KR100470669B1 - A method for manufacturing high strength cold-rolled enamel steel sheet with superior fishscale resistance - Google Patents

Abstract

The present invention relates to a method for manufacturing cold rolled steel sheet for enamel used as parts of home appliances, building exterior materials, bathtubs, etc., by adding an appropriate alloy element to medium carbon steel, and controlling the cold rolling amount and annealing for graphitization By finely dispersing graphite in the inside, it is to provide a method for producing an enamel steel sheet having excellent fish scale resistance, no fish scale after enamel annealing, PEI adhesion index ≥ 85 and yield strength ≥ 30 kg / mm 2. There is a purpose.

The present invention for achieving the above object,

By weight%, C: 0.1-0.5%, Mn: 0.1-0.5%, Al: 0.01-0.1%, N: 0.001-0.015%, B: 0.0005-0.005%, Si contains Mn + 0.06 / C- It is added to satisfy 0.21≤Si≤1.5%, and the hot rolled steel sheet containing the balance Fe and unavoidable impurities is wound to a temperature of 600 ° C or higher, pickled, and cold rolled to a cold rolling load of 10% or higher, and 600 to 720 ° C. Technical method for producing a high strength cold rolled enamel steel sheet having excellent fish scale resistance, characterized by annealing at a temperature range of.

Description

Manufacturing method of high strength cold rolled enamel steel sheet with excellent fish scale resistance {A METHOD FOR MANUFACTURING HIGH STRENGTH COLD-ROLLED ENAMEL STEEL SHEET WITH SUPERIOR FISHSCALE RESISTANCE}

The present invention relates to a method for manufacturing an enameled cold rolled steel sheet used as a component of home appliances or a building exterior material, a bath, etc. More specifically, the enameled cold rolled steel sheet having an excellent fish scale resistance and high strength after enamel processing It relates to a manufacturing method of.

One of the most important properties required for enamel steel sheet is fish scale resistance. Fish scale is a phenomenon in which the enamel layer falls on the surface of the steel sheet after being enameled, like meat scales. When fish scale occurs, the fish scale is not only bad in appearance but also exposed to the outside. Worse

The fish scale defect is a defect generated by hydrogen, and the causes are as follows. In other words, during enameling, the temperature of the steel sheet increases due to the high solubility of hydrogen.However, after firing, the temperature of the steel sheet decreases, causing the hydrogen to diffuse to the surface of the steel sheet, destroying the enamel layer and escaping into the atmosphere. It is.

In order to prevent such fish scale defects, it is necessary to make a position where hydrogen can be collected in the steel sheet, and in this case, inclusions or precipitates are mainly used.

For example, Japanese Patent Laid-Open No. 61-276958 and Korean Patent Application No. 97-42333 propose a method of using titanium mainly for fish scale prevention. However, titanium is a very oxidizing element, and when a large amount is added, oxides are frequently generated during continuous casting operations, causing nozzle clogging problems, thereby deteriorating continuous casting workability. In addition, titanium carbide reacts with the enamel layer to generate a lot of bubble defects after the enamel treatment.

As a technique for reducing such bubble defects and improving formability, Japanese Patent Application Laid-Open No. 10-17992 and Korean Patent Application No. 98-35267 propose a method of extremely low carbon content and controlling titanium. Silver strength is so low that it is easily deformed even with a small impact, there is a disadvantage that the thickness of the product is also thick.

As a technique for solving the problem of the strength degradation, Korean Patent Application No. 97-62730 is adding a large amount of P, P is difficult to manufacture slabs due to severe segregation during slab manufacturing, there is also a problem that the strength increase is not enough.

Accordingly, the present inventors have repeatedly conducted research and experiments to solve the problems of the prior arts, and based on the results, the present invention adds an appropriate alloy element to the medium carbon steel, And finely distribute the graphite in the steel sheet by controlling the annealing for graphitization, so that the fish scale is excellent and the fish scale does not occur after the enamel annealing treatment and the PEI adhesion index ≥ 85 and the yield strength ≥ 30 kg / mm 2 To provide a method for producing a steel sheet, the purpose is.

The present invention for achieving the above object, by weight%, containing C: 0.1 ~ 0.5%, Mn: 0.1 ~ 0.5%, Al: 0.01 ~ 0.1%, N: 0.001 ~ 0.015%, B: 0.0005 ~ 0.005% Si is added to satisfy Mn + 0.06 / C-0.21 ≦ Si ≦ 1.5%, and the hot rolled steel sheet containing the remaining Fe and unavoidable impurities is wound to a temperature of 600 ° C. or higher, pickled, and then subjected to cold pressure of 10% or more. Cold rolling in a quantity, and annealing in a temperature range of 600 ~ 720 ℃ relates to a method for producing a high strength cold rolled enamel steel sheet excellent in fish scale resistance.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

The inventors of the present invention comprehensively study the action of C, Si, B, Mn, Al, N and manufacturing conditions such as hot rolling, cold rolling and annealing in steels that affect the graphitization of cold rolled steel sheet. By properly adjusting the manufacturing conditions, it was found that steel with high fish scale resistance and high strength could be manufactured.

That is, the graphite particles in the steel are finely precipitated, and the carbon collected in the graphite particles during enamel annealing diffuses to the base, which causes the graphite particles to become vacant, thereby providing a space for storing hydrogen. Will greatly increase. To this end, within the range of easy cold working, the content of C and Si, which are graphitization promoting elements, is controlled, a very small amount of B, which acts as a graphitization nucleation site, is added, and the amount of Al added to the deoxidizer and the steel is present. N is appropriately controlled, the content of Mn which is a graphitization inhibiting element is suppressed, and hot rolling, cold rolling and annealing heat treatment conditions are appropriately controlled.

The reason for numerical limitation of the steel component is described below.

In the present invention, C is an element which plays the most important role in the graphitization of cementite, and if the content thereof is small, the amount of alloy addition increases, and it takes a long time to graphitize, so it is preferable to limit the lower limit to 0.1%. The higher the content of C, the higher the number of graphite particles and the higher the number of pores after enameling. However, the upper limit is preferably limited to 0.5% because the post-processing of the hot rolled steel becomes difficult and the weldability is greatly reduced.

Since Mn is an element that suppresses graphitization, the smaller the content thereof, the more favorable the graphitization, and the upper limit thereof is preferably limited to 0.5%. However, if the content is too small, the strength is low and the hot workability is deteriorated, so it is preferable to limit the lower limit to 0.1%.

Al is a deoxidizer and an element which promotes graphitization, but if the content is less than 0.01%, since it has almost no graphitization promoting effect, it is preferable to add 0.01% or more. However, if the content exceeds 0.1%, the graphitization promoting effect is almost saturated, and the inclusions in the steel increase and the workability of the steel sheet is lowered. Therefore, the upper limit is preferably set to 0.1%.

N acts most effectively as a nucleation site of graphite and is an element that reacts with boron to form BN. If the content is too small, it is difficult to form BN, and therefore it is preferable to contain it at 0.001% or more. However, if the content is too high, the ductility of the steel sheet is lowered, so it is preferable to limit the upper limit to 0.015%.

B is an important element for increasing the graphitization rate and finely distributing the graphite by acting as a nucleation position of the graphite, and is added at least 0.0005% to obtain such an effect. However, if the content is too large, the effect is saturated and there is a fear of cracking during slab production. Therefore, the upper limit is preferably limited to 0.005%.

Si is a graphitization promoting element, and the effect of graphite on the precipitation of graphite depends on the contents of C and Mn, so the content should be considered together with the contents of C and Mn. As a result, in order to enable graphite precipitation, the content of Si must satisfy the following formula (1).

[Relationship 1]

Si≥Mn + 0.06 / C-0.21

However, when the content of Si exceeds 1.5%, graphitization is considerably promoted, but the cold workability is lowered due to the increase in hardness of the material, which makes it difficult to post-process, and the red scale is generated so that the surface quality of the steel sheet is lowered, so the upper limit is 1.5. It is preferable to set it to%.

Next, manufacturing conditions used in the present invention will be described.

After reheating the slab formed as described above, in order to refine the structure through the hot rolling process, hot rolling is finished at a temperature of Ar ₃ or more, which is a normal hot rolling condition. Then, it winds up at the temperature of 600 degreeC or more, and then pickling is carried out, cold-rolled at the cold reduction rate of 10% or more, and annealed in the temperature range of 600-720 degreeC.

The reason why the winding temperature is set to 600 ° C. or higher is that when the temperature is less than 600 ° C., the strength of the hot rolled steel sheet increases and the load during cold rolling increases.

In addition, the reason why the cold rolling reduction rate is set to 10% or more is that when cold rolling is performed, not only voids occur between the base and the precipitate BN, but also carbon is easily moved through the generated potential to promote graphitization. This is because at least 10% of the cold reduction is required to obtain such an effect.

In addition, the reason for setting the annealing temperature in the temperature range of 600 ~ 720 ℃ is as follows. That is, if the annealing temperature is less than 600 ° C, the diffusion rate of C is slow and the graphitization progression rate is slow.

After processing the enamel parts using the steel sheet manufactured as described above, and firing at 800 to 850 ° C., which is the normal enamel firing temperature, sufficient vacancies necessary for suppressing fish scale generation can be secured. It is possible to obtain a high strength product without generating fish scale.

Hereinafter, the present invention will be described in more detail with reference to Examples.

(Example 1)

In order to investigate the effect of the alloy component on the graphite particle precipitation and enamel characteristics, the alloy component was changed as shown in Table 1 below, and the other manufacturing conditions were the same. In Table 1 below, the conventional steel (1) is a P-added high-strength cold-rolled enamel steel of Korea Patent Application No. 97-62730, the conventional steel (2) is a general cold-rolled enamel steel.

First, steels of the components shown in Table 1 below were heated to 1200 ° C. and maintained for 2 hours. Then, hot rolling was finished at a temperature of 900 ° C., and wound at 650 ° C. to prepare a hot rolled steel sheet having a thickness of 2.0 mm. Thereafter, pickling was performed and cold rolling was performed at a cold reduction rate of 40% to prepare a cold rolled steel sheet having a thickness of 1.2 mm. For graphitization of the thus produced cold rolled steel sheet, BAF annealing was performed at a temperature of 680 ° C. for 15 hours in a non-oxidizing atmosphere. On the other hand, conventional steels (1) and (2) are steels with a low carbon content, and are manufactured in the same manner as the above production method, but BAF annealing for graphitization is not performed, but continuous annealing is performed instead.

Then, the inventive steels (1) to (3) and the comparative steels (1) to (3) were cut to measure the amount of graphite, and the results are shown in Table 2 below.

In addition, in order to measure the degree of fish scale, the specimen of the cold-rolled steel sheet was immersed in 70 ° C, 10% sulfuric acid solution for 5 minutes and pickled, washed with warm water, and then heated to 85 ° C, 3.6 g / l sodium carbonate + 1.2 It was neutralized by dipping for 5 minutes in ag / l borax solution. Thereafter, glaze was applied to both surfaces of the specimen after the pretreatment was completed, dried at 200 ° C. for 10 minutes, calcined at 830 ° C. for 5 minutes, and then cooled by air to complete enameling. At this time, the dew point of a kiln atmosphere was made into harsh conditions in which fish scale is easy to generate | occur | produce at 30 degreeC. The enameled specimens were kept in a 200 ° C. holding furnace for 20 hours to visually investigate the number of fish scale defects occurring at 70 mm in width and 150 mm in length after fish scale acceleration treatment, and the results are shown in Table 2 below.

Enamel adhesion was evaluated by the PEI adhesion index using the adhesion test equipment (test equipment according to ASTM C313-78 standard), the results are shown in Table 2 below.

On the other hand, in order to measure the strength of the steel sheet after enamel processing, after the heat treatment corresponding to the enamel annealing condition that maintains the annealing steel sheet at 830 ℃ for 5 minutes, processed the specimen of ASTM-sub size to measure the yield strength, The results are shown in Table 2 below.

division Chemical composition (wt%) Minimum Si content (wt%) for graphitization C Si Mn Al B N Inventive Steel 1 0.2 0.5 0.2 0.05 0.002 0.007 0.29 Inventive Steel 2 0.3 0.5 0.2 0.05 0.0018 0.006 0.19 Invention Steel 3 0.4 0.5 0.2 0.05 0.0021 0.006 0.14 Comparative Steel 1 0.05 1.5 0.2 0.05 0.002 0.007 1.19 Comparative Steel 2 0.2 0.2 0.2 0.05 0.002 0.006 0.29 Comparative Steel 3 0.2 0.5 0.2 0.05 － 0.006 0.29 Conventional Steel 1 0.0015% C-0.13% Mn-0.075% P-0.03% S-0.072% Ti-0.0031% N ― Conventional Steel 2 0.0039% C-0.15% Mn-0.010% P-0.013% S-0.122% Ti-0.0075% N ―

division Steel plate characteristic Enamel Characteristics after enamel processing Graphite amount (%) Fish Scale Occurrences Enamel layer thickness (㎛) PEI adhesion index (%) Yield strength (kg / ㎡) Inventive Steel 1 0.6 0 130 95 33 Inventive Steel 2 0.9 0 120 91 31 Invention Steel 3 1.4 0 110 90 32 Comparative Steel 1 0 105 125 80 23 Comparative Steel 2 0 90 115 75 30 Comparative Steel 3 0 120 135 78 31 Conventional Steel 1 － 0 123 99 18.5 Conventional Steel 2 － 5 129 81 9.8

As shown in Tables 1 and 2, the hardened steels (1) to (3) had a large amount of graphite precipitated in the state of the steel sheet, and sufficient pore size could be ensured after the small-phase treatment, so that no fish scale was generated. In addition, the adhesion index was high and the yield strength after enamel treatment was equal to or higher than that of the comparative steel.

On the other hand, the comparative steel (1) has a high Si content but a low C content, no graphite was precipitated at all, thereby lowering the enameling properties and yield strength. The comparative steel 2 had a low content of Si, and the comparative steel 3 had no B added, so that no graphite was precipitated at all, and thus, the enamel property was not good.

On the other hand, the conventional steel (1) was excellent in enameling properties, but because of the low C content, the graphite particles desired in the present invention was not produced. Because of this, yield strength was low after enamel processing. In addition, the conventional steel (2) also had a low C content and a low yield strength after enameling.

Example 2

In order to examine the effect of the cold rolling amount on the graphite deposition amount and the fish scale generation, as shown in Table 3 below for the invention steel (1) of Example 1, the cold rolling amount is changed and the rest is the same as in Example 1 It was. Thereafter, the enameling treatment was carried out under the same conditions as in Example 1, and the degree of fish scale was investigated. The results are shown in Table 3 below.

division Cold rolling load (%) Graphite amount (%) Fish Scale Invention 1 0 0.12 0 Invention 2 20 0.14 0 Invention 3 40 0.15 0 Comparative Material 1 － 0.08 10

As shown in Table 3, the inventive materials (1) to (3) satisfy the conditions of the present invention with a cold reduction amount of 10% or more, and a large amount of graphite precipitated and no fish scale occurred. The reason is that if the cold reduction amount is 10% or more, the plastic deformation of the steel sheet causes interfacial separation at the interface between BN and the matrix, leading to precipitation of graphite, and easy movement of carbon along the dislocation, thereby increasing the graphite deposition rate. Because.

On the other hand, since the comparative material 1 was not cold rolled, it can be seen that the amount of graphite precipitated was small and fish scale was also generated.

Example 3

In order to determine the effect of the annealing temperature on the graphite amount and the fish scale resistance, as shown in Table 4 below, the annealing temperature of the invention steel (1) of Example 1 was changed to the same manner as in Example 1. Thereafter, enameling was performed under the same conditions as in Example 1, and the degree of fish scale was investigated. The results are shown in Table 4 below.

division Annealing Temperature (℃) Graphite amount (%) Fish Scale Invention 4 620 0.2 0 Invention 5 650 0.4 0 Invention 6 680 0.6 0 Invention 7 710 0.5 0 Comparative Material 2 560 0.02 40 Comparative Material 3 590 0.08 20 Comparative Material 4 740 0 110

As shown in Table 4, in the annealing for graphitization, the invention materials (4) to (7) that satisfies the 600 ~ 720 ℃ temperature conditions of the present invention range, a lot of graphite precipitates, fish scale It can be seen that it did not occur.

On the other hand, the comparative materials (2) and (3) having annealing temperature lower than 600 ° C. had a small amount of graphite precipitated, resulting in fish scale, and the comparative materials (4) having an annealing temperature higher than 720 ° C. showed austenite. Since a solid solution does not precipitate in graphite, many fish scales generate | occur | produced.

As described above, according to the present invention, since fine particles of graphite are precipitated in the cold rolled steel sheet, the pores can be easily secured during the enameling process, thereby completely preventing the occurrence of fish scale, which is one of the biggest problems in the enameled product. Can be. In addition, it is possible to obtain a high-strength enamel product in accordance with the re-stocking of graphite during the firing process. Accordingly, the impact resistance of home appliances or building exterior materials is also improved, and the weight of the product can be reduced, thereby greatly contributing to the expansion of demand for enamel steel sheets.

Claims (1)

By weight%, C: 0.1-0.5%, Mn: 0.1-0.5%, Al: 0.01-0.1%, N: 0.001-0.015%, B: 0.0005-0.005%, Si contains Mn + 0.06 / C- It is added to satisfy 0.21≤Si≤1.5%, and the hot rolled steel sheet containing the balance Fe and unavoidable impurities is wound to a temperature of 600 ° C or higher, pickled, and cold rolled to a cold rolling load of 10% or higher, and 600 to 720 ° C. Method for producing a high strength cold rolled enamel steel sheet having excellent fish scale resistance, characterized by annealing in the temperature range of