JP2002523633A - Method for producing high adhesion enamel coated steel sheet having excellent formability - Google Patents

Abstract

(57) Abstract: A method for producing a cold rolled steel sheet that is enamel coated and used as a raw steel sheet for enamel coated products, such as components of bathtubs and appliances. Optimum contents of alloying elements such as S, P and N are realized, thereby satisfying enamel layer adhesion, fish scale resistance and formability. First, in order to form an aluminum killed steel, 0.004% by weight or less of C, 0.3% by weight or less of Mn, 0.02 to 0.05% by weight of S, and 0.005 to 0.03% by weight of P, 0.08 to 0.15% by weight of Ti, 0.004% by weight or less of N, 0.04% by weight or more of (Ti * = Ti- (48/32) S- (48/14) N- Prepare a steel composed of excess Ti * (defined in (48/12) C) and the balance iron and other unavoidable impurities. The aluminum-killed steel is reheated, and then hot-rolled at a finish rolling temperature of at least the Ar ₃ transformation point. Next, winding is performed in a usual manner, and then cold rolling is performed at a reduction of 50 to 85%. Continuous annealing is performed at or above the recrystallization temperature.

Description

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method for producing a cold-rolled steel sheet used as a raw steel sheet for an enamel-coated product such as a bathtub or a component of an electric appliance. More specifically, the present invention relates to a method for producing a cold-rolled steel sheet in which properties such as enamel adhesion, fish scale resistance, and formability are secured to a certain level or higher.

Generally, an enamel-coated cold-rolled steel sheet is pressed into various shapes, then an enamel coating is applied to its surface, followed by firing at a high temperature, thereby completing the production of the enamel-coated steel product. . Important properties required of the enamel-coated cold-rolled steel sheet are fish scale resistance, formability, and adhesion between the raw steel sheet and the enamel layer.

[0003] The enamel layer adhesion depends on the added element and the surface roughness. Fish scale refers to defects formed on the surface of the enamel coated product. That is, when producing an enamel-coated product, hydrogen dissolved in steel is released after or during cooling. Due to the hydrogen pressure, the hardened enamel layer breaks down, forming defects similar to fish scales. In order to prevent the formation of fish scale, it is necessary to provide room for storing hydrogen in steel. This is greatly affected by the type and amount of precipitates or non-metallic inclusions present in the steel sheet.

In order to prevent the formation of fish scale, Ti, B, N, or O ₂ has been added to steels that have been proposed so far, and Ti sulfide, Ti nitride, which is known to be a hydrogen absorption source, is known. , Ti carbide, B nitride, Mn oxide, and the like. In this way, precipitates or oxides are deposited, or the high carbon steel is decarburized to ensure fish scale resistance. Therefore, the steel proposed so far is
Most are Ti-added steel, B-added steel, high oxygen steel, and decarburized steel.

[0005] On the other hand, the enamel-coated cold-rolled steel sheet must be pressed into a desired shape before enamel coating, and thus formability is also very important. JP-A-63-500 discloses a method for producing an enamel-coated steel sheet. In this steel, the composition is: C up to 0.005% by weight; Si up to 0.03% by weight; Mn up to 0.50% by weight; P up to 0.02% by weight;
0.005 to 0.01% by weight of N; 0.15% by weight or less of Ti [Ti
> (48 / 12C + 48 / 14N + 48 / 32S)]; 0.08% by weight or less of C
u: one or more elements selected from the group consisting of As, Sb, and Bi at 0.003 to 0.03% by weight of the total amount of the additives; and the balance contains iron and other unavoidable impurities. However, since this steel has a very high N content, the TiN precipitates are exposed to the steel sheet surface to form bubbles.

Meanwhile, Korean Patent Application No. 97-63270 discloses another method for manufacturing an enamel-coated steel sheet. In this method, the steel contains up to 0.01% by weight of C,
0.3 wt% or less Mn, 0.05-0.1 wt% P, 0.02-0.04 wt% S, 0.04-0.10 wt% Ti, 0.005 wt% The following N, Ti
The ratio of atoms of / (C + N + S) is 1.0 or more, and the balance is composed of iron and other unavoidable impurities. However, since this steel has a high P content, although the strength of the steel sheet is acceptable, the formability deteriorates.

As described above, the enamel-coated cold-rolled steel sheets that have been developed do not satisfy the enamel layer adhesion, fish scale resistance, and formability. Rather, there is the problem that certain characteristics must be sacrificed to ensure certain characteristics.

[0008] The present invention is intended to overcome the above deficiencies of the prior art. Accordingly, an object of the present invention is to provide an enamel-coated cold-rolled steel sheet in which an optimum content of alloying elements such as S, P, N, Ti, and effective Ti (Ti *) is realized. Yes, it satisfies the enamel layer adhesion, fish scale resistance, and moldability.

In order to achieve the above object, a method for producing an enamel-coated cold steel sheet according to the present invention comprises the steps of:
0.3 wt% or less Mn, 0.02-0.05 wt% S, 0.005-0.0
3 wt% P, 0.08-0.15 wt% Ti, 0.004 wt% or less N,
0.04% by weight or more of (Ti * = Ti- (48/32) S- (48/14) N-
Preparing a steel composed of excess Ti * (as defined in (48/12) C) and the balance iron and other unavoidable impurities, reheating the aluminum killed steel;
When the finish rolling temperature is equal to or higher than the Ar ₃ transformation point, hot rolling is performed, winding is performed in a usual manner, cold rolling is performed at a reduction rate of 50 to 85%, and the recrystallization temperature is higher. Including a step of performing continuous annealing.

[0010] A method for producing an enamel-coated cold steel sheet according to the present invention comprises the steps of forming an aluminum-killed steel by forming C of 0.004 wt% or less, Mn of 0.3 wt% or less,
. 02-0.05 wt% S, 0.005-0.03 wt% P, 0.08-0
. 15% by weight of Ti, 0.004% by weight or less of N, 0.04% by weight or more of (Ti
* = Preparing a steel composed of Ti- (48/32) S- (48/14) N- (48/12) C with excess Ti * and the balance being iron and other unavoidable impurities Re-heating the aluminum-killed steel, hot rolling at a finish rolling temperature of not less than the Ar ₃ transformation point, winding in a usual manner, 50-85.
%, And a step of performing continuous annealing at a recrystallization temperature or higher.

Hereinafter, the reasons for limiting the numerical values to the composition of the present invention will be described in more detail. If the C content is higher than 0.004%, the amount of dissolved carbon becomes excessive. Therefore, the development of texture during annealing is hindered, or the precipitation amount of fine Ti carbides becomes excessive. As a result, the crystal grains become fine and the formability deteriorates. Therefore, the C content must be limited to 0.004% or less.

In order to prevent thermal brittleness caused by the FeS film, Mn is added to precipitate S in the form of Mn sulfide as a solid solution. However, in the present invention, T
i is added to precipitate S in the form of Ti sulfide, thereby completely removing residual S. As a result, Mn does not need to be added separately. Furthermore, if Mn is present in the form of a solid solution, the strength of the steel increases, but the increase in strength is not so large, but rather deteriorates the formability. Therefore, Mn should preferably be limited to 0.3% or less.

In general, S is known as an element that deteriorates mechanical properties, but is added in the present invention to enhance fish scale resistance. S content is 0.02
%, The amount and size of Ti sulfide is insufficient, and as a result, fish scale resistance is not improved. If the S content is 0.05% or more, excess Ti
* Is too small to deteriorate the moldability. Therefore, the S content is preferably 0.02
Should be limited to ~ 0.05%. More preferably, it should be limited to 0.02 to 0.03% in order to ensure better moldability.

On the other hand, like S, P is an element that deteriorates the mechanical properties of steel. Therefore, its content should be as low as possible. However, in the present invention, P is T
It is added to improve fish scale resistance by reacting with i to form Ti (Fe, P) precipitates. If the P content is less than 0.005%, no Ti (Fe, P) precipitates are formed, and as a result, fish scale resistance is not improved. If the content is higher than 0.03%, the recrystallized grains become excessively fine due to the formation of fine Ti (Fe, P) precipitates, and consequently the formability deteriorates. Therefore, the P content should be limited to 0.005 to 0.03%.

On the other hand, Ti removes solid solution C and N in the form of Ti carbide and Ti nitride, thereby improving the formability of the raw steel sheet. Further, Ti is a Ti sulfide (Ti
By depositing S) and Ti (Fe, P) precipitates, fish scale resistance is improved. If its content is lower than 0.08%, it becomes impossible to improve fish scale resistance because Ti precipitates are precipitated in an excessively small amount. If the Ti content is higher than 0.15%, the amount of Ti precipitates increases and the fish scale resistance is improved, but the adhesion of the enamel layer is deteriorated due to the higher Ti *. Therefore, the Ti content should be limited to 0.08-0.15%.

On the other hand, N reacts with Ti and precipitates in the form of Ti nitride, thereby improving fish scale resistance. However, when the Ti nitride is exposed to the steel sheet surface, it is oxidized to generate N gas, which causes surface defects. Therefore,
The N content should be as low as possible. Thus, the content is 0.004
%, The Ti nitride is precipitated in small amounts and as a result is very unlikely to cause surface defects. Therefore, the N content should be less than 0.004%.

The excess titanium is Ti * = Ti− (48/32) S− (48/14) N− (48
/ 12) Defined by C. Ti reacts with N, S, and C to form TiN, TiS, and TiC. Under the assumption that all the added elements are precipitated, the excess Ti * corresponds to the residual Ti in solid solution. However, since not all of the added elements actually react completely, if the amount of excess Ti * increases, the completely dissolved residual C and N precipitate.

Further, Ti (Fe, P) precipitates are precipitated to improve fish scale resistance. If the excess Ti * is higher than 0.04%, since there is almost no solid solution of residual C and N, formability having an r value of 2.0 or more is ensured. If the r value is 2.0 or more, a complicated shape can be formed. In addition, an appropriate amount of Ti (Fe
, P) Sufficient fish scale resistance is ensured if there is a precipitate. Therefore,
The lower limit of excess Ti * should be 0.04%. In particular, 0.04% of excess Ti *
If it is lower, the fish scale resistance is reduced because no Ti (Fe, P) precipitate is formed.

Hereinafter, the production conditions of the steel of the present invention will be described. The aluminum killed steel having the above composition is reheated and hot rolled. Under these conditions, the finishing hot rolling temperature should be above the Ar ₃ transformation point. This is because when hot rolling is performed at a temperature lower than the Ar ₃ transformation point, rolled crystal grains are formed, thereby deteriorating the formability.

After hot rolling, winding is carried out in the usual way, and then cold rolling is carried out at a rolling reduction limited to 50-85%. The precipitates formed during hot rolling are broken or stretched during cold rolling. Small cavities are formed during this step, and these cavities remain in their original shape even after continuous annealing and serve as a hydrogen absorbing source.

In this regard, the rolling reduction of cold rolling needs to be controlled. That is, if the cold rolling reduction is lower than 50%, all the small holes become excessively small, and the hydrogen absorption is reduced, which may cause fish scale. On the other hand, if the reduction is higher than 85%, the small holes are crushed due to the high reduction. Thus, overall the void space is rather reduced, dramatically reducing hydrogen absorption capacity.

After cold rolling, the steel is subjected to continuous annealing in the usual way. That is, continuous annealing is performed at a temperature equal to or higher than the recrystallization temperature.

Here, the present invention will be described based on examples. (Example) Ingots having the composition shown in Table 1 were prepared, and after maintaining them at 1250 ° C for 1 hour in a furnace, hot rolling was performed. The hot rolling finish temperature was 900 ° C, and then the winding was performed at 650 ° C. Final thickness was 3.2 mm. The hot-rolled test piece was pickled to remove the surface oxide film. Next, cold rolling was performed at a rolling reduction of 70%.

The cold-rolled specimen was further processed into an enamel specimen and a tensile specimen.
Subsequently, these two types of test pieces were subjected to continuous annealing. The enamel specimen was cut into a size of 70 mm x 150 mm, and the tensile specimen was processed based on ASTM E-8 standard. Continuous annealing is performed at 830 ° C for 30 minutes.
Made for a second.

An Instron tensile tester (Model 6025) was used to test the tensile test pieces, and the yield strength, tensile strength, elongation, and r value were measured in this manner. The measurement results are shown in Table 2 below.

The enamel test piece was completely immersed in a sulfuric acid solution (10%, 70 ° C.) for 5 minutes, after the oil and fat were completely removed. Next, hot water washing was performed, and neutralization was performed by immersing the test piece in an aqueous solution of 3.6 g / L of sodium carbide and 1.2 g / L of sodium borate at 85 ° C. for 5 minutes.

[0027] Thus, the pretreatment was completed, and then the enamel was applied to the test specimen. Drying was then performed at 200 ° C. for 10 minutes, which completely removed moisture. After drying, the test specimen was left at 830 ° C. for 7 minutes, then fired, and then air-cooled, completing the enamel coating process.

Under these circumstances, the atmosphere of the firing furnace had a dew point temperature of 30 ° C. This creates a harsh environment where fish scales are most likely to occur. After enamel coating, the specimens were left in the oven at 200 ° C. for 20 hours to promote fish scale formation, and then visually inspected for the number of fish scale defects, the results of which are shown in Table 2 below. . Regarding the evaluation of the enamel layer adhesion, an adhesion index (adherence ind)
ex) was measured using an adhesion tester (based on ASTM C313-78).

[0029]

[Table 1]

[0030]

[Table 2]

As shown in Tables 1 and 2, Material 1-4 of the present invention shows an r value of 2.0 or more, and ensures high moldability. Furthermore, even under very severe conditions, no fish scale was generated, indicating excellent fish scale resistance. With respect to the enamel layer adhesion, those indices were higher than 95%.

On the other hand, Comparative Material 1 had a high S content of 0.042%, and as a result, no fish scale was generated, but the C content was as high as 0.0042%. Therefore, since the amount of excess Ti * was as low as 0.005%, the r value was only 1.7, and as a result, the formability was very low.

In the case of Comparative Material 2, since the amount of excess Ti * was as high as 0.057%, the r value was 2
. As a result, the moldability was excellent. However, when the S content is 0.01
As low as 2%, 25 fish scale defects were formed. Therefore, the steel sheet could not be used for enamel coating.

In Comparative Material 3, since the S content was 0.032%, no fish scale defects occurred. Further, the amount of excess Ti * is 0.115% and the r value is 2.37.
Thus, high moldability was realized. However, when the Ti content is 0
. Since it was as high as 182%, the enamel layer adhesion was as low as 83%. Therefore, this material cannot be used as an enamel coated steel sheet.

In Comparative Material 4, the S content was as high as 0.038%, but the Ti content was 0.038%.
As low as 72%, 38 fish scale defects resulted. Furthermore, since the amount of excess Ti * was as low as 0.002%, the r value was only 1.72, and as a result, the moldability was very low.

For the conventional materials 1 and 2, the Ti content was 0.122% and 0.11%, respectively.
It was as high as 0%, and the N content was as high as 0.0075% and 0.0082%. Therefore, fish scale defects did not occur because a coarse Ti compound was formed in the steel. Furthermore, the amount of excess Ti * was 0.061%, and the r value was as high as 2.12. As a result, the moldability was excellent. However, since a large amount of the coarse TiN compound was present on the surface of the steel sheet, the generation of large bubbles that had grown abnormally caused surface defects.

In the conventional material 3, since the P content was as high as 0.058%, the r value was low, and the yield strength was excessively high, and as a result, the formability was low. Thus, if such a steel sheet is used to form a complex shape, cracks will probably occur.

As described above, according to the present invention, the enamel layer adhesion, fish scale resistance, and moldability are excellent. Furthermore, the steel sheet of the present invention is suitable for pressing a complicated shape.

──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Song Jong Woo Korea, 790-300, Gyeongsang Book, Pohang-Shi, Nank, Cordon-Don, 1 Pohang Iron and Steele Shio. , Eltidi. (72) Inventor Song Won Ho South Korea, 790-300, Gyeongsang Book, Pohang-shi, Nank, Cordon-dong, 1 Pohang Iron and Steel Shioh. , Eltidi. F term (reference) 4K037 EA04 EA15 EA18 EA25 EA31 EB01 EC04 FA03 FC08 FE02 FE03 FG00 FH01 GA03

Claims (2)

[Claims] 1. A method for manufacturing an enamel-coated cold steel sheet, comprising the steps of forming an aluminum-killed steel by forming C of 0.004% by weight or less, Mn of 0.3% by weight or less, and 0.02 to 0.2% of Mn. 05 wt% S, 0.005 to 0.03 wt% P, 0.08 to 0.15 wt% Ti, 0.004 wt% or less N, 0.04 wt% or more (Ti * = Ti- (48/32) S- (48/14) N- (48/1
2) preparing a steel consisting of excess Ti * and the balance being iron and other unavoidable impurities (defined in C), reheating the aluminum killed steel, finishing rolling temperature above the Ar ₃ transformation point A step of performing hot rolling, a step of winding in a usual manner, a step of performing cold rolling at a reduction ratio of 50 to 85%, and a step of performing continuous annealing at a recrystallization temperature or higher. A method for producing an enamel-coated cold steel plate. 2. The method according to claim 1, wherein S is contained in an amount of 0.02 to 0.03%.