JP2001288536A - High workability hot rolled steel sheet free from generation of chatter mark defect and its production method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot rolled steel sheet for working in which the generation to chatter mark defects is eliminated by using low-cost B-added low carbon steel and sufficiently reducing the content of solid solution C and the content of solid solution N, and further, sufficient ductility is secured, and to provide its production method. SOLUTION: This high workability hot rolled steel sheet free from the generation of buckling has a composition containing, by weight, 0.015 to 0.08% C, 0.05 to 0.5% Mn, 0.003 to 0.5% Si, <=0.05% P, 0.005 to 0.02% S, 0.005 to 0.3% Al, <=0.006% N and B so as to satisfy 0.8<=B/N<=2.5, and the balance Fe with inevitable impurities, and in which the total of the content of solid solution C and the content of solid solution N is <=3 ppm. Moreover, in the method for producing the high workability hot rolled steel sheet free from the generation of buckling, a slab having the above components is reheated, is thereafter finished with rolling at the Ar3 transformation point of above, is cooled at a cooling rate of >=20 deg.C/sec and is coiled at >=580 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a soft-rolled hot-rolled steel sheet which does not cause breakage of a low-carbon steel to which B is added and a method for producing the same.

[0002]

2. Description of the Related Art In a conventional method for manufacturing a hot-rolled steel sheet for processing using a low-carbon Al-killed steel, a skin pass line is passed after winding. In the case of a thin material, there is also a purpose of shape correction, but there is also a purpose of preventing a stripe pattern called waist break, which is caused by a yield phenomenon due to the remaining solid solution C or solid solution N regardless of the sheet thickness. For waist break, pitch is several mm to several cm
One of the stripes that looks blackish has a width of about several mm, the stripes are locally deformed, and the portions between the stripes are hardly deformed. This waist break is caused by the yield phenomenon. By passing through the skin pass line, it is possible to give a uniform strain to the steel sheet, introduce a movable dislocation, suppress the yield phenomenon, and prevent the waist break.

[0003] The above method has disadvantages in that the number of threading steps is increased, the production cost is increased, and extra equipment is required. If there is no buckling, there is no need to pass through a skin pass line when using a steel sheet that does not need shape correction while hot-rolling it or using a leveler in an acid pickling line, reducing manufacturing costs. Can be suppressed. As a means for eliminating buckling, for example, as disclosed in JP-A-61-73836, C% is 0.010%.
Solid solution N by adding Ti, Nb and B to the following ultra low carbon steel
Is fixed with Ti and B, and solid solution C is fixed with Ti and Nb. However, in this method, C% is 0.1%.
There is a drawback that the production cost at the steelmaking stage is increased by the production of extremely low carbon steel of 010% or less and the addition of Ti and Nb.

[0004] As a method for eliminating the occurrence of hip breakage while keeping the cost in the steel making stage low, Japanese Patent Laid-Open No. 4-5673 is disclosed.
As described in Japanese Patent Publication No. 2 (2003), rolling is performed using the B-added steel at an Ar ₃ transformation point or higher, and after controlling the cooling rate after rolling, 300 to 50 is applied.
A method of winding at 0 ° C. has been proposed. In this method, BN is precipitated during rolling to fix solid solution N, and after winding, cementite is precipitated to fix solid solution C. However, since the precipitation nose of BN is about 800 ° C., Ar ₃ becomes about 830 to 850 ° C. in this component system.
It is difficult to sufficiently precipitate BN by rolling beyond the transformation point. Conversely, if rolling is performed at a temperature equal to or lower than the Ar ₃ transformation point at which BN tends to precipitate, coarse grains are generated in the surface layer and ductility is deteriorated.

[0005]

SUMMARY OF THE INVENTION In the present invention, low-cost B-added low-carbon steel is used, solid solution C and solid solution N are sufficiently reduced to eliminate the occurrence of breakage and to secure sufficient ductility. An object of the present invention is to provide a hot-rolled steel sheet for processing and a method for producing the same.

[0006]

Means for Solving the Problems (1) By weight%, N:
0.006% or less, B: made of ordinary low carbon Al-killed steel containing 0.8 ≦ B / N ≦ 2.5, and the amount of solid solution C and solid solution N
A hot-rolled hot-rolled steel sheet having no breakage, characterized in that the total amount is 3 ppm or less. (2) By weight%, C: 0.015 to 0.08%, Mn:
0.05-0.5%, Si: 0.003-0.5%,
P: 0.05% or less, S: 0.005 to 0.02%, A
l: 0.005 to 0.3%, N: 0.006% or less,
B: including 0.8 ≦ B / N ≦ 2.5, with the balance being Fe and unavoidable impurities, wherein the total amount of solid solution C and solid solution N is 3 ppm or less; Not high workability hot rolled steel sheet.

(3) C: 0.015 to 0.0% by weight
8%, Mn: 0.05 to 0.5%, Si: 0.003 to
0.5%, P: 0.05% or less, S: 0.005-0.
02%, Al: 0.005 to 0.3%, N: 0.006
% Or less, B: 0.8 ≦ B / N ≦ 2.5, the balance being F
After reheating the billet consisting of e and unavoidable impurities,
A method for producing a hot-rolled hot-rolled steel sheet having no breakage, characterized in that rolling is completed at an Ar ₃ transformation point or higher, cooled at a cooling rate of 20 ° C./second or more, and wound at 580 ° C. or more.

(4) C: 0.015 to 0.0% by weight
8%, Mn: 0.05 to 0.5%, Si: 0.003 to
0.5%, P: 0.05% or less, S: 0.005-0.
02%, Al: 0.005 to 0.3%, N: 0.006
% Or less, B: 0.8 ≦ B / N ≦ 2.5, the balance being F
e and slabs consisting of unavoidable impurities
After reheating in the temperature range of 50 ° C., the rolling was completed at the Ar ₃ transformation point or higher, and cooled at a cooling rate of 10 ° C./sec or higher.
There is provided a method for producing a hot-rolled hot-rolled steel sheet having no breakage, characterized by winding at 0 ° C. or higher.

Hereinafter, the present invention will be described in detail. The present inventors conducted experiments in which the hot rolling conditions were variously changed using the B-added low carbon steel. As a result, even if the rolling was completed at the Ar ₃ transformation point or higher, the subsequent cooling rate was sufficiently taken. 580 ° C
It has been found that the winding does not cause the waist to break. At this time, as shown in FIG. 1, in a steel plate in which no buckling occurs, the solid solution C which causes the yield phenomenon
And the total amount of dissolved N was 3 ppm or less. The total amount of the solid solution C and the solid solution N can be measured using an internal friction measuring device.

Under the above rolling conditions, BN precipitates after winding. After being wound around a coil at 580 ° C. or higher, the temperature drops at a very slow rate, so that BN can be sufficiently deposited in a temperature range away from the nose of BN deposition. After winding, not only BN but also cementite is sufficiently precipitated. It is considered that BN precipitated in this temperature range is relatively coarse, and serves as a precipitation nucleus of cementite, so that cementite is easily precipitated. Therefore,
It is most important to increase the amount of BN precipitated after winding, and for that purpose, it is necessary to control the reheating temperature and the cooling rate after rolling.

The reheating temperature affects the state of precipitation of MnS, which is the nucleus of BN, and near the nose of MnS precipitation of about 1000 to 1150 ° C., MnS of the size that becomes the nucleus of BN is easily deposited. In addition, by increasing the cooling rate after rolling, the energy of precipitation due to undercooling is increased, and the amount of BN deposited increases. The presence or absence of occurrence of buckling depending on the reheating temperature and the cooling rate after rolling is as shown in FIG. 2. If the reheating temperature is about 1000 to 1150 ° C., the cooling rate may be 10 ° C./sec or more. 1150
If the reheating temperature is not less than 950 ° C. or not more than 950 ° C., 20
A cooling rate of at least ° C / sec is required. In addition, ductility is also improved by reducing solid solution C and solid solution N. Good elongation of 3% or more is obtained with respect to a steel sheet having a similar strength level.

The reasons for limiting the components and component ranges of the steel sheet to which the present invention is applied will be described. C is a hardening element and C
The smaller the amount, the more advantageous the processability. However, the lower limit of the C content is set to 0.015% in consideration of the economics of the decarburization treatment for reducing the C content.
And Further, when the amount of C increases, the material becomes hard and the workability deteriorates. Therefore, the upper limit of the amount of C is set to 0.08%. Mn is
MnS is an element necessary for imparting toughness, and also a constituent element of MnS serving as a nucleus of BN, and its amount is required to be 0.05% or more. Further, when the amount of Mn increases, the effect of curing and improving elongation is saturated, so the upper limit was made 0.5%.

[0013] Si is added as a deoxidizing agent for steel,
If the content increases, it hardens and deteriorates workability, so the range is made 0.003 to 0.5%. Since P is inevitably contained as an impurity and adversely affects elongation, the upper limit is set to 0.05%. S is an element necessary for precipitating MnS serving as a nucleus of BN, and is required to be 0.005% or more. However, when the content increases, the composition hardens to deteriorate the workability. Therefore, the upper limit is set to 0.02%.

Al is added as a deoxidizing agent for steel and is contained in steel. Since Al precipitates solute N in steel as AlN, it is an important element for reducing solute N together with B. And 0.005% or more is required. On the other hand, the elongation improves as the amount of Al increases, but if it exceeds 0.3%, it hardens and deteriorates workability.
5 to 0.3%. Although N is contained as an unavoidable impurity, if it remains as solid solution N, it may cause breakage. By adding B, it can be precipitated as BN. However, if the amount of N is large, the amount of BN must be increased, resulting in deterioration of the material. Considering the balance, the upper limit is 0.
006%.

B is the most important element in the present invention because it precipitates N in steel as BN and further uses it as a nucleus to precipitate cementite. BN precipitates preferentially, but AlN also partially precipitates.
If it is above, the solid solution N can be sufficiently reduced. However, if the B / N ratio is too high, only the amount of solid solution B increases, which leads to a decrease in economy and workability. Therefore, the upper limit is set to 2.5.

[0016]

Next, embodiments of the present invention will be described. The present invention is carried out in a normal continuous hot rolling line including a heating furnace, rough rolling, finish rolling, a cooling zone, and a winder. After winding, it may be shipped as it is, or may be directly passed through a pickling line. The molten steel may be produced by a normal blast furnace method, or may be a large amount of scrap as in the electric furnace method. The slab may be manufactured by a normal continuous casting process or may be manufactured by thin slab casting. A coil box that winds and holds a rough bar after rough rolling is effective for precipitation of MnS, and is advantageous for the present invention. Further, a so-called hot rolling continuous process in which the wound coarse bar is joined with the preceding rough bar when unwinding and rolling is performed is preferable from the viewpoint of material variation and productivity.

[0017]

EXAMPLES Hot-rolled steel sheets for processing were manufactured using the steel slabs having the components shown in Table 1 under the conditions shown in Table 2. The results are also shown in Table 2. E to I outside the component range of the present invention
Comparative Example No. manufactured under the manufacturing conditions specified in the present invention using the steel type No. Comparative Examples Nos. 7 to 11, which were manufactured using steel types having the component ranges specified in the present invention and deviating from the manufacturing conditions specified in the present invention. In 12 to 15, waist breaks have occurred. On the other hand, in the example of the present invention, In Nos. 1 to 6, hip break did not occur and elongation was good.

[0018]

[Table 1]

[0019]

[Table 2]

[0020]

As described above, according to the method of the present invention, a hot-rolled hot-rolled steel sheet free from breakage can be provided at low cost, and the skin pass step can be reduced. it can. Therefore, the economic merit is very large.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the total amount of solid solution C and solid solution N and the occurrence of buckling.

FIG. 2 is a diagram showing changes in the occurrence of buckling according to the heating temperature and the average cooling rate from rolling to winding.

Claims (4)

[Claims] 1. An ordinary low-carbon Al-killed steel containing N: 0.006% or less and B: 0.8 ≦ B / N ≦ 2.5 by weight%. A hot-rolled hot-rolled steel sheet having no breakage, characterized in that the total amount is 3 ppm or less. 2. In% by weight, C: 0.015-0.08%, Mn: 0.05-0.5%, Si: 0.003-0.5%, P: 0.05% or less, S: 0.005 to 0.02%, Al: 0.005 to 0.3%, N: 0.006% or less, B: 0.8 ≦ B / N ≦ 2.5, the balance being Fe and A highly workable hot-rolled steel sheet which is made of unavoidable impurities and has a total content of solid solution C and solid solution N of 3 ppm or less, and is free from bending. C: 0.015 to 0.08%, Mn: 0.05 to 0.5%, Si: 0.003 to 0.5%, P: 0.05% or less by weight%. S: 0.005 to 0.02%, Al: 0.005 to 0.3%, N: 0.006% or less, B: 0.8 ≦ B / N ≦ 2.5, the balance being Fe and After reheating the slab consisting of unavoidable impurities, rolling was completed at the Ar ₃ transformation point or higher, and 2
A method for producing a hot-rolled steel sheet having a high formability that does not cause buckling, wherein the hot-rolled steel sheet is cooled at a cooling rate of 0 ° C./sec or more and wound at 580 ° C. or more. 4. In% by weight, C: 0.015 to 0.08%, Mn: 0.05 to 0.5%, Si: 0.003 to 0.5%, P: 0.05% or less, S: 0.005 to 0.02%, Al: 0.005 to 0.3%, N: 0.006% or less, B: 0.8 ≦ B / N ≦ 2.5, the balance being Fe and After re-heating the slab consisting of unavoidable impurities in the temperature range of 1000 to 1150 ° C.,
_A method for producing a hot-rolled hot-rolled steel sheet having no breakage, characterized in that rolling is completed at _three or more transformation points, cooled at a cooling rate of 10 ° C./sec or more, and wound at 580 ° C. or more.