JP2000319731A - Production of hot rolled steel sheet for working excellent in fatigue characteristic - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve both the fatigue characteristics and workability of the steel sheet by subjecting a slab having a specified compsn. to rough rolling, thereafter executing high pressure descaling, subjecting it to hot finish rolling under specified conditions, executing heating and cooling and forming its structure into a specified one. SOLUTION: A slab composed of, by mass, 0.03 to 0.20% C, 0.1 to 1.4% 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, 0.0002 to 0.0020% B, and the balance Fe with inevitable impurities is subjected to rough rolling, is thereafter subjected to high pressure descaling and is subjected to hot finish rolling at >=Ar3 point. Then, it is retained in a range from the Ar3 point to the Ar1 point for 1 to 10 sec, is thereafter cooled at >=20 deg.C/sec and is coiled at <=350 deg.C. As a result, the steel sheet whose microstructure is the composite one composed of ferrite as the main phase and martensite as a 2nd. phase, and in which Cu in the ferritic phase exists in the precipitated and/or solid solution state in which the size of particles composed of Cu alone is <=2 nm can be obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[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 a method of manufacturing steel sheet characteristics for achieving both fatigue characteristics and workability, and can advantageously solve the above-mentioned problems of the prior art, and provides a method of manufacturing a hot-rolled steel sheet for processing excellent in fatigue characteristics. The purpose is to provide.

[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-1.0% 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. And a steel sheet having a composite structure having martensite as a second phase and having a ferrite phase in which the size of particles composed solely of Cu was changed.

The second phase here is mainly martensite, but it is permissible to partially contain retained austenite. FIG. 1 shows the results of a fatigue test performed on these steel sheets. From this result, in the steel sheet having a composite structure including ferrite and martensite and partially retained austenite, there is a strong correlation between the average size of particles composed solely of Cu in the ferrite phase and the fatigue limit ratio. Cu
It has been newly found that the fatigue limit ratio is remarkably improved when the average size of a single particle is 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-1.0% 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 including ferrite as a main phase and martensite as a second phase was obtained. FIG. 2 shows the results of a fatigue test performed on these steel sheets. From these results, 1.
Only for steel containing 0% Cu, there is a strong correlation between the B content concentration and the fatigue limit ratio.
From the above, it was newly found that the fatigue limit ratio was significantly improved.

[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 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. The present invention has been made based on the above findings, and the gist thereof is as follows.

(1) In mass%, C: 0.03 to 0.2
0%, Si: 0.1-1.4%, Mn: 0.5-3.
0%, P: ≦ 0.02%, S: ≦ 0.01%, Al:
0.005 to 1.0%, Cu: 0.2 to 2.0%, B:
After the rough rolling of a steel slab containing 0.0002 to 0.0020%, the balance being Fe and inevitable impurities, high-pressure descaling is performed, and after hot finish rolling at the Ar ₃ transformation point or higher, Ar _It stays in the temperature range from the ₃ transformation point to the Ar ₁ transformation point for 1 to 10 seconds, and then is cooled at a cooling rate of 20 ° C./s or more, and is wound up at a winding temperature of 350 ° C. or less. This is a composite structure having ferrite as a main phase and martensite as a second phase, and the presence state of Cu in the ferrite phase is defined as a precipitation state and / or a solid solution state in which the size of a particle composed of Cu alone is 2 nm or less. A method for producing a hot-rolled steel sheet for processing having excellent fatigue characteristics, characterized by obtaining a steel sheet which is a steel sheet.

(2) The steel further comprises N
i: The method for producing a hot-rolled steel sheet for processing having excellent fatigue properties according to the above (1), which comprises 0.1 to 1.0%. (3) The steel further comprises, by mass%, Ca:
0.005 to 0.02%, REM = 0.005 to 0.2
(1) or (2), the method for producing a hot-rolled steel sheet for processing excellent in fatigue characteristics according to (1) or (2). (4) The steel further contains, by mass%, Mo: 0.05 to 1.0% and V: 0.02 to 0.2.
%, Ti: 0.01-0.2%, Nb: 0.01-0.
1%, Cr: 0.01-1.0%, Zr: 0.02-
The method for producing a hot-rolled steel sheet for processing according to any one of the above (1) to (3), characterized by containing 0.2% of one or more kinds.

[0019]

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. The microstructure of the steel sheet is a composite structure having ferrite as a main phase and martensite as a second phase in order to ensure excellent workability. However, the second phase permits the inclusion of partially retained austenite. It should be noted that good ductility and 70% to guarantee good workability.
%, The volume fraction of ferrite is preferably 50% or more and the volume fraction of retained austenite is less than 5%. Here, the volume fractions of ferrite, martensite and retained austenite are defined as 20% by an optical microscope at a thickness of 1/4 of the cross-sectional thickness in the rolling direction of the steel sheet.
Defined by the area fraction of those microstructures in the microstructure observed from 0 to 500 times.

The presence state 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, it is possible to improve the fatigue properties while suppressing an increase in static strength that leads to deterioration in workability, that is, without impairing the excellent workability of the composite structure steel sheet of ferrite and martensite. On the other hand, Cu in the ferrite phase
If the size of a single particle exceeds 2 nm, C
Since the static strength of the steel sheet is significantly increased by the precipitation strengthening of u, 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 the content is less than 0.03%, the volume ratio of martensite in the structure is reduced, and the strength is reduced. Si has the effect of promoting ferrite transformation and increasing the C concentration in untransformed austenite to form a composite structure. However, below 0.1%, the effect is lost,
Even if added in excess of 1.4%, the effect is saturated. Therefore, S
The content of i is more than 0.1% and 1.4% or less.

Mn is required to be 0.5% or more in order to obtain the desired second phase, martensite. Also,
If added in excess of 3.0%, slab cracks occur, so 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 it is less than 0.2%, the effect is small, and even if it is added more than 2.0%, the effect is saturated. Therefore, the addition 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, high-pressure descaling is performed after rough rolling is completed. It is preferable that the condition of the high-pressure descaling satisfies the condition of collision pressure P (MPa) of high-pressure water on the steel sheet surface × flow rate L (liter / cm ² ) ≧ 0.0025. The collision pressure P of the high-pressure water on the steel plate surface is described as follows. ("Iron and Steel" 1991 vol. 77 N
o. 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. The subsequent finish rolling needs to be completed in a temperature range where the final pass temperature (FT) is equal to or higher 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.

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 generated, and a microstructure having the target ferrite as the main phase and martensite as the second phase cannot be obtained. In addition, the temperature range in which the retention is performed for 1 to 10 seconds is performed in order to facilitate the ferrite transformation.
Desirably, the temperature is from the r ₁ transformation point to 800 ° C., and for that purpose, it is preferable to quickly reach the temperature range at a cooling rate of 20 ° C./s or more after finish rolling.

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 bainite (including bainitic ferrite, the same applies hereinafter) is generated, and sufficient martensite cannot be obtained, and the desired ferrite is used as the main phase, and the martensite becomes the second phase. Microstructure is not obtained. If the winding temperature exceeds 350 ° C,
The formation of bainite does not provide sufficient martensite and the desired microstructure with ferrite as the main phase and martensite as the second phase cannot be obtained. Is large size C
Since the precipitation of u may occur, the winding temperature is 350
Limit to below ° C. 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.

[0033]

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), high-pressure descaling (condition: collision pressure P = 2.7 MPa, flow rate L = 0.001 liter / cm ² ) is performed after rough rolling, and at the finishing rolling temperature (FT) shown in Table 2 as well. After rolling to a plate thickness of 1.2 to 5.4 mm,
After staying for the time shown in Table 2, the cooling rate (CR) shown in Table 2
And wound up at a winding temperature (CT). However, the indication of the chemical composition in the table is% by mass. The tensile test of the hot rolled sheet obtained in this way
First, it was processed into No. 5 test piece described in JISZ 2201.
It carried out according to the test method of JISZ2241.
Table 2 shows the volume ratio of the structure of the steel sheet and the test results. 1/4 thickness of the cross-section thickness in the rolling direction of the steel sheet is 200-5
Table 2 also shows the volume ratio of the tissue observed at × 00.

[0034]

[Table 1]

[0035]

[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, A-4,
B-1, C-1, D-1, F-1, H-1, I-1, J
-1, J-2, J-4, K-1, L-2, N-1, P-
1, R-1, S-1, T-1, W-1, X-1, Z-1
No. 21 steel and Cu in the ferrite phase which is the main phase
A hot-rolled steel sheet for processing excellent in fatigue characteristics in which the size of a single particle is 2 nm or less has been obtained.

Other steels are outside the scope of the present invention for the following reasons. That is, since the finishing rolling temperature (FT) of the steel A-2 is out of the range of the present invention, not only the strain remains in the ferrite grains and the ductility is reduced, but also the low yield ratio (YR) is not obtained. . In steel A-3, the desired second phase martensite cannot be sufficiently obtained because the winding temperature (CT) after hot rolling is out of the range of the present invention. 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.

Since the cooling rate (CR) of the steel A-5 after the stagnation is out of the range of the present invention, pearlite is generated and the target ferrite is the main phase and martensite is the second phase. No structure was obtained, and a sufficient fatigue limit ratio was not obtained at a low yield ratio (YR). In steel E-1, the content of P is out of the range of the present invention, so that P segregates at the grain boundary to lower the grain boundary strength, so that a sufficient fatigue limit ratio is not obtained. In steel G-1, the content of Cu is out of the range of the present invention, so that the effect of improving the fatigue characteristics is small and a sufficient fatigue limit ratio is not obtained.

Since steel J-3 and steel L-1 have a residence time after finish rolling outside the range of the present invention, a microstructure having the intended ferrite as the main phase and martensite as the second phase is obtained. No sufficient yield limit ratio was obtained at a low yield ratio. Since the content of B in steel M-1 is out of the range of the present invention, 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 steel O-1 and the steel Q-1 have a Si content exceeding the upper limit of the present invention, the properties of the scale deteriorate and the surface properties deteriorate, so that a sufficient fatigue limit ratio cannot be obtained.

In steel U-1, since the Si content is below the lower limit of the present invention, the effect of promoting ferrite transformation and the effect of forming a composite structure by enriching the C element in untransformed austenite cannot be obtained. A microstructure having the target ferrite as the main phase and martensite as the second phase was not obtained, and a sufficient fatigue limit ratio was not obtained. In steel V-1, the content of Mn is out of the range of the present invention, so that the desired martensite of the second phase is not sufficiently obtained, and a low yield ratio is not obtained. In steel Y-1, the content of C is out of the range of the present invention, so that the volume fraction of martensite in the microstructure is not sufficient, and a low yield ratio and a sufficient fatigue limit ratio are not obtained.

[0042]

As described in detail above, the present invention provides a method for producing a hot-rolled steel sheet for processing having excellent fatigue properties. Therefore, the present invention can be said to be an invention having high industrial value, since it is possible to expect a great improvement in fatigue characteristics while sufficiently securing workability.

[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.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor: Manabu Takahashi 20-1 Shintomi, Futtsu-shi, Chiba F-term in the Technology Development Division, Nippon Steel Corporation 4K032 AA01 AA02 AA04 AA05 AA08 AA11 AA14 AA15 AA16 AA17 AA19 AA22 AA23 AA27 AA29 AA31 AA32.

Claims (4)

[Claims] 1. In mass%, C: 0.03 to 0.20%, Si: 0.1 to 1.4%, 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 After the rough rolling of the slab made of impurities, high-pressure descaling is performed, and after hot finish rolling is completed at the Ar ₃ transformation point or higher, the temperature range from the Ar ₃ transformation point to the Ar ₁ transformation point is 1 to 10 seconds. Stay, and then cooled at a cooling rate of 20 ° C./s or more,
Winding at the following winding temperature, the microstructure is a composite structure having ferrite as a main phase and martensite as a second phase, and the presence state of Cu in the ferrite phase is determined by the size of a particle composed of Cu alone. A method for producing a hot-rolled steel sheet for processing having excellent fatigue properties, characterized in that a steel sheet having a precipitation state and / or a solid solution state having a thickness of 2 nm or less is obtained. 2. The hot-rolling process according to claim 1, wherein the steel further contains, by mass%, Ni: 0.1 to 1.0%. Steel plate manufacturing method. 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 method for producing a hot-rolled steel sheet for processing according to claim 1 or 2, which is excellent in 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 method for producing a hot-rolled steel sheet for processing having excellent fatigue properties according to claim 3.