JP2002129285A - Steel sheet with strain induced transformation type composite structure having excellent burring workability and its production method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot rolled steel sheet excellent in fatigue characteristics and burring workability (hole expandability) and having tensile strength of >=540 MPa and to provide a production method for inexpensively and stably producing the same steel sheet. SOLUTION: This steel sheet with a strain induced transformation type composite structure excellent in burring workability is composed of steel containing 0.01 to 0.3% C, 0.01 to 2% Si, 0.05 to 3% Mn, <=0.1% P, <=0.01% S and 0.005 to 1% Al, and whose microstructure is composed of the composite one containing retained austenite of 5 to 25% by volume fraction, and the balance mainly ferrite and bainite, in which the ferrite average grain size is 2 to 20 μm, and the value obtained by dividing the retained austenite average grain size by the ferrite average grain size is 0.05 to 0.8, and also, the concentration of carbon in the retained austenite is 0.2 to 3%, and, in the method for producing the same steel sheet, steel having the above components is subjected to hot finish rolling so as to be finished at the Ar3 transformation point temperature to the Ar3 transformation point temperature +100 deg.C, is thereafter retained in the temperature range of the Ar3 transformation point temperature to the Ar3 transformation point temperature for 1 to 20 sec, is subsequently cooled at a cooling rate of >=20 deg.C/s and is coiled at a coiling temperature in the temperature range of >350 to <450 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel plate having excellent burring workability and a tensile strength of 540 MPa or more, and more particularly to a method for producing the same. The present invention relates to a work-induced transformation type composite structure steel sheet excellent in burring workability, which is suitable as a material requiring both burring workability and ductility and durability, 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, although light metals such as Al alloys have the advantage of high specific strength, their application is limited to special applications because they are significantly more expensive than steel. Accordingly, there is a strong demand for the use of inexpensive high-strength steel sheets in order to promote a reduction in the weight of automobiles over a wider range.

[0003] In response to such demands for high strength, steel sheets having both strength and deep drawability in the field of cold-rolled steel sheets used for white bodies and panels that occupy about 1/4 of the body weight. And bake-hardening steel plates have been developed, 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 increasing strength generally degrades material properties such as formability (workability), the key to developing high-strength steel sheets is how to increase strength without deteriorating material properties. In particular, burring workability, ductility, fatigue durability, corrosion resistance, and the like are important as characteristics required for steel sheets for structural members and suspension members, and it is important to balance high strength with these characteristics in high dimensions. .

For example, burring workability and fatigue durability are particularly regarded as important characteristics required for a steel plate for a road wheel disc. This is because the burring process (hole enlarging 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 in wheel member characteristics. It is.

At present, as a high-strength hot-rolled steel sheet for a road wheel disk, a 590 MPa class ferrite-martensite composite structure steel sheet (so-called Dual Phase) having excellent fatigue characteristics 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, it is said that not only the burring workability tends to decrease with the increase in strength, but also the composite structure steel sheet is disadvantageous in terms of the burring workability due to its uneven structure. Therefore, the burring processability which was not a problem in the 590 MPa class may become a problem in the 780 MPa class.

That is, in applying a high-strength steel plate to a part around a foot such as a road wheel, in addition to fatigue durability and ductility, burring workability is also an important subject to be studied. However, the invention describing a high-strength steel sheet with a microstructure of ferrite-martensite or the like in order to improve fatigue durability, and having excellent burring workability is hardly found except for some exceptions. Is the current situation. For example, Japanese Patent Application Laid-Open No. Hei 5-179396 discloses that the microstructure is made of ferrite and martensite or retained austenite to ensure fatigue durability, and ferrite is made of Ti.
By strengthening with precipitates of C and NbC, the difference in strength between the ferrite grains and the martensite phase is reduced, local concentration of deformation on the ferrite grains is suppressed, and hole expandability (burring workability) is secured. Techniques are disclosed.

[0007]

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 ductility and a high level of fatigue durability is very important. However, with the above-mentioned prior art, satisfactory characteristics cannot be obtained. It is also important to provide a manufacturing method that can be manufactured stably at low cost even if both characteristics are satisfied, and it cannot be said that the above conventional technology is insufficient. That is, Japanese Patent Application Laid-Open No.
In 179396, elongation cannot be sufficiently obtained because the ferrite grains are precipitation-strengthened. Further, addition of Ti and Nb is not preferable because it causes an increase in manufacturing cost. Accordingly, the present invention can advantageously and stably produce a hot-rolled steel sheet having a tensile strength of 540 MPa or more, which is excellent in fatigue characteristics and burring workability (hole expanding properties), and which can advantageously solve the above-mentioned problems of the prior art. It is intended to provide a manufacturing method.

[0008]

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 burring workability, ductility and fatigue properties. As a result, the microstructure is a composite structure including residual austenite having a volume fraction of 5% or more and 25% or less, and the balance is mainly composed of ferrite and bainite, and has an average ferrite particle size of 2 μm to 20%.
The burring processability is improved when the value obtained by dividing the average particle size of the retained austenite by the average particle size of the ferrite is 0.05 to 0.8 and the carbon concentration of the retained austenite is 0.2% to 3%. The present inventors newly found that the present invention is very effective, and made the present invention.

That is, the gist of the present invention is as follows. (1) In mass%, C: 0.01 to 0.3%, Si:
0.01-2%, Mn: 0.05-3%, P: ≦ 0.1
%, S: ≦ 0.01%, Al: 0.005 to 1%, the balance being Fe and unavoidable impurities, the microstructure of which is 5% or more and 25% or less by volume. And a residual structure mainly composed of ferrite and bainite. The average ferrite grain size is 2 μm to 20 μm, and the value obtained by dividing the average austenite grain size by the average ferrite grain size is 0.05 to 0. 0.8 or less and the carbon concentration of the retained austenite is 0.8.
A work-induced transformation-type composite structure steel sheet having excellent burring workability, characterized in that the steel sheet content is 2% or more and 3% or less.

(2) The steel further comprises, by mass%, C
u: 0.2 to 2%, (1)
A work-induced transformation type composite structure steel sheet having excellent burring workability according to item 1. (3) The steel further contains B: 0.000% by mass.
(1) characterized by containing 2 to 0.002%.
Or a work-induced transformation type composite structure steel sheet excellent in burring workability according to (2).

(4) The steel further comprises N
i: containing 0.1 to 1%, (1)
Or a work-induced transformation type composite structure steel sheet excellent in burring workability according to any one of (3) to (3). (5) The steel further contains Ca: 0.00% by mass.
05-0.002%, REM: 0.0005-0.02
% Or one or two kinds,
The work-induced transformation composite structure steel sheet having excellent burring workability according to any one of (1) to (4).

(6) The steel further comprises, by mass%,
i: 0.05 to 0.5%, Nb: 0.01 to 0.5%,
Mo: 0.05-1%, V: 0.02-0.2%, C
The burring according to any one of (1) to (5), characterized by containing one or more of r: 0.01 to 1% and Zr: 0.02 to 0.2%. A work-induced transformation-type composite structure steel sheet with excellent workability.

(7) In the hot rolling of a steel slab having the component described in any one of (1) to (6), hot finishing is performed at an Ar ₃ transformation point temperature or higher and an Ar ₃ transformation point temperature + 100 ° C. or lower. After the end of the rolling, it is kept for 1 to 20 seconds in a temperature range from the Ar ₁ transformation point temperature to the Ar ₃ transformation point temperature, and
Cooling at a cooling rate of at least 350 ° C / s
A microstructure including a retained austenite with a volume fraction of 5% or more and 25% or less, and a balance mainly composed of ferrite and bainite. Is 2 μm or more and 20
μm or less, a steel sheet having a value obtained by dividing the average retained austenite grain size by the average ferrite grain size of 0.05 to 0.8 and a carbon concentration of the retained austenite of 0.2% to 3%. A method for producing a work-induced transformed composite structure steel sheet having excellent burring workability. Upon inter (8) the hot rolling, after rough rolling termination, perform high pressure descaling, characterized in that to end the hot finish rolling at Ar ₃ transformation point temperature or more Ar ₃ transformation temperature + 100 ° C. or less (7) The present invention relates to a method for producing a work-induced transformed composite structure steel sheet having excellent burring workability as described above.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The results of basic research that led to the present invention will be described below. First, the effects of the average ferrite grain size and the retained austenite grain size on the burring workability were investigated. The test material for that was prepared as follows. That is, 0.07% C-1.6% Si
-2.0% Mn-0.01% P-0.001% S-0.0%
After finishing the hot finish rolling at a temperature equal to or higher than the Ar ₃ transformation point, the ingot obtained by adjusting the composition to 03% Al is
It stayed for 1 to 15 seconds in any temperature range from the Ar ₁ transformation point temperature to the Ar ₃ transformation point temperature, then cooled at a cooling rate of 20 ° C./s or more, and wound at 550 ° C. to normal temperature.

FIG. 1 shows the results of the hole expansion test performed on these steel sheets, with respect to the relative sizes of the average ferrite grain size and the retained austenite grain size with respect to the ferrite grain size. From these results, there is a strong correlation between the relative size of ferrite average particle size Df and the relative size of retained austenite particle size to ferrite particle size (value obtained by dividing residual austenite average particle size dm by ferrite average particle size Df) and hole expansion value. When the relative sizes of the average ferrite particle diameter Df and the residual austenite particle diameter dm with respect to the ferrite particle diameter Df are respectively 2 μm or more and 20 μm or less and 0.05 or more and 0.8 or less, the hole expansion value is significantly improved. Newly discovered.

Although the mechanism is not always clear, if the relative size of the retained austenite grain size to the ferrite grain size is too large, voids are likely to be formed at the interface between the retained austenite and the parent phase, and the starting point of the crack at the time of hole expansion is increased. When it is too small, the local ductility, which is correlated with the hole expansion rate, is reduced, and it is presumed that the hole expansion rate is improved at an optimum size and interval. Also, if the average ferrite grain size is too small, the yield stress increases and the area of the fractured surface of the punched hole increases, and the roughness of the punched surface may increase and the hole expansion value may decrease. It is considered that the uniformity was lost and the local ductility, which is correlated with the hole expansion rate, was reduced. The method for measuring the average ferrite grain size is described in JI
The cutting method described in the Ferrite grain size test method for SG0552 steel was used. Further, the average particle diameter of retained austenite was defined as an average circle equivalent diameter, and a value obtained from an image processing apparatus or the like was adopted.

Next, as shown in FIG. 1 in the results of this test, the ferrite particle diameter Df having an average ferrite particle diameter Df of 2 μm or more and 20 μm or less and a residual austenite particle diameter dm is shown.
Although the hole expansion value was low even when the relative size with respect to 0.05 or more and 0.8 or less, the influence of the carbon concentration of retained austenite on the hole expansion value was further investigated. FIG. 2 shows the results obtained by rearranging the hole expansion values of the above steel sheets by the carbon concentration of retained austenite. From this result, there is a strong correlation between the carbon concentration of retained austenite and the hole expansion value, and the average ferrite particle diameter Df is 2 μm.
When the residual austenite particle size dm is not less than 20 μm and the relative size with respect to the ferrite particle size Df is not less than 0.05 and not more than 0.8, the hole is expanded when the residual austenite carbon concentration is not less than 0.2% and not more than 3%. It was newly found that the value was significantly improved.

Although this mechanism is not always clear, if the carbon concentration of the retained austenite is too high, the strength difference between the retained austenite or martensite after the work-induced transformation and the parent phase becomes large, and voids occur at the interface at the time of punching. It is easy to cause cracks when expanding holes. On the other hand, if the carbon concentration of the retained austenite is too low, the carbon concentration of the ferrite phase inevitably increases,
Since the ductility decreases and the local ductility, which is correlated with the hole expansion ratio, decreases, the hole expansion ratio decreases. Therefore, it is presumed that the hole expansion ratio is improved at the optimum retained austenite carbon concentration.

However, if the carbon concentration of the retained austenite is more than 2.4%, welding defects such as spot welding may become remarkable, so that the carbon concentration of the retained austenite is 0.2% or more and 2.4% or less. Is more desirable. The hole expandability (burring workability) was evaluated by a hole expansion value in accordance with the hole expansion test method described in the Japan Iron and Steel Federation Standard JFS T1001-1996.

Next, the microstructure of the steel sheet and the carbon concentration of retained austenite in the present invention will be described in detail. The microstructure of the steel sheet contains a retained austenite with a volume fraction of 5% or more and 25% or less in order to achieve both fatigue properties and ductility and burring workability (hole expandability), and the rest is a composite structure mainly composed of ferrite and bainite. And However, the inclusion of unavoidable pearlite and martensite is permitted. In order to secure good fatigue characteristics, the volume fraction of pearlite is desirably 5% or less. Further, in order to obtain good ductility, the volume fraction of ferrite is desirably 40% or more, and the volume fraction of martensite is desirably less than 5%.

Here, the volume fraction of retained austenite, ferrite, bainite, pearlite and martensite refers to a sample cut from a 1 / 4W or 3 / 4W position of the steel sheet width, which is polished to a cross section in the rolling direction, and a nital reagent and Etching is performed using the reagent disclosed in JP-A-5-163590, and 200 to
It is defined as the area fraction of the microstructure at 1 / 4t of the plate thickness observed at a magnification of 500 times.

On the other hand, austenite has a different crystal structure from ferrite and can be easily identified crystallographically. Therefore,
The volume fraction of retained austenite can also be experimentally determined by an X-ray diffraction method. In other words, the method is a method of easily obtaining the volume fraction of the austenite and ferrite from the difference in the reflection surface strength between austenite and ferrite using the Mo Kα ray and the following equation. Vγ = (2/3) ｛100 / (0.7 × α (211) /
γ (220) +1)｝ + (１ ／) ｛100 / (0.7
8 × α (211) / γ (311) +1) where α (211), γ (220) and γ (31
1) is the X-ray reflection surface intensity of ferrite (α) austenite (γ), respectively. As for the volume fraction of retained austenite, almost the same value was obtained by using either the optical microscope observation method or the X-ray diffraction method, and any measurement value may be used.

The carbon concentration of the retained austenite is X
It can be obtained experimentally by a line diffraction method or Mossbauer spectroscopy. For example, in the X-ray diffraction method, the carbon concentration of the retained austenite can be measured from the relationship between the carbon concentration and the change in the lattice constant that occurs because C, which is an intrusive solid solution element, is coordinated with the austenite crystal lattice. That is, the lattice constants were measured using Kα rays of Co, Cu, and Fe, and the austenite (002), (022), (113),
The reflection angle of the (222) plane is measured, and is obtained by a method of calculating the lattice constant from the reflection angle described in the literature ("Introduction to X-ray Diffraction": BD Culliity (Gentaro Matsumura), Agne Corporation). be able to.

Since there is a linear relationship between cos ² θ (where θ is the reflection angle) and the lattice constant a, the true lattice constant a ₀ is obtained by extrapolating this straight line to cos ² θ = 0. can get. Further, based on the value of the true lattice constant a ₀ , the relationship between the lattice constant of austenite and the carbon concentration in austenite, for example, literature (RC Ruhl and MC)
ohen, Transaction of the M
et alurgical Society of A
It can be obtained by using a ₀ = 3.572 + 0.033% C (carbon concentration) described in IME, vol 245 (1969) pp 241).

Next, the reasons for limiting the chemical components of the present invention will be described. C is an element necessary for obtaining a desired microstructure. However, if the content exceeds 0.3%, the workability deteriorates, so the content is set to 0.3% or less. Also, 0.2
%, The weldability deteriorates, so that 0.2% or less is desirable. On the other hand, if it is less than 0.01%, the strength is reduced. Further, in order to stably obtain a sufficient amount of retained austenite for obtaining good ductility, 0.05% or more is desirable.

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, the content needs to be 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 strength as a solid solution strengthening element. To obtain the desired strength, 0.05% or more is required. Mn is an austenite stabilizing element, and its addition amount is desirably 0.05% or more in order to stably obtain a sufficient amount of retained austenite for obtaining good ductility. On the other hand, if added over 3%, slab cracks occur, so the content is set to 3% or less.

P is an impurity and is preferably as low as possible.
If the content exceeds 0.1%, the workability and the weldability are adversely affected and the fatigue characteristics are also reduced. S is an impurity and is preferably as low as possible. If it is too high, A-based inclusions that deteriorate local ductility and burring workability are generated. Therefore, S should be reduced as much as possible.
Below is 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%. Also,
If it is added in an excessively large amount, nonmetallic inclusions are increased and elongation is deteriorated.

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 is added as necessary since it is effective to increase the fatigue limit by being combined with Cu. 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 set to 0.0002% or more and 0.002% 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
Is an element that becomes a starting point of destruction or changes the form of nonmetallic inclusions that degrade workability and renders them harmless. However, if less than 0.0005% is added, there is no effect.
If a exceeds 0.002%, and if REM exceeds 0.02%, the effect is saturated.
It is desirable to add 0.002% and REM: 0.0005 to 0.02%.

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.
If it is less than 02%, 0.01% or 0.02%, the effect cannot be obtained. Also, 0.5% and 0.5%, respectively.
%, 1%, 0.2%, 1%, and 0.2%, the effect is saturated even if added. The effect of the present invention can be obtained even if Sn is added, and its content is not particularly limited, but may be flawed during hot rolling.
5% or less is desirable.

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 the 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 more, the scale-off amount becomes large and the yield decreases, so the reheating temperature is 1400 ° C.
Desirably less than ° C. Further, since the heating at a temperature lower than 1000 ° C. significantly impairs the operation efficiency on a schedule, the reheating temperature is desirably 1000 ° 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 from the Ar ₃ transformation point temperature to the Ar ₃ transformation point temperature + 100 ° C. or less. is there. This is because during hot rolling the rolling temperature is Ar
_{If the} temperature is lower than the ₃ transformation point temperature, strain remains, ductility is reduced and workability is deteriorated. If the finishing temperature is higher than the Ar ₃ transformation point temperature + 100 ° C., the austenite grain size after finish rolling becomes large. 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 Ar
_3, transformation point temperature or higher Ar ₃ transformation temperature + 100 ° C. or less. Here, when high-pressure descaling is performed after the completion of rough rolling, it is desirable that the condition of collision pressure P (MPa) of high-pressure water on the steel sheet surface × flow rate L (liter / cm ² ) ≧ 0.0025 is 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 The distance between the flows L is described as follows: L (liter / cm ² ) = V / (W × v)

V (liter / min): Nozzle flow rate W (cm): Width of spray liquid per nozzle hitting steel sheet surface v (cm / min): Passing speed The upper limit of collision pressure P × flow rate L is as follows. In order to obtain the effects of the present invention, it is not particularly necessary to determine, but increasing the flow rate of the nozzle causes inconveniences such as intensified wear of the nozzle.
It is desirable to set it to 0.02 or less.

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 the finish rolling, the steps first are as follows:
It stays for 1 to 20 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 20 seconds, pearlite is generated, and the desired microstructure cannot be obtained. In addition, the temperature range in which the stagnation is performed for 1 to 20 seconds is Ar ₁ in order to facilitate the ferrite transformation.
The temperature is preferably from the transformation point to 800 ° C., and for that purpose, it is desirable to quickly reach the temperature range at a cooling rate of 20 ° C./s or more after finish rolling.

Further, the residence time for 1 to 20 seconds is desirably 1 to 10 seconds in order not to significantly reduce the productivity. In order to satisfy these conditions, it is necessary to quickly reach the temperature range at a cooling rate of 20 ° C./s or more after finishing rolling. The upper limit of the cooling rate is not particularly defined, but 300 ° C./s or less is a reasonable cooling rate in view of the capacity of the cooling equipment. Further, if the cooling rate is too high, the cooling end temperature cannot be controlled, and there is a possibility of overshoot and overcooling to the Ar ₁ transformation point or less. Therefore, the cooling rate here is desirably 150 ° C./s or less. .

Next, cooling is performed at a cooling rate of 20 ° C./s or more from the temperature range to the winding temperature (CT).
If the cooling rate is less than s, bainite containing a large amount of pearlite or carbide is generated, and sufficient retained austenite cannot be obtained, and a desired microstructure cannot be obtained.
Although the effect of the present invention can be obtained without particularly setting the upper limit of the cooling rate to the winding temperature, it is preferable to set the cooling rate to 300 ° C./s or less because there is a concern about warpage due to thermal strain.

Next, if the winding temperature is 450 ° C. or higher, bainite containing a large amount of carbide is generated, and sufficient retained austenite cannot be obtained, and the desired microstructure cannot be obtained. Limited to less than. If the winding temperature is 350 ° C. or lower, a large amount of martensite is generated, and sufficient retained austenite cannot be obtained, and a desired microstructure cannot be obtained. Therefore, the winding temperature is limited to over 350 ° C. After the hot rolling process, pickling is performed if necessary,
Thereafter, skin rolling with a rolling reduction of 10% or less or cold rolling to a rolling reduction of about 40% may be performed in-line or off-line.

[0040]

The present invention will be further described below with reference to examples. The steels A to O 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 at a finish rolling temperature (FT) shown in Table 2 to a sheet thickness of 1.2 to 5.4 mm, winding at a winding temperature (CT) shown in Table 2 respectively. I took it. For some parts, after rough rolling, the collision pressure was 2.7MP.
a, High-pressure descaling was performed under the conditions of a flow rate of 0.001 liter / cm ² . However, the indication of the chemical composition in the table is% by mass.

[0041]

[Table 1]

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 Z2201, and the tensile test was performed according to the test method described in JIS Z2241. Table 2 shows the test results. In the microstructure of Table 2, "Others" was pearlite or martensite. Here, the volume fraction of retained austenite, ferrite, bainite, pearlite and martensite refers to a sample cut from a 1/4 W or 3/4 W position of a steel sheet width, polished into a section in the rolling direction, and using a Nital reagent and JP -163590 using a reagent disclosed in JP-A-163590, and using an optical microscope to 200-
It is the area fraction of the microstructure at 1 / 4t of the plate thickness observed at a magnification of 500 times. However, some values include values obtained by the above-mentioned X-ray diffraction method. The method for measuring the average ferrite particle size is based on the cutting method described in JIS G0552 Steel Ferrite Grain Size Test Method, and the average retained austenite particle size is defined as the average circle equivalent diameter, and the value obtained from an image processing device or the like. It was adopted.

The carbon concentration of the retained austenite is X
It was determined experimentally by the line diffraction method. Measurement of lattice constant is Co,
Using Kα rays of Cu and Fe, (00
2), (022), (113), and (222) plane reflection angles
And the literature ("Introduction to X-ray diffraction": BD Cul)
(written by Gentaro Matsumura, Agne Co., Ltd.)
It can be obtained by calculating the lattice constant from the reflection angle.
Wear. Where cos ^Twoθ (where θ is the reflection angle) and
Since the lattice constant a has a linear relationship, the true lattice constant
a₀Calculates this line as cos^TwoIt is obtained by extrapolating to θ = 0.

Further, from the value of the lattice constant a ₀ of this true austenite, the carbon concentration of austenite can be determined, for example, according to the literature (RC Ruhl and M. Cohen, T.C.
transaction of the Metallu
rgical Society of AIME, vo
l 245 (1969) pp 241), which is a value obtained by using a ₀ = 3.572 + 0.033% C (carbon concentration) which is a formula showing the relationship between the austenite lattice constant a ₀ and the austenite carbon concentration. However, the measured number of retained austenite grains was 5 or more, and the carbon concentration was the average value.

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 are determined by changing the fatigue limit σ _W at 10 × 10 ⁷ times to the tensile strength σ of the steel sheet.
Evaluation was made by dividing by _B (fatigue limit ratio σ _W / σ _B ). However, the surface of the fatigue test piece was a pickled surface without any grinding or the like. On the other hand, regarding burring workability (hole expanding property), Japan Iron and Steel Federation Standard JFS T1001-1996
The evaluation was based on the hole expansion value according to the described hole expansion test method.

[0046]

[Table 2]

According to the present invention, steels A-1, E, I,
9 steels of J, K, L, M, N, and O, containing a predetermined amount of steel components, and having a microstructure having a volume fraction of 5% or more and 25% or more.
% Of residual austenite, the remainder being a composite structure mainly composed of ferrite and bainite, having an average ferrite grain size of 2 μm to 20 μm, and a value obtained by dividing the average retained austenite grain size by the average ferrite grain size of 0.05.
A work-induced transformation-type composite structure steel sheet excellent in burring workability, characterized in that the carbon content of retained austenite is not less than 0.8 and not more than 0.2% and not more than 3%.

The steels other than those described above were used for the following reasons.
Out of the range of light. That is, steel A-2 is finished at the finish rolling.
Temperature (FT) is lower than the range of the present invention, strain remains
Low strength-ductility balance (TS × El)
The value (λ) is also low. Steel A-3 has a finish rolling end temperature (F
T) is higher than the range of the present invention, and the target microstructure is
Strength-ductility balance (TS × El)
Low fatigue limit ratio (σ _W/ Σ_B) Is also low. Steel A-4 is
Retention temperature (MT) is lower than the range of the present invention,
Strength-ductility balance due to lack of microstructure
(TS × El) is low and the fatigue limit ratio (σ_W/ Σ_B) Also low
No.

[0049] Steel A-5 has a higher retention temperature (MT) than the range of the present invention and has a low strength-ductility balance (TS x El) because the desired microstructure is not obtained, and a fatigue limit ratio (σ). _W / σ _B ) is also low. Steel A-6 has a residence time (M
T), and the desired microstructure was not obtained, so that the strength-ductility balance (TS × El) was low and the fatigue limit ratio (σ _W / σ _B ) was low. Also, a sufficient hole expansion value (λ) has not been obtained. Steel A-7 has a cooling rate after retention (CR)
However, since the desired microstructure was not obtained, the strength-ductility balance (TS × El) was low and the fatigue limit ratio (σ _W / σ _B ) was low. Also, a sufficient hole expansion value (λ) has not been obtained.

The steel A-8 has a higher winding temperature (CT) than the range of the present invention, and has a low strength-ductility balance (TS × El) because a desired microstructure is not obtained. Steel A-
In No. 9, the winding temperature (CT) was lower than the range of the present invention, and the strength-ductility balance (TS × El) was low because the desired microstructure was not obtained. In steel B, since the content of C is out of the range of the present invention, a desired microstructure cannot be obtained, and a sufficient strength (TS) and a fatigue limit ratio (σ _W / σ _B ) cannot be obtained. Steel C has sufficient strength (TS) and fatigue limit ratio (σ) since the content of Si is out of the range of the present invention.
_W / σ _B ) has not been obtained.

In steel D, the Mn content was out of the range of the present invention, and the desired microstructure was not obtained, so that the strength-ductility balance (TS × El) was low and the fatigue limit ratio (σ
_W / σ _B ) is also low. Steel F does not have a sufficient fatigue limit ratio (σ _W / σ _B ) because the content of P is out of the range of the present invention. Steel G does not have sufficient hole expansion value (λ) and fatigue limit ratio (σ _W / σ _B ) because the content of S is out of the range of the present invention. Steel H does not have sufficient elongation (El), hole expansion value (λ), and fatigue limit ratio (σ _W / σ _B ) because the content of C is outside the range of the present invention.

[0052]

As described above in detail, the present invention provides a work-induced transformed 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, it is possible to expect a significant improvement in burring workability (hole expanding property) while sufficiently securing fatigue properties and ductility, and thus 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 relation to the relative sizes of ferrite average grain size and retained austenite grain size with respect to ferrite grain size and hole expansion values.

FIG. 2 is a diagram showing the results of preliminary experiments leading to the present invention in the relationship between the carbon concentration of retained austenite and the hole expansion value.

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

Continuing from the front page (72) Inventor Hiroyuki Okada 5-3 Tokai-cho, Tokai-shi, Aichi F-term in Nippon Steel Corporation Nagoya Works (reference) 4K032 AA01 AA02 AA04 AA05 AA08 AA11 AA14 AA15 AA16 AA17 AA19 AA22 AA23 AA27 AA29 AA31 AA32. JA07

Claims (8)

[Claims] 1. In mass%, C: 0.01 to 0.3%, Si: 0.01 to 2%, Mn: 0.05 to 3%, P: ≦ 0.1%, S: ≦ 0.01%, Al: 0.005 to 1%, the balance being Fe and inevitable impurities, the microstructure of which contains retained austenite with a volume fraction of 5% or more and 25% or less. The remainder is a composite structure mainly composed of ferrite and bainite, the average ferrite grain size is 2 μm or more and 20 μm or less, the value obtained by dividing the residual austenite average grain size by the ferrite average grain size is 0.05 or more and 0.8 or less, The carbon concentration of the retained austenite is 0.
A work-induced transformation-type composite structure steel sheet having excellent burring workability, wherein the steel sheet content is 2% or more and 3% or less. 2. The steel according to claim 1, further comprising:
2. The steel sheet according to claim 1, wherein the steel sheet contains 0.2 to 2% of burring workability. 3. 3. The steel according to claim 1, further comprising:
The work-induced transformation type composite structure steel sheet having excellent burring workability according to claim 1 or 2, characterized by containing 0.0002 to 0.002%. 4. The steel according to claim 1, further comprising:
The multi-structure steel sheet according to any one of claims 1 to 3, wherein the multi-structure steel sheet has excellent burring workability, comprising 0.1 to 1%. 5. The steel further comprises, in mass%, one or two of Ca: 0.0005 to 0.002% and REM: 0.0005 to 0.02%. The work-induced transformation type composite structure steel sheet according to any one of claims 1 to 4, which is excellent in burring workability. 6. The steel further comprises, by mass%, Ti: 0.05 to 0.5%, Nb: 0.01 to 0.5%, Mo: 0.05 to 1%, V: 0. 6. One to two or more of 0.02 to 0.2%, Cr: 0.01 to 1%, and Zr: 0.02 to 0.2%. A work-induced transformation-type composite structure steel sheet having excellent burring workability according to any one of the preceding claims. 7. Hot-rolling of a slab having the composition according to claim 1 at a temperature between the Ar ₃ transformation point temperature and the Ar ₃ transformation point temperature + 100 ° C. or less. Is completed, and is retained for 1 to 20 seconds in a temperature range from the Ar ₁ transformation point temperature to the Ar ₃ transformation point temperature, and then 20
Cooling at a cooling rate of at least 350 ° C / s
A microstructure including a retained austenite with a volume fraction of 5% or more and 25% or less, and a balance mainly composed of ferrite and bainite. Is 2 μm or more and 20
μm or less, a steel sheet having a value obtained by dividing the average retained austenite grain size by the average ferrite grain size of 0.05 to 0.8 and a carbon concentration of the retained austenite of 0.2% to 3%. A method for producing a work-induced transformed composite structure steel sheet having excellent burring workability. Upon wherein said hot rolling after rough rolling end performs high pressure descaling, Ar ₃ transformation point temperature or more Ar ₃
The method for producing a work-induced transformation-type composite structure steel sheet excellent in burring workability according to claim 7, wherein the hot finish rolling is completed at a transformation point temperature of + 100 ° C or lower.