KR101294447B1

KR101294447B1 - Method of manufacturing cold-rolled steel sheet for enamel with excellent formability

Info

Publication number: KR101294447B1
Application number: KR1020130032395A
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 excellent moldability is provided to obtain a hydrogen occlusion site inside the steel sheet through a thermal treatment process control after cold-rolling, thereby obtaining the fish scale resistant property. CONSTITUTION: A method for manufacturing a cold-rolled steel sheet for a porcelain enamel with excellent moldability includes the following steps of: directly firing the steel sheet inside a direct fired furnace up to a temperature of 630-730°C after cold-rolling the steel sheet (S110, S120); increasing the temperature of the direly fired steel sheet up to a temperature of 810-840°C at the average heating speed slower than the average heating speed when directly firing the steel sheet (S130); annealing the heated steel sheet by maintaining the steel sheet at a temperature of 810-840°C (S140); cooling the steel sheet up to a temperature of 420-440°C (S150); and over-aging processing the cooled steel sheet at a temperature of 420-440°C (S160). [Reference numerals] (AA) Start; (BB) Finish; (S110) Cold-rolling; (S120) Direct-fire heating; (S130) Temperature increasing; (S140) Annealing; (S150) Cooling; (S160) Over-aging processing

Description

Method for manufacturing enameled cold rolled steel sheet with excellent moldability {METHOD OF MANUFACTURING COLD-ROLLED STEEL SHEET FOR ENAMEL WITH EXCELLENT FORMABILITY}

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 formability and a method of manufacturing the same.

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 of a 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.

An object of the present invention is to provide an enameled cold rolled steel sheet excellent in formability, and a method of manufacturing the same, through alloying components 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): 0.05 to 0.15%, 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%, nitrogen (N): more than 0% to 0.005% or less and oxygen (O): more than 0% to 0.005% or less, the steel sheet consisting of the remaining iron (Fe) and unavoidable impurities Cold rolling; (b) direct heating the cold rolled steel sheet to 630 to 730 ° C. in a direct fire furnace; (c) heating the direct heated steel sheet to a temperature increase rate of 810 to 840 ° C. at a slower rate than the average temperature increase rate at the time of direct heating; (d) maintaining the heated steel sheet at 810˜840 ° C. to perform annealing; (e) cooling the annealed steel sheet to 420-440 ° C .; And (f) overaging the cooled steel sheet at 420 to 440 ° C.

In the step (a), the cold rolling may be carried out with a reduction ratio of 60% or more.

In the step (b), the direct heating may be carried out at an average temperature increase rate of 15 ~ 40 ℃ / sec. In addition, in the step (c), the temperature increase may be carried out at an average temperature increase rate of 5 ℃ / sec or less.

Also, in the step (d), the annealing treatment may be performed for 50 to 150 seconds.

In addition, the step (e) is the step of first cooling the annealed steel sheet to a first average cooling rate to 590 ~ 650 ℃, and the first cooled steel sheet, a second slower than the first average cooling rate It may include the step of secondary cooling to 530 ~ 420 ℃ at an average cooling rate. In this case, the primary cooling may be performed by a gas jet method, and the secondary cooling may be 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 (f), 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): 0.05 to 0.15%, 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%, nitrogen (N): more than 0% to 0.005% or less and oxygen (O): more than 0% to 0.005% or less, and the remaining iron (Fe) and inevitable impurities, tensile strength (TS): 270 ~ 350 MPa and elongation of 45% or more.

According to the method for manufacturing an enameled cold rolled steel sheet according to the present invention, the hydrogen storage site in the steel sheet can be secured by controlling the alloy components of carbon, manganese, aluminum, titanium, etc., and controlling the heat treatment after cold rolling. Excellent scale characteristics

In addition, in the case of the enameled cold rolled steel sheet produced by the above method, it can exhibit an elongation of 45% 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.
FIG. 2 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): 0.05 ~ 0.15%, 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%, nitrogen (N): Greater than 0% and up to 0.005% and oxygen (O): greater than 0% and up to 0.005%.

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 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 amount of carbon added exceeds 0.003% by weight, bubble defects may be caused on the enamel surface, and moldability may be reduced.

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 0.05 ~ 0.15% by weight of the total weight of the steel sheet. When the amount of manganese added is less than 0.05% by weight, the effect of adding manganese is insufficient. On the contrary, when the addition amount of manganese exceeds 0.15 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.

It is preferable that the said sulfur is contained in 0.04 to 0.06 weight% of the total weight of a steel plate. When the content of sulfur is less than 0.04% by weight, the above effect is insufficient. On the contrary, when the content of sulfur exceeds 0.06% by weight, it may bind with iron and cause rather red brittle 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, a problem of inhibiting moldability may occur.

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 cold rolled steel sheet for enamel according to the present invention may combine with manganese to serve as a hydrogen storage source. However, when oxygen is excessively contained, the generation of oxides in the steel can be greatly increased, thereby degrading the formability 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.

Cold rolled steel sheet for enamel according to the present invention having the alloy composition can exhibit a tensile strength (TS): 270 ~ 350MPa and elongation of 45% or more through the above-described alloy composition and the process control described later, excellent formability Can be represented.

In addition, in the case of enameled cold rolled steel sheet according to the present invention, it is possible to suppress the generation of fish scale during enamel firing after the coating of the enamel paint by the appropriate amount of titanium, manganese, sulfur and the like.

Method of manufacturing cold rolled 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 Figure 1, the enameled cold rolled steel sheet manufacturing method is shown in the cold rolling step (S110), direct heating step (S120), heating step (S130), annealing treatment step (S140), cooling step (S150) and overaging A processing step S160 is included.

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): 0.05 ~ 0.15%, 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%, nitrogen (N): 0% A steel sheet containing more than 0.005% or less and oxygen (O): more than 0% and 0.005% or less and consisting of the remaining iron (Fe) and unavoidable impurities is cold rolled.

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%. If the reduction ratio of cold rolling is less than 60%, it may be difficult to secure formability.

Next, in the direct heating step (S120), 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 exceeds 730 ° C., the elongation of the cold rolled steel sheet to be produced may decrease, thereby reducing the moldability characteristics.

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, the temperature raising step (S130) is heated to 810 ~ 840 ° C at an average temperature increase rate slower than the average temperature increase rate at the time of direct heating the steel plate heated by 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 annealing treatment step (S140), the elevated temperature steel sheet is maintained at 810 to 840 ° C. to perform annealing treatment.

It is preferable that annealing treatment temperature is 810-840 degreeC. When the annealing treatment temperature is less than 810 ° C., it is difficult to secure a sufficient elongation and the moldability of the steel sheet produced may be lowered. On the contrary, even if the annealing treatment temperature exceeds 840 ° C, the effect of increasing the elongation is not large.

Annealing treatment may be performed for 50 to 150 seconds. If the annealing treatment time is less than 50 seconds, it is difficult to secure a sufficient austenite phase. On the contrary, when the annealing treatment time exceeds 150 seconds, the strength of the steel sheet produced due to excessive austenite phase generation excessively rises, and the moldability may decrease.

Next, in the cooling step (S150), the annealing steel sheet is cooled to 420 ~ 440 ℃.

Cooling may include primary cooling and secondary cooling, as in the example shown in FIG. 2. In the primary cooling step S151, the annealing steel sheet is first cooled to 590 to 650 ° C at a first average cooling rate. In the secondary cooling step (S152), 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 (S160), 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 cold rolled 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 to 3.

[Table 3]

Referring to Table 3, both of the cold rolled steel sheet according to an embodiment of the present invention satisfied the tensile strength of 270 ~ 350MPa and elongation of 45% or more, the fish scale did not occur during enamel firing.

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

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: Direct heating step
S130: temperature increase step
S140: Annealing Treatment Step
S150: cooling step
S160: Overaging 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): 0.05 to 0.15%, phosphorus (P): 0% More than 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%, nitrogen (N): more than 0% to 0.005% Cold rolling of a steel sheet comprising less than 0% and less than 0.005% of oxygen (O), and consisting of remaining iron (Fe) and unavoidable impurities;
(b) direct heating the cold rolled steel sheet to 630 to 730 ° C. in a direct fire furnace;
(c) heating the direct heated steel sheet to a temperature increase rate of 810 to 840 ° C. at a slower rate than the average temperature increase rate at the time of direct heating;
(d) maintaining the heated steel sheet at 810˜840 ° C. to perform annealing;
(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,
In the step (b), 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.

The method of claim 3,
In the step (c), 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 1,
In the step (d), the annealing treatment
Enameled cold rolled steel sheet manufacturing method characterized in that carried out for 50 to 150 seconds.

The method of claim 1,
The step (e)
First cooling the annealed steel sheet to a 590 to 650 ° C. at a first average cooling rate;
And cooling the first cooled steel sheet to a second average cooling rate lower than the first average cooling rate to 530 to 420 ° C. for a second enameled cold rolled steel sheet.

The method according to claim 6,
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.

8. The method of claim 7,
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 (f), the overaging treatment
Enameled cold rolled steel sheet manufacturing method characterized in that carried out for 100 to 200 seconds.

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