KR100280724B1 - A method for manufacturing a hot-dip galvanized hot-rolled steel sheet excellent in thermal resistance and plating adhesion - Google Patents

Abstract

The present invention prevents the deterioration of the HGI material caused by the HGI heat treatment before plating to secure the surface cleanliness of the hot-rolled steel sheet as a plating material and secure the plating adhesion, thereby improving the variety of steels for processed steels and high- And to provide a method of manufacturing a hot-dip galvanized hot-rolled steel sheet excellent in plating adhesion.

To this end, a method for manufacturing a hot-dip galvanized hot-rolled steel sheet comprising a hot-rolled steel sheet as a plating material, comprising the steps of: pre-treating a surface of a base material after pickling the hot- - a HGI heat treatment step of heating to 460 캜, and a step of dipping in a plating bath at a temperature of 420 - 460 캜 to perform hot dip galvanizing, and a hot-dip galvanizing A method of manufacturing a hot-rolled steel sheet is provided.

Description

A method for manufacturing a hot-dip galvanized hot-rolled steel sheet excellent in thermal resistance and plating adhesion

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of regenerating hot rolled galvanized iron (hereinafter referred to as " HGI ") using a hot-rolled steel sheet as a plating material, Galvanized hot-rolled steel sheet, which prevents deterioration of materials such as plate bending and aging hardening phenomenon caused by pre-plating heat treatment for securing adhesion and is excellent in plating adhesion.

Since the hot-dip galvanized steel sheet can be produced at a relatively low cost at the same time as excellent rust-preventive properties, the use of the hot-dip galvanized steel sheet is diversified. In particular, HGI materials using hot-rolled steel sheets as plating materials do not require a cold rolling process as compared to cold-rolled galvanized iron (hereinafter referred to as "CGI") using cold-rolled galvanized steel sheets, The heat treatment process is simple and the manufacturing cost is low, and the demand thereof is increasing. Therefore, HGI materials have been increasing in demand for high-grade steels such as processed steels and high tensile steels in conventional low-carbon steels and structural steels.

Since CGI materials use cold rolled steel sheets as plating materials, high temperature annealing at 700-800 ° C is required to secure the quality of the coated steel sheets. Accordingly, CGI can be manufactured in various steel types by changing the heat treatment process according to the steel composition and purpose of the plating material.

On the other hand, HGI materials use hot-rolled steel sheets that are annealed simultaneously with hot-rolling, so they do not require high-temperature annealing to secure the quality of steel before plating, such as CGI materials that use cold-rolled steel sheets as plating materials. In order to secure the adhesion of the plating through the reduction of the oxide film formed after the removal of the foreign oxide film and the removal of foreign substances, the steel sheet is heated only at a temperature lower than the A ₁ transformation point of 500 - 600 ° C in the heating zone, There is no change in the material of the steel.

However, as shown in FIG. 1, the HGI material exhibits increased yield strength and elongation at yield point due to conventional plating heat treatment as shown in FIG. 1, resulting in sheet bending and material deterioration during processing, and in particular, It is impossible to produce high-grade steel such as machined steel or high-strength steel.

In order to investigate the cause of material deterioration of HGI material by the HGI heat treatment process, 0.038% C-0.15% Mn (which improves workability by lowering of solid solution of ferrite due to precipitation of Cr-carbide and softening of ferrite base) - The properties of steel before and after heat treatment of HGI for 0.5% Cr Cr-added hot-rolled steel were investigated.

First, Cr is added hot rolling processing molten steel tensile strength of 32.6 kg / mm ^2, yield strength of 21 kg / mm ^2, a tensile strength of 35 kg / mm ² or less, materials with an elongation 47% for processing lecture based on yield strength of 22 kg in the hot-rolled steel sheet condition / mm ² or less, a satisfactory elongation at least 40%, in the case to be heat-treated by two help HGI gayeoldae temperature 520 ℃ as hot dipping, plating material of the hot-rolled steel sheet compared to the yield strength and tensile strength of 6 kg / mm, respectively ² and 2.5 kg / mm ² , and elongation decreased by 9% from 47% to 38%. In addition, the elongation at yield point was reproduced at 7.5%, and the age hardening index for evaluating the aging property of steel sheet was 3.8 - 4.1 kg / mm ² , which was 2.5 kg / mm ² higher than that of hot - rolled steel sheet.

The amount of dissolved carbon and nitrogen in the hot - rolled steel sheet was found to be less than 1 ppm in the hot - rolled steel sheet, 4.7 ppm.

According to FIG. 2 showing the material characteristics according to the HGI heat treatment temperature, the tensile strength is gradually increased and the elongation rate is sharply lowered as the temperature of the HGI heating stand is increased. At the same time, it was confirmed that the yield strength remarkably increased and the yield point elongation rises and increases.

As a result of investigating the material change behavior before and after the HGI heat treatment, it can be confirmed that the material deterioration due to the HGI heat treatment is caused by the following causes.

It can be seen that the material deterioration due to the HGI heat treatment, that is, the rise in the yield strength, the reappearance and the increase in the yield point elongation, and the decrease in the elongation tendency are caused by the age hardening phenomenon by the hiring element. The aging phenomenon of steel is a phenomenon in which interstitial solute elements such as nitrogen or carbon in the steel interact with dislocations. It can be seen that the increase in strength and the increase in elongation at yield point are proportional to the amount of employment elements.

In particular, the behavior of the yield strength increase, the yield point elongation rate, and the elongation rate decrease, which is caused by heating, is accelerated by the HGI heat treatment temperature of the steel, and this is because the HFC ash content is further increased as compared with the hot- It can be seen that there is a correlation with the HGI heat treatment temperature. In other words, the age hardening phenomenon of HGI steel is related to the increase of the solid element by heating in addition to the amount of solid element.

Therefore, the unstable carbide in the hot rolled steel sheet, which is the plating material of the HGI as the factor of lowering the elongation, is partially reused due to the increase of the heat treatment temperature of HGI, and the increased employment element interacts with the dislocation as compared with the hot rolled state.

Conventional methods for preventing age hardening include: 1) a method of adjusting a component system to an extremely low carbon steel which inhibits generation of cementite in a ferrite and a solid content of a hot-rolled steel sheet; 2) a method of adding a carbonitride- (3) a method of coating Ni or a Ni-based alloy by a pretreatment of plating to reduce the heating temperature before plating to suppress redissolution of a solid element (Japanese Patent Laid- 44168) method is proposed.

However, the method 1) of modifying the steel component system to extremely low carbon steel to suppress the generation of solid carbon can prevent the deterioration of the HGI material due to aging hardening to some extent, In the case of hot rolling, since the Ar3 transformation point is high, a fine grain and a coarse grain are mixed due to the temperature difference between the outside and inside of the steel sheet, resulting in a reduction in the elongation. Also, making all steel grades into extremely low carbon steels is uneconomical as decarburization load and manufacturing cost increase.

In addition, the method 2) for stabilizing the solid element by adding the element for forming the carbon and nitride is not economical because of addition of expensive Ti, Nb, V and the like. Moreover, the deterioration of the plating adhesion and the change in the material And thus it is not preferable.

In the CGI process in which the cold-rolled steel sheet containing the hard-coating elements such as Si, Cr, Mn and the like is subjected to the high-temperature annealing before plating, the alloying elements such as Si, Cr and Mn It is diffused to the surface of the steel sheet to form an oxide film of these elements. Therefore, it is widely used as a method for solving the problems of unplated and plating peeling caused by lowering of wettability with hot zinc during zinc plating. However, since the HGI process using a hot-rolled steel sheet as a plating material in which the material is adjusted simultaneously with hot-rolling is a low-temperature heat treatment, the problem of plating detachment due to surface enrichment and oxidization of the elements is difficult. Therefore, Pre-Coating of complex processes such as, for example, leads to increased operating costs.

Particularly, the pre-coating elements have a large hardness and a low ductility. When the amount of plating is large, there is a problem that plating peeling occurs during processing.

Generally, as shown in FIG. 1 and FIG. 3 (a), a hot dip galvanized hot-rolled steel sheet is manufactured by a method in which a hot rolled steel sheet S ₁₁ , After the pickling treatment (S ₁₂ ) is carried out, the temperature of the steel sheet in the heating stand (S ₁₃₂ ) is heated to 500 - 600 ° C in order to reduce the adhesion of the plating due to reduction of the oxide film formed after the pickling, The steel sheet is cooled in the cooling stage (S ₁₃₃ ) at an appropriate temperature of 420 to 46 ° C. At this time, since it is not easy to cool accurately to the target temperature in the cooling zone, the steel sheet temperature is lowered to a temperature lower than the target temperature, and the steel sheet is heated and maintained at the target temperature in the crack zone ( _S134 ) (S ₁₄ ) by immersing the plating bath in the plating bath by optimizing the deposition temperature of the steel sheet by reducing the temperature deviation in the direction of the plating.

However, as the temperature of the heating stage (S ₁₃₂ ) is higher, the reduction of the thin oxide film remaining in the pickling process (S ₁₂ ) and the removal of the foreign substances are easy and it is possible to secure the plating adhesion by surface cleaning and activation. The strength is increased compared with the hot-rolled steel sheet, which is a plating material, and the material deterioration phenomenon in which the elongation rate is lowered can not be avoided.

The present invention has been made in order to solve the problems of the above-described prior art, and it is an object of the present invention to prevent the deterioration of the material of the HGI material caused by the HGI heat treatment before plating for securing the surface cleanliness of the hot- It is another object of the present invention to provide a method for manufacturing a hot-dip galvanized hot-rolled steel sheet which is capable of diversifying steel grades to high-grade steel as well as low-carbon steels and high-

In order to accomplish the above object, the present invention provides a method for scaling a hot-rolled steel sheet as a plating material, comprising the steps of pickling a hot-rolled steel sheet, removing the smut attached in the pickling process and the oxide film of the thin film remaining after pickling, A pretreatment step of plating the surface of the base material with 1.5 - 2 g / m ² surface grinding treatment to grind and activate the surface by grinding with a grinder roll, etc., and to prevent material deterioration due to heat treatment at 500 - 600 ℃ before plating An HGI heat treatment step of performing a low temperature heat treatment of the heating plate steel plate at a temperature of 420 to 46O < 0 > C before plating to ensure plating adhesion, and a step of dipping in a plating bath having a bath temperature of 420 to 46O < 0 & A method for manufacturing a hot-dip galvanized hot-rolled steel sheet excellent in internal material thermal resistance and plating adhesion.

Further, in the method for manufacturing a hot-dip galvanized hot-rolled steel sheet excellent in thermal resistance and plating adhesion of the present invention, the HGI heat treatment step is performed in two stages of a preheating-heating zone.

FIG. 1 is a graph showing a comparison between the present invention and a conventional heat-treatment process in a hot-dip galvanized hot-rolled steel sheet manufacturing process. FIG.

FIG. 2 is a graph showing a change in material according to a heat treatment temperature in a method of manufacturing a hot-dip galvanized steel sheet;

3 is a view showing a comparison between the present invention and a conventional method of manufacturing a hot-dip galvanized hot-rolled steel sheet

Figure 3 (a) is a block diagram of a conventional process.

3 (b) is a block diagram of the method of manufacturing the present invention.

DESCRIPTION OF THE REFERENCE NUMERALS

S ₁₁ , S ₂₁ : hot-rolled steel sheet S ₁₂ , S ₂₂ : Pickling treatment

S ₁₃ , S ₂₄ : heat treatment S ₁₃₁ , S ₂₄₁ :

S ₁₃₂ , S ₂₄₂ : Heating stand S ₁₃₃ : Cooling stand

S ₁₃₄ : SUSPENSION S ₂₃ : Surface grinding

S ₁₄ , S ₂₅ : plating

Hereinafter, a method of manufacturing a hot-dip galvanized hot-rolled steel sheet excellent in thermal resistance and plating adhesion of the present invention will be described in detail.

In FIG. 1, the heat treatment process in the manufacturing method according to the present invention is shown in comparison with the heat treatment process in the prior art, and the third (b) A method of manufacturing a hot-rolled steel sheet is shown in block diagrams.

As shown, to the present invention, in order to prevent the material deterioration caused by performing the reduction and impurity removal of the thin oxide film remaining in the pickling process in a hydrogen atmosphere at a high temperature, pickling process (S ₂₂₎ and HGI heat treatment step (S ₂₄ ), the surface of the plating material is ground by 1.5-2 g / m ² (S ₂₃ ) by a physical method such as a brush roll or a grinder roll to remove the residual oxidation film and the smut, Since the surface activation is given to the steel sheet, the steel sheet temperature in the heating stand (S ₂₄₂ ) is subjected to the low-temperature heat treatment at 420 to 460 캜, which is the same as that of the plating bath. Therefore, it is possible to prevent deterioration of the HGI steel sheet due to redeposition of the solid carbon by HGI heating temperature of 500 - 600 ° C, and to secure the adhesion of the plating.

The reason for limiting the surface grinding throughput to 1.5 - 2 g / m ² as a pretreatment method to prevent material deterioration of HGI materials is as follows.

When the surface grinding amount is less than 1.5 g / m ^2, it is impossible to completely remove the oxide film and smut of 2500 - 5000A remaining after the pickling. Therefore, HGI heat treatment is performed at 500 - 600 ° C for reduction of the oxide film as in the conventional method The strength and elongation at yield point are increased, and deterioration of the material in which elongation is reduced can not be prevented.

On the other hand, when the surface grinding amount is 2 g / m ² or more, it is possible to completely remove the oxide film and the smut, but since the excessive surface irregularities are formed, uneven active points are given. As a result, it is not preferable because a large number of grinding rolls are required because it is necessary to induce the formation of Fe-Zn alloy layer locally on the plated layer / substrate iron interface and to perform a low speed operation for excessive surface grinding. On the contrary, in the case of the surface grinding amount of 1.5 - 2 g / m ² as defined in the present invention, the remaining oxide film can be completely removed and the surface smoothness of the plating material can be ensured, thereby preventing formation of a local alloy layer. Therefore, in the present invention, the surface grinding amount of the plating pretreatment is limited to 1.5 - 2 g / m ² .

In the present invention, a surface grinding (S ₂₃₎ and then processes the HGI conventional third heat treatment (S ₂₄₎ (a) also as shown, for example, tropical (S ₁₃₁₎ - gayeoldae (S ₁₃₂₎ - cooling zone ( S ₁₃₃ ) -Sk ₁₃₄ , the third step (b) simplifies the process by the two-step process of the preheating zone (S ₂₄₁ ) -heating zone (S ₂₄₂ ) as shown in FIG. It is limited to 420 - 46 ° C.

In the conventional hot-dip galvanized hot-rolled steel sheet manufacturing method, the temperature of the heating stand is heated up to 600 ° C in order to clean and activate the oxide film on the surface of the base material formed after the pickling. However, It is possible to clean and activate the surface of the plating material by grinding treatment. As in the prior art, it is not necessary to perform the heat reduction treatment at a high temperature of 500 - 600 ° C.

However, when the steel sheet temperature of the heating zone is lower than 420 캜, when the steel sheet is immersed in the plating bath, the temperature of the plating bath is lowered and the temperature of the plating bath is lowered to 419 캜 or lower, which is the melting point of zinc. Wettability can not be ensured and thus unplated or deteriorated plating adhesion. Therefore, since the plating bath must be continuously heated to maintain the plating bath temperature, the load of the plating bath is increased, which is undesirable. On the other hand, when the steel sheet temperature of the heating zone is 460 DEG C or higher, as shown in Fig. 2, the yield strength increases and the elongation rate sharply decreases due to the redissolution and grain growth of carbide, thereby deteriorating the quality of the steel for processing. Therefore, in the present invention, the temperature of the heating zone of the HGI heat treatment process is set to 420 to 460 DEG C, which is the same as the temperature of the plating bath.

Since the temperature of the heating zone can be lowered from 420 to 46O < 0 > C in the conventional hot-dip galvanized steel sheet manufacturing process, it is heated up to 600 DEG C It is possible to omit the cooling zone (S ₁₃₃ ) and the crack zone (S _l34 ) which serve to adjust the temperature to 420 to 460 ° C, which is suitable for plating. Therefore, the HGI heat treatment process of the present invention can simplify the process to two stages of the preheating-heating zone in the fourth stage of the conventional preheating-heating zone-cooling zone-crack zone as shown in FIG. 3 (b) There is an additional advantage that productivity can be improved by saving and improving the line speed.

As described above, since the smut attached in the pickling process for scaling off the hot-rolled steel sheet as the plating material and the oxide film of the thin film remaining after the pickling are removed by the surface grinding process such as the brush roll and the grinder roll, Since the activation is achieved, the steel sheet temperature in the HGI heating zone is only 420 - 460 ° C, which is the minimum temperature for displaying the wettability. Therefore, it is possible to prevent deterioration of material and deterioration of plating adhesion due to high-temperature heat treatment before plating, so that it is possible to produce not only conventional low carbon steel but also processed steel and high tensile steel HGI material, have.

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

[Example]

The presence or absence of deterioration of the material according to the temperature of the HGI heat treatment process was measured using three kinds of test materials having the compositions shown in Table 1, that is, general low carbon steel used as a comparative material, Cr added processed steel, and Si added high tensile steel as test materials. Was subjected to a tensile test to evaluate its mechanical properties after being processed into a tensile specimen (JIS No. 5 test piece).

The presence or absence of material deterioration was judged by satisfying the material criteria for each steel type, and the results are shown in Table 2.

And the HGI heating zone temperature satisfies the silver halide range of the present invention. It is possible to prevent deterioration of material due to HGI heat treatment because the increase in strength and the decrease in elongation are insignificant in the No. 4 steel grade (A grade), No. 9,10 (B grade grade) and No. 13,14 (C grade grade). However, when the temperature of the HGI heating zone deviates from the temperature range of the present invention, an increase in the amount of solid carbon due to the redissolution of the carbide causes an increase in the yield strength and deterioration of the material in which the elongation decreases rapidly.

The hot-rolled scale of each steel type, which is the test material, was immersed in a hydrochloric acid pickling bath, and the surface was ground by adjusting the amount of surface grinding as shown in Table 3, in order to remove the remaining oxide film and smut remaining on the brush roll.

The test specimens subjected to the surface grinding treatment in the pretreatment step were subjected to a hot grinding treatment in a hot-dip galvanizing tester at 10 to 20% hydrogen and atmospheric gas containing the remainder nitrogen for 30 to 60 seconds in the heating zone temperature condition (420 to 460 deg. After heat treatment, the substrate was immersed in a plating bath containing 0.2 wt. 96 Al for 3 seconds and then subjected to gas wiping treatment to adjust the plating adhesion amount of the cross section to 150 g / m ² .

The evaluation of the adhesion of the plating with each condition after the plating was judged as to whether or not the plating occurred and whether or not the plating was peeled off. In other words. The presence or absence of unplated plating was visually observed. Whether or not the plating was peeled off was evaluated by naked eyes after the bending test. The results are shown in Table 3.

In No.4 and No.5 are each HGI gayeoldae case where a temperature increases only the low temperature heat treatment in order to prevent the material deterioration due to the pre-plated surface if a 1.5 g / m ² or less of the amount of grinding, removing the oxide film remaining after the pickling It is impossible to secure the adhesion of the plating. In addition, No. 7 has a surface grinding amount of 2 g / m ² or more, and it is possible to completely remove the oxide film and foreign matter after the pickling, but the formation of the Zn / Fe alloy layer due to excessive unevenness on the surface of the plating material ensures plating adhesion Is impossible. No. 6 satisfies the conditions of the present invention, it is possible to prevent the material from being thermally oxidized and to display the adhesion of the plating.

According to the method for producing a hot-dip galvanized hot-rolled steel sheet excellent in thermal resistance and plating adhesion of the present invention, it is possible to lower the temperature of the heating zone of the HGI heat treatment process to 420 to 46O < Since the process can be simplified, energy savings and diversification of the steel grade can be achieved. Therefore, according to the method of the present invention, an effect of economically manufacturing a hot-dip galvanized hot-rolled steel sheet having no material thermal degradation and excellent in plating adhesion can be obtained.

Claims (1)

Grinding the hot-rolled steel sheet to a surface grinding throughput of the surface of the base material in the range of 1.5 to ² g / m < ² >; and preheating the ground hot-rolled steel sheet at a temperature of 130 to 170 & And a hot-dip galvanizing step of immersing the hot-rolled steel sheet in a galvanizing bath at 420 to 460 ° C for hot-dip galvanizing and hot- A method of manufacturing a steel sheet.