours, and the maximum value of the load stress of the molded member test pieces that did not undergo delayed fracture (cracking) after the immersion was set. , Critical load stress.

It should be noted that the determination of delayed fracture is performed visually and with an image magnified at a magnification of 20 times with a stereomicroscope, and when no crack with a length of 200 μm or more is confirmed, it can be evaluated that there is no delayed fracture (crack). ..

Next, a method for manufacturing a steel sheet according to an embodiment of the present invention will be described.

A method for manufacturing a steel sheet according to an embodiment of the present invention is as follows.
A steel material having a pearlite of 95% or more with respect to the entire steel structure and an average lamellar spacing of the pearlite structure of 300 nm or less is subjected to cold rolling conditions that satisfy all of the following conditions 1) to 4). It has a cold rolling process in multiple directions.
1) Number of rolling directions n: n is an integer, n ≧ 2
2) Minimum total reduction rate in one rolling direction r: r ≧ 10%
3) Total reduction rate R: R ≧ 90%
4) When reading the angle between the 1st (initial) cold rolling direction and the 1st (initial) cold rolling direction of each of the 2nd to nth rolling directions after the 1st (initial) cold rolling, which is more than 0 ° and 90 ° or less. Angle that is the largest angle in the rolling direction Xmax: 60 ° ≤ Xmax ≤ 90 °
Area ratio of steel material pearlite: 95% or more Since the pearlite area ratio of steel material before cold rolling is the same as the area ratio of rolled pearlite after cold rolling, high strength is obtained after cold rolling. Moreover, from the viewpoint of eliminating the old austenite grain boundary and obtaining good delayed fracture resistance, the area ratio of pearlite of the steel material is 95% or more. It is preferably 97% or more, and more preferably 99% or more. The upper limit is not particularly limited and may be 100%.

The pearlite referred to here is a structure composed of ferrite and needle-like cementite.

The structure of the steel material according to one embodiment of the present invention may contain other metallic phases other than pearlite. Here, if the area ratio of the other metal phase is less than 5%, it is acceptable.
Examples of other metal phases include ferrite, martensite, and bainite.

Here, the area ratio of each phase is measured as follows.

That is, a test piece is collected from the base material region of the steel sheet so that the L cross section parallel to the rolling direction is the test surface. Then, the test surface of the test piece is mirror-polished and the structure is exposed with a nital solution. The test surface of the test piece revealing the structure is observed by SEM at a magnification of 1500 times, and the area ratio other than pearlite at the plate thickness 1/4 position is measured by the point counting method.

The area ratio of pearlite is calculated by subtracting the area ratio other than pearlite from 100%.

Average lamellar spacing of pearlite From the viewpoint of obtaining high strength after cold rolling, the average lamellar spacing of pearlite made of steel material shall be 300 nm or less. It is preferably 280 nm or less, and more preferably 250 nm or less. The lower limit is not particularly limited, but if it is too fine, the strength becomes excessive and a sufficient cold rolling ratio cannot be obtained. Therefore, from the viewpoint of obtaining good delayed fracture resistance, 50 nm or more is preferable.

The measurement of this average lamella interval can determine the average width from SEM observation. Here, the lamellar spacing of pearlite means the average distance between the center points in the thickness direction of the adjacent ferrite layers and cementite layers constituting the lamellar. For the average distance, for example, one ferrite layer and one cementite layer are regarded as one set of layers, and in microstructure observation, how many sets of layers are formed by a line segment having a predetermined length in the direction perpendicular to the extension direction of the layers. It may be obtained by measuring whether or not it is cut. Layers that are not completely cut by the line segment at both ends of the line segment are not measured. That is, it is calculated by lamella spacing = line segment length / (number of sets cut by line segment × 2).
The structure can be measured by etching a cross section parallel to the rolling direction by nital or electrolytic polishing, photographing 3 or more fields at 5000 times or more using SEM, and measuring by a method such as image analysis.

Cold rolling process Number of rolling directions n: n is an integer, n ≧ 2
Pearlite is easy to process depending on the direction of the lamella. If the rolling direction is one direction, pearlite having a lamellar direction perpendicular to the rolling direction is difficult to process, so that the hardness is low and the hardness ratio in all directions of the plate thickness is large. As a result, the delayed fracture resistance is deteriorated. Therefore, the number of rolling directions is 2 or more. It is preferably 3 or more. The upper limit of the number of rolling directions is not particularly limited, but the number of rolling directions is preferably 10 or less from the viewpoint of gaining the total rolling reduction ratio in one rolling direction. More preferably, it is 8 or less.

Minimum total reduction rate in one rolling direction r: r ≧ 10%
If the minimum total rolling reduction ratio in one rolling direction is less than 10%, the hardness of pearlite having the lamellar direction in that direction is insufficient, so that the hardness is low and the hardness ratio in all directions of the plate thickness is large. As a result, the delayed fracture resistance is deteriorated. Therefore, the minimum total rolling reduction r in one rolling direction is set to 10% or more. It is preferably 15% or more. The upper limit of the minimum total reduction rate r in one rolling direction is not particularly limited, but from the viewpoint of gaining a reduction rate in other rolling directions, the minimum total reduction rate r in one rolling direction is preferably 50% or less.
Here, the total rolling reduction r is the total rolling ratio in the same direction (one rolling direction). If the rolling direction is ± 5 ° or less, it may be regarded as the same direction.

Total reduction rate in all directions R: R ≧ 90%
In order to obtain high-strength characteristics and improve delayed fracture resistance, it is necessary to perform strong processing so that the old austenite grain boundaries are reduced. Therefore, the total reduction rate R in all directions is 90% or more. It is preferably 95% or more. The upper limit of the total rolling reduction ratio R is not particularly limited, but the total rolling reduction ratio R is preferably 120% or less due to the manufacturing limit of the cold rolling mill.
Here, the total rolling reduction ratio R is the total rolling ratio in all rolling directions.

First (initial) cold rolling When reading the angle of each rolling direction after the first (initial) cold rolling direction with respect to the first (initial) cold rolling direction, which is more than 0 ° and 90 ° or less, in each of the rolling directions. Xmax: 60 ° ≤ Xmax ≤ 90 °
The angle between the largest angle in the 2nd to nth directions and the initial rolling direction (the direction of the first cold rolling) is less than 60 °, that is, rolling within ± 30 ° in the vertical direction of the initial rolling direction. It means that it has not been done. As a result, pearlite having a lamellar direction perpendicular to the rolling direction is difficult to process, so that the hardness is low and the hardness ratio in all directions of the plate thickness is large. Therefore, the largest angle in the 2nd to nth directions and the angle in the initial rolling direction are set to 60 ° or more. It is preferably 65 ° or more. Since the maximum is 90 °, the largest angle in the 2nd to nth directions and the angle in the initial rolling direction are 90% or less.

A method for manufacturing a steel sheet according to an embodiment of the present invention is as follows.
The steel material is held at a heating temperature of 900 ° C. or higher for 1 minute or longer, then cooled at an average cooling rate of 10 ° C./sec or higher, and then cooled at (850-380 [C] ^1/2 ) ° C. or higher (950-380 [C]. ] ^1/2 ) It has a heat treatment step of cooling to room temperature after holding at ℃ or less for 20 minutes or more. Here, [C] is the C content contained in the steel material, which is the same as the C content in the steel sheet obtained after cold rolling.
The room temperature is −5 to 40 ° C.

Heat treatment step In order to obtain a steel material having pearlite of 95% or more in area ratio and an average lamellar spacing of the pearlite structure of 300 nm or less, it is preferable to perform the following heat treatment before cold rolling.

Heating temperature 900 ° C. or higher In order to obtain a pearlite structure in the final structure, it is preferable to eliminate ferrite and heat in the austenite single-phase region. Therefore, the heating temperature is preferably 900 ° C. or higher. More preferably, it is 920 ° C. or higher. The upper limit is not particularly limited, but if excessive scale is generated, the plate thickness before cold rolling becomes thin, and it becomes difficult to obtain the cold rolling ratio. Therefore, the upper limit of the heating temperature is preferably 1100 ° C. or lower.

Heating time 1 minute or more In order to obtain a pearlite structure in the final structure, it is preferable to completely eliminate ferrite and heat in the austenite single-phase region. For that purpose, it is preferable to keep it for 1 minute or more in the austenite temperature range. More preferably, it is 5 minutes or more. The upper limit is not particularly limited, but if excessive scale is generated, the plate thickness before cold rolling becomes thin, and it becomes difficult to obtain the cold rolling ratio. Therefore, the upper limit of the heating time is preferably 7200 seconds or less.

Average cooling rate of 10 ° C./sec or more In order to obtain a pearlite structure in the final structure, it is preferable to cool faster than ferrite is formed. For that purpose, the average cooling rate is preferably 10 ° C./sec or more. More preferably, it is 12 ° C./sec or higher. The upper limit is not particularly limited, and the faster the speed, the better. It may be 2000 ° C./sec or higher, which is equivalent to water cooling.

Holding temperature (850-380 [C] ^1/2 ) ° C or higher (950-380 [C] ^1/2 ) ° C or lower In order to keep the average lamellar spacing of the pearlite structure to 300 nm or less, holding near the pearlite nose is necessary. preferable. Since the pearlite nose fluctuates depending on the chemical composition and is particularly affected by carbon, it is preferable to select the optimum temperature according to the amount of carbon. When the temperature rises or falls below 50 ° C from the pearlite nose, it becomes difficult to keep the average lamellar spacing of the pearlite structure below 300 nm, so the holding temperature is (850-380 [C] ^1/2 ) ° C or higher (950-380 []. C] ^1/2 ) ° C or lower. It is preferably (870-380 [C] ^1/2 ) ° C. or higher and (930-380 [C] ^1/2 ) ° C. or lower.

Retention time 20 minutes or more In order to obtain a pearlite structure in the final structure, a longer retention time is preferable. Therefore, the holding time is preferably 20 minutes or more. More preferably, it is 25 minutes or more. Although the upper limit is not particularly limited, the upper limit of the holding time is preferably 120 minutes or less in order to prevent the carbides from becoming coarse and deteriorating the delayed fracture resistance.

The heating temperature and the holding temperature may be constant during holding, and may not always be constant during holding as long as they are within the above temperature range. The same applies to the holding of the slab.

The conditions of each step other than the above are not particularly limited, and a conventional method may be followed.

Further, a tempering process may be performed after the cold rolling step as long as the characteristics are not changed. Further, the steel sheet may be subjected to a plating treatment such as Zn-based plating or Al-based plating. Further, after the cold rolling step, the annealing step, or the plating treatment, the steel sheet may be subjected to temper rolling for shape adjustment.

A steel material having the component composition shown in Table 1 (the balance is Fe and unavoidable impurities) is heat-treated under the conditions shown in Table 2 (heating temperature, heating time, average cooling rate, holding temperature, holding time), and then heat-treated. Obtained a steel plate. After holding at the holding temperature and holding time shown in Table 2, the mixture was cooled to room temperature (25 ° C.).
The obtained heat-treated steel sheet was ground and then cold-rolled (cold-rolled in 1 to 5 directions) under the conditions shown in Table 3 to obtain a cold-rolled steel sheet.

Further, the blanks of each element in Table 1 indicate that the element is not intentionally added, and not only when the element is not contained (0% by mass) but also when the element is inevitably contained. Including cases.

Here, the structure of the obtained steel sheet was measured as follows.
A test piece was taken from the base metal region of the steel sheet so that the L cross section parallel to the rolling direction was the test surface. Then, the test surface of the test piece was mirror-polished and the structure was exposed with a nital solution. The test surface of the test piece in which the structure appeared was observed by SEM at a magnification of 1500 times, and the area ratio other than the rolled pearlite at the plate thickness 1/4 position was measured by the point counting method.
In the SEM image, martensite exhibits a white tissue. Further, among the martensites, the tempered martensite has fine carbides precipitated inside. Ferrites have a black structure. In bainite, white carbides are precipitated in the black structure. From these points, each phase was identified in the SEM image. However, depending on the surface orientation of the block grains and the degree of etching, it may be difficult for carbides inside to appear. In that case, sufficient etching was performed to confirm.
The area ratio of rolled pearlite was calculated by subtracting the area ratio other than rolled pearlite from 100%.
It was confirmed that the hardness of the rolled pearlite was 300 or more in terms of HV. As for the specific conversion method, Vickers hardness HV and hardness measurement described later were performed using pearlite samples with different hardness, and it was judged that the pearlite was rolled because the hardness of pearlite was 300 or more in terms of HV. did.

The results are shown in Table 4.

In the tissue identification (point counting method), a 16 × 15 grid was placed on the observation area (82 μm × 57 μm area) by SEM so that the intervals were even. Then, the number of points of each phase at the grid points was counted, and the ratio of the number of grid points occupied by each phase to the total number of grid points was defined as the area ratio of each phase. The area ratio of each phase was taken as the average value of the area ratio of each phase obtained from three separate SEM images.
It was confirmed by the same method that the area ratio of the rolled pearlite of the steel material before cold rolling was the same as the area ratio of the rolled pearlite after cold rolling in Table 4.

Further, in the measurement of the average lamellar interval of pearlite before cold rolling, the average width was determined from the SEM observation. Here, the lamellar spacing of pearlite means the average distance between the center points in the thickness direction of the adjacent ferrite layers and cementite layers constituting the lamellar. For the average distance, for example, one ferrite layer and one cementite layer are regarded as one set of layers, and in microstructure observation, how many sets of layers are formed by a line segment having a predetermined length in the direction perpendicular to the extension direction of the layers. It may be obtained by measuring whether or not it is cut. Layers that are not completely cut by the line segment at both ends of the line segment are not measured. That is, it is calculated by lamella spacing = line segment length / (number of sets cut by line segment × 2).
The structure was measured by etching a cross section parallel to the rolling direction by nital or electrolytic polishing, photographing 3 or more fields at 5000 times or more using SEM, and measuring by a method such as image analysis.

Furthermore, the maximum hardness / minimum hardness of the steel sheet was evaluated as follows. The results are also shown in Table 4.

That is, a test piece for observing the metallographic structure is collected from the central portion of the plate width of the base material region of the steel plate. Then, the test piece for observing the metallographic structure was polished, and under a load of 500 μN (loading 10s, unloading 10s), a measurement area of 50 μm × 50 μm in the cross section parallel to the rolling direction and in the plate thickness direction was 3 μm. Measured at intervals. This measurement was carried out from the front surface to the back surface of the steel sheet. From this result, the maximum hardness / minimum hardness was obtained with the largest value among the measured values as the maximum hardness and the smallest value as the minimum hardness. The hardness was measured using a TriboScope / TriboIndenter manufactured by Hysiron. The indentation size was 500 nm in diagonal length, and the indentation interval was 5.0 μm.

The tensile properties were evaluated as follows.

That is, a JIS No. 5 test piece having a distance between gauge points of 50 mm and a width between gauge points of 25 mm is collected from the central portion of the plate width of the base metal region of the steel plate so that the rolling direction is the longitudinal direction. Then, using the collected JIS No. 5 test piece, a tensile test is performed in accordance with the provisions of JIS Z 2241 (2011), and the tensile strength (TS) and the yield strength (YS) are measured. The tensile speed is 10 mm / min.

The delayed fracture resistance was evaluated as follows.

That is, a 110 mm × 30 mm test piece was collected from the steel sheet obtained as described above so that the shear end face was in the longitudinal direction, and the test piece was subjected to V-shaped bending in the longitudinal direction. Then, using bolts, nuts and taper washers, the V-shaped bent test piece is bolted from both sides of the plate surface, and various load stresses at 10 MPa intervals from 1200 MPa to 2400 MPa are applied to the V-shaped bent portion. Various molded member test pieces were prepared so as to cover the above. Here, the load stress was adjusted using the correlation between the load stress and the bolt tightening amount. The correlation between the load stress and the bolt tightening amount was obtained by CAE analysis using the YU model. In the CAE analysis, the stress-strain curve obtained by the tensile test was used.

Then, the various molded member test pieces produced were immersed in a hydrochloric acid aqueous solution having a pH of 3 (25 ° C.) for 96 hours, and the maximum value of the load stress of the molded member test pieces that did not undergo delayed fracture (cracking) after the immersion was set. , Critical load stress. The obtained critical load stress is also shown in Table 1.

In addition, the judgment of delayed fracture is performed visually and with an image magnified at a magnification of 20 times with a stereomicroscope, and if no crack with a length of 200 μm or more is confirmed, it is said that there is no delayed fracture (crack). , Length: When even one crack of 200 μm or more was confirmed, it was determined that there was delayed fracture (crack).

Then, based on the obtained critical load stress, the delayed fracture resistance was evaluated according to the following criteria.

Pass (excellent): Critical load stress is yield stress YS or more Fail: Critical load stress is less than yield stress YS

As shown in Table 4, all of the steel sheets of the invention examples had high strength and excellent delayed fracture resistance.

On the other hand, in the comparative example, the strength was low or sufficient delayed fracture resistance was not obtained.

Claims (5)

By mass%,
C: 0.20% or more and 0.90% or less,
Si: 0.50% or less,
Mn: 1.50% or less,
P: 0.050% or less,
S: 0.020% or less,
Al: 0.10% or less,
N: 0.010% or less, and the balance has a component composition consisting of Fe and unavoidable impurities.
The area ratio of the entire steel structure is 95% or more for rolled pearlite.
The maximum hardness / minimum hardness, which is the ratio of the maximum hardness to the minimum hardness measured in the plate thickness direction, is 1.0 or more and 2.0 or less.
A steel sheet having a tensile strength of 1500 MPa or more. As the component composition, further, by mass%,
Cr: 4.0% or less,
Mo: 4.0% or less,
The steel sheet according to claim 1, which contains at least one selected from V: 0.5% or less and Ni: 0.10% or less. As the component composition, further, by mass%,
Nb: 0.020% or less,
Ti: 0.020% or less,
Cu: 0.20% or less,
B: 0.0020% or less,
The steel sheet according to claim 1 or 2, which contains at least one selected from Sb: 0.10% or less and Sn: 0.10% or less. A steel material having the component composition according to any one of claims 1 to 3, having pearlite having an area ratio of 95% or more with respect to the entire steel structure, and having an average lamella spacing of the pearlite structure of 300 nm or less. A method for manufacturing a steel sheet, which comprises a cold rolling step of performing cold rolling in a plurality of directions under cold rolling conditions satisfying all of the following conditions 1) to 4).
1) Number n in the rolling direction: n is an integer, n ≧ 2
2) Minimum total rolling reduction in one rolling direction r: r ≧ 10%
3) Total reduction rate in all directions R: R ≧ 90%
4) When reading the angle of each rolling direction after the first cold rolling with respect to the first cold rolling direction, which is more than 0 ° and 90 ° or less, it is the largest angle in each rolling direction. Angle Xmax: 60 ° ≤ Xmax ≤ 90 ° The steel material having the component composition according to any one of claims 1 to 3 is held at a heating temperature of 900 ° C. or higher for 1 minute or longer, and then cooled at an average cooling rate of 10 ° C./sec or higher (850-380). After holding at [C] ^1/2 ) ° C. or higher (950-380 [C] ^1/2 ) ° C. or lower ([C] is the C content contained in the steel material) for 20 minutes or longer, the temperature reaches room temperature. The heat treatment process to cool and
A method for manufacturing a steel sheet, which comprises a cold rolling step of performing cold rolling in a plurality of directions under cold rolling conditions satisfying all of the following conditions 1) to 4).
1) Number n in the rolling direction: n is an integer, n ≧ 2
2) Minimum total rolling reduction in one rolling direction r: r ≧ 10%
3) Total reduction rate in all directions R: R ≧ 90%
4) When reading the angle of each rolling direction after the first cold rolling with respect to the first cold rolling direction, which is more than 0 ° and 90 ° or less, it is the largest angle in each rolling direction. Angle Xmax: 60 ° ≤ Xmax ≤ 90