JP2000319756A - Hot rolled steel sheet for working excellent in fatigue characteristic and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the fatigue characteristics and workability of the steel sheet by allowing it to have a specified elemental compsn. and a microstructure of the composite one consisting of ferrite as the main phase and allowing Cu in the ferritic phase to exist in the precipitated state and/or solid solution state in which the size of the grains composed of Cu alone is controlled to the value equal to or below a specified one. SOLUTION: The steel compsn. is composed of, by mass, 0.03 to 0.20% C, >1.4 to 2.5% Si, 0.5 to 3.0% Mn, <=0.02% P, <=0.01% S, 0.005 to 1.0% Al, 0.2 to 2.0% Cu and 0.0002 to 0.0020% B, and the balance Fe with inevitable impurities. Moreover, its microstructure is formed of the composite one consisting of ferrite as the main phase and martensite or martensite and bainite as a 2nd phase. Then, Cu in the ferritic phase exists in the precipitated state and/or solid solution state in which the size of the grains composed of Cu alone is controlled to <=2 nm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled steel sheet for processing having excellent fatigue characteristics and a method for producing the same, and more particularly, to achieving both durability and workability of undercarriage parts and road wheels of automobiles. The present invention relates to a hot-rolled steel sheet for processing excellent in fatigue properties suitable as a required material and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, the application of light metals such as Al alloys and high-strength steel sheets to automobile members has been promoted for the purpose of weight reduction in order to improve fuel efficiency of automobiles. However, light metals such as Al alloys have the advantage of high specific strength,
Due to their significant cost compared to steel, their application has been limited to special applications. In order to reduce the weight of automobiles in a wider range, there is a strong demand for the use of inexpensive high-strength steel sheets. In general, the higher the strength of a material, the lower the ductility and workability (formability) as well as the notch sensitivity. Therefore, when applying a high-strength steel plate to a vehicle undercarriage having a complicated shape, not only the formability but also the fatigue durability are important considerations.

As a high-strength hot-rolled steel sheet having excellent workability, an invention for obtaining a high-strength steel sheet having a low yield ratio and excellent ductility in a microstructure mainly composed of ferrite and martensite has been disclosed, for example, in Japanese Patent Application Laid-Open No. H10-163,897. JP-A-58-6937 and JP-A-6-1983.
No. 0-112225. In particular, an invention for obtaining a high-strength steel sheet having excellent stretch flangeability (hole expanding property) with a microstructure mainly composed of ferrite and bainite has been disclosed in, for example, JP-A-57-145965 and JP-A-61-1986. It is disclosed in, for example, Japanese Patent No. 96057.
Furthermore, a high-strength steel sheet that combines these characteristics
The invention obtained with a microstructure mainly composed of ferrite, bainite and martensite is disclosed in, for example, Japanese Patent Application Laid-Open No. Hei 3-2646.
No. 45, for example.

Further, high strength hot rolled steel sheets having excellent fatigue properties are disclosed in JP-A-4-276016 and JP-A-5-3.
In JP-A-31591, JP-A-6-145792, JP-A-8-60240, etc., attention is paid to a specific additive element in order to improve fatigue properties, and solid solution strengthening of P and / or precipitation strengthening of Cu are performed. Is disclosed. That is, Japanese Patent Application Laid-Open No. Hei 4-276016 discloses a technique for improving the fatigue strength by solid solution strengthening of P and precipitation strengthening of Cu.

In Japanese Patent Application Laid-Open No. Hei 5-335991, the microstructure is made of ferrite and martensite or ferrite, martensite and retained austenite, and ε-Cu is precipitated in the ferrite phase to improve fatigue strength and stretch flangeability. Techniques are disclosed. In Japanese Patent Application Laid-Open No. 6-145792, the microstructure is made of three phases of ferrite, bainite and martensite, the strength and stretch flangeability are secured by defining the volume fraction of each phase, and precipitation strengthening of Cu is performed. Discloses a technique for improving fatigue characteristics.

Further, in Japanese Patent Application Laid-Open No. H8-60240, the microstructure is made of three phases of ferrite, bainite and martensite, the volume fraction of each phase is defined, the strength-ductility balance is secured, and the winding temperature is reduced. A technique for improving fatigue characteristics by strengthening the precipitation of Cu at 400 ° C. or higher is disclosed. On the other hand, in Japanese Patent Application Laid-Open No. Hei 9-137349, the microstructure is made of three phases of ferrite, bainite and martensite, the volume fraction of each phase is specified, and the ferrite phase is precipitated and strengthened with carbides of Ti and Nb. A technique has been disclosed in which the ferrite grain size near the surface and the roughness of the steel sheet surface are specified to improve the fatigue properties.

[0007]

However, in some parts such as a disk of a road wheel, elongation,
Fatigue durability is very important together with workability such as a low yield ratio, and it cannot be said that satisfactory characteristics cannot be obtained with the above-mentioned conventional technology. That is, Japanese Patent Application Laid-Open No.
In the invention described in Japanese Patent Publication No. 16, since it is essential to add 0.05 to 0.12% of P which segregates at crystal grain boundaries and causes grain boundary embrittlement, the grain boundary fracture which is a starting point of fatigue fracture is required. If this occurs, the fatigue properties may be significantly degraded.

Further, the document does not disclose the addition of B which suppresses grain boundary embrittlement due to P.
Further, in the invention described in the above-mentioned Japanese Patent Application Laid-Open No. 5-331591, since ε-Cu is precipitated in the ferrite phase, ductility may be reduced and workability may be deteriorated. Further, in the invention described in JP-A-6-145792, the volume fraction of each phase of ferrite, bainite and martensite is liable to fluctuate due to heat history and the like, so that properties such as ductility are greatly affected. There is a problem that the material tends to vary in the longitudinal direction and the width direction of the steel sheet.

Further, in the invention described in JP-A-8-60240, since the winding temperature is specified to be 400 ° C. or higher, a large amount of bainite or pearlite is generated in the microstructure, and sufficient martensite is formed. Not only is it not possible to obtain a low yield ratio, but also a sufficient fatigue limit ratio cannot be obtained. further,
In the invention described in JP-A-9-137349,
Heating at a high solution temperature is required in a heating furnace process before hot rolling in order to obtain Ti and Nb carbides effective for precipitation strengthening, which is not preferable from the viewpoint of operation cost and energy saving. Therefore, the present invention clarifies the properties of a steel sheet for achieving both fatigue properties and workability and a method for producing the same, and can advantageously solve the above-mentioned problems of the prior art, and has excellent fatigue properties having a strength of 590 MPa or more. It is an object of the present invention to provide a hot-rolled steel sheet for use and a method for producing the same.

[0010]

Means for Solving the Problems The present inventors considered the production process of a hot-rolled steel sheet produced on an industrial scale by a continuous hot-rolling equipment which is currently usually used, and considered the production process of the hot-rolled steel sheet. We conducted intensive research to achieve both fatigue characteristics and workability. As a result, they have found that a solid solution of Cu or a Cu precipitate composed of Cu alone and having a particle size of 2 nm or less is very effective in improving fatigue properties and does not impair workability, and the present invention has been achieved. It was done.

The results of the basic research that led to the present invention will be described below. First, an investigation was made on the effect of the size of a particle composed of Cu alone in the ferrite phase on fatigue characteristics. The test material for that was prepared as follows. That is, 0.05% C-1.6% Si-
1.4% Mn-1.0% Cu-0.5% Ni-0.00
A steel sheet rolled at room temperature by hot-rolling a slab smelted and adjusted to a composition of 03% B is kept at 100-600 ° C. for 1 hour, and then subjected to a heat treatment of furnace cooling, and the microstructure becomes ferrite. Having a composite structure of martensite, or martensite and bainite as a second phase,
A steel sheet was obtained in which the size of particles composed solely of Cu in the ferrite phase was changed.

Here, the second phase is mainly martensite or martensite and bainite, but it is permissible to partially contain retained austenite. In the following, bainite also includes bainitic ferrite. FIG. 1 shows the results of a fatigue test performed on these steel sheets. From these results, in the steel sheet consisting of ferrite and martensite, or martensite and bainite, and a composite structure including partially retained austenite, in the ferrite phase, the average size and the fatigue limit ratio of particles composed of Cu alone in the ferrite phase are: There was a strong correlation, and it was newly found that the fatigue limit ratio was significantly improved when the average size of the particles composed of Cu alone in the ferrite phase was 2 nm or less.

Although this mechanism is not always clear, solid solution of Cu or Cu precipitates having a particle size of 2 nm or less consisting of Cu alone suppresses cross-slip under repeated load in the ferrite phase, and It is presumed that the form of the sliding step of the surface caused by the load is changed from a coarse and deep state to a dense and shallow state, and the stress concentration there is reduced, thereby improving the resistance to fatigue crack initiation. It has also been newly found that a steel sheet having an average size of particles of only Cu of 2 nm or less in the ferrite phase can be manufactured by limiting the hot rolling conditions and the like.

Next, the effect of the element B on the fatigue characteristics was investigated. The test material for that was prepared as follows. That is, 0.05% C-1.6% Si
Based on -1.4% Mn-0.5% Ni steel.
In addition to the steel containing 0% Cu and the steel containing no Cu, the slabs obtained by adjusting the composition of the steel with the changed B content are hot-rolled and rolled at room temperature to obtain a microstructure. Thus, a steel sheet having a composite structure having ferrite as a main phase and martensite or martensite and bainite as a second phase was obtained. FIG. 2 shows the results of a fatigue test performed on these steel sheets. From these results, it is found that there is a strong correlation between the B content concentration and the fatigue limit ratio only for steel to which 1.0% Cu is added, and that the fatigue limit ratio is significantly improved when the B content concentration is 2 ppm or more. I found out.

[0015] The mechanical properties obtained by the tensile test were measured using a No. 5 test piece described in JIS Z 2201 according to JI.
It was measured by the test method described in SZ2241. Further, the fatigue properties of the steel sheet are as shown in FIG.
Using a fatigue test piece having a notch with a radius of curvature of 30 mm, a two-sided flat bending fatigue test was performed using a full swing.
Evaluation was made by dividing the fatigue strength σW at × 10 ⁶ times by the tensile strength σB of the steel sheet (fatigue limit ratio σW / σB).

In the case of particles composed solely of Cu in the ferrite phase, a transmission electron microscope sample was taken from a quarter of the thickness of the test steel and subjected to energy dispersive X-ray spectroscopy (Energy Dispersive X-ray).
Spectroscope (EDS) or electron energy loss spectroscopy (Electron Energy Loss)
Field emission type electron gun (Field Emission Gun: FEG) with an accelerating voltage of 200 kV, which has a composition analysis function of s Spectroscope (EELS).
Observed by a transmission electron microscope equipped with. The composition of the observed particles was determined by EDS and EELS.
u alone. Further, the size of the particles composed solely of Cu in the ferrite phase defined in the present invention is the average value in one field of view of each of the observed particle sizes.

The present invention has been made based on the above findings, and its gist is as follows. (1) In mass%, C: 0.03 to 0.20%, Si:
More than 1.4 to 2.5%, Mn: 0.5 to 3.0%, P: ≦
0.02%, S: ≦ 0.01%, Al: 0.005 to
1.0%, Cu: 0.2 to 2.0%, B: 0.0002
A microstructure having ferrite as a main phase, martensite, or martensite and bainite as a second phase. The presence state of Cu in the ferrite phase is characterized in that particles composed of Cu alone are in a precipitated state and / or a solid solution state having a size of 2 nm or less, and are hot rolled for processing excellent in fatigue characteristics. steel sheet.

(2) The steel further comprises N
i: The hot-rolled steel sheet for processing described in (1), which is excellent in fatigue characteristics, containing 0.1 to 1.0%. (3) The steel further contains, by mass%, Ca: 0.00
(1) or (2), characterized by containing one or two kinds of REM: 0.005 to 0.2%, and having excellent fatigue properties. Rolled steel sheet. (4) The steel further contains Mo: 0.05 in mass%.
1.0%, V: 0.02 to 0.2%, Ti: 0.01
-0.2%, Nb: 0.01-0.1%, Cr: 0.0
The fatigue characteristics according to any one of the above (1) to (3), wherein one or two or more of Zr: 0.02 to 0.2% are contained. Excellent hot rolled steel sheet for processing.

(5) When hot rolling a steel slab having the components described in any one of (1) to (4) above, Ar
After finishing the hot finish rolling at the ₃ transformation point or more, stay in the temperature range from the Ar ₃ transformation point to the Ar ₁ transformation point for 1 to 10 seconds, and then cool at a cooling rate of 20 ° C./s or more, 3
Wound at 50 ° C. below the winding temperature, microstructure, the ferrite as a main phase, a martensite or martensite and bainite composite structure of the second phase,
The presence state of Cu in the ferrite phase is defined as a precipitation state in which the size of the particle composed of Cu alone is 2 nm or less and / or
Alternatively, a method for producing a hot-rolled steel sheet for processing excellent in fatigue characteristics, characterized by obtaining a steel sheet in a solid solution state. (6) In the hot rolling described above, after rough rolling is completed, high-pressure decaling is performed, and hot finish rolling is completed at an Ar ₃ transformation point or higher. It is in a method of manufacturing a hot-rolled steel sheet.

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.
First, the microstructure and the presence of Cu of the steel sheet of the present invention will be described. In order to ensure excellent workability, the microstructure of the steel sheet is a composite structure having ferrite as a main phase and martensite or martensite and bainite as a second phase. However, the second phase permits the inclusion of partially retained austenite. In addition,
In order to ensure good ductility which guarantees good workability, the volume fraction of ferrite is 50% or more and the total volume fraction of bainite and retained austenite is 40%.
The following is preferred. In order to obtain a low yield ratio of 70% or less, it is more preferably 25% or less. Here, the volume fraction of ferrite, martensite, bainite and retained austenite is 1 / th of the cross-sectional thickness in the rolling direction of the steel sheet.
Defined by the area fraction of those structures in the microstructure observed at 200-500 times with an optical microscope at 4 thickness.

The state of the presence of Cu in the ferrite phase is a precipitation state and / or a solid solution state in which the size of the particle composed of Cu alone is 2 nm or less. Thereby, while suppressing an increase in static strength leading to deterioration in workability, that is, impair the excellent workability of a composite structure steel sheet having ferrite as a main phase and martensite, or martensite and bainite as a second phase. Without this, the fatigue characteristics can be improved. On the other hand, if the size of the particles composed solely of Cu in the ferrite phase exceeds 2 nm, the static strength of the steel sheet is significantly increased due to the precipitation strengthening of Cu, so that the workability is significantly deteriorated.
In addition, with such precipitation strengthening of Cu, the fatigue limit does not improve as much as the increase in static strength, so that the fatigue limit ratio decreases. Therefore, the size of the particle composed of Cu alone in the ferrite phase needs to be 2 nm or less.

Next, the reasons for limiting the chemical components of the present invention will be described. If the content of C exceeds 0.20%, the workability and the weldability deteriorate, so the content is set to 0.20% or less. If it is less than 0.03%, the volume fraction of martensite or martensite and bainite, which is the second phase, is reduced, and the strength is reduced. S
i has the effect of promoting the transformation of ferrite and the concentration of C in untransformed austenite, and is effective in increasing the strength as a solid solution strengthening element. . In order to obtain a desired strength in the composite structure, the content needs to be more than 1.4%. However, if the content exceeds 2.5%, the workability deteriorates. Therefore, the content of Si exceeds 1.4% and 2.5%
The following is assumed.

Mn is required to be 0.5% or more in order to obtain the desired second phase, martensite, or martensite and bainite. Further, if added over 3.0%, slab cracks occur, so the content is made 3.0% or less. If P is added in excess of 0.02%, it not only adversely affects the workability and weldability, but also segregates at the grain boundaries, lowering the grain boundary strength and causing grain boundary embrittlement. .

If the content of S is too large, it causes cracking during hot rolling, so it should be reduced as much as possible, but if it is 0.01% or less, it is in an acceptable range. Al needs to be added in an amount of 0.005% or more for deoxidation of molten steel. However, the cost is increased, so the upper limit is set to 1.0%. Further, if added in an excessively large amount, nonmetallic inclusions are increased and elongation is deteriorated, so that the content is preferably 0.5% or less.

Cu is one of the most important elements of the present invention, and has the effect of improving the fatigue properties by forming a solid solution or a precipitate having a particle size of 2 nm or less. However,
If the content is less than 0.2%, the effect is small. Even if the content exceeds 2.0%, the effect is saturated, so the content range is limited to 0.2 to 2.0%. B is one of the most important elements of the present invention, and has an effect of increasing the fatigue limit by being combined with Cu. However, if it is less than 0.0002%, it is insufficient to obtain the effect, and 0.0020%
%, Slab cracking occurs. Therefore, the addition of B is set to 0.0002% or more and 0.0020% or less.

Ni is added to prevent hot brittleness due to the inclusion of Cu. However, if the content is less than 0.1%, the effect is small, and if the content exceeds 1.0%, the effect is saturated. Therefore, the content is set to 0.1 to 1.0%. Ca and REM are
It is an element that changes the form of non-metallic inclusions that become the starting point of destruction or deteriorates workability and renders them harmless. However,
Even if added less than 0.005%, there is no effect. If Ca is added more than 0.02%, and if REM is added more than 0.2%, the effect is saturated, so Ca = 0.005-0.02. %,
REM = 0.005 to 0.2%.

Further, in order to impart strength, Mo,
One or two or more elements of precipitation strengthening or solid solution strengthening of V, Ti, Nb, Cr and Zr may be added. However, 0.05%, 0.02%, 0.01%,
If it is less than 0.01%, 0.01% or 0.02%, the effect cannot be obtained. In addition, 1.0%,
Even if it exceeds 0.2%, 0.2%, 0.1%, 1.0% and 0.2%, the effect is saturated.

Next, the reasons for limiting the production method of the present invention will be described in detail below. In the present invention, a slab obtained by casting molten steel whose components have been adjusted so as to have a target component content may be directly sent to a hot rolling mill as a high-temperature slab, or a heating furnace after cooling to room temperature. And then hot-rolled. The reheating temperature is not particularly limited, but if it is 1350 ° C. or higher, the scale-off amount becomes large and the yield decreases, so the reheating temperature is 1350 ° C.
Desirably less than ° C.

In the hot rolling step, finish rolling is performed after rough rolling is completed, but it is necessary to finish the final pass temperature (FT) in a temperature range not lower than the Ar ₃ transformation point. This is because if the rolling temperature falls below the Ar ₃ transformation point during hot rolling, strain remains in the ferrite grains and ductility decreases. Here, when high-pressure descaling is performed after the completion of rough rolling, the collision pressure P (MPa) of high-pressure water on the steel sheet surface × flow rate L (liter / liter)
cm ² ) ≧ 0.0025 is preferably satisfied.

The collision pressure P of the high-pressure water on the steel plate surface is described as follows. ("Iron and Steel" 1991 vol. 77
No. 9 P1450) P (MPa) = 5.64 × P ₀ × V / H ² where P ₀ (MPa): liquid pressure V (liter / min): nozzle flow H (cm): steel sheet surface and nozzle Distance between

The flow rate L is described as follows. L (liter / cm ² ) = V / (W × v), where V (liter / min): Nozzle flow amount W (cm): Width of jet liquid per nozzle hitting steel sheet surface v (cm / min): Stripping speed

Further, the maximum height R of the steel sheet after the finish rolling is performed.
y is 15 μm (15 μm Ry, 12.5 mm, ln1
2.5 mm) or less. This is apparent from the fact that the fatigue strength of a hot-rolled or pickled steel sheet is correlated with the maximum height Ry of the steel sheet surface, as described in, for example, Handbook of Fatigue Design for Metallic Materials, edited by The Society of Materials Science, Japan, page 84. is there. Further, the subsequent finish rolling is performed in order to prevent scale from being formed again after descaling.
It is desirable to do this within seconds.

After finishing the finish rolling, the process
It stays for 1 to 10 seconds in the temperature range from the r ₃ transformation point to the Ar ₁ transformation point (two-phase region of ferrite and austenite).
The retention here is performed to promote ferrite transformation in the two-phase region, but if it is less than 1 second, sufficient ductility cannot be obtained because the ferrite transformation in the two-phase region is insufficient. On the other hand, if it exceeds 10 seconds, pearlite is formed, and a microstructure having the intended ferrite as the main phase and martensite or martensite and bainite as the second phase cannot be obtained. Further, the temperature range in which the stagnation is performed for 1 to 10 seconds is desirably from the Ar ₁ transformation point to 800 ° C. in order to easily promote the ferrite transformation. For that purpose, the cooling rate is 20 ° C./s or more after the finish rolling. Preferably, the temperature range is reached quickly.

Next, cooling is performed at a cooling rate of 20 ° C./s or more from the temperature range to the winding temperature (CT).
At a cooling rate of less than s, pearlite or a large amount of bainite is generated, and sufficient martensite cannot be obtained, and the intended ferrite is used as a main phase, and martensite,
Alternatively, a microstructure having martensite and bainite as a second phase cannot be obtained. If the winding temperature exceeds 350 ° C,
A large amount of bainite is generated and sufficient martensite cannot be obtained, so that not only the desired ferrite as a main phase and a microstructure with martensite or martensite and bainite as a second phase cannot be obtained, but also winding. The winding temperature is limited to 350 ° C. or lower because there is a possibility that Cu of a size having a large precipitation strengthening ability in static strength may later be generated. The lower limit of the winding temperature is not particularly limited, but if the coil is in a wet state for a long time, the appearance may be poor due to rust.

[0035]

The present invention will be further described below with reference to examples. The steels A to Z having the chemical components shown in Table 1 were melted in a converter and continuously cast, and then heated at a temperature shown in Table 2 (SR
T), and after rough rolling, after rolling to a plate thickness of 1.2 to 5.4 mm at the finish rolling temperature (FT) also shown in Table 2, after stagnation for the time shown in Table 2, shown in Table 2 Cooling rate (C
R) and cooled at a winding temperature (CT).
In addition, after rough rolling, high pressure descaling was performed under the conditions of a collision pressure of 2.7 MPa and a flow rate of 0.001 liter / cm ² . However, the indication of the chemical composition in the table is% by mass. In the tensile test of the hot-rolled sheet thus obtained, the test material was first processed into a No. 5 test piece described in JIS Z 2201, and was subjected to a test method described in JIS Z2241. Table 2 shows the test results. The thickness of the 1/4 thickness of the cross section in the rolling direction of the steel sheet is 200 to 50
Table 2 also shows the volume ratio of the tissue observed at 0 magnification.

[0036]

[Table 1]

[0037]

[Table 2]

Further, as shown in FIG.
A plane bending fatigue test of complete swinging was performed on a plane bending fatigue test piece having a width of 38 mm, a minimum cross section width of 20 mm, and a notch with a radius of curvature of 30 mm. The fatigue properties of the steel sheet were evaluated by the value obtained by dividing the fatigue strength σW at 2 × 10 ⁶ times by the tensile strength σB of the steel sheet (fatigue limit ratio σW / σB).
For particles composed of Cu alone in the ferrite phase, a transmission electron microscope sample was taken from a quarter of the thickness of the test steel and subjected to energy dispersive X-ray spectroscopy (EDS).
Observation was made with a transmission electron microscope equipped with a field emission electron gun (FEG) with an accelerating voltage of 200 kV and a function of composition analysis of electron energy loss spectroscopy (EELS). The observed particle composition was determined by the EDS and EEL described above.
It was confirmed by S that Cu was solely Cu. Further, the size of a particle composed of Cu alone in the ferrite phase defined in the present application is an average value in one visual field of each of the measured particle sizes.

According to the present invention, steels A-1, C-1,
D-1, E-1, F-1, F-2, F-3, I-1, J
-1, K-1, L-1, M-1, O-1, P-1, Q-
1, Q-2, Q-3, Q-4, Q-5, Q-6, Q-
7, R-1 is a 22 steel with a ferrite as a main phase, a martensite or a composite structure with martensite and bainite as a second phase, and the presence state of Cu in the ferrite phase is composed of Cu alone. A hot-rolled steel sheet for processing having excellent fatigue characteristics in which the size of the particles is 2 nm or less in a precipitated state and / or a solid solution state is obtained.

Other steels are outside the scope of the present invention for the following reasons. That is, since the content of C in steel B-1 is out of the range of the present invention, the volume ratio of martensite in the microstructure is not sufficient, and a sufficient yield limit at a low yield ratio is not obtained. In steel G-1, the content of P is out of the range of the present invention, so that P segregates at the grain boundary and lowers the grain boundary strength, so that a sufficient fatigue limit ratio is not obtained. Steel H-1
However, since the content of Cu is out of the range of the present invention, the effect of improving the fatigue properties is small and a sufficient fatigue limit ratio cannot be obtained.

In steels K-2 and K-5, the residence time after finish rolling is out of the range of the present invention, so that the target ferrite is the main phase and martensite or martensite and bainite are the second phase. A microstructure that does not have a high yield ratio and a sufficient fatigue limit ratio cannot be obtained. Steel K-
No. 3 is a microstructure in which pearlite is formed because the cooling rate (CR) after stagnation is out of the range of the present invention and the intended ferrite is the main phase, and martensite or martensite and bainite are the second phase. And a sufficient yield limit was not obtained at a low yield ratio (YR).
Since the finish rolling temperature (FT) of the steel K-4 is out of the range of the present invention, not only the strain remains in the ferrite grains to reduce the ductility, but also the low yield ratio (YR) is not obtained.

The steel K-6 has a winding temperature (C) after hot rolling.
Since T) is out of the range of the present invention, the desired second phase martensite cannot be obtained. Further, the size of the particle composed of Cu alone becomes 2 nm or more. Therefore, a sufficient fatigue limit ratio (σW / σB) has not been obtained. Steel N
-1, the target strength was not obtained because the Si content was outside the range of the present invention. In steel S-1, the content of B is out of the range of the present invention, so that the effect of improving the fatigue properties, which is manifested by being combined with Cu, cannot be obtained, and a sufficient fatigue limit ratio has not been obtained. Since the content of Mn of steel T-1 is out of the range of the present invention, the desired martensite of the second phase is not sufficiently obtained, and a low yield ratio is not obtained.

[0043]

As described in detail above, the present invention provides a 590
It is intended to provide a hot-rolled steel sheet for processing having excellent fatigue properties having a strength of not less than MPa and a method for producing the hot-rolled steel sheet. By using these hot-rolled steel sheets, it is possible to sufficiently secure workability such as elongation. Since significant improvement in fatigue characteristics can be expected, the present invention can be said to be an invention having high industrial value.

[Brief description of the drawings]

FIG. 1 is a diagram showing the results of preliminary experiments leading to the present invention in the relationship between the size of particles composed of Cu alone and the fatigue limit ratio.

FIG. 2 is a diagram showing the results of preliminary experiments leading to the present invention in the relationship between the concentration of B element and the fatigue limit ratio.

FIG. 3 is a diagram illustrating the shape of a fatigue test piece.

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[Procedure amendment]

[Submission date] May 26, 1999 (1999.5.2
6)

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0037

[Correction method] Change

[Correction contents]

[0037]

[Table 2]

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C22C 38/58 C22C 38/58 (72) Inventor Manabu Takahashi 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation 4K037 EA01 EA02 EA05 EA06 EA09 EA11 EA13 EA15 EA16 EA17 EA19 EA20 EA23 EA25 EA28 EA31 EA32 EA35 EA36 EB05 EB07 EB09 EB11 FB04 FB10 FC07

Claims (6)

[Claims] 1. In mass%, C: 0.03 to 0.20%, Si: more than 1.4 to 2.5%, Mn: 0.5 to 3.0%, P: ≦ 0.02 %, S: ≦ 0.01%, Al: 0.005 to 1.0%, Cu: 0.2 to 2.0%, B: 0.0002 to 0.0020%, the balance being Fe and inevitable Microstructure having a main phase of ferrite and martensite or a composite phase of martensite and bainite as a second phase.
The hot-rolled steel sheet for processing excellent in fatigue characteristics, characterized in that u exists in a precipitated state and / or a solid-solution state in which the size of particles composed of Cu alone is 2 nm or less. 2. The hot-rolling process according to claim 1, wherein the steel further contains, by mass%, Ni: 0.1 to 1.0%. steel sheet. 3. The steel according to claim 1, further comprising one or two of Ca: 0.005 to 0.02% and REM: 0.005 to 0.2% by mass%. The hot-rolled steel sheet for processing according to claim 1 or 2, having excellent fatigue properties. 4. The steel further comprises, by mass%: Mo: 0.05 to 1.0%, V: 0.02 to 0.2%, Ti: 0.01 to 0.2%, Nb : 0.01 to 0.1%, Cr: 0.01 to 1.0%, Zr: 0.02 to 0.2%, one or more of the following: A hot-rolled steel sheet for processing according to any one of claims 3 to 5, which is excellent in fatigue properties. 5. The method as claimed in claim 1, wherein the first and second steps are different from each other.
In hot rolling of a slab having the components described in_ThreeStrange
After finishing hot finish rolling at or above the _ThreeA transformation point
Ar₁Stay in the temperature range up to the transformation point for 1 to 10 seconds.
After that, it is cooled at a cooling rate of 20 ° C./s or more,
Winding at the following winding temperature, the microstructure becomes ferrite
With martensite or martensite or
And bainite as the second phase.
The existence state of Cu in the iron phase is composed of Cu alone.
Particles having a size of 2 nm or less and / or
Fatigue characteristics characterized by obtaining a steel plate in a solid solution state
Excellent method of manufacturing hot rolled steel sheet for processing. 6. The method according to claim 5, wherein high-pressure descaling is performed after the rough rolling in the hot rolling, and the hot finish rolling is completed at an Ar ₃ transformation point or higher. Manufacturing method of hot rolled steel sheet for processing.