KR100345703B1 - A method of manufacturing high strength steel with good for mability for enamel application - Google Patents

Abstract

PURPOSE: A method for manufacturing high strength enamel steel sheet with superior formability and resistance against fish scale is provided. CONSTITUTION: The method includes the steps of reheating an aluminum killed steel comprising 0.01 wt.% or less of C, 0.3 wt.% or less of Mn, S 0.02 wt.% to 0.04 wt.%, P 0.05 wt.% to 0.10 wt.%, Ti 0.04 to 0.10 wt.%, 0.005 wt.% or less of N, a balance of Fe and incidental impurities, wherein the atomic ratio of Ti/(C+N+S) is greater than 1; hot rolling the reheated aluminum killed steel, wherein finish hot rolling temperature is maintained higher than Ar3 transformation temperature; coiling the steel sheet; cold rolling the steel sheet at a reduction ratio of 50 to 85 %; and continuous annealing the cold rolled steel sheet at a temperature higher than recrystallization temperature.

Description

A method of manufacturing high strength steel with good for mability for enamel application}

The present invention relates to a method for manufacturing an enameled cold rolled steel sheet used as a part of a home appliance or a building exterior material, and more particularly, has excellent fish scale and moldability, and particularly high strength after enameling. The present invention relates to a method for producing an enameled cold rolled steel sheet.

Conventional enamel steel sheets are steel grades developed to prevent fish scale defects known as the most fatal defects in enamel products. Fishcale means that hydrogen, which has been dissolved in steel before firing or at high temperature firing in the enamel product manufacturing process, must be released to the surface again. At this time, the enamel layer on the surface of the steel sheet is already hardened, so the enamel layer is hydrogen pressure. Refers to a defect that breaks into fish scales. In order to prevent fish scale defects, it is necessary to make a position to adsorb hydrogen inside the steel, and inclusions or precipitates are known to have such a function. Until now, steel grades proposed to prevent fish scale have added titanium, boron, nitrogen and oxygen to precipitate or oxides or precipitates known as hydrogen storage sources such as titanium sulfide, titanium nitride, titanium carbide, boron nitride or manganese oxide. Most of these are titanium-added steel, boron-added steel, high-oxygen steel and decarbonized steel which have decarbonized steel and high carbon steel to secure fish scale resistance.

These conventional steels have a low strength after enamel processing at high temperature, so the enamel product is easily deformed even with a small impact, and there is a problem that the enamel layer is dropped. In particular, in applications requiring strength, such as building exterior materials or bathtubs, there is a problem that the weight of the product is very heavy using a thick steel sheet to solve the problem of strength.

Accordingly, the present invention has been made to solve the above-mentioned conventional problem, while improving the strength of the enamel product after enamel processing, satisfies the enamel characteristics such as fish scale resistance, sagging resistance, surface defects and enamel adhesion In addition, an object of the present invention is to provide a method for producing a high strength enamel steel sheet excellent in molding processability.

The manufacturing method of the present invention for achieving the above object, in weight%, C: 0.01% or less, Mn: 0.3% or less, S: 0.02% -0.04%, P: 0.05% -0.10%, Ti: 0.04-0.10 %, N: 0.005% or less, and the atomic ratio of Ti / (C + N + S) satisfies 1.0 or more, and reheats the aluminum-kilted steel made of the remaining Fe and other unavoidable impurities to change the finish rolling temperature to Ar ₃ transformation point. After hot rolling under the above conditions, it is wound by a usual method, followed by cold rolling at a 50-85% reduction rate, and then continuous annealing at a temperature above the recrystallization temperature.

Hereinafter, the present invention will be described in detail.

When C is added at 0.01% or more, the amount of solid solution in steel is high so that it hinders the development of the aggregate structure during annealing or the amount of fine titanium carbide is precipitated.

The Mn is added to prevent hot shortness due to solid solution of precipitated sulfur in steel as manganese sulfide, but in the case of the present invention steel, titanium is added to precipitate titanium sulfide to completely remove sulfur remaining in solid solution. This eliminates the need to add manganese separately. In addition, manganese has the effect of increasing strength when present in solid solution in the steel, but the reinforcing effect is not so large that the formability impairs the formability and is limited to 0.3% or less.

S is generally known as an element that inhibits the physical properties of steel, but in the case of the present invention steel, titanium sulfide is added to secure fish scale resistance. Less than 0.02% of sulfur content, the amount and size of titanium sulfide is not enough to secure fish scale resistance. If it is added more than 0.04%, surface defects may occur due to redness and brittleness, and moldability decreases, so it is added at 0.02-0.04%. It is desirable to.

P is known as an element in the cold rolled steel sheet that effectively increases the strength without significantly reducing the formability. In the present invention, it is added in the range of 0.05-0.1% because the solid solution strengthening effect of phosphorus in the specific addition range and the effect of improving fish scale resistance obtained by reacting with titanium to form Ti (Fe, P) precipitates simultaneously appear. In other words, if the P content is less than 0.05%, deformation easily occurs due to low yield strength after firing.In the case of 0.1% or more, the strength is high after enameling, but the yield strength of the material before processing increases and the value of plastic anisotropy index is low, so that the molding processability Since it is lowered, it is preferable to add it at 0.05-0.1%.

The Ti improves the formability of the steel sheet by precipitating and removing solid carbon and nitrogen in the steel with titanium carbon and nitride, and precipitates titanium sulfide (TiS) and Ti (Fe, P) precipitates to improve fish scale resistance. As an element, when the added amount is 0.04% or less, the amount of titanium precipitates precipitated is small so that the fish scale resistance cannot be secured. When 0.10% or more is added, the amount of titanium precipitates is large, the fish scale resistance can be secured, but enamel adhesion It is preferable to add it at 0.04-0.10% because it lowers.

The N is combined with titanium to precipitate as titanium nitride to improve the fish scale resistance, but in the present invention, titanium nitride and titanium (Fe, P) precipitates to ensure sufficient enough fish scale resistance, titanium nitride It is not necessary to enhance fish scale resistance by. Therefore, in the case of N, if it remains in solid solution in the steel, it is desirable to reduce the moldability of the steel sheet as low as possible. In the case of the present invention, when the content of 0.005% or more is contained, the moldability is greatly lowered, so it is 0.005% or less.

The atomic ratio of Ti / (C + N + S) is limited to 1 or more to completely remove solid solution carbon or nitrogen that inhibits the development of an aggregate structure favorable for workability during precipitation and annealing, thereby improving moldability. If the atomic ratio of Ti / (C + N + S) is 1.0 or less, since the solid solution carbon and nitrogen remain and moldability becomes low, it is made into 1.0 or more.

Re-heating the aluminum-kilted steel, which is formed as described above, is hot rolled. In this case, the hot rolling is preferably performed at a finish rolling temperature of at least Ar ₃ transformation. The reason for this is that hot rolling at an Ar ₃ transformation temperature lowers workability due to the formation of rolled grains.

As described above, the hot rolling is carried out and wound in a usual manner, followed by cold rolling. The cold rolling reduction is preferably limited to 50-85%. It is produced during hot rolling, and the fine cracks are generated in the process of breaking or stretching in the cold rolling process, and these gaps remain mostly after continuous annealing and serve as an important hydrogen storage source. Is needed. In other words, if the cold reduction rate is less than 50%, the formation area of fine gaps is small, and the hydrogen absorption ability is low, and thus the probability of fish scale generation is high. This is because the area of the gap is rather reduced and the hydrogen storage capacity is drastically reduced.

Cold rolling as above and then continuous annealing in the usual manner. That is, it is good to continuously anneal above reheating temperature.

Hereinafter, the present invention will be described in detail through examples.

EXAMPLE

The steel ingot prepared to be dissolved so as to satisfy the composition shown in Table 1 was maintained for 1 hour in a 1250 ℃ heating furnace and then hot rolled. At this time, the hot finishing rolling temperature was 900 ℃, winding temperature was 650 ℃, the final thickness was 3.2mm. The hot rolled specimens were pickled to remove the oxide film on the surface and then cold rolled at a 70% reduction rate. At this time, the comparative steel 6 for investigating the effect of cold reduction ratio made cold reduction rate 40%.

Cold rolled specimens were processed into enameled specimens for enameling and tensile specimens for mechanical properties, followed by continuous annealing. The enameled specimens were cut to 70mm × 150mm, and the tensile specimens were processed into standard specimens according to ASTM (ASTM E-8 standard). Continuous annealing was performed at an annealing temperature of 830 ° C.

As described above, the tensile specimens of the specimens in which continuous annealing was completed were measured using a tensile tester (INSTRON, Model 6025) to measure yield strength, tensile strength, elongation, and r, and the results are shown in Table 2 below.

The enameled specimens were completely degreased and then acid-treated for 5 minutes at 70 ° C. and 10% sulfuric acid solution, washed with warm water, and then maintained at 85 ° C., 3.6 g / l sodium carbonate + 1.2 g / l. It was neutralized by dipping in borax aqueous solution for 5 minutes. After the pretreatment was completed, the glaze (Haewang ceramics, glaze firing temperature of 830 ° C for the first enamel) was applied and dried at 200 ° C for 10 minutes to completely remove moisture. The dried specimens were kept at 830 ° C. for 7 minutes and subjected to calcination, followed by enameling of air cooling. At this time, the atmosphere condition of the kiln is a harsh condition where fish scale defects are most likely to occur at a dew point temperature of 30 ° C. The enameled specimens were kept in a 200 ° C. holding furnace for 20 hours to visually examine the number of fish scale defects generated after the fish scale acceleration treatment, and the results are shown in Table 2 below.

At this time, the enamel adhesion evaluation was measured by using the adhesion test equipment (test equipment according to ASTM C313-78 standard). Meanwhile, in order to measure the strength of the steel sheet after enameling, the yield strength was measured after heat treatment corresponding to the enameling condition of maintaining the continuously annealed steel sheet at 830 ° C for 7 minutes.

Steel grade Chemical composition (% by weight) Ti / (C + N + S) atomic ratio Cold rolling reduction (%) C Mn P S Ti N Invention steel One 0.0015 0.13 0.075 0.030 0.072 0.0031 1.17 70 2 0.0038 0.13 0.068 0.028 0.074 0.0032 1.09 3 0.0023 0.15 0.080 0.031 0.071 0.0024 1.11 4 0.0014 0.20 0.078 0.032 0.088 0.0015 1.50 Comparative steel One 0.0042 0.28 0.014 0.012 0.052 0.0030 1.15 2 0.0032 0.75 0.072 0.043 0.095 0.0045 1.02 3 0.0023 0.26 0.073 0.012 0.052 0.0030 1.39 4 0.0025 0.14 0.072 0.038 0.032 0.0021 0.43 5 0.0133 0.24 0.082 0.032 0.112 0.0022 1.03 6 0.0038 0.13 0.068 0.028 0.074 0.0032 1.09 40 Conventional Steel One 0.0039 0.15 0.010 0.013 0.122 0.0075 2.39 70

Psalter Enamel Mechanical properties Yield strength after enamel processing (kgf / mm ² ) Remarks Fish Scale Outbreaks () Enamel thickness (μm) Cohesion Index (%) Yield strength (kgf / mm ² ) Tensile strength (kgf / mm ² ) Elongation (%) r value Invention One 0 123 99 24.5 36.3 44.6 1.92 18.5 Inventive Steel 1 2 0 103 100 23.3 35.2 46.2 1.89 17.4 Inventive Steel 2 3 0 131 98 25.3 36.9 45.3 1.88 20.4 Invention Steel 3 4 0 118 97 23.8 35.9 46.9 2.08 17.9 Inventive Steel 4 Comparative material One 0 130 97 18.8 29.4 49.3 1.92 9.6 Comparative Steel 1 2 0 136 94 21.9 38.9 42.0 1.65 20.8 Comparative Steel 2 3 42 104 93 20.6 36.3 46.8 1.87 17.5 Comparative Steel 3 4 38 120 99 19.9 35.1 45.0 1.42 16.4 Comparative Steel 4 5 0 99 85 25.6 43.7 39.4 1.49 24.8 Comparative Steel 5 6 55 112 99 16.4 34.9 47.2 1.62 12.8 Comparative Steel 6 Conventional One 5 129 81 17.2 30.5 47.3 1.98 9.8 Conventional Steel

As shown in Tables 1 and 2, the invention material (1-4) has a yield strength of more than 17kgf / mm ² after enamel processing, significantly higher than the 9.8kgf / mm ² of the conventional material, the strength of the plastic deformation is significantly higher Edo enamel product was found to be much less damage. In addition, the enamel adhesion index was more than 97%, and the enamel adhesion was excellent. Even in the harshest conditions, there was no occurrence of fish scale defect, which is a fatal defect of the enamel. In the case of the r value indicating the workability of the base steel sheet, almost all of the enameled products could be easily processed to 1.8 or more.

On the contrary, the comparative material (1) has excellent mechanical and enamel properties, but can be damaged even with a small impact of yield strength of 9.6 kgf / mm ² after enameling. This is because the phosphorus content was much less than the range of the present invention steel. In addition, the comparative material (2) has a high manganese content higher than the scope of the present invention, the strength is high after enamel processing, but the r value, which is a workability index is very low, so the crack is very likely to occur when processing a product having a complex shape. In addition, in the case of the comparative material (3), the sulfur content is lower than the range of the present invention generated a fish scale which is a fatal defect of the enamel product. In addition, the comparative material 4 has a titanium content is lower than the scope of the present invention, the precipitation amount of titanium precipitates less, the fish scale was generated, Ti / (C + N + S) atomic ratio of 0.43 is very low than the scope of the present invention r The value is very low, 1.42. In addition, in the case of the comparative material 5, the carbon content is higher than the range of the present invention, the r value is very low, and the titanium content is high, so that the enamel adhesion index is very low, which is 85%. In addition, in the case of the comparative material 6, the cold reduction rate is lower than the range of the present invention, a fish scale defect occurred and the r value was also low.

In addition, the conventional material (1) is an example of conventional steel, but the r value is rather high, but the yield strength after enamel processing is much lower than that of the present invention steel, which may damage the product even with a small impact. In addition, the enamel adhesion was also lower than the inventive steel. In addition, the fish scale is generated in the harsh atmosphere, the fish scale may occur when the enamel treatment in a high humidity environment such as summer.

As described above, according to the present invention, it is possible to provide a method for producing an enameled cold rolled steel sheet which is excellent in fish scale and moldability, and particularly high in strength after enameling. It is used as parts of household appliances such as gas stoves and microwave ovens, or as steel sheets for building exterior materials, which can reduce the weight of products.

Claims (1)

In the manufacturing method of the cold rolled steel sheet for enamel, By weight%, C: 0.01% or less, Mn: 0.3% or less, S: 0.02% -0.04%, P: 0.05% -0.10%, Ti: 0.04-0.10%, N: 0.005% or less and Ti / (C + N + S) satisfies 1.0 or more, and reheats the aluminum-kilted steel made of the remaining Fe and other unavoidable impurities and hot-rolls under the condition that the finish rolling temperature is above the Ar ₃ transformation point. After winding, it is cold-rolled at a 50-85% reduction rate and then continuously annealed at a temperature above the recrystallization temperature.