KR20050105778A - A continuous hot-dip galvanizing method for manufacturing a high strength steels - Google Patents

Abstract

The present invention relates to a method of manufacturing a hot-dip galvanized steel sheet having a uniform coating layer, excellent adhesion, and excellent appearance of the final product by plating the molten zinc after the alloy elements are completely reduced in a strong reducing component crisis during the hot dip galvanizing process.

Method for producing a hot-dip galvanized steel sheet according to the present invention is any one of Mo 5.0wt% or less, Cr 5.0wt% or less, Mn 4.0wt% or less, Si 1.0wt% or less, Ti 0.7wt% or less, Al 0.1wt% or less Or preparing a cold rolled steel sheet including two or more alloying elements, the balance being Fe and other unavoidable impurities; Completely degreasing and drying the steel sheet; Reducing annealing of the stripped steel sheet in a continuous furnace formed of a heating zone, a cracking zone, and a cooling zone while heating, cooling, and reducing annealing in a strong reducing atmosphere; Coating the plated layer on the reduced annealed steel sheet in a molten zinc bath or a molten zinc bath containing alloy elements; Solidifying the plating layer coated on the steel sheet; It provides a method of manufacturing a hot-dip galvanized steel sheet comprising a.

Description

Manufacturing method of high strength hot-dip galvanized steel sheet {A CONTINUOUS HOT-DIP GALVANIZING METHOD FOR MANUFACTURING A HIGH STRENGTH STEELS}

The present invention relates to a method for producing a high strength hot dip galvanized steel sheet, and more particularly, to a continuous hot dip galvanizing method of a high strength hot dip galvanized steel sheet having excellent plating properties and surface appearance.

In general, the method for manufacturing hot-dip galvanized steel sheet is to degrease the cold-rolled carbon steel sheet, reduce and anneal it, and then dip the molten zinc or alloy element on the surface of the steel sheet by immersing it in a plating bath in which metal zinc or its alloy is molten. Cooling while controlling this to the required thickness to produce a hot-dip galvanized steel sheet.

In the method for manufacturing a hot-dip galvanized steel sheet, a method of hot-dip galvanizing by continuously degreasing, reducing and annealing without using flux is disclosed in US Patent No. 2,110,893 (hereinafter referred to as "Sensimir process") and US patent publication. 3,320,085 (hereinafter referred to as "cells process").

In the Sensimir process, the steel sheet is combusted in a general oxidation furnace without control of the atmosphere, and then heated and degreased by heating or degreasing the temperature of the steel sheet to 370 to 480 ° C by electric resistance or electric induction heating. Oxidize to In a reduction furnace with hydrogen, nitrogen and dew point under reduced atmosphere, the steel sheet is reduced and annealed for about 30 seconds in a temperature range of 730 to 930 ° C, and then reduced to about 460 ° C, which is the temperature of the hot-dip galvanizing bath. The molten zinc plating bath is passed through the hot dip galvanizing bath for about 3 seconds in the state of rapid cooling until it is melted and then solidified by shaving with an air knife to the desired coating amount.

In the Cellars process, the steel sheet is sufficiently heated and reduced to a temperature of 430 to 700 ° C. in an oxidization-free furnace by combustion of fuel gas, and then the steel sheet temperature is reduced to 480 ~ in a reduction atmosphere in which hydrogen and nitrogen and dew point are adjusted. After the reduction annealing while heating for about 30 seconds in the range of 1040 ℃ and the hot dip galvanized bath through the molten zinc plating bath for about 3 seconds while rapidly cooling to a temperature of about 460 ℃, the temperature of the hot dip galvanizing bath. The hot-dip galvanized layer is formed by shaping and solidifying by air knife at the desired plating amount.

In the above two processes, when hot-dip galvanized layer solidifies immediately after air nipping, the steel sheet is reheated to 500 to 550 ° C. for 10 to 30 seconds, and the alloyed hot-dip galvanized layer is formed by mutual diffusion of base iron and zinc. May be formed.

In this process, iron as a main component remains on the surface of the steel sheet during the hot-dip plating process, so that it has good wettability and adhesion with the hot dip zinc plating solution (hot dip zinc containing 0.125 to 0.135 or 0.18 to 0.20 wt% of aluminum alloy component). Because of the presence of trace amounts of impurities that are more oxidizing than iron on the surface of steel sheets, Mn, Si and Al are present in trace amounts, most of them are in the state of internal oxides and are in contact with molten zinc. Since it is covered by the surrounding plating layer, it hardly affects the wettability, plating adhesion and alloying property of the plating layer.

However, in the conventional cold rolled steel sheet continuous hot dip galvanizing process, the iron oxide on the surface of the substrate is insufficiently reduced, or a small amount of highly oxidizing impurities such as Mn and Si are locally segregated to generate residual oxide. When the residual oxides are generated in this way, they are transferred to the outside of the shape of the base surface through the plating layer formed after the plating or alloying plating on the residual oxides, thereby causing various types of patterns to be exposed, resulting in a poor appearance of the surface. .

On the other hand, in recent years, in the case of automotive steel sheets that have rapidly increased in demand, high strength of the steel sheet itself is required to improve the weight and collision resistance. However, since the heat treatment of the continuous molten zinc process using the existing steel grades alone cannot satisfy such high strength, low alloy steel sheets for high strength and various heat treatment technologies have been developed. Such steel grades include dual phase steel and metamorphic organic plastic (TRIP) steel.

Mn, Si, Al, Ti, Cr, Mo, P and the like are used as main alloy elements of the high strength low alloy steel sheet. However, these alloying elements are much more oxidative than iron, which is the main component of the base material. Therefore, although the content is generally lower than 5.0 wt%, it is highly oxidizable, and when the conventional process is applied, an external oxide film is formed on the surface of the steel sheet, resulting in poor molten zinc plating solution and wettability and even unplated parts. Even with some plating, a problem of inferior adhesion occurs.

US Patent Publication No. 3,925,579 is proposed as a method for improving the disadvantages of the poor plating properties of such a low alloy steel sheet.

In US Patent No. 579, the redox method oxidizes the surface of a low alloy steel sheet containing 3.0 wt% or less of Al, 1.0% or less of Ti, 2.0% or less of Si, or 5.0% or less of Cr. When the redox method is applied to the Sensimir process, the steel sheet temperature on the exit side is increased to 600 ~ 760 ℃ by oxidative heating, and when applied to the cellar process, the oxygen is injected up to 6.0% by volume instead of the excess fuel by direct combustion. With this direct combustion, the exit steel sheet temperature is increased to 760 ~ 910 ℃. When the surface of the steel sheet is oxidized as described above, the surface of the steel sheet forms an oxide layer composed of iron oxide, which is a main component, and low alloying elements that are finely distributed or solid solution therein. When the oxide layer is reduced under the existing reduction furnace conditions and quenched to the temperature of the hot dip galvanizing bath, only iron oxide is reduced to ferrous metal, and the strong oxidizing low-alloy oxide is dispersed in the form of internal oxide. In the cooling process, alloying elements diffuse from the inside to the reducing surface, further oxidation and surface concentration can be ignored. In addition, the reduced steel sheet surface has been disclosed that it is possible to form the alloyed hot-dip layer by the hot-dip plating layer and, if necessary, conventional alloying treatment with good melt plating solution and wettability and bonding property.

However, in the redox method described above, the iron oxides on the surface of the oxidized low-alloy steel sheet should be sufficiently reduced in order for the reduced steel plate surface to wet well with the molten plating solution. As it is difficult to maintain, there is a problem that is not sufficiently reduced.

In addition, in order to sufficiently reduce the oxidized low alloy steel sheet must be maintained for a considerable time in a high temperature reducing atmosphere. However, the highly oxidizable alloy component diffuses from the inside of the steel sheet to the surface, especially along the grain boundary, and forms an alloying oxide around the surface grain boundary. Therefore, it occupies a considerable part on the surface. In addition, there is a problem that the iron oxide is not completed to reduce the molten plating solution and the wettability, so that the poor plating adhesion or even the portion that is not plated.

In addition, when alloying the hot-dip galvanized steel sheet having poor plating adhesion, there is a problem in that the mutual diffusion of the base iron and the zinc in the coating layer is non-uniform, causing various surface appearance defects.

The present invention is to solve the above problems, an object of the present invention is to completely reduce the alloying elements in a strong reducing component during the hot dip galvanizing process to plate the molten zinc to make the plating layer uniform and excellent adhesion and final product It is to provide a method for producing a hot-dip galvanized steel sheet excellent in appearance.

Method for producing a hot-dip galvanized steel sheet according to the present invention for achieving the object of the present invention is Mo 5.0wt% or less, Cr 5.0wt% or less, Mn 4.0wt% or less, Si 1.0wt% or less, Ti 0.7wt% or less, Manufacturing a cold rolled steel sheet including any one or less than 0.1 wt% of Al or two or more alloying elements, the balance being Fe, and other unavoidable impurities; Completely degreasing and drying the steel sheet; Reducing annealing of the stripped steel sheet in a continuous furnace formed of a heating zone, a cracking zone, and a cooling zone while heating, cooling, and reducing annealing in a strong reducing atmosphere; Coating the plated layer on the reduced annealed steel sheet in a molten zinc bath or a molten zinc bath containing alloy elements; Solidifying the plating layer coated on the steel sheet; It provides a method of manufacturing a hot-dip galvanized steel sheet comprising a.

In the present invention, the strong reducing atmosphere is 95 vol% or more of hydrogen, the balance is inert gas and is composed of a gas maintained at a dew point of -30 ° C. or lower. % And the remainder is nitrogen gas and the dew point is maintained at -30 ℃ or less in the gas atmosphere to heat the temperature of the steel sheet in the range of 600 ~ 940 ℃ and crack for 15 to 180 sec, cooling and snout in continuous furnace The step of reducing annealing by passing through the step is quenching the temperature of the steel sheet in the range of 500 to 300 ° C. under a gas atmosphere in which the atmosphere is 95 vol% or more, the remainder is nitrogen gas and the dew point is maintained at −50 ° C. or lower.

In addition, the present invention is a cold-rolled steel sheet applied to the cold-rolled steel sheet is an abnormal structure or transformed organic plastic by heat treatment or high strength automotive steel sheet containing high Mn, Si.

And the present invention includes any one or more alloy elements of Mo 5.0 wt% or less, Cr 5.0 wt% or less, Mn 4.0 wt% or less, Si 1.0 wt% or less, Ti 0.7 wt% or less, Al 0.1 wt% or less. The remainder is Fe and cold rolling equipment for producing cold rolled steel sheet containing other unavoidable impurities; A degreasing apparatus for completely degreasing and drying the steel sheet; 95 vol% or more of hydrogen, the remainder being an inert gas and having a dew point of -30 ° C. or lower and partitioned into a heating zone, a crack zone and a cooling zone so that the degreasing steel sheet is reduced-annealed in a strong reducing atmosphere and discharged through a snout; A plating bath facility for coating the plating layer on the reduced annealed steel sheet in molten zinc or molten zinc containing alloy elements; And a cooling facility for solidifying the plating layer coated on the steel sheet; It also provides an apparatus for producing a hot-dip galvanized steel sheet comprising a.

In addition, the present invention includes any one or two or more alloy elements of Mo 5.0wt% or less, Cr 5.0wt% or less, Mn 4.0wt% or less, Si 1.0wt% or less, Ti 0.7wt% or less, Al 0.1wt% or less. The remainder is Fe, cold rolled steel sheet containing other unavoidable impurities. The remainder is reduced annealed in a strong reducing atmosphere where the hydrogen is 95 vol% or more and the remainder is an inert gas and the dew point is kept below -30 ° C. It also provides a hot-dip galvanized steel sheet prepared by plating the hot-dip zinc.

Hereinafter, with reference to the accompanying drawings will be described an embodiment of the present invention. These embodiments of the present invention are merely for illustrating the present invention, but the present invention is not limited thereto.

1 is an Ellingham diagram by metal thermodynamics showing the oxidation potential of each alloying element of a high strength alloy steel sheet at 800 ° C. 1 shows the state of external oxidation and reduction according to the oxidation potential of each alloy element at the furnace temperature of continuous hot dip galvanizing. The main alloying elements of the high strength alloy steel sheet used in the present invention are Mn, Si, Al, Ti, Cr and Mo.

FIG. 1A shows the oxidation potential of each alloying element in the pure state. As shown in (a) of FIG. 1, each alloying element is more oxidative than pure Fe in pure water (100 wt%) and is located at a lower oxidation potential. This indicates that each alloying element must be kept below the oxidation potential corresponding to the redox equilibrium in order to be hot dip galvanized.

FIG. 1 (b) shows the oxidation potential of each component when the alloying elements are present in the steel sheet in an ultra low alloy or impurity state of less than 0.1 wt%. As shown in (b) of FIG. 1, when a small amount of the alloying component is present, it can be seen that the oxidation potential is increased compared to the pure state as shown in (a) of FIG. 1. In the continuous molten zinc plating process, even though the alloying elements are present as internal oxides in the reducing atmosphere of the Fe component of the steel sheet and the external oxides are formed in the strong reducing atmosphere, the content of the alloying components is so small that the surface concentration is consequently reduced. It is less and for this reason the amount of oxide formation is also small, which hardly affects the wettability with the molten zinc plating solution. Therefore, there is no problem if the plating property of the ultra-low alloy steel sheet is a plating condition in which the basic Fe component maintains the reduction.

However, when the content of such a strong oxidizing alloy component is about 0.1 to 5.0 wt%, when the content is low in the oxidation potential region of each component in the reducing atmosphere of the base Fe component, diffusion concentration of the alloy component occurs on the surface of the steel sheet. And a surface oxygen and an external oxide film are formed. When the potential is high, oxygen atoms diffuse into the steel sheet to form alloying elements and internal oxides. That is, for each alloy component, there are external and internal oxidation boundary oxidation potentials by oxidation kinetics. In addition, when the external oxidation potential regions overlap for each alloy component, the steel sheet forms an external oxide film on the surface of the low alloy steel over the entire potential region from the region where the Fe component starts to reduce to the reduction region of the alloy component with the highest oxidation power. .

Figure 1 (c) shows the oxidation state of the surface of the steel sheet according to the oxidation potential of 800 ℃ in a high-strength steel sheet containing 1.0wt% Al, 1.0wt% Si and 3.0wt% Mn.

As shown in (c) of FIG. 1, as the oxidation potential decreases at the starting Fe reduction level, an area formed by sequentially overlapping the external oxide film of Mn, the external oxide film of Si, and the external oxide film of Al appears sequentially. . As such, the surface of the steel sheet having such a composition has a problem that the external oxide film is formed above the oxidized potential in which the Al component is completely reduced, resulting in poor adhesion with the molten zinc plating solution and poor adhesion even if partially plated. Will occur.

However, in the continuous hot dip galvanizing furnace, an external oxide film, that is, the most oxidizing alloy component and the concentration of the most oxidizing alloy, does not form an external oxide film. It is possible to realize hot dip galvanizing having. Furthermore, when the steel sheet is quenched using a reducing atmosphere gas that is stronger than the strong reducing atmosphere in the process step between the intense reduction annealing and immediately before the plating is started, hot dip galvanizing with better surface quality can be realized. When the plating is carried out in such a strong reducing atmosphere, the surface of the steel sheet is formed of a ferrous metal state and a metal state of most alloy components, or some alloy components formed during cooling are formed of internal oxides, thereby improving the wet zinc plating solution and wettability. As a result, the plating layer is uniformly coated, and thus a hot dip galvanized layer having excellent adhesion and few appearance defects is formed. After the heat treatment necessary for alloying in the process can be formed a high quality alloyed hot dip galvanized layer.

FIG. 1 (d) is a diagram showing the oxidation and reduction states of the steel sheet surface according to the oxidation potential at 800 ° C. for a low alloy steel sheet containing 0.1 wt% Al, 1.0 wt% Si, and 3.0 wt% Mn. . As shown in (d) of FIG. 1, as the oxidation potential decreases at the level of Fe reduction initiation level, a region formed by sequentially overlapping the outer oxide film of Mn, the outer oxide film of Si, and the inner oxide film of Al is sequentially shown. .

In this case, in an atmosphere capable of reducing 1.0 wt% of Si, the Fe and the main alloy elements Mn and Si are reduced and the impurity level Al is almost in the form of an internal oxide, so there is no problem in plating property.

The present invention derives the main alloying elements that can prevent the formation of the main external oxide film by annealing in the high strength steel sheet including the low carbon steel sheet in the strong reducing atmosphere as described above, and the reduced steel sheet surface is secured with the molten zinc plating solution and excellent wettability It is.

Hereinafter, the present invention will be described with reference to the attached continuous molten zinc plating process of FIG. 2.

The present invention first uncoils the high strength steel sheet (2) coil which causes a poor plating appearance by the oxide in the pay-off reel (1), and then passes through a plate welder and a compression luper. Then, the steel sheet 2 is degreased in the preliminary washing tank 3 and then completely degreased by passing through the electrolytic washing tank 4, and the hot water washing tank 5 is passed through to completely wash away the excess of the washing liquid. Pass through (6) and dry. In this way, by completely degreasing the steel sheet 2 by wet before entering into the continuous furnace, it is possible to reduce the degreasing load due to heating in the furnace and to immediately reduce heating and annealing.

Then, the steel sheet 2 completely degreased in this way is subjected to reduction annealing in a furnace (7, 8, 9). The continuous furnaces 7, 8, 9 according to the invention consist of a heating and cracking zone 7, a slow cooling stand 8 and a quenching stand 9.

The crack zone 7 of the continuous furnace according to the present invention is controlled in a strong reducing atmosphere, and controls the temperature of the steel sheet passed in the range of 600 ~ 940 ℃ and the residence time of the steel sheet 2 is controlled in the range of 15 ~ 180 seconds. .

The strong reducing atmosphere in the crack zone 7 is preferably at least 95% by volume of hydrogen, the rest being nitrogen gas, and controlled by maintaining the dew point at -30C or lower.

Thus, the amount of hydrogen is limited to 95% or more in order to maintain the strong reducing atmosphere in order to set a strong reduction range beyond the explosion range because the hydrogen explosion range is 5 to 95% by volume of hydrogen in the hydrogen reduction reactor.

The lower the dew point, the higher the reducing power of the atmosphere. However, when the steel sheet temperature is increased to 940 ° C. during annealing, the reducing power is also increased. In addition, in the present invention, the heating and uniform zone (7) is a strong reduction since the strong reducing gas flows into the main heating and cracking zone (7) from the quench zone (9) and slow cooling zone (8), which is the rear part of the continuous furnace. It is desirable to limit the upper limit of the dew point to about -30 ° C since the reducing power of the gas may be slightly lower than the front part.

The heating temperature in the cracking zone 7 is preferably limited to 940 ° C in order to sufficiently anneal the steel sheet to pass through. In simple process annealing, the heating temperature is preferably limited to about 600 ° C which is equal to or less than the process point temperature (727 ° C) of the carbon steel.

And the annealing time at the cracking zone 7 may vary depending on the width and thickness of the steel sheet and the line speed, but the required heat capacity can be secured, and a range of 15 to 180 seconds is preferable for sufficient annealing and reduction.

When the fully degreased steel sheet 2 is controlled as described above, when the heating radiator passes through the heating and cracking zone 7 provided in the furnace, all of the strong oxidizing alloy components such as Mn, Si, Cr, and Mo are reduced. The super oxidizing trace impurities such as Al and Ti remaining after the deoxidation of Si are formed as internal oxides, and the steel sheet is subjected to reduction annealing.

In accordance with the above process, the reduced-annealed steel sheet 2 is quenched in the subsequent quench zone 9 to control the structure and prepare for bathing in the plating bath.

Quenching the steel sheet in the quench zone 9 uses cooled gas. The cooling gas used in the quench zone (9) uses a strong reducing cold gas having a hydrogen content of 95% by volume or more, the rest being nitrogen gas and a dew point of -50 ° C or less. Controlling the dew point of the cooling gas used in the quench zone 9 is to lower the dew point lower than the crack zone in order to increase the reducing power of the atmosphere in the furnace because the oxidizing power of the alloying element is increased as the temperature of the steel sheet is lowered by the quenching. . When the steel sheet 2 passes through the quench zone 9 in this way, the temperature of the steel sheet is maintained at 500 to 300 ° C.

In the present invention, a short slow cooling stand 8 is formed in an intermediate region between the quenching stand 9 and the cracking stand 7.

In the present invention, it is preferable that the portion of the snout 10 which communicates the quench zone 9 with the plating bath 11 also uses the same atmosphere as the quench zone 9. The steel sheet cooled in the quench zone 9 is readjusted to about 460 ° C., which is the temperature of the hot dip galvanizing bath, passed through the snout 10 and bathed in the plating bath 11.

Here, the steel sheet 2 being processed is a high-strength steel reduced to the steel sheet because the temperature of the steel sheets in the quench zone 9 and the snout 10 is lower than the crack zone 7 before bathing in the plating bath 11 or The oxidative properties of the trace elements can be increased to create a situation of external oxidation.

However, since the temperature of the steel sheet is lowered to about 500 ~ 300 ° C., the alloying elements slow down the diffusion from the inside of the steel sheet to the surface, so that an external oxide film cannot be formed and takes the form of an internal oxide. Therefore, there is no big problem in plating property.

As described above, the steel sheet containing all of the components of the steel sheet or some internal oxide is bathed in the hot dip galvanizing bath 11.

The plating layer coated on the surface of the steel sheet 2 is controlled to the required thickness by the air knife 13 and then solidified in the cooling zone 16.

Since the solidified plating layer does not have an oxide on the surface of the steel sheet according to the process of the present invention, the molten zinc plating solution and the wettability are good to form a uniform plating layer and the adhesion is also excellent.

In the present invention, an induction heating device 14 or a crack 15 may be selectively installed between the air knife 13 and the cooling stand 16. When the steel plate 2 having the plated layer passes through the induction heater 14 or the crack 15, an alloying element such as Al is diffused between the base metal and the plated layer to perform alloyed hot dip galvanizing. At this time, the alloying conditions in the induction heater 14 or the crack zone 15 is maintained at a temperature of 500 ~ 550 ℃ plated steel sheet and heated for 10 to 30 seconds.

In FIG. 2, reference numeral 12 denotes a sink roll, 17 denotes a post treatment tank, and 18 denotes a tension reel.

In the present invention, as described above, by determining the components and contents of the alloys in which each component element of the steel sheet can be reduced or at least maintain the internal oxide form by the atmosphere in the continuous furnace (7, 8, 9) through the following experiments It was.

Table 1 shows the composition of the cold-rolled specimens prepared in the low-carbon steel as a test specimen according to the present invention by varying each alloy component for each single component and its concentration.

Specimens having a composition as shown in Table 1 were hot-plated using the hot dip galvanizing apparatus of the present invention under the process conditions shown in Table 2. These experiments were conducted using a hot dip galvanizing machine that can simulate hot dip galvanizing.

The plating properties of the hot dip galvanized specimens under the conditions shown in Table 2 were evaluated and the results are shown in Table 3.

As shown in Table 3, the content of the alloying elements that can be added in the high-strength alloy steel sheet which can ensure excellent plating property in the strong reducing annealing atmosphere according to the present invention is not more than 5.0wt% Mo, 5.0wt% Cr or less It can be seen that it is preferable to limit the Mn to 4.0 wt% or less, Si 1.0 wt% or less, Ti 0.7 wt% or less, and Al 0.1 wt% or less.

Hereinafter, embodiments of the present invention will be described.

Example 1

0.112 wt% C, 0.21 wt% Si, 1.87 wt% Mn, 0.04 wt% Sol. A steel sheet containing Al, 0.09 wt% Mo, 0.02 wt% Ti balance Fe, and other unavoidable impurities is annealed and quenched to form a dual phase (DP), and a cold rolled steel specimen having a tensile strength of 780 MPa or more. Prepared.

These specimens were completely degreased outside the furnace prior to charging in the furnace (7, 8, 9) and then charged in the furnace, heated, cracked and quenched under conventional conditions and immersed in a molten zinc bath through a snout. At this time, the process conditions of the continuous furnace is 15vol% hydrogen + 85vol% nitrogen and the dew point of the atmosphere gas is -30 ℃, the heating rate of the steel sheet is 20 ℃ / sec, the crack temperature and time of the steel sheet is 800 And 30 sec. The steel sheet thus heated and cracked was quenched to 465 ° C at a cooling rate of 30 ° C / sec. The steel plate passed through the snout is made of 0.13wt% Al + 99.87wt% Zn and attached to the hot dip galvanized layer while passing through a plating bath at a temperature of 460 ° C.Then, the thickness is controlled with an air knife and the plate is cooled by cooling. Solidified. On the other hand, the coating layer was coated under the same conditions, and then alloyed plating was performed by passing the alloying crack zone.

On the other hand, using the cold rolled steel sheet of the same conditions was carried out hot-dip plating is applied to the process conditions according to the present invention. The process according to the present invention maintains the total atmosphere of the continuous furnace at 95 vol% + nitrogen 5 vol% and the dew point of the atmosphere gas at -50 ° C, heats the steel sheet passed through 20 ° C / sec and then cracks at 800 ° C for 30 sec. Stout was passed through cooling to 465 ℃ at a cooling rate of 30 ℃ / sec. Under the same conditions, the surface of the steel sheet was reduced and immersed in a hot dip galvanized bath having a composition of 0.13wt% Al + 99.87wt% Zn and maintained at 460 ° C. to coat the plating layer of the steel sheet, and then passed through a cooling zone to form a plating layer. Solidified. On the other hand, the coating layer was coated under the same conditions, and then alloyed plating was performed by passing the alloying crack zone.

The testability of the plating layer was evaluated for the test specimens as described above, and the results are shown in Table 4 below.

division Plating Result Conventional Plating Process Unplated parts of diameter 1mm or more occurred on most surfaces Plating process of the present invention Unplated parts and plating appearance defective pattern does not occur at all

As shown in Table 4, the steel sheet having an abnormal structure according to the present invention was able to form a plating layer having excellent physical properties that plating is not possible by the conventional plating process. This is because the gas atmosphere according to the present invention forms a high hydrogen low dew point to maintain a strong reducing atmosphere, so that the surface of the steel sheet is oxidized cleanly so that various plating appearance defects after plating and alloying treatment due to residual oxides do not occur. Is analyzed. This could maintain excellent plating appearance.

Example 2

0.139 wt% C, 0.49 wt% Si, 2.46 wt% Mn, 0.04 wt% Sol. A steel sheet containing Al, 0.026 wt% Nb, balance Fe, and other unavoidable impurities was annealed and quenched, and transformed to organic transformation (TRIP) to prepare a cold rolled steel specimen having a tensile strength of 780 MPa or more.

These specimens were completely degreased outside the furnace prior to charging in the furnace (7, 8, 9) and then charged in the furnace, heated, cracked and quenched under conventional conditions and immersed in a molten zinc bath through a snout. At this time, the process conditions of the continuous furnace is 15vol% hydrogen + 85vol% nitrogen and the dew point of the atmosphere gas is -30 ℃, the heating rate of the steel sheet is 20 ℃ / sec, the crack temperature and time of the steel sheet is 800 And 30 sec. The steel sheet thus heated and cracked was quenched to 465 ° C at a cooling rate of 30 ° C / sec. The steel plate passed through the snout is made of 0.13wt% Al + 99.87wt% Zn and attached to the hot dip galvanized layer while passing through a plating bath at a temperature of 460 ° C.Then, the thickness is controlled with an air knife and the plate is cooled by cooling. Solidified. On the other hand, the coating layer was coated under the same conditions, and then alloyed plating was performed by passing the alloying crack zone.

On the other hand, using the cold rolled steel sheet of the same conditions was carried out hot-dip plating is applied to the process conditions according to the present invention. In the process according to the present invention, the total atmosphere of the continuous furnace is 100 vol% hydrogen and the dew point of the atmosphere gas is maintained at -60 ° C and the steel sheet passed through is heated to 20 ° C / sec and then cracked at 800 ° C for 30 sec and 30 ° C / sec Cooling down to 465 ℃ to pass through the stout. Under the same conditions, the surface of the steel sheet was reduced and immersed in a hot dip galvanized bath having a composition of 0.13wt% Al + 99.87wt% Zn and maintained at 460 ° C. to coat the plating layer of the steel sheet, and then passed through a cooling zone to form a plating layer. Solidified. On the other hand, the coating layer was coated under the same conditions, and then alloyed plating was performed by passing the alloying crack zone.

The testability of the plating layer was evaluated for the test specimens as described above, and the results are shown in Table 5 below.

division Plating Result Conventional Plating Process Unplated parts of diameter 1mm or more occurred on most surfaces Plating process of the present invention Unplated parts and plating appearance defective pattern does not occur at all

As shown in Table 5, in the case of the transformation organic plastic (TRIP) steel sheet according to the present invention, it was possible to form a plating layer having excellent physical properties even when the plating was not possible by the conventional plating process. This is because the gas atmosphere according to the present invention can maintain a strong reducing atmosphere, the surface of the steel sheet can be reduced cleanly to maintain excellent plating properties and appearance.

Example 3

0.15 wt% C, 1.0 wt% Si, 4.0 wt% Mn, 0.05 wt% Sol. High strength cold rolled steel specimens for high Mn, Si automobiles containing Al, balance Fe and other unavoidable impurities were prepared.

These specimens were completely degreased outside the furnace prior to charging in the furnace (7, 8, 9) and then charged in the furnace, heated, cracked and quenched under conventional conditions and immersed in a molten zinc bath through a snout. At this time, the process conditions of the continuous furnace is 15vol% hydrogen + 85vol% nitrogen and the dew point of the atmosphere gas is -30 ℃, the heating rate of the steel sheet is 20 ℃ / sec, the crack temperature and time of the steel sheet is 800 And 30 sec. The steel sheet thus heated and cracked was quenched to 465 ° C at a cooling rate of 30 ° C / sec. The steel plate passed through the snout is made of 0.13wt% Al + 99.87wt% Zn and attached to the hot dip galvanized layer while passing through a plating bath at a temperature of 460 ° C.Then, the thickness is controlled with an air knife and the plate is cooled by cooling. Solidified. On the other hand, the coating layer was coated under the same conditions, and then alloyed plating was performed by passing the alloying crack zone.

On the other hand, using the cold rolled steel sheet of the same conditions was carried out hot-dip plating is applied to the process conditions according to the present invention. In the process according to the present invention, the total atmosphere of the continuous furnace is 100 vol% hydrogen and the dew point of the atmosphere gas is maintained at -70 ° C and the steel sheet passed through is heated to 20 ° C / sec and then cracked for 30 seconds at 800 ° C Cooling down to 465 ℃ to pass through the stout. Under the same conditions, the surface of the steel sheet was reduced and immersed in a hot dip galvanized bath having a composition of 0.13wt% Al + 99.87wt% Zn and maintained at 460 ° C. to coat the plating layer of the steel sheet, and then passed through a cooling zone to form a plating layer. Solidified. On the other hand, the coating layer was coated under the same conditions, and then alloyed plating was performed by passing the alloying crack zone.

The testability of the plating layer was evaluated for the test specimens as described above, and the results are shown in Table 6 below.

division Plating Result Conventional Plating Process The entire surface is not plated Plating process of the present invention No unplated parts and plating appearance defect pattern on the whole surface

As shown in Table 6, in the case of the high-strength automotive steel sheet according to the present invention, a plating layer having excellent physical properties could be formed even when the plating was not possible by the conventional plating process. This is because the gas atmosphere according to the present invention can maintain a strong reducing atmosphere, the surface of the steel sheet can be reduced cleanly to maintain excellent plating properties and its appearance.

As described above, the hot-dip galvanizing method according to the present invention is to form a strong reducing atmosphere of high hydrogen low dew point as a heat treatment atmosphere to fully reduce the alloying elements to maintain a uniform coating and strong adhesion of the subsequent plating layer There is.

In addition, there was an effect of forming an excellent plating layer on the abnormal structure steel sheet or the transformation organic plastic (TRIP) steel sheet or high Mn, Si high-tension automotive steel sheet, which was not possible to form the plating layer by the conventional hot dip galvanizing method.

In addition, in the plating process of some high strength steel sheets, the reduction of the appearance defects is greatly reduced by controlling the conventional annealing cooling process in which various plating appearance defects caused by surface residual oxides are generated immediately before plating, using a strong reducing atmosphere according to the present invention. It has a very beneficial effect.

Although the present invention has been described above, it will be readily understood by those skilled in the art that various modifications and variations are possible without departing from the spirit and scope of the claims set out below.

1 is a conceptual diagram showing the oxidation state of the surface of the steel sheet according to the increase in the oxidation potential of each alloy element of the high strength alloy steel sheet at 800 ℃.

2 is a conceptual diagram showing a continuous hot dip galvanizing plant according to the present invention.

Claims (8)

Mo or less: 5.0 wt% or less, Cr 5.0 wt% or less, Mn 4.0 wt% or less, Si 1.0 wt% or less, Ti 0.7 wt% or less, 0.1 wt% or less, or any one or two or more alloying elements. Preparing a cold rolled steel sheet containing other unavoidable impurities; Completely degreasing and drying the steel sheet; Reducing annealing of the stripped steel sheet in a continuous furnace formed of a heating zone, a cracking zone, and a cooling zone while heating, cooling, and reducing annealing in a strong reducing atmosphere; Coating the plated layer on the reduced annealed steel sheet in a molten zinc bath or a molten zinc bath containing alloy elements; And Solidifying the plating layer coated on the steel sheet; Method for producing a hot-dip galvanized steel sheet comprising a. In claim 1, The method of producing a hot-dip galvanized steel sheet, characterized in that the strong reducing atmosphere is composed of a gas having a hydrogen of 95 vol% or more, the balance is an inert gas and the dew point is maintained below -30 ℃. In claim 2, The reduction annealing step during the heating and cracking in the continuous furnace is performed in a gas atmosphere in which the atmosphere is at least 95 vol% of hydrogen, the remainder is nitrogen gas, and the dew point is maintained at -30 ° C or lower in the range of 600 to 940 ° C. Method for producing a hot-dip galvanized steel sheet characterized in that the heating and cracking for 15 ~ 180sec. In claim 2, Reducing annealing by passing through the cooling and the snout in the continuous furnace is the temperature of the steel sheet 500 ~ 300 ℃ under a gas atmosphere in which the atmosphere is more than 95vol% hydrogen, the remainder is nitrogen gas and the dew point is maintained below -50 ℃ Method for producing a hot-dip galvanized steel sheet, characterized in that the quenching in the range of. In claim 1, The cold rolled steel sheet is a method of manufacturing a hot-dip galvanized steel sheet, characterized in that the cold-rolled steel sheet has an abnormal structure or transformation organic firing by heat treatment. In claim 1, The cold rolled steel sheet is a method for producing a hot-dip galvanized steel sheet, characterized in that the high-strength automotive steel sheet containing high Mn, Si. Mo or less: 5.0 wt% or less, Cr 5.0 wt% or less, Mn 4.0 wt% or less, Si 1.0 wt% or less, Ti 0.7 wt% or less, 0.1 wt% or less, or any one or two or more alloying elements. Cold rolling equipment for producing cold rolled steel sheets containing other unavoidable impurities; A degreasing apparatus for completely degreasing and drying the steel sheet; 95 vol% or more of hydrogen, the remainder being an inert gas and having a dew point of -30 ° C. or lower and partitioned into a heating zone, a crack zone and a cooling zone so that the degreasing steel sheet is reduced-annealed in a strong reducing atmosphere and discharged through a snout; A plating bath facility for coating the plating layer on the reduced annealed steel sheet in molten zinc or molten zinc containing alloy elements; And A cooling facility for solidifying the plating layer coated on the steel sheet; Apparatus for producing a hot-dip galvanized steel sheet comprising a. Mo or less: 5.0 wt% or less, Cr 5.0 wt% or less, Mn 4.0 wt% or less, Si 1.0 wt% or less, Ti 0.7 wt% or less, 0.1 wt% or less, or any one or two or more alloying elements. Cold rolled steel sheet containing other unavoidable impurities is reduced annealed in a strong reducing atmosphere in which hydrogen is 95 vol% or more and the balance is an inert gas and the dew point is maintained at -30 ° C or lower, followed by hot-dip zinc or molten zinc containing alloying elements. Hot-dip galvanized steel sheet, characterized in that produced by plating.