WO2023026582A1

WO2023026582A1 - Hot-rolled steel plate

Info

Publication number: WO2023026582A1
Application number: PCT/JP2022/017417
Authority: WO
Inventors: 耕平中田; 武豊田; 駿介小林
Original assignee: 日本製鉄株式会社
Priority date: 2021-08-24
Filing date: 2022-04-08
Publication date: 2023-03-02
Also published as: JPWO2023026582A1

Abstract

Provided is a hot-rolled steel plate that has a prescribed chemical composition, the microstructure thereof being, in terms of surface area ratio, at least 70% pearlite, 0 to 30% ferrite, and 0 to 30% bainite. The perlite has, per 10 μm2, less than 10 grains of cementite having a major-axis length over 0.3 μm and an aspect ratio of less than 3.0, and has a tensile strength of at least 900 MPa.

Description

hot rolled steel plate

TECHNICAL FIELD The present invention relates to a hot-rolled steel sheet, and more particularly, to a hot-rolled steel sheet used for structural members of automobiles and the like, which has high strength, uniform elongation, and excellent punched end face fatigue properties. .

In recent years, in the automotive industry, there has been a demand for weight reduction of parts such as chassis parts from the viewpoint of improving fuel efficiency. Increasing the strength of the steel plate used is one of the effective ways to achieve both weight reduction and collision safety of members.

On the other hand, as the strength increases, the uniform elongation and the fatigue properties of the punched end face, which are necessary for forming parts, generally decrease. In order to improve uniform elongation, TRIP steel using retained austenite has been devised, but in such steel materials, high-hardness and low-toughness martensite is generated near the end face by work-induced transformation during punching. However, the formation of such martensite can be a cause of deterioration in the fatigue properties of the punched end face.

In Patent Document 1, it has a predetermined chemical composition, and the metal structure has an area ratio of pearlite: 90 to 100%, pseudo pearlite: 0 to 10%, and proeutectoid ferrite: 0 to 1%, and the pearlite is 0.20 μm or less, and the average pearlite block diameter of the pearlite is 20.0 μm or less. In addition, Patent Document 1 describes that with the above configuration, it is possible to obtain a hot-rolled steel sheet having a high tensile strength of 980 MPa or more and excellent ductility, hole expansibility, and punchability. .

In relation to the pearlite-based structure as described in Patent Document 1, in Patent Document 2, in the case of a hot-rolled steel sheet, if the growth time during pearlite transformation is sufficient, it is possible to have an elongated form of lamellar cementite. stated to be common. Further, Patent Document 3 describes a hot-rolled steel sheet having a pearlite structure as a main phase, a ferrite structure in the residual structure of 20% or less, and a lamellar spacing of the pearlite structure of 500 nm or less.

WO2020/179737 Japanese Patent Publication No. 2020-509190 JP 2011-099129 A

Although Patent Document 1 teaches that a total elongation of 13% or more can be achieved in relation to ductility, it does not specifically show improvement in uniform elongation. Similarly, in Patent Document 1, in relation to punchability, although it is specifically disclosed that the occurrence of cracks on the end face during punching is suppressed, improvement in fatigue characteristics on the punched end face is not necessarily sufficient. No consideration has been made. Therefore, the hot-rolled steel sheet described in Patent Document 1 still has room for improvement in terms of uniform elongation and fatigue properties of punched edge faces.

Therefore, an object of the present invention is to provide a hot-rolled steel sheet with high strength and excellent uniform elongation and punched end face fatigue properties by a novel configuration.

The inventors investigated the chemical composition and microstructure of hot-rolled steel sheets in order to achieve the above objectives. As a result, the present inventors improved the strength of the steel sheet by utilizing the precipitation strengthening by Cu, and appropriately controlled the C and Cr contents in the steel sheet. It was found that uniform elongation can be improved by increasing the percentage of high pearlite, and on the other hand, fatigue characteristics of the punched end face can be improved by making the cementite in the pearlite layered (lamellar). perfected the invention.

The present invention that has achieved the above object is as follows.
(1) chemical composition, in mass %,
C: 0.20 to 0.30%,
Si: 0.01 to 2.00%,
Mn: 0.50-3.00%,
P: 0.100% or less,
S: 0.0100% or less,
Al: 0.005 to 3.000%,
N: 0.0100% or less,
O: 0.0100% or less,
Cr: more than 1.00 to 3.00%,
Cu: more than 1.00 to 3.00%,
Ti: 0 to 0.10%,
Nb: 0 to 0.10%,
V: 0 to 0.10%,
Ni: 0 to 2.00%,
Mo: 0 to 1.00%,
B: 0 to 0.0100%,
Ca: 0 to 0.0050%,
REM: 0-0.005%, and balance: Fe and impurities,
The microstructure is the area ratio,
Perlite: 70% or more,
ferrite: 0-30%, and bainite: 0-30%,
Less than 10 pieces of cementite per 10 μm ² having a major axis length of more than 0.3 μm and an aspect ratio of less than 3.0 in the pearlite,
A hot-rolled steel sheet characterized by having a tensile strength of 900 MPa or more.
(2) the chemical composition, in mass %,
Ti: 0.001 to 0.10%,
Nb: 0.001 to 0.10%,
V: 0.001 to 0.10%,
Ni: 0.001 to 2.00%,
Mo: 0.001 to 1.00%,
B: 0.0001 to 0.0100%,
Ca: 0.0001-0.0050%, and REM: 0.0001-0.005%
The hot-rolled steel sheet according to (1) above, comprising one or more selected from the group consisting of:
(3) The hot-rolled steel sheet according to (1) or (2) above, which has a thickness of 1.0 to 6.0 mm.

According to the present invention, it is possible to obtain a hot-rolled steel sheet having a high tensile strength of 900 MPa or more and excellent uniform elongation and punched edge fatigue properties.

BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic diagram of the test piece (the dimension unit in a figure: mm) used for evaluating a punching edge fatigue characteristic.

<Hot rolled steel plate>
A hot-rolled steel sheet according to an embodiment of the present invention is
The chemical composition, in mass %,
C: 0.20 to 0.30%,
Si: 0.01 to 2.00%,
Mn: 0.50-3.00%,
P: 0.100% or less,
S: 0.0100% or less,
Al: 0.005 to 3.000%,
N: 0.0100% or less,
O: 0.0100% or less,
Cr: more than 1.00 to 3.00%,
Cu: more than 1.00 to 3.00%,
Ti: 0 to 0.10%,
Nb: 0 to 0.10%,
V: 0 to 0.10%,
Ni: 0 to 2.00%,
Mo: 0 to 1.00%,
B: 0 to 0.0100%,
Ca: 0 to 0.0050%,
REM: 0-0.005%, and balance: Fe and impurities,
The microstructure is the area ratio,
Perlite: 70% or more,
ferrite: 0-30%, and bainite: 0-30%,
Less than 10 pieces of cementite per 10 μm ² having a major axis length of more than 0.3 μm and an aspect ratio of less than 3.0 in the pearlite,
It is characterized by having a tensile strength of 900 MPa or more.

As mentioned earlier, there is a problem that the uniform elongation and fatigue properties of the punched end faces, which are necessary for forming parts, generally deteriorate along with the increase in strength. Therefore, the present inventors first focused on pearlite, which has the highest work hardening ability among microstructures other than retained austenite, and appropriately controlled the contents of C and Cr in the steel sheet to make the pearlite an area ratio 70% or more, and if there is a residual structure, it is mainly composed of ferrite and / or bainite, thereby significantly improving the uniform elongation and improving the fatigue characteristics of the punched end face. I found what I can do. More specifically, a relatively high C content is generally required to produce high levels of pearlite. However, the inventors have found that increasing the Cr content to over 1.00% can extend the region of pearlite formation to the low carbon side, resulting in a relatively low carbon content of 0.20-0.30%. It was found that a high pearlite fraction of 70% or more in terms of area ratio can be achieved in spite of the low C content. In addition, the present inventors have found that in such a hot-rolled steel sheet having a relatively low C content of 0.20 to 0.30% and a pearlite-based microstructure with a Cr content of more than 1.00%, It was found that the fatigue characteristics of the punched end face can be improved by substantially not including martensite with high hardness and low toughness and retained austenite that generates the martensite by deformation-induced transformation.

Next, the present inventors have found such a relatively low C content of 0.20 to 0.30% and a Cr content of more than 1.00%, and an area ratio of 70% or more In order to further improve the fatigue properties of punched edges of hot-rolled steel sheets containing pearlite, studies were conducted focusing on the form of cementite in the pearlite. In the case of a steel sheet containing a relatively large amount of pearlite, when the steel sheet is punched, minute voids may occur starting from cementite or the interface between cementite and ferrite on the punched end face. These voids can be the cause of deterioration in the fatigue properties of the punched end face after punching. In contrast, the present inventors have found that the amount of coarse spheroidal cementite in pearlite should be reduced, more specifically pearlite having a long axis length of more than 0.3 μm and an aspect ratio of less than 3.0 By limiting the number density of cementite to less than 10 per 10 μm ² , the occurrence of voids during punching can be suppressed, and as a result, the fatigue properties at the punched end face can be significantly improved.

Although it is not intended to be bound by any particular theory, by making the cementite in the pearlite a more layered (more lamellar) form, when the steel plate is punched, in the region near the punched end face, It is believed that the strong stress applied during the punching process forms a structure in which the cementite lamellar structure is aligned in one direction along the punching direction. It is believed that due to the aligned structure of the punched end faces, the fatigue properties are significantly improved compared to conventional microstructures that do not contain such a pearlite structure. On the other hand, when a large amount of coarse and spherical cementite is contained in pearlite, that is, cementite having a long axis length of more than 0.3 μm and an aspect ratio of less than 3.0 has a number density of 10 or more per 10 μm ² It is considered that if there is such a structure, it becomes impossible to form such an aligned structural structure on the punched end face during punching, and as a result, the punched end face fatigue characteristics are degraded. In addition, the present inventors have found that in order to suppress the formation of such coarse spherical cementite in pearlite and promote the formation of layered cementite, it is necessary to secure a sufficient amount of carbon for forming pearlite. To this end, the carbide-forming alloying elements, more specifically Ti, Nb and V, are each limited to 0.10% or less so that the carbon in the steel is consumed by these alloying elements. It has been found that suppression is effective.

Ti, Nb and V are elements that generally contribute to the improvement of steel sheet strength through carbide precipitation. On the other hand, the hot-rolled steel sheet according to the embodiment of the present invention contains only a relatively low content of 0.20 to 0.30% of C, which is an element effective in improving the strength of the steel sheet, as described above. is. Therefore, it is generally difficult to achieve a high strength, specifically a tensile strength of 900 MPa or more, while suppressing the contents of these elements to relatively low contents. Therefore, the present inventors have found that, despite such a relatively low C content, Cu is contained in the steel sheet in an amount exceeding 1.00%, so that the steel sheet is strengthened by precipitation strengthening by Cu. It has been found that the strength can be maintained at a high level. By utilizing the precipitation strengthening by Cu, it is possible not to utilize or to limit the utilization of strength improvement by carbide precipitation of Ti, Nb and V. As a result, it becomes possible to suppress the formation of carbides due to Ti, Nb and V, and to ensure a sufficient amount of carbon necessary to form the desired pearlite structure according to the embodiment of the present invention.

To summarize the above, the present inventors mainly combined the following four findings to develop a hot-rolled steel sheet having a high tensile strength of 900 MPa or more, uniform elongation, and excellent punching edge fatigue characteristics. The combination of these findings and the fact that a hot-rolled steel sheet having high strength, uniform elongation and excellent punched end face fatigue characteristics can be obtained by this combination has not been known in the past, and this time, the present invention This is the first time it has been revealed by
(i) Despite the relatively low C content of 0.20 to 0.30%, the pearlite fraction of 70% or more in area ratio by including Cr in the steel sheet in an amount exceeding 1.00% can be achieved, and as a result uniform elongation can be improved,
(ii) The residual structure is composed of ferrite and/or bainite to improve the fatigue properties of the punched edge by being substantially free of martensite in the microstructure and retained austenite that produces said martensite by strain-induced transformation. that you can
(iii) limiting the content of Ti, Nb and V to suppress the formation of carbides by these elements to ensure a sufficient amount of carbon to form a pearlite structure containing lamellar cementite, resulting in pearlite The formation of coarse spheroidal cementite, i.e., cementite having a major axis length of more than 0.3 μm and an aspect ratio of less than 3.0, is limited to less than 10 per 10 μm ² to improve punching edge fatigue properties. and (iv) by utilizing precipitation strengthening by Cu, the formation of carbides by Ti, Nb, and V is suppressed, and the formation of the pearlite structure in (iii) above is promoted. Despite the relatively low C content of 30%, a high tensile strength of 900 MPa or more can be achieved.

　Hereinafter, the hot-rolled steel sheet according to the embodiment of the present invention will be described in more detail. In the following description, the unit of content of each element, "%", means "% by mass" unless otherwise specified. In addition, in this specification, the term "to" indicating a numerical range is used to include the numerical values before and after it as lower and upper limits, unless otherwise specified.

[C: 0.20 to 0.30%]
C is an essential element for ensuring the strength of the hot-rolled steel sheet. In order to sufficiently obtain such effects, the C content is made 0.20% or more. The C content may be 0.21% or more, 0.22% or more, or 0.23% or more. On the other hand, an excessive C content may deteriorate the weldability of the steel sheet. Therefore, the C content should be 0.30% or less. C content may be less than 0.30%, 0.29% or less, 0.28% or less, 0.27% or less, 0.26% or less, 0.25% or less or 0.24% or less . In the embodiment of the present invention, since the C content is relatively low as described above, it is advantageous from the viewpoint of weldability. Is possible.

[Si: 0.01 to 2.00%]
Si is an element used for deoxidizing steel. In order to sufficiently obtain such effects, the Si content is set to 0.01% or more. The Si content may be 0.10% or more, 0.20% or more, 0.30% or more, or 0.50% or more. On the other hand, if Si is contained excessively, the chemical convertibility deteriorates, and austenite remains in the microstructure of the steel sheet. Therefore, the Si content is set to 2.00% or less. The Si content may be 1.85% or less, 1.70% or less, 1.50% or less, or 1.40% or less.

[Mn: 0.50 to 3.00%]
Mn is an element effective for delaying phase transformation of steel and preventing phase transformation from occurring during cooling. In order to sufficiently obtain such effects, the Mn content is made 0.50% or more. The Mn content may be 0.60% or more, 0.80% or more, 1.00% or more, 1.30% or more, or 1.50% or more. On the other hand, when Mn is contained excessively, micro-segregation or macro-segregation tends to occur, which may reduce the hole expansibility. Therefore, the Mn content should be 3.00% or less. The Mn content may be 2.80% or less, 2.70% or less, 2.50% or less, or 2.20% or less.

[P: 0.100% or less]
P is an element mixed in during the manufacturing process. The lower the P content is, the better. If it is excessive, the formability and weldability may be adversely affected, and the fatigue properties may be deteriorated. Therefore, the P content should be 0.100% or less. It is preferably 0.090% or less or 0.070% or less, more preferably 0.050% or 0.040% or less. The P content may be 0%, but excessive reduction leads to increased costs. Therefore, the P content may be 0.0001% or more, 0.0005% or more, 0.001% or more, or 0.005% or more.

[S: 0.0100% or less]
S forms MnS and acts as a starting point of fracture, which may significantly reduce the hole expansibility of the steel sheet. Therefore, the S content is set to 0.0100% or less. The S content is preferably 0.0090% or less, more preferably 0.0085% or less or 0.0070% or less. The S content may be 0%, but excessive reduction leads to an increase in cost. Therefore, the S content may be 0.0001% or more, 0.0005% or more, 0.0010% or more, or 0.0020% or more.

[Al: 0.005 to 3.000%]
Al is an element used for deoxidizing steel. In order to sufficiently obtain such effects, the Al content is made 0.005% or more. The Al content may be 0.010% or more, 0.030% or more, 0.050% or more, 0.100% or more, or 0.300% or more. On the other hand, an excessive Al content may increase inclusions and deteriorate the workability of the steel sheet. Therefore, the Al content is set to 3.000% or less. The Al content may be 2.800% or less, 2.500% or less, 2.000% or less, 1.800% or less, 1.500% or less, or 1.200% or less.

[N: 0.0100% or less]
N combines with Al in the steel to form AlN, which prevents the pearlite block diameter from increasing due to the pinning effect, thereby improving the toughness of the steel. However, an excessive N content saturates the effect and may rather cause a decrease in toughness. Therefore, the N content is set to 0.0100% or less. The N content is preferably 0.0090% or less, 0.0080% or less or 0.0070% or less. From such a point of view, the N content may be 0%, but excessive reduction leads to an increase in cost. Therefore, the N content may be 0.0001% or more, 0.0005% or more, 0.0010% or more, or 0.0020% or more.

[O: 0.0100% or less]
O is an element mixed in during the manufacturing process. If O is contained excessively, coarse inclusions may be formed to lower the toughness of the steel sheet. Therefore, the O content is 0.0100% or less. The O content may be 0.0090% or less, 0.0080% or less, 0.0070% or less, or 0.0060% or less. The O content may be 0%, but excessive reduction leads to increased costs. Therefore, the O content may be 0.0001% or more, 0.0005% or more, 0.0010% or more, or 0.0020% or more.

[Cr: more than 1.00 to 3.00%]
Cr is an element that contributes to improving the strength of the steel sheet, and also has the effect of suppressing spheroidization of cementite. Therefore, in order to reduce the number density of coarse spheroidal cementite in the pearlite and suppress the generation of voids during punching, it is necessary to contain a certain amount or more of Cr. Furthermore, since Cr stabilizes cementite, the inclusion of Cr can expand the pearlite formation region toward the low carbon content side. Therefore, by including Cr in an appropriate amount, ie, an amount exceeding 1.00%, it is possible to achieve a pearlite fraction of 70% or more even with a relatively low C content. Cr content is 1.01% or more, 1.02% or more, 1.03% or more, 1.05% or more, 1.10% or more, 1.30% or more, 1.50% or more, or 1.70% or more. On the other hand, when Cr is contained excessively, the pearlite transformation is delayed, and a relatively large amount of hard structures such as bainite and martensite are generated, which may make it difficult to achieve a pearlite fraction of 70% or more. Alternatively, when Cr is contained excessively, the uniform elongation may be lowered as the tensile strength is improved. Therefore, the Cr content is set to 3.00% or less. The Cr content may be 2.80% or less, 2.70% or less, 2.50% or less, 2.20% or less, 2.00% or less, or 1.80% or less.

[Cu: more than 1.00 to 3.00%]
Cu is an element effective in improving strength by precipitation strengthening. Also, unlike Ti, Nb and V, Cu can improve the strength of the steel sheet without forming carbides. Therefore, Cu does not consume the carbon required to form the desired pearlite structure in which coarse spherical cementite is reduced. Therefore, Cu is an extremely important element for achieving both strength improvement and punch end face fatigue properties. In order to sufficiently obtain these effects, the Cu content should be more than 1.00%. Cu content is 1.01% or more, 1.02% or more, 1.03% or more, 1.05% or more, 1.10% or more, 1.20% or more, 1.30% or more, 1.50% or more or 1.70% or more. On the other hand, an excessive Cu content may cause minute cracks on the surface during hot working due to an increase in precipitates. Therefore, the Cu content is set to 3.00% or less. The Cu content may be 2.80% or less, 2.70% or less, 2.50% or less, 2.20% or less, 2.00% or less, or 1.80% or less.

The basic chemical composition of the hot-rolled steel sheet according to the embodiment of the present invention is as described above. Furthermore, the hot-rolled steel sheet may contain at least one of the following optional elements in place of part of the remaining Fe, if necessary. For example, hot-rolled steel sheets contain Ti: 0-0.10%, Nb: 0-0.10%, V: 0-0.10%, Ni: 0-2.00% and Mo: 0-1. It may contain at least one selected from the group consisting of 00%. Also, the hot rolled steel sheet may contain B: 0 to 0.0100%. Also, the hot-rolled steel sheet may contain at least one selected from the group consisting of Ca: 0 to 0.0050% and REM: 0 to 0.005%. These optional elements are described in detail below.

[Ti: 0 to 0.10%]
[Nb: 0 to 0.10%]
[V: 0 to 0.10%]
Ti, Nb and V are elements that contribute to the improvement of steel sheet strength by precipitation of carbides. The Ti, Nb and V contents may be 0%, but in order to obtain the above effect, one selected from these may be contained alone or two or more may be combined as needed. good. In order to obtain the above effects, the Ti, Nb and V contents are each preferably 0.001% or more, and may be 0.01% or more, 0.02% or more, or 0.03% or more. . On the other hand, if these elements are excessively contained, a large amount of carbide is generated, consuming the carbon necessary to form the desired pearlite structure with reduced coarse spherical cementite. As a result, the fatigue properties of the punched end face may be degraded. Therefore, the Ti, Ni and V contents are each preferably 0.10% or less. The Ti, Ni and V contents may each be 0.08% or less, 0.06% or less or 0.05% or less.

[Ni: 0 to 2.00%]
Ni is an element that dissolves in steel to increase strength without impairing toughness. Although the Ni content may be 0%, the Ni content is preferably 0.001% or more in order to obtain the above effects. The Ni content may be 0.01% or more, 0.10% or more, 0.20% or more, or 0.50% or more. On the other hand, Ni is an expensive element, and excessive addition causes an increase in cost. Therefore, the Ni content is preferably 2.00% or less. The Ni content may be 1.70% or less, 1.50% or less, or 1.20% or less.

[Mo: 0 to 1.00%]
Mo is an element that increases the strength of steel. Although the Mo content may be 0%, the Mo content is preferably 0.001% or more in order to obtain the above effects. The Mo content may be 0.01% or more, 0.02% or more, or 0.03% or more. On the other hand, if Mo is contained excessively, toughness may decrease as the strength increases. Therefore, the Mo content is preferably 1.00% or less. Mo content may be 0.80% or less, 0.50% or less, 0.30% or less, 0.10% or less, 0.08% or less, 0.06% or less, or 0.05% or less .

[B: 0 to 0.0100%]
B segregates at grain boundaries and has the effect of increasing grain boundary strength. Although the B content may be 0%, the B content is preferably 0.0001% or more in order to obtain the above effect. The B content may be 0.0003% or more, 0.0005% or more, or 0.0010% or more. On the other hand, even if B is contained excessively, the effect is saturated, which leads to an increase in raw material cost. Therefore, the B content is preferably 0.0100% or less. The B content may be 0.0080% or less, 0.0060% or less, or 0.0050% or less.

[Ca: 0 to 0.0050%]
Ca is an element that improves workability by controlling the form of non-metallic inclusions that act as starting points for fracture and cause deterioration in workability. Although the Ca content may be 0%, the Ca content is preferably 0.0001% or more in order to obtain the above effect. The Ca content may be 0.0003% or more, 0.0005% or more, or 0.0010% or more. On the other hand, even if Ca is contained excessively, the effect is saturated, which leads to an increase in raw material cost. Therefore, the Ca content is preferably 0.0050% or less. The Ca content may be 0.0045% or less or 0.0040% or less.

[REM: 0 to 0.005%]
REM is an element that improves the toughness of weld zones by adding a small amount of REM. Although the REM content may be 0%, the REM content is preferably 0.0001% or more in order to obtain the above effects. The REM content may be 0.0003% or greater, 0.0005% or greater, or 0.001% or greater. On the other hand, an excessive REM content may reduce weldability. Therefore, the REM content is preferably 0.005% or less. The REM content may be 0.004% or less or 0.003% or less. REM herein refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanoids lanthanum with atomic number 57 (La) to lutetium with atomic number 71 (Lu ), and the REM content is the total content of these elements.

In the hot-rolled steel sheet according to the embodiment of the present invention, the balance other than the above elements consists of Fe and impurities. The term "impurities" refers to components and the like that are mixed due to various factors in the manufacturing process, including raw materials such as ores and scraps, when hot-rolled steel sheets are industrially manufactured. Examples of impurities include Sn: 0.02% or less, Sb: 0.02% or less, W: 0.015% or less, and Co: 0.015% or less.

[Perlite: 70% or more]
The microstructure of the hot-rolled steel sheet contains 70% or more pearlite in terms of area ratio. Pearlite has the highest work hardening ability among microstructures other than retained austenite. it becomes possible to For example, if ferrite is excessively generated and as a result the area ratio of pearlite is less than 70%, sufficient strength may not be ensured. On the other hand, when bainite is excessively produced and as a result, the area ratio of pearlite is less than 70%, although the strength is improved, the uniform elongation may be deteriorated. Therefore, the area ratio of pearlite is 70% or more, and may be 72% or more, 75% or more, 77% or more, 80% or more, 85% or more, or 90% or more. The upper limit of the area ratio of pearlite is not particularly limited, and may be 100%. For example, the perlite area ratio may be 99% or less, 98% or less, 96% or less, 94% or less, 92% or less, 90% or less, 87% or less, 83% or less, or 79% or less.

[Ferrite: 0 to 30%]
[Bainite: 0 to 30%]
The remaining structure other than pearlite may have an area ratio of 0%, but if there is a remaining structure, it is composed of at least one of ferrite and bainite. Therefore, the area ratio of ferrite and bainite is set to 0 to 30%. Ferrite and bainite may each have an area ratio of 1% or more, 2% or more, 4% or more, or 6% or more. Similarly, ferrite and bainite may each have an area ratio of 25% or less, 20% or less, 15% or less, or 10% or less. The residual structure is composed of ferrite and/or bainite, that is, the residual structure does not contain or substantially does not contain high-hardness, low-toughness martensite or retained austenite that forms the martensite by deformation-induced transformation. Therefore, it is possible to secure good punching end face fatigue characteristics. As used herein, the expression "substantially absent" or "substantially free of" means that the total area fraction of martensite and retained austenite in the residual structure is less than 0.5%. is. It is difficult to accurately measure the total amount of such microstructures, and the effect thereof can be ignored. It is possible to judge The total area ratio of ferrite and bainite is 30% or less, 25% or less, 21% or less, 17% or less, 13% or less, 10% or less, 8% or less, 6% or less, 4% or less, 2 % or less, 1% or less, or 0%. On the other hand, if necessary, the total area ratio of ferrite and bainite is 1% or more, 2% or more, 4% or more, 6% or more, 8% or more, 10% or more, 13% or more, 17 % or more or 21% or more.

[Number density of cementite having a major axis length of more than 0.3 μm and an aspect ratio of less than 3.0 in pearlite: less than 10 per 10 μm ² ]
In the embodiment of the present invention, the number density of coarse spherical cementite among the cementites constituting pearlite is limited within a predetermined range. And cementite having an aspect ratio of less than 3.0 is less than 10 per 10 μm ² . The aspect ratio of cementite refers to a value obtained by dividing the length of the major axis of cementite appearing on the viewing surface by the length of the minor axis. Cementite having a major axis length of more than 0.3 μm and an aspect ratio of less than 3.0 is defined herein as coarse spheroidal cementite. Such coarse spheroidal cementite may become a starting point of void generation during punching of a steel plate, and these voids can be a cause of deterioration of fatigue properties at the punched end face after punching. Therefore, it is extremely important to reduce the amount of such coarse spheroidal cementite in order to improve the fatigue properties of the punched end face. In relation to this, in the embodiment of the present invention, since the number density of the coarse spherical cementite is limited to less than 10 per 10 μm ² , the generation of voids during punching can be reliably suppressed. As a result, it is possible to remarkably improve the fatigue properties of the punched end faces. The number density of coarse spherical cementites may be 8 or less, 6 or less, or 4 or less per 10 μm ² in pearlite. The number density of the coarse spherical cementite may be 0 per 10 μm ² in the pearlite, but may be, for example, 1 or more or 2 or more. Although details will be described later, the aspect ratio refers to the ratio of the length of the major axis to the length of the minor axis of an ellipsoid when ellipsoid approximation processing is performed on individual cementites by image processing.

[Method for measuring area ratio of microstructure]
The area ratio of the microstructure is determined as follows. First, a sample is taken from a position of 1/4 or 3/4 of the plate thickness from the surface of the steel plate so that the cross section parallel to the rolling direction and thickness direction of the steel plate becomes the observation surface. Subsequently, the observation surface is mirror-polished, corroded with a picral corrosive solution, and then subjected to structural observation using a scanning electron microscope (SEM). The measurement area is an area of 12,000 μm ² (for example, an area of 80 μm×150 μm), and the area ratios of pearlite and ferrite are calculated from a structure photograph at a magnification of, for example, about 5000 times using the point counting method. Here, pearlite is a region surrounded by grain boundaries where the ferrite crystal orientation difference is 15° or more, where the ferrite phase and the cementite phase are mixed, and the cementite has a lamellar and/or spherical form. and certify. Therefore, for example, pearlite has a layered (lamellar) dispersed structure of ferrite phase and cementite, as well as a structure mainly composed of cementite dispersed in clusters. It also includes a structure containing more than 50% in terms of area ratio with respect to the total amount of cementite. Further, it is an aggregate of lath-shaped crystal grains, and has a plurality of iron-based carbides having a major axis of 20 nm or more inside the laths, and these carbides are a single variant, that is, an iron-based carbide elongated in the same direction. Those belonging to the group are identified as bainite. Inclusions observed in the pearlite structure are basically cementite, and using a scanning electron microscope with an energy dispersive X-ray spectroscope (SEM-EDS), etc., individual inclusions are identified as cementite or iron-based carbides. need not be identified. Only when there is doubt that the inclusion is cementite or iron-based carbide, the inclusion may be analyzed using SEM-EDS or the like separately from SEM observation, if necessary. Retained austenite has a cementite area fraction of less than 1% inside, and if such a structure exists, it is analyzed using electron back scatter diffraction (EBSD) after observing the structure with SEM. , fcc structure is determined as retained austenite.

[Method for measuring the number density of cementite having a major axis length of more than 0.3 μm and an aspect ratio of less than 3.0 in pearlite]
The number density of coarse spherical cementite is determined as follows. First, a sample is taken from a position of 1/4 or 3/4 of the plate thickness from the surface of the steel plate so that the cross section parallel to the rolling direction and thickness direction of the steel plate becomes the observation surface. Subsequently, the observation surface is mirror-polished, corroded with a picral corrosive solution, and then subjected to structural observation using a scanning electron microscope (SEM). The measurement area is a 12,000 μm ² (for example, an area of 80 μm×150 μm) SEM photograph of about 5000 times, and the image of the area recognized as pearlite is binarized, and the dark part is ferrite and the bright part is cementite. . Of these, for each cementite, ellipsoid approximation is performed by image processing, and the length of the major axis and the length of the minor axis of the ellipsoid are calculated as the length of the major axis and the length of the minor axis of each cementite. and the aspect ratio of each cementite is defined by the following equation.
[aspect ratio] = [long axis length] / [short axis length]
In one field of view of 80 μm × 150 μm, the number of cementites having a long axis length of more than 0.3 μm and an aspect ratio of less than 3.0 as defined by the above method was calculated by image processing. The value converted per 10 μm ² is determined as the number density of coarse spherical cementite defined in the present invention.

[Mechanical properties]
A hot-rolled steel sheet having the above chemical composition and structure can achieve a high tensile strength, specifically a tensile strength of 900 MPa or more. The tensile strength is preferably 910 MPa or higher or 920 MPa or higher, more preferably 940 MPa or higher or 980 MPa or higher, and most preferably 1000 MPa or higher or 1080 MPa or higher. Although it is not necessary to specify the upper limit, for example, the tensile strength may be 1500 MPa or less or 1400 MPa or less. Similarly, the hot-rolled steel sheet having the above chemical composition and structure can achieve high uniform elongation, more specifically 7.0% or more, preferably 7.5% or more, and more Preferably, a uniform elongation of 8.0% or more can be achieved. Although it is not necessary to specify the upper limit, for example, the uniform elongation may be 20.0% or less or 15.0% or less. The tensile strength and uniform elongation are measured by taking a No. 5 tensile test piece of JIS Z2241:2011 from the direction perpendicular to the rolling direction of the hot-rolled steel sheet and performing a tensile test in accordance with JIS Z2241:2011. be. Uniform elongation means plastic elongation (%) at maximum test force defined in JIS Z2241:2011.

Similarly, a hot-rolled steel sheet having the chemical composition and structure described above can achieve high punch end face fatigue properties. More specifically, in the punching fatigue limit ratio corresponding to the value (σ _F /TS) obtained by dividing the fatigue limit σ _F (MPa) determined based on the fatigue test of the punched end face by the tensile strength TS (MPa) A punched edge fatigue property of 0.28 or higher can be achieved. The punching fatigue limit ratio is preferably 0.30 or more, more preferably 0.32 or more. Although there is no need to specify the upper limit, for example, the punching fatigue limit ratio may be 0.42 or less or 0.40 or less. The punching fatigue limit ratio is determined by the following method using a plate-shaped test piece having the dimensions shown in FIG. First, a plate-shaped test piece (30 mm × 90 mm) sampled from a hot-rolled steel plate so that the rolling direction is the long side is punched in the center with a punch diameter of 10 mm and a punching clearance of 12%, and then a constant stress amplitude. A double-sided plane bending fatigue test is performed at σ (MPa). The plate-shaped test piece may be chamfered at the corners of the plate end surface as shown in FIG. A double-sided plane bending fatigue test was performed until the number of repetitions N reached 10 ⁷ times, and the maximum stress amplitude among the tests that did not lead to fracture was defined as the fatigue limit σ _F (MPa), and the fatigue limit σ _F (MPa ) divided by the tensile strength TS (MPa) (σ _F /TS) is determined as the punching fatigue limit ratio.

[Thickness]
A hot rolled steel sheet according to an embodiment of the present invention generally has a thickness of 1.0 to 6.0 mm. Although not particularly limited, the plate thickness may be 1.2 mm or more, 1.6 mm or more, or 2.0 mm or more, and/or may be 5.0 mm or less, 4.0 mm or less, or 3.0 mm or less. .

<Method for manufacturing hot-rolled steel sheet>
Next, a preferred method for manufacturing the hot-rolled steel sheet according to the embodiment of the present invention will be described. The following description is intended to exemplify a characteristic method for manufacturing a hot-rolled steel sheet according to embodiments of the present invention, wherein the hot-rolled steel sheet is manufactured by a manufacturing method as described below. It is not intended to be limited to what is manufactured.

A preferred method for manufacturing a hot-rolled steel sheet according to an embodiment of the present invention includes:
heating a slab having the chemical composition described above in relation to hot rolled steel to 1150° C. or above;
A hot rolling step including finish rolling of a heated slab, wherein the delivery side temperature of the finish rolling is 820 to 920 ° C.;
The obtained steel sheet is primary cooled from the finish rolling delivery side temperature to the primary cooling end temperature of 720 ° C. or less at an average cooling rate of 50 ° C./sec or more, and then to the coiling temperature at an average cooling rate of 10 ° C./sec or less. a cooling step comprising secondary cooling with
coiling the steel sheet at a coiling temperature of 580-650°C; and an additional cooling step comprising cooling the coiled steel sheet to a steel sheet temperature of 550°C or less, wherein the steel sheet temperature is 550°C after coiling. It is characterized by including an additional cooling step with a time to ℃ of 30-180 minutes. Each step will be described in detail below.

[Slab heating process]
First, a slab having the chemical composition described above in relation to hot rolled steel is heated prior to hot rolling. The heating temperature of the slab is set to 1150° C. or higher so that Ti carbonitrides and the like are fully dissolved again. Although the upper limit is not particularly specified, it may be 1250° C., for example. Also, the heating time is not particularly limited, but may be, for example, 30 minutes or more and/or 120 minutes or less. The slab to be used is preferably cast by continuous casting from the viewpoint of productivity, but may be produced by ingot casting or thin slab casting.

[Hot rolling process]
(rough rolling)
In this method, for example, the heated slab may be subjected to rough rolling before finish rolling in order to adjust the plate thickness or the like. Conditions for the rough rolling are not particularly limited as long as the desired sheet bar dimensions can be secured.

(finish rolling)
The heated slab or the slab that has been rough rolled as necessary is then subjected to finish rolling, and the delivery side temperature in the finish rolling is controlled at 820 to 920°C. If the delivery-side temperature of finish rolling exceeds 920°C, the accumulation of work strain in austenite during cooling is insufficient, pearlite transformation is delayed, and a pearlite fraction of 70% or more cannot be achieved. For this reason, the upper limit of the outlet temperature of the finishing temperature is 920°C, preferably 915°C, more preferably 910°C. From this point of view, there is no particular need to set a lower limit to the delivery side temperature of finish rolling if the Ar is 3 points or more, but the lower the temperature, the greater the deformation resistance of the steel sheet, which places a great burden on the rolling mill and causes equipment trouble. can cause Therefore, the lower limit of the delivery side temperature of finish rolling is set to 820°C.

[Cooling process]
After finish rolling, the steel sheet is cooled. The cooling process is further subdivided into primary cooling and secondary cooling.

(Primary cooling: cooling at 50°C/sec or more to 720°C or less)
In the cooling step, first, the steel sheet is primarily cooled from the finish rolling delivery side temperature to the primary cooling end temperature of 720° C. or less at an average cooling rate of 50° C./second or more. If the average cooling rate to the primary cooling end temperature is less than 50°C/sec or the primary cooling end temperature is more than 720°C, a large amount of ferrite is generated and a pearlite fraction of 70% or more is achieved. I can't do it. The average cooling rate of primary cooling may be 52° C./second or more. Although the upper limit of the average cooling rate is not particularly limited, for example, the average cooling rate of the primary cooling is preferably 200 ° C./sec or less in order to obtain the desired structure, and even if it is 100 ° C./sec or less. good.

(Secondary cooling: cooling to winding temperature at 10°C/sec or less)
Subsequently, in secondary cooling, the steel sheet is cooled from the primary cooling end temperature to the coiling temperature (that is, the temperature range of 580 to 650° C.) at an average cooling rate of 10° C./sec or less. If the average cooling rate of the secondary cooling is higher than 10°C, temperature unevenness is likely to occur in the thickness direction and the width direction of the steel sheet, resulting in variations in the metal structure. By setting the average cooling rate of the secondary cooling to 10° C./sec or less, it is possible to reliably reduce such variations in the metal structure in the plate thickness direction and plate width direction of the steel plate. The average cooling rate of secondary cooling is preferably 9° C./sec or less. The lower limit of the average cooling rate is not particularly limited, but from the viewpoint of productivity, the average cooling rate of secondary cooling is set to 1° C./second or more, and may be 2° C./second or more. In order to obtain the effect of dividing the cooling process into two stages, the secondary cooling is preferably performed immediately after the primary cooling is completed.

[Winding process]
After the cooling process, the steel sheet is coiled. The temperature of the steel sheet during winding is 580 to 650°C. If the coiling temperature is lower than 580° C., a large amount of bainite is produced, making it impossible to achieve a pearlite fraction of 70% or more. On the other hand, if the coiling temperature is higher than 650° C., the precipitated Cu particles become coarse, and the precipitation strengthening ability of Cu cannot be obtained sufficiently. As a result, it becomes impossible to achieve a tensile strength of 900 MPa or more. Alternatively, too much ferrite is produced and sufficient tensile strength cannot be achieved. By controlling the coiling temperature to 580 to 650° C., it is possible to achieve a pearlite fraction of 70% or more while miniaturizing the Cu particles precipitated with such pearlite transformation. Therefore, the precipitation strengthening effect of Cu can be fully exhibited, and as a result, the steel sheet strength can be significantly improved, and more specifically, a tensile strength of 900 MPa or more can be achieved. The coiling temperature may be 584°C or higher and/or may be 640°C or lower.

[Additional cooling step: 30 to 180 minutes until the temperature reaches 550°C after winding]
In this manufacturing method, an additional cooling step, that is, a cooling step after winding, is performed after the winding step, and in the cooling step after winding, the steel sheet is cooled from the winding temperature to a steel sheet temperature of 550 ° C. or less. . During this cooling, it is important to control the time required for the temperature of the steel sheet to reach 550° C. within the range of 30 to 180 minutes after winding. If the steel sheet temperature reaches 550°C in less than 30 minutes, a large amount of bainite is generated and pearlite is not sufficiently generated, so that a pearlite fraction of 70% or more cannot be achieved. On the other hand, if this time exceeds 180 minutes, the cementite in the produced pearlite will coarsen and become spherical. In this case, the number density of cementite in which the length of the long axis in pearlite is more than 0.3 μm and the aspect ratio is less than 3.0 cannot be limited to less than 10 per 10 μm ² , and voids are formed when punching a steel plate. It becomes impossible to sufficiently suppress the occurrence. On the other hand, after the coiling process is properly performed as described above, the cooling time until the steel plate temperature reaches 550 ° C. is controlled within the range of 30 to 180 minutes, so that the pearlite fraction is 70% or more. It is possible to obtain a desired pearlite structure in which coarse spherical cementite is reduced while surely achieving the above. As a result, the occurrence of voids during punching can be reliably suppressed, and the fatigue properties of the punched end faces can be significantly improved. For example, the time required for the steel sheet temperature to reach 550° C. may be 35 minutes or more and/or may be 150 minutes or less. The time required for the steel plate temperature to reach 550°C can be adjusted by any appropriate method. For example, if the coiling temperature is around 580°C, cover the coil with a heat insulating cover, etc., as necessary, in order to ensure that the steel sheet temperature reaches 550°C for at least 30 minutes. may

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

In the following examples, hot-rolled steel sheets according to embodiments of the present invention were produced under various conditions, and the mechanical properties of the obtained hot-rolled steel sheets were investigated.

First, a slab having the chemical composition shown in Table 1 was manufactured by continuous casting. Then, from these slabs, hot-rolled steel sheets with a thickness of 2.5 mm were produced under the heating, hot rolling, cooling, coiling and additional cooling conditions shown in Table 2. The balance other than the components shown in Table 1 is Fe and impurities. In addition, the chemical composition obtained by analyzing the sample taken from the manufactured hot-rolled steel sheet is the same as the chemical composition of the slab shown in Table 1, especially the Sn and Sb contents in the impurities are 0.02% or less, The W and Co contents were 0.015% or less.

A No. 5 tensile test piece of JIS Z2241: 2011 was taken from the hot-rolled steel sheet thus obtained in a direction perpendicular to the rolling direction, and a tensile test was performed in accordance with JIS Z2241: 2011 to determine the tensile strength ( TS) and uniform elongation (uEl) were measured. The fatigue properties of punched end faces were evaluated by the following method using plate-shaped test pieces having the dimensions shown in FIG. First, a plate-shaped test piece (30 mm × 90 mm) sampled from a hot-rolled steel plate so that the rolling direction is the long side is punched in the center with a punch diameter of 10 mm and a punching clearance of 12%, and then a constant stress amplitude. A double-sided plane bending fatigue test was performed at σ (MPa). The plate-shaped test piece may be chamfered at the corners of the plate end surface as shown in FIG. A double-sided plane bending fatigue test was performed until the number of repetitions N reached 10 ⁷ times, and the maximum stress amplitude among the tests that did not lead to fracture was defined as the fatigue limit σ _F (MPa), and the fatigue limit σ _F (MPa ) divided by the tensile strength TS (MPa) (σ _F /TS) was determined as the punching fatigue limit ratio. A hot-rolled steel sheet having a TS of 900 MPa or more, a uEl of 7.0% or more, and a punching fatigue limit ratio of 0.28 or more was evaluated as a hot-rolled steel sheet having high strength, uniform elongation, and excellent punching edge fatigue characteristics. . The results are shown in Table 3 below.

Referring to Table 3, in Comparative Example 9, a large amount of ferrite was generated due to the low average cooling rate of the primary cooling in the cooling process, and a pearlite fraction of 70% or more could not be achieved. As a result, TS decreased. In Comparative Example 10, since the cooling end temperature of the primary cooling was high, a large amount of ferrite was similarly generated, and a pearlite fraction of 70% or more could not be achieved. As a result, TS decreased. In Comparative Example 11, since the coiling temperature was low, a large amount of bainite was generated, and a pearlite fraction of 70% or more could not be achieved. As a result, the TS improved, but the uEl decreased. In Comparative Example 12, a large amount of ferrite was generated due to the high coiling temperature, and a pearlite fraction of 70% or more could not be achieved. Moreover, in Comparative Example 12, the coiling temperature was high, so that the precipitated Cu particles became coarse, and it is considered that the precipitation strengthening ability of Cu was not sufficiently exhibited. As a result, TS decreased. In Comparative Example 13, a large amount of bainite was generated because the time until the temperature reached 550° C. after winding was short, and although the TS was improved, the uEl was lowered. In Comparative Example 14, since it took a long time to reach 550° C. after winding, the cementite in the pearlite was coarsened and spheroidized, and the number density of such coarse spherical cementite could not be reduced. rice field. As a result, the punching fatigue limit ratio was lowered. In Comparative Examples 19 and 20, since the C and Cr contents were low, pearlite was not formed sufficiently, while ferrite was formed in a large amount, resulting in a decrease in TS. In Comparative Example 21, although the strength was improved due to the high Cr content, the uEl decreased accordingly. In Comparative Example 22, since the Cu content was low, the precipitation strengthening ability of Cu was not sufficiently exhibited, resulting in a decrease in TS. In Comparative Examples 24, 26 and 28, since the Ti, Nb and V contents were high, the formation of coarse spheroidal cementite could not be sufficiently reduced, and the punching fatigue limit ratio was lowered. This result is believed to be due to the consumption of carbon necessary to form the desired pearlite structure with reduced coarse spheroidal cementite due to the formation of a large amount of carbide due to excessive Ti, Nb and V. be done.

In contrast, Examples 1-8, 15-18, 23, 25, 27, and 29-36 have a given chemical composition and microstructure, and in addition, coarse spheroids in pearlite at that microstructure. By reducing the amount of cementite, it is possible to achieve a uEl of 7.0% or more and a punching fatigue limit ratio of 0.28 or more even though the TS has a high strength of 900 MPa or more, and therefore a high A hot-rolled steel sheet with high strength, uniform elongation, and excellent punched edge fatigue properties was obtained. In addition, a similar fatigue test was performed on a test piece from a welded member obtained by arc welding the steel plate of each example. As a result, in all examples, it was possible to achieve high fatigue properties equivalent to those without such welds. It is believed that this is mainly due to the relatively low C content of 0.30% or less.

Claims

The chemical composition, in mass %,
C: 0.20 to 0.30%,
Si: 0.01 to 2.00%,
Mn: 0.50-3.00%,
P: 0.100% or less,
S: 0.0100% or less,
Al: 0.005 to 3.000%,
N: 0.0100% or less,
O: 0.0100% or less,
Cr: more than 1.00 to 3.00%,
Cu: more than 1.00 to 3.00%,
Ti: 0 to 0.10%,
Nb: 0 to 0.10%,
V: 0 to 0.10%,
Ni: 0 to 2.00%,
Mo: 0 to 1.00%,
B: 0 to 0.0100%,
Ca: 0 to 0.0050%,
REM: 0-0.005%, and balance: Fe and impurities,
The microstructure is the area ratio,
Perlite: 70% or more,
ferrite: 0-30%, and bainite: 0-30%,
Less than 10 pieces of cementite per 10 μm 2 having a major axis length of more than 0.3 μm and an aspect ratio of less than 3.0 in the pearlite,
A hot-rolled steel sheet characterized by having a tensile strength of 900 MPa or more.
The chemical composition, in mass %,
Ti: 0.001 to 0.10%,
Nb: 0.001 to 0.10%,
V: 0.001 to 0.10%,
Ni: 0.001 to 2.00%,
Mo: 0.001 to 1.00%,
B: 0.0001 to 0.0100%,
Ca: 0.0001-0.0050%, and REM: 0.0001-0.005%
The hot-rolled steel sheet according to claim 1, comprising one or more selected from the group consisting of
The hot rolled steel sheet according to claim 1 or 2, characterized by having a thickness of 1.0 to 6.0 mm.