KR960005233B1 - Making method of hot rolling steel sheet - Google Patents

Abstract

The hot rolled galvanized iron for public works, household and automobile provides excellent deterioration resistance against sheet shaving of pipe and elevator rail. The low carbon steel sheet comprises, in weight percent, 0.01 to 0.02% carbon, 0.5 to 0.8% manganese, 0.02 to 0.06% aluminum, silicon, sulfur and phosphorus not exceeding 0.02%, nitrogen not exceeding 0.08%, and the balance of iron. The sheet is produced by the processes of pickling of the low carbon steel sheet having the same chemical composition as mentioned above as a starting material, heating and holding of the pickled sheet at 500 to 550deg.C for 40 to 150sec, and then galvanizing.

Description

Manufacturing method of hot-dip galvanized hot rolled steel sheet with excellent internal degradation

1 is a heat treatment curve showing an example of heat treatment of a hot dip galvanized cold rolled steel sheet.

2 is a heat treatment curve showing an example of heat treatment of hot-dip galvanized hot rolled steel sheet.

The present invention relates to a method for manufacturing hot-dip galvanized hot rolled steel sheet having excellent thermal degradation properties used in civil construction, home appliances, and automotive steel sheets, and more specifically, in plate bending and the like during the production of structures such as pipes or elevator rails. It relates to a method for producing hot-dip galvanized hot rolled steel sheet excellent in preventing deterioration of the material.

Among the various antirust methods of steel materials, hot-dip galvanized steel sheet is being used in various fields such as civil engineering building, home appliances and automobiles due to its excellent rust prevention and economical efficiency. In particular, the conventional hot-dip galvanized steel sheet refers to a cold-rolled galvanized iron (cold-rolled galvanized iron, hereinafter referred to as "CGI") as a cold-rolled steel sheet (hereinafter referred to as "cold-rolled steel sheet"), Recently, hot rolled steel sheets (hereinafter, referred to as "hot rolled steel sheets") are used due to an increase in demand for thick materials having a thickness of 2 mm or more, and thus they are subdivided into CGI and HGI.

The HGI material causes a difference in manufacturing process due to the difference between the CGI material and the plating material, which will be described with reference to FIGS. 1 and 2.

FIG. 1 and FIG. 2 are heat treatment curves showing an example of heat treatment of CGI and HGI materials, respectively. As shown in FIG. 1, CGI material uses a cold hard steel plate as a plating material to secure workability of the product. In order to perform the annealing treatment of 700-800 ℃ before the plating operation, there is a disadvantage that a high temperature material difference occurs according to the heat treatment process.

On the other hand, as shown in FIG. 2, since HGI material uses a hot rolled steel sheet which is annealed at the same time as rolling, it does not require high temperature annealing treatment to secure workability before plating, and only when reducing and plating the oxide film formed after pickling. Since only annealing is required below A ₁ transformation point to secure plating failure, the manufacturing process is simple, and theoretically, there is no change in mechanical properties before and after plating.

However, when ordinary low carbon steel is plated after low temperature heating treatment of HGI in HGI materials, material deterioration such as plate breakage during the fabrication of structures such as pipe or elevator rail due to the increase in yield strength and increase in yield point elongation. Is generated, so prevention of this is required.

In order to investigate the cause of such material deterioration, as a result of changing the temperature of the heating table during the HGI heat treatment process of FIG. 2, the following results were obtained.

First, yield strength, tensile strength and yield point elongation increased with increasing heat treatment temperature of heating zone. That is, the yield strength and tensile strength of 4kg / mm2 and 1kg / mm2 were increased when HGI was heat treated at a heating zone temperature of 550 ° C for general low carbon steel with a tensile strength of 35kg / mmm2. In addition, yield point elongation was reproduced from 0% to 6% when the plated material was a hot rolled steel sheet.

Second, as a result of quantitative analysis of carbide volume fraction by HGI heat treatment with an image analyzer, the number and volume fraction of carbide (Fe C, cementite) decreased with increasing HGI heat treatment temperature. It was found that carbon, which is an invasive element, was present as a carbide in a hot rolled steel sheet, and the number and volume fraction of carbide decreased as the carbide was re-used as a solid solution carbon after HGI heat treatment.

Finally, the internal friction test, which is most frequently used to quantitatively measure the amount of carbide re-used by HGI heat treatment, showed that the amount of dissolved carbon increased linearly with increasing HGI heat treatment temperature.

Based on the above test results according to the HGI heat treatment temperature, the material deterioration generated in the HGI material is re-carried into carbides and nitrides as solid carbon and solid nitrogen as the HGI heat treatment, and the increased solid carbon and solid nitrogen are displaced. It can be seen that it is caused by strain aging caused by interaction with dislocation to form a Cotrell atmosphere.

As a conventional technique proposed to prevent such strain aging phenomenon, a method of stabilizing solid carbon or solid nitrogen with carbohydrates or nitrides and a skin pass mill before processing HGI have been proposed. However, the method of stabilizing by adding Ti and Nb and V, which are carbide and nitride forming elements, can prevent strain aging, but additional Ti, Nb and V, which are carbide and nitride forming elements, must be added. It is not preferable because it is uneconomical and plating element deterioration and material change can be caused by the addition of these elements.

Therefore, in general, temper rolling is performed before and after HGI heat treatment in order to prevent material degradation of the HGI material, and as the temper rolling elongation increases during temper rolling, the dislocation increases, and the dislocation introduced by temper rolling increases the moving speed. As a result, the moving speed in the crystal coincides with the moving speed added from the outside, so that no yield point stretching occurs, and the yield strength is trivially deformed by relatively small stress. However, when the temper rolling elongation is more than 2%, the strain hardening occurs, the yield strength is increased again to cause material deterioration.

In the case of actual HGI material, in order to remove the yield point stretching phenomenon, rough rolling elongation of 1.1% or more for general low carbon steel and 0.9% or more for structural steel is required.In the case of temporal rolling elongation of 0.5% or more, the spangle of plating layer of HGI material is broken. Since long-term storage at high temperature and high humidity results in black stain, which is a surface defect of black spots, the work is carried out with a temper rolling elongation of 0.5% or less, but yield point stretching is not completely eliminated and yield strength The problem that material degradation occurs because it does not become a hot rolled state is reproduced.

Accordingly, the present invention has been proposed to solve the above-described problems, and does not cause an alloying element addition or plating damage caused by excessive temper rolling, and adjusts the HGI heat treatment temperature while adjusting the conventional steel components. It is an object of the present invention to provide a method for producing HGI having excellent material deterioration.

Hereinafter, the present invention will be described.

The present invention provides a method for producing HGI, in weight% C: 0.01-0.02%, Si: 0.02% or less, Mn: 0.5-0.08%, P: 0.02% or less, S: 0.02% or less, N: 0.008% or less , Al: 0.02-0.06%, and after pickling the low-carbon steel sheet adjusted to the balance Fe, and then heat-treated by maintaining at 500-550 ℃ for 40-150 seconds, excellent intrinsic degradation resistance, characterized in that hot-dip galvanizing It relates to a method for producing HGI.

The present invention is characterized by securing the mechanical properties of the material by inhibiting the formation of solid solution carbon and solid solution nitrogen which is the cause of material degradation of the HGI material to prevent material degradation and by adding manganese. Hereinafter, the reason for numerical limitation on the low carbon steel composition of the present invention will be described in detail.

The carbon content is preferably 0.01-0.02% by weight (hereinafter referred to as "%"), because the carbon content in the steel component is 0.01% or less, since solid solution carbon is not formed by HGI heat treatment. Although material deterioration does not occur, decarburization to less than 0.01% is not economically desirable because a lot of load and cost are consumed. This increased solid solution is due to the material degradation caused by strain aging caused by the formation of the Cottrell atmosphere in interaction with dislocations.

The silicon forms nitride and suppresses deformation aging by nitrogen, but before plating and heat treatment, the amount of Si on the surface of the steel sheet is concentrated by 10-100 times or more than in the bulk state, and the surface-concentrated Si is crystallized. Since it exists in the form of an oxide film of Si or SiO ₂ in grain boundaries and in the mouth, the wettability of molten zinc is reduced and plating adhesion is lowered. Therefore, when used as a material of a plated steel sheet in the present invention, the amount of Si in steel It is preferable to manage to 0.02% or less.

The manganese combines with sulfur, which is a harmful element that causes deoxidant and red light embrittlement, forms MnS, thereby preventing red light embrittlement, and at the same time, has excellent weldability and increases strength due to the refinement of pearlite. This is the most effective element to increase the strength at a low cost. Especially, it is an element that hardly affects the adhesion of plating, and it reduces the carbon content by adding 0.5-0.8% of Mn of 0.2-0.3%. It is desirable to correct the decrease in intensity.

The P and S have little effect on the strain aging as an impurity element, but when the P content is more than 0.02%, Fe ₃ P compounds are formed and segregated, so that they are very fragile and exhibit brittleness at room temperature. If the amount of silver Mn is not sufficient, FeS is formed at the grain boundary. Since FeS is a weak and low melting point compound, it exhibits red brittleness during high temperature processing, so the content of S is preferably limited to 0.02% or less.

Nitrogen contains much more carbon than modified age-generating elements such as carbon or carbon steel, and the aluminum-killed self-carbon phase is AlN combined with nitrogen to AlN. However, if the dissolved nitrogen content exceeds 0.008% or more, material degradation occurs, so the nitrogen content in the steel is preferably at most 0.008% or less.

The solid solution Al (Solute Al) represents Al that is dissolved in a residual amount after the deoxidation of Al added as a deoxidizer in the steelmaking process of aluminum-kilted steel, and there is a possibility that the deoxidation is not completely performed when the content of the solid solution Al is 0.02% or less. Complete deoxidation is possible at 0.06% or more, but it is uneconomical because a large amount of expensive Al must be added. Therefore, the dissolved Al content is preferably 0.02-0.06%.

Hereinafter, the heat treatment conditions of the low carbon steel of the present invention having the above composition will be described in detail.

In general, the material deterioration due to HGI heat treatment is influenced by the heating table temperature and the holding time in addition to the steel component. The higher the heating table temperature, the longer the holding time is advantageous for the reduction of the thin oxide film formed after pickling. As the heating zone temperature increases, the amount of carbon and nitrogen reclaimed increases, and the rate at which these solid atoms move to the potential depends on the temperature, thereby increasing the possibility of material deterioration.

In the present invention, when the temperature of the heating table is lowered to 480 ° C. or less, since the oxide film formed on the surface after pickling is not sufficiently reduced, resulting in a decrease in plating adhesion, heating the temperature of the heating table to 480 ° C. or more, more preferably 500 or more. Although it is preferable to heat it to 550 degreeC or more, since the amount of solid solution carbon increases further and there is a possibility of causing material deterioration, it is preferable to make heating zone temperature into 500-550 degreeC.

It is preferable to maintain 40-150 seconds in the above heating zone temperature, because if the holding time is less than 40 seconds, the oxide film formed after pickling cannot be completely reduced and plating adhesion is lowered. This is because it is excellent in plating adhesion due to the increment, but the workability is lowered, which is not preferable.

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

EXAMPLE

After pickling a general low carbon steel hot rolled steel sheet having the composition shown in Table 1 with the same steel sheet thickness of 2 mm, heat-treated at a steel sheet drawing temperature of 460 ° C. under the heat treatment conditions as shown in Table 1 below, and the plating solution composition was 0.2% Al-0.1% Pb-. After immersion for 3 seconds at 460 ℃ in a plating bath of 99.7% Zn and hot-dip galvanized so that the cross-sectional gold deposition amount is 150g / ㎡ is shown in Table 1.

The mechanical properties described above were evaluated by tensile strength, yield strength, yield point elongation, and material degradation of the present invention steel occurred at yield point elongation of 0.3% or more.

In addition, the above-described dissolved carbon content was measured by an internal friction tester, and material degradation occurred at 5 ppm or higher. On the other hand, the above-mentioned evaluation of plating adhesion was judged to be good when the size of the crack dock of the processed part (bending part) after the OT bending test was 10㎛ or less, and the tendency of material deterioration was the mechanical properties, the amount of carbon employed, and the pipe fabrication. It was evaluated by the occurrence of bending during the bending process.

TABLE 1

YS: Yield strength (Yield streught)

TS: Tensile Strenght

Yp-El: Yield point elongation

O: good

X: bad

As shown in Table 1, in the case of the comparative materials (2) and (7) that the heat treatment temperature is out of the range of the present invention, the plating adhesion is poor and the steel component of the comparative material (12-15) is out of the condition range of the present invention. In the case of material deterioration occurs, in the case of the invention materials (3-5) and (8-10) satisfying the conditions of the present invention, material deterioration did not occur and the coating adhesion was also excellent.

On the other hand, the inventive material can be seen that the mechanical properties are superior to the comparative materials (1), (6) and (11) that do not heat-treated the hot rolled steel sheet.

As described above, the present invention does not require the addition of expensive carbide forming elements by further adjusting the amount of carbon which is the cause of material deterioration in general low carbon steel, and correcting the decrease in strength with manganese, and further adding molten zinc. It is possible to prevent the damage of the sequins due to temper rolling after plating, and economically prevent the material deterioration by HGI heat treatment, and increase the temperature of the heating table. Therefore, it is possible to effectively reduce the surface oxide film formed after pickling. There is a significant improvement.

Claims (1)

In the manufacturing method of hot-dip galvanized hot rolled steel sheet, in weight%, C: 0.01-0.02%, Si: 0.02 or less, Mn: 0.5-0.8%, P: 0.02% or less, S: 0.02% or less, N: 0.008 After pickling the low carbon steel sheet composed of% or less, Al: 0.02% -0.06%, and the balance Fe, the heat treatment is performed by maintaining the heat treatment at 500-550 ° C. for 40-150 seconds, followed by hot dip galvanizing Method for manufacturing hot-dip galvanized hot rolled steel sheet with excellent chemical properties.