JPH07118799A - Hot rolled plate of sn-, ti-, and b-containing dead-soft steel excellent in surface characteristic and its production - Google Patents

Abstract

PURPOSE:To obtain a hot rolled plate of Ti-and B-containing dead-soft 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, <0.010% C, <=0.10% Si, <=0.10% Si, <=1.0% Mn, <=0.10% P, <=0.03% S, 0.01-0.40% Sn, 0.004-0.10% sol. Al, <=0.0050% N, 0.0005-0.0030% B, 0.005-0.10% Ti, 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 dead-soft 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 characteristic, can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultra-low carbon hot-rolled steel sheet and a method for producing the same, and more particularly to an ultra-low-carbon hot-rolled steel sheet having a good surface property in which strength is improved by using Sn as a strengthening element and the hot-rolled steel sheet is inexpensive The present invention relates to various manufacturing methods.

[0002]

2. Description of the Related Art It is well known that P, Mn and the like are strengthening elements for steel plates using ultra low carbon steel. Moreover, it is well known that Sn is also an effective strengthening element.

[0003]

However, a very low carbon hot-rolled steel sheet containing Ti and B as a strengthening element of Sn has not been produced and does not exist. The reason is that as a strengthening element of steel, Sn cannot be contained in steel until recently when an extremely expensive Sn ingot is used as a steelmaking raw material, so that a steel containing Sn as a strengthening element becomes extremely expensive. The idea is that it is possible to obtain a steel containing Sn as a strengthening element at a low cost because it is contained in a fixed concept and can be contained in steel by using inexpensive tin scrap and / or steel can scrap as a steelmaking raw material. This is because the effect of addition of Sn on the deterioration of the high temperature workability that causes a flaw on the surface of the steel sheet has not been clarified.

The present inventors have clarified the effect of Sn addition on the high temperature workability based on the above idea. The present invention was made on the basis of the above idea and the above-mentioned clarification, and Ti and B having excellent surface properties and improved strength by using Sn as a strengthening element.
An ultralow carbon hot-rolled steel sheet containing and an inexpensive manufacturing method of 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 contained Sn as a strengthening element, and contained Ti and B.
A new finding that a hot rolled steel sheet with good surface properties can be obtained in hot strip hot rolling in which normal heating is performed by limiting the Sn content of the ultra low carbon steel containing Cr to 0.4% or less The present invention has been made based on this knowledge and the above-mentioned idea, and the summary thereof is as follows. (1) Chemical composition is C <0.0100%, Si ≦
0.10%, Mn ≦ 1.00%, P ≦ 0.100%, S
≦ 0.03%, Sn: 0.01 to 0.400%, So
l. Al: 0.004 to 0.10%, N ≦ 0.0050
%, B: 0.0005 to 0.0030%, Ti: 0.0
It is a hot-rolled steel sheet of 05 to 0.10%, the balance being Fe and unavoidable impurity elements, and the Sn concentration of the steel sheet surface layer is 5%.
Sn, T with good surface properties characterized by
Ultra low carbon hot rolled steel sheet containing i and B.

(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).
Ultra low carbon hot rolled steel sheet containing n, Ti and B. (3) A slab produced by continuously casting molten steel having a chemical composition range as described in (1) or (2) obtained by using tin scrap and / or steel can scrap as iron scrap of a steelmaking raw material is 1330 ° C.
A method for producing an ultra low carbon hot-rolled steel sheet containing Sn, Ti and B having good surface properties, which comprises heat-retaining or heating in the following range 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 scraps and / or tin scraps are used as the iron scraps of the steelmaking raw material, the present inventors, based on the idea that it is possible to use only Sn as the playing card element in the steel, Ti and Various experiments were conducted on the effect of Sn on the surface properties of steel sheets using an ultra-low carbon steel containing B, and by controlling the Sn content to a normal 1
It was newly discovered that hot-rolled steel sheets with good surface properties can be obtained in the heating range of 050 ° C to 1330 ° C.

The inventors of the present invention have manufactured ultra-low carbon steel containing Ti and B with various Sn contents varying from steel can scrap and / or tin scrap as raw materials to produce a continuously cast slab having a thickness of 250 mm. Then, after charging into the heating furnace, it is heated under various heating conditions and hot-rolled with a hot strip mill.
By inspecting the surface properties of the steel sheet, the range of good surface properties was clarified. FIG. 1 shows the effect of the amount of Sn in the surface layer portion of the steel sheet of 1 μm on the surface properties of the steel sheet. According to this, it was newly found that when the Sn concentration in the steel sheet surface layer is 5% or less, surface flaws do not occur and the steel sheet surface property is 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 normal slab heating. 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 since Sn diffuses rapidly in Fe at high temperature, liquid is not produced in this heating condition range. It is considered that this is because the concentration is not increased to the concentration until the is created. Limit Sn concentration generated by this liquid (concentration of 1 μm surface layer portion analyzed by steel plate)
Was 5%.

In addition, 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 0.0020% C-0.15% Mn.
-0.04% Ti-0.0007% B It shows the relationship between the Sn content and the tensile strength of the hot-rolled steel sheet. According to this, the tensile strength is 20 N / mm ² per 0.1% Sn.
Shown to rise. Here, an example of a hot-rolled steel sheet is shown, but it has been confirmed that a cold-rolled / annealed sheet using the hot-rolled steel sheet as a starting material has the same effect.

The reasons for limiting the above steel components in the present invention are as follows. C: It is desirable that C be as low as possible in molten steel production in order to improve workability, and the upper limit was limited to 0.0100%. Si: If Si is contained, the descaling property of the primary scale generated in the heating furnace is deteriorated, so the upper limit was set to 0.10%. Mn: Mn is also an effective element for improving strength, but 1.
If it exceeds 00%, it will be difficult to manufacture molten steel, so the upper limit is 1.0.
It was set to 0%.

P: P is also an element effective for improving the strength, but if it exceeds 0.10%, the secondary workability deteriorates. Therefore, the upper limit was limited to 0.10% or less. 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%. Al: Al needs to be 0.004% or more for deoxidation,
If it exceeds 0.10%, the crystal grains become coarse and the strength is deteriorated, so the content is limited to 0.10% or less.

N: N is deteriorated by aging when it is contained.
It is desired to be as low as possible, and the upper limit was made 0.0050%. B: B is an element effective in improving the aging property in combination with N. In order to improve the aging property, it is necessary to contain at least 0.0005% or more, and if it is contained 0.0030% or more, surface defects of the slab occur. Therefore, the lower limit is 0.0
005% and the upper limit was 0.0030%. Ti: Ti is an element effective in suppressing the action of the interstitial solid solution element by forming a precipitate such as TiC or TiN in combination with C and N in steel. At least 0.005% or more is necessary for the effect to appear, and it is uneconomical if it is too much. Therefore, the lower limit is limited to 0.005% and the upper limit is limited to 0.10%.

Sn: Sn is an element which is a key point of the present invention, and is an element effective for improving 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 surface defects are generated. 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%,
From the viewpoint of economy, the upper limit was set to 0.3%. 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 steel sheet surface layer: Sn is concentrated in the steel surface layer portion by high temperature heating of the slab in an oxidizing atmosphere, and when the amount of this concentrated Sn is 5% or more when analyzed by the steel sheet, the molten phase is The upper limit of the Sn concentration in the surface layer portion is set to 5%, because it forms and penetrates into the grain boundaries and cracks during hot rolling, causing surface defects.
The steel plate surface layer concentration here means the average concentration of the steel plate surface layer of 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 0.031 to 0.4% Sn obtained by using tin scrap and / or steel can scrap as iron scrap of a steelmaking raw material. It is inexpensive to continuously cast ultra-low carbon steel to produce a slab.
This is to obtain an ultra low carbon steel slab containing 01 to 0.4% Sn in the above chemical composition range. The conditions for heating 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. 1330
When the temperature is higher than ℃, the subscale layer is thickly formed and the scale removal becomes insufficient due to the descaling. Also, 1330
If heating or heat retention is carried out for 6 hours or more even at a temperature of not higher than 0 ° C, Sn will concentrate and cause surface defects. Therefore, the heating or heat retention of the slab before hot rolling is limited to 1330 ° C. or less and 6 hours or less. The hot-rolled steel sheet of the present invention produced under the above conditions has good surface properties and contains 0.01 to 0.4% Sn as a strengthening element.
(Preferably 0.03 to 0.4%), the strength thereof is 2 to 8 as compared with the one not containing Sn.
It has improved by 0 N / mm ² .

[0017]

EXAMPLES Table 1 shows the chemical composition of the test steel. A1 to A5
Contains Sn in an amount of 0.02 to 0.
The present invention steels containing 35% of A6 to A8 are the present invention steels containing Ti and B-containing ultra-low carbon steel containing one or more kinds of Ni and Cr and further containing 0.28% of Sn. B1, B
Reference numeral 6 is a comparative steel containing no Sn, which has the same C, Mn, and Cr components as A1 and A6. C1 has a Sn content of 0.
It is a comparative steel as high as 41%. In addition, Cu as an unavoidable impurity element of these steels was contained by 0.01 to 0.02%. Table 2 shows steel having the chemical composition shown in Table 1 as a slab having a thickness of 250 mm by continuous casting, heating and hot rolling conditions when hot rolling with a hot strip mill, surface properties of steel sheet,
The Sn concentration of the surface layer of 1 μm and the tensile strength according to JIS No. 5 tensile test piece are shown.

[0018]

[Table 1]

[0019]

[Table 2]

The steel sheet symbols 1 to 5 are steel sheets containing Ti and B containing ultra-low carbon steels of steel types A1 to A5 containing 0.02 to 0.35% Sn, and are examples of the present invention having good surface properties.
Steel sheet symbols 6 to 8 are steel sheets obtained by hot-rolling a steel containing Ti and B-containing ultra-low carbon steel of steel types A6 to A8, containing at least one of Ni and Cr, and further containing 0.28% of Sn. Is a good example of the present invention. Steel plate symbols 9 and 10 are comparative steel plates containing no Sn. Steel plate symbol 9 (steel type B1)
Is 70 N / mm ² lower in strength than the steel plate symbol 5 (steel type A4) containing the same C, Mn and Sn.
Steel plate symbol 10 (steel type B6) has a strength of 56N as compared with steel plate symbol 6 (steel type A6) containing the same C, Mn and Sn.
/ Mm ² lower.

The steel sheet symbol 11 is a comparative example in which a steel having a high Sn content of 0.41% is hot-rolled, and the surface properties are poor and the surface layer Sn concentration of the steel sheet is as high as 5.5%. Steel plate symbol 12
Nos. 21 to 21 are steel sheets hot-rolled using steel type A4 under various heating conditions, steel plate symbols 12 to 19 are examples of the present invention having good surface properties, and steel plate symbols 22 and 21 have heating conditions within the scope of the present invention. This is a comparative example in which the surface properties that are not inside are poor. Steel plate symbol 20 is a comparative example in which the descaling property is poor and the surface property is poor because the heating temperature is high, and steel plate symbol 21 is a comparative example in which the surface property is poor because the heating time is long and the surface layer Sn is concentrated.

[0022]

The Ti and B-containing ultra low carbon hot-rolled steel sheet according to the present invention contains 0.01 to 0.4% Sn and has a surface Sn concentration of 5% or less. Is a Ti and B-containing ultra-low carbon hot-rolled steel sheet having an improved strength of ² to 80 N / mm ² and good surface properties. The method for producing the Ti- and B-containing ultra-low carbon hot-rolled steel sheet according to the present invention is an inexpensive tin scrap. And / or by using steel can scraps as iron scraps of steelmaking raw material, a slab manufactured from molten steel containing 0.01 to 0.4% Sn as a strengthening element at a low cost is used as a starting material and hot rolling conditions are set. , Especially surface S
Since the n concentration is limited to 5% or less, the strength is 2 to 80 N / m with Sn as a strengthening element at a low cost.
It is possible to manufacture a hot-rolled steel sheet having an improved m ^{2 and} good surface quality, and the economic and social effects of promoting the recycling of iron scraps such as tin scrap and steel can scrap are extremely large.

[Brief description of drawings]

FIG. 1 0.0020% C-0.15% Mn-0.04
% Ti-0.0007% B A diagram showing the effect of the steel sheet surface layer Sn concentration on the surface properties of the hot-rolled steel sheet,

FIG. 2 0.0020% C-0.15% Mn-0.04
It is a figure which shows the influence of Sn which acts on the tensile strength of the% Ti-0.0007% B hot rolled steel sheet.

Claims (3)

[Claims] 1. The chemical components are C <0.0100% Si ≦ 0.10% Mn ≦ 1.00% P ≦ 0.100% S ≦ 0.03% Sn: 0.01 to 0.400% Sol. ． Al: 0.004 to 0.10% N: ≤ 0.0050% B: 0.0005 to 0.0030% Ti: 0.005 to 0.10% A hot rolled steel sheet containing the balance Fe and unavoidable impurity elements. In addition, an Sn, Ti, and B-containing ultra-low carbon hot-rolled steel sheet having a good surface property, characterized in that the Sn concentration in the surface layer of the steel sheet is 5% or less. 2. A good surface property according to claim 1, which contains 0.02 to 0.3% of Ni and / or 0.02 to 0.5% of Cr as chemical components. Very low carbon hot rolled steel sheet containing Sn, Ti and B. 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 and / or steel can scrap as iron scrap of a steelmaking raw material. A method for producing an ultra-low carbon hot-rolled steel sheet containing Sn, Ti and B having good surface properties, which comprises heat-retaining or heating at 1330 ° C. or less for 6 hours or less and hot rolling with a hot strip mill.