KR101294575B1

KR101294575B1 - Method of manufacturing cold-rolled steel sheet for enamel with excellent fish scale resistance

Info

Publication number: KR101294575B1
Application number: KR1020130032397A
Authority: KR
Inventors: 박근웅; 임지형
Original assignee: 현대제철 주식회사; 현대하이스코 주식회사
Priority date: 2013-03-26
Filing date: 2013-03-26
Publication date: 2013-08-07

Abstract

PURPOSE: A method for manufacturing a cold-rolled steel sheet for a porcelain enamel with an excellent fish scale resistant property is provided to obtain the excellent adhesion between the steel sheet and a porcelain enamel paint and to prevent the generation of fish scale when burning the porcelain enamel by a thermal treatment process control after cold-rolling. CONSTITUTION: A method for manufacturing a cold-rolled steel sheet for a porcelain enamel with an excellent fish scale resistant property includes the following steps of: cold-rolling the steel sheet at a draft percentage of 60% or greater; annealing the cold-rolled steel sheet at a temperature of 840-860°C (S120); cooling the annealed steel sheet up to a temperature of 420-440°C (S130); and over-aging processing the cooled steel sheet at a temperature of 420-440°C (S140). [Reference numerals] (AA) Start; (BB) Finish; (S110) Arranging a steel sheet; (S120) Annealing the steel sheet; (S130) Cooling; (S140) Over-aging process

Description

Method for manufacturing enameled cold rolled steel sheet with excellent fish scale {METHOD OF MANUFACTURING COLD-ROLLED STEEL SHEET FOR ENAMEL WITH EXCELLENT FISH SCALE RESISTANCE}

The present invention relates to an enameled cold rolled steel sheet manufacturing technology, and more particularly, to an enameled cold rolled steel sheet excellent in fish scale resistance, deep drawing properties and adhesion to the enamel coating and its manufacturing method.

Enamel is a product that gives corrosion resistance, abrasion resistance, heat resistance and cleanliness by applying enamel coating on the surface of steel sheet. Such enamel is used in various fields such as home appliance kitchen products, building exterior panel products, industrial heat exchanger parts, photoluminescent signs, and hot water tanks.

In order to manufacture the enamel, the enamel coating is applied to the steel sheet and then fired. At this time, the most problematic in the process of manufacturing the enamel is fish scale (fish scale). Fish scale refers to a defect in which hydrogen dissolved in enamel firing is supersaturated during the cooling process, and hydrogen is released out of the enamel layer, resulting in fish scales in the enamel layer. This fish scale becomes a factor that hinders the surface properties of the enamel.

In addition, the enamel steel sheet is mainly used for flat enamel, because the moldability and deep drawing of the normal enamel steel sheet is not good.

Background art related to the present invention is an enamel steel sheet without a bubble defect disclosed in Republic of Korea Patent Publication No. 10-2010-0070683 (published on June 28, 2010) and a manufacturing method thereof.

SUMMARY OF THE INVENTION An object of the present invention is to provide an enameled cold rolled steel sheet excellent in fish scale resistance, deep drawing property, and adhesion to an enamel coating, and a method of manufacturing the same, through alloying and process control.

Method for producing a cold rolled steel sheet for enamel according to an embodiment of the present invention for achieving the above object is (a) wt%, carbon (C): more than 0% to 0.003% or less, silicon (Si): more than 0% to 0.03 % Or less, manganese (Mn): 0.9 to 1.1%, phosphorus (P): more than 0% to 0.02% or less, sulfur (S): 0.04% to 0.06%, soluble aluminum (Sol.Al): 0.02% to 0.04% , Titanium (Ti): 0.05 ~ 0.07%, calcium (Ca): 0.002 ~ 0.004%, nitrogen (N): more than 0% to 0.005% or less and oxygen (O): more than 0% to 0.005%, Cold rolling a steel plate comprising remaining iron (Fe) and unavoidable impurities; (b) annealing the cold rolled steel sheet at 840 to 860 ° C; (e) cooling the annealed steel sheet to 420-440 ° C .; And (f) overaging the cooled steel sheet at 420 to 440 ° C.

At this time, in the step (a), the cold rolling is preferably carried out at a reduction ratio of 60% or more.

In addition, the annealing treatment of step (b) comprises (b1) direct heating the cold rolled steel sheet to 630 to 730 ° C. in a direct fired furnace, and (b2) the direct heating steel sheet as described above. It may include the step of heating up to 840 ~ 860 ℃ at an average temperature increase rate slower than the average heating rate during direct heating, (b3) may include maintaining the elevated temperature of the steel sheet at 840 ~ 860 ℃.

At this time, in the step (b1), the direct heating may be carried out at an average temperature increase rate of 15 ~ 40 ℃ / sec. In addition, in the step (b2), the temperature increase may be carried out at an average temperature increase rate of 5 ℃ / sec or less. In addition, in the step (b3), the heating may be performed for 50 to 150 seconds.

In addition, the step (c) is (c1) the step of first cooling the annealed steel sheet to a first average cooling rate to 610 ~ 650 ℃, (c2) the first cooled steel sheet, the first average It may include the step of secondary cooling to 530 ~ 420 ℃ at a second average cooling rate slower than the cooling rate.

In this case, the primary cooling is preferably performed by a gas jet method, and the secondary cooling is performed by a roll quenching method. In this case, the first average cooling rate may be 7 ~ 15 ℃ / sec, the second average cooling rate may be 2 ~ 7 ℃ / sec.

In addition, in the step (d), the overaging treatment may be performed for 100 to 200 seconds.

Cold rolled steel sheet for enamel according to an embodiment of the present invention for achieving the above object by weight, carbon (C): more than 0% to 0.003% or less, silicon (Si): more than 0% to 0.03% or less, manganese ( Mn): 0.9 to 1.1%, phosphorus (P): more than 0% to 0.02% or less, sulfur (S): 0.04% to 0.06%, soluble aluminum (Sol.Al): 0.02% to 0.04%, titanium (Ti) : 0.05 to 0.07%, calcium (Ca): 0.002 to 0.004%, nitrogen (N): more than 0% to 0.005% or less and oxygen (O): more than 0% to 0.005% or less, remaining iron (Fe) And inevitable impurities.

At this time, the cold rolled steel sheet for enamel may have a tensile strength (TS): 270 ~ 350MPa and elongation of 40% or more.

According to the method for producing an enameled cold rolled steel sheet according to the present invention, by controlling the alloy components of carbon, manganese, aluminum, titanium, calcium, etc., and controlling the heat treatment process after cold rolling, the adhesion between the steel sheet and the enamel coating is excellent, In addition, it is possible to secure the hydrogen storage site in the steel sheet, it is possible to prevent the generation of fish scale during enamel firing.

In addition, in the case of the enameled cold rolled steel sheet produced by the above method, it can exhibit an elongation of 40% or more with a tensile strength of 270 ~ 350MPa, can exhibit a high formability, it can be utilized for the production of enamel of a complicated shape.

1 is a flow chart showing a method for producing a cold rolled steel sheet for enamel according to an embodiment of the present invention.
2 shows an example of the annealing treatment step shown in FIG. 1.
FIG. 3 shows an example of the cooling step shown in FIG. 1.

Advantages and features of the present invention and methods for achieving them will be apparent with reference to the embodiments described below in detail with the accompanying drawings. It should be understood, however, that the invention is not limited to the disclosed embodiments, but is capable of many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, To fully disclose the scope of the invention to those skilled in the art, and the invention is only defined by the scope of the claims.

Hereinafter, with reference to the accompanying drawings, a cold rolled steel sheet for enamel according to a preferred embodiment of the present invention and a manufacturing method thereof will be described in detail.

Cold rolled steel sheet for enamel

The cold rolled steel sheet for enamel according to the present invention in weight%, carbon (C): more than 0% to 0.003% or less, silicon (Si): more than 0% to 0.03% or less, manganese (Mn): 0.9 ~ 1.1%, phosphorus (P): more than 0% to 0.02% or less, sulfur (S): 0.04% to 0.06%, soluble aluminum (Sol.Al): 0.02% to 0.04%, titanium (Ti): 0.05 to 0.07%, calcium (Ca ): 0.002 to 0.004%, nitrogen (N): more than 0% to 0.005% or less and oxygen (O): more than 0% to 0.005% or less.

In addition to the above components, the remainder consists of iron (Fe) and inevitable impurities.

Hereinafter, the role and content of each component included in the enameled cold rolled steel sheet according to the present invention will be described.

Carbon (C)

Carbon (C) is an element contributing to the increase in strength of steel.

The carbon is preferably added in more than 0% to 0.003% by weight of the total weight of the steel sheet. When the carbon addition amount exceeds 0.003% by weight, bubble defects may be caused on the enamel surface, and the deep drawing property may be degraded.

Silicon (Si)

Silicon (Si) lowers the carbon solubility in the ferrite matrix, contributing to the formation of titanium-based precipitates, and serves to improve the adhesion to the enamel coating.

However, when too much silicon is added, the surface quality of the steel sheet may be lowered and the weldability may be reduced.

Therefore, in the present invention, the amount of silicon added is limited to more than 0% to 0.03% by weight of the total weight of the steel sheet.

Manganese (Mn)

Manganese (Mn) improves the strength of the steel of the steel sheet, and is particularly a major element in improving the adhesion of the enamel coating.

The manganese is preferably added in 0.9 to 1.1% by weight of the total weight of the steel sheet. When the amount of manganese added is less than 0.9% by weight, the effect of addition is insufficient. On the contrary, when the addition amount of manganese exceeds 1.1 weight%, there exists a problem which inhibits moldability.

Phosphorus (P)

Phosphorus (P) contributes to an increase in adhesion with the enamel coating, but when excessively included, it acts as a factor to lower the formability of the steel sheet.

Therefore, in the present invention, the content of phosphorus is limited to more than 0% to 0.02% by weight of the total weight of the steel sheet.

Sulfur (S)

Sulfur (S) forms a non-metallic inclusion such as MnS in combination with manganese to prevent redness brittleness, and forms TiS to reduce A3 transformation temperature and act as a hydrogen storage source, thereby contributing to improvement of fish scale resistance.

The sulfur is preferably contained in 0.04 to 0.06% by weight of the total weight of the steel sheet. If the content of sulfur is less than 0.04% by weight, the above effect is insufficient. On the contrary, if the content of sulfur exceeds 0.06% by weight, it may bind with iron and cause redness brittleness.

Soluble Aluminum (Sol.Al)

Soluble aluminum (Sol.Al) acts as a deoxidizer and contributes to the improvement of ductility by inhibiting the formation of oxides in steel, and contributes to fish scale prevention by acting as a hydrogen storage source by combining with nitrogen.

The aluminum is preferably added at 0.02 to 0.04% by weight of the total weight of the steel sheet. When the addition amount of aluminum is less than 0.02% by weight, the addition effect is insufficient. On the contrary, when the addition amount of aluminum exceeds 0.04% by weight, unwanted aluminum oxide remains in steel or on the steel surface, causing a decrease in ductility and a possibility of occurrence of surface defects.

Titanium (Ti)

Titanium (Ti) is combined with carbon, sulfur, nitrogen and the like to form a precipitate. Such titanium-based precipitates provide a large amount of pores in the steel sheet matrix structure, and the pores act as a hydrogen storage source to secure fish-scale characteristics.

The titanium is preferably added at 0.05 to 0.07% by weight of the total weight of the steel sheet. When the addition amount of titanium is less than 0.05% by weight, the addition effect is insufficient. On the contrary, when the addition amount of titanium exceeds 0.07% by weight, the A3 transformation temperature is disadvantageous to the production of the steel sheet due to the increase in strength, and may cause surface defects of the steel sheet.

Calcium (Ca)

Calcium (Ca) serves to prevent the MnS inclusions from elongating and causing central segregation by envisioning the shape of the MnS inclusions.

The calcium is preferably added in 0.002 ~ 0.004% by weight of the total weight of the steel sheet. When the added amount of calcium is less than 0.002% by weight, the addition effect is insufficient. On the contrary, when the added amount of calcium exceeds 0.004% by weight, a large amount of CaO is formed to deteriorate formability.

Nitrogen (N)

Nitrogen (N) combines with titanium to form a precipitate to suppress the formation of fish scale during enamel firing.

However, when the nitrogen is excessively contained, the solid solution nitrogen may increase to reduce the impact characteristics and the formability of the steel sheet.

Therefore, in the present invention, the content of nitrogen was limited to more than 0% to 0.005% by weight of the total weight of the steel sheet.

Oxygen (O)

Oxygen (O) acts as an inevitable impurity in a general steel sheet, but in the steel sheet for enamel according to the present invention may combine with manganese to serve as a hydrogen storage source. However, when the oxygen is excessively contained, the generation of oxides in the steel is greatly increased, which may lower the deep drawing property of the steel sheet.

Therefore, in the present invention, the oxygen content is limited to more than 0% to 0.005% of the total weight of the steel sheet.

Enamel steel sheet according to the present invention having the alloy composition can exhibit a tensile strength (TS): 270 ~ 350MPa and elongation of 40% or more through the above-described alloy composition and the process control described later, showing excellent formability Can be.

In addition, in the case of the enamel steel sheet according to the present invention, by containing the appropriate amount of titanium, calcium, manganese, sulfur, etc., it is possible to suppress the occurrence of fish scale during enamel firing after coating the enamel paint, high adhesion to the enamel paint Can be represented.

Method of manufacturing steel sheet for enamel

Hereinafter, a method for manufacturing an enameled cold rolled steel sheet according to the present invention having the alloy composition will be described.

1 is a flow chart showing a method for producing a cold rolled steel sheet for enamel according to an embodiment of the present invention.

Referring to FIG. 1, the method for manufacturing an enameled cold rolled steel sheet includes a cold rolling step S110, an annealing step S120, a cooling step S130, and an overage treatment step S140.

In the cold rolling step (S110) by weight%, carbon (C): more than 0% to 0.003% or less, silicon (Si): more than 0% to 0.03% or less, manganese (Mn): 0.9 ~ 1.1%, phosphorus (P ): More than 0% to 0.02% or less, sulfur (S): 0.04% to 0.06%, soluble aluminum (Sol.Al): 0.02% to 0.04%, titanium (Ti): 0.05 to 0.07%, calcium (Ca): A steel sheet comprising 0.002 to 0.004%, nitrogen (N): more than 0% to 0.005% or less, and oxygen (O): more than 0% to 0.005% or less, and cold rolled a steel sheet composed of remaining iron (Fe) and unavoidable impurities.

The hot rolled steel sheet to be cold rolled can be manufactured from a steel slab through a conventional hot rolling process, that is, reheating the slab performed at about 1000 to 1300 ° C., hot rolling at a temperature of Ar 3 or higher, and winding at about 400 to 700 ° C. have.

A pickling process may be included prior to cold rolling.

Cold rolling is preferably performed at a reduction ratio of 60% or more, and more preferably at a reduction ratio of 70 to 80%. When the rolling reduction rate of the cold rolling is less than 60%, it may be difficult to secure deep drawing property.

Next, in the annealing treatment step (S120), in order to adjust the strength and elongation of the enamel steel sheet, the cold rolled steel sheet is annealed at 840 ~ 860 ℃.

The annealing treatment step (S120) may include a direct heating step (S121), a temperature raising step (S122), and a heat holding step (S123), as in the example illustrated in FIG. 3.

First, in the direct heating step (S121), the cold rolled steel sheet is directly heated to 630 to 730 ° C. in a direct fired furnace. When the direct heating temperature is less than 630 ° C., the temperature increase time after the direct heating is long, which is not preferable in terms of productivity. On the contrary, when the direct heating temperature is higher than 730 ° C., the elongation of the cold rolled steel sheet to be manufactured may be decreased, thereby lowering the deep drawing property.

On the other hand, the direct heating is preferably carried out at an average temperature increase rate of 15 ~ 40 ℃ / sec. If the average heating rate applied during direct heating is less than 15 ° C./sec, the direct heating time is long, which is not preferable in terms of productivity. On the contrary, when the average temperature increase rate exceeds 40 ° C / sec, direct heating control may become difficult.

Next, in the temperature raising step (S122), the steel plate heated by direct heating is heated to 840 to 860 ° C at an average temperature increase rate slower than the average temperature increase rate during the direct heating.

At this time, it is preferable that temperature rising is performed at the average temperature increase rate of 5 degrees C / sec or less. When the average elevated temperature in the temperature increase step (S120) exceeds 5 ° C / sec, it may be difficult to accurately control the annealing temperature.

Next, in the heat-retaining step (S123), the elevated temperature steel plate is maintained at 840 to 860 ° C to perform annealing.

It is preferable that heating holding temperature is 840-860 degreeC. When the annealing treatment temperature is less than 840 ° C., it is difficult to secure a sufficient elongation, so that the deep drawing characteristics of the steel sheet manufactured may be lowered. On the contrary, even if the annealing treatment temperature exceeds 860 ° C, the effect of increasing the elongation is not large.

Heat retention can be carried out for 50 to 150 seconds. If the heat holding time is less than 50 seconds, it is difficult to secure a sufficient austenite phase. On the contrary, when the heat holding time exceeds 150 seconds, the strength of the steel sheet produced due to excessive austenite phase generation is excessively increased, and the moldability may be reduced.

Next, in the cooling step (S130), the annealing steel sheet is cooled to 420 ~ 440 ℃.

The cooling may include a first cooling step S131 and a second cooling step S132, as shown in the example illustrated in FIG. 3. In the primary cooling step (S131), the annealing steel sheet is first cooled to 610 to 650 ° C. at a first average cooling rate. In the second cooling step (S132), the first cooled steel sheet is secondarily cooled to 530 to 420 ° C. at a second average cooling rate slower than the first average cooling rate. When the secondary cooling end temperature exceeds 440 ° C, air cooling may be performed up to 420 ~ 440 ° C or cooling may be performed at an average cooling rate lower than the secondary cooling. By making the first average cooling rate of the primary cooling faster than the second average cooling rate of the secondary cooling, grain growth upon cooling after the annealing treatment can be suppressed.

At this time, it is preferable that the primary cooling is performed by the gas jet method, and the secondary cooling is performed by the roll quenching method. This is because the gas jet method has a higher cooling capacity than the roll quench method. However, this is only one example, and the primary cooling and the secondary cooling may be performed in various ways such as water cooling.

When the primary cooling is performed by the gas jet method and the secondary cooling is performed by the roll quenching method, the first average cooling rate applied to the primary cooling is 7 to 15 ° C / sec, and the first cooling rate is applied to the secondary cooling. 2 Average cooling rate may be 2 ~ 7 ℃ / sec.

Next, in the overage treatment step (S140), the cooled steel sheet is overaged.

It is preferable that overaging treatment is performed at 420-440 degreeC. If the overage treatment temperature exceeds 440 ° C, the strength may be too low. On the other hand, if the overaging temperature is less than 390 ℃, it may be difficult to secure a high elongation.

The overaging treatment can be performed for 100 to 200 seconds. If the overage treatment time is less than 100 seconds, it may be difficult to secure a high elongation. Conversely, even if the overaging treatment time exceeds 200 seconds, only productivity can be lowered without further effect improvement.

Example

Hereinafter, the configuration and operation of the present invention through the preferred embodiment of the present invention will be described in more detail. 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.

Details that are not described herein will be omitted since those skilled in the art can sufficiently infer technically.

1. Manufacture of steel sheet

After pickling the hot rolled steel sheet having the composition shown in Table 1, it was cold rolled at a 75% reduction rate to a steel sheet thickness of 0.65 mm, and then heat-treated under the conditions shown in Table 2.

[Table 1] (unit:% by weight)

[Table 2]

2. Evaluation of mechanical properties

Tensile strength (TS) and yield strength (YS) were measured by the tensile test based on JIS No. 5 test piece.

Whether or not fish scale was applied was coated with an enamel coating, dried, and then the enamel was calcined in a kiln maintained at 850 ° C. for 3 minutes, and then visually observed after 72 hours.

Table 3 shows the tensile test results and fish scale evaluation results of specimens 1-2.

[Table 3]

Referring to Table 3, all of the steel sheet specimens for enamel according to the embodiment of the present invention satisfied the tensile strength of 270 ~ 350MPa and elongation of 40% or more. Fish scale was not generated during enamel firing and showed very good adhesion.

According to the above results, in the case of cold rolled steel sheet for enamel according to the present invention, in addition to the target fish-fish scale characteristics through the process control capable of exhibiting high elongation with an alloy component having adequate hydrogen storage capability and an appropriate tensile strength, Drawability and adhesiveness can be satisfied.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments. Such changes and modifications are intended to fall within the scope of the present invention unless they depart from the scope of the present invention. Accordingly, the scope of the present invention should be determined by the following claims.

S110: cold rolling stage
S120: Annealing step
S121: Direct heating step
S122: temperature increase step
S123: heating step
S130: cooling step
S131: primary cooling stage
S132: secondary cooling stage
S140: overflow process step

Claims

(a) By weight%, carbon (C): more than 0% to 0.003% or less, silicon (Si): more than 0% to 0.03% or less, manganese (Mn): 0.9 to 1.1%, phosphorus (P): 0% More than 0.02% or less, sulfur (S): 0.04%-0.06%, soluble aluminum (Sol.Al): 0.02%-0.04%, titanium (Ti): 0.05-0.07%, calcium (Ca): 0.002-0.004% Cold rolling a steel sheet comprising nitrogen (N): more than 0% and less than 0.005% and oxygen (O): more than 0% and less than 0.005% and consisting of remaining iron (Fe) and unavoidable impurities;
(b) annealing the cold rolled steel sheet at 840 to 860 ° C;
(e) cooling the annealed steel sheet to 420-440 ° C .; And
(f) the step of overaging the cooled steel sheet at 420 ~ 440 ℃; enameled cold rolled steel sheet manufacturing method comprising a.

The method of claim 1,
In the step (a), the cold rolling is
Enameled cold rolled steel sheet manufacturing method characterized in that carried out at a rolling reduction of 60% or more.

The method of claim 1,
The annealing treatment of step (b)
(b1) direct heating the cold rolled steel sheet to 630 to 730 ° C. in a direct fired furnace;
(b2) heating the direct heated steel sheet to an average temperature increase rate slower than the average temperature increase rate at the time of direct heating, at 840 to 860 ° C .;
(b3) a method for producing cold rolled steel sheet for enamel, characterized in that it comprises the step of maintaining the heated steel sheet at 840 ~ 860 ℃.

The method of claim 3,
In the step (b1), the direct heating is
Method for producing a cold rolled steel sheet for enamel, characterized in that carried out at an average temperature increase rate of 15 ~ 40 ℃ / sec.

5. The method of claim 4,
In the step (b2), the temperature increase is
Method for producing a cold rolled steel sheet for enamel, characterized in that carried out at an average temperature increase rate of 5 ℃ / sec or less.

The method of claim 3,
In the step (b3), the heating maintenance
Enameled cold rolled steel sheet manufacturing method characterized in that carried out for 50 to 150 seconds.

The method of claim 1,
The step (c)
(c1) first cooling the annealed steel sheet to 610 to 650 ° C. at a first average cooling rate;
(c2) enameled cold rolled steel sheet manufacturing method comprising the step of second cooling the first cooled steel sheet to a second average cooling rate slower than the first average cooling rate to 530 ~ 420 ℃.

The method of claim 7, wherein
The primary cooling is performed by a gas jet method,
The secondary cooling is enameled cold rolled steel sheet manufacturing method characterized in that the roll quenching (Roll Quenching) method.

The method of claim 8,
The first average cooling rate is 7 ~ 15 ℃ / sec,
The second average cooling rate is enameled cold rolled steel sheet manufacturing method characterized in that 2 ~ 7 ℃ / sec.

The method of claim 1,
In the step (d), the overaging treatment
Enameled cold rolled steel sheet manufacturing method characterized in that carried out for 100 to 200 seconds.

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