KR101435035B1 - Manufacturing for hot rolled steel sheets with decreased surface crack by mini mill and hot rolled steel sheets fabricated using the same - Google Patents

Abstract

The present invention relates to a method for producing a hot-rolled steel sheet by a min-mill process and a hot-rolled steel sheet produced from the steel sheet. More particularly, the present invention relates to a steel sheet having a carbon equivalent (Ceq) of 0.07% Continuously casting molten steel having a manganese content of 50 or less and a sulfur content of 0.008 wt% or less to produce a slab; and rolling the slab to produce a steel sheet, and a method of manufacturing a hot- The present invention relates to a hot-rolled steel sheet produced from the above-
<Formula 1>
Carbon equivalent (Ceq) = carbon content + manganese content / 4 + silicone content / 7.

Description

Technical Field [0001] The present invention relates to a method of manufacturing a hot-rolled steel sheet by a mini-mill process in which cracks are reduced, and a hot-rolled steel sheet produced by the method. BACKGROUND OF THE INVENTION 1. Field of the Invention [0002]

The present invention relates to a method of manufacturing a hot-rolled steel sheet with reduced cracks by a min-mill process and a hot-rolled steel sheet produced from the steel sheet, and more particularly, The present invention relates to a method for manufacturing a hot-rolled steel sheet in which cracks are reduced by a mini-mill process capable of producing a hot-rolled steel sheet having an improved surface quality, and a hot-rolled steel sheet produced therefrom.

Compared with blast furnaces, the impregnation content of molten steel is high and it is difficult to control the steel sheet by the mini-mill using iron scrap as a raw material. Accordingly, techniques for improving the cleanliness inside the steel and improving the surface quality of the steel sheet have been developed.

In the production of a steel sheet by mini-milling, cracks may occur on the surface of the steel sheet, particularly at the end portion and / or the center portion of the steel sheet. The cracking rate tends to increase as the thickness of the steel sheet becomes thicker.

In order to minimize the occurrence of cracks, the solidification shell of the molten steel in the mold has been properly managed to prevent cracking. However, this may require costly equipment replacement and time to stabilize the operation.

Related Prior Art Korean Patent Publication No. 2000-0041265 (published on July 15, 2000, entitled &quot; Method of manufacturing high manganese steel hot-rolled steel sheet by mini-milling process to prevent both surface scrape defects and crack defects) is available.

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for manufacturing a hot-rolled steel sheet by a mini-mill process capable of preventing a crack occurring on the surface of a steel sheet in the production of hot-rolled steel sheets by a mini mill.

It is another object of the present invention to provide a method of manufacturing a hot-rolled steel sheet by a min-mill process capable of preventing cracks occurring at the end portion and / or the center portion of the steel sheet in the production of hot-rolled steel sheets by mini-milling.

It is still another object of the present invention to provide a method for manufacturing a hot-rolled steel sheet by a min-mill process capable of preventing cracking and improving the surface quality of the steel sheet.

It is still another object of the present invention to provide a method of manufacturing a hot-rolled steel sheet by a min-mill process which can easily prevent cracks only by designing chemical components of molten steel.

In order to accomplish the above object, the present invention provides a method for producing a hot-rolled steel sheet by a mini-mill process, the method comprising the steps of: (a) providing a steel sheet having a carbon equivalent (Ceq) of 0.07% or less, Continuously casting molten steel having a sulfur content of 0.008 wt% or less to produce a slab, and rolling the slab to produce a steel sheet.

<Formula 1>

Carbon equivalent (Ceq) = carbon content + manganese content / 4 + silicone content / 7.

Wherein the molten steel comprises 0.01-0.06 wt% of carbon (C), 0.01-0.15 wt% of silicon (Si), 0.05-0.25 wt% of manganese (Mn), 0.001-0.050 wt% of phosphorus (P) Weight percent and balance iron (Fe) and inevitable impurities.

Wherein the molten steel contains 0.001-0.050 wt% of aluminum (Al), 0.0001-0.010 wt% of boron (B), 0.001-0.010 wt% of titanium (Ti), 0.001-0.010 wt% of zirconium (Zr) 0.001-0.010% by weight of chromium (Cr), 0.001-0.010% by weight of chromium (Cr), 0.001-0.010% by weight of nickel (Ni), 0.001-0.010% by weight of niobium (Nb) By weight, and 0.0001 to 0.02% by weight of nitrogen (N).

The hot rolled steel sheet according to another aspect of the present invention may have a carbon equivalent (Ceq) of 0.07% or less, a ratio of manganese content to sulfur content of 50 or less, and a sulfur content of 0.008 wt% or less.

Wherein the hot rolled steel sheet comprises 0.01-0.06 wt% of carbon (C), 0.01-0.15 wt% of silicon (Si), 0.05-0.25 wt% of manganese (Mn), 0.001-0.050 wt% of phosphorus, 0.008 wt.% And a balance of iron (Fe) and unavoidable impurities.

According to the present invention, it is possible to provide a method of manufacturing a steel sheet by a mini-mill process capable of preventing cracks occurring at the end portion and / or the central portion of the steel sheet in the production of the steel sheet by the mini-mill and improving the surface quality of the steel sheet . In addition, the present invention has the effect of providing a method of manufacturing a steel sheet by a mini-mill process which can easily prevent cracks not only by design change, but also by designing chemical components of molten steel.

A method of manufacturing a hot-rolled steel sheet by a mini-mill process, which is one aspect of the present invention, will be described in detail with reference to one embodiment.

The method for producing a hot-rolled steel sheet according to an embodiment of the present invention is a method for controlling the ratio of manganese content (weight ratio, Mn / S) and sulfur content to carbon equivalent, sulfur content Thus, the surface quality of the steel sheet can be improved by remarkably lowering the cracking rate during production of the final steel sheet.

The carbon equivalent (Ceq) in the molten steel may be 0.07% or less. Within the above range, the deformation of the solidification shell of molten steel in the mold is minimized to form a uniform solidification shell, whereby cracking of the steel sheet can be reduced.

The carbon equivalent can be calculated from the following equation (1).

<Formula 1>

Carbon equivalent (Ceq) = carbon content + manganese content / 4 + silicone content / 7

Preferably, the carbon equivalent can be 0.05-0.07%.

The ratio of the manganese content to the sulfur content in the molten steel may be 50 or less, and the content of sulfur may be 0.008 wt% or less.

In general, reducing sulfur content in steel to reduce sulfur is effective in preventing cracks. However, this requires increasing the desulfurization time in the ladle, which may reduce productivity and not reduce the sulfur content to infinity.

In the method of manufacturing a hot-rolled steel sheet according to an embodiment of the present invention, when the ratio of manganese content to sulfur content (weight ratio, Mn / S) is 50 or less and the content of sulfur is 0.008 wt% (MnS) inclusions in the grain boundary to minimize the content of solid sulfur and thereby reduce the occurrence of cracks in the steel sheet.

Preferably, the ratio (by weight) of manganese content to sulfur content in the molten steel is 15-50 and the content of sulfur is 0.003-0.008% by weight.

The molten steel may include carbon (C), silicon (Si), manganese (Mn), phosphorus (P), sulfur (S), and the balance iron (Fe) and other unavoidable impurities.

The molten steel may contain 0.01-0.06 wt% of carbon, 0.01-0.15 wt% of silicon, 0.01-0.25 wt% of manganese, 0.001-0.050 wt% of phosphorus and 0.001-0.008 wt% of sulfur.

Preferably, carbon in the molten steel is 0.01-0.06 wt%, silicon is 0.01-0.07 wt%, manganese is 0.01-0.15 wt%, phosphorus is 0.001-0.050 wt%, and sulfur is 0.003-0.008 wt%.

More specifically, carbon may be included in the molten steel at 0.01-0.06 wt%. Within the above range, the carbon equivalent described above can be matched with silicon and manganese contained in the molten steel to prevent cracking.

Silicon may be included in the molten steel at 0.01-0.15 wt%. Within the above range, the carbon equivalent described above can be adjusted together with carbon and manganese contained in the molten steel to prevent cracking.

Manganese may be included in the molten steel in an amount of 0.05-0.25 wt%. Within the above range, the carbon equivalent described above together with carbon and silicon contained in molten steel can be matched to prevent cracking.

Phosphorus may be included in the molten steel in an amount of 0.001-0.050 wt%. Within the above range, cracks can be prevented.

Sulfur may be included in the molten steel in an amount of 0.008 wt% or less. Within this range, it is based on the fact that coarse manganese sulfur (MnS) inclusions are formed in the grain boundary to minimize the content of solid sulfur and thereby reduce cracking of the steel sheet.

The molten steel may be selected from the group consisting of aluminum (Al), boron (B), titanium (Ti), zirconium (Zr), calcium (Ca), copper (Cu), niobium (Nb), vanadium ), And nitrogen (N).

For example, 0.001-0.050 wt% aluminum, 0.0001-0.010 wt% boron, 0.001-0.010 wt% titanium, 0.001-0.010 wt% zirconium, 0.0001-0.010 wt% calcium, 0.001-0.50 wt% copper, 0.001-0.010 wt% , 0.001 to 0.010 wt% of vanadium, 0.001 to 0.010 wt% of vanadium, 0.001 to 0.010 wt% of chromium, 0.001 to 0.010 wt% of nickel, and 0.0001 to 0.02 wt% of nitrogen.

Within the above range, the surface quality of the steel sheet can be improved by significantly lowering the cracking rate during production of the final steel sheet.

According to an embodiment of the present invention, there is provided a method of manufacturing a steel sheet by a mini mill, comprising the steps of continuously casting the molten steel into a slab by a conventional method; And rolling the slab.

The continuous casting may include a step of passing molten steel taken in the tundish through a casting machine equipped with a mold to produce a slab.

A method of manufacturing a hot-rolled steel sheet according to another aspect of the present invention will be described in detail with reference to an embodiment.

The hot-rolled steel sheet according to an embodiment of the present invention may have a carbon equivalent (Ceq) of 0.07% or less, a ratio of manganese content to sulfur content of 50 or less, and a sulfur content of 0.008 wt% or less.

Wherein the hot rolled steel sheet comprises 0.01-0.06 wt% of carbon (C), 0.01-0.15 wt% of silicon (Si), 0.05-0.25 wt% of manganese (Mn), 0.001-0.050 wt% of phosphorus, 0.008 wt.% And a balance of iron (Fe) and unavoidable impurities.

Details of each element in the hot-rolled steel sheet are as described above.

The hot-rolled steel sheet may be manufactured by the manufacturing method according to one embodiment of the present invention.

Hereinafter, the present invention will be described in more detail with reference to the following examples and comparative examples. However, the present invention is not limited by the following examples.

Example  One

In the molten steel discharged from the electric furnace, the carbon equivalent expressed by the formula 1 was 0.07%, the ratio of the manganese content to the sulfur content was 50, and the sulfur content was 0.003 wt%.

Molten steel contains 0.03% by weight of carbon, 0.0175% by weight of silicon, 0.15% by weight of manganese, 0.050% by weight of phosphorus, 0.003% by weight of sulfur, and the balance iron and other unavoidable impurities.

The molten steel was continuously cast in a mold to prepare a slab, and rolled to produce a steel sheet.

Example  2

In the molten steel discharged from the electric furnace, the carbon equivalent was 0.07%, the ratio of the manganese content to the sulfur content was 50, and the sulfur content was 0.005 wt%. The molten steel was continuously cast in a mold to prepare a slab, and rolled to produce a steel sheet.

Comparative Example  1-8

A steel sheet was prepared in the same manner as in Example 1, except that molten steel as shown in Table 1 was used in place of carbon equivalent, ratio of manganese content to sulfur content, and sulfur content.

The occurrence of cracks in the steel sheets prepared in the above Examples and Comparative Examples was visually evaluated. The crack occurrence rate from the ratio of the number of cracked steel sheets to the total number of steel sheets was evaluated.

Carbon equivalent
(%) Mn / S Sulfur content
(weight%) Total number of steel plates Number of steel plates without cracks Number of cracked steel sheets Crack incidence
(%) Example 1 0.07 50 0.003 36 35 One 2.8 Example 2 0.07 15 0.005 122 118 4 3.3 Comparative Example 1 0.08 15 0.008 114 109 5 4.4 Comparative Example 2 0.09 15 0.008 209 200 9 4.3 Comparative Example 3 0.10 15 0.008 152 140 12 7.9 Comparative Example 4 0.12 15 0.008 65 60 5 7.7 Comparative Example 5 0.07 10 0.010 11 10 One 9.1 Comparative Example 6 0.07 51 0.005 47 44 3 6.4 Comparative Example 7 0.07 53 0.005 155 144 11 7.1 Comparative Example 8 0.07 52 0.005 12 10 2 16.7

As shown in Table 1, the hot-rolled steel sheet produced from molten steel satisfying the carbon equivalent, the ratio of manganese content to sulfur content, and the sulfur content according to one embodiment of the present invention had a significantly low cracking rate.

On the other hand, the steel sheet of Comparative Example 1-4 produced from molten steel having a carbon equivalent of 0.08-0.12% which does not satisfy the carbon equivalent according to one embodiment of the present invention has a relatively high cracking rate as compared with the present invention.

Further, even though the ratio of the carbon equivalent and the manganese content to the sulfur content according to one embodiment of the present invention is satisfied, the sulfur content is 0.010% by weight and the comparison made from the molten steel that does not satisfy the sulfur content according to one embodiment of the present invention In the steel sheet of Example 5, the crack occurrence rate was relatively high as compared with the present invention.

Further, even when the carbon equivalent and the sulfur content according to the present invention are satisfied, the ratio of the manganese content to the sulfur content is 55, which is comparable to that of the comparative example prepared from the molten steel which does not satisfy the ratio of the manganese content to the sulfur content according to one embodiment of the present invention In the steel sheets of 6-8, the crack occurrence rate was relatively higher than that of the present invention.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the scope of the present invention . Accordingly, the technical scope of the present invention should be defined by the following claims.

Claims (5)

Continuously casting a molten steel having a carbon equivalent (Ceq) of 0.05-0.07% and a manganese (Mn) content of 15-50 relative to the sulfur (S) content, as a slab, and
And rolling the slab to produce a steel sheet,
Wherein the molten steel contains 0.01-0.06 wt% of carbon (C), 0.01-0.15 wt% of silicon (Si), 0.05-0.15 wt% of manganese (Mn), 0.035-0.050 wt% of phosphorus (P) A method for producing a hot-rolled steel sheet by a min-mill process comprising iron (Fe) and unavoidable impurities in a weight%
<Formula 1>
Carbon equivalent (Ceq) = carbon content + manganese content / 4 + silicone content / 7. delete The method according to claim 1, wherein the molten steel comprises 0.001-0.050 wt% aluminum (Al), 0.0001-0.010 wt% boron (B), 0.001-0.010 wt% titanium, 0.001-0.010 wt% zirconium (Zr) 0.001-0.010 wt.% Of copper (Ca), 0.001-0.50 wt.% Of copper (Cu), 0.001-0.010 wt.% Of niobium N, 0.001-0.010 wt.% Of vanadium (V) 0.001 to 0.010% by weight of nickel (Ni), and 0.0001 to 0.02% by weight of nitrogen (N). (Ceq) expressed by the following formula 1 is 0.05-0.07%, the ratio of manganese (Mn) content to sulfur (S) content is 15-50,
(P), 0.003-0.008 wt.% Sulfur (S), and 0.001-0. 0 wt.% Sulfur (S) Hot rolled steel sheet containing a residual amount of iron (Fe) and unavoidable impurities:
<Formula 1>
Carbon equivalent (Ceq) = carbon content + manganese content / 4 + silicone content / 7. delete