JPH07118798A - Hot rolled plate of sn-and ti-containing dead-soft steel excelent in surface characteristic and its production - Google Patents

Abstract

PURPOSE:To obtain a hot rolled plate of Ti-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, >=1.0% Mn, <=0.10% P, <=0.03% S, >0.03-0.40% Sn, 0.004-0.10% sol.Al, <0.0050% N, 0.0050-0.10% 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 hor rolled plate of Ti-containing dead-soft steel, for which Sn in tin plate scrap and steel can as iron scrap for steelmkaing 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, an ultra-low carbon hot rolled steel sheet containing Ti as a strengthening element and containing Ti 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.
Based on the above idea, the inventors of the present invention have an effect on S
The effect of n addition was clarified. The present invention has been made based on the above idea and the above clarification, and an ultra low carbon hot rolled steel sheet containing Ti having a good surface property with Sn as a strengthening element and improved strength, and an inexpensive method for producing the hot rolled steel sheet. Is provided.

[0004]

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 the ultra low carbon steel containing Sn and Ti as strengthening elements By limiting the content to 0.4% or less, a new finding that a hot-rolled steel sheet having a good surface property can be obtained in hot strip hot rolling in which ordinary heating is performed is obtained. It was made based on the idea, and the summary is as follows. (1) Chemical composition is C <0.0100%, Si ≦ 0.10
%, Mn ≦ 1.00%, P ≦ 0.100%, S ≦ 0.0
3%, Sn: more than 0.03% to 0.400%, Sol. A
1: 0.004 to 0.10%, N <0.0050%, T
i: 0.005 to 0.10%, the balance being Fe and unavoidable impurity elements, which is a hot-rolled steel sheet, and moreover, Sn on the surface layer of the steel sheet.
A very low carbon hot rolled steel sheet containing Sn and Ti with good surface properties, characterized in that the concentration is 5% or less.

(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 and Ti. (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 ultralow carbon hot-rolled steel sheet containing Sn and Ti having good surface properties, which comprises heat-retaining or heating for 6 hours or less in the following range 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 is used as the iron scrap of the steelmaking raw material, the present inventors have considered that it is possible to use only Sn as the playing card element in the steel, and the Ti-containing material is contained. Various experiments were conducted on the effect of Sn on the surface properties of steel sheets using ultra-low carbon steel.
By limiting the content of
It was newly found that a hot-rolled steel sheet having good surface properties can be obtained in the heating range of 330 ° C.

The inventors of the present invention melt-produced Ti-containing ultra-low carbon steel having various Sn contents from steel can scraps and / or tin scraps as raw materials to manufacture 250 mm thick continuously cast slabs, After being charged into the steel, it was heated under various heating conditions, hot-rolled with a hot strip mill, and the surface properties of the steel sheet were inspected to clarify the range of good surface properties. Fig. 1 shows the surface layer of steel sheet 1μ which affects the surface properties of steel sheet.
The influence of the amount of Sn of m is shown. According to this, the steel plate surface layer S
It was newly found that when the n concentration is 5% or less, surface defects 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 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. The limit Sn concentration (concentration of the surface layer portion 1 μm analyzed by the steel sheet) produced by this liquid 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 0.0020% C-0.15% Mn.
Fig. 3 shows the relationship between the Sn content and the tensile strength of a -0.04% Ti hot rolled steel sheet. This shows that the tensile strength increases by 20 N / mm ² per 0.1% Sn. 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 is deteriorated, so 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: Since N deteriorates with age when N is contained,
It is desired to be as low as possible, and the upper limit was made 0.0050%. 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 it is necessary to contain at least 0.030% or more. 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%, 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 of the steel due to 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 according to the above (1) or (2), which contains more than 0.030% and 0.4% or less of Sn obtained by using tin scrap and / or steel can scrap as iron scrap of a steelmaking raw material. The reason for producing a slab by continuously casting ultra low carbon steel in the range is to obtain an inexpensive ultra low carbon steel slab containing the above Sn content of more than 0.030% and 0.4% or less. Is. 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. 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. Therefore,
The heating or heat retention of the slab before hot rolling was 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 has a Sn content of 0.0
Since it contains more than 30% and 0.4% or less, its strength is 6 to 80 N / mm ^{2 as} compared with that containing no Sn.
Has improved.

[0016]

EXAMPLES Table 1 shows the chemical composition of the test steel. A1 to A5
Is 0.031 to 0.35% of Sn in Ti-containing ultra low carbon steel
The present invention steels containing A6 to A8 are Ti-containing ultra-low carbon steel containing one or more kinds of Ni and Cr and further containing 0.2% of Sn.
It is the steel of the present invention containing 8%. B1 and B6 are comparative steels containing no Sn and have the same C, Mn, and Cr components as A1 and A6. C1 is a comparative steel with a high Sn content of 0.41%. In addition, Cu as an unavoidable impurity element of these steels was contained by 0.01 to 0.02%. Table 2 shows a steel with the chemical composition shown in Table 1 with a thickness of 250 by continuous casting.
mm slab, heating and hot rolling conditions when hot rolling with a hot strip mill, surface properties of steel sheet, Sn of 1 μm surface layer
The concentration and the tensile strength according to JIS No. 5 tensile test piece are shown.

[0017]

[Table 1]

[0018]

[Table 2]

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

The steel plate 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 plate 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.

[0021]

The Ti-containing ultra-low carbon hot rolled steel sheet according to the present invention contains more than 0.030% and 0.4% or less of Sn, and the surface layer Sn concentration is 5% or less. Is a Ti-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-containing ultra-low carbon hot-rolled steel sheet according to the present invention is inexpensive tin scrap and / or steel can. By using scraps as iron scraps of steelmaking raw material, Sn can be inexpensively added as a strengthening element to 0.030.
%, The slab produced from the molten steel containing 0.4% or less of the starting material is used as the starting material, and the hot rolling conditions are limited so that the Sn concentration of the surface layer is 5% or less. It is possible to manufacture a hot rolled steel sheet having a good surface property with the strength improved by 6 to 80 N / mm ² as a strengthening element, and the economical and social effect of promoting the recycling of iron scrap such as tin scrap and steel can scrap. Is extremely large.

[Brief description of drawings]

FIG. 1 0.0020% C-0.15% Mn-0.04
The figure which shows the influence of the steel plate surface layer Sn density | concentration on the surface property of the% Ti hot rolled steel plate,

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 hot rolled steel sheet.

Claims (3)

[Claims] 1. The chemical composition is C <0.0100% Si ≦ 0.10% Mn ≦ 1.00% P ≦ 0.100% S ≦ 0.03% Sn: more than 0.03% to 0.400. % Sol. Al: 0.004 to 0.10% N <0.0050% Ti: 0.005 to 0.10% A hot-rolled steel sheet with the balance Fe and unavoidable impurity elements, and a Sn concentration of the steel sheet surface layer of 5%. A very low carbon hot rolled steel sheet containing Sn and Ti having good surface properties, characterized in that 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 Ti-containing ultra low carbon 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 obtained by using tin scrap and / or steel can scrap as iron scrap of a steelmaking raw material.
A method for producing a Sn- and Ti-containing ultra low carbon hot-rolled steel sheet having a good surface property, which comprises heat-retaining or heating in a range of 30 ° C. or lower for 6 hours or less, and hot rolling with a hot strip mill.