KR101143101B1 - High strength cold rolled steel sheet having excellent resistance to second work embrittleness and aging resistance, and process for producing the same - Google Patents

Abstract

A high strength cold rolled steel sheet used as a material for automobiles, home appliances and the like, and a manufacturing method thereof. The cold-rolled steel sheet according to the present invention is characterized by comprising 0.0005 to 0.003% of C, 0.03 to 0.2% of Mn, 0.03 to 0.06% of P, 0.003 to 0.025% of S, 0.01 to 0.1% of Al, 0.005 to 0.02 of N, Cu, and S satisfy the conditions Mn + Cu? 0.3, 0.5 * (Mn + Cu) / S: 2 to 20, and the balance Fe and other unavoidable impurities And the average size of the precipitates is 0.2 mu m or less. The cold-rolled steel sheet of the present invention has an aging index of 30 MPa or less and a low ductility-brittle transition temperature (DBTT).

Cold-rolled steel sheet, durability, high strength, secondary processing brittleness, in-plane anisotropy index, (Mn, Cu) S

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold rolled steel sheet and a method for manufacturing the same,

Fig. 1 is a graph showing changes in the amount of solid carbon in the crystal grains depending on the size of the precipitate.

2 is a graph showing the size of the precipitate with respect to the cooling rate,

      2A shows a case of 0.5 * (Mn + Cu) / S? 20,

      FIG. 2B shows the case of 0.5 * (Mn + Cu) / S? 20.

The present invention relates to a high-strength cold-rolled steel sheet having a strength of 340 MPa, which is used as a material for automobiles, household appliances and the like, and a manufacturing method thereof. More particularly, the present invention relates to a high-strength cold-rolled steel sheet excellent in resistance to secondary aging and secondary porosity by controlling the amount of solid carbon in the grain by precipitates, and a method for producing the same.

Cold rolled steel sheets used in automobiles and home appliances are required to have strength and moldability as well as resistance to aging. Aging is a kind of strain aging phenomenon that causes a defect called Stretcher Strain as hardening occurs as the solid element (C, N) sticks to the potential.

The durability of the cold-rolled steel sheet can be ensured by the application of aluminum-killed steel, but it has a disadvantage that the productivity is low due to the long annealing time and the material variation is large in each part. Therefore, Interstitial Free Steel, which is continuously annealed by adding strong carbon and nitride forming elements such as Ti and Nb, is mainly used.

In order to produce an IF steel, Ti, Nb, etc., which are strong carbon and nitride forming elements, are added, and these elements must be annealed at a high temperature because they raise the recrystallization temperature. This lowers productivity and increases energy costs, resulting in higher costs and more pollution. In addition, when annealing is performed at a high temperature, various defects such as fine scratches and shape defects are likely to occur. Also, since Ti and Nb are highly oxidative, many nonmetallic inclusions are generated in steel making to cause surface defects of the steel sheet. In addition, the IF steel has a disadvantage in that so-called secondary processing brittleness occurs in which the grain boundary is weak and brittleness is generated after processing. To prevent this, an element such as B is added to prevent brittle secondary processing. Particularly, in the case of IF steel, there are disadvantages that many defects are generated in a product which performs surface treatment such as plating and painting.

In order to solve this problem, Ti and Nb-free alloys without Ti or Nb have been proposed. For example, in JP-A Nos. 6-093376, 6-093377 and 6-212354, Ti and Nb are not added and instead, 0.0001 to 0.003% of B is added to 0.0001 to 0.003% It is a technology to improve Hyosung. However, in this prior art, endurance is not sufficient and quenching after annealing is recommended in order to ensure endurance. In this case, since most of the water is cooled, the pickling treatment is performed again to remove the oxide film formed during water- This is not good and costs extra. In addition, these steel types have disadvantages of low strength, high in-plane anisotropy, wrinkles, and ear inconvenience.

On the other hand, the present inventors have proposed a method of manufacturing a cold-rolled steel sheet having improved ductility without adding Ti and Nb to Korean Patent Publication No. 2000-0039137. A method for producing a cold-rolled steel sheet comprising the steps of: C: 0.0005 to 0.002%, Mn: 0.05 to 0.3%, S: 0.015% or less, P: 0.015% %, Steel slabs containing C + N + S + P not more than 0.025% and the remaining Fe and other inevitably contained elements are subjected to hot rolling at a finishing rolling temperature of not less than the Ar ₃ transformation point, Rolled at a temperature of 50 to 90%, and continuously annealed at a temperature in the range of 650 to 850 DEG C for 10 seconds or more. The cold-rolled steel sheet thus obtained is excellent in endurance and ductility while maintaining the plastic anisotropy index at a certain level or higher. However, this cold-rolled steel sheet strictly controls C + N + S + P to 0.025% or less, so that the desulfurization and denitrification capability should be strengthened in the steelmaking process. In terms of material, the yield strength is too low to use a thicker material, and the in-plane anisotropy index (? R value) during processing is too high to cause wrinkles and breaks.

In addition, the present inventors have proposed a method of manufacturing a cold-rolled steel sheet capable of improving the yield strength in a high-strength steel having a tensile strength of 35 kgf / mm ² without adding Ti and Nb in Korean Patent Publication No. 2002-0049667. A method for producing a cold-rolled steel sheet comprising the steps of: C: 0.0005 to 0.003%, Mn: 0.1% or less, S: 0.003 to 0.02%, P: 0.03 to 0.07%, Al: 0.01 to 0.1% , Cu: 0.05 to 0.3% and Cu / S atomic ratio of 2 to 10 at a temperature not lower than the Ar ₃ transformation point, cold rolling at a reduction ratio of 50 to 90% And then continuously annealing for 5 seconds to 5 minutes. The cold-rolled steel sheet thus obtained had a yield strength of 240 MPa . However, since the aging index is larger than 30 MPa, the aging property can not be guaranteed, and the plastic anisotropy index (r _m ) is 1.8, but the in-plane anisotropy index is as high as 0.5 or more.

The present invention relates to a cold-rolled steel sheet which has substantial anti-aging properties without addition of Ti and Nb and which is excellent in secondary brittleness and which has excellent anisotropy index in planes while maintaining a plastic anisotropy index of a certain level or higher, Method, which has its purpose.

In order to achieve the above object, the cold-rolled steel sheet of the present invention comprises 0.0005 to 0.003% of C, 0.03 to 0.2% of Mn, 0.03 to 0.06% of P, 0.003 to 0.025% of S, 0.01 to 0.1% Cu, and S satisfy the following conditions: Mn + Cu? 0.3, 0.5 * (Mn + Cu) / S: 2 to 20, and N: 0.005 to 0.02% Fe and other unavoidable impurities, and the average size of the precipitates is 0.2 탆 or less.

In the cold-rolled steel sheet of the present invention, C may be more than 0.002% to 0.003% or less. It is preferable that Al and N satisfy the condition 0.52 * Al / N: 1 to 5. The precipitate includes at least one kind of MnS, CuS, (Mn, Cu) S. The number of precipitates is preferably ³ x 10 ⁶ or more. The above 0.5 * (Mn + Cu) / S is 2 to 7, and the number of precipitates is preferably 3 × ^{10 8} or more. The cold-rolled steel sheet of the present invention may further contain at least one of Mo: 0.01 to 0.2% and V: 0.01 to 0.2%. It is most preferable that V and C satisfy 0.25 * V / C of 1 to 20.

Further, in the cold-rolled steel sheet manufacturing method of the present invention, the steel slab satisfying the above-mentioned stress is reheated at a temperature of 1100 ° C or higher, hot rolled at a finishing rolling temperature equal to or higher than the Ar ₃ transformation point, cooled at a rate of 300 ° C / Rolled at a temperature of 700 ° C or lower, cold-rolled, and continuously annealed.

Hereinafter, the present invention will be described in detail.

The inventors of the present invention have found the following new facts in the research process for improving the secondary machining brittleness having the endurance characteristics without adding Ti and Nb. In other words, precipitates of MnS, CuS, (Mn, Cu) S in the cold rolled steel sheet affect the secondary processability, plastic anisotropy index and in-plane anisotropy index as well as resistance to aging.

As shown in Fig. 1, as the precipitates are finely distributed in the cold-rolled steel sheet, the amount of solid carbon in the crystal grains is reduced, thereby improving the aging resistance characteristics. In order to secure the aging property, the amount of solid carbon in the crystal grains should be about 20 ppm or less, which can be stably ensured when the average size of the precipitates of MnS, CuS, (Mn, Cu) S is 0.2 탆 or less.

With this new fact in mind, we have been studying a way to finely distribute MnS, CuS, (Mn, Cu) S. As a result, it was found that (1) the contents of Mn, Cu, and S were adjusted to the following conditions Mn + Cu? 0.3, Mn content of 0.03 to 0.2%, S content of 0.003 to 0.025%, and Cu content of 0.005 to 0.2% It is necessary to adjust to satisfy 0.5 * (Mn + Cu) / S: 2 ~ 20, and (2) when the cooling rate is 300 ° C / ) The average size of the precipitates of S is 0.2 μm or less.                     

FIG. 2 (a) is a graph showing the relationship between the composition of 0.5 * (Mn + Cu) /S:5.67 and 0.0023% C-0.15% Mn- 0.04% P- 0.015% S- 0.068% Al- 0.013% N-0.02% FIG. 5 is a graph showing the size of the precipitate according to the cooling rate after hot rolling the steel. FIG. 2 (a), when the cooling rate is adjusted for 0.5 * (Mn + Cu) / S? 20, the precipitates of MnS, CuS, (Mn, Cu) It can be confirmed that the following can be satisfied.

According to the present invention, when the average size of the precipitate is 0.2 탆 or less, the secondary working brittleness is also excellent while basically ensuring the anti-aging property. When the precipitate becomes fine, an appropriate amount of carbon remains in the grain boundaries, and the grain boundaries are strengthened, so that the brittle fracture can be prevented due to the weak grain boundaries after processing. Further, the present inventors have found that as the number of single precipitates of MnS and CuS increases from the complex precipitates of MnS to Cu in the distribution of precipitates of MnS, CuS, (Mn, Cu) S, the fine precipitates are uniformly distributed as the number of single precipitates of MnS and CuS increases, It was confirmed that the inner cushion indices were lowered and the workability was greatly improved. That is, when the ratio of 0.5 * (Mn + Cu) / S is in the range of 2 to 7, the number of precipitates is increased as the number of single precipitates of MnS and CuS is larger than the number of precipitates of (Mn, Cu) Respectively.

The cold-rolled steel sheet of the present invention and its manufacturing method will be described in detail below.

[Cold rolled steel sheet of the present invention]                     

The content of carbon (C) is preferably 0.0005 to 0.003%.

When the content of carbon (C) is less than 0.0005%, the crystal grains of the hot-rolled steel sheet have a low strength and an in-plane anisotropy. When the content of carbon is 0.003% or more, it is difficult to secure endurance due to a large amount of solid carbon in the steel, and the crystal grains of the annealed plate become finer and the ductility is greatly lowered. Therefore, the content of carbon (C) is preferably 0.0005 to 0.003% or less. In the present invention, the content of C may be increased to 0.002 to 0.003% to ensure anticorrosiveness, and the decarburization treatment for lowering the carbon content in the steelmaking process may be omitted.

The content of manganese (Mn) is preferably 0.03 to 0.2%.

Manganese is known as an element to prevent hot shortness due to sulfur by precipitating solid sulfur in MnS. In the present invention, it is preferable to set the content of manganese to 0.03 to 0.2% on the basis of the result of research that improvement of yield strength and in-plane anisotropy is ensured by basically securing the anti-oxidative property by precipitating very fine MnS when the content of manganese and sulfur becomes appropriate Do. If the content of manganese is 0.03% or more, the above-mentioned effect can be exhibited. If the content of manganese is more than 0.2%, the content of manganese is high and coarse MnS precipitates are formed and the endurance is poor

The content of phosphorus (P) is preferably 0.03 to 0.06%.

When the content of phosphorus is less than 0.03%, it is difficult to secure the desired strength. When the content of phosphorus is more than 0.06%, the ductility and formability are lowered, so it is desirable to set the content to 0.03 to 0.06%.

The content of sulfur (S) is preferably 0.003 to 0.025%.

When the content of sulfur (S) is less than 0.003%, not only the precipitation amount of MnS, CuS, (Mn, Cu) S is small but also the size of precipitates precipitated becomes very large, When the content of sulfur is more than 0.025%, the content of sulfur dissolved is large, so that ductility and moldability are greatly lowered, and there is a fear of heat brittleness.

The content of aluminum (Al) is preferably 0.01 to 0.1%.

Aluminum is an element to be added as a deoxidizer and is added to precipitate nitrogen in the steel to enhance the effect. When the addition amount is less than 0.01%, there is no strengthening effect. When the addition amount exceeds 0.1%, the ductility decreases sharply.

The content of nitrogen (N) is preferably 0.005 to 0.02%.

Nitrogen is added in an amount of 0.005% or more for the purpose of strengthening by AlN precipitation. When the amount of addition exceeds 0.02%, the formability is lowered. Therefore, it is preferably 0.005 to 0.02%.

The content of copper (Cu) is preferably 0.005 to 0.2%.

Copper has a precipitation content of 0.2 탆 or less when the content ratio of Cu and S and the cooling rate before winding in the hot rolling process become appropriate, thereby decreasing the amount of solid carbon in the crystal grain to improve the aging property, the secondary process brittleness, Based on the research that it is improved. If the content of copper is 0.005% or more, it can be precipitated finely. If the content is more than 0.2%, copper precipitates excessively and the aging resistance is poor.

The sum of Mn and Cu is preferably 0.3% or less. When the sum of Mn and Cu is 0.3% or more, the size of the precipitate becomes large and it is difficult to ensure the aging property

The weight ratio of Mn, Cu and S is preferably 0.5 * (Mn + Cu) / S: 2 to 20.

S precipitates as MnS, CuS, (Mn, Cu) S in combination with Mn and Cu. The precipitation state of these precipitates varies according to the addition amount of Mn, Cu and S, and the aging index, secondary process embrittlement, In-plane anisotropy index. According to the study of the present invention, an effective precipitate is obtained when the addition ratio of Mn, Cu and S (0.5 * (Mn + Cu) / S (content of Mn, Cu and S is% by weight) (Mn + Cu) / S ratio is in the range of 2 to 20, the average size of the precipitates is 0.2 탆. When the ratio of 0.5 * (Mn + Cu) / S is in the range of 2 to 20, When the ratio of 0.5 * (Mn + Cu) / S is less than 7 (Mn, Cu) / S, the number of the precipitates and the number of the precipitates are significantly different from the ratio of 0.5 * (Mn + Cu) / S is larger than 7, the number of distributions decreases even though the size difference of the precipitates is small. (MN, Cu) S complex precipitates are increased in amount.

In the steel component system of the present invention, the addition ratio (0.52 * Al / N) of Al and N is more preferably 1 to 5. When the addition ratio of Al and N (0.52 * Al / N) is less than 1, aging due to solid solution N may occur.

The cold-rolled steel sheet of the present invention may further include Mo or / and V. [

The content of molybdenum (Mo) is preferably 0.01 to 0.2%.

Molybdenum is added as an element to increase the plastic anisotropy index. When the content is more than 0.01%, the plastic anisotropy index becomes large. When the content exceeds 0.2%, plastic anisotropy index does not increase any more and may cause hot brittleness.

The content of vanadium (V) is preferably 0.01 to 0.2%.

Vanadium is added to precipitate solute C to ensure non-aging properties. When the content is more than 0.01%, non-aging properties can be obtained. When the content exceeds 0.2%, the plastic anisotropy index is lowered.

It is more preferable that the weight ratio (V / C) of V and C satisfies 1 to 20. If the weight ratio of V and C is less than 1, the precipitation effect of the solid solution C is not large. If the weight ratio of V and C is more than 20, the plastic anisotropy index is low.

The average size of precipitates in the component system of the present invention is preferably 0.2 탆 or less.

According to the results of the present invention, the size of MnS, CuS and (Mn, Cu) S precipitates directly affect the aging index, secondary process brittleness, plastic anisotropy index and in-plane anisotropy index, In the case of exceeding 0.2 탆, the aging index increases sharply, and the secondary process brittleness, plastic anisotropy index and in-plane anisotropy index are not good.

Furthermore, when the distribution number of precipitates of 0.2 占 퐉 or less in the component system of the present invention is 3 × 10 ⁶ or more, the plastic anisotropy index is increased, and the in-plane anisotropy index is lowered, and the workability is greatly improved. Considering that the plastic anisotropy index generally increases and the in-plane anisotropy index rises and the plasticity anisotropy index is limited in terms of processability, the specific change in the plastic anisotropy index and in-plane anisotropy index is notable depending on the number of precipitates distributed . Further, in the present invention, when the ratio of 0.5 * (Mn + Cu) / S is 2 to 7, the distribution number of precipitates of 0.2 μm or less greatly increases to 3 × ^{10 8} or more, and the workability is further improved.

[Production method of cold-rolled steel sheet]

The present invention is characterized in that an average size of MnS, CuS, and (Mn, Cu) S precipitates in the cold rolled steel sheet is 0.2 μm or less through hot rolling and cold rolling, The average size of these precipitates in the cold-rolled sheet is directly influenced by the addition conditions, the reheating temperature, the coiling temperature, and especially the cooling rate after hot rolling.

[Hot rolling condition]

In the present invention, the steel satisfying the above-mentioned stress is reheated and hot-rolled. The reheating temperature is preferably 1100 DEG C or higher. When the reheating temperature is less than 1100 ° C, the coarse CuS produced during the continuous casting remains in a completely undissolved state due to the low reheating temperature, so that coarse precipitates remain even after the hot rolling.

The hot rolling is preferably carried out under the condition that the finishing rolling temperature is equal to or higher than the Ar ₃ transformation temperature. When the finishing rolling temperature is lower than the Ar ₃ transformation temperature, not only the workability is lowered due to the production of the pressure-relief but the ductility is greatly reduced.

The cooling rate before hot rolling is preferably 300 DEG C / min or more. According to the present invention, when the cooling rate is less than 300 占 폚 / min, the average size of the precipitates exceeds 0.2 占 퐉 even if 2? 0.5 * (Mn + Cu) / S? That is, as the cooling rate increases, a large number of nuclei are generated and the precipitates become finer. When the ratio of 0.5 * (Mn + Cu) / S is more than 20, precipitates are not fine because the number of coarse precipitates unreacted in the reheating step increases and the cooling rate is increased, -0.21% Mn-0.043% P-0.005% S-0.07% Al-0.014% N-0.05% Cu, 0.5 * (Mn + Cu) / S = 26. 2, it is not necessary to limit the upper limit of the cooling rate because the size of the precipitate becomes finer as the cooling rate is increased. However, even if the cooling rate is 1000 ° C / min or more, More preferably 300 to 1000 占 폚 / min.

[Winding condition]

After hot rolling as described above, winding is performed, and the winding temperature is preferably 700 DEG C or lower. When the coiling temperature exceeds 700 ° C, the precipitates grow too coarse and the anti-aging properties deteriorate.

[Cold rolling conditions]

The cold rolling is preferably performed at a reduction ratio of 50 to 90%. When the cold rolling reduction rate is less than 50%, the amount of annealed recrystallized nuclei is small, so that the grain size grows too large during annealing and the strength and formability are lowered due to coarsening of the annealed recrystallized grains. If the cold rolling reduction ratio exceeds 90%, the formability is improved but the amount of nucleation is too large, so that the annealed recrystallized grains are rather too fine and the ductility is lowered.

[Continuous Annealing]

The continuous annealing temperature plays an important role in determining the material of the product. In the present invention, it is preferable to carry out the reaction in a temperature range of 500 to 900 ° C. When the continuous annealing temperature is less than 500 캜, recrystallization is not completed and a desired ductility value can not be secured. When the annealing temperature is higher than 900 캜, the strength is lowered due to coarsening of recrystallized grains. The continuous annealing time is maintained so that the recrystallization is completed. When the time is about 10 seconds or longer, the recrystallization is completed. Preferably, the continuous annealing time is in the range of 10 seconds to 30 minutes.

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

In the examples, the mechanical properties were measured by processing cold-rolled steel sheets into standard specimens according to ASTM E-8 standard. Yield strength, tensile strength, elongation, plastic anisotropy index ( _rm value), in-plane anisotropy index (Δr value) and aging index (AI) were measured using a tensile tester (INSTRON, Model 6025). In the examples, the plastic anisotropy index (r _m ) and the in-plane anisotropy index (? R) were obtained by the following formulas. _{r m = (r 0 + 2r} 45 + r 90) / 4, Δr = (r 0 -2r 45 + r 90) / 2

The average size of the precipitates and the number of precipitates in the specimen are the sizes and distribution numbers of all the precipitates present in the substrate.

[Example 1]

Steel slabs of Specimen Nos. 1-7 of Table 1 were reheated at 1200 ° C, finishing hot-rolled, cooled at a rate of 400 ° C / min, and wound at 650 ° C. The finish rolling temperature of not less than Ar ₃ transformation point is 910 ℃, continuous annealing was performed by heating for 40 seconds to 750 ℃ to 10 ℃ / second.

Psalter
number Chemical composition (% by weight) Mn + Cu 0.5 *
(Mn + Cu) / S 0.52 * Al / N C Mn P S Al N Cu Ti 0.0005 to 0.003 0.03 to 0.2 0.03 to 0.06 0.003 to 0.025 0.01 to 0.1 0.005-0.2 0.005
~ 0.2 0.3 2 to 20 1-5 A1 0.0023 0.05 0.04 0.015 0.032 0.0097 0.03 - 0.08 2.67 1.72 A2 0.0018 0.1 0.042 0.012 0.042 0.0072 0.03 - 0.13 5.42 3.03 A3 0.0021 0.1 0.05 0.01 0.057 0.0080 0.08 - 0.18 9 3.71 A4 0.0025 0.15 0.05 0.008 0.065 0.0075 0.1 - 0.25 15.63 4.51 A5 0.0025 0.05 0.045 0.017 0.042 0.0072 0.01 - 0.06 1.76 3.03 A6 0.0022 0.15 0.04 0.009 0.038 0.0014 0.05 - 0.2 11.1 14.1 A7 0.0016 0.15 0.05 0.005 0.05 0.0070 0.2 - 0.35 35 3.71 A8 0.0024 0.4 0.07 0.01 0.04 0.0016 0.05 13

city
side
time
number Mechanical property Precipitation
water
Average
size
(탆) Precipitate
Number
(Pieces / mm ² ) Remarks surrender
burglar
(MPa) Seal
burglar
(MPa) year
God
rate
(%) Plasticity
Anisotropy
Indices
(r _m ) In-plane
Anisotropy
Indices
(R) prescription
Indices
(AI-
(MPa) Secondary
Processing
Brittle
(DBTT-C) A1 246 352 54 1.96 0.29 22 - 70 0.04 4.9X10 ⁸ Invention river A2 252 356 53 1.94 0.28 25 - 70 0.05 3.5X10 ⁸ Invention river A3 250 348 50 1.89 0.32 27 -60 0.07 3.2X10 ⁶ Invention river A4 255 350 48 1.86 0.35 22 - 60 0.09 4.1X10 ⁶ Invention river A5 243 340 43 1.68 0.39 36 - 70 0.21 9.2X10 ⁴ Comparative steel A6 223 328 48 1.89 0.32 27 - 70 0.09 9.3X10 ⁶ Comparative steel A7 238 342 43 1.72 0.34 38  - 70 0.32 9.3X10 ⁴ Comparative steel A8 210 353 40 1.73 0.58 0 + 0 0.21 1.5X10 ⁵ Conventional steel

As shown in Tables 1 and 2, A1-A4 (invention steel) has an average size of precipitates of 0.2 μm or less and an aging index of 30 Mpa or less to ensure aging, so that the plastic anisotropy index is 1.8 or more and the in- And the high strength is ensured. Especially, A1 ~ A2 with a ratio of 0.5 * (Mn + Cu) / S of 7 or less was very excellent in strength and formability, and it was confirmed that very fine precipitates precipitated by MnS or CuS alone were uniformly distributed. Due to the distribution characteristics of such precipitates, the plastic anisotropy index is high and the in-plane anisotropy index is low, thus exhibiting excellent processing characteristics. (Mn, Cu) S precipitates in the case of the ratio of 0.5 * (Mn + Cu) / S of not less than 7 despite the small difference in size of the precipitates.

A5 (comparative steel) has low strength, low moldability and high aging index because of low ratio of 0.5 * (Mn + Cu) / S.

In A6 (comparative steel), the content of N and the ratio of 0.52 * Al / N were out of the range of the present invention, and the target strength was not secured.

A7 (comparative steel) has a high ratio of 0.5 * (Mn + Cu) / S, so strength and formability are poor and the age index is high.

A8 (conventional steel) is a P-added high strength IF (Ti) cold rolled steel sheet, which has an excellent aging index but a high ductile-brittle transition temperature, so there is a high risk of fracture at impact even at room temperature.

[Example 2]

The steel slabs shown in Table 3 were reheated at 1200 ° C, finishing hot-rolled, cooled at a rate of 600 ° C / min, and wound at 650 ° C. The rolled hot rolled sheet was subjected to cold rolling and continuous annealing at a reduction ratio of 75%. The finish rolling temperature of not less than Ar ₃ transformation point is 910 ℃, continuous annealing was performed by heating for 40 seconds to 750 ℃ to 10 ℃ / second.

Specimen Number Chemical composition (% by weight) C Mn P S Al N Cu Mo Mn +
Cu 0.5 *
(Mn +
Cu)
/ S 0.52 *
Al / N 0.0005 ~
0.003 0.03 to 0.2 0.03 to 0.06 0.003 ~
0.025 0.01
~ 0.1 0.005 ~ 0.02 0.005
~ 0.2 0.01
~ 0.2 0.3 2 to 20 1-5 B1 0.0015 0.12 0.044 0.012 0.051 0.011 0.038 0.019 0.16 6.58 2.41 B2 0.0027 0.11 0.04 0.009 0.052 0.0092 0.053 0.082 0.16 9.06 2.94 B3 0.0017 0.09 0.042 0.01 0.055 0.0094 0.084 0.17 0.17 8.7 3.04 B4 0.0022 0.12 0.045 0.011 0.05 0.0095 0.11 0.27 0.23 10.5 2.74

Psalter
number Mechanical property Precipitation
water
Average
size
(탆) Number of precipitates
(Pieces / mm ² ) Remarks surrender
burglar
(MPa) The tensile strength
(MPa) year
God
rate
(%) Plasticity
Anisotropy index
(r _m ) In-plane
Anisotropy
Indices
(R) prescription
Indices
(AI-
(MPa) Secondary
Processing
Brittle
(DBTT-
° C) B1 244 350 54 2.32 0.39 18 - 70 0.05 5.2X10 ⁸ Invention river B2 255 353 53 2.29 0.36 27 - 70 0.04 6.4X10 ⁸ Invention river B9 250 352 51 2.39 0.41 16 - 70 0.06 4.4X10 ⁶ Invention river B4 254 357 48 1.83 0.29 22 - 70 0.08 3.3X10 ⁶ Comparative steel

As shown in Tables 3 and 4, in the case of B1 to B3, it was found that the addition of Mo improved workability. In the case of B4, Mo was added in an excessive amount, and the workability was rather poor.

[Example 3]

The steel slabs shown in Table 5 were reheated at 1200 ° C, finishing hot-rolled, cooled at a rate of 600 ° C / min, and wound at 650 ° C. The rolled hot rolled sheet was subjected to cold rolling and continuous annealing at a reduction ratio of 75%. The finish rolling temperature of not less than Ar ₃ transformation point is 910 ℃, continuous annealing was performed by heating for 40 seconds to 750 ℃ to 10 ℃ / second.

Specimen Number Chemical composition (% by weight) C Mn P S Al N Cu V Mn + Cu 0.5 *
(Mn +
Cu)
/ S 0.52 *
Al / N 0.25 *
V / C 0.0005 ~
0.003 0.03 ~
0.2 0.03
~ 0.06 0.003 to 0.025 0.01 to 0.1 0.005-0.2 0.005-0.2 0.01 to 0.2 0.3 2 to 20 1-5 1 to 20 C1 0.0018 0.1 0.041 0.009 0.045 0.0095 0.039 0.02 0.14 7.72 2.46 2.78 C2 0.0025 0.09 0.044 0.012 0.058 0.011 0.055 0.076 0.15 6.04 2.74 7.6 C3 0.0022 0.08 0.04 0.011 0.055 0.01 0.068 0.159 0.15 6.73 2.86 18.1 C4 0.0028 0.11 0.045 0.008 0.045 0.011 0.085 0.24 0.2 12.2 2.13 21.4

city
side
time
number Cognitive property Precipitation
water
Average
size
(탆) Number of precipitates
(Pieces / mm ² ) Remarks surrender
burglar
(MPa) The tensile strength
(MPa) year
God
rate
(%) Plasticity
Anisotropy
Indices
(r _m ) In-plane
Anisotropy
Indices
(R) prescription
Indices
(AI-
(MPa) Secondary
Processing
Brittle
(DBTT
- ° C) C1 195 349 53 1.93 0.27 0 - 70 0.05 4.5X10 ⁸ Invention river C2 204 350 52 1.92 0.26 0 - 70 0.05 5.4X10 ⁸ Invention river C3 210 355 51 1.89 0.22 0 - 70 0.05 4.8X10 ⁸ Invention river C4 252 374 46 1.8 0.21 0 - 70 0.09 1.3X10 ⁶ Comparative steel

As shown in Tables 5 and 6, the non-aging characteristics could be obtained by adding V. In the case of C4, it was found that the deterioration of workability was severe when V was added in excess.

[Example 4]

The steel slabs shown in Table 7 were reheated at 1200 ° C, finishing hot-rolled, cooled at a rate of 600 ° C / min, and wound at 650 ° C. The rolled hot rolled sheet was subjected to cold rolling and continuous annealing at a reduction ratio of 75%. The finish rolling temperature of not less than Ar ₃ transformation point is 910 ℃, continuous annealing was performed by heating for 40 seconds to 750 ℃ to 10 ℃ / second.

Specimen Number C Mn P S Al N Cu Mo V Mn + Cu 0.5 *
(Mn +
Cu)
/ S 0.52 * Al / N 0.25 * V / C 0.0005 ~
0.003 0.03 to 0.2 0.03 to 0.06 0.003 to 0.025 0.01 to 0.1 0.005 ~ 0.02 0.005-0.2 0.01 to 0.2 0.01
~ 0.2 0.3 2 to 20 1-5 1 to 20 D1 0.0016 0.11 0.042 0.01 0.042 0.01 0.049 0.018 0.016 0.16 7.95 2.18 2.5 D2 0.0023 0.09 0.046 0.009 0.055 0.0093 0.035 0.081 0.074 0.13 6.9 3.08 8.04 D3 0.0022 0.08 0.04 0.011 0.055 0.01 0.068 0.17 0.15 0.15 6.73 2.86 17 D4 0.002 0.1 0.041 0.008 0.04 0.012 0.08 0.22 0.23 0.18 11.3 1.73 28.8 D5 0.0018 0.08 0.038 0.01 0.058 0.009 0.054 0.38 0.084 0.134 6.7 3.35 11.7 D6 0.0025 0.09 0.045 0.012 0.041 0.01 0.078 0.075 0.34 0.168 7 1.025 34

Specimen Number Mechanical property Remarks Yield strength
(MPa) The tensile strength
(MPa) year
God
rate
(%) Plasticity
Anisotropy index
(r _m ) In-plane
Anisotropy
Indices
(R) prescription
Indices
(AI-
(MPa) Secondary
Processing
Brittle
(DBTT
- ° C) Precipitation
water
Average
size
(탆) Number of precipitates
(Pieces / mm ² ) D1 193 345 53 2.32 0.38 0 - 70 0.06 4.8X10 ⁸ Invention river D2 201 353 52 2.35 0.39 0 - 70 0.05 5.9X10 ⁸ Invention river D3 208 357 50 2.43 0.38 0 - 70 0.05 5.3X10 ⁸ Invention river D4 232 373 45 1.89 0.28 0 - 70 0.07 2.3X10 ⁶ Comparative steel D5 241 378 45 1.83 0.22 0 - 70 0.07 1.8X10 ⁶ Comparative steel D6 207 361 41 2.22 0.47 0 - 10 0.22 4.7X10 ⁴ Comparative steel

As shown in Tables 7 and 8, it can be seen that the combined addition of Mo and V improves workability and non-vitality.

As described above, the cold-rolled steel sheet provided according to the present invention has a ductile-to-brittle transition temperature of -60 ° C or less, excellent in secondary working brittleness, while having an aging index of 30 MPa or less, And the in-plane anisotropy index is low, and the workability is extremely excellent.

Claims (24)

0.003 to 0.025% of Al, 0.01 to 0.1% of Al, 0.005 to 0.02% of N, 0.005 to 0.02% of Cu, 0.005 to 0.03% of Al, Wherein the Mn, Cu and S satisfy the following conditions Mn + Cu? 0.3, 0.5 * (Mn + Cu) / S: 2 to 20, the balance Fe and other unavoidable impurities, And an average size of the MnS, CuS, (Mn, Cu) S precipitates is not more than 0.2 占 퐉, and the number of the precipitates is not less than ³ × 10 ⁶ / High-strength cold-rolled steel sheet with excellent brittleness. 2. The cold rolled steel sheet according to claim 1, wherein the C is 0.002% to 0.003% or less. 2. The cold rolled steel sheet according to claim 1, wherein Al and N satisfy the following condition: 0.52 * Al / N: 1 to 5. delete delete 2. The cold rolled steel sheet according to claim 1, wherein 0.5 * (Mn + Cu) / S is 2 to 7. 7. The cold rolled steel sheet according to claim 6, wherein the number of precipitates is 3 x ^{10 8} / mm ² or more. The high-strength, time-aged, cold-rolled steel sheet according to any one of claims 1 to 3, 6 or 7, further comprising 0.01 to 0.2% Mo. The high-strength, time-aged cold-rolled steel sheet according to any one of claims 1 to 3, 6 or 7, further comprising 0.01 to 0.2% of V. 10. The cold rolled steel sheet according to claim 9, wherein the V and C satisfy 0.25 * V / C in the range of 1 to 20. [ 9. The cold rolled steel sheet according to claim 8, wherein V is further added in an amount of 0.01 to 0.2%. 12. The cold rolled steel sheet according to claim 11, wherein the V and C satisfy 0.25 * V / C of 1 to 20. 15. A high strength, 0.003 to 0.025% of Al, 0.01 to 0.1% of Al, 0.005 to 0.02% of N, 0.005 to 0.02% of Cu, 0.005 to 0.03% of Al, And the steel comprising Mn, Cu and S satisfying the following conditions Mn + Cu? 0.3, 0.5 * (Mn + Cu) / S: 2 to 20 and composed of the remaining Fe and other unavoidable impurities, Rolled at a temperature not lower than the Ar ₃ transformation point, and then hot-rolled at a temperature of not less than the Ar ₃ transformation point, cooled at a temperature of 300 to 400 ° C / min and rolled at a temperature of 700 ° C or lower, then cold- (Mn, Cu) S precipitates are formed so that the number of the precipitates is 3 × 10 ⁶ / mm ² or more, and the average size of the MnS, CuS, (Mn, Cu) By weight based on the total weight of the steel sheet. 14. The method of claim 13, wherein C is in a range of more than 0.002% to less than 0.003%. 14. The method according to claim 13, wherein Al and N satisfy the following condition: 0.52 * Al / N: 1 to 5. delete delete 14. The method according to claim 13, wherein the 0.5 * (Mn + Cu) / S ratio is 2 to 7. 8. The method according to claim 7, The method according to claim 18, wherein the number of precipitates is 3 x ^{10 8} / mm ² or more. The steel sheet according to any one of claims 13 to 15, 18 and 19, further comprising 0.01 to 0.2% Mo. The present invention relates to a method for producing a high strength, Way. 20. The method of manufacturing a high strength, weather-resistant, cold-rolled steel sheet according to any one of claims 13 to 15, 18, 19, further comprising 0.01 to 0.2% of V. Way. The method according to claim 21, wherein V and C satisfy 0.25 * V / C of 1 to 20, respectively. The method of manufacturing a high strength intrinsic age cold rolled steel sheet according to claim 20, wherein V is further added in an amount of 0.01 to 0.2%. 24. The method according to claim 23, wherein V and C satisfy 0.25 * V / C in the range of 1 to 20. 20. A method of manufacturing a high strength intrinsic austenitic cold-rolled steel sheet,

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