JPH07157844A - Hot rolled steel sheet excellent in workability and production thereof - Google Patents

Abstract

PURPOSE:To produce a hot rolled steel sheet excellent in workability even if a steel contg. tramp elements such as copper and tin is used by specifying the componental compsn. of the steel and the producing conditions therefor. CONSTITUTION:A steel having a compsn. contg., by weight, 0.01 to 0.15% C, <=0.2% Si, 0.1 to 0.8% Mn, <=0.04% P, <=0.015% S, 0.01 to 0.07% sol.Al, <=0.012% N, <=0.40% Cu, <=0.040% Sn <=0.10% Ni, <=0.10% Cr and <=0.10% Mo and satisfying the inequality of Cu+10XSn<=0.40, and the balance Fe is melted. Moreover, this steel may be incorporated with one or more kinds among. 0.01 to 0.10% Ti, 0.01 to 0.10% Zr, 0.001 to 0.010% Ca and 0.001 to 0.010% rare earth metals as well. The steel is subjected to continuous casting to form into a slab, which is heated to >=1100 deg.C and is thereafter subjected to hot rolling at a finishing temp. in the temp. range of the Ar3 transformation point 100 to the Ar3+100 deg.C, and the obtd. steel strip is coiled at 500 to 700 deg.C. Thus, the hot rolled steel sheet excellent in workability and having <=50kgf/mm<2> tensile strength can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot rolled steel sheet having excellent workability and a tensile strength of 50 kgf / mm ² or less, and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, the amount of discarded iron scraps such as food cans and automobiles has increased, and in the steel manufacturing industry, the necessity of using such iron scraps as an iron source is increasing in view of environmental problems. . However, so-called Trump elements such as Cu, Sn, Mo, Ni and Cr contained in the iron scrap are less likely to be oxidized than Fe, so that they are hardly removed in the current ironmaking process and remain in the steel. It has been pointed out that, for example, the manufacturability, the material quality, and the surface quality characteristics are adversely affected in, for example, Japanese Patent Application Laid-Open Nos. 4-162943 and 4-371528.

Therefore, in the prior art, a technique of intentionally adding elements such as Cu, Cr and Ni as useful elements for the purpose of improving the corrosion resistance, strength and surface characteristics, for example, JP-A-4-325657. Except for the case of the technique described in Japanese Patent Application Laid-Open No. 4-365813 (hereinafter referred to as prior art), the Trump element is generally not contained in steel and is not added. However, the use of iron scrap has various restrictions.

[0004]

However, in the case of producing a steel product using iron scrap containing a playing card element, which is generated from a food can, an automobile, etc., as a steel raw material in view of the above-mentioned environmental problem countermeasures, it is An important issue was to prevent quality deterioration.

Therefore, it is an object of the present invention to include Cu in the steel composition.
An object of the present invention is to provide a hot-rolled steel sheet having excellent workability and a manufacturing method thereof even when using a steel containing a playing card element such as Sn and Sn.

[0006]

The present invention solves the above problems found in steel materials containing a playing card element and develops a hot-rolled steel sheet having excellent workability and a tensile strength of 50 kgf / mm ² or less. The summary is as follows.

The hot rolled steel sheet excellent in workability of the first invention is
% By weight, C: 0.01 to 0.15%, Si: 0.2% or less, Mn:
0.1 to 0.8%, P: 0.04% or less, S: 0.015% or less, so
l.Al: 0.01 to 0.07%, N: 0.012% or less, Cu: 0.040% or less, Sn: 0.040% or less, Ni: 0.10% or less, Cr: 0.10%
Or less and Mo: 0.10% or less, and
The following formula (1), Cu + 10 × Sn ≤ 0.40% ───────────────────────────────────────────────────────────────────────────―― is there.

The hot rolled steel sheet excellent in workability of the second invention is
% By weight, C: 0.01 to 0.15%, Si: 0.2% or less, Mn:
0.1 to 0.8%, P: 0.04% or less, S: 0.015% or less, so
l.Al: 0.01 to 0.07%, N: 0.012% or less, Cu: 0.40% or less, Sn: 0.040% or less, Ni: 0.10% or less, Cr: 0.10
% Or less and Mo: 0.10% or less, and further Ti: 0.01 to 0.10%, Zr: 0.01 to 0.10%, Ca:
0.001 to 0.010% and REM: Contains at least one of 0.001 to 0.010%, and satisfies the relationship of the following formula (1), Cu + 10 x Sn ≤ 0.40% ─── (1), and the balance is iron and It is characterized in that it has a chemical composition of an unavoidable impurity element other than the above elements.

In the method for producing a hot rolled steel sheet having excellent workability according to the third and fourth aspects of the invention, the steel having the chemical composition of the first and second aspects of the invention is melted, and then the slab is obtained by continuous casting. And then heating the slab cast by the continuous casting to a temperature of 1100 ° C. or higher, and then, with respect to the slab, an _{Ar 3} transformation point −100 to an _{Ar 3} transformation point + 100 ° C.
It is characterized in that hot rolling is carried out at a finishing temperature within the temperature range of 1), and the steel strip thus obtained is wound up within the temperature range of 500 to 700 ° C.

[0010]

The reason why the chemical composition of the steel sheet of the present invention is limited to the above range will be described. (1) C, Si, Mn: C, Si, Mn are basic elements that increase the strength of steel. However, the effect is insufficient if the C content is less than 0.01% and the Mn content is less than 0.1%, while the C content is more than 0.15% and the Si content is 0.2%.
If the content of Mn exceeds 0.8%, the tensile strength cannot be reduced to 50 kgf / mm ² or less. Further, if the content of Mn is less than 0.1%, flaws are likely to occur on the surface of the steel sheet. Therefore, the C content should be limited to 0.01 to 0.15%, the Si content to 0.2% or less, and the Mn content to 0.1 to 0.8%.

(2) S: S is preferably contained in a small amount in order to improve workability such as bending workability and stretch flangeability of the steel sheet. FIG. 1 is a graph showing the relationship between the hole expansion ratio showing the stretch flangeability of a steel sheet and the S content. As is clear from the figure, a high hole expansion rate is obtained in the region where the S content is 0.015% or less. The reason for this is considered to be that if the S content exceeds 0.015%, the amount of sulfide-based inclusions such as MnS in the steel increases, and it becomes a crack initiation point when the steel sheet is processed. Further, in order to secure weldability, it is important that the S content is 0.015% or less. Therefore, the content of S should be limited to 0.015% or less. In addition, even in steel containing a large amount of playing cards, low sulfurization is extremely effective in improving workability.

(3) P: P is an element that causes intergranular brittle fracture called secondary work cracking after press forming of a steel sheet, and it is desirable that its content is small, and the content of P is 0.
It should be limited to 04% or less.

(4) N: N is preferably contained in a small amount in order to improve workability, and the content should be limited to 0.012% or less.

(5) Al: Al is an element effective for deoxidizing steel, but if the content of sol.Al is less than 0.01%, its effect is insufficient. On the other hand, if its content exceeds 0.07%, the amount of inclusions, which are deoxidation products, increases and the workability deteriorates.
Therefore, the content of sol.Al should be limited to the range of 0.01 to 0.07%.

(6) Cu: Cu is an element contained as a playing card element. If the content is high, surface defects called Cu defects occur and the surface quality deteriorates. If the content exceeds 0.40%, Cu defects are likely to occur. Further, in a steel sheet containing Sn in addition to Cu, Sn is concentrated in the Cu-rich phase and the melting point of the Cu-rich phase is lowered, so that Cu-containing Sn is contained.
Cu defects are more likely to occur than in steel sheets not containing Cu, and the surface quality is significantly deteriorated. Therefore, the Cu content should be limited to 0.40% or less, and further, the Cu content should be limited so that Cu + 10 × Sn ≦ 0.40% is satisfied.

(7) Sn: Sn is an element contained as a playing card element. If the Sn content exceeds 0.040%, the surface quality and workability deteriorate. Further, in the steel sheet containing Cu in addition to Sn, as in the phenomenon described above, Sn is concentrated in the Cu-rich phase and the melting point of the Cu-rich phase is lowered,
Cu flaws are more likely to occur than in a steel sheet containing Sn and not containing Cu, and the surface quality is significantly deteriorated. Therefore, the Sn content should be limited to 0.040% or less, and further, the Sn content should be limited so as to satisfy Cu + 10 × Sn ≦ 0.40%.

(8) Ni: Ni is an element contained as a playing card element. However, it should be contained within an appropriate amount because it is effective in preventing Cu defects. Further, Ni is effective in improving hardenability. However, if its content exceeds 0.10%, the strength increases and the workability deteriorates. Therefore, the Ni content should be limited to 0.10% or less.

(9) Cr, Mo: Cr and Mo are playing card elements. In all cases, if the content exceeds 0.10%, the workability deteriorates due to the increase in strength. Therefore, the contents of Cr and Mo should be limited to 0.10% or less.

(10) Ti, Zr, Ca, REM: Ti, Zr, Ca and REM (rare earth elements) are all effective elements for controlling the sulfide morphology, and are steel sheets containing a large amount of Trump elements. Also in the above, the effect of controlling the sulfide morphology exerts a remarkable effect in improving stretch flangeability and bendability. However, if the content of Ti and Zr is less than 0.01%, and that of Ca and REM is less than 0.001%, the effect is insufficient. However, the effect reaches saturation even if the content of Ti and Zr exceeds 0.10%, and that of Ca and REM exceeds 0.010%, respectively, while the effect reaches saturation, while the cost increase and steel Cause deterioration of cleanliness. Therefore, the Ti and Zr contents are both in the range of 0.01 to 0.10%, and the Ca and REM contents are both 0.001 to
It should be limited to within 0.010%.

Next, the reason why the method for manufacturing a steel sheet according to the present invention is limited within the range of the above-mentioned conditions will be described. For the melting and refining of steel, either a converter method or an electric furnace method may be used, and for the production of slabs, the continuous casting method is advantageous in terms of its quality, yield and production efficiency. Cast the slab by.

Regarding the heating temperature of the slab, the usual method is 1100.
It may be at least ° C. For the finishing temperature of hot rolling,
When the finishing temperature is below the _{Ar 3} transformation point −100 ° C., strain is applied to the ferrite grains of the steel sheet to form a mixed grain structure and the ductility deteriorates. On the other hand, if the finishing temperature is above the _{Ar 3} transformation point + 100 ° C,
It is difficult to perform rolling due to the facility capacity, and the quality of the surface of the steel sheet deteriorates due to the scale generated during the hot rolling process. Therefore, the finishing temperature for hot rolling is the _{Ar 3} transformation point −100.
˜A _r3 transformation point should be limited to within + 100 ° C. The coiling temperature should be limited to the usual temperature range of 500 to 700 ° C.

[0022]

EXAMPLES Next, the present invention will be described by way of examples in comparison with comparative examples. Inventive Examples Nos. 1 to 12 and Comparative Example No. 2 having the chemical composition within the scope of the present invention shown in Table 1 and the chemical composition having at least one element outside the scope of the present invention Comparative Examples Nos. 1 and 3 to 7 having
Steel was melted in an electric furnace, and then a slab was cast by a continuous casting method. Then, as shown in Table 2, for the slabs of the present invention examples Nos. 1 to 12 and the comparative examples No. 3 to 7, the production conditions were within the scope of the present invention, and the comparative example No. The slabs 1 and 2 were hot-rolled under manufacturing conditions outside the scope of the present invention to prepare hot-rolled steel strips with a plate thickness of 3.2 mm. Hot rolled steel sheet was manufactured by subjecting to 1% temper rolling.

[0023]

[Table 1]

[0024]

[Table 2]

A test material was sampled from the hot-rolled steel sheet produced as described above to prepare a tensile test piece (JIS No. 5 test piece), a hole-expansion test piece and a bending test piece, and these tests were conducted. The results are also shown in Table 2.

For the bending test, 180 degree contact bending was performed. For hole expansion test, measure the hole diameter before and after the test,
The hole expansion ratio was calculated by the following equation (2). The hole diameter setting value before the test was 10 mm. Hole expansion rate (λ), (%) = {(hole diameter after test-hole diameter before test) / hole diameter before test} × 100 ── (2) Regarding surface quality, surface defects of hot-rolled steel sheet Depending on the presence or absence, it was evaluated as ○ (no occurrence) and × (occurrence).

The following matters were clarified from Tables 1 and 2. In Comparative Examples Nos. 1 and 2, the finish rolling temperature was a low temperature outside the range of the present invention, so that the microstructure became a ferrite mixed grain structure and ductility was significantly deteriorated. Comparative example No.
In 1, 3, 4, 6, and 7, the S content was a high value outside the range of the present invention, so the hole expansion ratio was low, and cracking occurred in the bending test. On the other hand, in Comparative Example No. 2, although the hole expansion ratio was high because the S content was low, the total elongation was low, so that roughening occurred in the bending test. In Comparative Examples Nos. 3, 4, 6 and 7, the contents of Cu and Sn alone were within the scope of the present invention, but since Cu + 10 × Sn was excessive, surface defects occurred in the hot rolled steel sheet, The surface quality has deteriorated. Further, Comparative Example No. 5 had a Mn content, and Comparative Example No. 7 had a large C content exceeding the range of the present invention, respectively, so that the tensile strength was 50 kgf / mm.
^{The number} exceeds ² , which is outside the target range of this invention.

On the other hand, all of the invention examples Nos. 1 to 12 in which the chemical composition of the steel and the manufacturing conditions are within the scope of the present invention, the elongation, the hole expansion ratio, the bending test result and the steel plate surface quality are all An excellent hot-rolled steel sheet was obtained, and its tensile strength was 50 kgf / mm ² or less.

As described above, with regard to the chemical composition and manufacturing conditions of steel, hot-rolled steel sheet, even one of which is outside the scope of the present invention, deteriorated in workability or surface quality, whereas The hot rolled steel sheet within the scope of the present invention was excellent in both workability and surface quality.

[0030]

When iron scrap containing a conventional playing card element is manufactured as a steel material, the workability or surface quality of the steel material is deteriorated due to the remaining playing card element contained in the steel. Therefore, the use of iron scrap has various restrictions. However, since the present invention is configured as described above, even when using a steel slab containing a playing card element, the tensile strength is 50 kgf / mm ² or less without sacrificing the quality of the product. It is possible to provide a soft hot-rolled steel sheet having excellent properties and an excellent surface quality, and a method for producing the same, which can bring industrially beneficial effects.

[Brief description of drawings]

FIG. 1 is a graph showing a relationship between a hole expansion ratio showing stretch flangeability of a steel sheet and S content.

Claims (4)

[Claims] 1. By weight%, carbon (C): 0.01 to 0.15%, silicon (Si): 0.2% or less, manganese (Mn): 0.1 to 0.8%, phosphorus (P): 0.04% or less, sulfur (S) ): 0.015% or less, soluble aluminum (sol.Al): 0.01 to 0.07%, nitrogen (N): 0.012% or less, copper (Cu): 0.40% or less, tin (Sn): 0.040% or less, nickel (Ni) : 0.10% or less, Chromium (Cr): 0.10% or less, and Molybdenum (Mo): 0.10% or less, and the relationship of the following formula (1), Cu + 10 × Sn ≤ 0.40% ─── (1) And a balance having a chemical composition of iron and unavoidable impurity elements other than the above elements, the hot-rolled steel sheet having excellent workability. 2. By weight%, carbon (C): 0.01 to 0.15%, silicon (Si): 0.2% or less, manganese (Mn): 0.1 to 0.8%, phosphorus (P): 0.04% or less, sulfur (S) ): 0.015% or less, soluble aluminum (sol.Al): 0.01 to 0.07%, nitrogen (N): 0.012% or less, copper (Cu): 0.40% or less, tin (Sn): 0.040% or less, nickel (Ni) : 0.10% or less, chromium (Cr): 0.10% or less, and molybdenum (Mo): 0.10% or less, further, titanium (Ti): 0.01 to 0.10%, zirconium (Zr): 0.01 to 0.10% , Calcium (Ca): 0.001 to 0.010%, and REM: 0.001 to 0.010%, and at least one of them is contained, and the following formula (1), Cu + 10 × Sn ≦ 0.40% ─── (1) And the balance has a chemical composition that consists of iron and inevitable impurity elements other than the above elements. 3. After smelting the steel having the chemical composition according to claim 1, casting a slab by continuous casting, and then heating the slab cast by the continuous casting to a temperature of 1100 ° C. or higher. Then, the slab is hot-rolled at a finishing temperature within a temperature range of _{Ar 3} transformation point −100 to _{Ar 3} transformation point + 100 ° C., and the steel strip thus obtained is subjected to 50
Characterized by winding within a temperature range of 0 to 700 ° C,
A method for producing a hot rolled steel sheet having excellent workability. 4. After smelting the steel having the chemical composition according to claim 2, casting a slab by continuous casting, and then heating the slab cast by the continuous casting to a temperature of 1100 ° C. or higher. Then, the slab is hot-rolled at a finishing temperature within a temperature range of _{Ar 3} transformation point −100 to _{Ar 3} transformation point + 100 ° C., and the steel strip thus obtained is subjected to 50
Characterized by winding within a temperature range of 0 to 700 ° C,
A method for producing a hot rolled steel sheet having excellent workability.