JP2001164322A - Thin steel sheet and method for producing thin steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a producing method capable of obtaining a high strength thin steel sheet excellent in workability including stretch-flanging properties, also uniform in mechanical properties and having various strength levels. SOLUTION: This method for producing a high strength thin steel sheet has a stage in which a continuously cast slab having a C content of <=0.8 wt.% is subjected to rough rolling, a stage in which the slab is subjected to finish rolling at >=(Ar3 transformation point -20) deg.C, a stage in which the same is subjected to primary cooling to 500 to 800 deg.C at >120 deg.C/sec, a stage in which the same is let to cool for 1 to 30 sec, a stage in which the same is subjected to secondary cooling at >=20 deg.C/sec and a stage in which the same is coiled at <=650 deg.C. Moreover, it is possible that the continuously cast slab is heated to <=1230 deg.C without executing cooling to a room temperature, is thereafter subjected to rough rolling and is subjected to induction heating directly before or in the process of the finish rolling, within >0.1 to 1.0 sec after the finish rolling, the primary cooling is started, and the variation of the temperature in the width direction of the coil and in the longitudinal direction thereof after the rapid cooling (maximum value-minimum value) is controlled to <=60 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the production of thin steel sheets having various characteristic levels such as hot-rolled steel sheets and cold-rolled steel sheets, in particular, thin steel sheets having excellent workability and uniform mechanical properties. About the method.

[0002]

2. Description of the Related Art Thin steel sheets, such as hot-rolled steel sheets and cold-rolled steel sheets,
It is used in a wide range of fields such as automobiles, home appliances, and industrial machines. Since such thin steel sheets are often used after being subjected to some kind of processing, various workabilities are required. For example, a high-strength hot-rolled steel sheet having a strength of 340 MPa or more and not subjected to drawing is required to have high stretch flangeability during burring.

[0003] In recent years, the quality requirements of customers for thin steel sheets have become more and more severe, and not only the above-mentioned workability has been further improved, but also the mechanical properties of products wound in a coil shape. There is also a strong demand for uniformity.

In response to such demands from consumers, for example, Japanese Patent Publication No. 61-15929 and Japanese Patent Publication No. 63-67524 disclose a high-strength heat control by controlling a cooling rate after hot rolling and a winding temperature. Methods for improving the workability of rolled steel sheets and the like are also disclosed in
No. 241742 proposes a method of improving the uniformity of mechanical properties in a hot-rolled coil by continuous hot-rolling.

[0005]

[Problems to be solved by the invention]
The high-strength hot-rolled steel sheets produced by the methods described in JP-A-61-15929 and JP-B-63-67524 could not obtain sufficiently excellent stretch flangeability. Even when the method described in JP-A-9-241742 is applied to a high-strength thin steel sheet, excellent uniformity of mechanical properties cannot be obtained.

SUMMARY OF THE INVENTION The present invention has been made to solve such a problem, and is intended to produce thin steel sheets which are excellent in workability including stretch flangeability and have various strength levels with uniform mechanical properties. The aim is to provide a method.

[0007]

Means for Solving the Problems The above-mentioned problem is that the C content is low.
A step of roughly rolling a continuously cast slab of 0.8 mass% or less to produce a rough bar; and a step of finish rolling the rough bar at a finishing temperature of (Ar ₃ transformation point -20) ° C. or more to produce a steel strip. A step of primary cooling the steel strip after the finish rolling to a temperature of 500 to 800 ° C. at a cooling rate exceeding 120 ° C./sec, and a step of allowing the steel strip after the primary cooling to cool for 1 to 30 sec. A step of secondary cooling the steel strip after the cooling is performed at a cooling rate of 20 ° C./sec or more, and a step of winding the steel strip after the secondary cooling at a winding temperature of 650 ° C. or less. The problem is solved by a method for producing a thin steel sheet having excellent and uniform mechanical properties.

[0008] If a continuous cast slab having a C content of 0.8 mass% or less is roughly rolled, and the coarse bar is finish-rolled at a finishing temperature of (Ar ₃ transformation point -20) ° C or more, the crystal grains immediately after the finish rolling are thinned. Since the grains can be formed, crystal grains can be refined in a subsequent step. As a result, crystal grains can be refined in subsequent steps, and workability such as strength-ductility balance and stretch flangeability can be improved.

After rolling, the steel strip is heated to a temperature of 500 to 800 ° C.
When primary cooling is performed at a cooling rate exceeding 0 ° C./sec, precipitates such as ferrite crystal grains and pearlite after transformation can be refined, so that workability can be improved.

After the primary cooling, the steel strip is allowed to cool for 1 to 30 seconds and then secondary cooled at a cooling rate of 20 ° C./sec or more, since the structure in the coil after winding can be made uniform. Can achieve uniform mechanical properties.

After the secondary cooling, if the steel strip is wound at a winding temperature of 650 ° C. or less, an appropriate low-temperature transformation phase can be obtained according to each component system of the high-strength thin steel sheet.

When the C content exceeds 0.8 mass%, (Ac
m Transformation point -20) Finish rolling at a finishing temperature of ℃ or more, and other conditions are the same as when the C content is 0.8 mass% or less, a thin film with excellent workability and uniform mechanical properties Steel sheet can be obtained.

Further, if the rough rolling is started after heating the continuous cast slab to a temperature of 1230 ° C. or less without cooling it to room temperature, the slab temperature before rolling can be made uniform, and the mechanical properties in the coil can be further improved. Can be uniform.

If the material to be rolled is heated by an induction heating device immediately before or during finish rolling, the temperature of the material to be rolled during rolling can be made more uniform, and the mechanical properties in the coil can be made more uniform. Can be achieved.

If the primary cooling is started within a time period of more than 0.1 sec and less than 1.0 sec after finish rolling, precipitates such as ferrite crystal grains and pearlite after transformation can be further refined, and workability can be further improved. .

In order to reduce the variation in the material of the hot-rolled steel strip to a more preferable level, the above-mentioned quenching stop temperature is set within the scope of the present invention, and the quenched coil width direction, longitudinal direction, etc. Temperature fluctuation (highest value-lowest value)
It must be within 60 ° C. The temperature in the coil width direction in the present invention indicates a range excluding 30 mm from both edges of the coil width in consideration of the measurement method of the temperature sensor.

As for the quenching ability, the heat transfer coefficient is 2000 kc
By performing cooling at al / m ² h ° C. or higher, the fluctuation in temperature after the rapid cooling can be reduced.

Thus, in the present invention, by reducing the temperature fluctuation in the coil, the fluctuation (the maximum value and the minimum value) of the tensile strength in the width direction and the longitudinal direction of the hot-rolled steel strip can be reduced. It is possible to obtain a thin steel sheet in which the tensile strength in the coil is within ± 8% of the average value. Such a steel sheet having a small variation has small press workability (spring back at the time of bending, etc.) within the coil and has excellent performance as a material.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, the steel components are not particularly limited, and conventional component systems of high-strength hot-rolled steel sheets and high-strength cold-rolled steel sheets having various strength levels can be applied.
That is, Ti, Nb, V, M
It can also be applied to steel sheets containing special elements such as o, Zr, Ca, and B.

The thin steel sheet of the present invention can be produced by a usual steelmaking-hot rolling process, but can also be applied to a direct rolling process in which a slab after continuous casting is directly hot-rolled without passing through a heating furnace. It is also effective for a continuous rolling process using a coil box or the like. When heating the material to be rolled by the induction heating device immediately before or during finish rolling, it is effective to perform edge heating.

In the hot rolling, if the finish rolling is preferably performed so that the difference in the finishing temperature in the material to be rolled is within 50 ° C., the structure in the steel strip immediately after the finish rolling can be made uniform.
Uniform mechanical properties after winding on the coil. When the C content is 0.8 mass% or less, the upper limit of the finishing temperature is (A r ₃ transformation point +5) from the viewpoint of refining and homogenizing the structure.
0) ° C. or lower, and when the C content exceeds 0.8 mass%, the temperature is preferably (Acm transformation point + 100) ° C. or lower.

In the primary cooling, in order to make the variation of the material to a more preferable level, within the scope of the present invention,
It is preferable that the start of the primary cooling be more than 0.5 sec. Regarding the cooling rate, it is preferable to cool at a cooling rate of preferably 200 ° C./sec or more, more preferably 400 ° C./sec or more, in order to obtain a finer structure. In order to reduce temperature fluctuations in the coil, a preferable heat transfer coefficient is 5000 kc.
al / m ² h ℃ or more, more preferable level is 8000 kcal / m ² h ℃
That is all.

Regarding the uniformity of the material, preferably, the performance at the consumer can be remarkably improved by controlling the fluctuation of the tensile strength to within ± 4%. In this case, by setting the fluctuation of the stop temperature of the rapid cooling (primary cooling) to within 40 ° C,
Variations in material can be reduced in this way. Further, in order to keep the variation in tensile strength within ± 2%, the variation in the quenching stop temperature should be kept within 20 ° C. The reduction in the variation in the material can be determined from the relationship between the temperature and the variation in the tensile strength.

It is more preferable to set the secondary cooling rate to 100 ° C./sec or more in order to improve workability by making the structure finer.

If the obtained hot-rolled coil is annealed after cold rolling, a cold-rolled steel sheet excellent in both workability and uniformity of mechanical properties can be obtained. At this time, it is more preferable to perform annealing by continuous annealing in order to achieve uniform mechanical properties.

[0026]

[Example 1] [Example 1] A hot rolled coil having a thickness of 3 mm was prepared by melting steel Nos. 1 to 5 having the composition shown in Table 1 under the hot rolling conditions shown in Table 2.
Nos. 1 to 11 were prepared. The heat transfer coefficient in the primary cooling in the example of the present invention is 3000 to 4000 kcal / m ² h ° C.

[0027]

【table 1】

[0028]

[Table 2]

Tensile test specimens were taken from five locations in the longitudinal direction of the hot-rolled coil, and the average tensile strength (TS), total elongation (El), variation in tensile strength (ΔTS), and variation in total elongation (ΔEl) Was measured. For some hot-rolled coils, the hole expansion ratio (λ) was used to evaluate stretch flangeability.
And its variation (Δλ) were measured. Table 3 shows the results.

[0030]

[Table 3]

As is clear from the comparison between the present invention example and the comparative example in each component system, the ΔTS, ΔEl, and Δλ of the present invention example were smaller in all the component systems, and the mechanical properties in the coil were smaller. The uniformity of hot rolled coil
El and λ are high and workability is excellent.

Example 2 Steel Nos. 1 to 5 having the chemical components shown in Table 1 were rolled under hot rolling conditions shown in Table 4, and hot-rolled coils No. 12 to 22 having a thickness of 3 mm. Was manufactured. In addition,
The heat transfer coefficient during primary cooling is 120 in Example Nos. 12 to 17 of the present invention.
00kcal / m ² h ℃, is 1000kcal / m ² h ℃ Comparative Example Nanba18～22.

[0033]

[Table 4]

The mechanical properties of these hot rolled coils were measured in the same manner as in Example 1. Table 5 shows the results.

[0035]

[Table 5]

Steel sheets Nos. 12 to 1 of the present invention in each component system
As is clear from the comparison between the steel sheets No. 18 and No. 18 to No. 22 of the comparative example, the variations ΔTS and ΔEl of the mechanical properties of the example of the present invention are smaller in all the component systems. On the other hand, in the steel sheets Nos. 18 to 22 of the comparative examples, one or more of the production conditions defined in the present invention are not satisfied, and the steel sheets No. 12 to Compared with 17, the uniformity of mechanical properties or workability is poor.

In the example of the present invention, the variation in the quenching (primary cooling) stop temperature in the coil is smaller than that in the comparative method using the conventional laminar cooling, and the variation in mechanical properties is reduced to a more preferable level. ing. The cooling method in the present invention is a multi-jet type cooling method having a high heat transfer coefficient.

[0038]

Since the present invention is constructed as described above, it is possible to obtain thin steel sheets which are excellent in workability including stretch flangeability and have various characteristic levels in which the mechanical properties in the coil are uniform. Can be provided.

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Hiroyasu Kikuchi 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Inside Nihon Kokan Co., Ltd. (72) Inventor Yoichi Honashiki 1-1-2, Marunouchi, Chiyoda-ku, Tokyo No. Nihon Kokan Co., Ltd. F term (reference) 4E002 AA02 AA07 AB01 AD02 AD04 BC07 BD03 BD07 BD08 CB01 4K037 EA04 EA05 EA06 EA07 EA08 FA01 FA02 FA03 FB10 FD03 FD04 FD08 FE01 FE02

Claims (9)

[Claims] 1. A step of roughly rolling a continuous cast slab having a C content of 0.8 mass% or less to produce a rough bar, and forming the rough bar at a finishing temperature of (Ar ₃ transformation point -20) ° C. or higher. Step of finish-rolling to produce a steel strip, the steel strip after the finish-rolling, the temperature of 500 ~ 800 ℃ 120 ℃ /
a step of primary cooling at a cooling rate exceeding sec, a step of allowing the steel strip after the primary cooling to cool for 1 to 30 seconds, and a step of cooling the steel strip after the cooling at a cooling rate of 20 ° C / sec or more. A method for producing a thin steel sheet, comprising: a step of performing secondary cooling; and a step of winding the steel strip after the secondary cooling at a winding temperature of 650 ° C. or less. 2. A step of roughly rolling a continuous cast slab having a C content of more than 0.8 mass% to produce a rough bar, and finishing the rough bar at a finishing temperature of (Acm transformation point -20) ° C or higher. Rolling the steel strip to produce a steel strip and the finish-rolled steel strip, a step of primary cooling at a cooling rate of more than 120 ° C./sec to a temperature of 500 to 800 ° C., and the steel strip after the primary cooling, Step of allowing to cool for 1 to 30 seconds, and step of secondary cooling the steel strip after the cooling, at a cooling rate of 20 ° C / sec or more, and steel strip after the secondary cooling, 650 ° C or less. Winding at a winding temperature. 3. The method of producing a thin steel sheet according to claim 1, wherein the continuous casting slab is heated to a temperature of 1230 ° C. or less without cooling to room temperature, and then rough rolling is started. 4. The method for producing a thin steel sheet according to any one of claims 1 to 3, wherein the material to be rolled is heated by an induction heating device immediately before or during finish rolling. 5. The method for producing a thin steel sheet according to claim 1, wherein after the finish rolling, primary cooling is started within a time period of more than 0.1 sec and less than 1.0 sec. 6. The method for producing a thin steel sheet according to claim 1, wherein the steel strip after being left to cool is secondarily cooled at a cooling rate of 100 ° C./sec or more. 7. The method according to claim 1, wherein the difference between the maximum value and the minimum value of the temperature in the width direction and the longitudinal direction of the steel strip after the primary cooling is within 60 ° C. The method for producing a thin steel sheet according to the above. 8. The heat transfer coefficient during primary cooling is 2000 kcal / m ² h ° C.
The method for producing a thin steel sheet according to claim 7, wherein the cooling is performed as described above. 9. The method for manufacturing a thin steel sheet according to claim 7, wherein the variation in the tensile strength in the width direction and the longitudinal direction is within ± 8% of the average value of the tensile strength in the coil. Features thin steel sheet.