KR100627474B1 - The precipitation hardening cold rolled steel sheet having excellent yield strength and yield ratio, and the method for manufacturing the same - Google Patents

Abstract

The present invention relates to a steel plate mainly used as a structural member for a seat rail for automobiles.

In the present invention, by weight%, C: 0.08 to 0.12%, Mn: 1.8 to 2.2%, P: 0.02% or less, S: 0.005% or less, N: 0.005% or less, Si: 0.1% or less, acid value Al: 0.1 ~ 0.15%, Nb: 0.06 ~ 0.09%, B: 0.0005 ~ 0.001%, Mo: 0.05 × Mn [%], balance Fe and other unavoidable impurities, yield strength, characterized in that the recrystallization ratio is 65-75%. And it provides a precipitation strengthening cold rolled steel sheet excellent in yield ratio and a method of manufacturing the same.

The present invention has the effect of providing a precipitation-reinforced cold rolled steel sheet of 750 MPa or more, and yield ratio of 75% or more.

Precipitation strengthening, cold rolled steel, yield strength, yield ratio, recovery recrystallization annealing, mailing speed

Description

The precipitation hardening cold rolled steel sheet having excellent yield strength and yield ratio, and the method for manufacturing the same}

The present invention relates to a steel sheet mainly used as a structural member for a seat rail for automobiles, and more particularly, a precipitation-reinforced cold rolled steel sheet capable of securing a yield strength of at least 750 MPa and a yield ratio of at least 75%, and its It relates to a manufacturing method.

Recently, as the impact safety regulations of automobiles are spreading, precipitation hardening type high strength steel sheets are widely used in structural members such as members, seat rails, and pillars to improve impact resistance of the vehicle body. Since the precipitation hardening type high strength steel sheet is designed to absorb the collision energy of the automobile, the yield strength is high, that is, the yield ratio (TS / TS) is high.

In general, the method of reinforcing steel can be summarized as the method of strengthening solid solution, strengthening precipitation, strengthening by transformation of grains, and transformation. However, the strengthening method by solid solution strengthening and grain refinement is very difficult to manufacture high strength steel of 490MPa or more based on tensile strength, and the transformation strengthening method requires a large amount of alloy components to secure strength and form transformation structure, Since it is made of bainite or martensite, the yield strength is lower than the tensile strength, that is, the yield ratio is low, which is not suitable for parts requiring impact resistance in a car crash.

On the other hand, precipitation-reinforced high-strength steel is a steel that improves its strength 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. It has the advantage that it can be achieved. The precipitation strengthening method is a phenomenon in which steel is first subjected to solution treatment at a high temperature, and then a large number of fine precipitates are formed during cooling to be strengthened by the stress field around the precipitate. Usually, since these fine precipitates are formed in a large amount during winding after hot rolling, the recrystallization temperature is very rapidly increased by the fine precipitates during annealing after cold rolling, and thus has a burden of high temperature annealing. In addition, a large amount of alloying elements should be added in order to obtain a solid solution effect by adding Mn and P together with the reinforcing effect by fine precipitates. Recently, since the automobile company strictly limits its content, it does not contribute significantly to the strength contribution. On the other hand, manganese (Mn) in steel increases the recrystallization temperature at the time of annealing when a large amount is added, it is absolutely necessary for high temperature annealing, but Mn-based oxides are eluted by the high temperature annealing on the surface to act as a main cause of surface defects.

Representative prior arts for precipitation-reinforced high strength steels include Japanese Patent Application Laid-open No. 56-84422, Hei 4-221015, Hei 3-140412 and Hei 11-241119.

Japanese Laid-Open Patent Publication No. 56-84422 of the prior arts contains one or two or more kinds of Ti, Nb, V, etc., based on low carbon steel containing 0.15% or less of C as a basic component system, and final hot rolling finish. The temperature is set at 750 to 950 ° C. and the winding temperature is controlled at 450 ° C. or less to produce precipitation strengthened high strength steel. However, the prior art has not only obtained a yield strength of more than 750MPa despite the extremely high strength contribution effect by forming a very fine precipitate due to the winding temperature is too low, cold rolling due to the increase of the residual stress around the precipitate Overload is often a problem that occurs.

In addition, Japanese Patent Application Laid-open No. Hei 4-221015 discloses a method for producing a precipitation-reinforced steel having an excellent strength increase effect by accelerated cooling after hot rolling using Nb or V precipitate formation elements. In this case, however, the coiling temperature is set to 400 ° C. or lower, so that not only the above-mentioned problem occurs but also bainite or martensite structure is formed instead of uniform ferrite structure, so that the yield ratio (yield strength / tensile strength) is increased. It has a problem of deterioration.

In addition, Japanese Patent Laid-Open Nos. Hei 3-140412 and Hei 11-241119 among the related arts suggest a method of manufacturing a precipitation strengthening high strength steel using Cu precipitates. However, the prior art not only causes poor alloying of the coated steel sheet by Cu-based precipitates, but also has poor weldability, which causes many problems in field applications.

The present invention is to solve the problems of the prior art, the precipitation-reinforced cold-rolled steel sheet excellent in yield strength and yield ratio by adding the appropriate level of Nb, Mo, B, and appropriately control the winding temperature and recovery recrystallization temperature during annealing And to provide a method for manufacturing the same, there is a purpose.

The present invention for achieving the above object, in weight%, C: 0.08 ~ 0.12%, Mn: 1.8 ~ 2.2%, P: 0.02% or less, S: 0.005% or less, N: 0.005% or less, Si: 0.1% Hereinafter, the acid-soluble Al: 0.1 ~ 0.15%, Nb: 0.06 ~ 0.09%, B: 0.0005 ~ 0.001%, Mo: 0.05 × Mn [%], the balance Fe and other unavoidable impurities,
It relates to a precipitation strengthening cold rolled steel sheet excellent in yield strength and yield ratio, characterized in that the recrystallization ratio is 65 ~ 75%.

In addition, the present invention is in weight%, C: 0.08 to 0.12%, Mn: 1.8 to 2.2%, P: 0.02% or less, S: 0.005% or less, N: 0.005% or less, Si: 0.1% or less, acid-soluble Al : 0.1 ~ 0.15%, Nb: 0.06 ~ 0.09%, B: 0.0005 ~ 0.001%, Mo: 0.05 × Mn [%], the balance Fe and other end of hot rolling a steel slab which is the composition as inevitable impurities in more than Ar ₃ transformation point It is then wound at 550 ~ 600 ℃, cold rolling at a rolling rate of 50% or more, followed by recrystallization annealing at a continuous sheet annealing speed 150 ~ 200mpm and annealing temperature 770 ~ 810 ℃,
It relates to a method for producing a precipitation-reinforced cold rolled steel sheet having excellent yield strength and yield ratio, characterized in that the recrystallization ratio is 65 to 75%.

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

The present invention is characterized by providing a precipitation-reinforced cold rolled steel sheet having excellent yield strength and yield ratio by adding appropriate levels of Nb, Mo, and B, and appropriately controlling the coiling temperature and the recovery recrystallization temperature upon annealing. First, look at the reasons of the limited ingredients of the present invention.

C: 0.08 to 0.12% by weight (hereinafter referred to as '%' only)

The C plays an important role as a precipitate forming element in the present invention, and if the content is less than 0.08%, it is difficult to obtain a sufficient precipitation effect, and it is difficult to secure the desired yield strength, and the yield ratio is increased since NbC carbides tend to be coarse. If it exceeds 0.12%, there is a high possibility that cracks will occur in the cast steel during the steelmaking process, as well as bainite structure is formed during coil winding after hot rolling, which significantly increases the strength of the hot rolled sheet. There is a problem that brings load at the time. Therefore, the content of C is preferably limited to 0.08 to 0.12%.

Mn: 1.8-2.2%

The Mn not only contributes to the strength increase as a solid solution element, but also precipitates S in MnS and plays an important role in suppressing plate breakage and high temperature embrittlement caused by S during hot rolling. When the content of Mn is less than 1.8%, even if recovery annealing is performed, it is difficult to secure a target yield strength of 750 MPa. When the content of Mn exceeds 2.2%, Mn inclusions are eluted to the surface of the steel sheet during annealing. In addition, the surface properties of the steel are not only significantly reduced, but the C equivalent welding index (C + Mn / 6) is increased to cause problems in weldability. Therefore, the content is preferably limited to 1.8 to 2.2%.

P: 0.02% or less

The P is the most advantageous element to secure the strength without significantly deteriorating the formability, but if the content exceeds 0.02%, the possibility of brittle fracture is significantly increased to increase the possibility of generating plate break in the slab during hot rolling. Since there is a problem that also inhibits the surface properties of the steel sheet, it is preferable to limit the content to 0.02% or less.

S: 0.005% or less, N: 0.005% or less

Since S and N are inevitably added elements as impurity elements in steel, it is preferable to manage them as low as possible. In addition, it is preferable to manage the content as low as possible in order to secure excellent welding properties, but there is a problem in that the refining cost of the steel is increased to lower the content. Therefore, it is preferable to limit to S: 0.005% or less and N: 0.005% or less, which are the ranges in which operating conditions are possible.

Si: 0.1% or less

If the content of Si exceeds 0.1%, it is very disadvantageous to the surface properties of the steel sheet, it is preferable to limit the content to 0.1% or less.

Acid value Al: 0.1 ~ 0.15%

The acid soluble Al is an element added to refine the particle size of the steel and deoxidation. If the content is less than 0.1%, fine AlN precipitates are not formed and thus the effect of increasing strength is insufficient. If the content exceeds 0.15%, the grain refinement effect is very advantageous to increase the strength, but excessive inclusions are formed during steelmaking operation. In addition to increasing the likelihood that a defect will occur on the surface, there is a problem that increases the manufacturing cost, it is preferable to limit the content to 0.1 ~ 0.15%.

Nb: 0.06-0.09%

Nb is an important component in performing recovery recrystallization annealing in combination with B in the present invention. Nb added in the present invention is to form a large amount of very fine NbC precipitates in the crystal grains by working with the solid solution C during hot rolling, whereby the interaction with B (Aciqular Ferrite) crystal structure is formed Done. These ecuculal ferrite tissues act as a major factor in increasing the recrystallization temperature during annealing. When the content of Nb is less than 0.06%, the fine precipitates for securing strength are not sufficiently precipitated, so that the desired strength cannot be obtained, and the annealing is carried out at low temperature annealing, and exceeds 0.09%. When it is disadvantageous to secure the surface properties of the steel sheet and the rolling load is increased by a large amount of fine precipitates during cold rolling, it is preferable to limit the content to 0.06 ~ 0.09%.

B: 0.0005-0.001%

B is an element which refines crystal grains and improves weld toughness. When the content of B is less than 0.0005%, the effect of grain refinement is small, and thus it does not affect the improvement of the weld toughness. When the content of B is more than 0.001%, there is a burden of cost increase when controlling the steelmaking components, and it is accompanied by a decrease in elongation. It is preferable to limit to 0.0005 to 0.001%.

Mo: 0.05 × Mn [%]

The Mo is added for some solid solution strengthening effect, the content is preferably limited to satisfy 0.05 × Mn [%]. If the Mo content is less than 0.05 × Mn [%], the recrystallization temperature is not sufficiently raised and low temperature annealing is performed to secure the desired strength. Therefore, the workability is significantly reduced due to mismatch with the connecting coil during annealing. If Mn [%] is exceeded, the recrystallization temperature is effective to increase, but there is a burden of increasing manufacturing cost and accompanied with a decrease in elongation.

The present invention is composed of Fe and other unavoidable impurities in addition to the above components.

Hereinafter, the manufacturing method of this invention is demonstrated in detail.

First, the steel slab formed as described above is hot rolled at a temperature of Ar ₃ or more, which is normal hot rolling conditions, and wound up at 550 to 600 ° C. When the coiling temperature is less than 550 ℃, it is more advantageous for the formation of fine NbC precipitate in the steel, but the distortion of the plate-like frequently occurs, which is very likely to cause equipment trouble, and if it exceeds 600 ℃, the buckling defect of the hot rolled coil is generated , The winding temperature is preferably limited to 550 ~ 600 ℃.

Thereafter, the wound hot rolled sheet is cold rolled at a rolling reduction ratio of 50% or more. According to the basic experiment of the present invention, as the cold reduction rate is increased, the formation of fine precipitates is more easily performed, which advantageously increases the strength. However, when the cold reduction rate is less than 50%, since the grain nucleation site is small during recovery recrystallization, the recrystallization temperature is increased. Therefore, the cold reduction rate is preferably limited to 50% or more.

Subsequently, the cold rolled sheet is recrystallized and annealed in a continuous annealing furnace at a line speed of 150 to 200 mpm and an annealing temperature of 770 to 810 ° C. If the mailing speed is less than 150mpm, the annealing structure is very likely to be completely recrystallized even at low temperature annealing, so that the target 750MPa yield strength is highly secured. It is more advantageous for strength, but it puts a load on the facility capacity. In addition, when the annealing temperature is less than 770 ℃ the strength is secured, but the elongation is sharply lowered and exceeds 810 ℃ can not secure the desired yield strength due to complete recrystallization.

In the present invention, if the recrystallization ratio (the rate at which the tissue is completely recrystallized during annealing) is limited to 65 to 75%, it is possible to secure an excellent elongation at the same time as the yield strength of 750 MPa or more.

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

EXAMPLE

The specimens were prepared under the conditions shown in Table 2 below using the inventive steels (A, B) and comparative steels (A, B) as shown in Table 1 below. Then, the steel sheet material properties of the prepared specimens were evaluated, and the results are shown in Table 3 below. At this time, the tensile test was conducted in the C direction using the DIN standard.

Steel grade Ingredient Content (wt%) C Mn P S N Si Acid Value Al Nb B Mo Inventive Steel A 0.11 1.85 0.015 0.0045 0.0028 0.08 0.12 0.07 0.0006 0.1 Inventive Steel B 0.09 1.92 0.018 0.0044 0.0027 0.09 0.10 0.085 0.0007 0.11 Comparative Steel A 0.09 2.06 0.015 0.0048 0.004 0.08 0.06 0.042 - - Comparative Steel B 0.10 1.3 0.016 0.0046 0.0028 0.09 0.04 0.13 - 0.019

division Steel grade Hot rolled Cold rolled steel Recrystallization annealing Material characteristics Finish Hot Rolled (℃) Coiling temperature (℃) Cold rolling reduction (%) Mail speed (mpm) Annealing Temperature (℃) Yield strength (MPa) Elongation (%) Yield Ratio (%) Recrystallization cost (%) Invention 1   Inventive Steel A 912 570 52 160 780 762 14 77 68 Invention 2 917 580 55 165 785 774 13 76 72 Comparative Material 1 921 640 53 220 772 685 17 73 73 Invention 3 912 590 57 164 795 782 13 77 68 Invention 4 917 583 53 155 782 776 14 78 67 Comparative Material 2  Inventive Steel B 922 612 51 152 824 676 16 75 85 Comparative Material 3 907 590 45 163 752 805 8 86 56 Invention 5 906 575 54 155 779 779 13 78 68 Invention 6 899 560 52 182 784 783 12 76 67 Comparative Material 4 Comparative Steel A 715 570 55 168 756 689 16 68 73 Comparative Material 5 721 582 53 165 779 695 17 72 72 Comparative Material 6  Comparative Steel B 723 540 43 155 838 565 21 75 98 Comparative Material7 718 580 50 182 778 596 19 74 69 Comparative Material 8 709 620 53 223 785 635 18 76 68

In Table 2, in the case of the invention materials (1-6) manufactured according to the production method of the present invention using the invention steel (A, B) satisfying the component range of the present invention, the yield strength of 750MPa or more, and yield ratio More than 75% could be secured. In addition, it was found that in the case of the invention material having a recrystallization ratio of 65 to 75%, an elongation of 10% or more can be secured.

However, even when using the comparative steel (A, B) that does not satisfy the component range of the present invention, or using the invention steel (A, B) that satisfies the component range of the present invention is not produced according to the manufacturing method of the present invention In the case of the comparative materials (1-8), the yield strength of 750 MPa or more could not be secured, or the yield ratio of 75% or more could not be secured. In addition, in the case of Comparative Material 3, the recrystallization ratio was smaller than the limit of the present invention was confirmed that the elongation is sharply reduced.

As described above, according to the present invention has an effect that can provide a precipitation-reinforced cold rolled steel sheet yielding strength of 750MPa or more and yield ratio of 75% or more.

Claims (4)

By weight%, C: 0.08 to 0.12%, Mn: 1.8 to 2.2%, P: 0.02% or less, S: 0.005% or less, N: 0.005% or less, Si: 0.1% or less, acid soluble Al: 0.1 to 0.15% , Nb: 0.06 to 0.09%, B: 0.0005 to 0.001%, Mo: 0.05 × Mn [%], balance Fe and other unavoidable impurities, A precipitation strengthening cold rolled steel sheet having excellent yield strength and yield ratio, characterized in that the recrystallization ratio is 65 to 75%. delete By weight%, C: 0.08 to 0.12%, Mn: 1.8 to 2.2%, P: 0.02% or less, S: 0.005% or less, N: 0.005% or less, Si: 0.1% or less, acid soluble Al: 0.1 to 0.15% Steel slab composed of Nb: 0.06 ~ 0.09%, B: 0.0005 ~ 0.001%, Mo: 0.05 × Mn [%], balance Fe and other unavoidable impurities, and finish hot rolling above Ar ₃ transformation point and then 550 ~ 600 Winding at a ℃, cold rolling at a reduction rate of 50% or more, followed by recrystallization annealing at a continuous sheet annealing speed of 150 ~ 200mpm and annealing temperature of 770 ~ 810 ℃, A method for producing a precipitation-reinforced cold rolled steel sheet having excellent yield strength and yield ratio, wherein the recrystallization ratio is 65 to 75%. delete