JP2001303187A - Dual-phase steel sheet excellent in burring property, and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a hot rolled steel sheet having >=540 MPa tensile strength and excellent in fatigue characteristic and burring property (bore expandability) and also to provide a manufacturing method for stably manufacturing this steel sheet at a low cost. SOLUTION: The dual-phase steel sheet excellent in burring property is composed of steel having a composition containing, by mass, 0.01-0.2% C, 0.01-2% Si, 0.05-3% Mn, <=0.1% P, <=0.01% S and 0.005-1% Al and has a microstructure consisting of a dual-phase structure in which a phase having maximum volume fraction is composed of ferrite and a second phase is composed essentially of martensite; a value given by dividing the volume fraction of the second phase by the average grain size of the second phase is 3-12; and a value given by dividing the average value of the hardness of the second phase by the average value of the hardness of ferrite is 1.5-7. This steel sheet can be manufactured by finishing the hot finish rolling of the steel with the above composition at a temperature between the Ar3 transformation point and (Ar3 transformation point + 100 deg.C), holding the resultant steel sheet in the temperature region between the Ar1 transformation point and the Ar3 transformation point for 1-20 s, cooling the steel sheet at >=20 deg.C/s cooling rate, and then coiling it at <=350 deg.C coiling temperature.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a composite steel sheet having excellent burring workability and a tensile strength of 540 MPa or more, and a method for producing the same. The present invention relates to a composite structure steel sheet excellent in hole expandability (burring workability) and a method for producing the same, which is suitable as a material required to achieve both balance and durability.

[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, although light metals such as Al alloys have the advantage of high specific strength,
Due to their considerable cost compared to steel, their application is limited to special applications. Therefore, in order to promote weight reduction of automobiles in a wider range, there is a strong demand for the use of inexpensive high-strength steel sheets.

[0003] In response to such demands for high strength, in the field of cold rolled steel sheets used for white bodies and panels that occupy about 1/4 of the body weight, they have both strength and deep drawability. Development of steel sheets and bake-hardening steel sheets has been promoted, which has contributed to weight reduction of vehicle bodies. However, at present, the object of weight reduction is shifting to structural members and undercarriage members occupying about 20% of the vehicle body weight, and there is an urgent need to develop high-strength hot-rolled steel sheets used for these members.

However, since high strength generally degrades material properties such as formability (workability), how to achieve high strength without deteriorating material properties is a key to the development of high strength steel sheets. Become. In particular, as properties required for steel sheets for structural members and suspension members, hole expandability, fatigue durability, corrosion resistance, and the like are important, and high strength and how to balance these properties in high dimensions are important.

[0005] For example, as properties required for a steel plate for a road wheel disc, hole expandability and fatigue durability are particularly important. This is because the burring process (hole expansion process) in hub hole forming is particularly severe in the road wheel disc forming process, and the fatigue durability is controlled by the strictest standards for wheel member characteristics. is there.

At present, as a high-strength hot-rolled steel sheet for a road wheel disc, a 590 MPa class ferrite-martensite composite structure steel sheet (so-called dual phase steel sheet) having excellent fatigue properties with emphasis on fatigue durability of members.
However, the strength level required for these steel sheets for members is from 590 MPa class to 780 MPa class, and the strength is being further increased. On the other hand, hole expandability not only tends to decrease with higher strength,
Composite steel sheets are said to be disadvantageous in terms of hole expandability due to their uneven structure. Therefore, 590 MPa
The hole expandability that was not a problem in the 780 MPa class may be a problem in the 780 MPa class.

That is, in applying a high-strength steel plate to a part around a load such as a road wheel, in addition to fatigue durability, hole expandability is an important consideration. However, the invention describing a high-strength steel sheet having a microstructure of a ferrite-martensite composite structure in order to improve fatigue durability, and also having excellent hole expandability is hardly found except for some exceptions. Is the current situation.

For example, Japanese Patent Application Laid-Open No. 5-179396 discloses that the microstructure is made of ferrite and martensite or retained austenite to ensure fatigue durability, and that ferrite is strengthened by TiC or NbC precipitates to form ferrite grains and martensite. A technique has been disclosed in which the difference in strength from the phase is reduced, the concentration of local deformation on the ferrite grains is suppressed, and the hole expandability is ensured.

[0009]

However, in a steel plate for some parts such as a disk of a road wheel, a balance between formability such as burring workability and a high level of fatigue durability is very important. With the conventional technology, characteristics satisfying this cannot be obtained. Even if both characteristics are satisfied, it is important to provide a manufacturing method that can be manufactured stably at a low cost, and the above-mentioned conventional technology has to be said to be insufficient.

That is, Japanese Patent Application Laid-Open No. Hei 5-179396 discloses that not only a sufficient elongation cannot be obtained due to precipitation strengthening of ferrite grains, but also that high-density mobile dislocations introduced around the martensite phase at the time of production are not sufficient. Since the migration is hindered by the precipitates, the characteristic characteristic of the ferrite-martensite composite structure of low yield ratio cannot be obtained. Also, T
Addition of i and Nb is not preferable because it increases the production cost.

Accordingly, the present invention provides a hot-rolled steel sheet having a tensile strength of 540 MPa or more excellent in fatigue characteristics and burring workability (hole-expandability), which can advantageously solve the above-mentioned problems of the prior art, and can stably produce the steel sheet at low cost. It is an object of the present invention to provide a manufacturing method which can be manufactured by using the method.

[0012]

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. In order to achieve both burring workability and fatigue characteristics, intensive research was conducted. As a result, the microstructure is a composite structure in which the phase having the largest volume fraction is ferrite and the second phase is mainly martensite, and the volume fraction of the second phase is divided by the average particle size of the second phase. A value of 3 or more and 12 or less, and a value obtained by dividing the average value of the hardness of the second phase by the average value of the hardness of the ferrite is 1.5 or more and 7 or less is very effective in improving burring workability. The present invention has been newly found, and the present invention has been made.

That is, the gist of the present invention is as follows. (1) In mass%, C: 0.01 to 0.2%, Si: 0.01 to 2%, Mn: 0.05 to 3%, P ≦ 0.1%, S ≦ 0.01%, Al: A steel containing 0.005 to 1%, with the balance being Fe and unavoidable impurities, the microstructure of which is ferrite as the phase having the largest volume fraction and mainly martensite as the second phase. It is a composite structure, and the value obtained by dividing the volume fraction of the second phase by the average particle size of the second phase is 3 or more and 12 or less, and the average value of the hardness of the second phase is divided by the average value of the hardness of the ferrite. The composite structure steel sheet having excellent burring workability, characterized in that the obtained value is 1.5 or more and 7 or less. (2) The steel further contains, by mass%, Cu: 0.2 to 2
%, Wherein the composite structure steel sheet is excellent in burring workability according to the above (1). (3) The steel further contains B: 0.0002 by mass%.
The composite structure steel sheet having excellent burring workability according to the above (1) or (2), characterized in that the steel sheet contains 0.1 to 0.002%. (4) The steel further contains Ni: 0.1 to 1 in mass%.
%. The composite structure steel sheet having excellent burring workability according to any one of the above (1) to (3), wherein (5) The steel further contains, by mass%, Ca: 0.0005 to 0.002%, REM: 0.00
The composite structure steel sheet having excellent burring workability according to any one of the above (1) to (4), characterized in that the steel sheet contains one or two kinds of steels having a burring property of from 0.05 to 0.02%. (6) The steel further contains, in mass%, Ti: 0.05 to 0.5%, Nb: 0.01 to 0.5%, Mo: 0.05 to 1%, and V: 0.02 to 0%. 2%, Cr: 0.01% to 1%, Zr: 0.02% to 0.2%, one or more of the above (1) to (5). 2. A composite structure steel sheet having excellent burring workability according to the above item.

(7) When hot rolling a steel slab having the components described in any one of the above (1) to (6),
After finishing hot finish rolling at the r3 transformation point temperature or higher and the Ar3 transformation point temperature + 100 ° C or lower, the Ar1 transformation point temperature is higher than the Ar1 transformation point temperature.
3 Stay for 1 to 20 seconds in the temperature range below the transformation point temperature, then cool at a cooling rate of 20 ° C / s or more and wind it up at a winding temperature of 350 ° C or less. A composite structure in which the largest phase is ferrite and the second phase is mainly martensite. The value obtained by dividing the volume fraction of the second phase by the average particle size of the second phase is 3 or more and 12 or less, and A method for producing a composite structure steel sheet excellent in burring workability, wherein a value obtained by dividing an average value of phase hardness by an average value of ferrite hardness is 1.5 or more and 7 or less. (8) In the hot rolling described above, after the rough rolling is completed, high-pressure descaling is performed, and the hot finish rolling is completed at an Ar3 transformation point temperature or higher and an Ar3 transformation point temperature + 100 ° C or lower. A method for producing a composite structure steel sheet having excellent burring workability.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The results of basic research that led to the present invention will be described below. First, the influence of the average ferrite grain size and the size of the second phase grain on the hole expandability was investigated. The test materials for that were prepared as follows. That is, 0.07% C-1.6% Si-2.0% Mn-
After the hot finish rolling of the ingot obtained by adjusting the composition to 0.01% P-0.001% S-0.03% Al at any temperature not lower than the Ar3 transformation point temperature, the Ar1 transformation is performed. In the temperature range from the point temperature to the Ar3 transformation point temperature.
It was kept for 15 seconds, then cooled at a cooling rate of 20 ° C./s or more, and wound at normal temperature. The results of the hole expansion test performed on these steel sheets were determined by dividing the volume fraction Vs of the second phase by the average particle diameter dm of the second phase and the average hardness Hvs of the second phase by the hardness of the ferrite. FIG. 1 shows the values obtained by dividing the average values Hvf.

From the results, the value obtained by dividing the volume fraction of the second phase by the average particle size of the second phase, and the value obtained by dividing the average value of the hardness of the second phase by the average value of the hardness of the ferrite were obtained. There is a strong correlation between the hole expandability and the value obtained by dividing the volume fraction of the second phase by the average particle size of the second phase, and the average value of the hardness of the second phase by the average value of the hardness of the ferrite. It has been found that when the divided values are 3 or more and 12 or less, and 1.5 or more and 7 or less, the hole expandability is remarkably improved.

Although this mechanism is not always clear, if the value obtained by dividing the volume fraction of the second phase by the average particle size of the second phase (the size of the second phase grains) is too large, the uniformity of the microstructure may be reduced. Lost, voids tend to occur at the interface between the second phase and the parent phase, tend to be the starting point of cracks at the time of hole expansion, and if too small, the local ductility, which is correlated with the hole expansion rate, decreases, so at the optimal value It is estimated that the hole expansion rate is improved. Also, if the average value of the hardness of the second phase divided by the average value of the hardness of the ferrite (the difference in strength between the ferrite and the second phase) is too large, voids are generated at the interface between the second phase and the mother phase. This is considered to be a starting point of cracks during hole expansion, and the effect of the second phase, which is effective in stopping fatigue cracks that are too small, is lost, and it is considered difficult to achieve both hole expansion properties and fatigue characteristics.

The method of measuring the average grain size of ferrite is described in J.
The cutting method described in the method for testing ferrite grain size of IS G 0552 steel was used. Further, the average particle diameter of the second phase was defined as an average circle equivalent diameter, and a value obtained from an image processing apparatus or the like was adopted. The hardness is measured in accordance with JIS Z 2244.
Vickers hardness test described-measured according to test method. However, the test force is 0.049 to 0.098 N, and the holding time is 15 seconds. Further, regarding the hole expandability (burring workability), Japan Iron and Steel Federation Standard JFST 1001
-Evaluated according to the hole expansion test method described in 1996.

Next, the microstructure of the steel sheet according to the present invention will be described in detail. The microstructure of the steel sheet was a composite structure in which the phase having the largest volume fraction was ferrite and the second phase was mainly martensite in order to achieve both fatigue characteristics and burring workability (hole expanding properties). However, the second phase permits inclusion of unavoidable bainite and retained austenite. In order to secure good fatigue characteristics, the volume fraction of bainite and / or retained austenite is preferably 5% or less. Here, the volume fraction of the ferrite and the second phase is the area fraction of those structures in the microstructure observed at a magnification of 200 to 500 times with an optical microscope at a quarter of the thickness in the rolling direction of the steel sheet. Defined.

Next, the reasons for limiting the chemical components of the present invention will be described. The component content is% by mass. C is an element necessary for obtaining a desired microstructure. However,
If the content exceeds 0.2%, workability and weldability deteriorate, so the content is made 0.2% or less. If it is less than 0.01%, the strength is reduced.

[0021] Si is necessary for obtaining a desired microstructure and is effective for increasing the strength as a solid solution strengthening element. In order to obtain a desired strength, it is necessary to contain 0.01% or more. However, if the content exceeds 2%, the workability deteriorates. Therefore, the content of Si is set to 0.01% or more and 2% or less.

Mn is effective for increasing the strength as a solid solution strengthening element. To obtain the desired strength, 0.05% or more is required. Further, if added over 3%, slab cracks occur, so the content is made 3% or less.

P is an impurity and is preferably as low as possible.
If the content exceeds 0.1%, workability and weldability are adversely affected, and fatigue characteristics are also reduced.

S is an impurity and is preferably as low as possible. If it is too large, A-based inclusions which deteriorate the hole expandability are generated. Therefore, the content of S should be reduced as much as possible. is there.

[0025] Al must be added in an amount of 0.005% or more for deoxidation of molten steel, but the upper limit is set to 1.0% because it causes an increase in cost. On the other hand, if you add too much,
Since increasing non-metallic inclusions and deteriorating elongation, 1.0%
%, More preferably 0.5% or less.

Since Cu has an effect of improving fatigue characteristics in a solid solution state, Cu is added as necessary. However, if the content is less than 0.2%, the effect is small, and even if the content exceeds 2%, the effect is saturated. Therefore, the content of Cu is set in the range of 0.2 to 2%.

B has an effect of increasing the fatigue limit by being added in combination with Cu, so B is added as necessary.
However, if it is less than 0.0002%, it is insufficient to obtain the effect, and if it exceeds 0.002%, slab cracking occurs. Therefore, the addition of B is 0.0002% or more,
02% or less.

Ni is added as necessary 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%, the effect is saturated. Therefore, the content is set to 0.1 to 1%.

Ca and REM are elements that become the starting point of destruction or change the form of nonmetallic inclusions that degrade workability and render them harmless. However, even if added less than 0.0005%, there is no effect, and if Ca is added, 0.002%
If the content of REM is more than 0.02%, the effect saturates even if it is added more than 0.02%.
= 0.0005 to 0.02% is preferably added.

Further, in order to impart strength, Ti, N
One, two or more of precipitation strengthening or solid solution strengthening elements of b, Mo, V, Cr and Zr may be added. However,
0.05%, 0.01%, 0.05%, 0.0% respectively
If it is less than 2%, 0.01% or 0.02%, the effect cannot be obtained. Also, 0.5%, 0.5%,
Even if it exceeds 1%, 0.2%, 1%, and 0.2%, the effect is saturated.

It is to be noted that Sn is not particularly required to obtain the effects of the present invention, but is desirably 0.05% or less because flaws may occur during hot rolling.

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 1400 ° C or higher, the scale-off amount becomes large and the yield decreases, so the reheating temperature is 1400 ° C.
Less than is desirable. Heating at a temperature lower than 1000 ° C. significantly impairs the operation efficiency according to the schedule.
The temperature is preferably 000 ° C or higher.

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 Ar3 transformation point temperature and not higher than the Ar3 transformation point temperature + 100 ° C. . This is because the rolling temperature during hot rolling is A
If the r3 transformation point temperature is reduced, strain remains and ductility is reduced to deteriorate workability. If the finishing temperature is higher than the Ar3 transformation point temperature + 100 ° C., the austenite grain size after finish rolling becomes large. In addition, the promotion of ferrite transformation in the two-phase region performed in the subsequent cooling step becomes insufficient, and the desired microstructure cannot be obtained. Therefore, the finishing temperature is not less than the Ar3 transformation point temperature and not more than the Ar3 transformation point temperature + 100 ° C.

When high-pressure descaling is performed after the completion of rough rolling, the collision pressure P (MP
It is preferable that the condition of a) × flow rate L (liter / cm ² ) ≧ 0.0025 is satisfied. The collision pressure P of high-pressure water on the steel sheet surface is described as follows ("Iron and Steel", 1991, vol.
7, 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): distance between steel plate surface and nozzle

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

The upper limit of the collision pressure P × the flow rate L does not need to be particularly determined in order to obtain the effect of the present invention. However, if the flow rate of the nozzle is increased, inconvenience such as intensified wear of the nozzle occurs. It is preferable to be 0.02 or less.

Further, the maximum height Ry of the steel sheet surface after finish rolling is 15 μm (15 μm Ry, 12.5 mm, ln
12.5 mm) or less. This is because the fatigue strength of a hot-rolled or pickled steel sheet has a correlation with the maximum height Ry of the steel sheet surface, as described in, for example, “Handbook for Designing Fatigue of Metallic Materials” edited by The Society of Materials Science, Japan, page 84. It is clear from Further, the subsequent finish rolling is desirably performed within 5 seconds in order to prevent scale from being generated again after descaling.

After finishing the finish rolling, the process is first performed by using Ar
3 Stay in the temperature range from the transformation point to the Ar3 transformation point (two-phase region of ferrite and austenite) for 1 to 20 seconds. 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 2
If it exceeds 0 seconds, pearlite is generated, and a composite structure in which the phase having the intended maximum volume fraction is ferrite and the second phase is mainly martensite cannot be obtained.

The temperature range in which the stagnation is maintained for 1 to 20 seconds is desirably from the Ar1 transformation point to 800 ° C. in order to facilitate the ferrite transformation. It is preferable to quickly reach the temperature range. Further, the residence time of 1 to 20 seconds is 1 to 20 in order not to reduce productivity extremely.
Preferably, it is 10 seconds.

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 is generated, and sufficient martensite cannot be obtained, and a microstructure in which the intended ferrite is the phase having the maximum volume fraction and martensite is the second phase is obtained. Absent. The upper limit of the cooling rate to the winding temperature can obtain the effect of the present invention without any particular limitation, but is preferably 200 ° C./s or less because there is a concern about sheet warpage due to thermal strain.

If the winding temperature is higher than 350 ° C., bainite is formed and sufficient martensite cannot be obtained, and the microstructure in which the intended ferrite is the phase having the maximum volume fraction and the martensite is the second phase is formed. Since it cannot be obtained, the winding temperature is limited to 350 ° C. or less. 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.

[0042]

The present invention will be further described below with reference to examples. The steels A to Q 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 (SRT).
After the rough rolling, after rolling to a sheet thickness of 1.2 to 5.4 mm at the finish rolling temperature (FT) also shown in Table 2,
Each was wound at a winding temperature (CT) shown in Table 2. 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 obtained in this manner, the test material was first processed into a No. 5 test piece described in JIS Z 2201, and was subjected to the test method described in JIS Z 2241. Table 2 shows the test results. Here, the volume ratios of the ferrite and the second phase are 200 to 50 by an optical microscope at a thickness of 1/4 of the cross section in the rolling direction of the steel sheet.
It is defined as the area fraction of those structures in the microstructure observed at 0x.

The method for measuring the average ferrite grain size is described in J.
The average particle diameter of the second phase was defined as an average circle equivalent diameter, and a value obtained from an image processing device or the like was adopted according to the cutting method described in the ferrite crystal grain size test method of IS G 0552 steel. The hardness was measured according to the Vickers hardness test method described in JIS Z 2244. However, the test force is 0.049 to 0.098 N, and the holding time is 15 seconds.

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 a value obtained by dividing the fatigue limit σW at 10 × 10 ⁷ times by the tensile strength σB of the steel sheet (fatigue limit ratio σW / σB).
However, the surface of the fatigue test piece was not pickled at all, and was left as pickled. On the other hand, regarding the burring workability (hole expanding property), Japan Iron and Steel Federation Standard JFST 1001
-Evaluated according to the hole expansion test method described in 1996.

According to the present invention, steels A, B, C-6,
G, K, L, M, N, O, P, and Q steels, each containing a predetermined amount of a steel component, and have a microstructure in which the phase having the maximum volume fraction is ferrite and the second phase is ferrite. The composite structure is mainly martensite, and the value obtained by dividing the volume fraction Vs of the second phase by the average particle diameter dm of the second phase is 3 or more and 12 or less, and the average value Hvs of the hardness of the second phase is A composite structure steel sheet excellent in burring workability, characterized in that a value obtained by dividing the average ferrite hardness value Hvf from 1.5 to 7 is obtained.

Other steels are outside the scope of the present invention for the following reasons. That is, the finish rolling temperature (FT) of steel C-1 is higher than the range of the present invention, and the size (Vs / dm) of the second phase grains is out of the range of the present invention. (Λ) and fatigue limit ratio (σW / σB) were not obtained. Steel C-2 has a finish rolling end temperature (FT)
Is lower than the range of the present invention, and the strength difference between the ferrite and the second phase (Hvs / Hvf) is out of the range of the present invention, so that a sufficient hole expansion ratio (λ) and a fatigue limit ratio (σW / σB) are obtained. Not been. Further, the strain remains and the ductility (El)
Also decrease.

The steel C-3 has a cooling rate (CR) after staying lower than the range of the present invention and a winding temperature (CT) higher than the range of the present invention. Therefore, since the size (Vs / dm) of the second phase grains is out of the range of the present invention, a sufficient hole expansion ratio (λ) and a fatigue limit ratio (σW / σB) are not obtained. Steel C-
No. 4 has a sufficient hole expansion ratio (λ) and fatigue limit because the retention temperature (MT) is lower than the range of the present invention and the strength difference between the ferrite and the second phase (Hvs / Hvf) is out of the range of the present invention. The ratio (σW / σB) was not obtained. Steel C-5 has no residence time (Time) and the difference in strength between the ferrite and the second phase (Hvs / Hvf) is out of the range of the present invention. Therefore, a sufficient hole expansion ratio (λ) and fatigue limit ratio ( σW / σB) is not obtained.

In steel D, since the content of C is out of the range of the present invention, a desired microstructure cannot be obtained, and sufficient strength (TS) and fatigue limit ratio (σW / σB) cannot be obtained. . Steel E has a sufficient strength (TS) and a sufficient fatigue limit ratio (σW / σ) since the content of Si is out of the range of the present invention.
B) was not obtained. In steel F, since the content of Mn is out of the range of the present invention and the size of the second phase grains (Vs / dm) is out of the range of the present invention, sufficient strength (TS) and hole expansion ratio ( λ) and the fatigue limit ratio (σW / σB) are not obtained.

In steel H, since the content of S is out of the range of the present invention, a sufficient hole expansion ratio (λ) and a fatigue limit ratio (σW)
/ ΣB) is not obtained. Steel I has a sufficient fatigue limit ratio (σW / σ) since the content of P is out of the range of the present invention.
B) was not obtained. Steel J does not have sufficient elongation (El), hole expansion ratio (λ), and fatigue limit ratio (σW / σB) because the content of C is outside the range of the present invention.

[0051]

[Table 1]

[0052]

[Table 2]

[0053]

As described in detail above, the present invention provides a composite structure steel sheet excellent in burring workability and having a tensile strength of 540 MPa or more, and a method for producing the same. By using these hot-rolled steel sheets, Since the burring processability (hole expanding property) can be expected to be greatly improved while ensuring sufficient fatigue properties, the present invention has a high industrial value.

[Brief description of the drawings]

FIG. 1 shows the results of preliminary experiments leading to the present invention obtained by dividing the volume fraction of the second phase by the average particle diameter of the second phase, and calculating the average value of the hardness of the second phase as the average of the hardness of the ferrite. It is a figure shown by the value divided by the value and the relationship of a hole expansion rate.

FIG. 2 is a diagram illustrating the shape of a fatigue test piece.

Continued on the front page F term (reference) 4K037 EA01 EA02 EA05 EA06 EA09 EA11 EA13 EA15 EA16 EA17 EA19 EA20 EA23 EA25 EA27 EA28 EA31 EA32 EA35 EA36 EB06 EB07 EB08 EB09 EB11 FC04 JA03 FC03 FC04

Claims (8)

[Claims] 1. Mass%, C: 0.01 to 0.2%, Si: 0.01 to 2%, Mn: 0.05 to 3%, P ≦ 0.1%, S ≦ 0.01 %, Al: 0.005 to 1%, with the balance being Fe and unavoidable impurities, the microstructure of which is ferrite as the phase having the largest volume fraction and mainly martensite as the second phase. The composite structure is a site, and the value obtained by dividing the volume fraction of the second phase by the average particle size of the second phase is 3 or more and 12 or less, and the average value of the hardness of the second phase is the average of the hardness of the ferrite. A composite structure steel sheet having excellent burring workability, characterized in that the value divided by the value is 1.5 or more and 7 or less. 2. The composite structure steel sheet according to claim 1, wherein the steel further contains Cu: 0.2 to 2% by mass%. 3. The composite structure steel sheet having excellent burring workability according to claim 1, wherein the steel further contains B: 0.0002 to 0.002% by mass%. 4. The composite having excellent burring workability according to claim 1, wherein the steel further contains Ni: 0.1 to 1% by mass%. Texture steel sheet. 5. The steel according to claim 1, wherein the steel further contains one or two types of Ca: 0.0005 to 0.002% and REM: 0.0005 to 0.02% by mass%. Item 5. The composite structure steel sheet having excellent burring workability according to any one of Items 1 to 4. 6. The steel further contains, by mass%, Ti: 0.05 to 0.5%, Nb: 0.01 to 0.5%, Mo: 0.05 to 1%, V: 0.02. The composition according to any one of claims 1 to 5, wherein the composition contains one or more of the following: 1 to 0.2%; Cr: 0.01 to 1%; Zr: 0.02 to 0.2%. A composite structure steel sheet having excellent burring workability according to item 1. 7. The hot rolling of a steel slab having the composition according to claim 1 after finishing hot finish rolling at a temperature between the Ar3 transformation point temperature and the Ar3 transformation point temperature + 100 ° C. or less. , Stay in the temperature range from the Ar1 transformation point temperature to the Ar3 transformation point temperature for 1 to 20 seconds,
Cooling at a cooling rate of s or more and winding at a winding temperature of 350 ° C. or less, the microstructure is a composite structure in which the phase having the maximum volume fraction is ferrite and the second phase is mainly martensite. The value obtained by dividing the volume fraction of the second phase by the average particle size of the second phase is 3 or more and 12 or less, and the value obtained by dividing the average value of the hardness of the second phase by the average value of the ferrite hardness is 1 A method for producing a composite structure steel sheet having excellent burring workability, wherein the steel sheet has a burring workability of 5 or more and 7 or less. 8. During the hot rolling, after the rough rolling is completed, high-pressure descaling is performed, and the Ar3 transformation point temperature or higher is determined.
8. The method for producing a composite structure steel sheet having excellent burring workability according to claim 7, wherein the hot finish rolling is completed at a transformation point temperature of + 100 ° C. or lower.