JPH07118800A - Hot rolled plate of sn-and ti-b-containing high strength steel excellent in surface characteristic and its production - Google Patents

Abstract

PURPOSE:To obtain a hot rolled plate of Ti- and B-containing high strength steel excellent in surface characteristic by specifying the chemical components of a steel stock and also limiting Sn concentration in the surface layer of a steel plate. CONSTITUTION:A steel stock, having a composition consisting of, by weight, >0.04-0.20% C, >=0.10% Si, <=2.50% Mn, 0.004-0.10% sol.Al, <=0.03% P, <=0.03% S, 0.03-0.25% Ti, 0.0005-0.0030% B, 0.01-0.40% Sn, and the balance Fe with inevitable impurities, is used. If Sn concentration in the steel is <=0.4, Sn concentration in the surface layer of a steel plate can be controlled to <=5% and the occurrence of surface flaw can be prevented. By this method, the hot rolled plate of Ti- and B-containing high strength steel, for which Sn in tin plate scrap and steel can as iron scrap for steelmaking material is utilized as a reinforcing element and which has superior surface characteristics, can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength hot-rolled steel sheet and a method for producing the same, and more particularly to a high-strength hot-rolled steel sheet with good surface properties and improved strength with Sn as a strengthening element and an inexpensive method for producing the same. Is.

[0002]

2. Description of the Related Art C, N, P, S are the strengthening elements of steel.
It is well known that there are i, Mn, etc.
It is well known that the addition of Ti is effective for strengthening by precipitation of TiC, and the addition of B is also effective for improving the hardenability, and that Sn is also an effective strengthening element.

[0003]

However, Sn is used as a strengthening element, and Ti and B are used as precipitation strengthening elements.
The high-strength hot-rolled steel sheet added with is not manufactured and does not exist. The reason is that when Sn is added as a strengthening element of steel, it was conventionally expensive because it was added as a Sn ingot. For example, by using inexpensive tin scrap and steel can scrap as a steelmaking raw material, There is no idea that a steel containing Sn as a strengthening element can be contained at a low cost, and therefore the effect of addition of Sn on the deterioration of the high temperature workability that causes surface defects on the steel sheet has not been clarified. Is.

The present inventors have clarified the effect of Sn addition on the high temperature workability based on the above idea. The present invention has been made based on the above idea and the above clarification, and provides a Ti-B-containing high-strength hot-rolled steel sheet having Sn as a strengthening element and good surface properties, and a method for producing the hot-rolled steel sheet.

[0005]

In view of the above situation, the present inventor investigated the effect of addition of Sn alone on the high temperature workability and found that S of a low carbon steel containing Sn as a strengthening element.
By limiting the content of n to 0.4% or less, 1
A new finding was obtained that a Ti-B-containing high-strength hot-rolled steel sheet having a good surface property can be obtained in hot strip hot rolling in which heating is performed at 330 ° C. or lower, and the present invention is based on this finding. The summary is as follows.

(1) The chemical component is C: more than 0.04% to 0.
2%, Si ≦ 0.10%, Mn ≦ 2.50%, Sol.
Al: 0.004 to 0.10%, P ≦ 0.03%, S ≦
0.03%, Ti: 0.03 to 0.25%, B: 0.0
005 to 0.0030%, Sn: 0.01 to 0.400
%, The balance being Fe and the unavoidable impurity element, which is a hot-rolled steel sheet, and in which the Sn concentration in the steel sheet surface layer is 5% or less, Sn and Ti having good surface properties.
-B-containing high strength hot rolled steel sheet.

(2) As a chemical component, 0.02 to 0.3
% Of Ni and / or 0.02 to 0.5% of Cr, and S having a good surface property according to (1).
n and Ti-B containing high strength hot rolled steel sheet. (3) A slab produced by continuously casting molten steel having a chemical composition in the range of (1) or (2) obtained by using tin scrap or steel can scrap as iron scrap of a steelmaking raw material is 1200 ° C to
A method for producing a Sn- and Ti-B-containing high-strength hot-rolled steel sheet having a good surface property, which is characterized by heat-retaining or heating in a range of 1330 ° C for 6 hours or less and hot-rolling with a hot strip mill.

The present invention will be described below. Considering that only steel can scrap and / or tin scrap are used as iron scrap, the present inventors have considered that it is possible to use only Sn as the playing card element in the steel, and thus the content of Ti and B is low. Various experiments were conducted on the influence of Sn on the surface properties of steel sheets using carbon steel, and the content of Sn was limited to the normal 1200 ° C to 133 ° C.
It was newly found that a hot-rolled steel sheet having good surface properties can be obtained in the heating range of 0 ° C.

The present inventors have made various changes in the content of Sn using steel can scraps and / or tin scraps as a raw material C:
Melt 0.01 to 0.200% Ti-B added steel,
After producing a continuous cast slab with a thickness of 50 mm and charging it in a heating furnace,
By heating in the heating range of 1250 ° C. to 1330 ° C., hot rolling with a hot strip mill, and inspecting the surface properties of the steel sheet, the range of good surface properties was clarified. Fig. 1 shows the surface layer of steel sheet having a surface layer of 1 μm.
3 shows the effect of the Sn content of According to this, the steel sheet surface layer Sn
It was newly found that when the concentration is 5% or less, surface flaws do not occur and the steel sheet surface properties are good. Further, it was newly found that if the Sn concentration of steel is 0.4% or less, the Sn concentration of the steel sheet surface layer becomes 5% or less in the range of slab heating from 1250 ° C to 1330 ° C.

These results show that when Sn is added up to 0.4%, it is concentrated in iron immediately below the scale by heating in an oxidizing atmosphere, but because Sn diffuses rapidly in Fe at high temperatures, this heating condition It is considered that this is because the concentration is not increased to the concentration until the liquid is prepared in the range. Limit S generated by this liquid
The n concentration (concentration of the surface layer portion 1 μm analyzed by the steel plate) was 5%.

Furthermore, S. L. Gertsman,
H. P. Tardif was in Iron Age, 169 (19
52) Feb. 14, 136-140, C: 0.15
~ 0.20%, Si: 0.14 to 0.25%, Mn:
0.14 to 0.40%, P: 0.008 to 0.014
%, S: 0.022 to 0.042%, a sample in which the Sn content was changed from 0.011 to 1.00% was subjected to a high temperature bending test, and the occurrence of flaws was observed, and the Sn content was 0. .02
It was reported that 8% did not cause flaws and 0.049% did cause flaws. This seems to be inconsistent with the findings of the above-mentioned inventors, but when examined in detail, Armc was used as a raw material.
o iron was used, but the components of Armco iron are not mentioned in this document,
A. B. Shelmerdine et al. I. S. I.
(1965) p. 40, the amount of Cu in Armco iron is described as 0.12%. L. Gert
It is considered that the Cu content of the sample used such as sman was also contained at about 0.10%, and it cannot be said that this is the finding of the effect of Sn alone inclusion. Therefore, the inventors of the present invention found the effect of Sn alone inclusion. Is not in contradiction with.

FIG. 2 shows C0.09% -Mn 1.90% -T.
i0.20% -B0.0015% Sn of high strength hot rolled steel sheet
It shows the relationship between the content and the tensile strength. According to this, the tensile strength per Sn0.1% is 20 N / mm.
Shown to rise by ² .

The reasons for limiting the above steel components in the present invention are as follows. C: C is desirably as low as possible in molten steel production in order to improve workability, and is the most effective element in order to obtain high tensile strength, and it is contained more than 0.04% and more. There is a need to. However, since the workability, toughness, and weldability decrease with the increase of C, the upper limit is set to 0.
Limited to 20%. Si: Si is useful as a strengthening element, but 0.10%
When added in excess of, a low melting point scale is formed by heating, and the scale is not easily removed by descaling performed prior to hot rolling, and the surface properties are degraded.
The upper limit was limited to 0.10%. Mn: Mn is also an effective element for improving strength, but 2.
If it exceeds 50%, it will be difficult to manufacture molten steel, so the upper limit is 2.5.
It was set to 0%.

Al: Al is required to be 0.004% or more for deoxidation, but if it exceeds 0.10%, the crystal grains become coarse and the strength is deteriorated, so it is limited to 0.10% or less. Since P: P lowers the toughness when it is contained, it is desired that it be as low as possible, and the upper limit was made 0.03%. S: S is an element that deteriorates hot workability and cold workability when contained, so it is desired that it be as low as possible, and the upper limit was made 0.03%.

Sn: Sn is an element which is the key point of the present invention, and is an element effective for improving the strength, and at least 0.01
% Or more, and 0.03% or more is necessary to make the improvement of strength more effective.
The upper limit is 0.4 because it reduces toughness and causes surface defects.
%. Ti: Ti is an element effective for strengthening because it precipitates as C and TiC, and it is necessary to contain at least 0.01% or more,
Furthermore, 0.02% or more must be added to make the improvement of strength more effective. The upper limit is 0.2% because it lowers the toughness. B: B is added to improve the hardenability, but its effect becomes clear, so 0.0005% or more is required to be added. If it is too large, defects will occur during the production of cast slabs, so the upper limit was made 0.0030%.

Ni: Ni may be added for the purpose of improving toughness, but the lower limit of the addition amount necessary for the effect to appear is 0.02% and the upper limit is 0.3% from the viewpoint of economy. Cr: Cr is added for the purpose of improving the scale composition, improving the surface properties after annealing, improving the corrosion resistance, and lowering the transformation point.
The range was up to 0.5%.

Sn concentration in the surface layer of the steel sheet: Sn is concentrated in the surface layer by heating the slab at high temperature in an oxidizing atmosphere,
If the amount of this concentrated Sn is 5% or more when analyzed in a steel sheet, a molten phase is formed and penetrates into the grain boundaries and causes surface defects during hot rolling. Therefore, the upper limit of the Sn concentration in the surface layer portion is set to 5 %.
The term "concentration of steel sheet surface layer" as used herein means the average concentration of the steel sheet surface layer 1 µm.

Next, the reasons for limiting the manufacturing conditions will be described. The chemical composition range according to the above (1) or (2), which contains 0.01 to 0.4% Sn obtained by using tin scrap and / or steel can scrap as the iron scrap of the steelmaking raw material. Ti-
The reason why the B-containing low carbon steel is continuously cast to produce a slab is to obtain an inexpensive Ti-B-containing low carbon steel slab containing 0.01 to 0.4% Sn in the above chemical composition range. Is.

The heating conditions of the slab affect the composition of the scale and the Sn concentration between the base iron and the scale, and as a result, the occurrence of surface defects during hot rolling. If the temperature is higher than 1330 ° C, the subscale layer is thickly formed, and descaling causes insufficient scale removal. Further, even if the temperature is 1330 ° C. or lower, if the heating or heat retention is performed for 6 hours or more, Sn is concentrated and causes a surface flaw. If the heating temperature is lower than 1200 ° C, Ti
C cannot be solutionized and Ti cannot effectively function as a strength improving element. Therefore, heating of the slab before hot rolling is 1200
It was limited to 6 ° C to 1330 ° C for 6 hours or less. The hot rolled steel sheet of the present invention produced under the above conditions has good surface properties,
The strengthening element Sn is 0.01 to 0.4% (preferably 0.
(03 to 0.4%), the strength is improved by ² to 80 N / mm ² as compared with the one not containing Sn.

[0020]

EXAMPLES Table 1 shows the chemical composition of the test steel. A1 to A4
Is 0.02-0.35% Sn in Ti-B containing low carbon steel.
The steel of the present invention containing A5 to A7 contains at least one kind of Ni and Cr in the Ti-B containing steel, and has a Sn content of 0.18.
It is the steel of the present invention containing ~ 0.25%. B1 and B7 are S
The comparative steel does not contain n and has the same chemical composition as A1 and A7 except Sn. C1 is a comparative steel with a high Sn content of 0.41%. Note that Cu was contained as an unavoidable impurity element of these steels in an amount of 0.01 to 0.02%.
Table 2 shows the steel having the chemical composition shown in Table 1 as a slab with a thickness of 250 mm by continuous casting, the heating and hot rolling conditions when hot rolling with a hot strip mill, the surface properties of the steel sheet, and the surface layer 1 μm.
2 shows the Sn concentration and the tensile strength according to JIS No. 5 tensile test piece.

[0021]

[Table 1]

[0022]

[Table 2]

Steel plate symbols 1 to 4 are Ti-types of steel types A1 to A4.
It is a steel sheet containing 0.02 to 0.35% Sn in B-containing low-carbon steel, and is an example of the present invention with good surface properties. Steel plate symbols 5 to 7 are Ti and B containing steels of steel types A5 to A7, and Ni and C.
1 or more types of r are contained, and Sn is 0.10-0.35.
% Steel is a steel sheet hot-rolled, and is an example of the present invention having good surface properties. Steel plate symbols 8 to 9 are comparative steel plates containing no Sn. Steel plate symbol 1 (steel type A1) is the same C,
The strength is 4 N / mm ² higher than the steel plate symbol 8 (steel type B1) which is Mn, Ti-B and does not contain Sn,
Steel plate symbol 7 (steel type A7) is the same C, Mn, Ti-B and S
The strength is 70 N / mm ² higher than that of steel plate code 9 (steel type B7) that does not contain n.

Steel plate symbols 10 to 11 are comparative examples using steel type A4 and heating conditions not within the scope of the present invention and poor surface properties. Steel plate symbol 10 is a comparative example having poor descaling property and poor surface quality due to high heating temperature.
No. 1 is a comparative example in which the heating time is long and the surface layer Sn concentration is high and the surface properties are poor. Steel plate symbol 12 has a Sn content of 0.41%
In a comparative example in which steel having a high temperature is hot-rolled, the surface properties are poor and the surface layer Sn concentration of the steel sheet is as high as 5.5%.

[0025]

The Ti-B-containing high-strength hot-rolled steel sheet according to the present invention contains 0.01 to 0.40% Sn and has a surface Sn concentration of 5% or less. A Ti-B-containing high-strength steel sheet having improved strength of ² to 80 N / mm ² and good surface properties.
The manufacturing method of the B-containing high-strength steel sheet uses molten tin steel containing 0.01 to 0.4% of Sn as a strengthening element at low cost by using inexpensive tin scrap and steel can scrap as iron scrap of the steelmaking raw material. In addition, since the hot rolling conditions are limited so that the Sn concentration in the surface layer is 5% or less, the strength is 2 to 80 N / n with Sn as a strengthening element at a low cost.
It is possible to manufacture Ti-B-added high-strength hot-rolled steel sheets with improved surface properties improved by mm ² , and the economic and social effects are extremely large by promoting the recycling of iron scraps such as tin scraps and steel can scraps. .

[Brief description of drawings]

FIG. 1 0.09% C-1.90% Mn-0.20% T
The figure which shows the influence of the steel plate surface layer Sn density | concentration on the surface property of the i-0.0015% B hot rolled steel plate,

FIG. 2 0.09% C-1.90% Mn-0.20% T
i-0.0015% B S on tensile strength of hot-rolled steel sheet
It is a figure which shows the influence of n.

Claims (3)

[Claims] 1. A chemical component is C: more than 0.04% to 0.20% Si ≦ 0.10% Mn ≦ 2.50% Sol. Al: 0.004-0.10% P <= 0.03% S <= 0.03% Ti: 0.03-0.25% B: 0.0005-0.0030% Sn: 0.01-0. It is a hot-rolled steel sheet containing 400% of the balance Fe and unavoidable impurity elements, and the Sn concentration of the steel sheet surface layer is 5% or less. Hot rolled steel sheet. 2. Sn having good surface properties according to claim 1, characterized in that it contains 0.02 to 0.3% of Ni and / or 0.02 to 0.5% of Cr as chemical components.
And a Ti-B-containing high strength hot rolled steel sheet. 3. A slab produced by continuous casting of molten steel having a chemical composition range according to claim 1 or 2, which is obtained by using tin scrap or steel can scrap as iron scrap of a steelmaking raw material, at 1200 ° C. A method for producing a Sn- and Ti-B-containing high-strength hot-rolled steel sheet having good surface properties, which comprises heat-retaining or heating in a range of 1330 ° C for 6 hours or less and hot-rolling with a hot strip mill.