WO2000012773A1

WO2000012773A1 - Method for manufacturing high adherence enamel-coating steel sheet with superior formability

Info

Publication number: WO2000012773A1
Application number: PCT/KR1999/000477
Authority: WO
Inventors: Jeong Bong Yoon; Jeong Woo Son; Won Ho Son
Original assignee: Pohang Iron & Steel Co., Ltd.
Priority date: 1998-08-28
Filing date: 1999-08-24
Publication date: 2000-03-09
Also published as: ID25493A; KR100360095B1; CN1091161C; JP2002523633A; JP3366904B2; KR20000015390A; CN1275173A

Abstract

A method for manufacturing a cold rolled steel sheet to be enamel-coated and to be used as raw steel sheets for enamel-coated products such as bath tubs and components of electric appliances is disclosed. The optimum contents of the alloying elements such as S, P and N are realized, thereby satisfying the enamel layer adherence, the fishscale resistance and the formability. First, there is prepared a steel composed of, in weight %, 0.004 % or less of C, 0.3 % or less of Mn, 0.02-0.05 % of S, 0.005-0.03 % of P, 0.08-0.15 % of Ti, 0.004 % or less of N, 0.04 % or more of an excess Ti* (being defined to be Ti* = Ti -(48/32)S -(48/14)N -(48/12)C), and a balance of Fe and other unavoidable impurities so as to form an aluminum killed steel. The aluminum killed steel is reheated, and then a hot rolling is carried out, with the finish rolling temperature being above the Ar3 transformation point. Then a coiling is carried out in the normal manner, and then, a cold rolling is carried out with a reduction rate of 50-85 %. A continuous annealing is carried out above the recrystallization temperature.

Description

METHOD FOR MANUFACTURING HIGH ADHERENCE ENAMEL-COATING STEEL SHEET WITH SUPERIOR FORMABILITY

FIELD OF THE INVENTION The present invention relates to a method for manufacturing a cold rolled steel sheet to be used as raw steel sheets for enamel-coated products such as bath tubs and components of electric appliances- More specifically, the present invention relates to a method for manufacturing a cold rolled steel sheet, in which there are ensured more than certain levels in the characteristics such as the enamel adherence, the fishscale resistance, the formability and the like.

BACKGROUND OF THE INVENTION

Generally, the enamel-coating cold rolled steel sheet is press-formed into various shapes, then an enamel coating is carried out on the surface, and then, a firing is carried out at a high temperature, thereby completing the manufacture of the enamel-coated steel products. The major characteristics which are demanded to the enamel- coating cold rolled steel sheet are the fishscale resistance, the formability and the adherence between the raw steel sheet and the enamel layer. The enamel layer adherence depends on the additive elements and the surface roughness.

The fishscale refers to the defects which are formed on the surface of the enamel-coated products. That is, when manufacturing the enamel-coated products, the hydrogen which is solid-solute within steel is released after or during the cooling. Due to the hydrogen pressure, the hardened enamel layer is ruptured to form defects resembling the scales of fish. In order to prevent the fishscaling, it is required to provide rooms for storing the hydrogen within the steel. This is much influenced by the kind and amount of the precipitates or the non-metallic inclusions which are present within the steel sheet. In the steels which have been proposed so far for preventing the fishscaling, there are added Ti, B, N or 0₂ so as to form Ti sulfides, Ti nitrides, Ti carbides, B nitrides, Mn oxides or the like, which are all known to be hydrogen absorption sources. In this manner, the precipitates or oxides are precipitated, or a high carbon steel is decarburized, to ensure the fishscale resistance. Thus so far, the proposed steels are mostly a Ti-added steel, a B-added steel, a high oxygen steel, or decarburized steel .

Meanwhile, the formability is also very important, because the enamel-coating cold rolled steel sheet has to be press-formed into the required shapes before the enamel- coating.

Japanese Patent Publication Gazette Sho-63-500 discloses a method for manufacturing an enamel-coating steel sheet. In this steel, the composition includes in weight %: 0.005% or less of C; 0.03% or less of Si; 0.50% or less of Mn; 0.02% or less of P; 0.03% or less of S; 0.005-0.01% of N; 0.15% or less of Ti [Ti > (48/12C + 48/14N + 48/32S)]; 0.08% or less of Cu; 0.003-0.03% of the sum addition of one or more elements selected from a group consisting of As, Sb and Bi; and a balance of Fe and other unavoidable impurities. In this steel, however, the content of N is very high, and therefore, if the TiN precipitates are exposed on the surface of the steel sheet, then bubbles are formed.

Meanwhile, Korean Patent Application No. 97-63270 discloses another method for manufacturing an enamel- coating steel sheet. In this method, the steel is composed of in weight %: 0.01% or less of C, 0.3% or less of Mn, 0.05-0.1% of P, 0.02-0.04% of S, 0.04-0.10% of Ti, 0.005% or less of N, the atomic ratio of Ti/(C+N+S) being 1.0 or more, and a balance of Fe and other unavoidable impurities. In this steel, however, the content of P is high, and therefore, the formability is aggravated, although the strength of the steel sheet is acceptable.

As described above, the enamel-coating cold-rolled steel sheets which have been developed so far do not satisfy the enamel layer adherence, the fishscale resistance and the formability. Rather there is the problem that a certain property has to be sacrificed in securing a certain property.

SUMMARY OF THE INVENTION

The present invention is intended to overcome the above described disadvantages of the conventional techniques .

Therefore it is an object of the present invention to provide an enamel-coating cold-rolled steel sheet in which the optimum contents of the alloying elements such as S, P,

N, Ti and effective Ti(Ti*) are realized, thereby satisfying the enamel layer adherence, the fishscale resistance and the formability. In achieving the above object, the method for manufacturing an enamel-coating cold steel sheet according to the present invention includes the steps of: preparing a steel composed of, in weight %, 0.004% or less of C, 0.3% or less of Mn, 0.02-0.05% of S, 0.005-0.03% of P, 0.08-0.15% of Ti, 0.004% or less of N, 0.04% or more of an excess Ti* (being defined to be Ti* = Ti -(48/32)S - (48/14)N -(48/12)C), and a balance of Fe and other unavoidable impurities so as to form an aluminum killed steel; reheating the aluminum killed steel; carrying out a hot rolling, with the finish rolling temperature being above the Ar₃ transformation point; carrying out a coiling in the normal manner; carrying out a cold rolling with a reduction rate of 50 - 85%; and carrying out a continuous annealing above the recrystallization temperature.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The method for manuf cturing an enamel-coating cold steel sheet according to the present invention includes the steps of: preparing a steel composed of, in weight %, 0.004% or less of C, 0.3% or less of Mn, 0.02-0.05% of S, 0.005-0.03% of P, 0.08-0.15% of Ti, 0.004% or less of N, 0.04% or more of an excess Ti* (being defined to be Ti* = Ti -(48/32)S -(48/14)N -(48/12)C), and a balance of Fe and other unavoidable impurities so as to form an aluminum killed steel; reheating the aluminum killed steel; carrying out a hot rolling, with the finish rolling temperature being above the Ar₃ transformation point; carrying out a coiling in the normal manner; carrying out a cold rolling with a reduction rate of 50 - 85%; and carrying out a continuous annealing above the recrystallization temperature.

Hereinafter, numerical value limiting reasons for composition of the present invention will be described more in detail.

If the content of C is more than 0.004%, then the solid-solute carbon becomes excessive. Therefore, during the annealing, the development of the texture is impeded, or the amounts of the precipitates of fine Ti carbides become excessive. Consequently, the grains become fine so as to aggravate the formability. Therefore, the content of C has to be limited to 0.004% or less. Mn is added to precipitate the solid-solute S in the form of Mn sulfides, so as to prevent the hot shortness which is caused by the FeS film. However, in the present invention, Ti is added to precipitate the S in the form of Ti sulfides, thereby completely removing the residual S. Therefore, Mn needs not be separately added. Further, if Mn is present in a solid-solution state, the strength of the steel is increased, but this strength increase is not significant, but rather aggravates the formability. Therefore, Mn should be preferably limited to 0.3% or less.

Generally, S is known to be an element which aggravates the mechanical properties, but in the present invention, it is added to reinforce the fishscale resistance. If the content of S is less than 0.02%, the amount and size of Ti sulfides become insufficient, with the result that the fishscale resistance is not improved. If the content of S is 0.05% or more, the excess Ti* becomes too small so as to aggravate the formability. Therefore, the content of S should be preferably limited to 0.02-0.05%. More preferably it should be limited to 0.02-0.03% to ensure more superior formability. Meanwhile, P is also an element which aggravates the mechanical properties of the steel, like S. Therefore its content should be as low as possible. In the present invention, however, P is added to improve the fishscale resistance by forming Ti(Fe,P) precipitates after reacting with Ti. If the content of P is less than 0.005%, the Ti(Fe, P) precipitates are not formed, and therefore, the fishscale resistance cannot be improved. If its content is more than 0.03%, the recrystallized grains become too fine due to the formation of fine Ti(Fe, P) precipitates, with the result that the formability is aggravated. Therefore, the content of P should be limited to 0.005-0.03%.

Meanwhile, Ti removes the solid-solute C and N in the form of Ti carbides and Ti nitrides, thereby improving the formability of the raw steel sheet. Further, Ti improves the fishscale resistance by precipitating Ti sulfide (TiS) and Ti(Fe, P) precipitates. If its content is less than 0.08%, the Ti precipitates are precipitated in too small amounts, thereby making it impossible to improve the fishscale resistance. If the content of Ti is more than 0.15%, then the Ti precipitates become large so as to improve the fishscale resistance, and the enamel layer adherence is aggravated due to the higher Ti* . Therefore the content of Ti should be limited to 0.08-0.15%. Meanwhile, N is reacted with Ti to be precipitated in the form of a Ti nitride, thereby improving the fishscale resistance. However, if the Ti nitride is exposed on the surface of the steel sheet, an oxidation occurs to generate N gases so as to cause surface defects . Therefore, the content of N should be as low as possible. Thus, if its content is less than 0.004%, the Ti nitride is precipitated in small amounts, and therefore, the probability of causing the surface defects is very low. Therefore, the content of N should be 0.004% or less.

The excess titanium is defined to be Ti* = Ti - (48/32)S -(48/14)N -(48/12)C. Ti is reacted with N, S and C to form TiN, TiS and Tie . Under the assumption that the total added elements are precipitated, the excess Ti* refers to the residual solid-solute Ti. Actually, however, the total added elements are not completely reacted, and therefore, as the amount of the excess Ti* is large, so much completely the residual solid-solute C and N are precipitated. Further, Ti(Fe, P) precipitates are precipitated to improve the fishscale resistance. If the excess Ti* is more than 0.04%, then the residual solid-solute C and N are almost absent, and therefore, a formability with an r value of 2.0 or more can be ensured. With the r value of 2.0 or more, complicated shapes can be formed. Further, with proper amounts of Ti(Fe, P) precipitates, a sufficient fishscale resistance can be secured. Therefore, the lower limit of the excess Ti* should be 0.04%. Particularly, if the excess Ti* is less than 0.04%, the Ti(Fe, P) precipitates are not formed, and therefore, the fishscale resistance is lowered.

Hereinafter, a manufacturing condition of the steel of the present invention will be described.

The aluminum-killed steel having the composition as described above is reheated, and hot-rolled. Under this condition, the finish hot rolling temperature should be above the Ar₃ transformation point. The reason is that if the hot rolling is carried out below the Ar₃ transformation point, rolling grains are formed, thereby aggravating the formability.

After the hot rolling, a coiling is carried out in the usual manner, and then, a cold rolling is carried out, with the reduction ratio being limited to 50-85%. The precipitates which have been formed during the hot rolling are destroyed or elongated during the cold rolling. During this process, tiny cavities are formed, and these cavities remain intact even after the continuous annealing so as to act as hydrogen absorption sources . In this connection, the reduction rate of the cold rolling needs be controlled. That is, if the cold rolling reduction rate is less than 50%, the total tiny cavities are too small, so as to decrease the hydrogen absorption, thereby making it likely to cause the fishscale. On the other hand, if the reduction rate is more than 85%, the tiny cavities are collapsed due to the high reduction rate. Therefore, the total tiny cavity spaces are rather decreased, to drastically decrease the hydrogen absorption capability. After the cold rolling, the steel is made to undergo a continuous annealing in the normal manner. That is, the continuous annealing is carried out at a temperature above the recrystallization temperature.

Now the present invention will be described based on an actual example.

Ingots having the compositions of Table 1 were prepared, and were subjected to hot rollings after maintaining them at 1250 °C for 1 hour within a furnace.

The hot rolling finish temperature was 900°C, and then, a coiling was carried out at 650 °C. The final thickness was 3.2 mm. The hot rolled specimens were pickled to remove the surface oxide films. Then a cold rolling was carried out at a reduction rate of 70%. The cold rolled specimens were further worked into enamel specimens and tensile specimens. Then these two kinds of specimens were subjected to a continuous annealing.

The enamel specimens were cut into a 70 mm x 150 mm size, while the tensile specimens were formed based on the ASTM E-8 standard. The continuous annealing was carried out at 830°C for 30 seconds.

A tensile test machine of Instron company (Model 6025) was used for testing the tensile specimens, and thus, the yield strength, the tensile strength, the elongation, the r values were measured. The measured results are shown in Table 2 below.

The enamel specimens were completely fat-removed, and then were dipped into a sulfuric acid solution (10%, 70°C) for 5 minutes. Then a hot water wash was carried out, and then a neutralization was carried out by dipping the specimens into an aqueous solution of 3.6g/L-sodium carbide + 1.2g/L-borax at 85°C for 5 minutes. Thus the pre- treatment was completed, and then, an enamel was applied on the specimens. Then a drying was carried out at 200°C for 10 minutes, thereby completely removing the moisture. After the drying, the specimens were left at 830 °C for 7 minutes, then a firing was carried out, and then an air cooling was carried out, thereby completing the enamel coating process.

Under this condition, the atmosphere of the firing furnace had a dew point temperature of 30°C. This forms a severe adversity in which the fishscale is most likely to occur. After the enamel coating, the specimens were left within a furnace at 200°C for 20 hours in order to accelerate the fishscaling, and then, the number of the fishscale defects was visually inspected, the results being shown in Table 2 below.

As to the evaluation of the enamel layer adherence, an adherence tester (based on ASTM C313-78) was used to measure the adherence index.

As shown in Tables 1 and 2, the inventive materials 1-4 showed an r value of 2.0 or more to ensure a high formability. Further, the fishscale did not occur under high adversity conditions to show a superior fishscale resistance. As to their enamel layer adherence, their indices were more than 95%.

In contrast to this, the comparative material 1 had an S content as high as 0.042%, and therefore, the fishscale did not occur, but its C content was as high as 0.0042%. Therefore, the amount of the excess Ti* was as low as 0.005%, and therefore, the r value was just 1.7, with the result that the formability was very low.

In the case of the comparative material 2, the amount of the excess Ti* was as high as 0.057%, and therefore, the r value was 2.25, with the result that the formability was superior. However, the S content was as low as 0.012%, and therefore, 25 fishscale defects were formed. Therefore, the steel sheet could not be used for the enamel-coating .

In the comparative material 3 , the S content was 0.032%, and therefore, the fishscale defects did not occur. Further, the amount of the excess Ti* was 0.115%, and the r value was 2.37, thereby realizing a high formability. However, the Ti content was as high as 0.182%, and therefore, the enamel layer adherence was as low as 83%. Therefore, this material cannot be used as an enamel-coating steel sheet.

In the comparative material 4, the S content was as high as 0.038%, but the Ti content was as low as 0.072%, with the result that 38 fishscale defects occurred. Further, the amount of the excess Ti* was as low as 0.002%, and therefore, the r value was just 1.72, with the result that the formability was very low.

As to the conventional materials 1 and 2, the Ti contents were as high as 0.122% and 0.110% respectively, and the N contents were also as high as 0.0075% and 0.0082%. Therefore, owing to the formation of coarse Ti compounds within the steel, fishscale defects did not occur. Further, the excess Ti* was 0.061%, and the r value was as high as 2.12, with the result that the formability was superior. However, large amounts of coarse TiN compounds were present on the surface of the steel sheet, and therefore, the surface defects occurred due to the generation of abnormally grown big bubbles.

In the conventional material 3, the P content was as high as 0.058%, and therefore, the yield strength was too high, as well as showing a low r value, with the result that the formability was low. Therefore when this kind of steel sheet is used in forming a complicated shape, cracks probably will occur.

According to the present invention as described above, the enamel layer adherence, the fishscale resistance and the formability are superior. Further, the steel sheet of the present invention is suitable for press-forming a complicated shape.

Claims

WHAT IS CLAIMED IS:

1. A method for manufacturing an enamel-coating cold steel sheet, comprising the steps of: preparing a steel composed of, in weight %, 0.004% or less of C, 0.3% or less of Mn, 0.02-0.05% of S, 0.005- 0.03% of P, 0.08-0.15% of Ti, 0.004% or less of N, 0.04% or more of an excess Ti* (being defined to be Ti* = Ti - (48/32)S -(48/14)N -(48/12)C), and a balance of Fe and other unavoidable impurities so as to form an aluminum killed steel; reheating the aluminum killed steel; carrying out a hot rolling, with a finish rolling temperature being above an Ar₃ transformation point; carrying out a coiling in a normal manner; carrying out a cold rolling with a reduction rate of 50 - 85%; and carrying out a continuous annealing above a recrystallization temperature.

2. The method as claimed in claim 1, wherein S is contained in an amount of 0.02 - 0.03%.