JPH07118803A - Sn and P-containing low carbon hot-rolled steel sheet having good surface properties and method for producing the same - Google Patents

Abstract

(57) [Summary] [Object] The present invention provides a P-containing low carbon hot-rolled steel sheet having improved surface strength with Sn as a strengthening element and having good surface properties, and an inexpensive method for producing the hot-rolled steel sheet. [Constitution] (1) 0.010 to Sn as a strengthening element
A P-containing low carbon hot rolled steel sheet containing 0.400%,
Moreover, the strength improvement due to Sn is ² to 80 N / mm ² , characterized in that the steel sheet surface layer Sn concentration is 5% or less,
Moreover, a low-carbon hot-rolled steel sheet containing Sn and P that has good surface properties. (2) By using inexpensive tin scrap and / or steel can scrap as iron scrap of a steelmaking raw material, a slab manufactured from molten steel containing Sn in an amount of 0.010 to 0.400% at a temperature of 1330 ° C or lower Heat retention or heating is performed for 6 hours or less within a range, and hot rolling is performed with a hot strip mill to produce the Sn and P-containing low carbon hot rolled steel sheet of (1) above.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a low-carbon hot-rolled steel sheet and a method for producing the same, and particularly to a low-carbon hot-rolled steel sheet having a good surface property and improved strength with Sn as a strengthening element, and an inexpensive production of the hot-rolled steel sheet. It is about the method.

[0002]

2. Description of the Related Art It is well known that C, P, Mn and the like are reinforcing elements for steel, and that Sn is also an effective strengthening element.

[0003]

However, a P-containing low carbon hot rolled steel sheet containing Sn as a strengthening element has not been produced and does not exist. The reason is that Sn is used as a strengthening element for steel.
Has been trapped in the fixed concept that steels containing Sn as a strengthening element would be extremely expensive until recently because extremely expensive Sn ingots could not be contained in the steel unless they were used as raw materials for steelmaking. There is no idea that scrap and / or steel can scrap can be contained in steel by using it as a raw material for steelmaking, and steel containing Sn as a strengthening element can be obtained at a low cost. S that affects deterioration of high temperature workability
This is because the effect of n addition has not been clarified.

Based on the above idea, the present inventors have clarified the effect of Sn addition on the high temperature workability. The present invention has been made based on the above idea and the above clarification, and provides a 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 there.

[0005]

In view of the above situation, the present inventor investigated the effect of the addition of Sn alone on the high temperature workability and found that the P-containing low carbon steel containing Sn as a strengthening element contains Sn. By limiting the amount to 0.4% or less, a new finding was obtained that a hot-rolled steel sheet with good surface properties can be obtained in hot strip hot rolling in which normal heating is performed, and the present invention provides this finding and the above-mentioned idea. It was made based on the following, and the summary is as follows.

(1) The chemical composition is C: 0.01 to 0.04% Si ≦ 0.02% Mn ≦ 1.50% sol. Al: 0.004 to 0.10% N ≦ 0.0050% P: more than 0.030% to 0.100% S ≦ 0.03% Sn: 0.01 to 0.400% balance Fe and inevitable impurities A low-carbon hot-rolled steel sheet containing Sn and P, which is an elemental hot-rolled steel sheet and has a good surface property, wherein the Sn concentration in the surface layer of the steel sheet 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).
A low carbon hot rolled steel sheet containing n and P. (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 a Sn- and P-containing low carbon hot-rolled steel sheet having a good surface property, 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 scraps and / or tin scraps are used as the iron scraps of the steelmaking raw material, the present inventors have found that it is possible to use only Sn as the playing card element in the steel. Various experiments were carried out on the effect of Sn on the surface properties of steel sheets using 04-0.10% low carbon steel to limit the content of Sn to 105
It was newly discovered that hot-rolled steel sheets with good surface properties can be obtained in the heating range of 0 ° C to 1330 ° C.

The inventors of the present invention have variously changed the Sn content from steel can scraps and / or tin scraps to 0.01%.
~ 0.04% C-0.23 to 0.45% Mn-0.07
% P steel is melted to produce a 250 mm thick continuous cast slab, which is charged into a heating furnace, heated under various heating conditions, hot-rolled by a hot strip mill, and inspected for surface properties of steel sheets. By doing so, 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 normal slab heating.

These results show that when Sn is added up to 0.4%, it is concentrated in iron immediately below the heating scale in an oxidizing atmosphere, but because Sn diffuses rapidly in Fe at high temperatures, this heating condition range It is considered that this is because the liquid is not concentrated to the concentration until the liquid is created. Limit Sn produced by this liquid
The 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 high temperature bending test was performed on a sample in which the Sn content was changed from 0.011 to 1.00%, 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 0.02% C-0.23% Mn-
It shows the relationship between the Sn content and the tensile strength of 0.07% P hot rolled steel sheet. According to this, it is shown that the tensile strength increases by 20 N / mm ² per 0.1% of 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: C is desirably as low as possible in the production of molten steel in order to improve workability, and is the most effective element for increasing tensile strength. The lower limit is 0.01% to satisfy the tensile strength, and the upper limit is 0.04 to secure workability.
Limited to%. Si: If Si is contained, the chemical conversion treatability deteriorates, so the upper limit was made 0.02%. Mn: Mn is also an effective element for improving strength, but 1.
If it exceeds 50%, it will be difficult to manufacture molten steel, so the upper limit is 1.5.
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. 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 the strength, and it is necessary to exceed 0.030% at least, and the lower limit is limited to 0.030%. However, 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%.

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 lowers toughness and causes surface defects.
%. 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.
%, The upper limit was set to 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 crystal grain boundaries and cracks during hot rolling to cause 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 0.01 to 0.4% Sn obtained by using tin scrap and / or steel can scrap as the iron scrap of the steelmaking raw material. Continuous casting of low carbon steel to produce a slab is inexpensive 0.01
This is to obtain a low carbon steel slab containing 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. 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 contains 0.01 to 0.4% (preferably 0.03 to 0.4%) of the reinforcing element Sn. because there is,
Its strength is 2 to 80 N / compared to that which does not contain Sn.
mm ² has been improved.

[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% of Sn in a C-Mn-P steel, A6-A8 are Ni in a C-Mn-P steel,
The steel of the present invention contains at least one kind of Cr and further contains 0.28% of Sn. B1 and B6 are comparative steels that do not contain 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, C as an unavoidable impurity element of these steels
The content of u was 0.01 to 0.02%. 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 plate symbols 1 to 5 are CM of steel types A1 to A5.
It is a steel plate containing 0.02 to 0.35% Sn in n-P steel, and is an example of the present invention with good surface properties. Steel plate symbol 6-
8 is Ni-Cr 1 to C-Mn-P steel of steel types A6 to A8
It is a steel sheet obtained by hot-rolling steel containing at least one kind 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 of 7 compared to steel plate symbol 5 (steel type A4) containing the same C, Mn and Sn.
0 N / mm ² lower, steel plate symbol 10 (steel type B
6) is a steel plate symbol 6 containing the same C, Mn and Sn
The strength is 56 N / mm ² lower than (Steel type A6).

The steel sheet symbol 11 is a comparative example in which 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 sheet symbols 12 to 19 are examples of the present invention having good surface properties, and steel sheet 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.

[0023]

The low carbon hot rolled steel sheet according to the present invention is 0.01
.About.0.4% Sn and the surface layer Sn concentration is 5% or less.
A low carbon hot rolled steel sheet with improved surface properties improved by 80 N / mm ^{2. The} method for producing a low carbon hot rolled steel sheet according to the present invention is to use inexpensive tin scrap and / or steel can scrap as iron scrap as a steelmaking raw material. In this way, 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 the hot rolling conditions are restricted so that the Sn concentration in the surface layer is 5% or less. Therefore, it is possible to inexpensively produce a hot-rolled steel sheet having a good surface property in which the strength is improved by ² to 80 N / mm ² by using Sn as a reinforcing element, and iron scrap recycling such as tin scrap and steel can scrap The economic and social effects of promotion are extremely large.

[Brief description of drawings]

FIG. 1 0.02% C-0.23% Mn-0.07% P
The figure which shows the influence of the steel plate surface layer Sn density | concentration on the surface property of a hot rolled steel plate,

FIG. 2 0.02% C-0.23% Mn-0.07% P
It is a figure which shows the influence of Sn which acts on the tensile strength of a hot rolled steel sheet.

Claims (3)

[Claims] 1. A chemical component is C: 0.01 to 0.04% Si ≦ 0.02% Mn ≦ 1.50% Sol. Al: 0.004 to 0.10% N ≦ 0.0050% P: more than 0.030% to 0.100% S ≦ 0.03% Sn: 0.01 to 0.400% balance Fe and inevitable impurities A low-carbon hot-rolled steel sheet containing Sn and P, which is an elemental hot-rolled steel sheet and has a good surface property, wherein the Sn concentration in the surface layer of the steel sheet is 5% or less. 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 P-containing 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 P-containing low-carbon hot-rolled steel sheet having good surface properties, 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.