KR20050032721A - Ultra high strength steel of 120kgf/㎟ grade having excellent formability - Google Patents

Abstract

Provided are a steel to which large quantities of carbon and silicon are added to prevent deterioration of plating property and weldability, and which has high strength as well as excellent formability by obtaining tensile strength of 120 kgf/mm^2 grade or more and yield ratio of 0.6 or less, and a method for manufacturing the same. A method for manufacturing an ultra high strength steel of 120 kgf/mm^2 grade having excellent formability comprises: a step of reheating a steel slab having a composition comprising 0.10 to 0.15 wt.% of C, 0.7 to 1.7 wt.% of Si, 4.0 to 7.0 wt.% of Mn, 0.03 wt.% or less of P, 0.005 wt.% or less of S, 0.05 to 0.15 wt.% of Al, 0.003 wt.% or less of N, 0.0001 to 0.002 wt.% of B, 0.01 to 0.05 wt.% of Mo and the balance being Fe and other inevitable impurities at a temperature of 1,200 to 1,250 deg.C for 60 to 180 minutes; a step of hot finish rolling the reheated slab at a temperature of Ar3 to Ar3+100 deg.C, and coiling the hot finish rolled steel sheet at a temperature of 560 to 680 deg.C; and a step of cold rolling the coiled hot rolled steel sheet to a reduction ratio of 15 to 75%, batch annealing the cold rolled steel sheet at a temperature of 620 to 650 deg.C for 1 to 12 hours, and air cooling the batch annealed steel sheet.

Description

Ultra high strength steel of 120 방법 Gｆ / mm2 grade with excellent workability and manufacturing method {Ultra high strength steel of 120kgf / mm2 grade having excellent formability}

The present invention, more particularly, the strength and 120kgf / a The part molded easily and workability is excellent mm ² class, with about the ultra-high strength steel of a 120 kgf / mm ² grade is used in the shock absorber in the bumper reinforcement or the door It relates to an ultra high strength steel and a method of manufacturing the same.

The bumper reinforcement of the vehicle or the shock absorber in the door is a part directly related to the safety of the passenger in the event of a vehicle collision, and high strength cold rolled steel sheets having a tensile strength of 80 kgf / mm ² or more are mainly used. This cold rolled steel sheet should have high tensile strength and low yield ratio and high elongation to improve workability. In addition, the use of high strength steels is increasing in order to increase fuel economy in response to the increasingly severe environmental pollution regulations. Recently, research on the commercialization of ultra high strength steel of 120kgf / mm ² or more is increasing.

High-strength steel for automobiles is typically Transformation Induced Plasticity (hereinafter simply referred to as TRIP) steel and Dual Phase (hereinafter referred to as DP) steel.

The manufacturing process of high strength steel for automobile is divided into hot rolling, cold rolling and annealing processes. The hot rolled sheet having ferrite and pearlite is cold rolled to be processed to the final thickness of the product, and then heated to an annealing temperature of A ₁ transformation point or more. And cool. At this time, the austenite formed in the annealing process is transformed into martensite or bainite by controlling the speed in the cooling process. In this case, the steel is called an ideal tissue steel. The abnormal tissue steel has an increase in strength as the fraction of martensite increases and ductility increases as the ferrite ratio increases. If the martensite ratio is too large for the strength increase, the ratio of ferrite decreases and the ductility decreases. That is, when manufacturing high strength steel of 120 kgf / mm ² or more as in the present invention has a drawback that the elongation is sharply reduced.

On the other hand, there is a method to increase the strength and ductility of the metamorphic tissue steel at the same time by forming austenite in the annealing process and controlling the cooling rate and the cooling end temperature in the cooling process and remaining austenite at room temperature. . That is, when the residual austenite is transformed into martensite by processing during plastic deformation, the transformation phase formed by the plastic organic transformation along with the strength increases the ductility by relaxing local stress concentration. (TRansformation Induced Plasticity Steel, TRIP Steel) (CAMP-ISIJ vol. 1, p. 877). The TRIP steel is widely used as a high strength steel because it has excellent strength and ductility at the same time.

In the metamorphic organic plastic steel, it is important to maintain a metastable austenite at a predetermined fraction or more at room temperature. To this end, Si, Al, P, etc. are added to increase the carbon activity in the ferrite, and the formation of carbides is suppressed to increase the amount of carbon in the austenite so that austenite remains. In order to suppress carbide formation, that is, bainite transformation, there is a method of retaining austenite by adding an element that inhibits austenite transformation into bainite during continuous cooling after annealing and delaying bainite transformation. However, in order to obtain a large amount of retained austenite by the above method, at least 0.2% by weight of carbon and at least 2.0% by weight of Si must be added. Therefore, the flash-butt weldability is very deteriorated by carbon, silicon, etc. when manufacturing a hot rolled steel sheet. In addition, when welding the cold rolled sheet material for cold rolling, there is a difficulty in the process as well as the use of the parts to be manufactured by welding after manufacturing to the final product.

The present invention is to solve the above problems of the prior art, it is possible to prevent the deterioration of plating and weldability due to the addition of a large amount of carbon and silicon, the tensile strength of 120kgf / mm ² or more grade and yield ratio of 0.6 or less In order to provide a steel exhibiting high strength and excellent workability at the same time and a method of manufacturing the same, an object thereof is provided.

The present invention for achieving the above object by weight, C: 0.10 ~ 0.15%, Si: 0.7 ~ 1.7%, Mn: 4.0 ~ 7.0%, P: 0.03% or less, S: 0.005% or less, Al: 0.05 0.15% or less, N: 0.003% or less, B: 0.0001 to 0.002%, Mo: 0.01 to 0.05%, and remaining Fe and other unavoidable impurities.

In addition, the present invention is in weight%, C: 0.10 to 0.15%, Si: 0.7 to 1.7%, Mn: 4.0 to 7.0%, P: 0.03% or less, S: 0.005% or less, Al: 0.05 to 0.15%, N : 0.003% or less, B: 0.0001 to 0.002%, Mo: 0.01 to 0.05%, reheating the steel slab composed of remaining Fe and other unavoidable impurities at 1200 to 1250 ° C. for 60 to 180 minutes;

Hot-rolling the reheated slab at Ar ₃ to Ar ₃ + 100 ° C. and winding at 560 to 680 ° C .; And

And cold rolling the wound hot rolled sheet at a reduction ratio of 15 to 75%, and annealing for 1 to 12 hours at 620 to 650 ° C., followed by air cooling.

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

The present invention is characterized in preventing the plating and weldability deterioration due to silicon and carbon, and producing steel having strength and excellent workability of 120 kgf / mm ² or more. In addition, in the present invention, 4 to 7% by weight of Mn is added. In this case, there is a problem in that the structure of the hot rolled sheet is changed to martensite and the load in cold rolling increases. In addition, after annealing, the microstructure becomes a layered structure of the retained austenite and martensite formed in the martensite las boundary, and since the overall structure is coarse, the elongation tends to decrease in comparison with the hot rolled steel sheet despite the high residual austenite fraction. . Therefore, there is a feature to solve the problem of adding a large amount of Mn by optimizing the cold reduction amount, annealing time and cooling conditions proposed in the present invention.

C: 0.10 to 0.15 wt%

Carbon [C] in steel is the most important component in steel materials, and it has the greatest influence on the properties of steel that is closely related to all physical and chemical properties such as strength, toughness and corrosion resistance. In the case of the metamorphic organic-plastic steels such as the present invention, especially when the amount of carbon is less than 0.10% by weight, the residual austenite stability is low and the fraction is reduced, thereby greatly reducing the strength and ductility, and when exceeding 0.15% by weight, the weldability decreases and the second phase fraction Since there is a problem such as deterioration of workability due to the sharp increase of the content, it is preferable to limit the content to 0.10 ~ 0.15% by weight.

Si: 0.7-1.7 wt%

The Si contributes to strength as a ferrite stabilizing element which is dissolved in ferrite, and is generally added as a deoxidizer. In the case of metamorphic organic plastic steel, Si is an important element that increases the amount of carbon in austenite by increasing the activity of carbon in ferrite, thereby improving the austenite stability. If the content of Si is less than 0.7% by weight, the fraction of retained austenite is reduced and the strength is lowered. If the content of Si is more than 1.7% by weight, not only the hot roll scale is caused, but also the weldability is deteriorated. It is preferable to limit the weight percentage.

In addition, in this invention, said Si is a component which contributes to formation of residual austenite by suppressing bainite transformation when cooling rate is moderately controlled.

Mn: 4-7% by weight

The Mn is an austenite stabilizing element that facilitates formation of low-temperature transformation phases such as acicular ferrite and bainite, that is, increases hardening ability and increases strength. As in the present invention, carbon and silicon are added in the range of 0.10 to 0.15% by weight and 0.7-1.7% by weight, and in order to have a strength of 120kgf / mm ² or more, more than 4% by weight should be added, and if it is added in excess of 7% by weight, weldability This decreases, and not only causes hydrogen organic brittleness by forming inclusions, but also forms a segregation zone in the center of the sheet during hot rolling, so that the content thereof is preferably limited to 4 to 7% by weight.

P: 0.03 wt% or less

If the P is added in excess of 0.03% by weight, brittleness increases and weldability is lowered, so the content thereof is preferably limited to 0.03% by weight or less.

S: 0.005 wt% or less

When the S content exceeds 0.005% by weight, coarse TiS and MnS are formed in the hot rolled sheet, thereby decreasing workability and toughness. Therefore, the content is preferably limited to 0.005% by weight or less.

Al: 0.05 ~ 0.15 wt%

The Al is added for deoxidation and is an effective component for making fine grains of austenite at a high temperature by combining with N in steel to form AlN. If the amount of Al added is less than 0.05% by weight, the effect of the addition cannot be obtained. If the amount of Al is exceeded, the content of 0.05 to 0.15% is caused because the toughness of the coarse precipitate is formed and the production cost is increased. It is desirable to limit to%.

N: 0.003 wt% or less

When N is added in excess of 0.003% by weight, coarse nitrides are formed to reduce toughness, so the content thereof is preferably limited to 0.003% by weight or less.

B: 0.0001-0.002 wt%

B is a component that increases the hardenability of the steel even when a small amount is added to the steel, and facilitates the formation of low-temperature transformation phases such as ecicular ferrite and bainite. If it is added more than 0.0001% by weight, it segregates at the austenite grain boundary at high temperature, thereby suppressing ferrite formation, contributing to the hardenability of the steel, and when it is added more than 0.002% by weight, the weldability is degraded and the recrystallization temperature is excessively increased, which lowers the drawing ability. The content is preferably limited to 0.0001 to 0.002% by weight.

Mo: 0.01 ~ 0.05% by weight

The Mo is added to increase the strength through improving the hardenability of the steel, the amount of increase is less when the Mo content is less than 0.01% by weight, and when the content exceeds 0.05% by weight of the hard phase to increase the elongation to decrease In addition, the weldability not only decreases, but also increases the manufacturing cost, so that the content is preferably limited to 0.01 to 0.05% by weight.

In addition to the above components, the remainder is composed of Fe and other unavoidable impurities.

The slabs having the composition as described above are obtained by casting or continuous casting after obtaining molten steel through a steelmaking process. The slab is manufactured from a cold rolled steel sheet having a target mechanical property through a reheating process, a hot rolling process, a winding process, a cold rolling process, and an annealing process. Hereinafter, manufacturing conditions for each process will be described in detail.

Reheating process

The slab, which is formed as described above, is reheated at 1200 to 1250 ° C. for 60 to 180 minutes. If the reheating temperature is less than 1200 ℃ coarse carbides formed during continuous casting is difficult to re-dissolve, if it exceeds 1250 ℃ the cost required to maintain the furnace temperature is greatly increased, the reheating temperature is limited to 1200 ~ 1250 ℃ It is preferable. When the reheating time is less than 60 minutes, the coarse carbides formed during continuous casting are hardly re-dissolved as in the case where the temperature is less than 1200 ° C., and when the reheating time is longer than 180 minutes, the productivity is lowered, so the reheating time is limited to 60 to 180 minutes. It is preferable.

Hot rolling process

Thereafter, the heated slabs are finished hot rolled at Ar ₃ to Ar ₃ + 100 ° C. If the finishing hot rolling temperature and Ar _{3 is} less than a lot of dislocations are introduced into the ferrite formed during hot rolling, and these ferrites grow during cooling or winding to form surface coarse grains and increase the load of the rolling rolls when rolling at low temperatures. If it exceeds ₃ + 100 ℃ temperature operation difficulties the finish, so causing the hot rolling it is preferably limited to the _{Ar 3 ~ Ar 3 + 100 ℃} .

Winding process

Then, the hot rolled rolled sheet is wound at 560 ~ 680 ℃. If the coiling temperature is less than 560 ℃, not only the productivity is lowered, but also the delay of the precipitation of carbon in the steel, the drawing property is lowered, if it exceeds 680 ℃ carbides are formed too coarse to be difficult to re-use when annealing and there is a non-uniform structure inside Induces a great deterioration of workability, so the odor temperature is preferably limited to 560 ~ 680 ℃.

Cold rolling process

The wound hot rolled sheet is cold rolled at a reduction ratio of 15 to 75%. The cold reduction rate is a variable that greatly affects the microstructure in addition to the hot rolling conditions. If the cold reduction rate is less than 15%, the workability is low because the tissue is not uniform. If the cold reduction rate exceeds 75%, the cold reduction rate is increased. Is preferably limited to 15-75%.

In the present invention, looking at the effect of cold reduction rate on the microstructure, as the cold reduction rate increases, the lattice form formed in the hot rolling is decomposed and converted into granular form, thus forming 2 after annealing-cooling. Next-generation morphology (size, shape and distribution) will also change. At this time, the morphology of the retained austenite, the secondary phase formed, is also changed, and the mechanical properties are changed. In the case of residual austenite enclosed in the hard phase, even if sufficient deformation is applied, it cannot be converted into martensite and thus does not contribute to ductility.

Annealing Process

The cold rolled sheet is subjected to annealing for 1 to 12 hours at 620 to 650 ° C. If the annealing temperature is less than 620 ℃, it is difficult to restock enough to maintain the coarse structure at the time of hot rolling, so the workability is lowered, and if it exceeds 650 ℃ fine structure becomes coarse to decrease the strength, the annealing temperature is 620 ~ It is preferable to limit to 650 ° C. The annealing time greatly affects the strength and elongation, and the cold-rolled sheet must be sufficiently recrystallized to secure the elongation while having a strength of 120kgf / mm ² , so that the annealing time should be maintained for at least 1 hour. Since the energy consumption required for lowering and maintaining the high temperature increases and the manufacturing cost is increased, the annealing time is preferably limited to 1 to 12 hours.

Since cooling is preferably carried out by air cooling to prevent elongation decrease due to the increase in strength in consideration of the annealing conditions.

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

EXAMPLE

The slabs having the composition of Table 1 were reheated, finished hot rolled, wound, cold rolled and annealed under the conditions of Table 2 below, followed by air cooling.

Steel grade C Mn Si P S Al N Mo B Comparative Steel A 0.15 6.5 0.5 0.02 0.002 0.11 0.0017 0.05 0.0015 Inventive Steel A 0.15 6.5 1.0 0.02 0.002 0.10 0.0020 0.05 0.0013 Inventive Steel B 0.15 6.5 1.5 0.02 0.002 0.13 0.0019 0.05 0.0015 Comparative Steel B 0.1 4 0.5 0.02 0.002 0.11 0.0017 0.05 0.0015 Comparative Steel C 0.15 4 0.5 0.02 0.002 0.11 0.0017 0.05 0.0015 Comparative Steel D 0.2 4 0.5 0.02 0.002 0.11 0.0017 0.05 0.0015

division Steel grade Reheating Temperature (℃) Reheat time (minutes) Finishing Hot Rolling Temperature (℃) Winding temperature (℃) Cold rolling reduction (%) Annealing Temperature (℃) Annealing time (hours) Comparative Example 1 Comparative Steel A 1200 60 900 620 15 645 12 Comparative Example 2 1200 60 900 620 30 645 12 Comparative Example 3 1200 60 900 620 45 645 One Comparative Example 4 1200 60 900 620 60 645 One Inventive Example 1 Inventive Steel A 1200 60 900 620 30 645 12 Inventive Example 2 1200 60 900 620 45 645 12 Inventive Example 3 1200 60 900 620 60 645 12 Inventive Example 4 1200 60 900 620 75 645 12 Inventive Example 5 Inventive Steel B 1200 60 900 620 15 645 12 Inventive Example 6 1200 60 900 620 30 645 12 Inventive Example 7 1200 60 900 620 45 645 12 Inventive Example 8 1200 60 900 620 60 645 12 Inventive Example 9 1200 60 900 620 75 645 12 Comparative Example 5 Comparative Steel B 1200 60 900 620 45 620 12 Comparative Example 6 1200 60 900 620 45 645 12 Comparative Example 7 Comparative Steel C 1200 60 900 620 45 620 12 Comparative Example 8 1200 60 900 620 45 645 12 Comparative Example 9 Comparative Steel D 1200 60 900 620 45 620 12 Comparative Example 10 1200 60 900 620 45 645 12

Yield ratios representing strength, elongation and processability were calculated using the specimens prepared as described above, and the results are shown in Table 3 below.

division Yield strength (kgf / ㎡) Tensile strength (kgf / ㎡) Uniform elongation (%) % Total elongation Yield fee Comparative Example 1 69.01 113.20 29.44 34.72 0.61 Comparative Example 2 68.30 116.92 21.98 31.69 0.58 Comparative Example 3 82.75 107.68 26.79 32.40 0.77 Comparative Example 4 83.16 109.11 23.07 30.08 0.76 Inventive Example 1 65.40 124.92 14.49 24.20 0.52 Inventive Example 2 65.90 126.87 16.30 19.30 0.52 Inventive Example 3 66.00 129.33 13.66 18.00 0.51 Inventive Example 4 64.50 130.35 9.62 15.00 0.49 Inventive Example 5 64.20 129.50 15.32 20.60 0.50 Inventive Example 6 64.10 131.00 8.69 18.40 0.49 Inventive Example 7 63.80 135.20 12.90 15.90 0.47 Inventive Example 8 63.90 138.40 10.26 14.60 0.46 Inventive Example 9 62.40 138.70 8.12 13.50 0.45 Comparative Example 5 80.50 93.60 33.20 40.00 0.86 Comparative Example 6 80.50 93.60 33.20 40.00 0.86 Comparative Example 7 43.00 57.80 21.20 35.20 0.74 Comparative Example 8 38.60 57.70 24.00 37.40 0.67 Comparative Example 9 42.30 60.40 18.40 34.50 0.70 Comparative Example 10 39.50 59.90 22.50 37.20 0.66

The workability of the plate is closely related to the yield ratio of the material. The lower the yield ratio, the better the workability.

As can be seen in Table 3, Inventive Examples 1 to 9 according to the present invention can be seen that not only has a tensile strength of 120kgf / mm ² or more, but also has a yield ratio of 0.6 or less, very excellent workability. However, it can be seen that Comparative Examples 1 to 10 outside the scope of the present invention have a tensile strength of less than 120 kgf / mm ² or a yield ratio of 0.6 or more.

1 is a graph showing the tensile strength and yield strength according to the cold reduction rate of the inventive steels A, B and comparative steel A prepared according to the present invention. As can be seen in Figure 1, the invention steels A, B according to the present invention has a tensile strength of 120kgf / mm ² or more, it can be seen that the comparative steel A does not.

In addition, in the case of the invention steel, as the cold reduction rate increases and the Si content increases, the tensile strength and the yield strength increase, which means that the structure becomes finer as the cold reduction rate increases, and the stability of the austenite increases as the Si content increases. This is because the work hardening rate increases.

2 is a graph showing the total elongation and yield ratio according to the cold reduction rate of the inventive steels A, B and comparative steel A prepared according to the present invention. As can be seen in Figure 2, the invention steels A, B according to the present invention has a yield ratio of 0.6 or less, but very excellent workability, it can be seen that comparative steel A does not.

As described above, according to the present invention, a high-strength steel having excellent workability having a high tensile strength of 120 kgf / mm ² and a low yield ratio of 0.6 or less can be manufactured. There is an effect that can be easily manufactured parts.

1 is a graph showing the tensile strength and the yield strength according to the cold reduction rate of the inventive steel and comparative steel produced according to the present invention.

2 is a graph showing the total elongation and yield ratio according to the cold reduction rate of the inventive steel and comparative steel produced according to the present invention.

Claims (2)

By weight%, C: 0.10 to 0.15%, Si: 0.7 to 1.7%, Mn: 4.0 to 7.0%, P: 0.03% or less, S: 0.005% or less, Al: 0.05 to 0.15%, N: 0.003% or less, B: 0.0001 ~ 0.002%, Mo: 0.01 ~ 0.05%, 120kgf / mm grade ² super high strength steel with excellent workability including remaining Fe and other unavoidable impurities. By weight%, C: 0.10 to 0.15%, Si: 0.7 to 1.7%, Mn: 4.0 to 7.0%, P: 0.03% or less, S: 0.005% or less, Al: 0.05 to 0.15%, N: 0.003% or less, B: 0.0001 to 0.002%, Mo: 0.01 to 0.05%, reheating the steel slab composed of the remaining Fe and other unavoidable impurities at 1200 to 1250 ° C. for 60 to 180 minutes; Hot-rolling the reheated slab at Ar ₃ to Ar ₃ + 100 ° C. and winding at 560 to 680 ° C .; And Cold rolled hot rolled sheet at a reduction ratio of 15 to 75%, and annealing for 1 to 12 hours at 620 to 650 ° C., followed by air cooling; 120 kgf / mm grade ² high strength steel having excellent processability, including Manufacturing method.