KR20190036012A - Method for manufacturing ultra high strength steel having excellent surface characteristics - Google Patents

Abstract

Disclosed is a method for manufacturing an ultrahigh strength steel sheet with excellent surface characteristics, capable of suppressing a surface defect on an edge part of a steel sheet. According to one specific embodiment of the present invention, the method comprises the steps of: scarfing both edge parts of the upper surface of a slab material to form a scarfing surface, wherein the slab material comprises 0.08 to 0.20 weight percent of C, 0.1 to 2.0 weight percent of Si, 2.0 to 4.0 weight percent of Mn, greater than 0 to 0.02 weight percent or less of P, greater than 0 to 0.005 weight percent or less of S, 0.1 to 1.0 weight percent of Cr, 0.01 to 0.5 weight percent or less of Mo, and the balance of Fe and inevitable impurities; hot-rolling the slab material to manufacture a hot-rolled sheet; winding the hot-rolled sheet to manufacture a hot-rolled coil; uncoiling and cold-rolling the hot-rolled coil to manufacture a cold-rolled sheet; and annealing the cold-rolled sheet.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a method of manufacturing an ultra high tensile steel sheet having excellent surface characteristics,

The present invention relates to a method of manufacturing an ultra high tensile strength steel sheet having excellent surface characteristics. More particularly, the present invention relates to a method of manufacturing an ultra high tensile strength steel sheet capable of preventing the occurrence of surface defects appearing in corners of a steel sheet.

Recently, in the field of automobile industry, the ratio of ultra-high tensile steel applied to automobile parts is increasing due to the trend of lightening of the body and strengthening of various safety regulations.

In the case of such a steel sheet for automobile parts, the surface quality of the final steel sheet product must be ensured as it is required to have an excellent surface quality together with an extremely high tensile strength.

On the other hand, the slabs produced in the continuous casting machine are cut to a predetermined length, heated in the reheating furnace, and hot-rolled to produce hot-rolled coils. Various slabs produced in the continuous casting process, including heat cracks, have. Defects in the edge regions can cause defects such as edge scabs at both corners of the hot-rolled coil produced after hot rolling.

BACKGROUND ART [0002] The background art relating to the present invention is disclosed in Korean Patent Registration No. 10-1429982 (published on Aug. 18, 2018, entitled "Scarping Apparatus and Surface Treatment Method of Slab Using Same").

According to one embodiment of the present invention, there is provided an ultra high tensile strength steel sheet manufacturing method capable of suppressing the occurrence of surface defects in corner portions of a steel sheet.

According to one embodiment of the present invention, there is provided a method of manufacturing an ultra high tensile strength steel sheet which prevents a process load such as a rolling load and is excellent in productivity.

One aspect of the present invention relates to a method of manufacturing an ultra high tensile steel sheet having excellent surface characteristics. In one embodiment, the method for manufacturing an ultra high strength steel sheet includes 0.08 to 0.20 wt% of carbon (C), 0.1 to 2.0 wt% of silicon (Si), 2.0 to 4.0 wt% of manganese (Mn) (Fe) and other inevitable impurities (Fe) in an amount of not more than 0.02 wt%, sulfur (S): not less than 0.005 wt%, chromium (Cr): 0.1-1.0 wt%, molybdenum Scarfing both edges of the upper surface of the slab member to form a scarfing surface; Hot rolling the slab material to produce a hot rolled plate; Winding the hot-rolled sheet at a winding temperature of 450 to 650 ° C to produce a hot-rolled coil; Uncoiling and cold rolling the hot-rolled coil to produce a cold-rolled sheet; And a step of subjecting the cold-rolled sheet to annealing, wherein the scarping surface is formed in one plane, and a top scapping length (a) and a side scarping length (a) formed on one edge of the slab material b) satisfy the relations of the following formulas (1) and (2), respectively:

[Formula 1]

0.005A? A? 0.08A

(In the above formula (1), A is the width of the slab material)

[Formula 2]

b = a? tan (?)

(Wherein, in the formula 2,? Is an angle formed by the scarping surface and the upper surface of the slab member, and? Is from 10 to 70).

In one embodiment, the slab material may be reheated to a reheat temperature of 1150 to 1300 ° C, followed by scarring in the rolling direction corner.

In one embodiment, the hot rolling may be performed at a finishing rolling temperature of 870 to 1000 ° C.

In one embodiment, the cold rolling can be performed at a reduction ratio of 30 to 70%.

In one embodiment, the annealing is performed by heating the cold-rolled sheet to 800 to 840 占 폚; And cooling the heated cold-rolled sheet to 300 to 500 ° C.

In one embodiment, the slab material may further comprise at least one of aluminum (Al): 0.001 to 1.000 wt%, niobium (Nb): 0 to 0.1 wt%, and titanium (Ti) have.

It is possible to suppress the occurrence of surface color defects such as surface color difference and edge scab which are generated at the corners of a steel sheet, prevent process load such as rolling load, and provide excellent productivity can do.

1 shows a method of manufacturing an ultra high strength steel sheet according to one embodiment of the present invention.
2 schematically illustrates scarping treatment of a slab material according to one embodiment of the present invention.
3 shows a cross-section of a scalloped slab material according to one embodiment of the present invention.
4 (a) shows the scarping conditions of the slab material according to the present invention. Fig. 4 (b) shows the scarfing conditions of the slab material according to the comparative example of the present invention. Fig. 4 Comparative Example for the Present Invention This is a schematic diagram showing the scarping treatment condition of the slab material.
5 (b) shows a scarfing surface of a comparative slab material according to the present invention, and FIG. 5 (c) shows a scarfing surface of a slab material according to the present invention ≪ / RTI > is a photograph showing the scarfing surface of the slab material.
6 (a) shows a hot-rolled plate according to an embodiment of the present invention, FIG. 6 (b) shows a hot-rolled plate according to a comparative example of the present invention, .
FIG. 7 (a) is an optical microscope photograph of a comparative steel sheet edge portion of the present invention, and FIG. 7 (b) is an optical microscope photograph of a comparative steel sheet edge portion of the present invention.

Hereinafter, the present invention will be described in detail. In the following description of the present invention, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention rather unclear.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to be exemplary, self-explanatory, allowing for equivalent explanations of the present invention.

High-strength steel plate manufacturing method with excellent surface characteristics

One aspect of the present invention relates to a method of manufacturing an ultra high tensile steel sheet having excellent surface characteristics.

1 shows a method of manufacturing an ultra high strength steel sheet according to one embodiment of the present invention. Referring to FIG. 1, the method for fabricating an ultra high strength steel sheet comprises the steps of: (S10) scraping a slab; (S20) hot rolling step; (S30) a winding step; (S40) cold rolling; And (S50) an annealing heat treatment step.

More specifically, (S10), 0.08 to 0.20 wt% of carbon (C), 0.1 to 2.0 wt% of silicon (Si), 2.0 to 4.0 wt% of manganese (Mn) (Fe) and other unavoidable impurities, and the balance of iron (Fe) and other inevitable impurities, and the balance of iron (Fe) and other inevitable impurities. Scarfing both edges of the upper surface of the slab to form a scarfing surface; (S20) hot rolling the slab material to produce a hot rolled sheet material; (S30) winding the hot-rolled sheet at a winding temperature of 450 to 650 DEG C to produce a hot-rolled coil; (S40) uncoiling and cold-rolling the hot-rolled coil to produce a cold-rolled sheet; And (S50) annealing the cold-rolled sheet material.

Hereinafter, the method for manufacturing the ultra-high strength steel sheet will be described in detail step by step.

(S10) Slab material Scarping  step

Wherein said step comprises: 0.08-0.20 wt% carbon; 0.1-2.0 wt% silicon; 2.0-4.0 wt% manganese; P: over 0.02 wt% S: 0 to 0.005 wt% or less, Cr (Cr): 0.1 to 1.0 wt%, molybdenum (Mo): 0.01 to 0.5 wt%, and balance of iron (Fe) and other unavoidable impurities And scarfing the corner portion to form a scarping surface.

Hereinafter, the components included in the slab material will be described in detail.

Carbon (C)

The carbon (C) is added for securing the strength of the present invention. In one embodiment, the carbon is included in an amount of 0.08-0.20 wt% based on the total weight of the slab material. When the carbon content is less than 0.08 wt%, it is difficult to secure sufficient strength. When the carbon content is more than 0.20 wt%, the strength is increased but the toughness and weldability may be greatly reduced.

Silicon (Si)

The silicon (Si) serves as a deoxidizer in the steel and contributes to securing strength and elongation. In one embodiment, the silicon is comprised between 0.1 and 2.0 wt% based on the total weight of the slab material. If the silicon content is less than 0.1 wt%, the effect of the addition is insignificant. If the silicon content is more than 2.0 wt%, the strength of the present invention is lowered and the carbon equivalent is excessively increased.

Manganese (Mn)

The manganese (Mn) contributes to the improvement of the strength of the steel through the solid solution strengthening and the increased incombustibility. In one embodiment, the manganese is contained in an amount of 2.0 to 4.0% by weight based on the total weight of the slab material. If the manganese content is less than 2.0 wt%, it is difficult to secure the strength. If the manganese content is more than 4.0 wt%, a manganese band structure is formed and the segregation increases sharply, which hinders the workability of the steel and increases the carbon equivalent, .

In (P)

The phosphorus (P) contributes to enhance the strength of the steel by solid solution strengthening. In one embodiment, the phosphorous is included in an amount of more than 0 to 0.02 wt% based on the total weight of the slab material. When the phosphorus is contained in an amount exceeding 0.02% by weight, low-temperature embrittlement may occur.

Sulfur (S)

The sulfur (S) inhibits toughness and weldability and increases MnS non-metallic inclusions, thereby inhibiting bending workability. In one embodiment, the sulfur is included in an amount of more than 0 to 0.005% by weight based on the total weight of the slab material. If the sulfur content exceeds 0.005% by weight, coarse inclusions are increased to deteriorate the fatigue characteristics.

Chromium (Cr)

The chromium (Cr) contributes to the improvement of the strength of the steel through the solid solution strengthening and the ingot enhancement. In one embodiment, the chromium is included in an amount of 0.1 to 1.0 wt% based on the total weight of the slab material. When the chromium is contained in an amount of less than 0.1% by weight, the addition effect thereof is insufficient and it is difficult to secure the strength. When the chromium is contained in an amount exceeding 1.0 wt%, a surface oxide is formed during the heat treatment to deteriorate the plating ability and increase the carbon equivalent, so that the weldability may be deteriorated.

Molybdenum (Mo)

The molybdenum (Mo) contributes to the improvement of the strength of the steel through the solid solution strengthening and the increase of the ingot property. In one embodiment, the molybdenum is included in an amount of 0.01 to 0.5 wt% based on the total weight of the slab material. When the content of the molybdenum is less than 0.01% by weight, the effect of the addition is insufficient and it is difficult to ensure the strength. When the molybdenum is contained in an amount exceeding 0.5% by weight, the amount of martensite increases to increase the toughness and the toughness decreases. When the amount of the expensive element is large, the cost of the steel increases, which is disadvantageous to production.

In one embodiment, the slab material may further comprise at least one of aluminum (Al): 0.001 to 1.000 wt%, niobium (Nb): 0 to 0.1 wt%, and titanium (Ti) have.

Aluminum (Al)

The aluminum (Al) is an element used as a deoxidizing agent, and serves to stabilize austenite by improving the elongation rate of the ferrite and enhancing the amount of carbon concentration in the austenite.

In one embodiment, the aluminum may be included in an amount of 0.001 to 1.000 wt% based on the total weight of the slab material. With the above content, the effect of improving the elongation and stabilizing effect of austenite can be excellent.

Niobium (Nb)

The niobium (Nb) can improve the toughness and strength of the steel as precipitate-forming elements. Particularly, the niobium precipitates are solidified during hot rolling after being solidified in the reheating step of the slab material, so that the effect of refining the martensite structure can be excellent.

In one embodiment, the niobium may be present in an amount of greater than 0 to 0.1 weight percent based on the total weight of the slab material. Under the above conditions, the effect of improving the toughness and strength of the steel can be excellent while preventing the rolling load.

Titanium (Ti)

The titanium (Ti) is a strong carbonitride-forming element and improves the strength of the steel. In one embodiment, the titanium may be present in an amount of greater than 0 to 0.1 weight percent based on the total weight of the slab material. Under the above conditions, the effect of improving the strength of steel can be excellent.

The slab material of the present invention may have, for example, a hexahedral shape. In one embodiment, the slab material is reheated to a reheating temperature of 1150 to 1300 ° C, followed by scarfing of both corners in the rolling direction. When reheating under the above temperature conditions, the segregated components are reused during casting, and the homogenizing effect of the slab material can be excellent.

In the case of the slab material having the alloy composition according to the present invention, corner cracks may frequently occur due to ingot elements such as molybdenum and chromium. Therefore, the corner portion of the slab material can be scoured. The scarfing process is carried out for the purpose of removing surface defects of the slab material which may adversely affect hot rolling and cold rolling by sparging the edges of the slab material with high-pressure oxygen.

2 schematically shows a scarping process of a slab material S according to one embodiment of the present invention. Referring to FIG. 2, both corner portions may be subjected to scarping along the rolling direction of the slab material S.

3 shows a cross section of a scalloped slab material S according to one embodiment of the present invention. 3, the scarfing surface is formed in one plane. When the scarfing surface is formed into one or more planes, an edge scab may occur on the surface of the steel sheet.

3, the scalloping length (a) and the side scarping length (b) of the upper surface of one edge portion of the slab material satisfy the following equations (1) and (2)

[Formula 1]

0.005A? A? 0.08A

(In the above formula (1), A is the width of the slab material)

[Formula 2]

b = a? tan (?)

(Wherein, in the formula 2,? Is an angle formed by the scarping surface and the upper surface of the slab member, and? Is from 10 to 70).

In one embodiment, the width A of the slab material is not limited thereto, but may be between 600 mm and 2000 mm.

When the upper surface scarping length (a) is less than 0.005A, the improvement effect of the edge scab of the slab material is lowered. When the upper surface scarping length (a) is larger than 0.08A, the working time is prolonged, The rolling load may increase.

When scoring is performed to less than 10 degrees between the scarfing surface and the upper surface of the slab member, it is difficult to remove cracks formed at the edge of the slab member, so that an edge scab is generated on the surface of the steel sheet, °, scrapping is performed for a long period of time, resulting in a decrease in productivity, an increase in rolling load upon rolling, and an effect of improving the edge scap on the surface of the steel sheet may be deteriorated.

(S20) Hot rolling  step

The step is a step of hot-rolling the slab material to produce a hot-rolled sheet material. In one embodiment, the hot rolling may be performed at a finish rolling temperature of 870 to 1000 ° C. Under the above conditions, the grain refinement effect is excellent and the mechanical strength of the steel sheet can be excellent. For example, the thickness of the hot-rolled sheet may be 2 to 4 mm.

(S30) Coiling  step

In this step, the hot-rolled coil is manufactured by winding the hot-rolled sheet at a winding temperature of 450 to 650 ° C. For example, the hot-rolled hot-rolled sheet can be cooled to the winding temperature range and then rolled to produce a hot-rolled coil. When the coiling temperature is cooled to a temperature of less than 450 캜 and the coiling is carried out, the strength of the hot rolled plate may be excessively increased and the productivity may be lowered due to an increase in the rolling reduction during cold rolling. The above coiling temperature is cooled to a temperature exceeding 650 캜 and surface defects may occur due to color difference on the surface of the steel sheet during winding.

(S40) Cold rolling step

The above step is a step of cold-rolling the hot-rolled coils to produce a cold-rolled sheet. For example, the hot-rolled coil may be uncoiled, pickled, and then cold-rolled to produce a cold-rolled sheet. In one embodiment, the cold rolling can be performed at a reduction ratio of 30 to 70%. Strength can be secured easily under the above conditions.

(S50) Annealing  Heat treatment step

The step of annealing the cold-rolled sheet is a step of heat-treating the cold-rolled sheet. In one embodiment, the annealing is performed by heating the cold-rolled sheet to 800 to 840 占 폚; And cooling the heated cold-rolled sheet to 300 to 500 ° C.

The carbon in the austenite is homogeneously distributed under the heating temperature condition, and the steel sheet of the present invention can have excellent mechanical properties. Further, the toughness and elongation of the steel sheet can be excellent under the cooling conditions.

Hereinafter, the configuration and operation of the present invention will be described in more detail with reference to preferred embodiments of the present invention. It is to be understood, however, that the same is by way of illustration and example only and is not to be construed in a limiting sense.

Example  And Comparative Example

Example  One

The slab material having a width of 162 mm and a thickness of 40 mm including the alloy component shown in Table 1 and the balance of iron (Fe) and other unavoidable impurities was reheated to a reheat temperature of 1190 캜.

Then, as shown in Fig. 4 (a), both edges of the upper surface of the slab member were scarfed along the rolling direction to form scaping surfaces having one plane. In this case, the angle θ between the scarfing surface and the upper surface of the slab material is 45 °. The top scarping length a of the one side edge portion of the slab material is 10 mm and the side scarping length b is 10 mm * tan 45 °) = 10 mm.

Then, the slab material was hot-rolled at a finish rolling temperature of 899 ° C to prepare a hot-rolled sheet, and rolled at a coiling temperature of 600 ° C to produce a hot-rolled coil. Then, the hot-rolled coil was pickled after uncoiling, and cold rolled under the conditions of a reduction rate of 30 to 70% to produce a cold-rolled sheet. Then, the cold-rolled sheet was subjected to annealing (cold-rolled sheet material was heated up to 831 占 폚 and then cooled down to 325 占 폚) to produce an ultra-high-strength steel sheet.

Example  2 to 3

High-tensile steel sheets were prepared in the same manner as in Example 1, except that the alloy components and the contents in Table 1 were applied and the process conditions of Table 2 were applied.

Comparative Example  1, 3, 5

The slab material in the form of a hexahedron having a width of 162 mm and a thickness of 40 mm including the alloy component shown in the following Table 1 and the remaining amount of iron (Fe) and other unavoidable impurities was reheated under the temperature condition shown in Table 2 below.

Then, as shown in Fig. 4 (b), both corner portions of the upper surface of the slab member were scarfed along the rolling direction to form scaping surfaces having two planes. Then, hot-rolled and rolled according to the following Table 2 to prepare a hot-rolled sheet, which was uncoiled, pickled, and cold-rolled at a reduction ratio of 30 to 70% to prepare a cold-rolled sheet. And then subjected to annealing heat treatment under the conditions shown in Table 2 below to produce a steel sheet.

Comparative Example  2, 4, 6

The slab material in the form of a hexahedron having a width of 162 mm and a thickness of 40 mm including the alloy component shown in the following Table 1 and the remaining amount of iron (Fe) and other unavoidable impurities was reheated under the temperature condition shown in Table 2 below.

Next, as shown in Fig. 4 (c), both edges of the upper surface of the slab member were scarfed along the rolling direction to form scaping surfaces having two planes. Then, hot-rolled and rolled according to the following Table 2 to prepare a hot-rolled sheet, which was uncoiled, pickled, and cold-rolled at a reduction ratio of 30 to 70% to prepare a cold-rolled sheet. And then subjected to annealing heat treatment under the conditions shown in Table 2 below to produce a steel sheet.

Comparative Example  7

The slab material in the form of a hexahedron having a width of 162 mm and a thickness of 40 mm including the alloy component shown in the following Table 1 and the remaining amount of iron (Fe) and other unavoidable impurities was reheated under the temperature condition shown in Table 2 below.

Then, both edges of the upper surface of the slab member were subjected to scarfing along the rolling direction to form scaping surfaces having one plane.

The top scarping length a of the one side edge portion of the slab member is 10 mm and the side scarping length b of the slab member is 10 mm * tan (75) °) = 37.3 mm.

Then, hot-rolled and rolled according to the following Table 2 to prepare a hot-rolled sheet, which was uncoiled, pickled, and cold-rolled at a reduction ratio of 30 to 70% to prepare a cold-rolled sheet. And then subjected to annealing heat treatment under the conditions shown in Table 2 below to produce a steel sheet.

Comparative Example  8

The slab material having the width of 162 mm and the thickness of 40 mm including the alloy component of Table 1 and the remaining amount of iron (Fe) and other unavoidable impurities was reheated at the temperature condition shown in Table 2, Were scarfed along the rolling direction to form a scaping surface having one plane.

In this case, the angle θ between the scarfing surface and the upper surface of the slab material is 8 °, the top scarping length a of the one side edge of the slab material is 10 mm, and the side scarping length b is 10 mm * tan 8 °) = 1.41 mm.

Then, hot-rolled and rolled according to the following Table 2 to prepare a hot-rolled sheet, which was uncoiled, pickled, and cold-rolled at a reduction ratio of 30 to 70% to prepare a cold-rolled sheet. And then subjected to annealing heat treatment under the conditions shown in Table 2 below to produce a steel sheet.

With respect to the steel sheets of Examples 1 to 3 and Comparative Examples 1 to 8, the occurrence of edge scabs at the corners of the steel sheet was visually observed, and the results are shown in Table 2 below.

Referring to Table 2, in the case of Examples 1 to 3 according to the present invention, edge scaps were not generated in the edge portions of the steel sheet in the range of 1 to 8, which were outside the scarfing conditions of the present invention. Could know.

5 (b) shows a scarfing surface of a slab material of Comparative Example 1 of the present invention, and Fig. 5 (c) shows a scarfing surface of a slab material of Example 1. Fig. Is a photograph showing the scarfing surface of the slab material of Comparative Example 2 of the present invention.

6 (a) shows a hot-rolled plate of Example 1 according to the present invention, FIG. 6 (b) shows a hot-rolled plate of Comparative Example 1 according to the present invention, and FIG. 5 Comparative Example 2: A photograph showing a hot-rolled sheet material. Referring to FIG. 5, in the case of Comparative Examples 1 and 2 in which the edge scap was not generated on the surface of the hot-rolled sheet according to Example 1 of the present invention but the scarfing condition of the present invention was out of order, .

7 (a) is an optical microscope photograph showing the edge portion (edge portion) of Comparative Example 3 steel sheet, and Fig. 7 (b) is an optical microscope photograph showing the end portion of Comparative Example 4 steel sheet. 6, in Comparative Examples 3 and 4 except for the scarping conditions of the present invention, oxides of various sizes were formed in the lower part of the line-shaped edge scap formed at the edge of the manufactured steel sheet , And thus the surface quality was deteriorated.

It will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims (6)

(P): more than 0 and not more than 0.02% by weight, sulfur (S): 0.08 to 0.20% by weight, silicon (Si): 0.1 to 2.0% (Fe) and other unavoidable impurities are contained in the upper surface of the slab material at an angle greater than 0 to 0.005 wt%, Cr (Cr) 0.1 to 1.0 wt%, molybdenum (Mo) Scarfing to form a scarping surface;
Hot rolling the slab material to produce a hot rolled plate;
Winding the hot-rolled sheet at a winding temperature of 450 to 650 ° C to produce a hot-rolled coil;
Uncoiling and cold rolling the hot-rolled coil to produce a cold-rolled sheet; And
Annealing the cold-rolled sheet material;
The scarifying surface is formed in one plane,
Wherein an upper scarping length (a) and a side scarping length (b) formed at one edge of the slab material satisfy the following expressions (1) and (2)
[Formula 1]
0.005A? A? 0.08A
(In the above formula (1), A is the width of the slab material)
[Formula 2]
b = a? tan (?)
(Wherein, in the formula 2,? Is an angle between the scarping surface and the upper surface of the slab member, and? Is from 10 to 70).
The method according to claim 1,
Wherein the slab material is reheated at a reheating temperature of 1150 to 1300 占 폚 and scarfed in a corner portion in the rolling direction.
The method according to claim 1,
Wherein the hot rolling is performed at a finishing rolling temperature of 870 to 1000 占 폚.
The method according to claim 1,
Wherein the cold rolling is performed at a reduction ratio of 30 to 70%.
The method according to claim 1,
The annealing heat treatment is performed by heating the cold-rolled sheet to 800 to 840 占 폚; And
And cooling the heated cold-rolled sheet to 300 to 500 占 폚.
The method according to claim 1,
Wherein the slab material further comprises at least one of aluminum (Al): 0.001 to 1.000 wt%, niobium (Nb): 0 to 0.1 wt% or less, and titanium (Ti) High strength steel plate manufacturing method.

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