KR100711359B1 - Bake-hardening thin steel sheet having excellent anisotropy and the method for manufacturing the same - Google Patents

Abstract

Mainly used for automotive exterior plates, there is provided a hardening type high strength steel sheet having excellent in-plane anisotropy and a method of manufacturing the same.

This thin steel sheet is in weight%, C: 0.0015 to 0.0025%, Mn: 0.03 to 0.2%, P: 0.06 to 0.1%, S: 0.003 to 0.015%, Sol.Al: 0.01 to 0.08%, N: 0.006 to 0.01 %, Cu: 0.005-0.2%, Sb: 0.02-0.1%, remaining Fe and other inevitable impurities, wherein Mn, Cu and S are 6.7≤ (Mn / 1.7S) + (Cu / 1.96S) ≤14.6 75% or more of MnS, CuS, and (Mn, Cu) S precipitates having a size of 20 nm or less are satisfied.

According to the present invention, it is possible to provide a hardened hardened steel sheet having a tensile strength of 390 MPa or more and an in-plane anisotropy (Δr) of 0.15 or less.

In-plane anisotropy, hardening hardening, sheet steel, high strength, MnS, strain organic dynamic transformation, hot rolling

Description

Bake-hardening thin steel sheet having excellent anisotropy and the method for manufacturing the same

Domestic patent application number 2005-0088518

Domestic patent application number 2005-0088517

Domestic patent application number 2004-0111706

The present invention relates to a steel sheet which is mainly used for automobile shell plates. More specifically, the present invention relates to a hardened hardened steel sheet and a method of manufacturing the same, which secure a tensile strength of 390 MPa or more and have excellent in-plane anisotropy.

Steel sheets, which are mainly used as automotive exterior materials, require excellent dent resistance. In order to secure such dent resistance, a hardening hardened steel sheet is mainly used. The hardened hardened steel sheet is a steel having a high yield point by retaining an appropriate amount of solid solution carbon in the steel sheet and using the heat of the coating element bush to fix the potential generated during press forming.

There are Al-Killed steel and IF steel (Interstitial Free Steel).

In the case of Al-Killed steel, which is an ordinary annealing material, a small amount of solid carbon remains, and it has a hardening hardening capacity of about 10 to 20 Mpa after the quenching treatment while securing aging characteristics. The upper annealing material has the disadvantages of low yield strength and low productivity after baking.

In the case of IF steel, Ti and Nb are added to completely precipitate the carbon or nitrogen dissolved in the steel to improve moldability. The hardening hardening characteristic is given to the IF steel by hardening. The baking hardening type IF steel controls the adding amount of Ti or Nb and the adding amount of carbon so that an appropriate amount of carbon remains in the steel to give the baking hardening characteristic.

In the case of small hardening IF steel, in order to employ an appropriate amount of carbon, not only the amount of carbon added, but also the amount of Ti or Nb added, as well as the amount of sulfur and nitrogen that react with Ti and Nb to form precipitates are controlled in a very narrow range. Since it is difficult to secure stable quality, there is a problem that also takes a lot of production costs.

Conventional techniques for improving this are domestic patent applications No. 2005-88518 and 2005-88517. The prior arts add about 0.08 to 0.12% of Al content to ultra-low carbon steels of about 25 ppm or less to produce steels having greatly improved secondary processing brittleness by grain refining effect (more than ASTM No. 9) by AlN precipitates. Doing.

However, inferior in workability due to unprecipitated remaining solid solution carbon in the steel, the in-plane anisotropy is disadvantageous at the level of 1.5 to 1.6 based on the r-value, and thus there is a problem in that the processability is not greatly improved in forming the automobile parts.

Meanwhile, in Korean Patent Application No. 2004-111706, in order to manufacture hardened hardened steel, 0.1% Cu is added without adding carbon and nitride (Ti, Nb, etc.) forming elements at all to secure strength, and N content is weakened. It is proposed a method of producing hardened hardened steel that has a strength compensation effect by AlN precipitation by adding more than 70ppm.

However, the above-mentioned prior art exhibits a workability, in particular, an in-plane anisotropy of 0.3 or more due to the residual solid solution C, and thus has a disadvantage inherent in solving the problem of workability.

The present invention is to improve the above-mentioned conventional problems, by appropriately controlling the size and content of the precipitates, and hot rolling process, to provide a small-hardened type steel sheet and a manufacturing method having a high strength of 390MPa or more and excellent in-plane anisotropy There is a purpose.

The present invention for achieving the above object, in weight%, C: 0.0015 ~ 0.0025%, Mn: 0.03 ~ 0.2%, P: 0.06 ~ 0.1%, S: 0.003 ~ 0.015%, Sol.Al: 0.01 ~ 0.08% , N: 0.006% to 0.01%, Cu: 0.005% to 0.2%, Sb: 0.02% to 0.1%, remaining Fe and other unavoidable impurities, wherein Mn, Cu, and S are 6.7≤ (Mn / 1.7S) + (Cu /1.96S) ≤ 14.6 and relates to a small hardening type steel sheet having excellent in-plane anisotropy containing 75% or more of MnS, CuS, (Mn, Cu) S precipitates having a size of 20nm or less.

In addition, the present invention is a weight%, C: 0.0015 ~ 0.0025%, Mn: 0.03 ~ 0.2%, P: 0.06 ~ 0.1%, S: 0.003 ~ 0.015%, Sol.Al: 0.01 ~ 0.08%, N: 0.006 ~ 0.01%, Cu: 0.005-0.2%, Sb: 0.02-0.1%, and is composed of the remaining Fe and other unavoidable impurities, wherein Mn, Cu and S are 6.7≤ (Mn / 1.7S) + (Cu / 1.96S) ≤ After reheating the steel slab that satisfies the relationship of 14.6, and controlling the average grain size of the austenitic structure to be 50 µm or less before finishing rolling, finish with a total reduction ratio of 60% or more in the temperature range of Ae ₃ to Ar ₃ . Hot rolled ferrite grain size is maintained at 5㎛ or less by rolling and then wound at a temperature of 680 ~ 720 ℃, cold rolled at 75% or more reduction rate, and then annealing hardening type with excellent in-plane anisotropy which is continuously annealed at 780 ~ 830 ℃. It relates to a method for producing a thin steel sheet.

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

The inventors of the present invention studied the method for improving the in-plane anisotropy of the hardened steel sheet without Ti and Nb, and used the Strain Induced Dynamic Transformation (SIDT) during hot rolling by appropriately controlling the hot rolling process. As a result, dynamic recrystallization of the microstructures occurred, and at the same time, the microstructures were formed to ensure excellent in-plane anisotropy, and the present invention was completed based on the results.

In addition, the present invention is characterized by ensuring the high strength by controlling the size and content of MnS, CuS, (Mn, Cu) S precipitates. First, the composition range of the steel component of the present invention will be described.

C: 0.0015 to 0.0025% is preferable.

The C is added to ensure the baking hardening of 30 ~ 50MPa. If the content is less than 0.0015% does not secure the baking hardening target of the present invention, if the content exceeds 0.0025% baking hardening increases but the aging resistance is poor, the possibility of surface wrinkles during processing increases. The content of C is preferably limited to 0.0015 ~ 0.0025%.

Mn: 0.03-0.2% is preferable.

Mn is known as an element that precipitates solid solution S in steel as MnS to prevent hot shortness due to solid solution S. In the present invention, the strength and in-plane anisotropy are greatly improved by controlling the content of Mn and S to manage Mn content of 0.03 to 0.2% so that very fine MnS can be precipitated. If the content is less than 0.03%, it is difficult to secure the above effects, while if the content exceeds 0.2%, coarse MnS precipitates are formed, and the aging resistance is likely to be poor, so the Mn content is limited to 0.03 to 0.2%. It is desirable to.

P: 0.06 to 0.1% is preferable.

The P is a representative solid solution element added to increase the strength with Mn. As the amount of P is increased, the strength synergistic effect is increased, while excessive P addition is likely to cause a problem of secondary brittleness in the IF steel. Therefore, it is important to add an appropriate amount of P. In the present invention, it is possible to lower the P content in the steel somewhat by using a technique for securing strength using fine precipitates. If the content is less than 0.06%, the strength targeted by the present invention cannot be secured, whereas if the content exceeds 0.1%, the possibility of secondary processing brittleness increases, so the content of P is preferably limited to 0.06 to 0.1%. .

S: 0.003-0.015% is preferable.

When the content of S is less than 0.003%, the content of MnS, CuS, and (Mn, Cu) S precipitates is not only low, but the precipitates are very coarse, which is likely to inhibit strength and aging resistance. On the other hand, if the content exceeds 0.015%, the content of solid solution S increases and greatly inhibits the ductility and formability, and there is a fear that red brittleness may occur, it is preferable to limit the content of S to 0.003 ~ 0.015%.

Sol. Al: 0.01 to 0.08% is preferred.

The Sol.Al significantly contributes to the strength improvement by depositing the added N and AlN while keeping the dissolved oxygen content in the steel sufficiently low. From this effect, in the present invention, by lowering the content of solid solution strengthening elements (P, Mn, etc.) it can greatly contribute to the improvement of secondary processing brittleness and plating surface properties. When the content of Sol.Al is less than 0.01%, an effective AlN precipitate cannot be formed. When the content of Sol.Al is greater than 0.08%, the AlN precipitate is coarsened to reduce the strength-contributing effect. Thus, the content of Sol.Al is 0.01 to 0.08%. It is desirable to limit.

N: 0.006-0.01% is preferable.

When N is in a solid solution state, it affects the baking hardening characteristics, and when precipitated with AlN, a strength-contributing effect is exhibited. If the content is less than 0.006%, the strength contribution due to the fine AlN precipitation cannot be secured, whereas if the content is more than 0.01%, the aging resistance may be inferior due to the excessive solid solution N. Therefore, the content of N is preferably limited to 0.006 ~ 0.01%.

Cu: 0.005-0.2% is preferable.

The Cu increases the strength of the steel sheet, and if the content is less than 0.005%, it is difficult to secure the target strength in the present invention, whereas if it exceeds 0.2%, Cu-based precipitates are coarsened, which is greatly advantageous in terms of strength improvement. In addition, manufacturing cost increases. Therefore, the content of Cu is preferably limited to 0.005 ~ 0.2%.

Sb: 0.02-0.1% is preferable.

The Sb improves the plating characteristics by preventing the Si and Mn oxides from eluting to the surface of the steel sheet during annealing. That is, after hot rolling, the Sb mainly segregates at the grain boundary and blocks the movement path of Mn and Si oxide through the grain boundary to lower surface defects, thereby securing excellent plating characteristics.

If the content is less than 0.02%, there is little effect of inhibiting the passage of Mn and Si oxides, whereas if the content is more than 0.1%, excess Sb exists in solid solution and inhibits the stretching property of the steel, so the content of Sb is 0.02 ~ 0.1%. It is preferable to limit to.

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

In the present invention, Mn, Cu, and S satisfy a relationship of 6.7≤ (Mn / 1.7S) + (Cu / 1.96S) ≤14.6.

When the relational value is less than 6.7, there is little precipitation effect, so it is difficult to secure the target strength in the present invention, whereas when the relational value exceeds 14.6, a large amount of coarse precipitates are formed to inhibit the strength improvement, so the relational expression is limited to 6.7 to 14.6. It is desirable to.

In addition, the steel sheet of the present invention contains 75% or more of MnS, CuS, (Mn, Cu) S precipitates having a size of 20nm or less.

If the size of the precipitate exceeds 20nm, it does not contribute significantly to securing the strength, even if the amount of the precipitate is less than 75% does not secure the strength targeted in the present invention. Therefore, it is desirable to limit the precipitates having a size of 20 nm or less to 75% or more.

In addition, the present invention uses a strain induced dynamic transformation (SIDT) during hot rolling to improve the in-plane anisotropy of the hardened hardened steel sheet, which will be described below.

SIDT means that when the steel is processed in the temperature range of Ae ₃ ~ Ar ₃ to give a deformation to the structure, the austenite phase is easily transformed to ferrite by deformation. In the present invention, a part of the ferrite is already formed at the moment of finishing the finish rolling by using SIDT in the hot rolling process, and then the ferrite transformation is promoted in the cooling process to form very fine grains.

Hereinafter, the manufacturing method of the thin steel plate which has the steel comprised as mentioned above is demonstrated in detail.

First, after reheating the steel slab formed as described above, it is controlled so that the average grain size of the austenite structure is 50 µm or less before finishing rolling. If it exceeds 50㎛, it is highly likely that the dynamic transformation does not occur effectively. Therefore, in order to effectively use strain organic dynamic transformation by hot working steel of austenitic structure, the average grain size of the austenitic structure before finishing rolling should be below 50㎛. It is preferable to limit to.

Then, finish rolling is performed at a total reduction ratio of 60% or more in the temperature range of Ae ₃ to Ar ₃ to maintain the hot-rolled ferrite grain size at 5 μm or less. The finely formed structure maintains fine grains even after cold rolling and annealing to develop {111} texture structure which is advantageous for the workability of steel, and also increases r45 value of plastic anisotropy in each direction. It can be prepared.

If the finish rolling temperature is lower than Ar ₃ is when the river tissue 2 sangyeok (ferrite + austenite) is ferrite coarse the interval is the non-transformed, the residue may have a final grain coarse, the excess of the Ae ₃ Austenite As the temperature increases in the single phase region, coarse austenite is likely to be transformed into coarse ferrite, so the finish rolling temperature is preferably limited to Ae ₃ to Ar ₃ .

When the total reduction ratio is less than 60%, the possibility of the rapid growth of ferrite grain growth during cooling increases, so it is preferable to limit the total reduction ratio to 60% or more.

Subsequently, the hot rolled sheet is wound at a temperature of 680-720 ° C., cold rolled at a reduction ratio of 75% or more, and then continuously annealed at 780-830 ° C. When the coiling temperature is less than 680 ° C, Cu precipitates are not effectively formed, and the solid solution C content is increased, whereas when the coiling temperature exceeds 720 ° C, the precipitates grow too coarse and there is little contribution to improving strength. Therefore, the winding temperature is preferably limited to 680 ~ 720 ℃.

In addition, as the cold rolling rate increases, the r-value, which is a workability evaluation index, increases, but it is desirable to limit the minimum rolling reduction to 75% in consideration of the roll load in the field work.

In addition, when the continuous annealing temperature is less than 780 ℃, it is difficult to secure a high strength steel with excellent stretching characteristics, while if the continuous annealing temperature is higher than 830 ℃ due to high temperature annealing, there is a very high risk of problems such as stripping of strips in operation plating properties Since it is inferior to, the continuous annealing temperature is preferably limited to 780 ~ 830 ℃.

In the present invention, the steel of the present invention continuously annealed can be alloyed by a conventional method.

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

EXAMPLE

The steel slab formed as shown in Table 1 was reheated at 1200 ° C., finished rolled at a reduction ratio of 60% or more under the manufacturing conditions as shown in Table 2, cold rolled, rolled up at 700 ° C., and then continuously annealed.

The obtained annealing plate was processed into a standard specimen according to the ASTM E-8 standard to investigate the mechanical properties. The specimen was measured for tensile strength, elongation, plastic anisotropy index (rm value) and in-plane anisotropy index (Δr value) using a tensile tester (INSTRON, Model 6025).

Where r _m = (r ₀ + 2r ₄₅ + r ₉₀ ) / 4 and Δr = (r ₀ −2r ₄₅ + r ₉₀ ) / 2. The evaluation of the secondary workability brittleness (DBTT) was evaluated by measuring the Ductile-Brittle Transition Temperature (DBTT) by dropping the weight and dropping the weight of the cup formed under the condition of processing ratio 1.9. .

Table 2 shows the mechanical properties and secondary work resistance of the inventive steel and the comparative steel.

Sample number Chemical composition (% by weight) C Mn P S Al N Cu Sb Inventive Steel 1 0.0017 0.07 0.07 0.006 0.05 0.007 0.11 0.03 Inventive Steel 2 0.0018 0.11 0.075 0.008 0.045 0.0071 0.11 0.03 Invention Steel 3 0.0020 0.15 0.072 0.0082 0.04 0.0074 0.10 0.03 Inventive Steel 4 0.0023 0.18 0.068 0.0085 0.045 0.0068 0.09 0.03 Inventive Steel 5 0.0018 0.15 0.071 0.012 0.05 0.0065 0.11 0.03

division Remarks Operating conditions Mechanical property Finish rolling (℃) Cold rolling reduction (%) Annealing Temperature (℃) Hot Rolled Grain Size (㎛) Tensile strength (MPa) Elongation (%) R-value △ r in-plane anisotropy DBTT (℃) Precipitate distribution (≤20nm) Inventive Steel 1 Invention 1 878 76 805 3.5 401 37.5 1.78 0.11 -50 78% Inventive Steel 1 Invention 2 869 77 803 3.2 398 38.1 1.75 0.10 -50 77% Inventive Steel 2 Invention 3 875 77 802 3.3 395 37.6 1.81 0.12 -50 81% Invention Steel 3 Invention 4 869 76 795 4.2 396 36.8 1.82 0.08 -55 80% Invention Steel 3 Invention 5 871 78 810 4.5 402 38.2 1.85 0.13 -50 82% Inventive Steel 4 Invention 6 870 75 802 3.8 401 38.5 1.78 0.03 -50 79% Inventive Steel 5 Invention Material7 868 77 806 3.6 403 38.7 1.83 0.08 -50 77% Inventive Steel 1 Comparative Material 1 922 78 803 7.8 396 37.8 1.69 0.45 -20 55% Inventive Steel 2 Comparative Material 2 924 78 807 9.6 395 38.2 1.65 0.39 -30 45%

As shown in Tables 1 and 2, in the case of the inventive materials (1-7) manufactured according to the production method of the present invention using the inventive steels (1-5) satisfying the component range of the present invention, 20 nm or more, 20 nm The following fine precipitates were formed to secure high strength steel with a tensile strength of 390 MPa or more. In addition, the inventive materials (1 to 7) having fine hot-rolled grains of 4.5 µm or less secured excellent in-plane anisotropy with R-value of plastic anisotropy index of 1.75 or more and Δr value of in-plane anisotropy index of 0.12 or less.

However, in the case of the comparative materials (1, 2) that do not satisfy the manufacturing conditions of the present invention, it does not satisfy the finish rolling temperature of the present invention, it exhibits coarse hot-rolled ferrite grains and low fine precipitate distribution R-value and in-plane anisotropy for heat Indicated.

As described above, according to the present invention, by appropriately controlling the size and content of the precipitates, and the hot rolling process, there is an effect that it is possible to provide a small hardened steel sheet having excellent in-plane anisotropy while having a high strength of 390 MPa or more of tensile strength.

Claims (2)

By weight%, C: 0.0015 to 0.0025%, Mn: 0.03 to 0.2%, P: 0.06 to 0.1%, S: 0.003 to 0.015%, Sol.Al: 0.01 to 0.08%, N: 0.006 to 0.01%, Cu: 0.005 to 0.2%, Sb: 0.02 to 0.1%, remaining Fe and other inevitable impurities, and Mn, Cu and S satisfy the relationship of 6.7≤ (Mn / 1.7S) + (Cu / 1.96S) ≤14.6 And, a small hardening type steel sheet having excellent in-plane anisotropy containing 75% or more of MnS, CuS, (Mn, Cu) S precipitates having a size of 20nm or less. By weight%, C: 0.0015 to 0.0025%, Mn: 0.03 to 0.2%, P: 0.06 to 0.1%, S: 0.003 to 0.015%, Sol.Al: 0.01 to 0.08%, N: 0.006 to 0.01%, Cu: 0.005 to 0.2%, Sb: 0.02 to 0.1%, remaining Fe and other inevitable impurities, and Mn, Cu and S satisfy the relationship of 6.7≤ (Mn / 1.7S) + (Cu / 1.96S) ≤14.6 After reheating the steel slab, and controlling the average grain size of the austenitic structure to be 50 µm or less before finishing rolling, performing hot rolling at a total reduction ratio of 60% or more in the temperature range of Ae ₃ to Ar ₃ Method for producing a small hardened type steel sheet having excellent in-plane anisotropy which is maintained at a grain size of 5 μm or less, then wound at a temperature of 680-720 ° C., cold-rolled at a reduction ratio of 75% or more, and continuously annealed at 780-830 ° C. .