KR100957959B1 - V-Zr Added Bake Hardenable Steel Sheet with Excellent Strain Aging Resistance and Manufacturing Method Thereof - Google Patents

Abstract

The present invention, in weight percent, C: 0.003% or less (excluding 0), Si: 0.01% to 0.1%, Al: 0.01% to 0.08%, P: 0.01% to 0.1%, S: 0.02% or less, other impurities and residues Fe, including: Mn: 0.05-0.2%, Cu: 0.005-0.2% or Mn: 0.05-0.2% and Cu: 0.005-0.2%, where V: 0.05-0.2%, Zr: 0.01-0.03 It provides MnS precipitated hardened hardened steel, CuS precipitated hardened hardened steel, and CuMnS precipitated hardened hardened steel further comprising% or V: 0.05-0.2% and Zr: 0.01-0.03%. In this case, 0.58 * Mn / S is 10 or less relationship between Mn and S in MnS precipitation type hardened steel, and 0.5 * Cu / S is 1-10 between Cu and S in CuS precipitation type hardened steel. It is preferable that 0.5 * (Mn + Cu) / S satisfy | fills 2-20 relationship between phosphorus relationship and Cu, Mn, and S in CuMnS precipitation type hardened steel.

As in the present invention, when preparing a bake hardened steel in which stable solid solution carbide is formed by adding an appropriate amount of V and Zr to the bake hardened steel, it is possible to suppress aging deterioration and stretcher strain defect of the bake hardened steel by efficient solid solution stabilization of carbide. Can be.

Baking Hardened Steel, Aging Degradation, Carbide, Plated Steel Sheet, Hot Dip Galvanized

Description

V-Zr Added Bake Hardenable Steel Sheet with Excellent Strain Aging Resistance and Manufacturing Method Thereof}

The present invention relates to a hardening type MAFE high strength alloyed hot-dip galvanized steel sheet that can be used as an inner and outer shell of an automobile. More particularly, the present invention relates to strength over time due to solid solution carbon and solid solution nitrogen present in a steel sheet during steel sheet manufacture. The present invention relates to a method for manufacturing a steel sheet stabilized with solid solution C in order to suppress processing defects due to aging that increases hardness.

Recently, as environmental issues have emerged as social issues, research on technologies using high-strength hot dip galvanized steel sheets has been increasing in the automobile field in terms of automobile stability, light weight, and low fuel consumption. In particular, research and development on hardened hardened steel has been increasing as a steel grade that can satisfy this purpose, and particularly in the case of very low carbon hardened hardened steel (Bake Haredening Steel), aging defects that may occur when the coil is transported at a long distance ( Various efforts are underway to prevent Aging Defects.

The generation of aging defects in ultra low carbon steel has been studied in the past. Particularly, in case of hardened steel that does not precipitate solid solution C, when the amount of invasive solid solution elements such as carbon or nitrogen increases, it shows uneven yield behavior during tensile test, and a stretcher strain occurs during press molding. It can be difficult to use in the car body shell that the surface appearance is important is a problem. Therefore, there has conventionally been a technique to solve the above-mentioned problems by generating many movable potentials on the surface of the steel sheet by adding a special component capable of controlling the solid solution C or by performing appropriate temper rolling. However, even with this method, there is still a problem in that a deformation aging phenomenon occurs in which C, N, etc. move to dislocations and fix the dislocations after time elapses after temper rolling.

Until the 1970s, aging defects were solved by batch-annealing low-carbon Al-Killed steels.However, in the 1980s, due to the introduction of CAL (Continuous Annealing Line) and the development of steelmaking technology, 2) The aging and formability problems were simultaneously solved by the development of IF steel, which alone or in combination, added Nb as either carbide or nitride.

In recent years, because of the rapid development of steelmaking field, the hardening steel has been developed due to the technology that can control the working carbon below 15ppm in the steelmaking sector, and this hardening steel is carbide by controlling the solid solution carbon below 30ppm in steelmaking. It is not necessary to add alloy elements such as Ti or Nb, and it is possible to produce plating products superior to IF steel when plating by reducing the amount of alloying elements such as Mn and Si which cause strip thickening. Therefore, such hardened hardened steel is easier to control aging than IF steel, and has the advantages of excellent surface products and less material deviation.

However, such hardened hardened steels use sulfide-based reinforcing mechanisms such as CuS and MnS, rather than carbides, and thus exhibit excellent surface quality unlike IF steels. However, conventional methods for suppressing aging and strain strain The reality is that the effect is not good. This is because, unlike the IF steel, in the hardened steel, the hardened carbon is controlled to 30ppm at the steelmaking stage and does not add an alloying element that precipitates carbides. In aging operations such as cold rolling, there is a need to suppress aging and stretcher strain in new ways.

Accordingly, the present invention solves the above-mentioned problems and also prevents the aging deterioration due to the presence of solid solution C in the hardened hardened steel with even better surface quality than the IF steel, and prevents the strainer strain experience. To provide.

In order to solve this problem, the present invention was intended to solve the aging deterioration and the strainer strain defects by forming carbides using the appropriate addition of V and Zr, by weight, C: 0.003% or less (excluding 0), Si : 0.01 to 0.1%, Mn: 0.05 to 0.2%, Al: 0.01 to 0.08%, P: 0.01 to 0.1%, S: 0.02% or less, including other impurities and balance Fe, V: 0.05 to 0.2% and Zr : Provides MnS precipitation type hardened steel, characterized in that it further comprises one or two selected from 0.01 ~ 0.03%. In this case, it is preferable that a relationship of 0.58 * Mn / S is 10 or less between Mn and S.

Furthermore, in the present invention, by weight%, C: 0.003% or less (excluding 0), Si: 0.01% to 0.1%, Cu: 0.005% to 0.2%, Al: 0.01% to 0.08%, P: 0.01% to 0.1%, and S: Provides CuS precipitation type hardened hardened steel further comprising at least 0.02%, other impurities and residual Fe, and additionally one or two selected from V: 0.05 to 0.2% and Zr: 0.01 to 0.03%. . In this case, it is preferable that the relationship of 0.5 * Cu / S is 1-10 between said Cu and S.

Furthermore, in the present invention, by weight%, C: 0.003% or less (excluding 0), Si: 0.01 to 0.1%, Mn: 0.05 to 0.2%, Cu: 0.005 to 0.2%, Al: 0.01 to 0.1%, P: 0.01-0.1%, S: 0.02% or less, containing other impurities and balance Fe, CuMnS precipitation characterized in that it further comprises one or two selected from V: 0.05-0.2% and Zr: 0.01-0.03% In this case, it is preferable that 0.5 * (Mn + Cu) / S satisfy | fills the relationship of 2-20.

Further, the present invention, the steel slab consisting of the above components, the re-heating at 1100 ℃ or more and the hot rolling step of the finish rolling temperature is more than the Ar ₃ transformation temperature, the winding step wound up to 700 ℃ or less, with a reduction ratio of 50 ~ 90% It provides a method for producing a hardened hardened steel, characterized in that it comprises a cold rolling step of cold rolling and a continuous annealing step of continuous annealing at 500 ~ 900 ℃.

Furthermore, the present invention is a steel slab consisting of the above components, the re-heating at 1100 ℃ or more and the hot rolling step of the finish rolling temperature is more than the Ar ₃ transformation temperature, the winding step wound up to 700 ℃ or less, cold rolling at a reduction ratio of 50 ~ 90% Cold rolling step of rolling Provides a method for producing a hardened plated hardened steel sheet comprising a continuous annealing step of continuous annealing at 500 ~ 900 ℃ and a plating step of hot dip galvanizing in a hot dip plating line.

As described above, when the bake hardened steel of the present invention is produced by adding the appropriate amounts of V and Zr, it is possible to suppress the aging deterioration of the bake hardened steel and the defect due to the stretcher strain by efficient solid solution stabilization of carbide.

Bevel hardened steel can improve workability by precipitating / removing to maintain an appropriate amount of solid solution C and N, but unlike IF steel whose surface quality is somewhat degraded due to the addition of a large amount of alloying elements such as Ti and Nb, The content of C at the time of tapping is controlled to 15 ppm or less without removing, and sulfide-based precipitation can be used to maintain proper strength.

In order to maintain the properties of the hardened hardened steel and to suppress the aging defects and the strainer strain defects, the present inventors have studied that the amount of solid solution C can be changed by using a small amount of V and Zr elements without adding a large amount of alloying elements. The conclusion was reached that this can be stabilized, and the present invention has been completed according to this principle.

Hereinafter, the component system constituting the steel of the present invention will be described in detail.

C: 0.003% or less (excluding 0)

In the present invention, C is kept as low as possible below 0.003%. If C exceeds 0.003%, aging deterioration may occur and the BH property may deteriorate.

Si: 0.01 ~ 0.1%

Si is an element that has a good solid solution effect and low elongation, and thus, when used in a steel for controlling precipitates as in the present invention, high strength can be ensured, so that 0.01% or more is added. However, when the content of Si exceeds 0.1%, the plating quality may be drastically lowered. The upper limit is 0.1%.

Mn: 0.02-0.2%

Mn precipitates solid solution S in steel as MnS and functions as an element preventing hot shortness due to solid solution S. In particular, in the present invention, Mn is fine MnS and / or (Mn, Cu) S is precipitated when the ratio of S and / or Cu and cooling rate are appropriate. These fine precipitates segregate carbon around the grain boundaries or precipitates, rather than in the grains, to impart hardening hardening during the coating baking process. In the present invention, in order to exhibit such an effect, Mn is added at least 0.02%. However, if the content of Mn is excessively excessive and exceeds 0.2%, coarse precipitates may be generated and the hardening hardening characteristic may be lowered, so the Mn content is limited to 0.02 to 0.2%. In addition, in the present invention, when Mn is added alone without addition of Cu, it is more preferable to control the content of Mn to 0.05 to 0.2%.

Al: 0.01% to 0.1%

Al is an element to be added as a deoxidizer, but in the present invention, Al is added in an amount of 0.01% or more because precipitation of N in steel serves to prevent formability deterioration due to solid solution N. If the Al content is less than 0.01%, the amount of solid solution nitrogen will increase and the moldability will decrease. On the contrary, if the Al content exceeds 0.1% and is excessively excessive, the amount of Al present in the solid state will increase. Ductility may be reduced. However, in the CuS precipitated steel and MnCu precipitated steel without Mn addition, preferable Al content is about 0.01 to 0.08%. In addition, when the content of N in the present invention is high as 0.005 ~ 0.02% can obtain a strengthening effect by the AlN precipitates, it is possible to manufacture a high strength steel sheet.

P: 0.01 ~ 0.1%

P is an element having a high solid solution strengthening effect and a small decrease in the r (plasticity anisotropy index) value, which guarantees high strength in steels that control precipitates. Therefore, in order to secure the high strength by P, it is preferable to add P content 0.01% or more. However, when the content of P is excessively excessively more than 0.1%, the ductility may be lowered.

S: 0.02% or less, 0.58 * Mn / S: 10 or less, 0.5 * Cu / S: 1-10

The content of S is preferably added to 0.02% or less for precipitation strengthening, but basically needs to vary depending on the type of precipitation steel.

First, in the case of MnS precipitated steel, it is preferable to add S so that the weight ratio of Mn and S satisfies 0.58 * Mn / S ≦ 10 (wherein Mn and S are by weight%). Mn is combined with S and precipitated as MnS, and the precipitated MnS precipitates vary depending on the amount of Mn and S added, thereby affecting the hardening hardening properties, yield strength, and in-plane anisotropy index. When the value of 0.58 * Mn / S exceeds 10, MnS precipitates are coarsened, so that the hardening hardening characteristic is lowered, which may lower the in-plane anisotropy index.

Moreover, it is preferable that the value of 0.5 * Cu / S (Cu, S is weight%) is 1-10 about CuS precipitation steel. Cu binds to S and precipitates as CuS. The CuS precipitates also affect the baking hardening property, plastic anisotropy index, and in-plane anisotropy index because the precipitated state varies depending on the amount of Cu and S added. In the present invention, the effective CuS precipitates may be precipitated when 0.5 * Cu / S is 1 or more, but when it exceeds 10, the CuS precipitates may be coarse to deteriorate the hardening property, plastic anisotropy index, and in-plane anisotropy index. More preferably, as a value for stably securing CuS of 0.1 µm or less, the value of 0.5 * Cu / S satisfies the range of 1-3.

Mn + Cu: 0.3% or less, 0.5 * (Mn + Cu) / S: 2-20

When Mn and Cu are added simultaneously, the sum of Mn and Cu is limited to 0.3% or less. When the sum of Mn and Cu exceeds 0.3%, the size of the precipitate may be too large to secure the baking hardening characteristics. In addition, the value of 0.5 * (Mn + Cu) / S (Mn, Cu and S are weight%) is limited to 2-20. This is also to control the (Mn, Cu) S precipitates so that the precipitates can improve the baking hardening characteristics, plastic anisotropy index, in-plane anisotropy index and the like as described above. When 0.5 * (Mn + Cu) / S is 2 or more, an effective precipitate can be obtained. On the other hand, when it exceeds 20, the precipitate is coarse, which may cause degradation of the hardening property, plastic anisotropy index, and in-plane anisotropy index.

In particular, in the present invention, when the ratio of 0.5 * (Mn + Cu) / S satisfies the range of 2 ~ 20, the average size of the precipitate can be finened to 0.1 ㎛ or less. In this case, the precipitate is preferably distributed in 2x10 ⁶ or more. In particular, the ratio of 0.5 * (Mn + Cu) / S is significantly different from the starting point of 7, and the number of distributions thereof is significantly different. Single precipitates of MnS and CuS, which are very fine than the composite precipitates of, are uniformly distributed.On the other hand, when the ratio is larger than 7, the number of complex precipitates of (Mn, Cu) S increases, so that the number of distribution decreases even though the size difference of the precipitates is small. It is characteristic.

Cu: 0.005 ~ 0.2%

In the present invention, Cu forms fine precipitates when the content ratio with S and / or Mn and the cooling rate before winding in the hot rolling process are appropriate. These fine precipitates segregate carbon around the grain boundaries or precipitates rather than in the grains, thus providing hardening hardening during the coating baking process. In the present invention, the content of Cu is added to 0.005% or more in order to exert such effects. However, when the Cu content exceeds 0.2%, the precipitates are coarsened, which may degrade the baking hardening characteristic. In addition, in the present invention, when Cu is added alone without adding Mn, the content of Cu is preferably controlled to 0.01 to 0.2%.

V: 0.05-0.2%

V is an element that has a high precipitation strengthening effect and improves the problem of aging deterioration at room temperature, and is an important element for ensuring high strength in steels for controlling precipitates as in the present invention. In the present invention, when the content of V is 0.05% or more, such an effect can be obtained, and the effect can be obtained even at room temperature aging. However, when the amount of V added is excessively greater than 0.2%, the production cost increases and the baking hardening characteristic may also be deteriorated.

Zr: 0.01 ~ 0.03%

Zr functions as an important element to prevent aging deterioration and stretcher strain defects together with V. In the present invention, this effect can be obtained even when V is added alone, but when added together with Zr, the effect is further improved. It is advantageous to add more than 0.01% of the content, but the upper limit is limited to 0.03% because adding more than 0.03% saturates the effect and increases the cost.

Hereinafter, a method of manufacturing the steel of the present invention will be described in detail.

Hot rolling condition: Reheat above 1100 ℃, finish rolling temperature is Ar ₃ Abnormal transformation temperature

In the present invention, the steel that satisfies the above-mentioned emphasis is reheated and hot rolled. The reheating temperature is controlled at 1100 ℃ or higher. If the reheating temperature is lower than 1100 ℃, the coarse precipitates produced during continuous casting may remain completely insoluble and many coarse precipitates remain after hot rolling. Can be.

In addition, the hot rolling is a finish rolling temperature of Ar ₃ Carry out under the transformation temperature. Finish rolling temperature is Ar ₃ If the temperature is lower than the transformation temperature, the workability and ductility of the rolled grain may be greatly reduced.

Winding Condition: Below 700 ℃

As described above, after performing hot rolling, winding is performed, and the winding temperature is preferably 700 ° C. or lower. If the coiling temperature exceeds 700 ℃, precipitates may grow coarse to degrade the baking hardening characteristics.

Cold Rolling Condition: 50 ~ 90% Rolling Rate

Cold rolling is rolled to the required thickness, in the present invention is carried out at a reduction ratio of 50 ~ 90%. If the cold reduction rate is less than 50%, the amount of nucleation of the annealing material crystals may be reduced so that the grains may grow too large during annealing, thereby decreasing the strength and formability due to coarsening of the annealing recrystallized grains. On the other hand, if the cold reduction rate exceeds 90%, the moldability is improved, but the amount of nucleation is too large, so the annealing recrystallized grains become too fine and the ductility may be reduced, so that the reduction rate is controlled to 50 to 90%.

Continuous Annealing: 500 ~ 900 ℃

Continuous annealing temperature plays an important role in determining the material of the product. In this invention, it is preferable to carry out in the temperature range of 500-900 degreeC. If the continuous annealing temperature is lower than 500 ° C, the recrystallized grain becomes too fine to obtain a target ductility value. If the annealing temperature exceeds 900 ° C, the strength may decrease due to coarsening of the recrystallized grain. have. On the other hand, the continuous annealing time is maintained to complete the recrystallization, if the recrystallization is completed in about 10 seconds or more.

Plating: Hot dip galvanized

Hot-dip plating of the steel sheet is carried out by hot dip galvanizing in a plating solution containing Al. In this case, the amount of Al contained in the plating solution for GA alloying should be 0.1% or more. However, if the content exceeds 0.3%, foreign matter may be generated on the plating surface, so the upper limit thereof is limited to 0.3%.

The present invention will be described in more detail with reference to the following examples.

V: 0.05%, 0.1% and 0.2% were added to the inventive steels 1 to 5 of the conventional CuMnS precipitated hardened steel, and Zr was added to the inventive steels 1 and 6 by 0.03% and 0.01%, respectively. V and Zr were not added to the comparative steel which is a bake hardened steel. (Table 1) was then subjected to annealing, SPM (based on 1.4% of Tenm Roll) in the same operation method applied to the comparative steel (see Fig. 1). The aging evaluation method used a tensile test, AI (Aging index) and BH (Bake hardenability) test (Figs. 2, 3 and 4).

Remarks C Mn Si P S Al Cu N Zr V Comparative steel 0.0015 0.09 0.06 0.06 0.012 0.04 0.1 0.0025 - - Inventive Steel 1 0.0015 0.09 0.06 0.06 0.012 0.04 0.1 0.0025 0.03 - Inventive Steel 2 0.0015 0.09 0.06 0.06 0.012 0.04 0.1 0.0025 - 0.05 Invention Steel 3 0.0015 0.09 0.06 0.06 0.012 0.04 0.1 0.0025 - 0.1 Inventive Steel 4 0.0015 0.09 0.06 0.06 0.012 0.04 0.1 0.0025 - 0.2 Inventive Steel 5 0.0015 0.09 0.06 0.06 0.012 0.04 0.1 0.0025 0.01 0.05

The AI (Aging index) test is to measure the existence of the yield point elongation through the tensile test after heat-treating the existing materials and inventions in 100 ℃ water for 2 hours, a condition that can guarantee about 6 months of aging at room temperature (Fig. 2). And 3). In addition, the BH evaluation was heat-treated for 20 minutes at 180 ℃, to calculate the difference in the YP value before and after the heat treatment, was set based on the current yield point as the current mass production standards (Fig. 4).

Accelerated aging test with a small amount of V and / or Zr added to stabilize the solid solution C showed an operating range (additional elements and amount of addition) in the YP-El curve that did not yield yield point and at the same time prevented the occurrence of stretcher strain. In order to improve the operating potential density, it was possible to set the appropriate working range for the temper reduction rate. As optimum conditions, when V: 0.05% and Zr: 0.01% were added, very favorable results were obtained for the aging resistance, and the BH value after the heat treatment was maintained at 30 MPa or more (Table 2).

division Before heat treatment After heat treatment BH Comparative steel 250.02 284.60 34.58 Inventive Steel 1 272.76 308.39 35.63 Inventive Steel 2 266.28 304.08 37.80 Invention Steel 3 268.49 307.28 38.79 Inventive Steel 4 268.16 305.33 37.17 Inventive Steel 5 266.16 294.89 38.73

Looking at the Table 2, V and Zr was also effective when added alone, especially V: 2000ppm alone was also effective in improving the aging resistance through the stabilization of solid solution C. However, considering the economics such as cost, the combination of V and Zr was found to be the most excellent in terms of aging resistance and economic efficiency.

1 is a view showing an annealing heat cycle of the hardened hardened steel

2 is a graph showing the yield point elongation after 100 ° C 2h accelerated aging

3 is a graph showing no yield point elongation after 100 ° C 2h accelerated aging

4 is a graph relating to 170 ° C. 20min BH measurement.

Claims (10)

delete delete By weight%, C: 0.003% or less (excluding 0), Si: 0.01 to 0.1%, Cu: 0.005 to 0.2%, Al: 0.01 to 0.08%, P: 0.01 to 0.1%, S: 0.02% or less, etc. Impurity and balance Fe, V: 0.05% to 0.2% and Zr: 0.01% to 0.03% of the selected hardened steel further comprising one or two selected. The bake hardened steel according to claim 3, wherein the Cu and S have 0.5 * Cu / S of 1 to 10. By weight%, C: 0.003% or less (excluding 0), Si: 0.01-0.1%, Mn: 0.05-0.2%, Cu: 0.005-0.2%, Al: 0.01-0.1%, P: 0.01-0.1%, S: 0.02% or less, containing other impurities and balance Fe, V: 0.05% to 0.2% and Zr: 0.01% to 0.03% of the selected hardened steel further comprising one or two selected. The bake hardened steel according to claim 5, wherein Cu, Mn, and S have 0.5 * (Mn + Cu) / S of 2 to 20. The baking hardening steel according to claim 5 or 6, wherein the sum of Cu and Mn is 0.3% or less. For steel slabs consisting of the components of claim 3 or 5, Reheating at 1100 ° C. or higher and finishing rolling temperature of at least Ar ₃ transformation temperature; A winding step of winding up at 700 ° C. or lower; Cold rolling step of cold rolling at a reduction ratio of 50 to 90%; And Continuous annealing step of continuous annealing at 500 ~ 900 ℃; Method for producing a sinter hardened steel comprising a. For steel slabs consisting of the components of claim 3 or 5, Reheating at 1100 ° C. or higher and finishing rolling temperature of at least Ar ₃ transformation temperature; A winding step of winding up at 700 ° C. or lower; Cold rolling step of cold rolling at a reduction ratio of 50 to 90%; Continuous annealing step of continuous annealing at 500 ~ 900 ℃; And A plating step of hot dip galvanizing in a hot dip plating line; Method for producing a hardened plated hardened steel sheet comprising a. 10. The method of claim 9, wherein the hot dip galvanizing is performed in a plating solution containing 0.12 to 0.2% of Al.

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