KR20080061855A - Dual phase steel having superior deep drawing, and method for manufacturing of it - Google Patents

Abstract

Composite steel having a superior deep drawing characteristic is provided to realize a high r value while maintaining high strength of the composite steel by improving the deep drawing property. Composite steel having a superior deep drawing function comprises 0.01 to 0.03 weight percent of C, 0.3 weight percent or less of Si, 1.0 to 2.0 weight percent of Mn, 0.01 to 0.06 weight percent of P, 0.015 weight percent or less of S, 0.2 to 0.8 weight percent of soluble Al, 0.003 weight percent or less of N, 0.2 to 1.0 weight percent of Mo, 0.5 weight percent or less of Cr, 0.0005 to 0.0015 weight percent of B, Fe and inevitable impurities. The composite steel has a dual phase structure of martensite and ferrite.

Description

 Dual phase steel having superior deep drawing, and method for manufacturing of it}

1 is a graph showing the influence of Mo on the material in 0.015% carbonized steel.

2 is a graph showing the effect of Mo on the material in 0.03% carbonized steel.

3 is a schematic diagram illustrating the mechanism by which Mo addition causes an r value improvement.

4 is a graph showing the influence of the transformation fraction of the second phase on the r value.

5 is a graph showing the effect of the metamorphic tissue size of the second phase on the r value.

6 is a microstructure photograph of a hot rolled sheet and an annealed plate by Al addition.

7 is a microstructure photograph of a hot rolled sheet and an annealed plate according to the hot rolled coiling temperature on the r value.

Japanese Laid-Open Patent Publication 56-139654

Japanese Patent Application Publication No. 55-10650

Japanese Patent Application Publication No. 1-35900

Japanese Laid-Open Patent Publication 2003-64444

The present invention relates to a composite tissue steel sheet and a method of manufacturing the same used for interior and exterior plate materials of automobiles. More specifically, the present invention relates to a composite structured steel sheet capable of appropriately maintaining martensite while minimizing degradation of {111} aggregate structure by Mo and Al in low carbon steel and a method of manufacturing the same.

Recently, the demand for fuel economy improvement of automobiles is gradually increasing to regulate CO ₂ emissions in terms of conservation of the global environment. In addition, in order to secure the safety of the passengers in the event of a collision, the improvement of safety centering on the collision characteristics of the vehicle body is also required. As described above, technology development is being progressed in two directions: hardening and reinforcing automobile bodies.

In order to satisfy both the weight reduction and the reinforcement of the automobile body at the same time, it is most desirable to reduce the rigidity of the parts and increase the weight of the parts by reducing the thickness of the parts. Since the weight reduction effect increases as the tensile strength of the steel sheet used increases, efforts are being made to employ high strength steel sheets in the automobile industry, and recently, it has been applied to panel panels up to 490Mpa grade composite steel.

On the other hand, since most of the automotive parts made of steel sheet are formed by press working, excellent press formability is demanded. As a method for improving the press formability, although there are some differences depending on the parts, it is usually necessary to improve the stretchability and the deep drawing property. As a method of improving the stretchability through high strength, there is a method of complex organization of the steel sheet. Composite steel is composed of two phases of soft ferrite and hard martensite in steel, so it has low yield strength and high work hardening rate. However, high strength steel sheet is inferior to the general soft steel sheet as the strength is increased, especially deep drawing property is inferior to the moldability deterioration in order to prevent the deterioration of the deep drawing property r value is required to secure a certain level or more.

In general, a method of increasing strength while having a high r value includes adding carbonitride-forming elements such as Ti and Nb to ultra-low carbon steel, and then adding solid solution strengthening elements such as Si, Mn, and P. However, the ultra-low carbon high strength steel sheet has a high yield strength, which may simultaneously increase the press formability, and may cause a problem of deterioration of plating properties due to elemental addition of Mn, P, and Si. Therefore, in order to solve these problems, efforts have recently been made to maintain r-value while maintaining the excellent advantages of composite steel.

As described above, as a method for improving r value in high strength steel sheet, carbon dioxide forming elements such as Ti and Nb are added to ultra low carbon steel to completely remove solid solution elements in the steel, and then solid solution strengthening elements such as Si, Mn, and P are removed. Add.

In the case of JP-A-56-139654, tensile strength having a composition of C 0.002-0.015%, Nb: C% x3 ~ C% x8 + 0.02%, Si 1.2% or less, Mn 0.04 ~ 0.8%, P 0.03 ~ 0.1% It describes a method for producing an average r value of 1.7 using a 340 ~ 440Mpa grade unaging high strength cold rolled steel sheet. However, as described above, in the technique of adding a solid solution element based on ultra low carbon steel, when the tensile strength of 440 MPa or more is attempted, the amount of alloying element is increased. There are several problems. In addition, if a large amount of solid solution is added, there is a possibility that the r value will fall. In addition, in order to reduce the carbon content to an ultra-low carbon region of less than 0.01%, vacuum degassing is required in steelmaking, which may cause various problems such as a large amount of CO ₂ gas in the manufacturing process and an increase in manufacturing cost.

In order to solve this problem, a high strength steel sheet proposed as an alternative is a composite high-strength steel sheet. However, the composite tissue steel has a problem that the r value is degraded by the hard martensite as described above. Methods to improve r value in composite steel have been suggested since 1980. Japanese Patent Publication No. S55-10650 discloses a method in which low-carbon steels are annealed at a recrystallization temperature to an AC3 transformation point after cold rolling, and then tempered after heating at 700 to 800 ° C. for the production of composite tissue steel. However, this method has a problem in that the manufacturing cost is increased by performing annealing twice after continuous annealing. In addition, in the technique of JP-A-55-100934, in order to obtain a high r value, annealing is performed after cold rolling, and the temperature at this time is made into a two-phase region of ferrite-austenite, followed by continuous annealing. In this technique, Mn is concentrated from a ferrite phase to austenite by an annealing step, and then a Mn thickened phase is preferentially changed into an austenite phase by subsequent annealing, thereby obtaining a composite structure in the cooling process, which is the next step. . However, this method requires a long time annealing operation at a relatively high temperature for the concentration of Mn in the annealing process, and also because of the large number of processes, it is not only economical in terms of manufacturing cost, but also adhesion between steel sheets, generation of temper color and furnace body Problems such as deterioration of the life of the inner cover may occur.

As a technique recently developed to improve r value in composite steel, Japanese Unexamined Patent Publication No. Hei 1-35900 proposes a technique for improving r value by promoting proper carbon content and V content. In other words, before recrystallization annealing, carbon in the steel is precipitated as V-based carbide to reduce the solid solution carbon as much as possible to achieve a high r value. After that, the V-based carbide is dissolved again by heating in the two-phase region of ferrite-austenite. The carbon content inside is increased to secure the martensite phase through cooling. However, V is very expensive, which leads to a significant increase in manufacturing cost. In addition, VC generated in the hot rolled sheet increases the deformation resistance during cold rolling, which is likely to cause trouble of the device. In addition, Japanese Patent Laid-Open No. 2003-64444 obtains a high-strength steel sheet containing predetermined carbon and having an average r value of 1.3 or more, and having at least 3% of bainite, martensite, and austenite in the structure of 3% or more in total. The production method is to develop a texture by forming a cold rolling rate of 30-95% followed by forming clusters or precipitates of Al and N. However, this method requires annealing to obtain a good r value after cold rolling and a heat treatment to form a structure, respectively, which leads to deterioration in productivity, and a relatively high two-phase fraction of the resulting structure to ensure a good strength-ductility balance. It is difficult.

The present invention is to solve the above problems, to provide a composite structure steel of martensite and ferrite that can implement a high r value while maintaining a high strength.

Composite tissue steel of the present invention for achieving the above object, by weight% C: 0.01-0.03%, Si: 0.3% or less, Mn: 1.0-2.0%, P: 0.01-0.06%, S: 0.015% or less, Soluble Al: 0.2-0.8%, N: 0.0030% or less, Mo: 0.2-1.0%, Cr: 0.5% or less and B: 0.0005-0.0015%, and are composed of the remaining Fe and other unavoidable impurities It is a two-phase organization of sites and ferrites.

In the present invention, as a composite tissue steel sheet capable of appropriately maintaining martensite while minimizing deterioration of {111} aggregate structure, r value of desired conditions can be obtained by appropriately controlling the fraction of martensite and / or the size of martensite. . According to one embodiment of the present invention, in terms of securing a more stable high r value, the martensite may have a size of 2.2 µm or more and a fraction of 2-5%, and the present invention is not limited thereto.

In the present invention, Mo affects the fraction of {111} aggregate structure and martensite by precipitation and re-dissolution of carbides during annealing, and Mo carbides also exist in the steel sheet after annealing.

The composite structured steel sheet of the present invention may be applied to various types of steel sheets, and the most preferable steel sheet is a cold rolled steel sheet or a plated steel sheet having a zinc-based molten zinc layer formed on at least one surface thereof.

According to one embodiment of the present invention, the composite steel sheet has a tensile strength of 440 ~ 490MPa grade while the elongation is more than 32% and the r value is to implement 1.4 or more.

According to the present invention, by weight% C: 0.01-0.03%, Si: 0.3% or less, Mn: 1.0-2.0%, P: 0.01-0.06%, S: 0.015% or less, Soluble Al: 0.2-0.8 Steel slab containing%, N: 0.0030% or less, Mo: 0.2 ~ 1.0%, Cr: 0.5% or less and B: 0.0005-0.0015%, and composed of the remaining Fe and other unavoidable impurities, after homogenizing heat treatment at 1200 ℃ or higher Hot-rolled at 900-950 ℃ finish rolling temperature and wound at 700-750 ℃ to obtain hot rolled sheet, cold rolled at 70-80% cold rolling rate, in the temperature range of 800-850 ℃ Continuous annealing and temper rolling below 1.0%. In the present invention, the hot rolled sheet includes Mo carbide.

Hereinafter, the present invention will be described in detail.

In general, in order to improve the r value in cold rolled steel sheets, a method of developing a {111} recrystallized texture structure by reducing the solid solution carbon or nitrogen remaining before recrystallization annealing after cold rolling or by miniaturizing the hot rolled sheet structure has been used. However, in the composite steel sheet, since a certain amount of solid carbon is required to form martensite, the {111} aggregate structure is degraded by such carbon, resulting in low r value.

Accordingly, the present inventors have completed the present invention by focusing on the fact that r value can be increased in the composite tissue steel if the amount of martensite is properly maintained while minimizing the degradation of the {111} recrystallized texture of the parent ferrite. That is, in the present invention, the carbon content is slightly lower than that of the conventional composite steel sheet, that is, the amount of martensite is appropriately controlled by appropriately controlling the Mn and Mo content in the range of about 0.01 to 0.03% by weight (hereinafter simply referred to as%). At the same time, it minimizes the deterioration of the {111} recrystallized texture.

1 and 2 show the change in tensile strength, elongation and r value while simultaneously changing the Mn content and Mo content in the carbon content of 0.015% and 0.03%. As shown in the figure, both tensile strength and r value increase and elongation decrease with increasing Mo content of carbon content 0.015% and 0.03%. In addition, as the Mn content increased, the strength increased but elongation and r value gradually decreased. These results indicate that Mo not only increases the strength but also increases the r value.

The present inventors have conducted various experiments to confirm the influence of Mo on the r-value, and have come to the following conclusions. That is, the present inventors observed the precipitates in each step of the heat treatment, that is, the temperature rising-cracking-slow cooling-quenching, etc. in each step of the continuous annealing process performed after cold rolling in order to investigate the influence of Mo, and present in the hot-rolled sheet state. Mo-based carbides were found to be re-dissolved at the elevated temperature of the heat treatment. The remelting temperature of the Mo-based carbides shown by these experiments was found to be about the arbitrary temperature at which recrystallization of the ferrite is progressing. This phenomenon means that the Mo-based carbide can prevent the degradation of the recrystallized texture structure due to carbon to some extent by fixing the solid solution carbon at the initial stage of recrystallization of the ferrite. In other words, Mo-based precipitates, which existed in the hot-rolled sheet state, exist in the early stage of annealing recrystallization in which {111} aggregates are formed, and thus, {111} aggregates are developed because they partially fix the solid solution carbon in the steel. By re-dissolving to secure sufficient amount of solid carbon required to form martensite, it is obtained in composite steel in the final process.

FIG. 3 illustrates the above-described phenomenon as a mechanism in which Mo affects r value improvement and thermodynamic behavior of Mo-based carbides in the inventive steel. As a result of thermodynamic analysis of Mo-based precipitates to confirm the behavior of Mo-based carbides as shown in the experiment, the precipitation temperature of Mo-based carbides is about 680 ° C and is higher than the initial temperature at which recrystallized texture develops. It can be seen that it is possible to improve the teeth. However, Cr is a carbide-like element similar to Mo, but as shown in the figure, the precipitation temperature is less than 500 ℃, so it is expected that it will be dissolved before the recrystallized texture develops and will not affect the development of {111} texture. In fact, the present inventors have found a slight difference depending on the content, but the addition of Cr was found to have little effect on the improvement of the r value. However, as shown in Fig. 1 and Fig. 2, it can be seen that the change in the r value according to the increase in Mo amount is linked with Mn. In general, Mn is known as a hardenability improving element in the metamorphic steel, and the martensite content increases as the Mn content increases. Therefore, even though the r value of the composite steel is increased by adding Mo in the present invention steel, if the Mn content is excessively increased, the martensite content is increased in the steel, and the r value is decreased. In addition, it can be seen that the distribution control of the metamorphic martensite is very important. In consideration of this aspect, in the present invention, the range of addition of Mn and Mo is controlled.

4 and 5 show how the distribution of martensite affects the r value of the composite steel. 4 and 5 show that the present inventors have changed various components and operating conditions, and compared the martensite produced at this time with the r value as a function of fraction and size. As shown in these two results, it can be seen that as the martensite fraction increases, the mar value decreases as the martensite size decreases. Therefore, it is preferable to appropriately select the fraction and size of martensite in consideration of the strength characteristics along with the r value. From the above results, it can be seen that it is most preferable to set the martensite fraction to 2 to 5% and the martensite size to at least 2.2 µm if the target value of r is 1.4 or more.

On the other hand, by properly controlling the Mo and Mn content in Figures 1 and 2 it is possible to improve the r value in the composite steel. In addition, if the martensite fraction and its size are controlled in FIG. 4, a higher r value can be obtained more stably. Furthermore, the present inventors considered addition of another element in addition to controlling the content of Mo and Mn in order to more stably secure a high r value. As a result of examining the material properties using various elements, the inventors found that Al significantly improved the r value. In particular, it was found that Al significantly increased the martensite size while controlling the fraction of martensite to 2-5%, for example. Figure 6 is the result of observing the behavior of Al in terms of microstructure. That is, when the Al content was added at 0.04%, which is a normal level, very coarse pearlite was present in the hot rolled sheet, but when the Al content was increased to 0.4% included in the range of the present invention, the hot rolled pearlite was added. Markedly reduced. Al generally acts to increase the mobility of carbon. Therefore, when the Al content is increased to 0.2% or more as in the present invention, the hot rolled pearlite is dispersed by the carbon mobility. This dispersion effect of pearlite is effective in preventing the recrystallized texture deterioration upon annealing the composite tissue steel after cold rolling. On the other hand, Al addition does not interfere with annealing recrystallized grain growth due to the dispersion effect of carbon as shown in FIG. 6, resulting in coarsening of the annealing ferrite grains, thereby increasing the martensite size by decreasing the grain size. . Therefore, Al addition leads to coarsening of ferrite and coarsening of martensite particles by the dispersion effect of hot rolled pearlite, resulting in a significant improvement in r value. According to the research of the present inventors, when adding 0.2% or more of Al, the r value is increased by at least 0.1 and the r value is confirmed to be improved up to 0.2. However, the addition of Al above 0.8% could not be expected to further increase the r value, and also caused problems such as increased oxide in the steelmaking operation due to excessive Al addition and a decrease in alloying degree in the manufacture of hot-dip galvanized steel sheets. .

Another factor considered to prevent deterioration of the r value in the inventive steel is the coiling temperature of hot rolled steel. That is, according to the research of the present inventors, it was found that the structure of the hot rolled sheet has a great influence on the r value after annealing. That is, in general, in the case of composite steel, the hot rolled sheet is a two-phase structure of ferrite and ferrite or is composed of metamorphic ferrite or metamorphic ferrite and pearlite. However, according to the research of the present inventors, when the hot rolled structure is composed of ferrite and perlite, there is a problem that the r value decreases due to the residual of perlite during annealing. However, this problem was solved through the addition of Al as described above. However, even when the hot rolled structure was composed of pure metamorphic ferrite without pearlite, it was found that the r value was deteriorated even after annealing. This is because the strain stress field due to the carbon segregation adversely affects the development of the {111} aggregate structure in the annealing recrystallization together with the non-uniform carbon segregation present in the metamorphic tissue. Therefore, it is very important to prevent metamorphic ferrite in the hot-rolled tissue in order to secure a certain level of r value in the composite tissue steel required by the present invention. A method of preventing the formation of metamorphic ferrite is a method of controlling the hot rolling temperature. Figure 7 shows the change in the hot-rolled structure according to the change of the hot-rolled coiling temperature in Mo-added steel, the transformation ferrite produced at a low temperature of 650 ℃ was extinguished at the winding temperature of 700 ℃, rather the ferrite structure of non-uniform grain size at 750 ℃ It turned out that it was obtained. The r value was very low as 1.0 at low temperature, but increased as the coiling temperature increased, and the coiling temperature of 750 ℃ was almost similar to that of 700 ℃. From these results, it can be seen that the winding temperature controlled to improve the r value in the composite tissue steel also has a certain range, and this condition was 700 to 750 ° C in the present invention steel.

Hereinafter, a detailed description of the present invention steel.

The content of carbon (C) is preferably 0.01-0.03%.

Carbon is one of the very important elements in the present invention steel. Carbon promotes high strength and promotes the formation of martensite in composite steel. As the carbon content increases, the martensite content in the steel increases. Therefore, in order to ensure the r value while containing the appropriate amount of martensite required by the inventive steel, it is necessary to appropriately control the content of carbon. If the amount of carbon added is less than 0.01%, it is difficult to form a martensite phase for forming a composite steel. However, in order to more stably secure a certain amount of martensite, it is preferable to control the amount of carbon added to 0.015% or more. However, when the amount of carbon exceeds 0.03%, excessive martensite amount is formed in the steel grade, so it is easy to secure the composite structure steel, but the tensile strength required by the present invention exceeds 440 to 490 Mpa, and due to the excessive solid carbon and martensite in the steel Drawability is degraded.

The content of silicon (Si) is preferably 0.3% or less.

Silicon promotes ferrite transformation and raises the carbon content in untransformed austenite, making it easy to form a composite structure of ferrite and martensite, and inducing a solid-solution strengthening effect of Si itself. However, when the Si content is increased, the wettability of the plating is deteriorated, and the wettability of the plating is deteriorated, and the plating quality is degraded. Therefore, SI is preferably managed as low as possible when producing the molten plating material. Therefore, the content of Si may not necessarily be less than 0.3% if the plating characteristics are not taken into consideration or if a method of solving the deterioration of plating properties due to the addition of Si is adopted. It is preferable to limit the addition amount of Si to 0.3% or less if the element is managed in the minimum possible amount to reduce the hot-plating property.

The content of manganese (Mn) is preferably 1.0-2.0%.

Manganese is an element that refines particles without damaging ductility, precipitates sulfur in steel completely with MnS, prevents hot brittleness due to the formation of FeS, and strengthens the steel, while at the same time lowers the critical cooling rate at which the martensite phase is obtained in composite steel. It serves to form martensite more easily. In order to play such a role, Mn is preferably contained at least 1.0% or more. However, when the amount of Mn added exceeds 2%, the formability deteriorates and the moldability deteriorates. In particular, during the annealing process during the production of hot-dip galvanized steel, a large amount of oxides such as MnO are formed on the surface, thereby deteriorating plating adhesion and streaking. A large amount of plating defects such as the like may occur, resulting in deterioration of product quality.

The content of phosphorus (P) is preferably 0.01-0.06%.

Phosphorus is the substitution type alloy element with the largest solid solution strengthening effect. It plays a role in improving in-plane anisotropy and strength. For this purpose, it is preferable that it is 0.01% or more of P addition amount. However, when the addition amount of P exceeds 0.06%, P increases with strength and segregates at grain boundaries, degrading secondary work brittleness and weldability. In addition, in the case of the hot-dip plating material, the alloying degree is lowered by suppressing the diffusion of Fe from the steel plate to the plating layer at the interface with the plating layer during the alloying treatment after the hot dip plating, so that the addition amount is preferably limited to 0.01-0.06%.

The content of sulfur (S) is preferably 0.015% or less.

Sulfur is an element that should be precipitated as sulfide of MnS at high temperature to prevent hot brittleness by FeS. However, when the content of S is excessive, there is a possibility that S remaining after precipitation into MnS embrittles grain boundaries and causes hot brittleness. In addition, even if the amount of S added to completely precipitate the MnS precipitate, when the S content is large, material degradation due to excessive precipitates occurs, so it is preferable to limit the added amount to 0.015% or less.

The content of aluminum (Al) is preferably 0.2-0.8%.

Aluminum is usually added for deoxidation of steel and serves to prevent the deterioration of aging resistance by nitrogen by fixing solid solution nitrogen in steel. In addition, Al is a ferrite forming element, and serves to control the fraction of the biphase region of ferrite-austenite during annealing. In the present invention, however, Al disperses the pearlite in the hot rolling step to minimize deterioration of the recrystallized texture due to the non-uniform distribution of carbon during annealing. In addition, the annealing grains are coarsened by the addition of Al, and the martensite is increased at the same fraction, thereby playing a very useful role in increasing the r value. In order to exhibit this effect, it is desirable to add Al content of at least 0.2%. However, when Al exceeds 0.8%, the structure becomes fine due to excessive addition of Al, and the surface quality decreases due to deterioration of formability and increase of oxidation inclusions in steelmaking. In addition, it is preferable to limit the addition amount to 0.2-0.8% because it will cause an increase in manufacturing cost due to excessive Al addition.

The content of nitrogen (N) is preferably 0.003% or less.

Nitrogen deteriorates the formability of the steel by being in solid solution before or after annealing, and is required to be fixed by Ti or Al because the aging deterioration is much larger than other invasive elements. In general, nitrogen has a very fast diffusion rate compared to carbon, and therefore, when present as solid nitrogen, the deterioration of room temperature aging resistance is very serious compared to that of solid carbon. In addition, since the yield strength increases and elongation and r value deteriorate due to the remaining of solid solution nitrogen, it is preferable to limit the content to 0.0030% or less as in the present invention.

The content of molybdenum (Mo) is preferably 0.2-1.0%.

Molybdenum is one of the very important elements considered in the present invention steel. Mo is dissolved in steel to improve strength or to form Mo-based carbides. In addition, like Mn, it reduces the critical cooling rate at which the martensite phase is obtained, thereby easily serving the martensite phase upon cooling after annealing. The present invention uses the carbide forming role of Mo together with the role of manufacturing the composite steel of Mo. That is, the added carbon is formed into Mo-based carbide in the hot rolling step to reduce the amount of solid solution carbon in the steel and re-dissolve in the crack maintenance step after the formation and development of the [111] texture by recrystallization in the annealing process, thereby improving the hardenability. By doing so, we want to secure complex texture and drawability at the same time. In order to achieve such a role of Mo, the amount thereof is preferably at least 0.2% or more. However, if Mo addition amount exceeds 1.0%, Mo-based carbide increases, but due to excessive increase in hardenability, a large amount of martensite is produced in the steel, so that the effect of hardenability is greater than the synergistic effect of r-value. Occurs. In addition, Mo is added to the element is very expensive, so it is better to manage as low as possible to minimize the rapid rise in manufacturing costs. Therefore, the present invention is to present a range of 0.2-1.0% of the components satisfying these conditions.

The content of chromium (Cr) is preferably 0.5% or less.

Chromium also plays a role similar to Mo as a carbide-forming element as well as a hardening element such as Mo. However, since the temperature at which the carbides dissolve is very low, Cr-based carbides are formed in the hot rolling phase to reduce the dissolved carbon in the steel, but Cr is dissolved before recrystallization starts during annealing. I can't give it. However, Cr is very useful for the production of martensite, and in particular, since it is inexpensive compared to Mo, it is preferable to use Cr appropriately for making composite tissue steel. In the present invention, the steel is controlled to 0.5% or less as an additive amount for planning such a role of Cr. If the Cr content exceeds 0.5%, the martensite content in the steel is significantly increased, leading to an increase in emphasis and material degradation.

The content of boron (B) is preferably 0.005-0.0015%.

Boron exists as an invasive element in steel and is dissolved in grain boundaries or combines with nitrogen to form nitrides of BN. B also plays an effective role in forming martensite like Mn. B is an element having a large influence of the material relative to the amount added, and it is preferable to strictly limit the amount added. That is, when a certain amount of B is added to the steel, the metamorphic tissue develops from the hot rolling stage, which causes deterioration of the r value after annealing. In the present invention, the steel is considered to be 0.0005-0.0015% in consideration of such characteristics and the ability of steelmaking to be added to current B.

Alloy elements may be added to the steel formed as described above as needed. However, it is necessary to manage the addition amount of alloying elements such as Ti and Nb that may affect the above-described action of Mo. Additions can be considered within a range that is harmless to the action of Mo.

The steel of the present invention is a composite steel structure composed of two phases, martensite and ferrite. Therefore, the strength and r value of the steel are affected by the fraction of martensite and ferrite. That is, as described above with reference to FIGS. 4 and 5, as the martensite fraction decreases, the r value gradually increases as the martensite size increases. Therefore, if the fraction and size of martensite are determined in consideration of the r value, appropriate values can be selected in FIGS. 4 and 5. In the case of the fraction of martensite, 1-11% is preferably 2-8%, more preferably 2-5%. The size of martensite is 1 µm or more, preferably 2.2 µm or more.

The type of steel sheet is not particularly limited as long as it has the above-described component system and has the fraction and size of martensite in consideration of high strength and high r value. Hot rolled steel sheets, cold rolled steel sheets and plated steel sheets are available, and steel pipes can be used if necessary. According to one embodiment of the present invention, the plated steel sheet. As the plating layer, hot dip zinc plating is preferable.

According to an embodiment of the present invention, in the present invention by using the Mo-based carbide in addition to the reinforcing effect by Mo using Mo, while ensuring the resistance ratio and high work hardenability, which is an advantage of the composite structure steel Al at least 0.2% By adding it, it is possible to provide a high strength cold rolled steel sheet having a tensile strength of 440 to 490 MPa class structure with excellent elongation and an average r value of 1.4 or more due to the dispersion effect of the hot rolled pearlite and the coarsening of the annealing grains.

Hereinafter, the manufacturing method of the above-mentioned steel according to this invention is demonstrated. This manufacturing method corresponds to the manufacturing method of cold rolled steel sheet and hot dip galvanized steel sheet. In this manufacturing method, the hot rolled winding temperature is controlled relatively high in the range of 700-730 ° C. so that the hot rolled grain is controlled as a ferrite single phase rather than a metamorphic structure, and the addition of Al facilitates the dispersion effect of the hot rolled pearlite and uses Mo. It is a manufacturing method that can promote the development of recrystallized texture when annealing and heating.

With the above composition, the steel slab is heated at 1200 ° C. or more, where the austenite structure before hot rolling can be sufficiently homogenized to finish hot rolling in the temperature range of 900-950 ° C., which is directly above the Ar ₃ temperature.

If the slab temperature is less than 1200 ℃, the steel structure does not become uniform austenite grains, and the mixture is generated, causing deterioration of the material. In addition, when the hot finishing temperature is less than 900 ° C, the top, tail and edges of the hot rolled coil become single-phase regions, thereby increasing in-plane anisotropy and degrading formability. In addition, when the temperature exceeds 950 ° C, remarkable coarse grains occur and defects such as orange peel are likely to occur on the surface after processing.

After hot rolling as described above, high temperature winding is performed in the temperature range of 700-750 ° C. which is one of the main features of the present invention. In the case of steels with a large amount of hardenable elements such as Mo, such as the present invention, when the winding temperature is lower than 700 ° C., transformed tissues are generated in the hot rolled sheet to hinder the development of recrystallized texture when annealing. On the other hand, when the coiling temperature exceeds 750 ℃, ferrite grain growth is very activated, grains are coarsened and their size is uneven, so the material deteriorates. Therefore, the present invention is to limit the appropriate range of the winding temperature to 700-750 ℃ to prevent these problems.

After hot rolling, the steel is pickled in the usual manner and then cold rolled at a cold rolling rate of 70-80%. In order to maximize the development of the recrystallized texture at the time of annealing in the steel of the present invention, the cold reduction rate is preferably 70% or more. However, when the cold reduction rate is more than 80%, the grain refinement is very large due to excessive rolling rate, which causes hardening of the material, and the r value gradually decreases due to excessive cold rolling rate increase.

Cold-rolled steel is subjected to continuous annealing operation by a conventional method in the temperature range of 800-800 ℃. At this time, the annealing temperature is suitable in the temperature range of 800-850 ℃ that can achieve the excellent elongation and r value required by the present invention steel. In other words, if the annealing temperature is lower than 800 ℃, the temperature is somewhat lower to maximize the [111] recrystallization texture. If the annealing temperature is higher than 850 ℃, the moldability is improved, but problems such as workability deterioration and strength drop due to excessive annealing are achieved. Occurs.

Temper rolling is carried out at the minimum reduction ratio for controlling the shape by using the composite tissue steel produced by the above production method. Such conditions are preferably 1.0% or less in the present invention. If the temper rolling ratio is higher than 1.0%, the yield strength and the elongation decrease. In addition, when the inventive steel is produced as a hot-dip galvanized steel sheet, the plating adhesion is degraded due to excessive temper rolling and peeling of the plating layer occurs. Therefore, it is preferable to perform temper rolling of 10% or less, which is an appropriate condition to solve these problems. .

The steel of the present invention can also be manufactured by hot-dip galvanized steel sheet, and the conditions such as annealing temperature and temper rolling ratio are also the same as the conditions for producing cold rolled steel sheet.

Hereinafter, the present invention will be described in detail through examples.

EXAMPLE

Table 1 below shows the chemical composition of the inventive steel and the comparative steel in which the amounts of carbon, Mn, Sol.Al, Mo, and Cr are strictly controlled. Steels 1-10 are invention steels and steels 11-16 are comparative steels.

Table 2 shows the performance and material performance of the steel produced using the steel of Table 1. After hot rolling, rolling was performed at a cold rolling rate of about 75% and continuously annealed at an annealing temperature of 830 ° C., and tempered rolling was performed at about 0.8% after alloy plating at a melting plating temperature of 460 ° C. to measure tensile strength, elongation and average r value. In addition, the observation of the microstructure shows the result of measuring the fraction and the average size of the phase martensite metamorphic tissue. The coiling temperature (CT) changed for each steel was 540-620 ℃ for low temperature quenching, and high temperature winding for 700 ~ 730 ℃.

Steel grade Chemical composition (wt%) Remarks C Si Mn P S Sol.Al Mo Cr N B One 0.015 0.049 1.63 0.011 0.0030 0.40 0.30 0.10 0.0022 0.0005 Invention steel 2 0.015 0.051 1.61 0.011 0.0028 0.38 0.25 0.10 0.0017 0.0005 Invention steel 3 0.015 0.052 1.63 0.020 0.0029 0.29 0.35 0.12 0.0020 0.0007 Invention steel 4 0.015 0.051 1.63 0.020 0.0030 0.49 0.25 0.21 0.0015 0.0006 Invention steel 5 0.022 0.053 1.72 0.012 0.0035 0.42 0.30 0.09 0.0013 0.0007 Invention steel 6 0.021 0.053 1.69 0.025 0.0036 0.51 0.41 0.22 0.0021 0.0008 Invention steel 7 0.025 0.062 1.73 0.030 0.0021 0.39 0.54 0.20 0.0027 0.0003 Invention steel 8 0.021 0.051 1.41 0.040 0.0041 0.44 0.58 0.15 0.0021 0.0005 Invention steel 9 0.027 0.043 1.53 0.020 0.0031 0.40 0.62 0.21 0.0019 0.0005 Invention steel 10 0.020 0.044 1.61 0.050 0.0029 0.50 0.60 0.35 0.0027 0.0006 Invention steel 11 0.051 0.052 1.71 0.015 0.0031 0.29 0.21 0.12 0.0026 0.0007 Comparative steel 12 0.0052 0.041 1.63 0.021 0.0024 0.31 0.35 0.21 0.0025 0.0005 Comparative steel 13 0.019 0.051 1.92 0.023 0.0035 0.42 0.05 0.61 0.0019 0.0008 Comparative steel 14 0.016 0.048 2.01 0.024 0.0021 0.04 0.24 0.10 0.0027 0.0006 Comparative steel 15 0.024 0.031 1.24 0.014 0.0041 0.07 0.02 0.31 0.0021 0.0003 Comparative steel 16 0.041 0.027 1.41 0.021 0.0027 0.31 0.07 0.21 0.0018 0.0007 Comparative steel

Steel grade CT TS (MPa) El (%) r-bar. 2 phase fraction (%) 2-phase size * (㎛) Remarks One 720 ℃ 448.2 36.9 1.57 3.5 2.6 Invention steel 550 ℃ 479.3 32.1 1.08 3.1 1.7 Comparative steel 2 700 ℃ 447.7 40.0 1.59 3.2 3.1 Invention steel 620 ℃ 462.3 35.7 1.10 2.9 1.5 Comparative steel 3 750 ℃ 451.8 36.4 1.54 3.8 2.5 Invention steel 540 ℃ 481.2 31.0 1.01 3.7 1.0 Comparative steel 4 710 ℃ 451.3 36.8 1.50 3.1 3.4 Invention steel 5 700 ℃ 502.9 32.3 1.44 4.2 2.5 Invention steel 6 700 ℃ 503.3 32.7 1.41 4.3 2.6 Invention steel 7 720 ℃ 512.7 33.4 1.46 4.4 2.8 Invention steel 8 730 ℃ 497.3 34.5 1.52 3.9 3.0 Invention steel 9 700 ℃ 521.4 32.2 1.43 4.7 2.9 Invention steel 10 700 ℃ 499.6 34.7 1.49 4.2 2.9 Invention steel 11 720 ℃ 541.2 26.1 1.02 7.2 1.3 Comparative steel 12 730 ℃ 451.3 27.2 1.41 0 0 Comparative steel 13 700 ℃ 462.3 37.4 1.21 2.7 2.1 Comparative steel 14 700 ℃ 452.1 35.4 1.31 3.2 1.8 Comparative steel 15 700 ℃ 502.1 33.2 1.03 3.7 1.6 Comparative steel 16 700 ℃ 512.3 34.1 1.02 6.7 1.1 Comparative steel The second phase means martensite.

The steel grade (1-10) has a tensile strength of 447.7-521.4 MPa, an elongation of 32.2-40.0%, and an average r value of 1.41-1.59. It can be seen that the value is satisfied. In addition, due to the appropriate combination of Mo and Al, the two-phase fraction 2-5 suggested by the present invention is not only a high r value but also a fraction of martensite in two phases of 3.2-4.7% and an average size of martensite phase of 2.5-3.4 µm. %, The average size of two phases of 2.2㎛ or more was satisfied.

On the other hand, when wound in the temperature range of 540 ~ 620 ℃ by using steel grades (1 ~ 3), the strength increased and elongation was decreased due to the formation of metamorphic tissues by low temperature winding and decrease in grain size, especially in the hot rolling stage Due to the low temperature metamorphosis, the average r value deteriorated to about 1.0.

In addition, in the case of the comparative steel 11, the other components satisfy the component conditions of the present invention, but 0.051% of the carbon content higher than 0.015-0.030% suggested by the present invention was added. The carbon content was so high that martensite in the steel was fine and the fraction was very high at 7.2%. Therefore, this increased the strength and the elongation did not satisfy more than 32% presented in the present invention.

In contrast to steel grade 11, steel grade 12 is a steel to which only 0.0052% of carbon is added, which is much lower than the present conditions of the present invention. Due to the absolute reduction of the added carbon, no composite steel was obtained, but only a single phase of ferrite. In addition, the material shown in steel 12 was different from that of composite steel as the material performance of ferrite single phase steel.

Comparative steel 13 was 0.05% Mo, 0.61% Cr, and did not satisfy Mo 0.2-1.0%, Cr 0.5% or less, which are the present conditions of the present invention. Compound steel was obtained by increasing the amount of Cr instead of Mo, but the Cr-based carbide had a very low remelting temperature during annealing compared to the Mo-based carbide, and thus the development of the recrystallized texture did not work effectively. Therefore, the r value was very low as 1.21 and the martensite size was also 2.1 mu m, which did not satisfy the conditions of the inventive steel.

Comparative steel 14, the Sol.Al content is outside the range of the present conditions of the present invention. Al acts to disperse hot rolled pearlite and coarsen martensite during annealing, but Al was added as 0.04% as the comparative steel 14, which is lower than the standard suggested by the present invention steel, and the r value was somewhat lower as 1.31. The size of martensite was also 1.8 μm, which did not satisfy the conditions of the present invention.

Comparative steel 15 is a steel in which the contents of Sol.Al and Mo are out of the range of the present invention steel. Due to the lack of Al addition, coarsening of martensite during dispersion and annealing of hot rolled pearlite could not be expected, and development of annealing [111] texture by Mo-based carbides could not be expected due to the reduction of Mo. As a result, the r value was 1.03 and the martensite size was also very low, 1.6 μm.

Steel grade 16 had a very high carbon content of 0.041% and a Mo content of 0.07%, which is beyond the conditions of the present invention. The martensite content was very high due to the excessive addition of carbon, but the increase in r value could not be expected because Mo effect was minimal.

As described above, in the present invention, the drawing composite structure, which is a typical property of the composite steel, has a low yield ratio, excellent elongation, and at the same time improves the deep drawing property, and is applicable to the amount of hardening of the automobile material and the internal and external board materials. A high strength steel sheet and a method of manufacturing the same are provided.

Claims (7)

By weight% C: 0.01-0.03%, Si: 0.3% or less, Mn: 1.0-2.0%, P: 0.01-0.06%, S: 0.015% or less, Soluble Al: 0.2-0.8%, N: 0.0030 It contains% or less, Mo: 0.2 ~ 1.0%, Cr: 0.5% or less and B: 0.0005-0.0015%, and is composed of the remaining Fe and other unavoidable impurities, and has excellent deep drawing property as a two-phase structure of martensite and ferrite. Composite tissue steel plate. The method of claim 1, The martensite has a size of more than 2.2㎛, the fraction is 2-5% excellent deep drawing composite steel sheet. The method of claim 1, The steel sheet is a composite steel sheet excellent in deep drawing properties, characterized in that Mo carbide is included. The composite tissue sheet according to claim 1, wherein the composite tissue sheet is selected from a cold rolled steel sheet or a plated steel sheet having a zinc-based molten zinc layer formed on at least one surface thereof. The method of claim 1, The steel sheet has a tensile strength of 440 ~ 490MPa grade, the elongation is more than 32%, r-structured cold rolled steel sheet, characterized in that more than 1.4. By weight% C: 0.01-0.03%, Si: 0.3% or less, Mn: 1.0-2.0%, P: 0.01-0.06%, S: 0.015% or less, Soluble Al: 0.2-0.8%, N: 0.0030 Steel slab containing% or less, Mo: 0.2 ~ 1.0%, Cr: 0.5% or less and B: 0.0005-0.0015%, and composed of the remaining Fe and other unavoidable impurities, after homogenizing heat treatment at 1200 ℃ or higher, at 900-950 ℃ Hot-rolled under the condition of finish rolling temperature and wound up at 700-750 ℃ to obtain hot-rolled sheet, cold-rolled at 70-80% cold rolling rate, continuous annealing and 1.0% in the temperature range of 800-850 ℃. A method for producing a composite structured cold rolled steel sheet having excellent deep drawing properties including the following temper rolling. The method of claim 6, The hot rolled sheet is a method for producing a composite structured cold rolled steel sheet having excellent deep drawing properties, characterized in that Mo carbide is included.

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