JP2000063955A - Production of thin dual-phase hot rolled steel strip - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a thin dual-phase hot rolled steel strip of <=3.2 mm sheet thickness capable of obtaining uniform characteristics over the whole body of the steel strip. SOLUTION: This production method has a stage in which a rough bar or thin slab of steel contg., by weight, 0.03 to 0.15% C, 0.3 to 2.5% Si, 0.5 to 2.5% Mn, <=0.07% P, <=0.02% S, 0.005 to 0.08% sol.Al and <=0.008% N is produced, a stage in which, by heating the rough bar or thin slab, the finishing temp. in the steel strip is controlled to the range of the Ar3 transformation point to (the Ar3 transformations point +50 deg.C), and finish rolling is executed a stage in which the steel strip after the finish rolling is cooled to the temp. range of 600 to 780 deg.C at a cooling rate of >=20 deg.C/s and is held in the temp. range for >=3 sea and a stage in which the steel strip after the holding in the temp. range is cooled at a cooling rate of >=20 deg.C/s and is coiled at <=250 deg.C.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a thin two-phase hot-rolled steel strip composed of ferrite and martensite and having a plate thickness of 3.2 mm or less.

[0002]

2. Description of the Related Art In recent years, structural members for automobiles have been required to be lighter for energy saving and to have higher strength to improve safety, and the opportunities for applying high-tensile steel sheets have been increasing. Among them, a hot-rolled steel sheet having a two-phase structure composed of ferrite and martensite, which has a low yield ratio and an excellent strength-ductility balance, is often used for members requiring formability. However, if there is variation in the material between the coils or within the coil, cracks will occur during press forming and the springback amount will differ, so there is a demand for a steel sheet with less variation in the material.

As a method for producing this hot rolled steel sheet having a two-phase structure, there is a method in which a steel strip after hot rolling is heat-treated using a continuous annealing equipment or the like, but it is more cost-effective to produce the hot rolled steel sheet as it is. Desirably, Japanese Patent Laid-Open No. 61-79730, Japanese Patent Laid-Open No. 4-235219, Japanese Patent Laid-Open No. 4-289126, Japanese Patent Laid-Open No. 9-67641 disclose methods therefor. In any method, in order to obtain a proper two-phase structure of ferrite + martensite,
It is necessary to keep the finishing temperature just above the Ar ₃ transformation point and strictly control the cooling conditions after finish rolling.

A technique of heating a rough bar in order to control the finishing temperature of the hot-rolled steel strip just above the Ar ₃ point is disclosed in, for example, Japanese Patent Laid-Open No. 9-225517. However, in a two-phase steel sheet of the type that is manufactured by hot rolling as it is, since the transformation structure is controlled under hot rolling conditions, it is necessary to control the cooling conditions in addition to the finish rolling temperature more strictly than general steel. That is, if the runout cooling, especially the intermediate holding condition, varies, the ratio of the ferrite phase to the martensite phase changes, and the variation in the material becomes large. As described above, a uniform material cannot be obtained only by uniformizing the finishing temperature.

[0005]

Regarding variations in materials, it is necessary to control the finishing temperature and the intermediate holding conditions as described above. Among them, rolling is preferable at a constant speed in order to keep the runout cooling, especially the intermediate holding time as constant as possible and to secure an appropriate ratio of the ferrite phase and the martensite phase.

However, the plate thickness is thin, especially 3.2 m.
When a thin hot-rolled steel strip having a thickness of m or less is manufactured at a constant rolling speed, the temperature drop during rolling is remarkable at the edges of the steel strip, and the entire steel strip can be finished rolled at the Ar ₃ transformation point or higher required for workability. Or even if finish rolling can be performed at the Ar ₃ transformation point or higher, variations in finish rolling temperature occur, and uniform properties cannot be obtained over the entire steel strip.

[0007] On the other hand, there is a technique that focuses on equalizing the finishing temperature and makes the finish rolling temperature constant by accelerated rolling (zooming). The method is a technique for increasing the speed during rolling for the purpose of suppressing the temperature decrease at the rear end of the steel strip. However, even with this method, it is difficult to control the finishing temperature when the sheet thickness is thin, and when trying to make the finishing temperature uniform, the sheet thickness is thin, and therefore the rolling speed must be changed greatly, for example, from 500 to 800 mpm. is necessary. Therefore, the cooling and holding conditions from finishing to winding are different in the coil longitudinal direction. Especially, the intermediate holding time depends on the rolling speed, and in the above example, 1.6 depending on the position.
As it is twice as large, it is largely different in the longitudinal direction of the coil, and the uniformity of the material cannot be obtained.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a method for manufacturing a thin-duplex hot-rolled steel strip having uniform properties over the entire steel strip.

[0009]

[Means for Solving the Problems] The above problems are in wt%,
C: 0.03 to 0.15%, Si: 0.3 to 2.5%,
Mn: 0.5 to 2.5%, P: 0.07% or less, S:
0.02% or less, sol. Al: 0.005-0.08
%, N: 0.008% or less, and a step of producing a steel rough bar or thin slab, and heating the rough bar or thin slab to achieve a finishing temperature in the steel strip of Ar ₃ at a constant rolling speed. A step of finishing rolling within the range of transformation point to (Ar ₃ transformation point + 50 ° C.), and the steel strip after the finishing rolling is 20 times.
A step of cooling to a temperature range of 600 to 780 ° C. at a cooling rate of ℃ / s or more and holding the temperature range for 3 seconds or more; and a steel strip after being held in the temperature range at a cooling rate of 20 ° C./s or more. It is solved by a method for producing a thin thin two-phase structure hot rolled steel strip, which comprises a step of cooling and winding at a temperature of 250 ° C. or less.

The reasons for limiting the components and manufacturing conditions will be described below. C: When it exceeds 0.15 wt%, it becomes hard and has low ductility,
If it is less than 0.03 wt%, it is necessary to add a large amount of alloying elements in order to obtain a predetermined strength, resulting in high cost.

Si: It is necessary to appropriately add Si depending on the target strength level, but if it exceeds 2.5 wt%, the weldability deteriorates, and if it is less than 0.3 wt%, a two-phase structure of ferrite + martensite cannot be obtained. .

Similar to Mn: Si, it is necessary to add an appropriate amount depending on the target strength level, but if it exceeds 2.5 wt%, the weldability deteriorates, and if it is less than 0.5 wt%, a two-phase structure of ferrite + martensite occurs. I can't get it.

P: P is added as appropriate in order to increase the strength and improve the corrosion resistance, but if it exceeds 0.07 wt%, it has low ductility and low toughness, so it was made 0.07 wt% or less. When not intentionally added, the preferable range is 0.
It is 015 wt% or less.

S: If it exceeds 0.02 wt%, low ductility and low toughness will result, so the content was made 0.02 wt% or less. A preferred range is 0.005 wt% or less.

Sol. Al: 0.005 wt% or more is necessary for stable deoxidation of steel, but 0.08 wt
If it exceeds%, the effect will be saturated and the cost will increase, so
It was made 0.08 wt% or less. The preferred range is 0.01
~ 0.05 wt%.

N: When it exceeds 0.008 wt%, low ductility
Since it has low toughness, it is set to 0.008 wt% or less. A preferred range is 0.005 wt% or less.

Other elements can be contained within the range not hindering the present invention. For example, in order to improve stretch flangeability, Ca is 0.006 wt% or less, REM:
0.1 wt% or less, Cr for the hardenability improvement etc. 0.
8 wt% or less, B: 0.01 wt% or less, Ni: 0.5
wt% or less, W: 0.5 wt% or less, Mo: 0.7 wt
It can be added within the range of less than%. Further, for the purpose of precipitation strengthening or prevention of HAZ softening of the welded portion, Nb: 0.
06 wt% or less, Ti: 0.15 wt% or less, V: 0.
1 wt% or less, Zr: 0.1 wt% or less may be added, Sn: 0.1 wt% or less, for improving corrosion resistance,
Cu: Even if added appropriately within the range of 0.5 wt% or less, the effect of the present invention is not hindered.

When the steel containing such components is finish-rolled, it is necessary to produce a rough bar or a thin slab having a thickness corresponding to the rough bar after the steel is melted. The production method is not particularly limited, but usually, after smelting steel, it is made into a slab by continuous casting or ingot-decomposition rolling, and a rough bar is directly or directly reheated in a heating furnace to be rough rolled,
After the melting, a thin slab having a thickness corresponding to the coarse bar is manufactured by continuous casting.

When the slab is reheated, low temperature heating at 1250 ° C. or lower is preferable in order to prevent scale defects from occurring and to refine the austenite grains before finish rolling.

As described above, when a hot-rolled steel strip having a thin plate thickness is manufactured at a constant rolling speed, the temperature of the steel strip at the end thereof is remarkably lowered during rolling, and the finishing temperature cannot be secured. Uniform characteristics cannot be obtained throughout.
Therefore, the inventors of the present invention examined conditions under which uniform properties can be obtained over the entire steel strip. In order to obtain uniform properties over the entire steel strip, the finishing temperature in the steel strip was kept constant at a constant rolling speed in order to obtain uniform properties. It is necessary to carry out finish rolling within a range of ₃ transformation points to (Ar ₃ transformation point + 50 ° C), and as a means for that, it is effective to heat a rough bar or thin slab before finish rolling. It was revealed.

As a method of securing the finishing temperature while keeping the finishing rolling speed constant, for example, a method of winding the rough bar prior to the finishing rolling can be considered. However, it is difficult to control the finishing temperature at a constant rolling speed and in the narrow range as described above in a steel strip such as a thin steel strip in which the temperature drop at the end is remarkable.

Further, since the method is not a technique for positively raising the temperature, it is impossible to secure a predetermined finishing temperature when the slab heating temperature is too low. Further, even if wound around the coil, unless heating is performed, the temperature lowers at the front end and the rear end of the coil, and it is difficult to obtain a uniform finishing temperature in the entire longitudinal direction. In that respect, in the method of heating the rough bar or the thin slab, it is possible to change the heating conditions according to the temperature distribution in the longitudinal direction of the rough bar or the thin slab, and it is easy to make the finishing temperature of the steel strip uniform. is there.

In the present invention, it is important to heat the rough bar or thin slab, and it is also possible to wind the rough bar or thin slab into a coil and then heat the coil as it is, or to wind the coil into a coil. It is also possible to heat before, or after coiling and then unwinding,
In either case, the effect of uniformizing the finishing temperature by heating the coarse bar or thin slab is not impeded. In addition to heating the rough bar or the thin slab in the longitudinal direction, optionally using an edge heater together does not hinder the present invention.

As mentioned above, it is important to keep the rolling speed constant. However, if the rolling speed is less than 300 mpm, the productivity is remarkably reduced, so 300 mpm or more is preferable. On the other hand, if the rolling speed is too fast, it becomes difficult to control the temperature during rolling, the plate thickness control and the cooling conditions after rolling, especially the intermediate holding temperature, and it is difficult to obtain material uniformity in the longitudinal direction and the width direction. , 850 mpm or less is preferable.

Further, when the rolling speed exceeds 450 mpm, the amount of temperature decrease at the rear end of the steel strip becomes large, and the temperature of the rough bar or thin slab must be increased for temperature compensation, resulting in an increase in cost. It is more preferably 450 mpm or less. The constant rolling speed described in the present invention changes according to the target intermediate holding time, but the present invention is not impaired up to a tolerance of about 100 mpm or less.

If the finishing temperature is less than the Ar ₃ transformation point, the workability is significantly deteriorated and uniform properties cannot be obtained over the entire steel strip. Also, (Ar ₃ transformation point +50
(° C), the processing strain is released and the austenite grains become large and the ferrite nucleation sites decrease, so that it becomes difficult to obtain uniform properties over the entire steel strip and the yield strength increases. As described above, the finishing temperature is set to the Ar ₃ transformation point over the entire steel strip.
By controlling the temperature to (Ar ₃ transformation point + 50 ° C.) and keeping the intermediate holding condition constant, the material can be made uniform over the entire steel strip. The value is 50 MPa, further 30 MPa or less due to variations in tensile strength.

The heating temperature of the rough bar or thin slab is appropriately determined according to the temperature of the rough bar or thin slab before heating, the transformation point of steel, the rolling speed, and the like. Since the temperature of the coarse bar or the thin slab usually differs depending on the position, it is preferable to change the heating conditions of the coarse bar or the thin slab according to the temperature distribution.

After the finish rolling, in order to precipitate an appropriate amount of fine ferrite grains according to the strength, it is cooled to a temperature range of 600 to 780 ° C. at a cooling rate of 20 ° C./s or more, and this temperature range is used for 3 seconds. It is necessary to hold more than this. Because, if it is out of this range, the low yield ratio (Y
R is about 70% or less) and high ductility cannot be obtained. It should be noted that a uniform material cannot be obtained within this range, and the cooling conditions in the steel strip must be made as uniform as possible within the above range according to the predetermined material. Especially, to reduce the variation of the intermediate holding time,
In controlling the ratio of ferrite phase and martensite phase,
is important.

Further, in order to stably transform the untransformed austenite phase into the martensite phase when kept in the temperature range of 600 to 780 ° C., after the holding, it is cooled at a cooling rate of 20 ° C./s or more, It is necessary to wind at a temperature of 250 ° C or lower.

The finishing temperature in the steel strip is set to the Ar ₃ transformation point to (Ar ₃
If the temperature is set within the range of the transformation point + 30 ° C) and finish rolling is performed, more uniform properties can be obtained over the entire steel strip.

If the rough bar or the thin slab is heated by a rough bar heating device capable of heating the rough bar or the thin slab in its entire width direction, the temperature can be made uniform without impairing the productivity. .

When the rough bar or the thin slab is heated by using the induction heating coil, the heating can be performed quickly, so that the productivity can be further improved.

[0033]

EXAMPLES Steels a to e having the components shown in Table 1 were melted,
Approximately 250mm thick slab is manufactured by continuous casting, and
After heating to 210 ° C., it was rolled into a rough bar having a thickness of about 30 mm by a rough rolling machine. Then, under the conditions shown in Table 2, the rough bar is sequentially heated, finished rolled, and cooled, and wound at room temperature to obtain a plate width of 800 mm and a strength level of 550 to 800 MPa.
Phase-structure hot-rolled steel strips 1 to 20 were produced. The rough bar heating was carried out by using a rough bar heating device equipped with an induction coil type heating means and heating while passing the rough bar through the coil.

Then, two JIS No. 5 test pieces (in the direction perpendicular to the rolling direction) were formed from the widthwise central portions of the manufactured steel strips 1 to 20 in the longitudinal direction (T), the central portion (M), and the rear end portion (B). The average value of the tensile strength TS is obtained and the variation of the strength of the steel strip ΔT
S was evaluated by the difference between the maximum value and the minimum value of the strength in T, M and B.

The results are shown in Tables 2, 3 and 4. Table 5 shows the yield strength, tensile strength, and elongation value of the B part of each steel sheet.
Shown in.

Steel strip No. which is an example of the present invention. 3, 4, 7,
All of 8, 11, 12, 15, 16, 19, and 20 have ΔTS of 50 MPa or less, and uniform characteristics can be obtained in the longitudinal direction. Particularly, the finishing temperature is set to the Ar ₃ transformation point to (A
or in the range of r ₃ transformation point + 30 ° C.), the rolling speed 30
Within the range of 0-450 mpm, ΔTS is 30MP
It becomes a or less and is more excellent in uniformity.

On the other hand, steel strip No. which is a conventional example in which accelerated rolling is not performed. In Nos. 1, 5, 9, 13, and 17, the finishing temperature cannot be controlled sufficiently, ΔTS becomes large, and the uniformity in the longitudinal direction is poor. In addition, steel strip No. which is a conventional example subjected to accelerated rolling. Nos. 2, 6, 10, 14, and 18 have improved controllability of finishing temperature as compared with the conventional example, but cooling, holding conditions,
In particular, the holding time varies depending on T, M, and B, and the uniformity is poor.

[0038]

[Table 1]

[0039]

[Table 2]

[0040]

[Table 3]

[0041]

[Table 4]

[0042]

[Table 5]

[0043]

EFFECTS OF THE INVENTION Since the present invention is constructed as described above, it is possible to provide a method for producing a thin two-phase hot-rolled steel strip having uniform properties over the entire steel strip.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) C21D 9/00 101 C21D 9/00 101N // C22C 38/00 301 C22C 38/00 301W 38/06 38 / 06 (72) Inventor Kunikazu Tomita Marunouchi 1-2-2 Marunouchi, Chiyoda-ku, Tokyo Japan Steel Pipe Co., Ltd. (72) Inventor Jun Masaki Marunouchi 1-2-2 Marunouchi, Chiyoda-ku, Tokyo F Nippon Steel Pipe Co., Ltd. F Term (reference) 4E002 AA07 AB01 AD04 BC02 BC07 BD07 BD08 4K037 EA01 EA05 EA06 EA15 EA16 EA18 EA23 EA25 EA27 EA28 EB06 EB09 FB07 FC07 FD03 FD04 FD08 FE01

Claims (6)

[Claims] 1. C: 0.03 to 0.15% by weight,
Si: 0.3 to 2.5%, Mn: 0.5 to 2.5%,
P: 0.07% or less, S: 0.02% or less, sol. A
l: 0.005 to 0.08%, N: 0.008% or less of the step of manufacturing a steel rough bar or thin slab containing steel, by heating the rough bar or thin slab, at a constant rolling speed, A step of finishing rolling in which the finishing temperature in the steel strip is within the range of Ar ₃ transformation point to (Ar ₃ transformation point + 50 ° C.), and the steel strip after the finishing rolling is cooled at 20 ° C./s or more at a cooling rate of 60.
A step of cooling to a temperature range of 0 to 780 ° C. and holding the temperature range for 3 seconds or more;
A method for producing a thin hot-rolled steel strip having a two-phase structure, comprising a step of cooling at a cooling rate of 0 ° C./s or more and winding at a temperature of 250 ° C. or less. 2. The finishing temperature in the steel strip is set to the Ar ₃ transformation point to (A
The method for producing a thin strip two-phase hot-rolled steel strip according to claim 1, wherein the sheet is placed within a range of (r ₃ transformation point + 30 ° C) and finish-rolled. 3. The method for producing a thin strip two-phase hot-rolled steel strip according to claim 1, wherein the finish rolling is performed at a constant rolling speed within a range of 300 to 850 mpm. 4. The method for producing a thin strip two-phase hot-rolled steel strip according to any one of claims 1 to 3, wherein the finish rolling is performed at a constant rolling speed within a range of 300 to 450 mpm. 5. The heating of the rough bar or the thin slab is performed by a device capable of heating the entire width direction of the rough bar or the thin slab while conveying the rough bar or the thin slab.
Item 2. A method of manufacturing a thin hot-rolled steel strip having a two-phase structure. 6. The method according to claim 1, wherein the rough bar or the thin slab is heated by using an induction heating coil.
Item 2. A method of manufacturing a thin hot-rolled steel strip having a two-phase structure.