KR101489243B1 - High strength galvannealed steel sheet having excellent formability and coating adhesion and method for manufacturing the same - Google Patents

Abstract

The present invention relates to a microstructure having an excellent plating adhesion and an elongation of not less than 21% and a tensile strength of not less than 600 MPa, wherein the microstructure is composed of 61 to 70% of ferrite, 11 to 20% of martensite and 15 to 25% (Bainite) and controlling the ferrite average grain size to 2 탆 or less, and a method for producing the same.
The present invention relates to a high strength alloyed hot-dip galvanized steel sheet excellent in workability and plating adhesion, which is used for automobile interior and exterior panel applications, and a method for producing the same. 0.1 to 0.3 wt% Mn, 1.7 to 2.0 wt% Al, 0.01 to 0.03 wt% Al, 0.01 to 0.02 wt% P, 0.006 wt% or less S, 0.15 to 0.45 wt% Cr and 0.02 to 0.08 wt% And the balance being Fe and inevitable impurities and satisfying the following formula 1 = 3 <[Cr] / [Mo] <12 and containing 0.0025% or more of carbon in the solid state, Wherein the alloyed molten zinc-plated steel sheet has a composition of 10 to 15%, and the alloyed hot-dip galvanized layer has a delta (delta) phase area High-strength alloyed hot-dip galvanized steel sheet excellent in workability and plating adhesion of not less than 90% And a method for producing the same.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength alloyed hot-dip galvanized steel sheet excellent in workability and plating adhesion, and a method for manufacturing the same. BACKGROUND ART &lt; RTI ID = 0.0 &gt;

The present invention relates to a galvannealed galvanized steel sheet used as a material for automobiles, home appliances, and the like, and a manufacturing method thereof.

Generally, the steel sheets used for automobile interior and exterior panels are required to have excellent workability in addition to corrosion resistance to corrosion for processing. A hot-dip galvanized steel sheet having zinc-plated on the surface of the cold-rolled steel sheet to improve the corrosion resistance is a representative example of the steel sheet for this purpose. Generally, the corrosion resistance of the hot-dip galvanized steel sheet depends on the amount of the plating adhered thereto. However, if the amount of zinc adhered increases, the weldability is deteriorated. As a result, galvannealed steel sheet has been improved.

The galvannealed galvanized steel sheet is obtained by annealing a cold rolled steel sheet that has not undergone annealing in a continuous galvanizing line, plating it in a galvanizing bath, and then passing it through a furnace to alloy the iron (Fe) .

Such a galvannealed galvanized steel sheet has a disadvantage in that when a plating layer is present on a base steel sheet and the adhesion between the plating layer and the base steel sheet is weak, there is a problem that the plating layer is separated at the time of processing and workability is lowered. Nevertheless, since galvannealed galvanized steel sheet is excellent in corrosion resistance of the material itself by the plating layer, many automobile manufacturers at home and abroad are actively expanding the use of galvannealed galvanized steel sheets. Therefore, there is a demand for a technique for improving both the workability and the plating properties of the galvannealed steel sheet.

On the other hand, precipitation hardened steel, hardened hardened steel, solidified hardened steel, and transformed hardened steel have been used as high strength automotive materials. Among them, dual phase steel (hereinafter referred to as "DP steel") and Transformation Induced Plasticity (referred to as "TRIP steel") are used for improving the strength and formability of steel sheet, ), Silicon (Si), niobium (Nb), aluminum (Al), etc. These alloying elements are concentrated on the surface of the steel sheet during the cold rolling process, thereby deteriorating the plating characteristics. Therefore, there is a problem that a bare spot layer occurs, or defects such as micro dent are generated.

As a technique for solving the above problems, Patent Documents 1 to 3 disclose a method of manufacturing a semiconductor device, which includes carbon (C), silicon (Si), manganese (Mn), phosphorus (P), aluminum (Al), chromium (Cr), antimony (Sb) Discloses a zinc-plated steel sheet for improving the moldability and plating characteristics by limiting the content of certain elements such as tin (Sn).

However, these techniques also have problems in that the addition effect of specific elements and the metallurgical behavior are unclear and thus the manufacturing method is insufficient and the workability is lowered, so that the processability and plating property of the galvannealed steel sheet are further improved Technology is still required.

Japanese Laid-Open Patent Publication No. 2002-155317 Japanese Laid-Open Patent Publication No. 2001-064750 Japanese Patent Application Laid-Open No. 2002-294397

An object of the present invention is to provide a high strength alloyed hot-dip galvanized steel sheet having excellent processability and excellent plating adhesion, and a method for producing the same.

The inventors of the present invention have conducted intensive studies to obtain a high strength alloyed hot-dip galvanized steel sheet excellent in workability and plating adhesion. As a result, it has been found that by controlling the component system of a steel sheet and controlling the microstructure of the steel sheet by using ferrite, martensite, (Bainite), control the crystal grain size of ferrite in the steel sheet to ensure processability, and control the alloy phase fraction and control the degree of alloying to ensure that the galvannealed galvanized steel sheet can be produced simultaneously. And reached the present invention.

In one aspect of the present invention, there is provided a method of manufacturing a steel plate, comprising: 0.065 to 0.075 wt% of C, 0.10 to 0.30 wt% of Si, 1.7 to 2.0 wt% of Mn, 0.01 to 0.03 wt% of Al, 0.01 By weight, Cr: from 0.02 to 0.08% by weight, Fe, and other unavoidable impurities, wherein the content of Fe is from 0.001 to 0.02% by weight, S is from 0.006% by weight or less to 0.15 to 0.45% by weight, (% By weight of Mo) < 12, wherein the microstructure is composed of 61 to 70% of ferrite, 11 to 20% of martensite and 15 to 25% of bainite in an area fraction A high strength alloyed hot dip galvanized steel sheet excellent in workability and plating adhesion is provided.

Preferably, the alloyed hot-dip galvanized steel sheet of the present invention contains 0.0025 wt% or more of carbon in solid state, and the average crystal grain size of the ferrite is 2 탆 or less, the average crystal grain size of the martensite is 2.5 탆 or less And the alloyed hot-dip galvanized steel sheet has an alloy degree of 10 to 15%, and the alloyed hot-dip galvanized layer has a delta (delta) phase of 90% or more in an area fraction.

The galvannealed steel sheet of the present invention has a tensile strength (TS) of not less than 600 MPa and an elongation (EL) of not less than 21%, and has a TS EL value of 13,500 MPa% or more.

In another aspect of the present invention, there is provided a ferritic stainless steel comprising 0.065 to 0.075% by weight of C, 0.10 to 0.30% by weight of Si, 1.7 to 2.0% by weight of Mn, 0.01 to 0.03% By weight, Cr: from 0.02 to 0.08% by weight, Fe, and other unavoidable impurities, wherein the content of Fe is from 0.001 to 0.02% by weight, S is from 0.006% by weight or less to 0.15 to 0.45% by weight, &Lt; wt% of Mo &gt; &lt;12;

Hot-rolling the reheated slab at 870 to 905 占 폚;

Rolling the hot-rolled steel sheet at 560 to 580 占 폚;

Cold rolling the rolled steel sheet at a reduction ratio of 50 to 80%;

Heat-treating the cold-rolled steel sheet by annealing at 780 to 810 ° C for recrystallization;

Subjecting the annealed heat treated steel sheet to hot dip galvanizing; And

And hot-treating the hot-dip galvanized steel sheet at a temperature ranging from 470 to 550 ° C. The present invention also provides a method for producing a galvannealed steel sheet.

In the method for producing an alloyed hot-dip galvanized steel sheet according to the present invention, the annealing heat treatment step is preferably performed for 10 to 120 seconds.

According to the present invention, the microstructure of a steel sheet is controlled by ferrite of 61 to 70%, martensite of 11 to 20% and bainite of 15 to 25%, and ferrite existing in the steel sheet It is possible to provide a steel sheet whose average crystal grain size is controlled to 2 탆 or less. Such a steel sheet can secure a tensile strength of 600 MPa or more and an elongation of 21% or more, and the processability and plating adhesion An excellent high strength alloyed hot-dip galvanized steel sheet can be provided.

Hereinafter, the present invention will be described in detail.

The present invention relates to a galvannealed galvanized steel sheet, which is one aspect of the present invention. The types and contents of the components contained in the slab are as follows.

Carbon [C]: 0.065 to 0.075 wt%

C is an Austenite stabilizing element that minimizes the pearlite structure and the carbides inside the ferrite structure in the hot rolled steel sheet and refines the crystal grains, and the reuse of the composite precipitates is partially reused in the annealing process of the cold rolled steel sheet , A fine grain having a size of about 10 to 30 mu m is provided, and the volume fraction of martensite appearing in the crystal grain boundaries is limited to 20% or less, thereby developing a well-formed texture. The content of C is 0.065 to 0.075% by weight. When the content of C is less than 0.065% by weight, stable austenite can not be secured in the critical temperature region, and martensite is not produced in an appropriate fraction after cooling, so that it becomes difficult to secure proper strength. On the other hand, when the content of C exceeds 0.075% by weight, ductility can not be ensured and weldability is deteriorated.

Silicon [Si]: 0.10 to 0.30 wt%

The Si increases the strength by solid solution strengthening as a ferrite stabilizing element while suppressing the precipitation of cementite while maintaining the temperature at 350 to 600 ° C. after the annealing heat treatment and the carbon (C) is concentrated into austenite And contributes to formation of martensite and improvement of ductility upon cooling. The Si content is 0.10 to 0.30% by weight. When the Si content is less than 0.10 wt%, the above-described austenite stabilizing effect is lowered. On the other hand, when the Si content exceeds 0.30 wt%, the surface property is lowered, and the Si oxide is concentrated to lower both the weldability and the plating ability.

Manganese [Mn]: 1.7 to 2.0 wt%

The Mn is an element for stabilizing austenite, and it delays decomposition of austenite into pearlite during cooling to 300 to 580 캜 after annealing. This results in a martensite structure as a low-temperature transformation phase during cooling to room temperature, thereby stably producing the structure. Further, it has an effect of improving the strength by solid solution strengthening and is very effective in preventing hot cracking of the slab because it forms MnS inclusions in combination with sulfur (S) in steel. The content of Mn is 1.7 to 2.0% by weight. When the content of Mn is less than 1.7% by weight, the decomposition of austenite into pearlite is not effectively retarded. On the other hand, when the content of Mn is more than 2.0% by weight, the cost of the slab remarkably increases, and the weldability and moldability may be deteriorated.

Aluminum [Al]: 0.01 to 0.03 wt%

The Al is used as a deoxidizer and is an element that stabilizes austenite by inhibiting cementite precipitation and delaying the progress of transformation such as Si. The content of Al is 0.01 to 0.03% by weight. Al is segregated in the grain boundary in the high temperature region to finely make carbide in the hot-rolled steel grain. Therefore, by limiting the content of Al to not less than 0.01% by weight, which is the austenite stabilization minimum effect threshold value, unnecessary dissolved nitrogen (N) Can be precipitated. On the other hand, when the content of Al exceeds 0.03% by weight, nozzle clogging may occur during continuous casting, and hot brittleness and ductility may be remarkably lowered due to Al oxide or the like during casting, and surface defects are likely to occur.

Phosphorus [P]: 0.01 to 0.02 wt%

The P increases the strength by solid solution strengthening, and when added together with Si, it inhibits cementite precipitation while maintaining the temperature at 300 to 580 캜 after annealing, and promotes carbon enrichment with austenite. The content of P is 0.01 to 0.02% by weight. When the content of P is less than 0.01% by weight, the effect of stabilizing the austenite is deteriorated. When the content of P is more than 0.02% by weight, the secondary workability is deteriorated and the adhesion and the alloying property of the zinc plating deteriorate.

Sulfur [S]: not more than 0.006% by weight

The S is inevitably contained as an impurity, and its content is 0.006% by weight or less. S is combined with Fe to form FeS, thereby causing hot brittleness. Therefore, it is desirable to suppress the content to the maximum. In theory, it is advantageous not to contain S at all but it is essential that the upper limit of the content is limited because it is inevitably contained in the manufacturing process normally.

Cr [Cr]: 0.15 to 0.45 wt%

The chrome (Cr) is one of the most important elements in the present invention, and is very effective for improving the hardenability and stably forming the low temperature transformation phase, and it causes the refinement of the carbide to delay the spheroidization rate, It has an inhibition function and strengthens ferrite. It is also effective in suppressing the softening of the heat affected zone (HAZ) at the time of welding. The content of chromium (Cr) is 0.15 to 0.45% by weight, preferably 0.21 to 0.35% by weight. When the content of Cr is less than 0.15 wt%, the bond with carbon (C) becomes too small to be reused. When the Cr content exceeds 0.45 wt%, the hardness of the HAZ increases excessively.

Molybdenum [Mo]: 0.02 to 0.08 wt%

The Mo is an element which improves the plating and workability. Since the Mo dissolution temperature is low in the cooling process after hot rolling, it is redissolved in the annealing process so that the content of the Mo in the composite precipitate is re- 0.08% by weight, preferably 0.03% by weight to 0.07% by weight. When the content of Mo is less than 0.02 wt%, the effect of addition is deteriorated. When the content of Mo is more than 0.08 wt%, the stock capacity is decreased, which results in difficulty in forming a low temperature transformation phase and an increase in cost.

On the other hand, the ratio [weight% of Cr] / [weight% of Mo] influences the index λ (%) indicating elongation flangeability,

 3 < [wt% of Cr] / [wt% of Mo] < 12

, Excellent stretch flangeability is exhibited.

However, when the ratio of [Cr% by weight] / [Mo by weight%] is less than 3 or 12 or more, the value of? (%) Becomes 50% or less.

In addition, the carbon (C) in the solid state in the steel sheet has a fatigue resistance higher when subjected to a load by repetition of tensile and compression after cold working. The reason for this is not clearly known. However, when the potential generated by the cold working in the ferrite phase in which fatigue cracking occurs causes dislocation movement to the load due to repetition of tensile and compression, It is presumed that this is due to the delay of the process of changing the structure into a dislocation arrangement which is easy to cause plastic distortion, which is the nature of fatigue damage. The content of carbon in such a solid state is preferably 0.0025 wt% or more (for example, 0.0025 to 0.01 wt%). When the carbon content in the solid state is less than 0.0025% by weight, the effect of resistance is deteriorated. Therefore, in order to secure endothelial characteristic, it is effective that the carbon content is at least 0.0025% by weight.

The microstructure of the galvannealed steel sheet of the present invention includes ferrites of 61 to 70% in area fraction, 11 to 20% of martensite and 15 to 25% of bainite in an area fraction. Here, it is preferable that the average crystal grain size of the martensite is 2.5 탆 or less, preferably 2 탆 or less (for example, 0.5 to 2 탆). By controlling the martensite average crystal grain size in this manner, the stretch flangeability of the steel sheet can be further improved.

In the galvannealed steel sheet of the present invention, a galvannealed hot-dip galvanized layer is formed on the surface of the steel sheet having the above composition. The galvannealed hot-dip galvanized layer is formed by the following manufacturing method.

The alloying degree of the galvannealed zinc plating layer is preferably 10 to 15%. The degree of alloying is closely related to not only processability but also plating properties such as resistance to powdering. When the degree of alloying is more than 15%, iron (Fe) diffuses excessively in the steel sheet, resulting in deterioration of the plating adhesion, such as the powderiness of the plating layer. Preferably, the degree of alloying does not exceed 13%.

The delta (delta) phase of the alloyed hot-dip galvanized layer preferably has an area fraction of 85% or more, more preferably 90% or more. When the fraction of the delta (delta) phase is less than 85%, the powdering resistance tends to become weak.

Next, as another aspect of the present invention, the manufacturing method of the galvannealed steel sheet of the present invention will be described in detail for each step of the process.

(1) Slab reheating process

Based on the total 100 weight% of the above elements, 0.065 to 0.075 weight% of C, 0.1 to 0.3 weight% of Si, 1.7 to 2.0 weight% of Mn, 0.01 to 0.03 weight% of Al, 0.01 to 0.03 weight% of Al, [% By weight of Cr] / [% by weight of Cr]: 0.01 to 0.02% by weight, S: 0.006% by weight or less, Cr: 0.15 to 0.45% Wt% of Mo] < 12 is reheated in the steel slab. The reheating temperature is preferably 1150 to 1250 DEG C or higher.

(2) Hot rolling process

The slab having the above composition is hot-rolled. The hot rolling finishing temperature is 870 to 910 占 폚, preferably 880 to 905 占 폚, and then the cooling is controlled to make the hot-rolled structure finer. At this time, if the hot rolling temperature is low, coarse particles are generated in the crystal structure by the strain annealing to lower the drawability, so that hot rolling is performed at an appropriate rolling temperature to make the hot rolled structure finer. After hot rolling, it is preferable to use a high-pressure descaling device or remove the scale of the surface by strong pickling.

(3) Coiling process

The hot-rolled steel sheet is wound at a temperature of 560 to 580 캜. In the wound state, carbide is smoothly formed to minimize generation of solid carbon, and unnecessary solid nitrogen in the steel is precipitated in the form of AlN as much as possible to minimize the formation of solid nitrogen. The coiling temperature is preferably 560 to 580 캜 so as to obtain a structure having optimum mechanical properties after cold rolling and recrystallization heat treatment. When the coiling temperature is less than 560 DEG C, cold rolling is difficult due to bainite or martensite structure, and when the coiling temperature exceeds 580 DEG C, the final microstructure is coarsened, making it difficult to produce a steel sheet having sufficient strength.

(4) Cold rolling process

The rolled hot-rolled steel sheet is pickled and then cold-rolled. The cold rolling reduction is preferably 50 to 80%. The hot-rolled structure is deformed by the cold rolling process, and this strain energy becomes energy in the recrystallization process. When the cold reduction rate is less than 50%, the deformation effect is small. When the reduction rate exceeds 80%, rolling becomes difficult in practice. Further, in the hot-rolled steel sheet, the complex precipitates are decomposed during rolling, , The drawability is deteriorated, cracks are formed on the edge of the steel sheet, and the possibility of plate breakage increases.

(5) Continuous Annealing Process

In the continuous annealing of the cold-rolled steel sheet, the annealing is preferably carried out at a temperature of 780 to 810 占 폚 for 10 to 120 seconds. It is important that the continuous annealing process cools at a sufficient cooling rate so that the austenite phase produced in the two phase region is not transformed into pearlite or bainite. When the temperature is maintained at 780 to 810 캜 for less than 10 seconds, a sufficient amount of martensite can not be obtained due to insufficient formation of the austenite phase during heating. On the other hand, if it is maintained for more than 120 seconds, productivity is deteriorated.

(6) Hot dip galvanizing and alloying heat treatment process

The annealed steel sheet is subjected to a hot dip galvanizing step. The hot dip galvanizing process is preferably carried out by quenching to 400 to 470 캜 at a cooling rate of 5 to 50 캜 / sec. If the quenching termination temperature exceeds 470 캜, the ductility is reduced because it is transformed into a bainite phase. On the other hand, when the quenching finish temperature is lower than 400 캜, almost all of the quartz is transformed into a martensite phase. Therefore, it is preferable to control the quenching end temperature to 400 to 470 캜. When the hot dip galvanizing is completed, it is more preferable to reheat the alloy by the alloying heat treatment until reaching the temperature range of 470 to 550 ° C by a conventional method for the stable growth of the plating layer. Then, it is cooled to a temperature of 250 to 350 DEG C at a cooling rate of 5 to 50 DEG C / sec or more by a conventional method.

Hereinafter, the present invention will be described more specifically by way of examples. The following examples are only illustrative of the present invention in more detail, and the scope of the present invention is not limited by the examples.

Example

Inventive Examples 1 to 12 belonging to the scope of the present invention and Comparative Examples 1 to 12 deviating from the scope of the present invention were manufactured by varying the alloy composition contained in the slab as shown in the following Table 1. The slab was heated at a temperature of 1250 캜 For 2 hours.

In the production of the steel sheet, the inventive Inventive Examples 1 to 12 and Comparative Examples 1 to 12 were subjected to hot rolling at 870 to 905 占 폚 under different process conditions as shown in the following Table 2, Rolled at a cooling temperature, subjected to pickling treatment, and subjected to cold rolling at a reduction ratio of 50 to 80%. Then, the cold-rolled steel sheet was subjected to annealing at 760 to 810 ° C, followed by quenching to 470 ° C to perform hot-dip galvanizing and alloying heat treatment at 450 to 570 ° C to prepare specimens. In these specimens, specimens prepared under the process conditions according to the present invention were classified into inventive steels 1 to 12, specimens deviating from the scope of the present invention were compared with comparative steels 1 to 12, The processability and plating ability were evaluated.

Tensile strength (TS) and elongation (EL) of inventive steels 1 to 12 and comparative steels 1 to 12 were measured, and microstructures were observed. The results are also shown in Table 2. In the following Table 2, the formability evaluation is based on a tensile strength of not less than 600 MPa, an elongation (%) of not less than 21%, and a TS EL of not less than 13,500 MPa% △ when the condition is satisfied, and × when the condition is not satisfied. In addition, the evaluation of the plating performance is based on the alloying degree (%) of 10 to 15% and the delta phase fraction of not less than 90% as a standard, and satisfies all of the conditions. If not, it is marked with ×.

As can be seen from the experimental results shown in Table 2, in Inventive steels 1 to 12 having the alloy composition according to the present invention and according to the process conditions of the present invention, .

Claims (7)

Based on 100 wt% total,
0.10 to 0.30% by weight of C, 1.7 to 2.0% by weight of Mn, 0.01 to 0.03% by weight of Al, 0.01 to 0.02% by weight of P, 0.006% by weight or less of S, By weight to 0.45% by weight, Mo: 0.02 to 0.08% by weight, the balance Fe and other unavoidable impurities,
The following formula
3 < [% by weight of Cr] / [% by weight of Mo] < 12
Lt; / RTI &gt;
Wherein the microstructure contains ferrite of 61 to 70% in an area fraction, martensite of 11 to 20% and bainite of 15 to 25% in an area fraction, carbon in a solid state in a steel sheet, Wherein the alloyed hot-dip galvanized layer has a delta (delta) phase in an area fraction of 90% or more. delete The galvannealed steel sheet according to claim 1, wherein the average crystal grain size of martensite present in the steel sheet is 2.5 m or less. delete The galvannealed steel sheet according to claim 1 or 3, wherein the tensile strength (TS) is 600 MPa or more, the elongation (EL) is 21% or more, and the TS EL is 13,500 MPa% or more. delete delete