KR20060074691A - Manufacturing method of high strength hot dip galvanized steel sheet - Google Patents

Abstract

Provided are a method for producing a hot-dip galvanized steel sheet containing a non-plating component and a hot-dip galvanized steel sheet obtained therefrom. In the manufacturing method of the hot-dip galvanized steel sheet of the present invention, the step of winding the hot rolled steel sheet containing a non-galvanizing component and cooling to form a busite scale layer on the surface layer, eliminating the pickling treatment of the hot-rolled steel sheet and the bus in the reducing atmosphere Reducing heat treatment of the scale layer of tight, cold rolling the reduced heat treated steel sheet,

And re-annealing the cold rolled steel sheet to hot-dip zinc plating. According to the present invention, the pickling facility for pickling the hot rolled scale layer of the hot rolled steel sheet, the flushing device and the drying facility can be omitted, thereby eliminating environmental problems caused by the shortening of the manufacturing process line and the pickling process.

Molten Zinc, High Strength Steel Sheet, Bustite, Wetability, Hot Rolled Scale

Description

METHOD OF MANUFACTURING HIGH STRENGTH HOT DIP ZINC PLATED STEEL SHEET AND THE OBTAINED HIGH STRENGTH HOT DIP ZINC PLATED STEEL SHEET}

1 is a method of manufacturing a hot-dip galvanized steel sheet,

1A is a conventional method,

1B is a method of the present invention.

2 is a schematic view showing a comparison between the conventional hot rolling process and the hot rolling process of the present invention.

3 is an electron micrograph of the surface of the steel sheet according to the reduction ratio in the cold rolling process,

3a is a comparative example,

3B is an example of the invention.

4 is an appearance of a hot dip galvanized steel sheet

4A is a comparative example

4B is an example of the invention.

5 is a photograph evaluation of adhesion of the hot-dip galvanized steel sheet

      5A is a comparative example

      5B is an example of the invention.

The present invention relates to a hot-dip galvanized steel sheet containing a non-plating component and a method of manufacturing the same. More specifically, the selective reduction of the surface of the steel sheet is suppressed by allowing a porous reduced iron layer, which is reduced from the hot rolled scale layer, to be present between the cold rolled steel sheet and the galvanized layer, and forming a composite oxide between the porous reduced layer and the steel sheet and at grain boundaries. The present invention relates to a hot-dip galvanized steel sheet for improving the adhesion of hot-dip zinc on the surface of a porous reduced iron layer and a method of manufacturing the same. According to the present invention, the pickling step performed before the cold rolling step can be omitted.

As a task for the preservation of the global environment, fuel economy regulations have been strengthened, and automobiles have been designed to reduce the weight of automobiles as a countermeasure to improve the fuel efficiency of automobiles. As one of measures to reduce the weight of automobiles, weight reduction of automobile materials is effective by increasing the strength of steel sheet. Accordingly, the demand for high strength hot-dip galvanized steel sheets for automobile bodies is increasing. As the high strength hot-dip galvanized steel sheet, solid solution hardened steel using solid solution hardening elements such as phosphorus and manganese is widely used. However, there is a limit to the increase in machinability of solid steel.

Recently, transformed induced plasticity (hereinafter referred to as TRIP) steel has been attracting attention as a galvanized steel sheet because of its high strength and processability. TRIP steel contains a large amount of silicon, manganese and phosphorus. In particular, silicon is an element capable of maintaining ductility without significantly reducing the strength of steel, and manganese is an austenite stabilizing element, and thus an alloy element necessary for remaining austenite necessary for TRIP steel at room temperature. However, if the steel component contains more than 0.5% by weight of silicon or manganese, it is diffused to the surface of the steel sheet during the recrystallization annealing heat treatment process of the hot-dip galvanized steel sheet to form a silicon-manganese composite oxide so that the concentration is 10 to 100 than the bulk material. It is about twice as high. As such, the silicon and manganese concentrated in the grain boundary or in the mouth react with an extremely small amount of oxygen in the furnace atmosphere to form a silicon oxide film on the surface layer of the steel sheet, which greatly reduces the wettability of the molten zinc in the hot dip galvanizing process. . As a result, it is difficult to secure the wettability, and thus the unplating phenomenon is frequently caused, or even when hot-dip plating, the plating adhesion is deteriorated, thereby causing plating peeling during processing.

As a technique for solving the plating adhesion problem of a high strength steel sheet containing a non-plating component such as silicon and manganese, (1) lead metal technology (2) redox method and the like are known.

(1) plating technology

It is a method to secure the wettability with the molten zinc layer by performing pre-coating by electroless or electroplating of various alloying elements such as Fe, Ni, iron oxide film, etc. before hot dip plating of cold rolled steel sheet. Even if the alloying element is concentrated in the base iron interface by the concentration in the lower leading layer to block the reaction with moisture in the atmosphere during the annealing process or heating process.

However, the lead gold is a high hardness and insufficient ductility, there is a problem that the plating peeling occurs during processing when the plating deposition amount is large. In addition, when the lead is carried out by the electroplating method, additional equipment is required, and thus the equipment is complicated and the manufacturing cost is high, which is not economically desirable.

(2) redox method

The heating zone of the annealing furnace is adopted as a direct fired furnace method and the excess air is increased from 0.9 to 1.05 to increase the thickness of iron oxide.Reduction heat treatment in the cracking zone of the annealing furnace results in the formation of pure iron on the surface of the steel sheet. Surface thickening, etc. can be suppressed. However, if the oxide film remains thick in the direct furnace, plating peeling may occur. In addition, when silicon is concentrated on the surface of the steel sheet, the galvanized layer may not be strongly adhered to the surface of the steel sheet or unplated. Therefore, it is very important to properly control the thickness of the iron oxide in the stove, there is a problem that it is difficult to apply the site.

On the other hand, Korean Patent Publication No. 95-703070 discloses that after the continuous annealing of a high strength steel sheet containing 0.1 to 2.0% by weight of silicon to remove the silicon thickening layer by pickling and polishing, the steel sheet is again 650 ℃ or more ~ recrystallization temperature A high-strength hot dip galvanizing method has been proposed in which hot dip galvanization is carried out under reduced conditions to reduce unplated defects. However, when the silicon-containing TRIP steel is heated to 650 ° C. or higher, silicon may be segregated on the surface of the steel sheet again to cause unplating, and residual austenite and bainite may be thermally decomposed to significantly degrade the material.

The present invention has been made in order to solve the problems of the prior art as described above, by using a porous reduced iron layer that is left by reducing the pickling process of hot-rolled steel sheet as a reduction treatment of the non-plating component in the recrystallization annealing process The present invention relates to a method for producing a high strength hot dip galvanized steel sheet capable of improving plating adhesion and a high strength hot dip galvanized steel sheet obtained therefrom. If the pickling process is omitted in accordance with the present invention, it is possible to omit peripheral equipment for the management and maintenance of the pickling equipment, and it is possible to establish an environmentally friendly manufacturing process and reduce manufacturing costs in the future.

Method for producing a high strength hot dip galvanized steel sheet of the present invention for achieving the above object,

Winding and cooling the hot rolled steel sheet including the non-plating component to form a bustite scale layer on the surface layer,

Omitting the pickling treatment of the hot rolled steel sheet and reducing heat treatment of the scale layer of the busstatite in a reducing atmosphere;

Cold rolling the reduced heat treated steel sheet,

And re-annealing the cold rolled steel sheet to hot-dip zinc plating.

Cooling after the winding in the present invention is to form a busite hot rolled scale on the surface layer, one of the conditions is wound at a temperature of 570 ℃ or more to cool at a rate of 300 ℃ / min or more to a temperature range of 400 ~ 500 ℃ will be. When the winding and cooling are performed under such conditions, the thickness of the scale layer of the beustite satisfies a condition of 2 μm or less.

In the present invention, the reduction heat treatment of the scale layer is preferably such that the scale layer of the beustite is reduced by 80% or more. The cold rolling is preferably performed at a reduction ratio of 70% or more to reduce the size of the pores and cracks of the hot rolled scale.

In the present invention, recrystallization annealing is preferably performed in a reducing atmosphere.

The most non-plating components of the present invention are Si, Mn and P.

Hot-dip galvanized steel sheet obtained by the present invention,

The cold rolled steel sheet is formed of a three-layered structure of a reduced iron layer and a zinc plated layer, and a Fe-X-based composite oxide (X is a non-plating component) at the grain boundary of the steel sheet and the boundary between the steel sheet and the hot rolled scale layer. It exists. In the case where X is one or two of Si and Mn, the Fe-X oxide is one or two of Si-Fe and Si-Mn-Fe oxides.

Hereinafter, the present invention will be described in detail.

The present inventors have obtained in the course of research to improve the plating adhesion while omitting the pickling process in the hot dip galvanizing process for high strength cold rolled steel sheet containing a non-plating component. According to the present invention, if the porous reduced iron layer acts as a diffusion inhibitory layer of the non-plating component in the recrystallization annealing process of the cold-rolled steel sheet without removing the hot-rolled scale of the hot-rolled steel sheet containing the non-plating component (omitted by pickling), plating adhesion may be improved. The present invention has been completed by paying attention to the fact that it can be used.

1 shows a manufacturing process of a hot rolled steel sheet which schematically compares the conventional method with the method of the present invention. In the present invention, the hot rolled scale is formed of a busite scale layer, and instead of omitting the pickling step, a reduction heat treatment step is added to suppress surface concentration of the non-plated components by the hot rolled scale.

The steel plate which becomes the object of this invention is a steel plate containing a non-plating component. The non-plating component is an element for oxidation, which concentrates as an oxide in the surface layer in the manufacturing process of a steel plate. The non-plating components include Si, Mn, P, Cr, and the like, in particular, steel sheets containing Si and Mn are fatal to hot dip galvanizing. Hereinafter, it demonstrates using the steel plate which Si and Mn added as a non-plating component.

In the high strength steel sheet including Si and Mn, liquid iron-silicon oxide and iron-silicon-manganese composite oxide (Fe-Si-Mn oxide) crystal grain boundaries directly under the surface layer in a hot rolling process. It causes complex and various forms of selective oxidation along the back. Since the silicon-manganese-iron oxide is first formed at the boundary between the scale and the matrix structure, it greatly inhibits the mechanical peelability and chemical peelability (pickling) of the hot rolled scale and remains as a defect on the surface of the hot rolled product. Therefore, in the present invention, rather than removing the hot rolled scale, they are to be used as a selective oxide layer effective for improving the adhesion between the hot dip layer and the steel sheet. The manufacturing method for obtaining the porous reduced iron layer will be described for each process step.

[Hot rolling process-including winding process and cooling process]

In the present invention, after hot-rolling a steel containing a non-plating component, the hot-rolled steel sheet after the winding and cooling to have a hot-rolled scale of beustite (FeO). Bustite's hot rolled scale is easy to reduce.

A method for forming a hot rolled scale of busstatite is shown in FIG. 2. Finishing rolling in hot rolling process and winding at 570 ℃ or higher, and forced at speeds of 300 ℃ or more from 400 ℃ to 500 ℃ temperature range, so that it can remain at room temperature from the temperature range in which the vistaite (FeO) exists in the stable phase. Cool down. In this way, a hot rolled scale composed of 100% bustite can be obtained. Preferably the thickness is less than or equal to 2㎛.

It is also preferable to perform the degreasing treatment before the reduction heat treatment described later.

[Reduction Heat Treatment Process]

Reduced heat treatment of the hot rolled steel sheet having the hot rolled scale of the Busteite. In the steel industry, the reducing atmosphere is mainly nitrogen gas containing hydrogen. Preferred reduction gas in the present invention is to use nitrogen gas containing 10% or more of hydrogen.

In a reduction heat treatment process, hydrogen gas must be in direct contact with the iron oxide surface. That is, the reducing gas is diffused to the inside and the gas generated by the reaction should be released to the outside. However, in the case of reducing dense iron oxide, a reducing gas is generated by a topochemical reaction. Existing scale cracks will diffuse the reducing gas sufficiently into the internal oxide layer interface and the iron oxide will gradually be reduced to lower dimensional oxides.

The reduction process in the hot rolled sheet is shown in Equation 1.

[Relationship 1]

3 Fe ₂ O ₃ + H ₂ → 2 Fe ₃ O ₄ + H ₂ O

Fe ₃ O ₄ + H ₂ → 3 FeO + H ₂ O

FeO + H ₂ → Fe + H ₂ O

In the present invention, it is preferable to reduce the hot rolled scale layer to 80% or more. That is, it is preferable to reduce | reduce so that 80% of the hot-rolled scale layer of buste (FeO) may become FeO. Reduction heat treatment conditions for this is performed in a mixed gas atmosphere of hydrogen nitrogen for 60 to 240 minutes at a temperature of 620 ~ 700 ℃.

In the reduction heat treatment process according to the present invention, about 50% of the total interface between the matrix and the scale layer is reduced and the keying compound (silicon-manganese-iron), which is formed first at the grain boundary, adheres the scale layer and the matrix together. (keying) action to improve the adhesion of the scale layer to the matrix structure. In addition, the volume of the scale layer shrinks about 27% by the reduction reaction, and the pores and microcracks formed during the reaction become a size that can act as a starting point of the plating layer peeling after hot dip galvanizing. Therefore, it is desirable to reduce the size of the pores and microcracks in the cold rolling process described later.

[Cold rolling process]

In the cold rolling process, it is desirable to push down to reduce the size of the pores and microcracks of the reduced porous iron layer. If not down, there is a deep microcracks in the porous reduced iron layer, which acts as a starting point for the plating layer peeling crack during processing. For this purpose, the cold reduction rate is preferably 70% or more.

Recrystallization Annealing Process

The cold rolled steel sheet is subjected to recrystallization annealing, and the annealing at this time may be performed under ordinary annealing conditions. The annealing atmosphere is performed in a reducing atmosphere so that the reduced iron layer which has been reduced is not oxidized.

[Plating Process]

The recrystallized annealing steel sheet is hot dip plated. In the case of zinc plating, it can be performed in the zinc bath containing aluminum.

The hot-dip galvanized steel sheet obtained according to the present invention is formed of a three-layer structure of a cold rolled steel sheet, a porous reduced iron layer and a zinc plated layer reduced thereon, and a grain boundary of the cold rolled steel sheet and a boundary between the cold rolled steel sheet and the porous reduced iron layer. Fe-X-based composite oxide (X is a non-plating component) is present. The porous reduced iron layer is made to be vacant through reduction heat treatment, as shown in Figure 3b. X is one or two of Si and Mn, and the Fe-X oxide is one or two of Si-Fe and Si-Mn-Fe oxides.

According to the present invention, the three-stage pickling equipment, the washing equipment, and the drying equipment for pickling the hot rolled scale layer of the hot rolled steel sheet may be omitted, thereby eliminating environmental problems caused by the shortening of the manufacturing process line and the pickling process.

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

EXAMPLE

Including the chemical composition shown in Table 1, the TRIP hot rolled steel sheet composed of the remaining Fe and other unavoidable impurities was wound at 620 ° C, and then cooled to 50-400 ° C per minute to a medium-cooling temperature of 400-500 ° C. Formed.

Chemical composition (% by weight) ingredient C Si Mn P S content 0.14 1.60 1.60 0.022 0.01

The hot rolled sheet having a hot rolled scale (100 mm × 200 mm × 3.2 mm) was degreased with a scale, followed by 60 to 240 minutes of reduction heat treatment at a temperature of 620 to 700 ° C. in a reduction heat treatment furnace of a 30% hydrogen-nitrogen mixed gas atmosphere. 50-80% cold rolling was performed on the hot-rolled steel sheet reduced at a reduction rate of 60-90% by changing scale forming conditions and reducing heat treatment conditions. The cold-rolled steel sheet was used for hot-dip galvanizing simulator for 40 seconds at a temperature of 820 ° C. for recrystallization annealing. The annealed cold rolled steel sheet was immersed in a zinc plating bath containing 0.13% aluminum to perform hot dip galvanizing.

Plating adhesion to the obtained hot-dip galvanized steel sheet was subjected to a 0t bending test to observe the bending portion. In addition, whether or not unplated was visually evaluated. The evaluation results are shown in Table 2 together with the manufacturing conditions. Conventional example in Table 2 is a steel sheet manufactured by the manufacturing method of Figure 1a.

division Cooling rate (℃ / min) Intermediate quench temperature (℃) Reduction Rate (%) Cold Rolling Rate (%) Unplated Defect Adhesion     Remarks Inventive Example 1 300 400 80 70 ○ ○ 3b Inventive Example 2 400 400 80 70 ○ ○ 4b Inventive Example 3 400 400 80 80 ○ ○ 5a Inventive Example 4 300 400 90 70 ○ ○ Inventive Example 5 300 500 70 50 ○ ○ Comparative Example 1 300 400 80 50 △ × 3a Comparative Example 2 300 400 80 50 × × 5b Comparative Example 3 50 400 80 50  ○ × 4a Comparative Example 4 50 400 60 70  × × Conventional example 5 - -  × ×

As shown in Table 2, the invention examples obtained by cold-rolling a hot-rolled steel sheet subjected to reduction heat treatment at a reduction rate of 80% or more at 70% or more have excellent plating adhesion and no unplated defects. Plating adhesion and plating properties of the inventive examples have been significantly improved so as not to be compared with the prior art.

3 is a scanning micrograph of the surface of the steel sheet for Inventive Example 1 (FIG. 3B) and Comparative Example 1 (FIG. 3A). In Comparative Example 1 having a cold reduction rate of 50%, the surface roughness is very large because pores and microcracks are not completely smoothed but remain. In contrast, in the case of Inventive Example 1 having a cold reduction rate of 70%, it can be seen that most of the reduction shrinkage cavities were smoothed.

4 is a photograph showing the surface state of the hot-dip galvanized steel sheet for Inventive Example 2 (FIG. 4B) and Comparative Example 3 (FIG. 4A). In the case of Comparative Example 3 having a reduction rate of 60% in the reduction heat treatment, unplating occurred due to the lack of wettability at the unreduced scale. On the contrary, in the case of Inventive Example 2 having a reduction rate of 80% in the reduction heat treatment, good wettability was shown.

5 is a photograph showing the plating adhesion of Inventive Example 3 (FIG. 5B) and Comparative Example 2 (FIG. 5A). In the case of Comparative Example 2, the reduction was sufficient in the reduction heat treatment step, but the cold reduction rate is low. Referring to FIG. 5B, in the case of Comparative Example 2, the wettability of the molten zinc was improved due to high surface roughness, but the plating property was excellent, but the adhesion was deteriorated by acting as a starting point of the plating layer peeling crack during processing from the deep microcracks.

As described above, according to the present invention, a wet pickling process using an acid can be omitted by forming an easy-to-reduced scale on the surface of a hot rolled steel sheet in the process of manufacturing a high strength hot dip galvanized steel sheet. Omission of facilities and peripheral equipment can be omitted, which can greatly reduce cost and eliminate the possibility of environmental pollution by waste acid. In addition, the reduced porous iron layer formed on the surface of the cold rolled sheet by cold rolling suppresses the selective oxidation of the alloying elements such as silicon and manganese in the hot dip galvanized steel plate, thereby inhibiting wettability and plating adhesion of high strength steel. It is effective to greatly improve.

Claims (10)

Winding and cooling the hot rolled steel sheet including the non-plating component to form a bustite scale layer on the surface layer, Omitting the pickling treatment of the hot rolled steel sheet and reducing heat treatment of the scale layer of the busstatite in a reducing atmosphere; Cold rolling the reduced heat treated steel sheet, And re-annealing the cold rolled steel sheet to perform hot dip galvanizing. The method of claim 1, wherein the cooling after the winding is wound at a temperature of 570 ° C or higher and cooled at a rate of 300 ° C / min or more to a temperature section of 400 to 500 ° C. The method of manufacturing a high strength hot dip galvanized steel sheet according to claim 2, wherein the thickness of the bustite scale layer is 2 µm or less. The method of manufacturing a high strength hot-dip galvanized steel sheet according to claim 1, wherein the reduction heat treatment of the scale layer causes the scale layer of busstatite to be reduced by 80% or more. The method of manufacturing a high strength hot dip galvanized steel sheet according to claim 4, wherein the reduction heat treatment of the scale layer is performed in a mixed gas atmosphere of hydrogen nitrogen for 60 to 240 minutes at a temperature of 620 to 700 ° C. The method of manufacturing a high strength hot dip galvanized steel sheet according to claim 1, wherein the cold rolling is performed at a reduction ratio of 70% or more. The method of manufacturing a high strength hot dip galvanized steel sheet according to claim 1, wherein the recrystallization annealing is performed in a reducing atmosphere. The method of manufacturing a high strength hot dip galvanized steel sheet according to any one of claims 1 to 7, wherein the non-plating component is one or two of Si and Mn. In the hot-dip galvanized steel sheet obtained by the method of claim 8, The hot-dip galvanized steel sheet is formed of a three-layer structure of a cold rolled steel sheet and a reduced iron layer and a zinc plated layer reduced thereon, and the Fe-X at the grain boundary of the cold rolled steel sheet and the boundary between the cold rolled steel sheet and the porous reduced iron layer. Hot-dip galvanized steel sheet, characterized in that the composite oxide (X is a non-plating component). The melting method according to claim 9, wherein X is one or two of Si and Mn, and the Fe-X oxide is one or two of Si-Fe and Si-Mn-Fe oxides. galvanized steel.

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