KR20140083819A - High strength hot-rolled steel sheet, hot-dip galvanized steel sheet using the same, alloyed hot-dip galvanized steel sheet using the same and method for manufacturing thereof - Google Patents

Abstract

The present invention relates to a high strength hot-rolled steel sheet used as a material for a vehicle, home appliances, and building construction. In particular, the present invention relates to the high strength hot-rolled steel sheet used for manufacturing a hot-dip galvanized steel sheet, a hot-dip galvanized steel sheet using the same, and an alloyed hot-dip galvanized steel sheet, and a manufacturing method thereof. More particularly, when the hot-dip galvanized steel sheet is manufactured by using the high strength steel sheet, a hot-rolling process is prioritized to be controlled so that a buckling phenomenon can be prevented and the hot-rolled steel sheet having excellent cold rolling properties can be manufactured. When the hot-dip galvanized steel sheet is manufactured, conditions of an annealing process are controlled to effectively prevent elements, which are included in the steel sheet and easy to be oxidized, from being diffused. Therefore, the hot-dip galvanized steel sheet or the alloyed hot-dip galvanized steel sheet having excellent surface quality can be obtained.

Description

TECHNICAL FIELD [0001] The present invention relates to a high strength hot-rolled steel sheet, a hot-dip galvanized steel sheet using the same, a hot-rolled galvanized steel sheet using the same, AND METHOD FOR MANUFACTURING THEREOF}

More particularly, the present invention relates to a hot-dip galvanized steel sheet for use in the production of the hot-dip galvanized steel sheet, a hot-dip galvanized steel sheet using the hot-dip galvanized steel sheet, And a process for producing them.

The hot-dip galvanized steel sheet has excellent corrosion resistance and is widely used in building materials, structures, household appliances, and automobile bodies. Hot-dip galvanized steel sheet (GI steel sheet) and galvannealed galvanized steel sheet (GA steel sheet) are the most commonly used hot-dip galvanized steel sheet. Especially, GI steel sheet is a hot-dip galvanized steel sheet which is easily plated and corrosion- And is widely used as the material of the automobile itself.

In recent years, there has been a continuing demand for improvement of collision safety and fuel economy for steel sheets used for such automobile materials and household electrical appliances. However, when only the improvement of the strength of the steel sheet is sought, the ductility is relatively lowered, so it is necessary to improve the ductility while securing a certain strength.

In order to cope with the above, recently, it has been proposed to add Mn, Si and / or Al to the steel to form an ideal structure steel (dual phase steel, DP steel) having ferrite and martensite phase as a microstructure, ferrite, High strength steels such as complex phase steel (CP steel), transformed organo-plastic steel (TRIP steel) and TWIP (Twin Induced Plasticity) steel have been actively developed. As described above, when Mn, Si and / or Al is added to steel, it is possible to manufacture a steel sheet with improved strength and improved ductility.

Generally, a process for producing a hot-dip galvanized steel sheet comprises degreasing to remove oil and foreign substances on the surface during steelmaking, performance, hot rolling and cold rolled steel sheet (full hard steel sheet) in a pretreatment process, Annealing is carried out by heating to a temperature, cooling to an appropriate temperature, dipping in a hot-dip galvanizing bath to adhere zinc to the surface of the steel sheet, controlling the amount of coating with an air knife (air knife) .

In the hot rolling process in the continuous process, a slab heated to 1000 ° C or higher is rolled to a suitable thickness on a rolling roll, and then cooled and wound in a coil state. Here, the temperature of the cooling step after hot rolling is set at 650 to 800 DEG C in consideration of the subsequent cold rolling, and then the cooled steel sheet is wound and cooled by natural air cooling. At this time, martensite or bainite The strength of the hot-rolled steel sheet is lowered, thereby reducing the load applied to the rolling roll in the subsequent cold rolling process, thereby improving the rolling property.

On the other hand, in the case of a high-strength steel containing elements such as Mn, Si or Al which are easily oxidized in the steel, the temperature at which the austenite transforms into ferrite, bainite or martensite is low and the transformation takes a long time. However, if such a high-strength steel is cooled at a temperature of 650 to 800 ° C. in a normal winding step, that is, for a short period of time, the winding is performed in a state in which the transformation is not completed, so that a transformation occurs during natural cooling, The steel sheet wound up due to the rise of the temperature of the steel sheet can not suppress the formation of the quenched structure and buckling phenomenon that is distorted by its own weight occurs.

Therefore, in order to suppress the occurrence of the buckling phenomenon in the above-described high-strength steel, a method of rapidly cooling the steel sheet before winding to complete the transformation and winding the steel sheet is applied. However, if the coiling temperature is set to a low temperature , The structure of the produced hot-rolled steel sheet contains a lot of rapidly-cooled structure, which results in an increase in the strength of the hot-rolled steel sheet, resulting in a great load on the rolling roll in the subsequent cold rolling process.

The high-strength steel containing the elements such as Mn, Si or Al is hot-rolled and cold-rolled to produce a cold-rolled steel sheet. The cold-rolled steel sheet is subjected to annealing and hot-dip galvanizing to produce a hot-dip galvanized steel sheet Since the atmosphere of the annealing furnace in the annealing step is a reducing atmosphere and the dew point temperature is set to a low temperature of -30 DEG C or lower, iron in the steel is not oxidized. However, when Mn, Si or Al present in the steel reacts with a trace amount of oxygen or water vapor present in the annealing furnace at the time of annealing to form a single or composite oxide of these elements on the surface of the steel sheet, There is an unplated phenomenon in which the zinc is not locally or globally adhered to the surface of the coated steel sheet, thereby greatly reducing the surface quality of the coated steel sheet.

Thus, as the measures to prevent their easy-to-oxidation of the steel elements from diffusing into the surface during the annealing process, the reduction annealing after the leading gold to iron (Fe) in the annealing before the steel sheet in Patent Document 1 as a coating weight of 10g / m ² The present invention proposes a method capable of producing a steel sheet having a superior surface to be plated because the Si and Mn in the ferrous iron are diffused into the Fe line plating layer but can not diffuse to the surface during the usual annealing temperature and annealing time.

In the case of Patent Document 1, the thick Fe line plating layer does not allow Si and Mn present in the base steel below the pre-plating layer to diffuse to the surface to form a surface oxide, and thus the plating wettability is excellent and the oxide is dispersed in the pre- However, in order to prevent the oxidizing elements such as Si and Mn from diffusing to the surface during reduction annealing, it is necessary to form a thicker pre-plating layer. To this end, an electroplating facility is required to be large, There is a problem.

Patent Document 2 proposes a method of increasing the partial pressure of water vapor in the annealing furnace. When the partial pressure of water vapor in the annealing furnace is increased, oxygen permeates into the steel, and oxygen penetrates into the steel to react with Si and Mn To form internal oxides, so that diffusion of these elements to the surface of the steel sheet can be reduced to some extent. However, in the case of Patent Document 2, elements such as Si and Mn which have already been diffused to the surface of the steel sheet or directly under the surface before the internal oxide is formed in the initial stage of heating in the annealing step form oxides, Si, Mn, and the like exist, there is a problem that it is not possible to obtain excellent surface quality when such a steel sheet is plated.

Japanese Patent Application Laid-Open No. 2002-322551 Japanese Laid-Open Patent Publication No. 2004-323970

One aspect of the present invention relates to a hot-rolled steel sheet in which a hot-rolled steel sheet is produced by using a high-strength steel to control a hot rolling process and a buckling phenomenon is not generated, and an annealing process condition To suppress the surface diffusion of elements which are liable to be oxidized in the steel, and to provide a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet excellent in surface quality, and a method of manufacturing the same.

An aspect of the present invention is a method of manufacturing a semiconductor device comprising a semiconductor device including a first semiconductor layer including manganese (Mn) of 15% or less (including 0%), silicon (Si) Wherein at least two of Mn, Si and Al are limited to not less than 2.0%, not less than 0.5% and not less than 0.5%, and the balance Fe and inevitable impurities, Preparing a steel sheet by heating a steel slab having a total of two or more components satisfying 2 to 20% at a temperature not lower than the austenite temperature and performing hot rolling;

Cooling the hot-rolled steel sheet to 400 to 650 ° C and winding the hot-rolled steel sheet;

After being drawn into a BAF (Batch Annealing Furnace) having a dew point of -30 to 0 ° C and a volume ratio of 5% or less in a nitrogen atmosphere containing hydrogen, the steel sheet is tempered at 600 to 750 ° C for 20 to 120 minutes ; And

And cooling the steel sheet to a normal temperature after the tempering, thereby producing a high strength hot rolled steel sheet.

According to another aspect of the present invention, there is provided a method of manufacturing a cold-rolled steel sheet, comprising: cold-rolling a high-strength hot-

Heating the cold-rolled steel sheet at 700 to 850 ° C in an annealing furnace containing 3 to 10% of hydrogen at a volume ratio and composed of the remainder nitrogen, and then cooling; And

And a step of immersing the cooled cold-rolled steel sheet in a hot-dip galvanizing bath at 440 to 470 ° C for plating, thereby producing a high-strength hot-dip galvanized steel sheet having excellent surface quality.

Another aspect of the present invention provides a method of manufacturing a high strength alloyed hot-dip galvanized steel sheet having an excellent surface quality by performing the alloying heat treatment of the high-strength hot-dip galvanized steel sheet produced at the temperature of 470 to 600 ° C.

The present invention also relates to a method of manufacturing a semiconductor device, which comprises, by weight%, manganese (Mn): 15% or less (including 0%), silicon (Si): 3.0% At least two selected from the group consisting of Mn, Si and Al is limited to not less than 2.0%, not less than 0.5% and not less than 0.5%, and the balance Fe and unavoidable impurities, The sum of the above components satisfies 2 to 20%

A high-strength hot-rolled steel sheet comprising an inner oxide composed of Mn oxide, Si oxide, Al oxide, Fe oxide and composite oxide thereof at a depth of 2 to 20 탆 in a thickness direction from the surface to the steel sheet.

The present invention also relates to a cold-rolled steel sheet obtained by cold-rolling the above hot-rolled steel sheet; And a molten zinc plating layer formed on the surface of the cold-rolled steel sheet, wherein Mn oxide, Si oxide, Al oxide, Fe oxide, and a composite oxide thereof are formed at a depth of 0.1 to 3.0 탆 from the interface between the base steel sheet and the hot- And a high-temperature hot-dip galvanized steel sheet

The present invention also relates to a cold-rolled steel sheet obtained by cold-rolling the above hot-rolled steel sheet; And a galvannealed layer formed on the surface of the cold-rolled steel sheet, wherein the galvannealed layer has a degree of alloying of 7 to 13%, the galvannealed steel sheet having high surface quality.

According to the present invention, it is possible to provide a hot-rolled steel sheet excellent in cold rolling performance by performing tempering in the hot rolling process.

When the hot-rolled steel sheet is used to produce a hot-dip galvanized steel sheet or a galvannealed hot-dip galvanized steel sheet, it is possible to suppress diffusion of elements, which are likely to be oxidized in steel, to the surface of the steel sheet during annealing, by controlling annealing process conditions. It is possible to obtain a hot-dip galvanized steel sheet or a galvannealed hot-dip galvanized steel sheet having excellent surface quality even after plating.

The present inventors have found that when a hot-dip galvanized steel sheet is manufactured using a high-strength hot-rolled steel sheet containing easily oxidizable elements such as Mn, Si or Al, the buckling phenomenon that can occur in the hot rolling process is suppressed, As a result of reacting with a trace amount of oxygen or water vapor existing in the furnace to form Mn, Si or Al alone or a composite oxide on the surface of the steel sheet, the wettability of zinc during galvanization is disturbed and local plating As a result of intensive studies on the problem of greatly reducing the surface quality of the plated steel sheet, it has been found that a hot-rolled steel sheet having no defects such as a buckling phenomenon can be used by controlling the hot- By controlling the annealing conditions, the surface diffusion of Mn, Si or Al in the steel is suppressed, and oxides are formed in the steel So that by ensuring that the surface quality can be produced with excellent hot-dip galvanized steel sheet and a galvannealed steel sheet, and led to the present invention.

Hereinafter, a method for manufacturing a high-strength hot-rolled steel sheet according to the present invention, a hot-dip galvanized steel sheet produced using the hot-rolled steel sheet, and a galvannealed steel sheet will be described in detail.

(Mn): not more than 15% (including 0%), silicon (Si): not more than 3.0% (including 0%), and aluminum (Al) : Not more than 6.0% (inclusive of 0%), wherein at least two kinds of Mn, Si and Al are restricted to not less than 2.0%, not less than 0.5% and not less than 0.5%, and the balance Fe and unavoidable impurities, , And Si and Al satisfy 2 to 20% of the sum of at least two kinds of components.

At this time, if the content of two or more of the above components satisfies the lower limit value, the remaining steel may contain 0%.

At this time, manganese (Mn) is an element contributing to stabilization of austenite by lowering the martensitic transformation starting temperature (Ms) by promoting the formation of austenite, improving hardenability and achieving high yield strength. In order to obtain the above-mentioned effect, it is necessary to contain Mn at not less than 2.0%. However, when it is added in an excessive amount, Mn may be oxidized at a high temperature to cause cracks in the inside of the continuous casting or hot rolling, 15%. ≪ / RTI >

Silicon (Si) is an element that stabilizes ferrite and residual austenite at room temperature while improving the yield strength of the steel. That is, Si inhibits precipitation of cementite upon cooling from austenite, and contributes to stabilize a sufficient amount of the residual austenite to obtain a TRIP effect by significantly inhibiting the growth of carbide. In order to obtain the above effect, it is necessary to add Si at not less than 0.5%. However, if too much Si is added, the austenite temperature becomes high, so that hot rolling, which is usually carried out in the austenite region, becomes difficult. Therefore, it is preferable to limit the upper limit of the Si content to 3.0%.

Aluminum (Al), like Si, is an element that stabilizes ferrite during cooling of the steel sheet, accelerates the formation of ferrite, and refines the ferrite crystal. Therefore, it may be added instead of Si, or may be added together with Si. In order to obtain the above-mentioned effect, it is necessary to add Al at a content of 0.5% or more. However, if too much is added, there is a problem that the cost of the alloy increases. Therefore, the upper limit of the Al content is preferably limited to 6.0%.

As described above, when two kinds of components of Mn, Si and Al 3 are added, the present invention can be applied to the present invention without adding the remaining elements at all, and even when added in a small amount, there is no problem in exhibiting the desired effect.

The above-mentioned components are indispensable elements for ensuring strength and ductility of steel. When the total content of these components is less than 2%, the strength improvement effect is insignificant, and therefore, it is preferably 2% or more. In the case where the above components are contained in an amount of 2% or more, there is no problem in inhibiting the buckling phenomenon that may occur in the hot rolling process and forming oxides on the surface of the steel sheet during the annealing process. However, Since the total content is up to 20%, the upper limit of the total content of the components is limited to 20%.

Furthermore, in the case of producing a hot-dip galvanized steel sheet or a galvannealed hot-dip galvanized steel sheet, if undrawn phenomenon occurs and the wettability with zinc is lowered, there is a problem in a high-strength steel containing easily oxidizable elements such as Mn, Si or Al , It is preferable to use a steel material containing a certain amount of these elements to maximize the effect of the present invention.

The steel slab for producing the high-strength hot-rolled steel sheet according to the present invention may contain, in addition to the above-mentioned components, 0.05 to 0.30% of carbon, 0.5% or less of molybdenum, 1.0% or less of chromium (Cr) ): Not more than 0.03%, titanium (Ti): not more than 0.2%, niobium (Nb): not more than 0.2%, nickel (Ni): not more than 1.0%, copper (Cu): not more than 1.0%, vanadium And boron (B): 0.1% or less.

At this time, carbon (C) is essential for obtaining good mechanical properties, and it is necessary to add C to not less than 0.05% in order to promote hardenability and to obtain sufficient yield strength and to form stabilized austenite. However, the higher the content of C is, the more the austenite phase formed at a high temperature when cooled to room temperature is not transformed into martensite so that the austenite phase remaining at room temperature is increased, which is advantageous for increasing ductility. However, It is preferable to limit the upper limit value to 0.30%.

Although molybdenum (Mo) is not essential, there is no problem in obtaining the desired mechanical properties, but Mo of 0.5% or less promotes the formation of martensite and has a beneficial effect in increasing corrosion resistance. However, a large amount of Mo accelerates the phenomenon of low-temperature cracking in the welding zone and can lower the toughness of the steel, resulting in an increase in manufacturing cost. Therefore, when Mo is added, its content is preferably limited to 0.5% or less.

Chromium (Cr) has the effect of forming fine carbides in the steel to inhibit the occurrence of cracks during bending. However, when added too much, there is a problem that the wettability of zinc is inhibited. Therefore, the content of Cr is limited to 1.0% or less .

Phosphorus (P) increases the stability of residual austenite by suppressing precipitation of carbides together with Si, but a large amount of P causes a problem of brittleness of steel. Therefore, it is preferable to limit the content to 0.03% or less.

Titanium (Ti) is an advantageous element for increasing the yield strength. However, in order to avoid decrease in toughness due to Ti addition, its content is preferably limited to 0.2% or less.

Niobium (Nb) is an element that improves the precipitation of carbonitride and increases the yield strength. However, if too much Nb is added, the weldability and high-temperature moldability of the steel deteriorate. Therefore, the content of Nb is preferably limited to 0.2%.

Nickel (Ni) is an element which is advantageous for raising the yield strength, but if it is added in an excessively large amount, the production cost rises. Therefore, it is preferable to limit the content to 1.0% or less.

Copper (Cu) is an element which is advantageous for increasing the yield strength together with Ti and Ni, and is an element which interferes with surface diffusion of oxidizing elements such as Si, Mn or Al during the annealing process. However, since it is an expensive element, It is preferably added in an amount of 1.0% or less.

Vanadium (V) is an element which is advantageous for improving the yield strength by grain refinement and increasing the wettability of the steel. However, if the content exceeds 0.4%, the toughness of the steel tends to deteriorate and cracks may occur in the welded portion, so that the upper limit value of V is preferably limited to 0.4%.

Boron (B) is an element that greatly improves the yield strength even in a small amount. However, when added in a large amount, low-temperature embrittlement may occur, and there is a problem that the wettability of zinc is also deteriorated. Limited is preferred.

Other than the above-mentioned components, is composed of the remainder Fe and unavoidable impurities.

Steel slabs having the above-mentioned composition can be heated to perform hot rolling. At this time, the heating is preferably carried out at a temperature at which the hot rolling finishing temperature can be the minimum austenite phase. Therefore, it can be heated to a temperature higher than the austenite temperature.

Thereafter, the heated steel slab is hot-rolled, cooled to 400 to 650 ° C, and then wound.

If winding is performed in a state where the cooling end temperature after hot rolling exceeds 650 ° C, buckling occurs after winding and the coil may be crushed. On the other hand, when the cooling end temperature is too low, The bainite or martensite phase fraction is too high, so that the strength is too high even if the tempering is carried out thereafter, resulting in a problem that the cold rolling property is deteriorated in the subsequent cold rolling step.

Thereafter, the rolled hot-rolled steel sheet was put in a furnace maintained at a dew point of -30 to 0 DEG C and maintained at a nitrogen gas temperature, tempered at 600 to 750 DEG C for 20 to 120 minutes, .

The furnace may be any furnace capable of heat-treating a coil in a process, and a general continuous annealing furnace may be used, but there is a problem that the process cost is greatly increased. Therefore, BAF (Batch Annealing Furnace) can be used.

Generally, when the cooling temperature (coiling temperature) is low in the cooling process after hot rolling, the cooling rate becomes faster, so a part of the austenite is transformed into martensite or bainite phase and is present in the hot-rolled steel sheet. The strength of the final hot-rolled steel sheet is increased. However, in the present invention, when the hot-rolled steel sheet containing the martensite phase is manufactured by lowering the coiling temperature and tempering is performed in the BAF maintained at 600 to 750 ° C, the martensite is decomposed into pearlite (ferrite + cementite) The strength is lowered, and the ductility is increased.

If the heating temperature is less than 600 ° C., the tempering effect is insufficient. On the other hand, when the heating temperature is higher than 750 ° C., a low-strength austenite phase is formed, and buckling may occur during the tempering process. If the tempering time is less than 20 minutes, the tempering is insufficient and the strength of the steel sheet after the tempering is reduced and the effect of increasing the elongation is insufficient. On the other hand, if tempering is performed for more than 120 minutes, The strength reduction of the steel sheet and the synergistic effect of the elongation are excellent, but it is preferable to set 20 to 120 minutes in order to control the thickness of the internal oxide layer in terms of productivity.

When tempering is carried out in the BAF under the above-mentioned conditions, it is preferable that the inside of the BAF is controlled at a dew point temperature of -30 to 0 占 폚 and maintained in a nitrogen gas atmosphere. By doing so, oxygen is diffused into the steel below the iron oxide layer while forming an iron oxide layer in the surface layer of the hot-rolled steel sheet in the tempering process, thereby oxidizing a component having a higher affinity to oxygen than iron such as Mn, Si, So that oxides of these elements can be formed inside the steel. As a result, Mn, Si or Al is not present in the steel in which the single or composite oxide such as Mn, Si or Al is formed, or the content thereof is greatly reduced.

However, if the dew point temperature is less than -30 ° C, the oxygen partial pressure in the atmospheric gas is low at the tempering temperature, so that oxygen is not diffused to the inside of the steel below the iron oxide layer and internal oxides are not formed. On the other hand, The depth at which oxygen diffuses into the steel becomes too deep and the layer in which the internal oxide is formed becomes too thick, which may result in deterioration of strength and ductility of the final product.

After completion of the tempering by the above-described process, the hot-rolled steel sheet can be taken out to the furnace or the air and then air-cooled in the BAF. At this time, if the steel sheet is quenched at a rate exceeding 10 ° C / s, the strength lowered by tempering may increase again, thereby deteriorating the cold rolling property in the subsequent cold rolling step. Do.

The hot-rolled steel sheet after completion of tempering and cooling is subjected to pickling and then cold-rolled by a usual method.

The pickling process is for removing the hot-rolled oxide in the surface layer formed during the hot rolling process. The pickling process and the acid solution at this time are not particularly limited and can be carried out by a conventional pickling process.

The cold-rolled steel sheet produced by the cold-rolling can remove contaminants such as oil and iron particles on the surface of the steel sheet during the cold rolling process by the degreasing process. At this time, a conventional degreasing process can be used, but it is more preferable to apply the alkali degreasing process in consideration of cost.

Thereafter, the cold-rolled steel sheet can be drawn into the annealing furnace and subjected to annealing heat treatment.

At this time, the annealing heat treatment step is preferably performed by reducing annealing by heating at 700 to 850 캜 under an atmospheric gas containing 3 to 10% of hydrogen and residual nitrogen and inevitable impurities at a volume ratio. If the hydrogen content in the atmosphere gas of the annealing furnace is less than 3%, it is difficult to effectively reduce the iron oxide present on the surface of the steel sheet. The higher the hydrogen content, the better the reduction effect of the iron oxide. However, .

When the annealing heat treatment is completed, the hot dip galvanizing layer may be formed by dipping in a hot dip galvanizing bath. The method of forming the hot plat layer is not particularly limited.

For example, in the case of hot-dip galvanizing, hot-dip galvanizing can be carried out by immersing in 0.1 to 0.25 wt% of Al and the remainder of Zn in a hot-dip galvanizing bath at 450 to 480 캜 for 3 to 5 seconds.

In the present invention, a hot-dip galvanized steel sheet can be produced by dipping in a hot-dip galvanizing bath containing from 0.16 to 0.25% by weight of effective aluminum (Al) at 450 to 470 ° C and performing hot dip galvanizing, It is possible to control the amount of plating adhered to the surface. Here, the temperature of the plating bath is preferably set in consideration of the fluidity of the plating bath, evaporation of zinc, and dissolution of the steel sheet in the plating bath. However, in the case of producing a galvannealed steel sheet by the alloying heat treatment of the produced hot-dip galvanized steel sheet, it is preferable to use a hot-dip galvanizing bath containing 0.10-0.14% by weight of effective Al.

Generally, an alloying inhibiting layer (Fe-Al alloy phase) is formed on the interface between the hot-dip galvanizing layer and the hot-dip galvanized steel sheet formed by hot-dip galvanizing. The alloying suppressing layer becomes thicker as the Al content in the plating bath becomes higher, A high alloying temperature is required. Therefore, in the case of producing a galvannealed galvanized steel sheet, it is preferable to keep the effective Al content in the plating bath low as compared with the case of producing a hot-dip galvanized steel sheet. Therefore, in the present invention, in the case of producing a hot-dip galvanized steel sheet, the content of effective Al in the plating bath is controlled to 0.16 to 0.25%, and when the galvannealed steel sheet is produced, 0.14%.

It is preferable to maintain the temperature of the steel sheet at 460 to 550 DEG C before drawing the annealed heat treated steel sheet into the plating bath satisfying the above conditions. The temperature at which the steel sheet is introduced into the plating bath must be equal to or higher than the plating bath temperature, and the wettability of zinc can be excellent when plating. However, if the steel sheet temperature is too high, the temperature of the plating bath rises during plating, which makes it difficult to control the temperature of the plating bath. Therefore, it is preferable to maintain the temperature of the steel sheet before entering the plating bath at 460 to 550 ° C .

The present invention can further perform the alloying heat treatment at 480 to 600 ° C without cooling the hot-dip galvanized steel sheet produced above.

In the present invention, it is possible to produce an alloyed hot-dip galvanized steel sheet having an alloyed hot-dip galvanized layer containing 7 to 13% of Fe (degree of alloying) by the alloying heat treatment.

If the alloying temperature is less than 480 DEG C during the alloying heat treatment, it is difficult to secure the Fe content in the galvannealed galvanized layer to 7 to 13%, whereas if it exceeds 600 DEG C, the bainite or martensite phase formed in the annealing and cooling step is decomposed So that there is a problem that strength and ductility are deteriorated by forming carbide, which is not preferable. If the Fe content in the galvannealed galvanized layer formed by the alloying heat treatment is less than 7%, the friction characteristics, weldability, and paintability may be deteriorated when processed into a product. On the other hand, if the Fe content exceeds 13% there is a problem. Therefore, in the present invention, by limiting the alloying temperature to 480 to 600 ° C in the alloying heat treatment, it is possible to ensure the desired degree of alloying while securing excellent strength and ductility.

As described above, it is possible to improve the cold rolling property in the subsequent cold rolling step while imparting strength and ductility through tempering in the hot rolling step. In the production of the hot-dip galvanized steel sheet by using the cold- By controlling the process conditions, a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet having excellent surface quality can be produced.

Hereinafter, a high-strength hot-rolled steel sheet according to another aspect of the present invention and a hot-dip galvanized steel sheet produced using the hot-rolled steel sheet will be described in detail. In the present invention, the hot-dip galvanized steel sheet may be an alloyed hot-dip galvanized steel sheet having an alloy degree of 7 to 13%.

The high-strength hot-rolled steel sheet according to the present invention may include an inner oxide composed of Mn oxide, Si oxide, Al oxide, Fe oxide, and a composite oxide thereof at a depth of 2 to 20 μm from the surface in the thickness direction of the steel sheet.

When a hot-rolled steel sheet having an oxide formed inside the steel sheet but not on the surface of the steel sheet as described above is subjected to cold rolling to produce a hot-dip galvanized steel sheet, no oxide is formed or a small amount is formed inside the steel sheet, A steel sheet excellent in surface quality can be obtained.

Further, the hot-dip galvanized steel sheet according to the present invention is a cold-rolled steel sheet obtained by cold-rolling the above hot-rolled steel sheet; And a molten zinc plated layer formed on the surface of the cold-rolled steel sheet, wherein Mn oxide, Si oxide, Al oxide Fe oxide, and a composite oxide thereof are stacked at a depth of 0.1-3.0 m from the interface between the cold- ≪ / RTI >

Generally, in the hot-dip galvanized steel sheet produced by the conventional method, oxides are present at the interface between the hot-dip galvanized layer and the base steel sheet. However, when the hot-rolled steel sheet obtained according to the present invention is plated after cold rolling, oxides are present in the range of 0.1 to 3 μm in the thickness direction of the steel sheet at the interface between the steel sheet and the hot-dip galvanized layer. That is, in the case of the present invention, since oxides are present inside the base steel sheet rather than on the surfaces of the hot-dip galvanized layer and the base steel sheet as in the prior art, problems such as surface quality deterioration due to these oxides can be prevented.

However, in order to form oxides to a depth exceeding 3 μm in the thickness direction, the oxidizing atmosphere in the BAF annealing process becomes too high, and the surface of the steel sheet A large amount of Fe oxide is generated, which may lower the wettability of zinc in the subsequent plating process. Therefore, in forming the oxide in the steel sheet, it is preferable to form the oxide up to 3 mu m in the thickness direction from the interface.

On the other hand, when the hot-rolled steel sheet is cold-rolled, the thickness of the steel sheet is changed. Thus, in the hot-rolled steel sheet, the oxides existing within 2 to 20 μm in the thickness direction from the surface are in the range of 0.1 to 3 μm .

As described above, the hot-dip galvanized steel sheet according to the present invention is a steel sheet having not only sufficient alloying degree but also excellent surface quality because the surface diffusion of oxidizing elements in the steel is suppressed.

Hereinafter, the present invention will be described more specifically by way of examples. It should be noted, however, that the following examples are intended to illustrate the invention in more detail and not to limit the scope of the invention. The scope of the present invention is determined by the matters set forth in the claims and the matters reasonably inferred therefrom.

( Example )

A TRIP steel slab containing 1.5% by weight of Si, 2.0% by weight of Mn and 1.0% by weight of Al as a main alloy element was heated to 1200 캜 and subjected to rough rolling and finishing rolling to prepare a 3.5 mm- And rolled to produce a hot-rolled steel sheet. At this time, the finishing rolling temperature was set to 910 캜, and the cooling temperature was set differently as shown in Table 1 below. Thereafter, whether or not buckling occurred in the manufactured hot-rolled steel sheet was evaluated, and the results are shown in Table 1 below.

After cooling the hot-rolled steel sheet by natural air cooling, some cold-rolled steel sheets were subjected to BAF tempering under the conditions shown in Table 1, and then cooled to room temperature by air cooling.

In order to evaluate the cold rolling property of the hot rolled steel sheet subjected to / not subjected to the tempering, the yield strength of each hot rolled steel sheet after tensile test was measured and shown in Table 2 below.

In order to confirm the existence of oxide on the surface layer of the hot-rolled steel sheet, a thin film was formed on a cross section using a FIB (Focused Ion Beam), and then TEM (Transmission Electron Micrograph) and EDX (Energy Dispersive X-ray Spectrometer) The presence or absence of the oxide and the maximum depth of the inner oxide in the thickness direction from the surface of the steel sheet were measured. The results are shown in Table 2 below.

Thereafter, the produced hot-rolled steel sheet was pickled using an aqueous hydrochloric acid solution, and cold-rolled by a conventional method to produce a cold-rolled steel sheet having a thickness of 1.6 mm. After removing the foreign substances and rolling oil from the surface of the cold-rolled steel sheet, the steel sheet was heated to 800 ° C in an annealing furnace having an atmosphere containing hydrogen and residual nitrogen at a dew point temperature of -60 ° C and a volume ratio of 5% Followed by reduction annealing, followed by cooling to 500 ° C. Thereafter, the steel sheet was immersed in a hot dip galvanizing bath containing 0.2% by weight of Al to prepare a hot-dip galvanized steel sheet. However, in the case of producing a galvannealed steel sheet, a hot-dip galvanizing bath containing 0.13% by weight of Al was used.

The surface of the prepared hot-dip galvanized steel sheet was visually inspected to determine the surface quality depending on the presence or absence of unplated and the degree of surface roughness, and the results are shown in Table 2 below. In addition, as in the above hot-rolled steel sheet evaluation, a thin film was produced by FIB on the cross-section of a hot-dip galvanized steel sheet, and the maximum depth at which the inner oxide existed from the interface to the thickness direction was measured by observing a cross section with TEM and EDX. Are shown in Table 2.

The inventors of the present invention observed the surface of each of the produced hot-dip galvanized steel sheets with naked eyes to determine the surface quality according to the presence or absence of the unplated and the degree of surface roughness.

At this time, the surface quality was determined according to the following criteria.

Very good (⊚): When there is no unplated over the whole plated steel sheet

Excellent (∘): When no dot plating of less than 0.5 mm is observed for two or less per 100 cm ²

Defective (△): When a large number of non-spotted plating of 0.5mm to 3mm size is observed

Very poor (X): When unplated size exceeding 3mm is observed

division Coiling temperature
(° C) Occurrence of buckling Presence or absence of tempering Tempering conditions atmosphere
gas dew point
Temperature (℃) Heating temperature
(° C) Retention time
(min)

foot
persons
Yes One 450 radish U nitrogen -10 650 30 2 600 radish U nitrogen 0 600 100 3 550 radish U nitrogen -15 600 60 4 630 radish U nitrogen -25 700 40 5 600 radish U nitrogen -20 650 60 6 600 radish U nitrogen -15 600 90 7 600 radish U Nitrogen + hydrogen 3% -10 600 30
ratio
School
Yes One 750 U radish - - - - 2 200 radish radish - - - - 3 250 radish U nitrogen -15 600 60 4 500 radish radish - - - - 5 550 radish U nitrogen -60 600 60

division Hot-rolled steel sheet
Yield strength (MPa) Hot-rolled steel oxide
Maximum depth (μm) Plated steel oxide
Maximum depth (μm) Coated steel plate
Surface quality evaluation

foot
persons
Yes One 541 8.3 1.4 ◎ 2 524 17.1 2.5 ◎ 3 533 7.6 1.2 ◎ 4 512 3.5 0.8 ○ 5 521 3.2 0.8 ○ 6 531 7.8 1.2 ◎ 7 528 8.1 1.3 ◎
ratio
School
Yes One 574 0.9 0.05 △ 2 689 0 0 × 3 621 7.5 1.2 ◎ 4 653 0 0 × 5 534 0 0 ×

As shown in Tables 1 and 2, in Inventive Examples 1 to 7 satisfying the present invention, no buckling phenomenon occurred after winding, and all of the yield strength after tempering was as low as 541 MPa or less, thereby facilitating the cold rolling process . Further, as the tempering conditions satisfied all the requirements of the present invention, the internal oxides were formed within 2 to 20 m in the thickness direction of the hot-rolled steel sheet immediately below the surface thereof, so that even when hot- Excellent or excellent results were obtained.

On the other hand, in Comparative Example 1, the winding temperature of the hot-rolled steel sheet was higher than the range limited by the present invention, and the buckling phenomenon occurred severely due to heat generation after transformation. Also, during the air cooling after the hot rolling, internal oxides were formed within 0.9 mu m in the thickness direction from the surface layer portion of the hot rolled steel sheet, and these internal oxides were present even after pickling and cold rolling, resulting in poor surface quality after plating.

In Comparative Examples 2 and 3, when the coiling temperature of the hot-rolled steel sheet was lower than the range limited by the present invention, the buckling was suppressed at a low coiling temperature, but the yield strength was as high as 620 MPa or more, Poor.

In Comparative Example 2, as tempering was not performed after hot rolling, Si, Mn and Al elements in the steel were diffused to the surface to form oxides on the surface in the subsequent annealing step, resulting in poor quality of the surface after plating.

However, in Comparative Example 3, the surface diffusion of Si, Mn and Al elements in the steel was suppressed by tempering under the conditions of the present invention after hot rolling, and the surface quality after plating was excellent as the inner oxide was formed within 7 μm in the thickness direction Respectively.

In Comparative Example 4, the buckling phenomenon was suppressed by satisfying the range proposed in the present invention. However, since the subsequent tempering was not performed, the Si, Mn and Al elements existing in the steel in the subsequent annealing process were diffused to the surface Oxide was formed on the surface, which resulted in extremely poor surface quality after plating.

In Comparative Example 5, the buckling phenomenon was suppressed as the hot-rolling process satisfied the range suggested in the present invention, and the resulting product was tempered to have a low yield strength of 534 MPa, which was excellent in the cold rolling property. However, since the dew- In the annealing process after cold rolling, Si, Mn and Al elements in the steel were diffused to the surface to form a surface oxide, which resulted in poor surface quality after plating.

Claims (12)

(Mn): not more than 15% (including 0%), silicon (Si): not more than 3.0% (including 0%) and aluminum (Al): not more than 6.0% , Si and Al, respectively, is limited to not less than 2.0%, not less than 0.5% and not less than 0.5%, and the balance of Fe and inevitable impurities,
A steel slab having a total content of at least two of Mn, Si and Al of 2 to 20% is heated to austenite temperature or higher and hot rolled to form a steel sheet;
Cooling the hot-rolled steel sheet to 400 to 650 ° C and winding the hot-rolled steel sheet;
After being drawn into a BAF (Batch Annealing Furnace) having a dew point of -30 to 0 ° C and a volume ratio of 5% or less in a nitrogen atmosphere containing hydrogen, the steel sheet is tempered at 600 to 750 ° C for 20 to 120 minutes ; And
And cooling the steel sheet to a normal temperature after the tempering.
The method according to claim 1,
Wherein the steel slab contains 0.05 to 0.30% carbon, 0.5% or less of molybdenum, 1.0% or less of chromium, 0.03% or less of phosphorus, 0.2% or less of titanium, , Wherein at least one selected from the group consisting of not more than 0.2% of niobium (Nb), not more than 1.0% of nickel (Ni), not more than 1.0% of copper (Cu), not more than 0.2% of vanadium (V) A method of producing a high strength hot-rolled steel sheet, which further comprises at least one component.
The method according to claim 1,
Wherein the post-tempering cooling is performed at a cooling rate of 10 DEG C / s or less.
A method for manufacturing a cold-rolled steel sheet, comprising the steps of: cold-rolling a high-strength hot-rolled steel sheet produced by any one of claims 1 to 3;
Heating the cold-rolled steel sheet at 700 to 850 ° C in an annealing furnace containing 3 to 10% of hydrogen at a volume ratio and composed of the remainder nitrogen, and then cooling; And
And a step of dipping the cooled cold-rolled steel sheet in a hot-dip galvanizing bath at 440 to 470 ° C for plating, thereby obtaining a high-quality hot-dip galvanized steel sheet having excellent surface quality.
5. The method of claim 4,
Wherein said hot-dip galvanizing bath comprises 0.16 to 0.25% Al by weight in terms of surface quality.
A method of producing a high strength alloyed hot-dip galvanized steel sheet excellent in surface quality by alloying heat treatment of a hot-dip galvanized steel sheet produced by any one of claims 4 to 5 at 470 to 600 ° C.
(Mn): not more than 15% (including 0%), silicon (Si): not more than 3.0% (including 0%) and aluminum (Al): not more than 6.0% , Si and Al are respectively limited to not less than 2.0%, not less than 0.5% and not less than 0.5%, and the balance of Fe and inevitable impurities, wherein the sum of at least two of Mn, Si and Al is 2 To 20%
A high strength hot-rolled steel sheet comprising an inner oxide composed of Mn oxide, Si oxide, Al oxide, Fe oxide, and composite oxide thereof at a depth of 2 to 20 占 퐉 in a thickness direction of the steel sheet from the surface.
8. The method of claim 7,
Wherein the hot rolled steel sheet comprises 0.05 to 0.30% carbon, 0.5% or less of molybdenum, 1.0% or less of chromium, 0.03% or less of phosphorus, 0.2% or less of titanium, , Wherein at least one selected from the group consisting of not more than 0.2% of niobium (Nb), not more than 1.0% of nickel (Ni), not more than 1.0% of copper (Cu), not more than 0.2% of vanadium (V) A high strength hot-rolled steel sheet further comprising at least one component.
(Mn): not more than 15% (including 0%), silicon (Si): not more than 3.0% (including 0%) and aluminum (Al): not more than 6.0% , Si and Al are respectively limited to not less than 2.0%, not less than 0.5% and not less than 0.5%, and the balance of Fe and inevitable impurities, wherein the sum of at least two of Mn, Si and Al is 2 To 20% by weight of a cold-rolled steel sheet obtained by cold-rolling a hot-rolled steel sheet; And a hot-dip galvanized layer formed on the surface of the cold-rolled steel sheet,
Characterized by comprising an inner oxide composed of Mn oxide, Si oxide, Al oxide, Fe oxide, and a composite oxide thereof at a depth of 0.1 to 3.0 占 퐉 in the direction of the base steel sheet from the interface between the base steel sheet and the hot dip galvanized layer Excellent high strength hot dip galvanized steel sheet.
10. The method of claim 9,
Wherein the hot rolled steel sheet comprises 0.05 to 0.30% carbon, 0.5% or less of molybdenum, 1.0% or less of chromium, 0.03% or less of phosphorus, 0.2% or less of titanium, , Wherein at least one selected from the group consisting of not more than 0.2% of niobium (Nb), not more than 1.0% of nickel (Ni), not more than 1.0% of copper (Cu), not more than 0.2% of vanadium (V) A high strength hot-dip galvanized steel sheet further comprising at least one component and having excellent surface quality.
(Mn): not more than 15% (including 0%), silicon (Si): not more than 3.0% (including 0%) and aluminum (Al): not more than 6.0% At least two kinds of Si and Al are limited to not less than 2.0%, not less than 0.5% and not less than 0.5%, and the balance Fe and inevitable impurities, A cold rolled steel sheet which is cold rolled hot rolled steel sheet satisfying 20%; And an alloyed hot-dip galvanized layer formed on the surface of the cold-rolled steel sheet,
And the alloyed hot-dip galvanized layer has an alloy degree of 7 to 13%.
12. The method of claim 11,
Wherein the hot rolled steel sheet comprises 0.05 to 0.30% carbon, 0.5% or less of molybdenum, 1.0% or less of chromium, 0.03% or less of phosphorus, 0.2% or less of titanium, , Wherein at least one selected from the group consisting of not more than 0.2% of niobium (Nb), not more than 1.0% of nickel (Ni), not more than 1.0% of copper (Cu), not more than 0.2% of vanadium (V) A high strength alloyed hot-dip galvanized steel sheet further comprising one or more components and having excellent surface quality.