JP2001152255A - Method of manufacturing high strength thin steel sheet excellent in surface characteristic and workability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method of a high strength thin steel sheet, with which even in the case of adopting a direct feeding rolling process, such problem as the surface defect is not developed and this rolled product has excellent coiling shape and high workability and such problem as extending- flange crack is not developed. SOLUTION: The steel containing 0.05-0.2 wt.% C, <=0.15 wt.% Si, 0.4-2.0 wt.% Mn, <= 0.025 wt.% P, <= 0.005 wt.% O, <= 0.01 wt.% S, <=0.006 wt.% N, <= 0.004 wt.% Sn and satisfying Mn/S >=50, is continuously cast to make a slab. A heating process is applied to the obtained continuously cast slab, or without applying the heating process, a rougher-rolling is started and a hot-rolling is completed at the temperature of not lower than the Ar3 transformation point and successively, after cooling the obtained hot-rolled sheet to the range of 400-700 deg.C at 20-2,000 deg.C/sec cooling rate, this sheet is coiled.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention has excellent surface properties and workability, and is applicable to a direct rolling process in which hot rolling is performed directly from continuous casting with little occurrence of surface flaws.
The present invention relates to a method for producing a high-strength steel sheet of 0 MPa class or higher.

[0002]

2. Description of the Related Art A high-strength thin steel sheet having a strength of 340 MPa class or more is based on a normal carbon steel having an adjusted carbon equivalent and added with a precipitation strengthening element such as Ti, Nb, or V according to the strength. Manufactured. However, when such a high-strength thin steel sheet is manufactured, cracks may occur in the hot rolling step, and surface defects due to the cracks may occur in the product. This surface defect, particularly, C is 0.05 to
Notably occurs in carbon steel containing 0.2 wt%.

[0003] The cracks in the hot rolling process, which cause these surface defects, occur during continuous casting, and cracks occur on the slab surface or under the slab surface due to tensile stress applied when the slab undergoes bending deformation. It is believed that the rough rolling in the hot rolling process in which cracks continue to occur has greatly advanced. For this reason, the product yield is remarkably reduced, and slab care is indispensable, and the care cost is high.

On the other hand, as a process that can reduce costs, there is a direct-feed rolling process in which hot rolling is performed directly from continuous casting. However, in this direct-feed rolling process, slab care cannot be performed, and cracks generated in the slab are reduced. Since it remains in the product (hot rolled steel strip or cold rolled steel strip) as it is, at present, this direct rolling process cannot be applied to the high-strength thin steel sheet as described above.

[0005] As described above, the 340 MPa class high-strength thin steel sheet requires slab care and cannot be applied to the direct rolling process, so that there is a problem in manufacturing that sufficient cost reduction cannot be achieved.

[0006] The cracks generated in the hot rolling process of such a high-strength thin steel sheet are generally considered to be caused by red-hot embrittlement, but have been implemented without taking effective measures.

Further, the above-mentioned high-strength thin steel sheet contains pearlite, and therefore has a problem that its workability is essentially low. For example, when burring is performed on these high-strength thin steel sheets, elongation-flange cracking often occurs. Conventional techniques aimed at improving the workability of high-strength thin steel sheets include, for example, cooling after finishing hot rolling of steel disclosed in JP-B-61-15929 and JP-B-63-67524. There are technologies that feature control of speed and winding temperature.

[0008]

However, in the technique disclosed in Japanese Patent Publication No. 61-15929, the strength-ductility balance is increased as compared with conventional steel.
In the technology disclosed in JP-A-66752, the elongation at break and the toughness of the steel are increased, but none of the above-mentioned technologies has drastically solved the elongation-flange property, and the occurrence of surface defects has been reduced. No effective measures have been taken.
Further, in the above two technologies, no study has been made from the viewpoint of securing the coil shape after hot rolling. In fact, in the high-strength steel sheet targeted by the present invention, the coil shape may be defective depending on the hot rolling conditions, and when the coil shape is defective, it is difficult for the user to perform press forming with a high yield. Problem arises.

[0009] As described above, conventionally, there is a problem of surface defects which occur when a direct rolling process is applied to a manufacturing process of a high-strength thin steel sheet and an extension-flange which occurs when a burring process is performed on the high-strength thin steel sheet. The problem of cracking has not been solved, and there is also the problem of the shape of the coil. Development of a high-strength thin steel sheet that has solved these problems has been desired.

The present invention has been made in view of the above circumstances, and does not cause a problem of surface defects even when a direct rolling process is employed, has high workability, and has a problem of elongation-flange cracking. An object of the present invention is to provide a method for producing a high-strength thin steel sheet that does not generate. It is another object of the present invention to provide a method for producing a high-strength thin steel sheet in which these problems do not occur and the coil shape is further excellent.

[0011]

Means for Solving the Problems The present inventors have conducted intensive studies to solve the above-mentioned problems, and as a result, have found that P
By controlling the amount, O amount, S amount, N amount, Sn amount and Mn / S ratio to specific values or less, and adding an appropriate amount of Ca as necessary, the amount of Ca is generated on the slab surface or under the slab surface during continuous casting. Cracks that had been occurring, even when manufacturing high-strength thin steel sheets by the direct-feed rolling process,
It has been found that a high-strength thin steel sheet having excellent surface properties can be obtained. Furthermore, the present inventors control workability (elongation-flangeability) by controlling conditions such as a finishing temperature in finish rolling, a cooling rate after hot rolling (hereinafter also referred to as runout cooling), and a winding temperature. It has been found that a high-strength thin steel sheet excellent in quality can be obtained. Furthermore, the present inventors have found that a high strength thin steel sheet having an excellent coil shape can be obtained by controlling the rolling reduction of the final stand in finish rolling.

The present invention has been completed based on these findings and provides the following (1) to (5). (1) By weight%, C: 0.05 to 0.2%, Si:
0.15% or less, Mn: 0.4 to 2.0%, P: 0.0
Steel containing 25% or less, O: 0.005% or less, S: 0.01% or less, N: 0.006% or less, Sn: 0.004% or less, and satisfying the relationship of Mn / S ≧ 50. Into a slab by continuous casting, subjecting the obtained continuous cast slab to a heating step or without a heating step, starting rough rolling, ending hot rolling at a temperature of Ar ₃ points or more, and then A method for producing a high-strength thin steel sheet having excellent surface properties and workability, wherein the obtained hot-rolled sheet is cooled at a cooling rate of 20 to 2000 ° C / sec to a range of 400 to 700 ° C and then wound.

(2) C: 0.05 to 0.2% by weight
%, Si: 0.15% or less, Mn: 0.4 to 2.0%,
P: 0.025% or less, O: 0.005% or less, S:
0.01% or less, N: 0.006% or less, Sn: 0.0
A steel containing 0.4% or less and satisfying the relationship of Mn / S ≧ 50 is continuously cast into a slab, and the obtained continuously cast slab is subjected to a heating step or without a heating step.
Start rough rolling, finish temperature is Ar ₃ transformation point or higher, and
The final stand is subjected to finish rolling at a rolling reduction of 8% or more and 30% or less, and then the obtained hot rolled sheet is subjected to 20 to 2000 ° C / s.
A method for producing a high-strength thin steel sheet having excellent surface properties and workability, wherein the sheet is cooled after being cooled at a cooling rate of ec to a range of 400 to 700 ° C.

(3) In the above (1) or (2), after finishing rolling, cooling is started within 0.1 second to less than 1.0 second, and is excellent in surface properties and workability. Manufacturing method of high strength thin steel sheet.

(4) In any one of the above (1) to (3), the steel component further contains Ca: 0.005%
A method for producing a high-strength thin steel sheet having excellent surface properties and workability, characterized by adding:

(5) In any one of the above (1) to (4), after hot-rolled sheet is wound, at least cold rolling and annealing are performed, and excellent in surface properties and workability. Manufacturing method of high strength thin steel sheet.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. The high-strength steel sheet to which the present invention is applied is mainly used for a member for a mechanical structure or the like.
It is a steel sheet having characteristics of the MPa class or higher. First, the composition of the steel of the present invention will be described.

(1) Composition of Steel Sheet In the present invention, the basic composition of steel is expressed by weight% and C: 0.05.
-0.2%, Si: 0.15% or less, Mn: 0.4-
2.0%, P: 0.025% or less, O: 0.005% or less, S: 0.01% or less, N: 0.006% or less, S
n: 0.004% or less, and Mn / S ≧ 50. Hereinafter, the reasons for these limitations will be described.

C: 0.05 to 0.2% C is an additional element for securing the strength of the steel sheet. If the C content is less than 0.05%, generation of cracks on the slab surface or under the slab surface during continuous casting is suppressed, and a high-strength steel sheet having excellent surface properties cannot be obtained. On the other hand, 0.2%
Exceeding the above causes deterioration of workability. For this reason, the C content is set to 0.05 to 0.2%. In order to obtain a high-strength steel sheet having better workability, the C content is preferably set to 0.10% or less.

Si: 0.15% or less Si is a solid solution strengthening element, but if it exceeds 0.15%, the surface properties deteriorate, so the content of Si is set to 0.15% or less.

Mn: 0.4 to 2.0% Mn is an effective element capable of suppressing the occurrence of cracks on the slab surface or under the slab surface during continuous casting. Mn
If the amount is less than 0.4%, this effect cannot be obtained. on the other hand,
If it exceeds 2.0%, the workability intended by the present invention cannot be obtained. Therefore, the content of Mn is 0.4 to 2.0%.
And

P: 0.025% or less P is a harmful element that promotes cracking on the slab surface or under the slab surface during continuous casting. P content is 0.
If the content exceeds 025%, cracks are remarkably generated on the slab surface or under the slab surface during continuous casting, and the frequency of cracks generated by hot rolling increases. Therefore, the P content is set to 0.025% or less. In addition, a more preferable range of the P content for suppressing crack generation in hot rolling is 0.010% or less.

O: 0.005% or less O is an element that must be controlled in order to suppress the occurrence of cracks on the slab surface or under the slab surface during continuous casting. If O exceeds 0.005%, cracks in the slab become significant during continuous casting, and the workability intended by the present invention also deteriorates. Therefore, the O content is set to 0.005% or less.

S: not more than 0.01% S significantly accelerates the generation of cracks on the surface of the slab or under the slab surface during continuous casting in the steel targeted by the present invention. It is a harmful element that induces cracks during rolling to deteriorate the surface properties of the steel sheet and also deteriorates the workability intended in the present invention. S
If the content exceeds 0.01%, slab cracking during continuous casting becomes particularly significant, and also significant cracking occurs during hot rolling, and the workability intended in the present invention also deteriorates. Was 0.01% or less. In addition, the more preferable range of the S content for suppressing crack generation during hot rolling and improving the workability intended in the present invention is 0.005.
% Or less. Further, by setting the S content to 0.001% or less, cracks generated on the slab surface or under the slab surface during continuous casting are extremely reduced, and the workability intended in the present invention is further improved. 0.001
% Is particularly preferable.

N: 0.006% or less N is an element to be reduced in the steel to be subjected to the present invention in order to suppress the occurrence of cracks during hot rolling and to improve the workability intended in the present invention. If the N content is more than 0.006%, the occurrence of cracks during hot rolling and the deterioration of workability become problems.
06% or less. In addition, crack generation suppression during hot rolling,
The more preferable N content for improving workability intended in the present invention is 0.005% or less.

Sn: 0.004% or less Sn is a very harmful element that significantly promotes the generation of cracks on the slab surface or under the slab surface during continuous casting in the steel targeted by the present invention. In recent years, the use of scrap at the time of steelmaking has increased, and Sn has been increasing particularly to a level at which the content should be controlled. If the Sn content exceeds 0.004%, cracks are particularly remarkable on the slab surface or under the slab surface during continuous casting,
Since the frequency of occurrence of cracks during hot rolling increases, the Sn content is set to 0.004% or less.

Mn / S ≧ 50 Mn / S is a value that needs to be controlled to suppress the occurrence of cracks on the slab surface or under the slab surface during continuous casting. If Mn / S is less than 50, cracking on the slab surface or below the slab surface during continuous casting becomes a problem, so Mn / S is set to 50 or less.

Ca: 0.005% or less Ca is an effective element capable of suppressing the occurrence of cracks on the slab surface or under the slab surface during continuous casting, and is preferably added. When Ca is added in an amount of 0.005% or less, the tendency of crack generation on the slab surface or under the slab surface during continuous casting can be further suppressed. On the other hand, when Ca exceeds 0.005%, the tendency of crack generation under the surface layer of the slab is increased, so the preferable addition amount of Ca is 0.005% or less.

In the present invention, in addition to the components described above, one or more of Ti, Nb, V, and Mo may be added in a total amount of 0.01 to 0.2% as needed for strength adjustment. You may add 0.02-2% of Cu, and 0.0001-0.0100% of B. Even if these additional elements are added, the effect of the present invention is not impaired.

(2) Manufacturing Conditions Next, the manufacturing conditions of the present invention will be described. The excellent effects of the present invention can be obtained by combining the steel having the above-described steel components with the following manufacturing conditions. That is, steel having the above-described components is continuously cast into a slab, and the obtained continuous cast slab is subjected to a heating step, or without being subjected to a heating step, to start rough rolling,
Finish rolling is performed at a finishing temperature of at least the Ar ₃ transformation point, and then the obtained hot rolled sheet is cooled at a cooling rate of 20 to 2000 ° C./sec to a range of 400 to 700 ° C. (runout cooling)
After winding, a high-strength thin steel sheet having excellent surface properties and workability is manufactured.

In the present invention, rough rolling may be started after the continuous casting slab is subjected to the heating step, or rough rolling may be started without performing the heating step. The process of starting rough rolling without performing a heating step is a direct-feed rolling process that starts rough rolling without cooling the continuous casting slab to room temperature and performs hot rolling directly from continuous casting. By adopting such a direct rolling process, the manufacturing cost of a high-strength steel sheet can be effectively reduced. Conventionally, when a high-strength thin steel sheet was manufactured by a direct-feed rolling process, the problem of surface defects had occurred.In the present invention, the occurrence of cracks on the slab surface or under the slab surface during continuous casting was suppressed by the steel composition described above. In addition, even if it is manufactured by the direct rolling process, there is no problem of surface defects.

In the normal direct rolling process, the heating step is not performed when starting the rough rolling without cooling the continuous casting slab to room temperature. In this case, the present invention can be applied. After auxiliary heating to a temperature of 1250 ° C. or less during rolling, rough rolling may be started. Thereby, embrittlement of crystal grain boundaries caused by sulfides is suppressed, and a high-strength thin steel sheet having excellent workability and particularly excellent surface properties can be obtained. If the heating temperature before the rough rolling exceeds 1250 ° C., the risk of cracking at the corner of the slab during the rough rolling slightly increases. Heating temperature before rough rolling is 1250 ° C
In the case where the content is below, the embrittlement of the crystal grain boundary caused by the sulfide can be sufficiently suppressed. The lower limit of the heating temperature before the rough rolling is not particularly limited, but is preferably 1000 ° C. or more from the viewpoint of the rolling load of the rough rolling and the finish rolling.

The present invention is particularly effective in the above-mentioned direct-feed rolling process. However, the present invention is also applicable to a case where a normal process of cooling once to room temperature after continuous casting and then performing a heating step to start rough rolling is employed. Since slab care is not required, the yield is high and the manufacturing cost can be kept low.

The reason why the finish temperature of the finish rolling is set to the Ar ₃ transformation point or more is to reduce the crystal grain size of the hot-rolled sheet. Further, the cooling rate of the runout cooling is set to 20 to 2
The reason for setting the temperature to 000 ° C./sec is to reduce the ferrite crystal grain size after transformation and the pearlite to improve workability of the steel sheet. From the viewpoint of improving workability by ferrite crystal grain size and refinement of pearlite,
More preferably, the cooling rate in the run-out cooling is 50 ° C./sec or more. In particular, this cooling rate is
By setting the temperature to be higher than 0 ° C./sec, it is possible to obtain a high-strength thin steel sheet with excellent workability.

In the present invention, from the viewpoint of ensuring a good shape of the hot-rolled coil and sufficiently reducing the grain size of the hot-rolled sheet, the rolling reduction of the final stand in the finish rolling should be 8% or more and 30% or more.
% Is preferable. If the rolling reduction of the final rolling final stand is less than 8%, the grain size of the hot rolled sheet cannot be sufficiently reduced, and if it exceeds 30%, the shape of the hot rolled coil is adversely affected.

It is preferable that the cooling in the run-out is started within 0.1 second to less than 1.0 second after finishing rolling. By starting cooling in the run-out during less than 1.0 second after the finish rolling, the steel plate having excellent press formability is suppressed by suppressing the growth of austenite crystal grains before transformation after the finish rolling. Can be obtained. In order to obtain even more excellent press formability, it is preferable to start cooling in the runout within 0.5 seconds after the finish rolling. In addition, the shorter the time before starting cooling in the run-out, the higher the effect becomes. However, due to equipment limitations (the need to install measuring equipment, the cooling device must be placed close to the exit of the final stand of the finishing mill). However, considering the actual equipment, it is difficult to start cooling within 0.1 second after finishing rolling, so that it was set to more than 0.1 second after finishing rolling.

The reason why the winding temperature is set in the range of 400 to 700 ° C. is that if the winding temperature is lower than 400 ° C., the strength-ductility balance is deteriorated due to the formation of a low-temperature transformation phase. If the temperature exceeds ℃, coarse pearlite which is harmful to ductility is formed, and ductility is significantly reduced. By setting the winding temperature to 400 to 700 ° C., the excellent workability intended by the present invention can be obtained.

In the conventional high-strength thin steel sheet, there has been a problem that the elongation-flange crack often occurs during burring or the like. However, in the high-strength thin steel sheet according to the present invention, the end temperature of hot rolling, the run-out By controlling the cooling rate and the winding temperature in cooling, the workability (elongation-flangeability) is remarkably improved, and the occurrence of elongation-flange cracks can be effectively suppressed.

In the present invention, after the rough rolling, before the finish rolling or during the finish rolling, it is preferable to perform the finish rolling after heating the whole or edge portion of the rough bar. Thereby, the uniformity of workability intended in the present invention can be further improved.

In the present invention, the heating of the rough rolling bar can be effectively used for a continuous hot rolling process using a coil box or the like. In this case, the effect of the present invention can be sufficiently exhibited even if the rough hot rolling bar is heated before and after the coil box or between or after the rough rolling mills.

Even if the hot-rolled sheet manufactured as described above is further subjected to cold rolling and annealing to form a cold-rolled sheet, the effects of the present invention are not impaired at all. At this time, the conditions of cold rolling and annealing are not particularly limited, and may be performed according to a conventional method.

[0042]

Embodiments of the present invention will be described below. [Example 1] Steel No. 1 shown in Table 1 was used. Material Nos. 1 to 12 After melting and casting slabs of material Nos. 1 to 12, material Nos. Hot rolled coils (thickness: 3.0 mm) were obtained by performing hot rolling by a direct rolling process, performing runout cooling at the cooling rate shown in Table 2, and winding at the winding temperature shown in Table 2. Was. In addition, material No. After melting and casting, the slabs were heated at 1150 ° C., then hot-rolled, runout-cooled at a cooling rate shown in Table 2, and taken up at a winding temperature shown in Table 2. It was wound up to obtain a hot-rolled coil (sheet thickness: 3.0 mm).

The state of occurrence of surface defects of the obtained hot-rolled coil was evaluated over the entire length of the coil on a coil inspection line, and evaluated in three steps according to the frequency of occurrence of flaws. The hole expansion rate was measured as an evaluation of tensile strength and stretch flangeability. Table 2 shows the results of each evaluation. FIG. 1 is a graph showing the tensile strength TS (MPa) on the horizontal axis and the hole expansion rate (%) on the vertical axis, together with the evaluation of the frequency of occurrence of surface defects.

As shown in Table 2 and FIG. 1, the steel composition is within the scope of the present invention, and the production conditions satisfy the condition of the present invention.
o. The hot-rolled coils Nos. 1 to 4 are excellent in surface properties, and the hole expansion ratio is excellent in each strength level as compared with the comparative example. For these materials No. Each of 5 to 12 has an Mn amount less than the lower limit specified in the present invention, an Mn amount exceeds the upper limit specified in the present invention, and the S amount and O amount exceed the upper limit specified in the present invention. And the Mn / S value is less than the lower limit specified in the present invention, the P and N contents exceed the upper limit specified in the present invention, the O amount and the Sn amount are specified in the present invention. Those exceeding the upper limit, those in which the N amount exceeds the upper limit specified in the present invention, those in which the Sn amount exceeds the upper limit specified in the present invention, and those in which the value of Mn / S is the lower limit specified in the present invention In each case, the frequency of occurrence of surface defects or the rate of hole expansion is inferior to those of the present invention.

As described above, it has been confirmed that a high-strength thin steel sheet satisfying desired surface properties and workability (elongation-flangeability) can be obtained by satisfying the constitutional requirements specified in the present invention. .

[0046]

[Table 1]

[0047]

[Table 2]

Example 2 The steel No. described in Table 1 was used. 1,
Material No. 2 consisting of 13 to 20 were melted to form a continuous cast slab, and then hot-rolled under the manufacturing conditions shown in Table 3 to obtain a hot-rolled coil. The material No. As for the materials Nos. 13 to 16, the material Nos. Heating is performed before hot rolling as in the cases 2 and 3. Table 3 shows the results of evaluating the obtained hot-rolled coils in the same manner as in Example 1. FIG. 2 shows the tensile strength TS (MPa) on the horizontal axis from these evaluations,
It is the graph which showed the hole expansion rate (%) on the vertical axis, and the cooling rate in the run-out, and classified by steel.

As shown in Table 3 and FIG. 2, the material No. satisfying the manufacturing conditions of the present invention. 14, 15, 16, 18, 1
Nos. 9 and 20 are excellent in surface properties and the hole expansion ratio is excellent in each strength level. For these materials No. 13,
17 is a comparative example in which the run-out cooling rate after finish rolling is out of the range specified in the present invention, and the hole expansion ratio is a slightly lower value as compared with the inventive examples in Table 3; It is clear that the intended excellent workability has not been obtained.

As described above, in the present invention, all the production conditions specified in the present invention (particularly, the run-out cooling rate)
It has been confirmed that by controlling within the range of the present invention, a high-strength steel sheet excellent in surface properties and workability intended in the present invention can be obtained.

[0051]

[Table 3]

Example 3 Steel No. 1 shown in Table 1 was used. 1 to 1
Material No. 2 consisting of After smelting and casting slabs 21 to 32, hot rolling was performed under the conditions shown in Table 4, cooling was performed, run-out was performed, and the hot rolled coil (thickness 3.
0 mm). Table 4 shows the results of evaluating the obtained hot-rolled coil in the same manner as in Example 1, and the results of evaluating the shape of the hot-rolled coil. FIG. 3 shows the tensile strength TS (MPa) on the horizontal axis and the hole expansion ratio (%) on the vertical axis from these evaluations.
2 is a graph showing the evaluation of the frequency of occurrence of surface defects.

As shown in Table 4 and FIG. 3, the steel composition is within the scope of the present invention, and the production conditions satisfy the conditions of the present invention.
o. The hot-rolled coils 21 to 24 are excellent in the coil shape and surface properties, and the hole expansion ratio is excellent in each strength level as compared with the comparative example. In addition, material No. Nos. 23 and 24 have different runout cooling start times under the same runout cooling conditions. 23 is more excellent in strength and workability. For these materials N
o. Each of 25 to 32 has an Mn amount less than the lower limit specified in the present invention, an Mn amount exceeds the upper limit specified in the present invention, and the S amount and O amount exceed the upper limit specified in the present invention. , The amount of P exceeds the upper limit specified in the present invention, the amount of O and Sn exceeds the upper limit specified in the present invention, and the amount of N exceeds the upper limit specified in the present invention. , Where the amount of Sn exceeds the upper limit specified in the present invention, M
The value of n / S is less than the lower limit specified in the present invention, and in each case, the frequency of occurrence of surface defects or the hole expansion rate is inferior to those of the examples of the present invention. In addition, material No. 26
Since the rolling reduction of the final pass of the finish rolling exceeded 30%, the coil shape was also poor.

As described above, in the present invention, all of the manufacturing conditions specified in the present invention (particularly, the run-out cooling rate)
It was confirmed that by controlling the content within the range of the present invention, a high-strength steel sheet having excellent surface properties and workability intended in the present invention and excellent coil shape can be obtained.

[0055]

[Table 4]

[0056]

As described above, according to the present invention,
Since the steel composition can suppress slab cracks generated during continuous casting and cracks generated during hot rolling, cracks that have occurred on the slab surface or under the slab surface during continuous casting can be suppressed. Optimum control can improve the workability (elongation-flangeability) of the steel. Furthermore, according to the present invention, an excellent coil shape can be ensured. Therefore, even with the cost-effective direct-feed rolling process, 340MP having excellent surface properties and workability can be obtained.
It is possible to manufacture a high-strength thin steel plate of a or more.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing the strength-hole expansion balance in Example 1 in comparison between the present invention example and a comparative example.

FIG. 2 is an explanatory view (for each steel) showing the strength-hole expansion balance in Example 2 in comparison with the present invention example and a comparative example.

FIG. 3 is an explanatory view showing the strength-hole expanding balance in Example 3 in comparison between the present invention example and a comparative example.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yoichi Honashiki 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Inventor Hiroyasu Kikuchi 1-1-2 Marunouchi, Chiyoda-ku, Tokyo No. Nippon Kokan Co., Ltd. F term (reference) 4K037 EA05 EA06 EA09 EA15 EA18 EA23 EA25 EA27 FB08 FC07 FD03 FD04 FE01 FE02 FE03 FG03 FH01

Claims (5)

[Claims] 1. C .: 0.05 to 0.2% by weight, S
i: 0.15% or less, Mn: 0.4 to 2.0%, P:
0.025% or less, O: 0.005% or less, S: 0.0
1% or less, N: 0.006% or less, Sn: 0.004%
A steel containing the following and satisfying the relationship of Mn / S ≧ 50 is continuously cast into a slab, and the obtained continuous cast slab is subjected to a heating step or to a rough rolling without a heating step. , Ar hot rolling is completed at a temperature of ₃ points or more, and then the obtained hot rolled sheet is cooled to a range of 400 to 700 ° C. at a cooling rate of 20 to 2000 ° C./sec and then wound up. A method for producing high-strength thin steel sheets with excellent surface properties and workability. 2. In% by weight, C: 0.05-0.2%, S
i: 0.15% or less, Mn: 0.4 to 2.0%, P:
0.025% or less, O: 0.005% or less, S: 0.0
1% or less, N: 0.006% or less, Sn: 0.004%
A steel containing the following and satisfying the relationship of Mn / S ≧ 50 is continuously cast into a slab, and the obtained continuous cast slab is subjected to a heating step or to a rough rolling without a heating step. Finish rolling is performed at a finishing temperature of not less than the Ar ₃ transformation point and a rolling reduction of the final stand of 8% or more and 30% or less.
A method for producing a high-strength thin steel sheet having excellent surface properties and workability, wherein the high-temperature steel sheet is cooled after being cooled to a temperature in the range of 400 to 700 ° C. at a cooling rate of 1 ° C. 3. The surface properties and workability according to claim 1 or 2, wherein cooling is started within 0.1 second and less than 1.0 second after finishing rolling. Manufacturing method of high strength thin steel sheet. 4. The steel composition further comprises Ca: 0.00
The method for producing a high-strength thin steel sheet having excellent surface properties and workability according to any one of claims 1 to 3, wherein 5% or less is added. 5. The surface properties and workability according to claim 1, wherein at least cold rolling and annealing are performed after winding the hot-rolled sheet. Manufacturing method of high strength thin steel sheet.