KR101228701B1 - Bake Hardenable Steel Sheet with Excellent Strain Aging Resistance and Manufacturing Method Thereof - Google Patents

Abstract

In the present invention, by weight%, C: 0.003% or less (excluding 0), Si: 0.01 to 0.1%, Mn: 0.02 to 0.2%, P: 0.01 to 0.1%, S: 0.02% or less (excluding 0), sol.Al: 0.003-0.02%, Cu: 0.2% or less (excluding 0), the balance Fe and other inevitable impurities, the sum of the Mn and Cu (Mn + Cu) is 0.3 or less, the Mn, Cu And S is to provide a bake hardened steel sheet that satisfies the relationship of 0.5 * (Mn + Cu) / S: 2 ~ 20.

In addition, in weight%, C: 0.003% or less (excluding 0), Si: 0.01 to 0.1%, Mn: 0.02 to 0.2%, P: 0.01 to 0.1%, S: 0.02% or less (excluding 0), sol .Al: 0.003 ~ 0.02%, Cu: 0.2% or less (except 0), reheat the steel slab containing residual Fe and other unavoidable impurities, reheat it in the range of 1000 ~ 1300 ℃, and finish rolling temperature condition above Ar ₃ transformation temperature. After hot-rolling to produce a hot rolled steel sheet, the hot rolled steel sheet is wound in the range of 500 ~ 750 ℃, the hot rolled steel sheet is cold rolled to a reduction ratio of 30 ~ 90% to produce a cold rolled steel sheet, the cold rolled steel sheet An object of the present invention is to provide a method for producing a hardened hardened steel sheet, which includes the step of continuous annealing at 780 to 850 ° C.

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

Description

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 which can be used as an inner and outer surface of a vehicle. More specifically, the amount of solid solution nitrogen in the steel material is controlled by controlling the Al content of the steel sheet to 0.003 to 0.02% by weight. By stabilizing, it relates to a steel sheet capable of minimizing aging defects and material defects and a method for producing such steel sheet.

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 these objectives, and particularly in case of very low carbon hardened hardened steel (Bake Haredening Steel) Various efforts are underway to prevent Aging Defects.

Aging defects mean that carbon or nitrogen in solid solution moves around or is stuck to dislocations, which in turn causes stretcher strain to cause defects on the steel surface. Unlike IF (Interstitial Free) steels, which use Ti and Nb to precipitate elements such as solid solution to the outside, in the case of hardened steel that does not include such precipitation processes, the control of invasive solid elements such as carbon and nitrogen is very It is important. If the solid solution of carbon or nitrogen is not properly controlled, the amount of dissolved carbon or nitrogen may increase, resulting in uneven yielding behavior (lrudder band formation) during tensile test, and stretcher strain may occur during press molding. This is because it cannot be used for body shell etc.

Therefore, there have conventionally been techniques to solve the above-mentioned problems by generating many movable potentials on the surface of the steel sheet by adding a special component that can control the solid solution elements or by performing appropriate temper rolling. However, even with such a method, there is still a problem in that a deformation aging phenomenon (aging defect) in which C, N, etc., moves to a potential and adheres to a potential after time elapses due to long time transportation or storage 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, B) The aging and formability problems were simultaneously solved by the development of IF steel, in which Nb was added alone or in combination to precipitate both solid carbon and nitrogen nitrogen into carbides and nitrides.

In recent years, due to the rapid development of the steelmaking sector, due to the technology that can control the employment carbon in the steelmaking sector to 15ppm or less has led to the development of hardened steel that can replace the IF steel. Bake hardenability (BH) means that the steel sheet is processed into a desired shape in the automobile body manufacturing process, and then the baking treatment is performed for a predetermined time to dry it after painting, in which the yield strength of the steel sheet is It means an ascending character.

However, such a hardened hardened steel may have the advantage of ensuring proper moldability and dent resistance at the same time, but this property appears to increase the dissolved carbon within a certain range, an increase in the amount of dissolved carbon may cause aging defects. Can cause problems.

Therefore, there is a need for a method for providing sufficient bake hardenability while maintaining the amount of such solid solution at 30 ppm or less.

Accordingly, in the present invention, in order to solve the above problems, while maintaining the amount of solid solution carbon of 30 ppm or less, at the same time, it is possible to remove the solid solution nitrogen including the low content of Al, and stabilize the amount of nitrogen so as to sufficiently improve the baking hardenability, It is an object of the present invention to provide a quench hardened steel sheet and a method of manufacturing the same, which can obtain dent resistance and prevent stretcher strain defects.

The present invention is a temporary, by weight, C: 0.003% or less (excluding 0), Si: 0.01% to 0.1%, Mn: 0.02% to 0.2%, P: 0.01% to 0.1%, S: 0.02% or less (0 Sol.Al: 0.003-0.02%, Cu: 0.2% or less (excluding 0), the balance Fe and other unavoidable impurities, the sum of Mn and Cu (Mn + Cu) is 0.3 or less, The Mn, Cu and S is to provide a bake hardened steel sheet that satisfies the relationship of 0.5 * (Mn + Cu) / S: 2 ~ 20.

The baking hardening type steel sheet preferably includes one or more precipitates of AlN precipitates, CuMnS precipitates and CuS precipitates having an average diameter of 0.1 µm or less.

In another embodiment of the present invention, in weight%, C: 0.003% or less (excluding 0), Si: 0.01% to 0.1%, Mn: 0.02% to 0.2%, P: 0.01% to 0.1%, and S: 0.02% or less ( Reheat the steel slab containing sol.Al: 0.003 ~ 0.02%, Cu: 0.2% or less (except 0), balance Fe and other unavoidable impurities, and reheat in the range of 1000 ~ 1300 ℃, Ar ₃ After hot-rolling to manufacture hot rolled steel sheet under the finishing rolling temperature condition of transformation temperature or more, the hot rolled steel sheet is wound in the range of 500 to 750 ° C., and the hot rolled steel sheet is cold rolled at a rolling rate of 30 to 90% to form a cold rolled steel sheet. To prepare, to provide a method for producing a hardened hardened steel sheet comprising the step of continuous annealing the cold rolled steel sheet at 780 ~ 850 ℃.

However, it is preferable that the sum (Mn + Cu) of Mn and Cu is 0.3 or less, and Mn, Cu, and S satisfy a relationship of 0.5 * (Mn + Cu) / S: 2 to 20. In addition, it is preferable to further include a plating step of hot-dip galvanizing the continuously annealed steel sheet in a hot dip plating line.

As described above, the low content Sol. In the case of high-strength steel containing Al, it is not possible to precipitate all of the solid nitrogen due to the low Al content, but the annealing temperature can be increased by more than 780 ° C. to increase the AlN precipitation effect, thereby improving aging resistance. In addition, by maintaining a constant size of the grains, mechanical properties such as yield strength, tensile strength and elongation can also be maintained compared to conventional steel.

The calcined hardened steel of the present invention has a basic concept of stably adjusting the amount of solid solution nitrogen by controlling the amount of sol.Al in the steel to 0.003 to 0.02% by weight, and in particular, the amount of solid solution carbon is maintained at 30 ppm or less. Formability, dent resistance and baking hardening characteristics can be obtained.

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

(Hereinafter, by weight)

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 sharply lowered, so 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, when Mn and S and / or Cu content ratio and cooling rate is appropriate, fine MnS and / or (Mn, Cu) S is precipitated. 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, when the content of Mn is excessively excessive, exceeding 0.2%, coarse precipitates may be produced, which may lower the hardening property of Mn. Therefore, the Mn content is limited to 0.02 to 0.2%. In addition, in the present invention, when Mn is added alone without the addition of Cu, it is more preferable to control the content of Mn to 0.05 to 0.2%.

sol.Al: 0.003-0.02%

Al is an element added as a deoxidizer, but the sol.Al concentration is limited to the minimum in the present invention. This is because, when the sol.Al content is less than 0.02%, the AlN precipitation effect cannot be obtained properly. In other words, the concentration of sol.Al and N must be increased by increasing the annealing temperature to remove the solid solution nitrogen. If it is added more than 0,02%, solute nitrogen can be removed by sol.Al concentration, so it should be limited to 0.02%.

P: 0.01 ~ 0.1%

P is an element having a high solid solution strengthening effect and a small decrease in r (plastic anisotropy index) value, which guarantees high strength in steels that control precipitates. Therefore, in order to secure high strength by P, it is preferable to add P content of 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 (excluding 0)

In the case of MnS precipitated steel, it is preferable that the weight ratio of Mn and S satisfies 0.58 * Mn / S ≦ 10, where Mn and S are by weight. Mn binds to S and precipitates as MnS. The MnS precipitates vary depending on the amount of Mn and S added, affecting the hardening hardening properties, yield strength and in-plane anisotropy index. If the value of 0.58 * Mn / S is more than 10, MnS precipitates are coarsened, so that the hardening hardening characteristic is poor, and the in-plane anisotropy index is not good.

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

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 to 20, the average size of the precipitate may be refined to 0.1 ㎛ or less. In particular, the ratio of 0.5 * (Mn + Cu) / S described above is significantly different from the kinds of precipitates and the number of distribution thereof. 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: 0.2% or less (excluding 0)

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, to impart 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%.

N: 0.003% or less

N is an important element for producing fine AlN precipitates. If N is present in an amount greater than 0.003%, aging may be caused by solid solution nitrogen. Therefore, it is preferable to limit the amount of nitrogen added within 0.003%.

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

Hot rolling condition: Reheat above 1000 ~ 1300 ℃, 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 in the range of 1000 ~ 1300 ℃. If the reheating temperature is lower than 1000 ℃, the coarse precipitates produced during continuous casting may remain completely insoluble and the coarse precipitates may remain even after hot rolling. You can leave a lot.

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: 500 ~ 750 ℃ or less

As described above, after performing hot rolling, winding is performed. The winding temperature is preferably in the range of 500 to 750 ° C. If the coiling temperature is higher than 750 ℃, precipitates grow coarse and the baking hardening characteristics may be lowered.

Cold rolling condition: 30 ~ 90% rolling rate

Cold rolling is rolled to the required thickness, in the present invention is carried out at a reduction ratio of 30 ~ 90%. If the cold reduction rate is less than 30%, the amount of nucleation of the annealing recrystallization decreases, so that the grains may grow too large during annealing, and the coarsening of the annealing recrystallization grains may lower the strength and formability. On the other hand, when 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 lowered, so the reduction rate is controlled to 30 to 90%.

Continuous Annealing: 780 ~ 850 ℃

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 780-850 degreeC. When the continuous annealing temperature is higher than 780 ° C., AlN may be sufficiently precipitated even at low Al, but when the continuous annealing temperature exceeds 850 ° C., the strength may decrease due to coarsening of recrystallized grains. 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%.

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

The annealing temperature of the conventional CuMnS precipitated-baked hardened steel in the state in which the component system was fixed at 0.02% by weight in the amount of sol.Al in the inventive material 1 and the comparative material 1 was changed (Table 1). After the annealing, SPM (based on 1.4% of Temper Roll stretching) was performed 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).

In addition, OM analysis and TEM analysis were performed for Inventive Example 1 and Comparative Example 1 of Table 2, and the results for the OM analysis are shown in FIG. 5, and the results for the TEM analysis are shown in FIG. 6, respectively. In FIG. 5, (a) shows Comparative Example 1, and (b) shows Invention Example 1. FIG. In FIG. 6, (a) shows Comparative Example 1, and (b) shows Invention 1.

Annealing Temperature (° C) C
( ppm ) Si Mn P S Sol - Al Cu N
( ppm ) Remarks 700 20 0.06 0.15 0.06 0.008 0.02 0.1 25 Comparative Example 1 850 20 0.06 0.15 0.06 0.008 0.02 0.1 25 Inventory 1

Annealing Temperature (℃) YP (MPa) TS (MPa) AI (YP-EL) BH Remarks 700 216 350 0.42 82 Comparative Example 1 850 209 352 0.00 34 Inventory 1

As shown in Table 2, Inventive Example 1 conforming to the present invention can be seen that it is excellent in strength and baking hardenability.

As shown in FIG. 5, the grain size was found to be the same regardless of the change in the annealing temperature. In addition, in Figure 6, the type of precipitate was observed using TEM, but at an annealing temperature of 700 ° C., no nitride-based precipitate was present (meaning that the precipitate density was very small). . That is, it can be seen that as the annealing temperature increases, the grain size does not increase, but the nitride-based precipitate precipitates.

1 is a HEAT CYCLE graph showing a temperature change with time during an annealing process;

2 is a graph when no yield point elongation occurs after accelerated aging at 100 ° C. for 2 hours;

3 is a graph when the yield point elongation occurs after accelerated aging at 100 ° C. for 2 hours;

4 is BH (Bake Hardenability) measurement at 170 ° C. for 20 minutes;

Figure 5 (a) is an OM analysis picture of Comparative Example 1, (b) OM analysis picture of Inventive Example 1;

Figure 6 (a) is a TEM picture of Comparative Example 1, (b) is a TEM picture of Inventive Example 1.

Claims (5)

By weight%, C: 0.002% or less (excluding 0), Si: 0.01 to 0.1%, Mn: 0.02 to 0.2%, P: 0.01 to 0.1%, S: 0.02% or less (excluding 0), sol.Al : 0.003-0.02%, Cu: 0.2% or less (excluding 0), the balance Fe and other inevitable impurities, the sum of the Mn and Cu (Mn + Cu) is 0.3 or less, wherein the Mn, Cu and S is 0.5 * (Mn + Cu) / S: A bake hardened steel sheet which satisfies the relationship of 2 to 20. The method of claim 1, The baking hardening type steel sheet comprises at least one precipitate of AlN precipitates, CuMnS precipitates and CuS precipitates having an average diameter of 0.1 μm or less. By weight%, C: 0.002% or less (excluding 0), Si: 0.01 to 0.1%, Mn: 0.02 to 0.2%, P: 0.01 to 0.1%, S: 0.02% or less (excluding 0), sol.Al : 0.003-0.02%, Cu: 0.2% or less (excluding 0), the balance Fe and other unavoidable impurities, the sum (Mn + Cu) of the Mn and Cu is 0.3 or less, and the Mn, Cu and S 0.5 * (Mn + Cu) / S: Reheat the steel slab that satisfies the relation of 2 ~ 20 and reheat it in the range of 1000 ~ 1300 ℃, and hot-rolling the steel sheet under the finish rolling temperature condition above Ar ₃ transformation temperature to manufacture hot rolled steel sheet. Doing; Winding the hot rolled steel sheet in a range of 500 to 750 ° C; Manufacturing the cold rolled steel sheet by cold rolling the hot rolled steel sheet at a reduction ratio of 30 to 90%; And Method for producing a hardened type steel sheet comprising the step of continuously annealing the cold rolled steel sheet at 780 ~ 850 ℃. delete The method of claim 3, After the step of the continuous annealing, the method for producing a hardened type steel sheet further comprising a plating step of hot-dip galvanizing the steel sheet in a hot-dip plating line.

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