JP2001032041A - High strength hot rolled steel plate excellent in workability, and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength hot rolled steel plate excellent in workability, and its manufacturing method. SOLUTION: The high strength hot rolled steel plate has a structure which contains ferrite as a principal phase and also contains residual (γ) in an amount of >=5 vol.% on the average and in which the difference between the maximum and the minimum value of austenite content in the positions in plate-thickness direction in the region between a position at a depth of 0.1 mm from the surface of the steel plate and a position at a depth of 0.1 mm from the rear surface of the steel plate, that is ΔVγ is regulated to <=3 vol.%. It is preferable that hot finish rolling is performed at 780-980 deg.C rolling finishing temperature while applying lubrication in such a way that rolling load becomes <=80% of the rolling load at nonlubricating rolling and that the resultant steel plate is subjected to cooling at >=50 deg.C/s cooling rate down to 620-780 deg.C, to isothermal holding treatment for 1 to 10 sec or to slow cooling treatment at <=20 deg.C/s cooling rate, to forced cooling at >=50 deg.C/s cooling rate down to 300-500 deg.C, and then to coiling.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a high-strength hot-rolled steel sheet excellent in workability and suitable for use as a steel sheet for automobiles.

[0002]

2. Description of the Related Art In recent years, there has been a strong movement to control the exhaust gas of automobiles to protect the global environment, and the weight of automobiles has been reduced. As a steel sheet used for reducing the weight of automobiles, a high-strength thin steel sheet excellent in formability has been strongly demanded. In addition, a hot-rolled steel sheet is more economically advantageous than a cold-rolled steel sheet, and there is a strong demand for a high-strength hot-rolled steel sheet having excellent formability in consideration of economy.

In response to such a demand, various high-strength hot-rolled steel sheets have been developed in consideration of formability. For example, it has a composite structure of ferrite + martensite
There is Dual-Phase steel (hereinafter referred to as DP steel). DP steel is
Conventionally, it has been used as a steel having an excellent strength-elongation balance. However, strength-elongation balance of DP steel TS ×
El is at most about 20,000 MPa ·%, and cannot meet the recent demand for high strength-elongation balance (25,000 MPa ·% or more) stably.

Further, Japanese Patent Publication No. 6-41617, Japanese Patent Publication No.
JP-A-65566 and JP-B-5-67682 disclose a method for producing a high-workability, high-strength hot-rolled steel sheet containing ferrite, bainite, and retained austenite.
This hot rolled steel sheet is a so-called Transformation Induced Pla
It is a sticity steel (hereinafter referred to as TRIP steel) and contains 5% or more of retained austenite by volume in order to achieve both workability and high strength.

FIG. 3 shows a typical continuous cooling transformation curve (CCT diagram) of TRIP steel. The TRIP steel sheet precipitates 60 to 90% by volume of pro-eutectoid ferrite (also referred to as polygonal ferrite) by being slightly retained in the ferrite region (PF region in FIG. 2) after hot rolling, and at the same time, untransformed austenite. After promoting the concentration of solid solution carbon in the phase to increase the stability of austenite, it is led to a bainite region (region B in FIG. 2) and gradually cooled in this region to cause bainite transformation, It is manufactured by leaving a predetermined amount of austenite.

[0006]

In the recent press forming of steel plates for automobiles, beads are provided on a presser of a mold so that the steel plates pass through the beads during press forming to suppress wrinkles. are doing. Therefore, when the steel sheet passes through the bead, a bending-unbending deformation is applied to the steel sheet. If the bending-unbending deformation by the bead is severe, the steel sheet may crack. For this reason,
Usually, a bead having a bending radius that satisfies bending-bending conditions that does not cause cracking in the steel sheet is used. However, in recent years, press conditions have become more severe, and in order to suppress the occurrence of wrinkles, it has been aimed to provide a bead having a smaller bending radius in a plate retainer of a mold to perform press forming.

In the press forming under such severe conditions, the TRIP steel sheet described in Japanese Patent Publication No. 6-41617, Japanese Patent Publication No. 5-65566, Japanese Patent Publication No. 5-67682, etc. has a small bending radius. There was a problem that it was not able to withstand the bending-unbending deformation due to the bead, cracking might occur, and stable press molding could not be performed.

The present invention solves the above-mentioned problems of the prior art, and proposes a high-strength hot-rolled steel sheet excellent in workability, in which press formability is remarkably improved as compared with the conventional TRIP steel sheet, and a method for producing the same. With the goal.

[0009]

Means for Solving the Problems In order to achieve the above object, the present inventors have intensively studied factors affecting the bending characteristics of a hot-rolled steel sheet. As a result, it was found that the bending properties of the hot-rolled steel sheet were greatly affected by the distribution of the retained austenite in the thickness direction. The distribution of residual austenite in the thickness direction of the steel sheet differs due to the difference in the strain distribution in the thickness direction of the steel sheet introduced during hot rolling. It has been found that this is extremely important for the improvement of the bending characteristics.

The precipitation rate of pro-eutectoid ferrite (PF) shown in FIG. 3 largely depends on the amount of strain accumulated during rolling. In other words, the larger the amount of strain accumulated, the higher the precipitation rate of proeutectoid ferrite, and the higher the C content in the austenite phase.
Is sufficiently concentrated, and the amount of retained austenite increases. Conversely, if the amount of strain accumulated during rolling is small, the amount of retained austenite obtained will be small.

From these facts, the present inventors paid attention to the effect of lubrication on the amount of retained austenite, and as a result of examination, as a result of appropriate lubrication rolling, the amount of retained austenite was increased and the thickness of the steel sheet was increased. It was found that the direction becomes uniform. This means that when hot rolling is performed without lubrication, the amount of strain introduced into the steel sheet during rolling has a distribution in the thickness direction, whereas when hot rolling is performed with lubrication, Strain is introduced uniformly during rolling. For this reason,
It is considered that when rolling is performed while lubricating during hot rolling, the amount of retained austenite obtained is larger than when rolling is performed without lubrication, and is evenly distributed in the thickness direction.

First, an experiment conducted by the present inventors on the effect of lubrication on the distribution of retained austenite in the thickness direction will be described. Using a seven-stand finishing hot rolling mill, hot finish rolling was performed with various amounts of rolling oil, including non-lubrication, to produce hot-rolled steel sheets. The rolling load was measured at each stand during hot rolling, and the amount of retained austenite of the obtained hot-rolled steel sheet was examined by X-ray diffraction. The amount of retained austenite (volume%) was measured at each position in the thickness direction of the steel sheet, that is, from the steel sheet surface in the thickness direction.
0.1 mm position, 1/8 position of plate thickness, 2/8 position of plate thickness,
3/8 position of the plate thickness, 4/8 position of the plate thickness, 5/8 position of the plate thickness, 6/8 position of the plate thickness, 7/8 position of the plate thickness and 0.1 mm from the back of the plate At a total of 9 locations. Of the obtained amount of retained austenite at each position in the thickness direction of these steel sheets,
The maximum content Vmax and the minimum content Vmin were determined, and their difference ΔVγ (= Vmax−Vmin) was calculated.

FIG. 1 shows the relationship between ΔVγ and the rolling load ratio. Here, the rolling load ratio refers to the ratio of the rolling load at the time of lubrication and the rolling load at the time of non-lubrication obtained at each stand (rolling load at the time of lubrication / rolling load at the time of no lubrication). The rolling load ratio shown in the figure is 7
The average value at the stand was adopted. According to FIG. 1, when the rolling load ratio is reduced to 0.8 or less, that is, lubrication is performed so that the rolling load ratio is 0.8 or less, ΔVγ is reduced to 3.0% or less, and the distribution of residual austenite in the thickness direction is made uniform. You can see that it can be done.

The present invention has been completed based on the above findings and further studies. That is, the first present invention is a hot-rolled steel sheet having a structure containing ferrite as a main phase and containing retained austenite as a second phase, containing the retained austenite in an amount of 5% by volume or more on average and 0.1% from the steel sheet surface. mm and the maximum content V of the retained austenite at each position in the thickness direction between 0.1 mm and 0.1 mm from the back of the steel plate.
difference ΔVγ between max and minimum content Vmin (Vmax-Vmin)
Is less than 3.0% by volume and the total elongation equivalent to 2mm
It is a high-strength hot-rolled steel sheet with excellent workability characterized by being at least 34%.

In the first aspect of the present invention, the hot-rolled steel sheet has a mass% of C: 0.05 to 0.40%, Si: 0.1 to 3.0%,
Mn: preferably a hot-rolled steel sheet containing 0.6 to 3.0% and having a composition consisting of the balance of Fe and inevitable impurities,
The hot-rolled steel sheet has a mass% of C: 0.05 to 0.40%,
Si: 0.1-3.0%, Mn: 0.6-3.0%,
Preferably, the hot-rolled steel sheet has a composition containing one or two selected from P: 0.01 to 0.2% and Al: 0.01 to 0.3%.
And C: 0.05-0.40%, Si: 0.1-3.0%, Mn: 0.6-
3.0%, Ti: 0.005 to 0.25%, Nb: 0.00
Preferably, the hot-rolled steel sheet has a composition containing one or two selected from 3 to 0.1%, and the hot-rolled steel sheet has a mass% of C: 0.05 to 0.40%, S:
i: 0.1 to 3.0%, Mn: 0.6 to 3.0%,
P: 0.01 to 0.2%, Al: one or two selected from 0.01 to 0.3%, Ti: 0.005 to 0.25%, Nb: 0.003
It is preferable to use a hot-rolled steel sheet having a composition containing one or two selected from 0.1% to 0.1%. Also,
In the first invention, in addition to each of the above-mentioned compositions,
%, The composition may contain Ca: 0.01% or less.

[0016] The second present invention is characterized in that: C: 0.05 to 0.40 mass%,
After heating a steel slab having a composition containing Si: 1.0 to 3.0 mass% and Mn: 0.6 to 3.0 mass% to 1000 to 1300 ° C and rough rolling, the rolling load is reduced to 80% or less of the rolling load during lubrication-free rolling. While performing lubrication so as to achieve a finish rolling at a rolling end temperature in the range of 780 to 980 ° C., 50 ° C./s after completion of the finish rolling
After cooling to 620 to 780 ° C at the above cooling rate,
Isothermal holding treatment for 2 sec or slow cooling treatment at a cooling rate of 20 ° C / s or less is performed.
This is a method for producing a high-strength hot-rolled steel sheet excellent in workability, which is forcibly cooled to ~ 500 ° C and wound around a coil. In the second aspect of the present invention, in addition to the above composition, one or two kinds selected from P: 0.01 to 0.2% and Al: 0.01 to 0.3%, Ti: 0.005 to 0.25%, Nb: 0.00
One or two selected from 3 to 0.1%, Ca:
0.01 mass% or less may be contained alone or in combination.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be specifically described below. The hot-rolled steel sheet according to the first aspect of the present invention is a high-strength hot-rolled steel sheet having a structure including ferrite as a main phase and a second phase including retained austenite. The ferrite as the main phase is a pro-eutectoid ferrite precipitated by cooling after hot rolling as shown in FIG. 3, and is preferably contained in a volume fraction of 50% or more. The second phase contains retained austenite, and may additionally contain bainite or martensite.

The hot-rolled steel sheet of the present invention contains retained austenite in an average of 5% by volume or more. If the amount of retained austenite is less than 5% by volume on average, a good strength-elongation balance TS × El of 24000 MPa ·% or more cannot be obtained. On the other hand, in order to make the amount of retained austenite more than 20% by volume on average, it is necessary to add a large amount of alloying elements, which is not practical. Therefore, the amount of retained austenite is
It is preferable that the content be 20% by volume or less.

In the present invention, the amount of retained austenite is measured by X-ray diffraction. The average value of the retained austenite amount is an average value measured at each position in the thickness direction of the hot-rolled steel sheet. The position where the amount of retained austenite is measured is 0.1 mm from the steel sheet surface in the thickness direction.
And 0.1 mm from the back of the steel sheet in the thickness direction of the steel sheet, preferably in the thickness direction, at a position of 0.1 mm from the steel sheet surface, at a position of 1/8 of the sheet thickness, and at a position of 2/8 of the sheet thickness. Position, 3/8 of plate thickness
Position, 4/8 position of plate thickness, 5/8 position of plate thickness,
6/8 position, 7/8 position of plate thickness and 0.1mm from back of plate
It is preferable to use a total of 9 positions, i.e., from the viewpoint of simplicity in measurement, but the present invention is not limited to this.

The difference ΔVγ (= Vmax−Vmin) between the maximum content Vmax and the minimum content Vmin of the retained austenite is 3.
0% by volume or less In the present invention, 0.1 mm from the steel plate surface and 0.1 mm from the steel plate back surface
The difference ΔVγ between Vmax and Vmin of the amount of retained austenite at each position in the thickness direction is limited to 3.0% by volume or less. When ΔVγ exceeds 3.0% by volume, the critical bending radius Rmi
The ratio of n to the plate thickness t, Rmin / t, becomes 2 or more, and the bending properties are degraded and the press formability is degraded. Therefore, ΔVγ is
Limited to 3.0% by volume or less.

In the present invention, the amount of retained austenite is measured at each position in the thickness direction between 0.1 mm from the steel plate surface and 0.1 mm from the steel plate rear surface, and the maximum content Vmax and the minimum content Vmin are determined from the residual austenite amount. And ΔVγ
(= Vmax-Vmin) is calculated. The positions in the sheet thickness direction for measuring the amount of retained austenite for calculating ΔVγ are 0.1 mm from the steel sheet surface, 1/8 the sheet thickness, and 2/8 the sheet thickness in the sheet thickness direction. , 3/8 position of board thickness,
4/8 position of the plate thickness, 5/8 position of the plate thickness, 6/8 position of the plate thickness, 7/8 position of the plate thickness and 0.1 mm position from the back of the plate,
However, the present invention is not limited to this.

The thickness 2mm equivalent total elongation E _2: 34% or more hot rolled steel sheet of the present invention is a press-molding, the total elongation E ₂ equivalent thickness 2mm in order to ensure a good press formability
It is necessary to have more than 34%, and the total elongation E ₂ having a thickness of 2mm equivalent is limited to more than 34%. Total elongation E ₂ having a thickness of 2mm equivalent is defined by the following formula (1), the total elongation E ₁ obtained by the tensile test according to JIS Z 2241, a thickness of 2mm JI
S means the value converted to the total elongation when tested with a No. 5 test piece. E ₂ (%) = E ₁ [｛L ₁ × (50) ^1/2 ｝ / ｛50 × (A ₁ ) ^1/2 ｝] ^0.4 … (1) where E ₁ = based on JIS Z 2241 L ₁ = gauge length of tensile test specimen (mm) A ₁ = cross-sectional area of parallel part of tensile test specimen (mm ² ) Next, residual austenite amount and total elongation E _A description will be given of the components of the steel sheet suitable for setting ₂ to the above range. Hereinafter, mass% in the composition is simply described as%.

C: 0.05 to 0.40% C is an element useful for forming retained austenite and also contributes to strengthening of steel.
If the C content is less than 0.05%, the above-mentioned effects are not obtained. On the other hand, if it exceeds 0.40%, the weldability will decrease, so C
Is preferably limited to the range of 0.05 to 0.40%.

Si: 1.0 to 3.0% Si is an element indispensable for the generation of retained austenite, and for that purpose, it is desirable to contain at least 1.0%. On the other hand, if the content exceeds 3.0%, not only the ductility is reduced, but also the scale properties are reduced, and the surface quality becomes a problem. Therefore, the content of Si is preferably in the range of 1.0 to 3.0%.

Mn: 0.6-3.0% Mn is a useful element for producing retained austenite and also a useful element contributing to the strengthening of steel. However, if the Mn content is less than 0.6%, the above effects cannot be obtained. On the other hand, if it exceeds 3.0%, ductility is reduced. For this reason, Mn is preferably set in the range of 0.6 to 3.0%.

In the present invention, if necessary,
P: 0.01 to 0.2%, Al: one or two selected from 0.01 to 0.3%, Ti: 0.005 to 0.25%, Nb: 0.003 to
One or two selected from 0.1%, and Ca:
0.01% can be contained alone or in combination. Hereinafter, desirable contents of these elements will be described.

One or two selected from P: 0.01 to 0.2% and Al: 0.01 to 0.3% P and Al are useful as elements for forming retained austenite, and may be selected as necessary. Can be contained. P
If the content is less than 0.01%, the above effect is poor, while if it exceeds 0.2%, the secondary work brittleness resistance deteriorates. For this reason, P is desirably in the range of 0.01 to 0.2%.

Al, similarly to P, has a poor effect when its content is less than 0.01%, whereas when it exceeds 0.3%, the ductility is deteriorated. For this reason, Al is desirably in the range of 0.01 to 0.3 mass%. Nb: 0.003 to 0.1%, Ti: 0.005 to 0.25% One or two types selected from the group consisting of Nb and Ti are effective in improving strength by reducing the size of ferrite, which is the main phase. It is a contributing element and can be contained as needed.

If the contents of Nb and Ti are less than 0.003% and 0.005%, respectively, the effect cannot be expected. On the other hand, Nb, Ti
If the content exceeds 0.1% and 0.25%, respectively, the ductility is reduced. Therefore, Nb is 0.003 to 0.1% and Ti is 0.1%.
Preferably, it is in the range of 005 to 0.25%. Ca: 0.01% or less Ca is an element for improving stretch flangeability, and can be contained as necessary. However, if the content is large, the corrosion resistance is reduced. Therefore, when the content is large, the content is preferably limited to 0.01% or less.

The balance other than the above components is substantially Fe. As unavoidable impurities, S: 0.01% or less, N: 0.01% or less, and O: 0.01% or less are acceptable. Next, a method for producing a hot-rolled steel sheet according to the present invention will be described.
A steel slab having the above composition is heated and then hot-rolled under predetermined conditions to obtain a hot-rolled steel sheet.

Slab heating temperature: 1000 to 1300 ° C. If the slab heating temperature is less than 1000 ° C., the surface quality of the steel sheet deteriorates remarkably. On the other hand, if it exceeds 1300 ° C., the crystal grains become coarse, and the material homogeneity and This causes deterioration of ductility. For this reason, the heating temperature of the slab is preferably in the range of 1000 to 1300 ° C. Although the heating time is not particularly limited, it is preferable that the heating time is not more than about 60 min because heating for a long time significantly increases the crystal grain size.

Next, the slab is subjected to rough rolling to form a sheet bar. Rough rolling conditions may be ordinary conditions, and need not be particularly limited. After the rough rolling, the sheet bar is subjected to finish rolling. In the finish rolling step, it is preferable to perform lubrication rolling. As the lubrication condition, it is preferable to perform lubrication so that the rolling load is 80% or less of the rolling load when no lubrication is performed. The change in rolling load can be achieved by adjusting the amount of rolling oil for lubrication. In addition,
When performing finish rolling using multiple stands,
The average value of the rolling load ratio (rolling load at the time of lubrication / rolling load at the time of no lubrication) of all the stands may be 80% or less.

Rolling end temperature in finish rolling: 780 to
If the finish temperature of the 980 ° C finish rolling is lower than 780 ° C, the work structure remains in the steel sheet, resulting in deterioration of ductility. On the other hand, if the rolling end temperature exceeds 980 ° C., the structure of the steel sheet becomes coarse and the formability is reduced due to the delay of ferrite transformation. For this reason, the rolling end temperature in finish rolling is 780 to 980 ° C.
It is preferable to set it in the range.

Cooling after finishing rolling: From 620 to 780 ° C. at a cooling rate of 50 ° C./s or more, after finishing rolling, near the nose of the proeutectoid ferrite region (62
Cool rapidly to 0-780 ° C). Rapid cooling rate of 50 ℃ / s
If it is less than 50%, the effect of promoting the ferrite transformation is small, so that the rapid cooling rate is preferably 50 ° C./s or more. However, if the temperature exceeds 300 ° C./s, the shape of the steel sheet deteriorates.

After quenching to the vicinity of the nose of the pro-eutectoid ferrite region, isothermal holding treatment in this temperature region for 1 to 10 seconds or slow cooling treatment at a cooling rate of 20 ° C./s or less After quenching to a temperature region of 620 to 780 ° C. , In this temperature range
By maintaining the temperature isothermally for 10 seconds or gradually cooling at a rate of 20 ° C./s or less, proeutectoid ferrite as a main phase can be easily obtained. In order to carry out the isothermal holding process for more than 10 seconds, the air cooling zone on the exit side of the finishing rolling mill needs to be long, which is difficult in an actual machine line. If the isothermal holding time is less than 1 second, the amount of ferrite generated is small. In the case of slow cooling, if the cooling rate exceeds 20 ° C / s,
There is a problem that the amount of ferrite generated is small. In the case of the slow cooling treatment, if the cooling stop temperature of the slow cooling is lower than 600 ° C., pearlite transformation may occur and it may become impossible to effectively secure proeutectoid ferrite.
The temperature is preferably set to 0 ° C. or higher.

Then, at a cooling rate of 50 ° C./s or more,
Cool to a temperature range of 500 ° C. By this treatment, the austenite phase enriched with C undergoes bainite transformation in the bainite region at 300 to 500 ° C., and the austenite phase remains. If the cooling rate is less than 50 ° C./s, the austenite phase undergoing pearlite transformation increases, and it is not possible to obtain retained austenite of 5% or more on average.

[0037]

EXAMPLE Molten steel having the components shown in Table 1 was melted in a converter, and was made into a slab by a continuous casting method. After heating these slabs to 1200 ° C., rough rolling was performed, and then finish rolling was performed so that the rolling end temperature was 860 ° C., and then cooled to 700 ° C. at a cooling rate of 60 ° C./s. After holding for 2 seconds, it was cooled to 450 ° C. at a cooling rate of 60 ° C./s and wound into a coil to obtain a hot-rolled steel sheet. In addition, hold for 15 minutes after winding, and then
/ H to room temperature.

As shown in Table 2, the finish rolling was performed without lubrication (no lubrication) and with lubrication. The rolling loads in the case of lubricating rolling and non-lubricating rolling were measured, and the rolling load ratio (rolling load during lubricating rolling / rolling load during non-lubricating rolling) was determined. When the rolling load ratio was larger than 0.8, no lubrication was performed, and when the rolling load ratio was 0.8 or less, lubrication was performed. A test specimen was collected from the obtained hot-rolled steel sheet, and the amount of retained austenite at each position in the thickness direction was measured by X-ray diffraction. Using the measured amount of retained austenite at each position in the thickness direction, an average value is calculated, and the maximum content Vmax and the minimum content Vmin of the amount of retained austenite at each position in the thickness direction are determined, and ΔVγ (= Vmax-Vmin) was calculated. The measurement positions are 0.1 mm from the plate surface, 1/8 of the plate thickness, 2/8 of the plate thickness, and 3/8 of the plate thickness in the plate thickness direction.
Position, 4/8 position of plate thickness, 5/8 position of plate thickness,
6/8 position, 7/8 position of plate thickness and 0.1mm from back of plate
, A total of 9 locations. In addition to the amount of retained austenite, the amount of ferrite as a main phase was also measured.

A tensile test piece and a bending test piece were cut out from the obtained hot-rolled steel sheet, and a tensile test and a bending test were performed. In the tensile test, the yield strength (YS), tensile strength (TS) and total elongation (E ₁ ) were determined in accordance with JIS Z 22411. Wherein the total elongation values E ₁ obtained (1) in terms of total elongation E ₂ thickness 2mm equivalent based on type.

In the bending test, the bead radius R was changed.
After sandwiching the hot-rolled steel sheet with the apparatus shown in FIG. 2, the sheet is subjected to bending-unbending deformation by pulling out, observing the presence or absence of cracks, and determining the critical bending radius Rmin that can be pulled out without breaking,
The ratio of Rmin to the plate thickness tmm, Rmin / t, was used as an index of bending-unbending deformability. The bending-unbending deformation is the thickness t ×
The press oil viscosity 300sct the specimen of 20 mm × 100 mm was applied to both surfaces of the coating amount per surface 2 g / m ^2, pressing load 300 kg,
The drawing speed was 5 mm / s.

Table 2 shows the obtained results.

[0042]

[Table 1]

[0043]

[Table 2]

Examples of the present invention (steel sheets No. 1 to No. 3, No. 7, No. 9,
No.10, No.14, No.16-19) have tensile strength TS of 650M
Pa and high strength, and ΔVγ is 3.0% by volume or less and the amount of retained austenite in the thickness direction has a uniform distribution,
Rmin / t is 1.6 or less and good bending - unbending has deformability, also total elongation E ₂ is 34% or more, strength - elongation balance TS × E ₂ is chromatic excellent properties and 25000 MPa ·% or more are doing.

On the other hand, in Comparative Examples (steel Nos. 4 to 6) which are out of the range of the present invention, ΔVγ exceeds 3.0% due to insufficient lubrication conditions. 2.0
% Or more, and the bending-unbending deformability is reduced.
Further, the steel sheets No. 11 to No. 13 (Comparative Example) have low elongation because the amount of retained austenite (average value) in the thickness direction is small, and TS × E ₂ is 20,000 MPa ·% or less. The balance has deteriorated. bad. In addition, steel sheet No.15, No.2
0 (Comparative Example) has a lower total elongation E ₂ because the steel composition is outside the preferred range of the present invention.

[0046]

According to the present invention, a hot-rolled steel sheet having a good strength-elongation balance and good bending-unbending deformability and capable of withstanding severe press forming can be obtained. It is expected to expand applications, and has a significant industrial effect.

[Brief description of the drawings]

FIG. 1 is a graph showing a relationship between a rolling load and ΔVγ.

FIG. 2 is a schematic explanatory view of a bending test apparatus.

FIG. 3 is an explanatory diagram showing a typical continuous cooling transformation diagram (CCT) of hot-rolled TRIP steel.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Tetsuzumi Shimizu 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Prefecture Inside the Technical Research Laboratory, Kawasaki Steel Works (72) Inventor Osamu Furukun 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Address F-term in Kawasaki Steel Engineering Laboratory (reference) 4K037 EA01 EA05 EA06 EA09 EA11 EA15 EA16 EA19 EA23 EA25 EA27 EA28 EA31 EB05 EB08 EB09 EB12 FA02 FA03 FC03 FC04 FD03 FD04 FD08 FE01 JA02

Claims (6)

[Claims] 1. A hot-rolled steel sheet having a structure containing ferrite as a main phase and having retained austenite as a second phase, comprising at least 5% by volume of the retained austenite on average, and 0.1 mm from the surface of the steel sheet and the back surface of the steel sheet. Between the maximum content Vmax and the minimum content Vmin of the retained austenite (Vmax-Vmi) at each position in the thickness direction between 0.1 and 0.1 mm.
A high-strength hot-rolled steel sheet excellent in workability, wherein n) is not more than 3.0% by volume and the total elongation corresponding to a sheet thickness of 2 mm is not less than 34%. 2. The hot-rolled steel sheet has a mass% of C: 0.05 to 2.
0.40%, Si: 0.1-3.0%, Mn: 0.6-3.0%
The high-strength hot-rolled steel sheet having excellent workability according to claim 1, wherein the high-strength hot-rolled steel sheet has a composition consisting of a balance of Fe and unavoidable impurities. 3. In addition to the above composition, in mass%,
The high-strength hot-rolled steel having excellent workability according to claim 2, characterized in that it has a composition containing one or two selected from P: 0.01 to 0.2% and Al: 0.01 to 0.3%. steel sheet. 4. In addition to the composition, T
4. The high strength excellent in processability according to claim 2, wherein the composition has a composition containing one or two selected from i: 0.005 to 0.25% and Nb: 0.003 to 0.1%. Hot rolled steel sheet. 5. In addition to the above composition, in mass%, C
The high-strength hot-rolled steel sheet according to any one of claims 2 to 4, having a composition containing a: 0.01% or less. 6. C: 0.05 to 0.40 mass%, Si: 1.0 to 3.0
mass%, Mn: A steel slab having a composition containing 0.6 to 3.0 mass% is heated to 1000 to 1300 ° C, rough-rolled, and then lubricated so that the rolling load is 80% or less of the rolling load during unlubricated rolling. While performing, finish rolling is performed so that the rolling end temperature is in the range of 780 to 980 ° C., and after finishing the finish rolling, cooling is performed at a cooling rate of 50 ° C./s or more to 620 to 780 ° C., and then isothermally maintained for 1 to 10 seconds. Treatment or slow cooling at a cooling rate of 20 ° C / s or less,
Next, a method for producing a high-strength hot-rolled steel sheet excellent in workability, characterized in that it is forcibly cooled to 300 to 500 ° C. at a cooling rate of 50 ° C./s or more and wound around a coil.