JPH07118804A - Sn-containing dead-soft hot rolled steel sheet good in surface property and its production - Google Patents

Abstract

PURPOSE:To produce a hot rolled steel sheet improved in strength and good in surface properties by regulating the Sn concn. in the surface layer of a hot rolled steel sheet having specified chemical components to specified %. CONSTITUTION:The chemical components of this hot rolled steel sheet is constituted of <=0.0100% C, <=0.10% Si, <=1.00% Mn, 0.004 to 0.10% Sol.Al, <=0.0050% N, <=0.100% P, <=0.03% S, 0.01 to 0.400% Sn, and the balance Fe with inevitable impurity elements. Then, the Sn concn. in the surface layer of the steel sheet is regulated to <=5%. As the chemical components, 0.0005 to 0.030% B is furthermore incorporated. Moreover, as the chemical components, 0.02 to 0.3% Ni and/or 0.02 to 0.5% Cr is incorporated. Thus, the recycle of iron scraps such as tin plate scraps and steel can scraps can be promoted.

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 and a hot-rolled steel sheet having excellent surface properties with Sn as a strengthening element to improve strength. The present invention relates to an inexpensive manufacturing method.

[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 Sn as a strengthening element has not been manufactured,
not exist. The reason is that Sn is a strengthening element for steel.
Conventionally, extremely expensive Sn ingot cannot be contained in steel unless it is used as a steelmaking raw material.
The steel that has been used as a strengthening element is caught by the fixed concept that it is extremely expensive, and it can be contained in steel by using inexpensive tin scrap and / or steel can scrap as a steelmaking raw material. This is because there is no idea that it is possible to obtain the steel described above, and therefore, the effect of Sn addition on the deterioration of the high temperature workability that causes the surface defects 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 has been made based on the above idea and the above clarification, and provides 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 an inexpensive manufacturing method of the hot rolled steel sheet. Is.

[0005]

In view of the above situation, the present inventor investigated the effect of addition of Sn alone on high-temperature workability, and found that the content of Sn in the ultra-low carbon steel containing Sn as a strengthening element. By limiting the value to 0.4% or less,
Obtained new knowledge that a hot rolled steel sheet having a good surface property can be obtained in hot strip hot rolling in which normal heating is performed, and the present invention has been made based on this knowledge and the above-mentioned idea, and the gist thereof is as follows. Is. (1) Chemical composition is C <0.0100%, Si ≦ 0.10
%, Mn ≦ 1.00%, Sol. Al: 0.004 ~
0.10%, N ≦ 0.0050%, P ≦ 0.100%,
S ≦ 0.03%, Sn: 0.01 to 0.400%, hot-rolled steel sheet with balance Fe and unavoidable impurity elements, and characterized in that the Sn concentration in the steel sheet surface layer is 5% or less. Ultra low carbon hot rolled steel sheet containing Sn with good surface properties.

(2) As a chemical component, 0.0005-
It is characterized by containing 0.0030% B (1).
An ultralow carbon hot-rolled steel sheet containing Sn having good surface properties as described. (3) As a chemical component, 0.02 to 0.3% of Ni and / or 0.02 to 0.5% of Cr are contained, and the surface property is good as described in (1) or (2). Na S
Ultra low carbon hot rolled steel sheet containing n. (4) A slab produced by continuously casting molten steel having a chemical composition range as described in (1), (2) or (3), which is obtained by using tin scrap and / or steel can scrap as iron scrap of a steelmaking raw material. It is a method for producing a Sn-containing ultra low carbon hot rolled steel sheet having a good surface property, which is characterized by heat-retaining or heating at 1330 ° C. or less 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 have considered that it is possible to use only Sn as the playing card element in the steel, and thus the ultra-low By conducting various experiments on the effect of Sn on the surface properties of steel sheets using carbon steel and limiting the Sn content, the usual 1050 ° C to 1330 ° C
It was newly discovered that hot-rolled steel sheets with good surface properties can be obtained in the heating range of.

The inventors of the present invention melt-manufactured extra-low carbon steel with various Sn contents varying from steel can scraps and / or tin scraps as raw materials to manufacture 250 mm thick continuously cast slabs,
After charging into the heating furnace, heating was performed under various heating conditions, hot rolling was performed 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 Sn of 1 μm surface layer of steel sheet that affects the surface properties of steel sheet.
Shows the effect of the amount of. 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. In addition, the Sn concentration of steel is 0.4
It was newly found that the Sn concentration in the surface layer of the steel sheet is 5% or less in the range of normal slab heating if the content is 5% or less.

These results show that when adding up to 0.4% Sn, 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, the 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.
It shows the relationship between the Sn content and the tensile strength of the hot rolled steel sheet. This shows that the tensile strength increases by 20 N / mm ² per 0.1% Sn. here,
Although an example of a hot rolled steel sheet is shown, it has been confirmed that the same effect can be obtained with a cold rolled / annealed sheet using this hot rolled steel sheet as a starting material.

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%.

Al: Al needs 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 the content is limited to 0.10% or less. N: N is deteriorated by aging when it is contained, so it is desired to be as low as possible, and the upper limit was made 0.0050%. P: P is also an effective element for improving strength, but 0.10
If it exceeds%, the secondary workability deteriorates, so the upper limit was made 0.10% or less.

S: S is an element that reduces 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%. 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 exceeds 0.0030%, surface defects of the slab will occur. Therefore, the lower limit is made 0.0005% and the upper limit is made 0.0030%. 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 0.01% or more, and further 0.03% for improving the strength more effectively. The above additions are necessary. 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 property after annealing, improving the corrosion resistance, and lowering the transformation point, but the respective effects become clear 0.02
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 item (1), (2) or (3) containing 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. The continuous casting of ultra low carbon steel in the chemical composition range described to produce a slab,
This is to obtain an inexpensive ultra-low carbon steel slab containing 0.01 to 0.4% Sn and having 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. 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 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 the strengthening element Sn is 0.01
.About.0.4% (preferably 0.03 to 0.4%), the strength thereof is improved by ² to 80 N / mm.sup.2 as compared with the one not containing Sn.

[0018]

EXAMPLES Table 1 shows the chemical composition of the test steel. A1 to A5
Is a steel of the present invention containing 0.02 to 0.35% Sn in an ultra low carbon steel, and A6 to A8 are Ni and Cr in the ultra low carbon steel.
It is the steel of the present invention containing at least one kind and further containing 0.28% of Sn. A9 is a steel of the present invention which contains Ni, Cr and B in an ultra low carbon steel and further contains 0.28% of Sn. B1, B6 are comparative steels containing no Sn, A1,
It has the same C, Mn, and Cr components as 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 is 0.01 to 0.
02% was included. Table 2 shows a 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, Sn concentration of 1 μm surface layer and JIS5. The tensile strength of the No. 10 tensile test piece is shown.

[0019]

[Table 1]

[0020]

[Table 2]

Steel sheet symbols 1 to 5 are steel sheets containing 0.02 to 0.35% Sn in an ultra-low carbon steel of steel types A1 to A5, which are examples of the present invention having good surface properties. Steel plate symbols 6-8
Is a steel sheet obtained by hot rolling a steel containing one or more kinds of Ni and Cr in an ultra low carbon steel of steel types A6 to A8 and further containing 0.28% of Sn, and is an example of the present invention having a good surface property. The steel plate symbol 9 is a steel plate obtained by hot rolling a steel containing Ni, Cr and B and further containing 0.28% Sn in an ultra-low carbon steel of steel type A9, which is an example of the present invention having good surface properties. Steel plate symbols 10 and 11 are comparative steel plates containing no Sn. Steel plate symbol 10 (steel type B1) has a strength of 70 N / compared with steel plate symbol 5 (steel type A4) containing the same C, Mn and Sn.
mm ^{2 is} lower, and the steel plate symbol 11 (steel type B6) contains the same C, Mn and Sn, and the steel plate symbol 6 (steel type A).
The strength is 56 N / mm ² lower than that of 6).

The steel plate symbol 12 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 13
Nos. 22 to 22 are steel sheets hot-rolled using steel type A4 under various heating conditions, steel plate symbols 13 to 20 are examples of the present invention having good surface properties, and steel plate symbols 21 and 22 have heating conditions within the present invention range. This is a comparative example in which the surface properties that are not inside are poor. The steel plate symbol 21 is a comparative example in which the descaling property is poor and the surface property is poor because the heating temperature is high, and the steel plate symbol 22 is a comparative example in which the heating property is long and the surface property is poor due to the thickening of the surface layer Sn.

[0023]

The ultra low carbon hot rolled steel sheet according to the present invention is 0.0
Since it contains Sn of 1 to 0.4% and the surface layer Sn concentration is 5% or less, the strength is 2 with Sn as a strengthening element.
-80 N / mm ^{2 It} is an ultra low carbon hot rolled steel sheet with improved surface properties, and the method for producing an ultra low carbon hot rolled steel sheet according to the present invention uses inexpensive tin scrap and / or steel can scrap as iron scrap as a steelmaking raw material. By using, as a starting material, a slab produced from molten steel containing 0.01 to 0.4% Sn as a strengthening element at a low cost, and the hot rolling conditions, especially the Sn concentration of the surface layer, becomes 5% or less. Therefore, 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 is a diagram showing the effect of the steel sheet surface layer Sn concentration on the surface properties of 0.0020% C-0.15% Mn hot rolled steel sheet,

FIG. 2 is a diagram showing the effect of Sn on the tensile strength of a 0.0020% C-0.15% Mn hot rolled steel sheet.

Claims (4)

[Claims] 1. The chemical composition is C <0.0100% Si ≦ 0.10% Mn ≦ 1.00% Sol. Al: 0.004 to 0.10% N ≤ 0.0050% P ≤ 0.100% S ≤ 0.03% Sn: 0.01 to 0.400% In the hot-rolled steel sheet with the balance Fe and unavoidable impurity elements In addition, a Sn-containing ultra-low carbon hot-rolled steel sheet having good surface properties, characterized in that the Sn concentration in the surface layer of the steel sheet is 5% or less. 2. As a chemical component, 0.0005 to 0.0
The Sn-containing ultra-low carbon hot-rolled steel sheet according to claim 1, which contains 030% of B. 3. A good surface property according to claim 1 or 2, characterized by containing 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. 4. A tin scrap and / or a steel can scrap as an iron scrap of a steelmaking raw material.
A slab produced by continuously casting molten steel in the described chemical composition range is heat-treated or heated in a range of 1330 ° C. or less for 6 hours or less, and hot-rolled by a hot strip mill, which has a good surface property. A method for manufacturing an Sn-containing ultra low carbon hot rolled steel sheet.