KR20130070825A - Steel sheet having excellent spot weldabity, strength and ductility for automobile and method for manufacturing the same - Google Patents

Abstract

PURPOSE: A steel sheet for a vehicle which spot weldability, strength, and a ductility ratio are excellent and a manufacturing method thereof are provided to improve the stability and weight-lightening of a vehicle body when the steel sheet of 500Mpa class level is utilized for some components. CONSTITUTION: A manufacturing method of steel sheet for a vehicle which spot weldability, strength, and a ductility ratio are excellent comprises: a step of reheating a steel sheet including 0.04-0.3wt% of C, 0.8-2.0wt% of Si, 1-3wt% of Mn, 0.03wt% or less of P, 0.008wt% or less of S, 0.012-0.02wt% of N, 0.04-2wt% of Al, 0.005-0.02wt% of Ti , 20ppm or less of B(except for zero), 0.01-0.05wt% of Sb, and the rest of Fe and inevitable impurities; a step of hot-rolling the reheated steel sheet; a step of cooling the hot-rolled steel sheet at a cooling speed of 50-110°C/sec; a step of winding the cooled steel sheet at a temperature of 100-300°C; a step of cold-rolling the wound steel sheet at a reduction rate of 30% or less; and a step of annealing the cold-rolled steel sheet.

Description

Automotive sheet having excellent spot weldability, strength and elongation and manufacturing method {STEEL SHEET HAVING EXCELLENT SPOT WELDABITY, STRENGTH AND DUCTILITY FOR AUTOMOBILE AND METHOD FOR MANUFACTURING THE SAME}

The present invention relates to a steel sheet excellent in weldability, strength and elongation which can be used for automobile steel sheets and the like, and a method of manufacturing the same.

In recent years, due to the complexity and integration of automobile molded products, automotive steel sheets are required to have a higher level of formability. In particular, the bumper reinforcement or shock absorber in the door is a part that is closely related to the passenger's safety when the vehicle collides with a tensile strength of 780 MPa and an elongation of 24% or more. It is anticipated that the parts having will be required.

In addition, as the environmental pollution problem caused by automobile exhaust gas has emerged, technology development for improving fuel economy has been made, and research for achieving automobile weight reduction using ultra high strength steel has been increasing. In order to manufacture a high strength steel sheet, the content of C has been used to increase the content of the steel sheet, but due to the increase in the content of C, the spot weldability is deteriorated, causing problems such as fracture of the weld.

In addition, research is being conducted to improve strength and ductility at the same time. For this purpose, a steel sheet containing a large amount of retained austenite has been proposed. The steel sheet is not only excellent in uniform ductility due to the increase in ductility as the residual austenite is transformed into martensite by processing, and also in the local deformation as the residual austenite is transformed into martensite when subjected to local compression pressure such as drawing. The resistance to this rapidly increases. For this reason, there is a characteristic that drawing processing is possible even if the (222) aggregate structure is not developed, such as an ultra low carbon cold rolled steel sheet. Therefore, if the steel sheet containing a large amount of residual austenite having excellent ductility can be applied to the workpiece for drawing, its field of application will be considerably broadened.

A steel sheet containing a large amount of the retained austenite described above is usually formed by adding Si and Mn to carbon steel to form austenite during annealing, and to maintain the constant bainite temperature during the cooling process to simultaneously increase strength and ductility. It is prepared by the method. The residual austenite prepared by this method is transformed into martensite during plastic deformation, thereby increasing ductility by mitigating stress concentration due to plastic organic transformation with increasing strength, which is transformed plastic plastic (TRIP) It is called, and is used as high strength steel which has high strength and ductility.

In relation to such a technique, when examining a conventionally proposed technique, Patent Literature 1 has been proposed, but the technique has a problem that can not significantly improve the spot welding characteristics of the steel sheet. And although there exists patent document 2, the intensity | strength of the steel of the said invention is low at 780 Mpa class or less. In addition, Patent Document 3 has a very narrow range of 0.13 to 0.15 of the C component of the present invention and is not useful. Moreover, the above inventions mainly focus on lowering the lower limit of the C content and adding other alloying elements in order to simultaneously increase spot weldability and ductility, but there is an uneconomical problem due to the addition of alloys and at the same time It was not possible to increase the strength and ductility together.

Japanese Patent Publication No. 1993-070886 Japanese Patent Publication No. 2001-152287 Japanese Patent Publication No. 2004-269920

One aspect of the present invention is to provide an automotive steel sheet and a method of manufacturing the same that are excellent in both spot weldability, strength and elongation at the same time.

The automotive steel sheet having excellent spot weldability, strength, and elongation, which is one aspect of the present invention, is% by weight, C: 0.04-0.3%, Si: 0.8-2.0%, Mn: 1-3%, P: 0.03% or less, S : 0.008% or less, N: 0.012 to 0.02%, Al: 0.04 to 2%, Ti: 0.005 to 0.02%, B: 20 ppm or less (excluding 0), Sb: 0.01 to 0.05%, residual Fe and other unavoidable impurities It includes and satisfies the tensile strength (MPa) * elongation (%) ≥ 22000 (MPa *%) and the ductility ratio (Cross tensile strength / Tensile shear strength) ≥ 0.35. The microstructure of the steel sheet preferably includes 10 to 30% ferrite, 20 to 40% residual austenite and the balance martensite.

Another aspect of the present invention is the manufacturing method of the automotive steel sheet excellent in spot weldability, strength and elongation is by weight, C: 0.04 ~ 0.3%, Si: 0.8 ~ 2.0%, Mn: 1-3%, P: 0.03 % Or less, S: 0.008% or less, N: 0.012 to 0.02%, Al: 0.04 to 2%, Ti: 0.005 to 0.02%, B: 20 ppm or less (excluding 0), Sb: 0.01 to 0.05%, balance Fe and Reheating the slab containing other unavoidable impurities, hot rolling the reheated slab, cooling the hot rolled steel sheet at a cooling rate of 50 to 110 ° C / sec, and cooling the cooled steel sheet to 100 to 300 Winding at C °, cold rolling the wound steel sheet at a reduction ratio of 30% or less, and annealing the cold rolled steel sheet. The manufacturing method may further comprise the step of plating the annealed steel sheet.

According to an aspect of the present invention, it is possible to provide a steel sheet having a strength * elongation of 22000 or more and a ductility ratio of 0.35 or more. When grade grade steel sheet is used to replace some parts, the stability and weight reduction of automobile body can be expected.

At the same time, the inventors of the present invention have conducted studies to derive a method for manufacturing automotive steel sheets having improved strength, elongation, and weldability, and have found that the component system and manufacturing conditions can be appropriately controlled. Specifically, it was recognized that by lowering the content of C to improve weldability and adding N to prevent the decrease in strength, the stability and fractional increase of residual austenite can be increased to increase the strength and elongation. It leads to the present invention by recognizing that it can control the fine martensite structure to secure the strength and ductility, and to control the cold reduction rate low, to prevent the martensite structure from recrystallization during annealing to prevent the strength drop. It became.

EMBODIMENT OF THE INVENTION Hereinafter, the automotive steel plate excellent in the spot welding property, strength, and elongation which are one side of this invention is demonstrated in detail.

Carbon (C): 0.04-0.3 wt%

C is the most important component in high strength steel and closely related to strength and ductility, and affects the residual austenite fraction and stabilization. The higher the C content, the higher the fraction of retained austenite and the better the stability. In order to secure such an effect, the lower limit of the C in the present invention is preferably controlled to 0.04% by weight. However, if the C content is too high, it is easy to manufacture ultra high strength steel with a tensile strength of 980 MPa or more, but the weld strength may be significantly reduced due to the large nonuniformity between the nugget portion and the surrounding tissue during spot welding and the hardness difference of the weld portion. Therefore, it is preferable to control the upper limit to 0.3 weight%.

Si (silicon): 0.8-2.0 wt%

Si plays a role of suppressing carbide formation and securing an amount of solid carbon. In addition, Si facilitates floating separation of inclusions during steelmaking and increases the fluidity of the weld metal during welding. In low carbon TRIP steel, when the Si content is 0.8 wt% or more, carbide formation can be greatly suppressed by Si and the stability of residual austenite can be improved. However, when the content of Si exceeds 2.0% by weight, the hot rolled scale is greatly induced, the wettability is greatly deteriorated, the plating property is degraded, and the weldability is also degraded. Therefore, the content of Si is preferably limited to 0.8 to 2.0% by weight.

Mn (manganese): 1-3 wt%

In the present invention, Mn may serve to increase the hardenability and expand the temperature range in which austenite is formed. In addition, Mn facilitates the formation of low-temperature transformation phases such as acicular ferrite and bainite by increasing hardenability, increases strength, and stabilizes austenite, and is very effective for easily retaining austenite formed during annealing. to be. In the steel of the present invention having a low content of C, Mn is preferably contained by 1% by weight or more in order to secure the strength of the steel and to secure the TRIP characteristics. However, when the Mn content is more than 3% by weight, the weldability is greatly deteriorated, the martensite phase is sharply increased, and cracks are likely to occur at the edge part during cold rolling. In addition, the composition of the slag changes during the steelmaking process to increase the erosion of the refractories and to form manganese oxides in the grain boundaries near the surface layer of the steel ingot in the heating step before hot rolling, thereby causing surface defects after hot rolling. In hot rolling, a segregation zone is formed at the center of the plate material, and hydrogen embrittlement is caused by formation of inclusions. Therefore, the content of Mn is preferably controlled to 1 to 3% by weight.

N (nitrogen): 0.012-0.02 wt%

In general, N is dissolved in the steel and then precipitates to increase the strength of the steel, which is superior to C. The dissolved N component plays a similar role to C when the C content is low to ensure the stability of residual austenite. In order to obtain the above effect, a content of 0.012% by weight or more is required. However, when the content of N exceeds 0.02% by weight, there is a problem that defects occur such as bubbles are generated on the surface by N which is not dissolved. Therefore, the content of N is preferably controlled to 0.012 to 0.02% by weight.

Al (aluminum): 0.04-2 wt%

Al serves to suppress the concentration of Si and Mn on the surface of the steel sheet. In order to secure such an effect, it is preferable to include 0.04% by weight or more. However, the fraction of austenite in which the content exceeds 2% by weight is lowered, so the ductility is relatively lowered and the surface properties are worsened. Therefore, the content of Al is preferably controlled to 0.04 ~ 2% by weight.

Ti (titanium): 0.005 to 0.02 wt%

Ti suppresses the formation of AlN nitride through the bonding of Al to N, thereby forming TiN to allow Al to perform its original function. It is preferable that 0.005% by weight or more is included to secure such effects. However, if the content exceeds 0.02% by weight, the addition effect is no longer expected, and the formation of carbides increases and the content of solid solution carbon decreases. Therefore, the content of Ti is preferably controlled to 0.005 ~ 0.02% by weight.

B (boron): 20 ppm or less (excluding 0)

B can improve hardenability even if small amount is added to steel. However, when the content of B exceeds 20 ppm, the recrystallization temperature is raised to deteriorate the weldability. Therefore, the content of B is preferably controlled to 20ppm or less.

Sb (antimony): 0.01-0.05% by weight

Sb may serve to improve the surface properties of the steel sheet and prevent surface decarburization. In order to secure such an effect, it is preferable that 0.01 wt% or more is included. However, when the content exceeds 0.05% by weight, the surface is thickened and the surface characteristics are rather deteriorated. Therefore, the content of Sb is preferably controlled to 0.01 to 0.05% by weight.

The remainder of the present invention is iron (Fe). However, in the ordinary manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. Since these impurities are known to those skilled in the art, all details except for P and S are not specifically mentioned herein.

P (phosphorus): 0.03 wt% or less

P is an element that is inevitably contained during manufacture, and it is preferable to control it as low as possible, and in principle, it is possible to limit the content of P to 0%, but it is inevitably added in the manufacturing process. It is important to manage the upper limit, and if the content exceeds 0.03% by weight, it combines with Mn to form a non-metallic inclusion, thereby greatly reducing the toughness and strength of the steel. Therefore, the content of P is preferably limited to 0.03% by weight or less.

S: 0.008 wt% or less

Since S is an element that is inevitably contained at the time of manufacture, it is preferable to suppress the content as much as possible. In theory, it is possible to limit the content of S to 0%, but inevitably be added in the manufacturing process. It is important to manage the upper limit, and if the content exceeds 0.008% by weight, it forms a non-metallic inclusion by combining with Mn and the like, and thus, the problem of greatly reducing the toughness and strength of the steel is limited. It is desirable to.

The steel sheet satisfying the above component type and composition range is preferably a cold rolled steel sheet or a galvanized steel sheet. In addition, the tensile strength (MPa) * elongation (%) ≥ 22000 (MPa *%) and the ductility ratio (Cross tensile strength / Tensile shear strength) ≥ 0.35 is satisfied. In general, the higher the ductility ratio, the better the spot weldability test of 780MPa class or higher strength steel when the ductility ratio is less than 0.35, the spot weldability is poor.

In addition, the microstructure of the steel sheet preferably includes 10 to 30% ferrite, 20 to 40% residual austenite and the balance martensite. An important part of the microstructure of the steel sheet, which is one aspect of the present invention, is the residual austenite fraction. The fraction of retained austenite should be about 20 ~ 40% to bring about strength increase and ductility increase in 780MPa grade or more. Ferrite is a tissue that contributes to ductility, and a fraction of about 10 to 30% can give proper ductility. The balance is martensite and can contribute to the improvement of strength as a base material.

The steel sheet of the present invention is intended to obtain excellent mechanical properties through transformation when subjected to deformation by increasing and stabilizing the fraction of initially produced lath-shaped fine martensite and residual austenite of the second phase. Here, in order to obtain excellent strength and elongation, the characteristics of matrix structure, residual austenite stability, and fraction are the most influential factors in determining the properties.

Hereinafter, the manufacturing method of the automotive steel plate excellent in the spot welding property, strength, and elongation which is another aspect of this invention is demonstrated in detail. By increasing and stabilizing the fraction of the initially produced lath-shaped fine martensite and residual austenite in the second phase, it is desired to obtain excellent mechanical properties through transformation when subjected to deformation.

First, the slab which satisfies the above-described component system is reheated and hot rolled. The reheating step is not particularly limited in terms of conditions, and is subject to usual conditions. In addition, the conditions at the time of hot rolling are not specifically limited, either, It is preferable to carry out in the austenite region of A3 or more.

Cool the hot rolled steel sheet. At this time, the cooling rate is preferably controlled to 50 ~ 110 ℃ / sec. If the cooling rate is less than 50 ℃ / sec is slow cooling rate is difficult to secure the microstructure intended by the present invention. However, when the cooling rate exceeds 110 ° C / sec, the cooling rate is too fast to cause shrinkage or voids in the steel sheet, or may cause a problem of material warping.

The cooled steel sheet is wound up. At this time, the winding temperature is preferably controlled to 100 ~ 300 ℃. When winding is carried out at a general winding temperature of 500 ~ 600 ℃, to form a ferrite structure which is a normal structure, the strength of the steel sheet can be reduced. However, as controlled by the present invention, winding at 100 to 300 ° C. secures a fine martensite structure in a lath shape, and thus an increase in strength and ductility after hot rolling is much higher than that of ordinary steel. Done.

The wound steel sheet is cold rolled. At this time, it is preferable to control the cold reduction rate to 30% or less. In the future annealing of the fine martensite structure, which is an initial matrix structure obtained after the odor treatment process, the amount of deformation is generated so that no recrystallization occurs. If the cold reduction exceeds 30%, excessive strain may be given to recrystallize quickly upon annealing, and the initial martensite matrix may be recrystallized to cause a decrease in strength.

The cold rolled steel sheet is annealed. At this time, the annealing conditions are not particularly limited. However, after maintaining and cooling at 700 ~ 850 ℃ it is preferable to anneal at 300 ~ 450 ℃. It is preferable to anneal the cold rolled steel sheet in the temperature range of 700 ~ 850 ℃ fast C and Mn diffusion rate is necessary because the fast reaction of the distribution of C and Mn, at this temperature. Austenite is easily formed during annealing. If the annealing temperature is less than 700 ℃, it is difficult to secure the amount of carbon required to stabilize the austenite for strength and ductility increase, and the austenite transformation fraction is too small to secure sufficient TRIP characteristics. On the other hand, when the temperature exceeds 850 ° C., diffusion of Si is promoted to prevent the precipitation of carbides, thereby making it difficult to secure the stability of austenite. Therefore, the annealing temperature is preferably limited to 700 ~ 850 ℃. The annealing time is the time necessary to obtain the equilibrium at the annealing temperature, and if it is maintained for more than 50 seconds, the austenite can reach the equilibrium sufficiently in the temperature range. However, the time that can be applied to continuous annealing is preferably 50 seconds to 300 seconds. After the annealing step is cooled to 300 ~ 450 ℃ at a cooling rate of 10 ~ 200 ℃ / sec and maintained for 60 seconds or more. The final cooling can then be produced to produce a cold rolled steel sheet.

In addition, it is also possible to provide a galvanized steel sheet by performing zinc plating after the annealing step.

Through the above manufacturing method, the spot weldability, strength and ductility satisfying the tensile strength (MPa) * elongation (%) ≥ 22000 (MPa *%) and the cross tensile strength / Tensile shear strength ≥ 0.35 All of them can lead to an excellent steel sheet.

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)

To prepare a slab that satisfies the component system as shown in Table 1, after reheating and hot rolling, cooled, wound at a temperature shown in Table 2, cold-rolled at a reduction ratio shown in Table 2, followed by annealing Inventive Examples 1 to 4 and Comparative Examples 1 to 10 were prepared.

Tensile strength and elongation of Inventive Examples 1 to 4 and Comparative Examples 1 to 10 were measured and shown in Table 2 below. In addition, the value and ductility ratio of the product of tensile strength and elongation are shown together in Table 2 below. In this case, the ductility ratio is (CTS / TSS = CROSS TENSILE STRENGTH / TENSILE SHEAR STRENGTH), indicating that the higher the ductility ratio, the better the spot weldability.

Steel grade C Si Al Mn P S Ti N B Sb A 0.15 1.49 One 1.6 0.015 0.002 0.015 0.013 0.0007 0.02 B 0.19 1.47 0.08 1.8 0.016 0.003 0.018 0.02 0.0009 0.03 C 0.07 1.50 1.51 2.7 0.014 0.002 0.020 0.015 0.0008 0.02 D 0.18 1.51 0.8 1.7 0.018 0.002 0.012 0.016 0.0005 0.02 E 0.5 1.50 0.04 1.7 0.016 0.002 0.006 0.005 0.0007 0.01 F 0.3 0.1 1.1 1.5 0.015 0.002 0.002 0.004 0.0009 0.005 G 0.25 0.52 1.03 1.8 0.014 0.003 0.0015 0.007 0.0007 0.002

(However, the unit of each element is weight%.)

As shown in Table 1, steel grades A, B, and C are steel grades that satisfy all of the component ranges controlled by the present invention. However, steel grades D, E, F and G are steel grades outside the component range controlled by the present invention.

division Steel grade Coiling temperature
(℃) Cold rolled
Rolling reduction (%) The tensile strength
(MPa) Elongation
(%) Tensile Strength * Elongation
(MPa *%) Ductility ratio Comparative Example 1 A 620 25 830 22.6 18758 0.41 Inventory 1 A 240 28 987 28.5 28129.5 0.35 Comparative Example 2 B 615 30 865 22.9 19808.5 0.45 Inventive Example 2 B 198 29 992 28.2 27974.4 0.43 Comparative Example 3 C 590 25 923 22.9 21136.7 0.37 Inventory 3 C 287 30 998 28.8 28742.4 0.48 Comparative Example 4 D 550 28 845 22.1 18674.5 0.42 Honorable 4 D 148 30 988 27.1 26774.8 0.45 Comparative Example 5 E 600 50 831 22 18282 0.34 Comparative Example 6 E 300 50 899 24.6 22115.4 0.31 Comparative Example 7 F 620 30 670 32.1 21507 0.32 Comparative Example 8 F 200 30 721 30.7 221347 0.32 Comparative Example 9 G 600 60 754 25.9 19528.6 0.29 Comparative Example 10 G 150 55 780 23.7 18486 0.34

As shown in Table 2, Inventive Examples 1 to 4 are examples of satisfying both the component system and the manufacturing conditions of the present invention. After the microstructure is made into a microscopic martensite in a res shape through low-temperature winding, the stability of residual austenite By ensuring that the re-crystallization of the initial martensite matrix structure does not occur even after cold rolling and annealing, the tensile strength * elongation value of the present invention all intended to exceed 22000 MPa *%, since the ductility ratio is 0.35 or more, It was confirmed that the steel sheet is excellent in weldability, strength and ductility.

On the other hand, Comparative Examples 1 to 4 satisfy the component system controlled by the present invention, but are wound at a high temperature in manufacturing conditions, and thus do not secure the microstructure intended by the present invention, and thus the tensile strength * elongation values are all 22000 MPa *. It was confirmed that it was less than%. Comparative Example 5 did not satisfy the component system of the present invention, and was controlled by high temperature winding and high cold reduction rate, so that both the tensile strength * elongation value was less than 22000 MPa *%. Comparative Example 6 did not satisfy the component system of the present invention, but was wound at a low temperature, but a low ductility ratio was ensured due to the high reduction ratio. Comparative Example 7 did not satisfy the component system of the present invention and was wound up at high temperature, thereby ensuring a low ductility ratio. In Comparative Example 8, although the manufacturing conditions satisfied the present invention, the ductility ratio was ensured low because it did not satisfy the component system of the present invention. Comparative Example 9 did not satisfy the component system of the present invention, and the tensile strength * elongation value and ductility ratio were low due to high temperature winding and high rolling reduction. Comparative Example 10 did not satisfy the component system of the present invention, and due to the high reduction ratio, the ductility ratio was ensured low.

Tensile properties among the mechanical properties of the steel sheet depend on the component and microstructure fabrication conditions. Through the above examples, such contents can be confirmed, and through this, in order to obtain the strength and ductility targeted in the present invention, the appropriate component system of the present invention must be satisfied, and control at low temperature winding and low rolling rate is required. It was confirmed that the mechanical properties can be satisfied.

Claims (6)

By weight%, C: 0.04 to 0.3%, Si: 0.8 to 2.0%, Mn: 1 to 3%, P: 0.03% or less, S: 0.008% or less, N: 0.012 to 0.02%, Al: 0.04 to 2% , Ti: 0.005 to 0.02%, B: 20 ppm or less (excluding 0), Sb: 0.01 to 0.05%, balance Fe and other unavoidable impurities,
Automotive steel sheet having excellent spot weldability, strength, and elongation, which satisfies tensile strength (MPa) * elongation (%) ≥ 22000 (MPa *%) and cross tensile strength / Tensile shear strength ≥ 0.35.
The steel sheet of claim 1, wherein the microstructure of the steel sheet is excellent in spot weldability, strength, and elongation, including ferrite 10-30%, residual austenite 20-40%, and residual martensite.
The steel sheet for automobiles according to claim 1 or 2, wherein the steel sheet is a cold rolled steel sheet or a galvanized steel sheet.
By weight%, C: 0.04 to 0.3%, Si: 0.8 to 2.0%, Mn: 1 to 3%, P: 0.03% or less, S: 0.008% or less, N: 0.012 to 0.02%, Al: 0.04 to 2% Reheating the slab comprising Ti: 0.005% to 0.02%, B: 20 ppm or less (excluding 0), Sb: 0.01% to 0.05%, balance Fe and other unavoidable impurities;
Hot rolling the reheated slab;
Cooling the hot rolled steel sheet at a cooling rate of 50 to 110 ° C./sec;
Winding the cooled steel sheet at 100 to 300 ° C;
Cold rolling the wound steel sheet at a rolling reduction of 30% or less; And
The method of manufacturing a steel sheet for automobiles excellent in spot weldability, strength and elongation comprising annealing the cold rolled steel sheet.
The method of claim 4, wherein the annealing is performed at 700 to 850 ° C. for 50 to 300 seconds, and then cooled to 300 to 450 ° C. for 60 seconds or more. 5. .
The method of claim 4, wherein the manufacturing method further comprises plating the annealed steel sheet.