KR100775338B1 - Cold rolled steel sheet having high yield ratio and excellent formability and the method for manufacturing the same - Google Patents

Abstract

A high yield ratio type cold rolled steel sheet and a method for manufacturing the same are provided, wherein the cold rolled steel sheet has high yield ratio and excellent formability by controlling alloy elements and fine precipitates in a basic component system of ultra low carbon steel. A cold rolled steel sheet having high yield ratio and excellent formability has a composition comprising, by weight percent, 0.005% or less of C, 1.0% or less of Mn, 0.1% or less of P, 0.01% or less of S, 0.006 to 0.008% of N, 0.11 to 0.16% of acid-soluble Al, 0.04 to 0.07% of Ti, 0.0005 to 0.0015% of B, and the balance of Fe and other inevitable impurities, wherein 1x10^6 AlN precipitates having an average size of 20 nm or less are distributed in the cold rolled steel sheet. A method for manufacturing a cold rolled steel sheet having high yield ratio and excellent formability comprises: reheating a steel slab to a temperature of 1100 deg.C or more, the steel slab having a composition comprising, by weight percent, 0.005% or less of C, 1.0% or less of Mn, 0.1% or less of P, 0.01% or less of S, 0.006 to 0.008% of N, 0.11 to 0.16% of acid-soluble Al, 0.04 to 0.07% of Ti, 0.0005 to 0.0015% of B, and the balance of Fe and other inevitable impurities; finishing a hot finish rolling process of the reheated steel slab at an Ar3 transformation temperature or more; coiling the hot finish rolled steel sheet in a temperature range of 550 to 630 deg.C; cold rolling the coiled steel sheet; and continuously annealing the cold rolled steel sheet in a temperature range of 760 to 810 deg.C.

Description

Cold rolled steel sheet having high yield ratio and excellent formability and the method for manufacturing the same}

Korean Application No. 2003-0091535

Japanese Patent Laid-Open No. 4-280943

Japanese Patent Laid-Open No. 5-070836

Japanese Patent Laid-Open No. 5-263184

Japanese Unexamined Patent Publication No. 10-096051

The present invention relates to a high yield ratio cold rolled steel sheet mainly used as a shock-resistant material, such as automotive interior, exterior panels and some structural members. More specifically, the present invention relates to a cold rolled steel sheet having excellent formability and high yield ratio by fine AlN precipitates of Ti-added ultra low carbon steel and a method of manufacturing the same.

Precipitation-reinforced high-strength steel sheet is widely used for various applications such as panels, structural members such as members, beams, pillars, and inner plates to improve impact resistance of automobiles due to the proliferation of impact safety regulations in automobiles. Is being used. This is because it is required to use a steel sheet with excellent shock absorbing ability in an automobile accident. Precipitation-reinforced high-strength steel sheet is designed to absorb the collision energy of the automobile, characterized by high yield strength, that is, yield ratio (YS / TS).

In general, the method of strengthening the steel can be summarized as a method of strengthening solid solution, strengthening precipitation, strengthening by transformation of grains, and transformation.

High-strength steel manufactured by solid solution strengthening method includes P-added IF (interstitial free) steel, which secures workability by securing strength by P-adding, extremely inhibiting invasive elemental components, and immobilizing simultaneously. Is being used. Such solid solution reinforced steel has a lower yield strength than tensile strength, so it is advantageous to use it as an automobile part requiring workability, but it is not suitable for use as a component requiring impact resistance mentioned above. Known techniques for producing solid solution strengthening type high strength steel sheets include Japanese Patent Application Laid-Open Nos. 4-280943, 5-05836, 5-263184, and 10-096051.

Metamorphic reinforcement method requires not only a large amount of alloying elements to secure strength and formation of metamorphic structure, but also because its lower structure is composed of bainite or martensite, it has low yield strength compared to tensile strength, that is, yield ratio. It has the drawback that its use in parts is not appropriate.

Precipitation strengthening type high strength steel is a steel whose strength is enhanced by the precipitation strengthening effect by the addition of carbon and nitride forming elements such as Cu, Nb, Ti, V, etc., and the grain refinement effect, making it easy to achieve high strength even at low manufacturing costs. It has advantages

As a representative method for manufacturing a precipitation-reinforced high strength steel, there is a known Korean Patent Application No. 2003-0091535, which uses a method of increasing yield strength using a low carbon steel of 0.02 to 0.04% C as a basic component system. Even if the strength is secured, the moldability, that is, the r value remains at 1.4 level, has a problem of poor workability.

The present invention is to improve the conventional problems described above, by controlling the alloying elements and fine precipitates in the basic component system of the ultra-low carbon steel, to provide a high yield ratio cold-rolled steel sheet and manufacturing method having excellent yield strength and excellent moldability , Its purpose is.

The present invention for achieving the above object, by weight% C: 0.005% or less, Mn: 1.0% or less, P: 0.1% or less, S: 0.01% or less, N: 0.006 ~ 0.008%, acid value Al: 0.11 ~ 0.16%, Ti: 0.04% to 0.07%, B: 0.0005% to 0.0015%, remaining Fe and other unavoidable impurities, and AlN precipitates having an average size of 20 nm or less are distributed at 1 × 10 ⁶ pieces / mm 2 or more The present invention relates to a high yield ratio cold rolled steel sheet having excellent properties.

In the present invention, C: 0.005% or less, Mn: 1.0% or less, P: 0.1% or less, S: 0.01% or less, N: 0.006 to 0.008%, acid value Al: 0.11 to 0.16%, Ti: 0.04 to 0.07%, B: 0.0005 to 0.0015%, steel slabs composed of the remaining Fe and other unavoidable impurities are reheated to a temperature of 1100 ° C. or higher and the hot finish rolling is finished at Ar ₃ transformation point or higher, and then 550 to 630 ° C. The present invention relates to a method for producing a high yield ratio cold rolled steel sheet having excellent moldability after winding in a temperature range and cold rolling, followed by continuous annealing in the range of 760 to 810 ° C.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

The inventors of the present invention are seeking to improve yield ratio and formability while securing excellent yield strength, while increasing the yield strength by dispersing and precipitating fine AlN in grain boundaries and in the mouth, and at the same time lowering the C content to ultra low carbon steel level. Based on the results of research that lowering can greatly improve formability.

First, the composition range of the steel component of the present invention will be described.

C: 0.005% or less is preferable.

When the C is present in the ultra-low carbon steel by adding Ti to the presence of precipitates rather than in solid solution state, the {111} aggregate structure which is advantageous for workability is developed during annealing, thereby greatly improving workability. When the content of C exceeds 0.005%, the aging and moldability are not greatly improved, so the content of C is preferably limited to 0.005% or less.

Mn: 1.0% or less is preferable.

The Mn not only contributes to the increase in strength as a solid solution strengthening element, but also precipitates S in MnS and plays an important role of suppressing plate breakage and high temperature embrittlement caused by S during hot rolling. As the content of Mn is increased, the strength-improving effect is increased, whereas when the content exceeds 1.0%, the workability, that is, the r-value, drops sharply, so the Mn content is preferably limited to 1.0%.

P: 0.1% or less is preferable.

P is an element most advantageous for securing strength without significantly deteriorating formability. However, excessive addition of P significantly increases the possibility of brittle fracture, thereby increasing the possibility of slab breakage during hot rolling, and may act as an element that inhibits the plating surface properties. Therefore, the content of P is preferably limited to 0.1%.

S: 0.01% or less is preferable.

It is important to manage S as low as possible because it is an element that is inevitably added as an impurity. In addition, it is desirable to manage the content as little as possible in order to secure excellent welding properties, but the refining cost of the steel is high. Therefore, the content of S is preferably limited to 0.01% or less which is a possible operating condition range.

N: 0.006 to 0.008% is preferable.

N plays an important role in increasing the strength in the present invention. If the content is less than 0.006%, the effect of AlN fine precipitate is not sufficient, and it is difficult to entail an increase in yield strength. If it is more than 0.008%, aging guarantee by solid nitrogen may be difficult. Therefore, the content of N is preferably limited to 0.006 ~ 0.008%.

Acid value Al: 0.11-0.16% is preferable.

Acid value Al is an element normally added for refinement | miniaturization of the particle size of steel, and deoxidation. However, in the present invention, by using the fine precipitate effect of AlN acts as an important factor to increase the yield strength of the steel. If the acid-soluble Al content is less than 0.11%, it does not sufficiently secure the very fine AlN precipitates, and it does not contribute significantly to the increase in strength of the steel. have. Therefore, the content of the acid-soluble Al is preferably limited to 0.11 ~ 0.16%.

Ti: 0.04-0.07% is preferable.

The Ti improves the formability of the steel by the scavenging effect of completely depositing and fixing the solid solution C by forming a TiC precipitate by working with the solid solution C during hot rolling. If the content is less than 0.04%, it does not completely precipitate solid solution C, which is disadvantageous in terms of formability of the steel. If it exceeds 0.07%, not only Ti is precipitated but also TiC precipitate is so large that the grain refinement effect is reduced. Strength and plating properties can be greatly degraded. Therefore, the content of Ti is preferably limited to 0.04 ~ 0.07%.

B: 0.0005 to 0.0015% is preferable.

The B is an element having an effect of improving the surface properties of the plated steel sheet in conjunction with the Mo component. If the content is less than 0.0005%, the effect is insignificant and if the content exceeds 0.0015%, the recrystallization temperature rises sharply and acts as a factor of defects of the plated steel sheet due to high temperature annealing, ie, streaks and unalloyed, so the content of B is It is desirable to limit to 0.0015%.

In the steel sheet of the present invention, AlN precipitates having an average size of 20 nm or less are distributed at 1 × 10 ⁶ holes / mm 2 or more. In the component system of the present invention, the finer the distribution, the more advantageous. According to the results of the present invention, when the average size of the precipitate exceeds 20 nm, the strength is particularly low, which does not contribute significantly to securing yield strength.

Furthermore, in order to maximize the effect of increasing the strength in the component system of the present invention, the distribution number of AlN precipitates of 20 nm or less is preferably 1 × 0 ⁶ or more per mm 2, which greatly contributes to the increase in strength and greatly improves the r value, which is the plastic anisotropy index. Can be. Therefore, it is preferable to limit AlN precipitates having an average size of 20 nm or less to 1 × 10 ⁶ holes / mm 2 or more.

Hereinafter, the manufacturing method of the cold rolled steel sheet which has the steel comprised as mentioned above is demonstrated in detail.

First, the steel slab formed as described above is reheated to a temperature of 1100 ° C. or more, and then hot finish rolling is finished at an Ar ₃ transformation point or more.

When the reheating temperature is lower than 1100 ° C., coarse precipitates generated during continuous casting remain completely insoluble, and a large amount of coarse precipitates are present even after hot rolling, and thus do not contribute significantly to the strength increase effect. It is preferable to limit it to more than degreeC.

In addition, when the hot rolling finish temperature is less than the Ar ₃ transformation point temperature, not only the workability is lowered due to the formation of the rolled grain, but also the strength tends to be lowered. Therefore, the hot rolling finishing temperature is preferably limited to the Ar ₃ transformation point or more. .

Then, it is wound in a temperature range of 550 ~ 630 ℃, cold rolled. If the coiling temperature is less than 550 ℃, not only the precipitated solid solution C may not be completely precipitated, but also the problem of poor plate shape when winding up, and when the temperature exceeds 630 ℃ precipitates tend to be significantly coarse Yield effect is not great, yield rate can be lowered. Therefore, the winding temperature is preferably limited to 550 ~ 630 ℃.

In addition, the cold rolling after winding is more advantageous in terms of workability as the reduction ratio is higher, but the target formability is secured even under normal conditions due to the field application limit, so it is not necessary to limit the range.

Subsequently, the cold rolled steel sheet is continuously annealed at a temperature range of 760 to 810 ° C. The continuous annealing temperature plays an important role in determining the material of the product, but is preferably performed in the range of 760 to 810 ° C., which is a general operating condition. If the annealing temperature is less than 760 ℃, the recrystallization of the steel is secured, but the grain growth is not sufficiently achieved may cause problems in securing the moldability, if the temperature exceeds 810 ℃ the inclusions are eluted to the surface of the steel sheet to fine dent (Dent Can produce a large amount of) to impair surface quality. Therefore, the continuous annealing temperature is preferably limited to a temperature range of 760 ~ 810 ℃.

Hereinafter, the present invention will be described in more detail with reference to Examples.

EXAMPLE

The steel slab, as shown in Table 1 below, was reheated to a temperature of 1100 ° C. or higher and finished hot rolled at a temperature of Ar ₃ transformation point or more. Subsequently, rolling was carried out at a 78% level, which is a normal reduction ratio, after winding under the same manufacturing conditions as in Table 2, and annealing was performed under the continuous annealing conditions in Table 2 below.

The obtained annealing plate measured the mechanical properties of yield strength, tensile strength, elongation, r value, and in-plane anisotropy index (Δr value), and the tensile test was carried out using JIS No. 5 specimens. Mechanical properties are shown.

division C Mn P S N S.Al Ti B Inventive Steel A 0.0035 0.6 0.05 0.008 0.0063 0.13 0.06 0.0007 Inventive Steel B 0.0041 0.7 0.06 0.008 0.0065 0.14 0.055 0.0006 Invention Steel C 0.0032 0.5 0.05 0.007 0.0062 0.13 0.05 0.0008 Inventive Steel D 0.0028 0.8 0.07 0.007 0.0060 0.12 0.06 0.0011 Comparative Steel A 0.04 0.6 0.02 0.007 0.003 0.04 - -

division Coiling temperature (℃) Annealing Temperature (℃) Precipitate distribution (× 10 ⁷ pieces / mm2) Precipitate size (nm) Material characteristics Yield strength (MPa) Tensile Strength (MPa) Elongation (%) r value △ r Inventive Steel A Invention 1 580 790 2.7 18 231 354 39 1.78 0.28 Comparative Material 1 720 802 0.08 32 215 351 40 1.52 0.31 Inventive Steel B Invention 2 610 785 1.5 17 226 348 39 1.79 0.41 Invention Steel C Invention 3 620 805 2.2 18 227 349 38 1.82 0.43 Inventive Steel D Invention 4 615 802 2.3 17 232 361 39 1.92 0.38 Invention 5 595 796 1.9 19 228 354 41 1.86 0.29 Comparative Steel A Comparative Material 2 630 790 0.05 28 228 356 35 1.41 0.51

As shown in Tables 1 and 2, in the case of the inventive materials (1 to 5) manufactured according to the manufacturing method of the present invention using the inventive steels (A to D) satisfying the component range of the present invention, 1 × 10 ⁶ By forming fine precipitates of 20 nm or less in pieces / mm 2 or more, excellent yield strength of 220 MPa or more was secured, and excellent moldability was obtained with elongation of 38% or more and plastic anisotropy index (r value) of 1.7 or more.

However, Comparative Materials 1 and 2, which did not satisfy the component range or manufacturing method of the present invention, did not satisfy the distribution and size of the precipitates of the present invention, and exhibited thermal yield strength, and the plastic anisotropy index (r value). 1.52 and 1.41 showed thermal formability.

As described above, according to the present invention, there is an effect that can provide a high yield ratio cold rolled steel sheet having a yield strength of 220MPa or more and at the same time improved formability.

Claims (2)

By weight% C: 0.005% or less, Mn: 1.0% or less, P: 0.1% or less, S: 0.01% or less, N: 0.006 to 0.008%, acid value Al: 0.11 to 0.16%, Ti: 0.04 to 0.07%, B: A high yield ratio cold rolled steel sheet having excellent moldability, which is composed of 0.0005% to 0.0015%, the remaining Fe and other unavoidable impurities, and has an AlN precipitate having an average size of 20 nm or less and 1 × 10 ⁶ pieces / mm 2 or more. By weight% C: 0.005% or less, Mn: 1.0% or less, P: 0.1% or less, S: 0.01% or less, N: 0.006 to 0.008%, acid value Al: 0.11 to 0.16%, Ti: 0.04 to 0.07%, B: The steel slab composed of 0.0005% to 0.0015%, the remaining Fe and other unavoidable impurities is reheated to a temperature of 1100 ° C or higher, and finished hot finish rolling above the Ar ₃ transformation point, and then wound in a temperature range of 550 ° C to 630 ° C. The high yield ratio cold rolled steel sheet having excellent moldability, which is continuously annealed in the range of 760 to 810 ° C after cold rolling.