KR101156697B1 - Ferritic steel sheet and method for manufacturing the same - Google Patents

Abstract

The present invention relates to a ferritic steel sheet and a method of manufacturing the same. Ferritic steel sheet according to an embodiment of the present invention is more than 0wt% and less than 0.0055wt% carbon (C), 0.01wt% to 0.35wt% silicon (Si), 0.15wt% to 0.7wt% manganese (Mn), 0.007 wt% to 0.013 wt% sulfur (S), 0.04 wt% to 0.09 wt% phosphorus (P), 0.025 wt% to 0.065 wt% aluminum (Al), more than 0 wt% and no more than 0.005 wt% nitrogen (N) , 0.005wt% to 0.015wt% titanium (Ti), more than 0wt% and less than 0.06wt% copper (Cu), 0.01wt% to 0.05wt% niobium (Nb), more than 0.001wt% and less than 0.005wt% boron (B) and the remaining Fe and impurities. The contents of titanium (Ti), niobium (Nb), carbon (C), nitrogen (N) and sulfur (S) satisfy the following formula.

4C + 48N / 14 <Ti * <4C + 48S / 32, Ti * = Ti + 48Nb / 93

Ferritic steel, strength, phosphorus, boron

Description

Ferritic steel sheet and manufacturing method thereof {FERRITIC STEEL SHEET AND METHOD FOR MANUFACTURING THE SAME}

The present invention relates to a ferritic steel sheet and a method of manufacturing the same. More specifically, the present invention relates to a ferritic steel sheet excellent in strength and a method of manufacturing the same.

Ferritic steel sheet is widely used as a structural material, so it must have a certain level of strength or more. In order to increase the strength of the ferritic steel sheet, various methods such as hardening precipitates and strengthening solid solutions are used.

However, if the amount of the added element is not adjusted to increase the strength of the ferritic steel sheet, the physical properties of the steel sheet may be deteriorated except for the strength of the ferritic steel sheet.

An object is to provide a ferritic steel sheet having excellent strength. In addition, an object of the present invention is to provide a method for producing a ferritic steel sheet.

Ferritic steel sheet according to an embodiment of the present invention, more than 0wt% and less than 0.0055wt% carbon (C), 0.01wt% to 0.35wt% silicon (Si), 0.15wt% to 0.7wt% manganese (Mn ), 0.007 wt% to 0.013 wt% sulfur (S), 0.025 wt% to 0.065 wt% aluminum (Al), more than 0 wt% and less than 0.005 wt% nitrogen (N), 0.005 wt% to 0.015 wt% Titanium (Ti), more than 0 wt% and less than 0.06 wt% copper (Cu), 0.01 wt% to 0.05 wt% niobium (Nb), 0.04 wt% to 0.09 wt% phosphorus (P), more than 0.001 wt% A ferritic steel sheet containing boron (B) and remaining iron (Fe) and impurities of 0.005 wt% or less, wherein the titanium (Ti), the niobium (Nb), the carbon (C), the nitrogen (N), and the sulfur ( The content of S) is a ferritic steel sheet satisfying the following formula.

4C + 48N / 14 <Ti * <4C + 48S / 32, Ti * = Ti + 48Nb / 93

Contents of the copper (Cu), the manganese (Mn) and the sulfur (S) may satisfy the following formula.

 (Mn + Cu) / S> 30

The content of the boron (B) and the nitrogen (N) may satisfy the following formula.

0.001 <B <0.5 N

The content of aluminum (Al) and nitrogen (N) may satisfy the following formula.

Al / N> 10

The ferritic steel sheet includes BN precipitates and NbC precipitates, and the amount of NbC precipitates may be greater than the amount of BN precipitates. The ferritic steel sheet may include an AlN precipitate. BN precipitate may not be present in the ferritic steel sheet. On the other hand, the tensile strength of the ferritic steel sheet may be 340MPa to 500MPa.

Method for producing a ferritic steel sheet according to an embodiment of the present invention, more than 0wt% and less than 0.0055wt% carbon (C), 0.01wt% to 0.35wt% silicon (Si), 0.15wt% to 0.7wt% Manganese (Mn), 0.007 wt% to 0.013 wt% sulfur (S), 0.025 wt% to 0.065 wt% aluminum (Al), more than 0 wt% and less than 0.005 wt% nitrogen (N), 0.005 wt% to 0.015 wt% titanium (Ti), more than 0 wt% and less than 0.06 wt% copper (Cu), 0.01 wt% to 0.05 wt% niobium (Nb), 0.04 wt% to 0.09 wt% phosphorus (P), 0.0002 wt% Heating a slab comprising% to 0.0015 wt% of boron (B), and the remaining iron (Fe) and impurities, hot rolling the heated slab to provide a hot rolled steel sheet, and winding the hot rolled steel sheet. The method may include preparing a cold rolled steel sheet by releasing the wound hot rolled steel sheet and cold rolling the steel sheet, and annealing the cold rolled steel sheet at 700 ° C. to 850 ° C. to produce a ferritic steel sheet.

The method of manufacturing the ferritic steel sheet may further include cooling the manufactured ferritic steel sheet at a rate of 10 ° C./s to 100 ° C./s. In addition, in the providing of the cold rolled steel sheet, the slab may be rolled at a cold rolling reduction rate of 50% to 85%.

In the manufacturing of the ferrite steel sheet, the ferrite steel sheet includes a BN precipitate and NbC precipitates, the amount of the NbC precipitates may be greater than the amount of the BN precipitates.

A ferritic steel sheet having high strength can be produced. Therefore, a ferritic steel plate can be used for automobile steel plate etc.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a,” “an,” and “the” include plural forms as well, unless the phrases clearly indicate the opposite. As used herein, the term "comprising" embodies a particular characteristic, region, integer, step, operation, element, and / or component, and other specific characteristics, region, integer, step, operation, element, component, and / or group. It does not exclude the presence or addition of.

Unless defined otherwise, all terms including technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. Commonly used predefined terms are further interpreted as having a meaning consistent with the relevant technical literature and the present disclosure, and are not to be construed as ideal or very formal meanings unless defined otherwise.

1 shows a schematic flowchart of a method of manufacturing a ferritic steel sheet according to an embodiment of the present invention. The manufacturing method of the ferritic steel plate of FIG. 1 is only for illustrating this invention, Comprising: This invention is not limited to this.

As shown in FIG. 1, the method of manufacturing a ferritic steel sheet includes providing a slab (S10), heating the slab (S20), and hot rolling the slab (S30). In addition, the method of manufacturing the ferritic steel sheet may further include other steps.

First, in step S10 to provide a slab to produce a ferritic steel sheet. Here, the slab is more than 0wt% and less than 0.0055wt% carbon (C), 0.01wt% to 0.35wt% silicon (Si), 0.15wt% to 0.7wt% manganese (Mn), 0.007wt% to 0.013wt % Sulfur (S), 0.025 wt% to 0.065 wt% aluminum (Al), more than 0 wt% and less than 0.005 wt% nitrogen (N), 0.005 wt% to 0.015 wt% titanium (Ti), more than 0 wt% More than 0.06 wt% copper (Cu), 0.01 wt% to 0.05 wt% niobium (Nb), 0.04 wt% to 0.09 wt% phosphorus (P), more than 0.001 wt% and less than 0.005 wt% boron (B) And the remaining Fe and impurities.

The amount of carbon (C) is 0.0055 wt% or less. The carbon in the steel acts as an invasive solid solution to inhibit formation of {111} texture, which is advantageous for processability during the formation of the texture of the steel sheet during cold rolling and annealing, and to form carbonitride when the steel content exceeds 0.010%. It is also economically disadvantageous because the amount of titanium (Ti) or niobium (Nb), which is an element, needs to be increased. Therefore, the amount of carbon is limited to the above range.

The amount of silicon (Si) is 0.01wt% to 0.35wt%. If the amount of silicon is less than 0.01wt%, there is almost no strength improvement effect. If the amount of silicon is more than 0.35wt%, not only does silicon (Si) in the steel cause surface scale defects, but also temper color during annealing and unplating during plating. . Therefore, the amount of silicon is limited to the above range.

The amount of nitrogen (N) is 0.005 wt% or less. Nitrogen (N) in the steel is not only economically impaired workability when present in solid solution, but if the content exceeds 0.005%, the addition amount of titanium (Ti) or niobium (Nb) to fix the precipitate is uneconomical. Therefore, the amount of nitrogen is limited to the above range.

The amount of titanium (Ti) is 0.005 wt% to 0.015 wt% or less. Titanium (Ti) is added to precipitate carbon (C) and nitrogen (N) to prevent aging by solid carbon. If the amount of titanium is less than 0.005wt%, it is difficult to prevent aging phenomenon due to the large amount of solid solution carbon. If the amount of titanium exceeds 0.015wt%, the hot workability of the ferritic steel sheet is reduced. Therefore, the amount of titanium is adjusted to the above range.

The amount of niobium (Nb) is 0.01 wt% to 0.05 wt%. Niobium (Nb) is added to precipitate carbon (C) and nitrogen (N) to prevent aging by solid solution carbon. If the amount of niobium is less than 0.01wt%, it is difficult to prevent aging phenomenon due to the high amount of solid solution, and if the amount of niobium is more than 0.05wt%, the strength increases, so that the hot workability of the ferritic steel sheet is deteriorated. Therefore, the amount of niobium is adjusted to the above range.

Meanwhile, when niobium (Nb) is added, NbC may be precipitated more than BN by adjusting the amount of niobium (Nb) added to precipitate NbC.

In addition, the contents of titanium (Ti), niobium (Nb), carbon (C), nitrogen (N) and sulfur (S) satisfy the following formula (1).

4C + 48N / 14 <Ti * <4C + 48S / 32, Ti * = Ti + 48Nb / 93

In the case of 4C + 48N / 14> Ti *, that is, when carbon (C) and nitrogen (N) are more included than titanium (Ti) and niobium (Nb), workability is deteriorated. On the other hand, when Ti *> 4C + 48S / 32, that is, when titanium (Ti) and niobium (Nb) are included more than carbon (C) and sulfur (S), precipitation of MnS, CuS, and AlN may decrease, resulting in a decrease in strength. Can be economically disadvantageous.

The amount of copper (Cu) is 0.06 wt% or less. If the amount of copper (Cu) exceeds 0.06wt%, it may locally segregate in the surface layer portion of the slab to generate hot brittleness. Therefore, the amount of copper is limited to the above range.

The amount of manganese (Mn) is 0.15 wt% to 0.7 wt%. Manganese (Mn) in steel is added as a substitutional solid solution strengthening element to secure the strength, but when the amount of manganese is less than 0.15wt%, the effect of strength improvement is small. Degrades. Therefore, the amount of manganese is limited to the above range.

The amount of sulfur (S) is 0.007 wt% to 0.013 wt%. The amount of sulfur (S) should be limited to less than 0.007wt% to improve the workability of the steel. However, in the ferritic steel sheet according to the embodiment of the present invention, the precipitate of CuS or Cu ₂ S is formed by the added copper (Cu), so that even if the amount of sulfur is added to 0.007wt% or more, the workability does not deteriorate and it is effective in obtaining high strength. . On the other hand, when the amount of sulfur is less than 0.007wt%, hot brittleness may occur due to segregation and melting of pure copper (Cu). If the amount of sulfur exceeds 0.013 wt%, workability is drastically lowered. Therefore, the amount of sulfur is limited to the above range.

In addition, the contents of copper (Cu), manganese (Mn) and sulfur (S) satisfy the following formula (2).

(Mn + Cu) / S> 30

When (Mn + Cu) / S <30, that is, when the amount of manganese (Mn) or copper (Cu) is less than that of sulfur (S), the strength improvement effect is small and the workability is poor.

The amount of phosphorus (P) is from 0.04 wt% to 0.09 wt%. Phosphorus (P) is a representative solid solution strengthening element added to increase the strength, and the amount of phosphorus should be more than 0.04wt% to obtain some solid solution strengthening effect. However, when the amount of phosphorus exceeds 0.09 wt%, the elongation of the ferritic steel sheet decreases and secondary work brittleness tends to occur. Therefore, the amount of phosphorus is adjusted to the above range.

The amount of boron (B) is more than 0.001 wt% and less than 0.005 wt%. When boron is added, the grain boundary strengthening effect is obtained. If the boron content is less than 0.001 wt%, the effect is insignificant. Therefore, the amount of boron is adjusted to the above-mentioned range.

The amount of aluminum (Al) is 0.025 wt% to 0.065 wt% or less. Sol.Al in the steel precipitates with AlN together with nitrogen, thus reducing the content of solid solution nitrogen (N) to prevent aging. However, if the content of aluminum (Al) is less than 0.025wt%, the effect is too small. In addition, when the content of aluminum (Al) exceeds 0.065%, the size of AlN precipitates in the steel is coarsened, and the precipitation strengthening effect and the ferrite grain refining effect are reduced. Therefore, the amount of aluminum is adjusted to the above range.

As described above, when phosphorus (P) is added, there is a solid solution effect, but there is a problem in that brittleness increases during secondary processing. In this case, when boron (B) is added together with phosphorus (P), boron (B) first segregates at the grain boundaries to stabilize the grain boundaries and suppress grain boundary segregation of phosphorus (P).

However, when too much boron (B) is added, hot rolling and cold rolling may be degraded, and BN precipitates are formed to reduce the grain boundary stabilization effect of boron (B) and to reduce the generation of AlN precipitates, thereby lowering the strength improving effect.

Therefore, the content of boron (B), nitrogen (N) and aluminum (Al) should be adjusted to satisfy the following formulas (3) and (4).

0.001 <B <0.5 N

Al / N> 10

If the boron (B) is less than 0.001wt% strength improvement effect is small and secondary processing brittleness may occur. On the other hand, when B> 0.5N, that is, when boron (B) is contained more compared with nitrogen (N), the strength improvement effect is large, but workability worsens.

When Al / N <10, that is, containing less aluminum (Al) than nitrogen (N), the effect of improving strength is small.

Except for the above-mentioned elements, the rest of the ferritic steel sheet consists of iron (Fe) and other unavoidable impurities.

Next, in step S20, the slab having the above-described composition is heated. The slab may be heated to 1100 ° C to 1150 ° C. If the heating temperature of the slab is less than 1100 ℃, there is a problem that the casting structure in the slab remains or the segregation of alloying elements is not removed, and if it exceeds 1150 ℃, excessive grain growth of austenite grains occurs, resulting in poor workability or local manganese (Mn). ) And a high temperature ductility deteriorates due to segregation of copper (Cu).

Step S30 may further include the step of hot rolling the heated slab to produce a hot rolled steel sheet. Step S30 may further include winding the manufactured hot rolled steel sheet.

In addition to the above-described steps, cooling the manufactured ferritic steel sheet at a rate of 10 ° C / sec to 100 ° C / sec may be further included. If the cooling rate is slower than 10 ° C / sec, there is a problem that austenite grains grow excessively or coarse ferrite grains are formed before the phase transformation of the steel sheet is completed after hot rolling is completed. The temperature deviation of the steel sheet is severe and there is a high possibility of material deviation and shape defects.

In addition, the method for manufacturing a ferritic steel sheet according to an embodiment of the present invention may further include a step of manufacturing a cold rolled steel sheet by unrolling the cold rolled hot rolled steel sheet. Further, in the step of manufacturing the cold rolled steel sheet can be hot rolled steel sheet at a cold rolling reduction rate of 50% to 85%. If the rolling rate of the hot rolled steel sheet is too low, the thickness of the ferritic steel sheet is too thick, which makes secondary processing difficult. To solve this problem, when the thickness of the slab is rolled thinly, the excessive hot rolling rate causes an overload of the equipment or even shape. There is a problem that is difficult to manufacture. In addition, when rolling with a cold rolling rate of more than 85%, there is a problem that an overload of the cold rolling equipment occurs or the process cost increases. Therefore, the hot rolled steel sheet is cold rolled at a reduction ratio in the above-described range.

In addition, the method of manufacturing a ferritic steel sheet according to an embodiment of the present invention may further comprise the step of annealing the cold-rolled steel sheet at 700 ℃ to 850 ℃ manufacturing a ferrite steel sheet. If the annealing temperature is less than 700 ℃, there is a problem that the workability is poor due to the lack of recrystallization of the cold rolled plate, if the temperature exceeds 850 ℃ excessive grain growth of the ferrite grains occurs or there is a problem of poor workability due to the re-use or growth of the precipitate. The prepared ferritic steel sheet contains MnS, CuS, AlN precipitates and (Ti, Nb) (C, N) precipitates and the amount of (Ti, Nb) (C, N) precipitates can be greater than the sum of MnS, CuS, AlN precipitates. have.

The tensile strength of the ferritic steel sheet thus prepared may be 340MPa to 500MPa.

Through the above-described step (S10) to step (S30) it can be produced a ferritic steel sheet excellent in strength. Therefore, the ferritic steel sheet manufactured by the method mentioned above can be used as a steel sheet for automobiles.

Hereinafter, the present invention will be described in more detail with reference to experimental examples. These experimental examples are only for illustrating the present invention, and the present invention is not limited thereto.

Experimental Example

To prepare a ferritic steel sheet using a slab having a composition of Table 1.

Steel grade C Si Mn S Al N Ti Cu Nb P B Etc A 0.0022 0.07 0.47 0.01 0.06 0.003 0.0014 0.05 0.04 0.05 0.0012 Balance Fe and impurities B 0.0021 0.04 0.42 0.01 0.06 0.003 0.0013 0.04 0.04 0.05 0.0014 C 0.0028 0.2 0.65 0.011 0.03 0.003 0.0013 0.05 0.04 0.07 0.0014 D 0.0027 0.16 0.65 0.009 0.06 0.003 0.01 0.055 0.03 0.06 0.0012 E 0.0034 0.28 0.65 0.01 0.06 0.003 0.0013 0.05 0.05 0.09 0.0014 F 0.0037 0.31 0.58 0.012 0.06 0.003 0.011 0.05 0.03 0.08 0.0011 G 0.0028 0.04 0.45 0.011 0.06 0.003 0.015 0.04 0.03 0.05 0.0012 H 0.0028 0.04 0.45 0.011 0.06 0.003 0.0013 0.04 0.03 0.05 0.0012 I 0.0021 0.03 0.42 0.012 0.05 0.003 0.009 0.055 0.02 0.06 0.0008 J 0.0023 0.09 0.48 0.012 0.06 0.003 0.0014 0.05 0.02 0.05 0.0013 K 0.0019 0.09 0.28 0.011 0.06 0.003 0.0022 0.02 0.03 0.05 0.0013 L 0.003 0.17 0.61 0.009 0.06 0.002 0.0012 0.05 0.04 0.07 0.0011 M 0.0029 0.19 0.63 0.008 0.06 0.002 0.0014 0.05 0.03 0.07 0.0014 N 0.0032 0.17 0.61 0.012 0.04 0.003 0.0014 0.05 0.04 0.07 0.0012 O 0.003 0.17 0.6 0.012 0.04 0.003 0.002 0.05 0.04 0.08 0.0025 P 0.0035 0.25 0.99 0.011 0.06 0.002 0.012 0.05 0.02 0.08 0.0014 Q 0.0038 0.27 0.97 0.012 0.03 0.003 0.0014 0.05 0.05 0.09 0.0012 R 0.0036 0.34 0.92 0.013 0.03 0.003 0.0013 0.05 0.05 0.09 0.0014

Experimental Example 1

A slab having a composition of steel A of Table 1 was manufactured to a thickness of 60 mm and a width of 180 mm, and then heated at 1130 ° C. for 60 minutes. The heated slab was hot rolled at a rolling start temperature of 1100 ° C. and a rolling end temperature of 920 ° C. to prepare a hot rolled steel sheet having a thickness of 3.2 mm. The hot rolled steel sheet thus prepared was cooled to 600 ° C. at a rate of 60 ° C./sec and then wound up at 550 ° C. The wound hot rolled steel sheet was unrolled and cold rolled to produce a cold rolled steel sheet, and the prepared cold rolled steel sheet was annealed at 700 ° C. to produce a ferritic steel sheet.

Experimental Example 2

Except that the steel B of Table 1 was used in the same manner as in Experiment 1.

Experimental Example 3

Except for using the steel C in Table 1 was the same experiment as Experimental Example 1.

Experimental Example 4

Except for using the steel grade D in Table 1 was the same experiment as in Experiment 1.

Experimental Example 5

Except for using E steel grades in Table 1 was the same experiment as in Experiment 1.

Experimental Example 6

Except for using the steel F in Table 1 was the same experiment as Experimental Example 1.

Comparative example

Steel grades G to R of Table 1 were used to compare with Experimental Examples 1 to 6 of the present invention, and the experiments were performed in the same manner as in Experimental Example 1.

Component parameters of the ferritic steel sheet manufactured by the above-described method are shown in Table 2 below, and mechanical properties and ductile-brittle transition temperature (DBTT) were measured and the results are shown in Table 3 below. DBTT is a measure of the temperature at which the fracture mode at the impact transitions from ductile to brittle fracture. The lower the temperature, the better the secondary processing brittleness.

Steel grade 4C + 48N / 14 4C + 48S / 32 Ti * (Mn + Cu) / S 0.5N Al / N A 0.019086 0.0238 0.019465 52 0.0015 19.33333 B 0.018686 0.0234 0.021945 46 0.0015 20.33333 C 0.021486 0.0277 0.021945 63.63636 0.0015 10.66667 D 0.021086 0.0243 0.022903 78.33333 0.0015 19 E 0.023886 0.0286 0.027106 70 20.33333 F 0.025086 0.0328 0.026484 52.5 0.0015 19 G 0.021486 0.0277 0.030484 44.54545 0.0015 18.33333 H 0.021486 0.0277 0.016784 44.54545 0.0015 18.33333 I 0.019486 0.0272 0.01069 44.16667 0.0015 19.33333 J 0.018457 0.0236 0.016884 85 0.001 30 K 0.023086 0.0308 0.022045 55 0.0015 13.33333 L 0.018686 0.0264 0.019323 39.58333 0.0015 17.33333 M 0.017886 0.0241 0.019232 27.27273 0.0015 19.33333 N 0.018857 0.0255 0.021845 73.33333 0.001 31 O 0.022286 0.03 0.023677 54.16667 0.0015 13.33333 P 0.020857 0.0305 0.022323 94.54545 0.001 30 Q 0.025486 0.0332 0.027206 85 8.333333 R 0.024686 0.0339 0.027106 74.61538 9.333333

Ti * = Ti + 48 Nb / 93

Steel grade Yield strength The tensile strength Elongation r DBTT Experimental Example 1 A 225 352 42.5 1.86 -60 Experimental Example 2 B 220 348 43 1.87 -55 Experimental Example 3 C 275 402 38 1.66 -45 Experimental Example 4 D 281 397 37.6 1.67 -50 Experimental Example 5 E 320 452 35 1.55 -35 Experimental Example 6 F 316 448 34 1.53 -30 Comparative Example 1 G 257 375 38.5 1.78 -50 Comparative Example 2 H 227 355 42.1 1.76 -55 Comparative Example 3 I 228 350 41.5 1.72 -55 Comparative Example 4 J 280 411 37.8 1.53 -40 Comparative Example 5 K 276 399 39.1 1.5 -45 Comparative Example 6 L 232 355 42 1.80 -35 Comparative Example 7 M 215 345 42.5 1.85 -55 Comparative Example 8 N 279 408 38.2 1.65 -45 Comparative Example 9 O 285 420 38.2 1.64 -50 Comparative Example 10 P 314 445 34 1.5 -35 Comparative Example 11 Q 309 430 36 1.55 -35 Comparative Example 12 R 313 435 36 1.5 -30

Experimental Example 1  Experimental Results from 6 to 6

As shown in Table 3, Experimental Examples 1 to 6 having component compositions according to the examples of the present invention not only exhibited excellent mechanical properties, but also r-values of drawing properties of 1.53 or more and DBTT showing secondary workability. It showed up to 60 degreeC and showed the outstanding workability.

Comparative Example 1

In Comparative Example 1, a large amount of titanium (Ti) and niobium (Nb) was included so that Ti * exceeded the upper limit. In Comparative Example 1, all of the solute atoms in solid solution were present as (Ti, Nb) (C, N) precipitates, which increased yield strength and tensile strength, while elongation decreased, resulting in poor workability. In addition, since Comparative Example 1 contains a lot of titanium (Ti), economic efficiency is poor.

Comparative Example 2  To 5

Comparative Examples 2 to 5 contained less titanium (Ti) and niobium (Nb), so that Ti * was smaller than the lower limit. In Comparative Examples 2 to 5, solute atoms in a solid solution state existed, resulting in poor workability.

Comparative Example 6

In Comparative Example 6, the boron (B) is contained 0.0008wt% less than 0.001wt%. In Comparative Example 6, both the elongation and the r-value were high, whereas the DBTT was -35 ° C., indicating that the secondary work brittleness was very bad.

Comparative Example 7

Comparative Example 7 contained less manganese (Mn) and copper (Cu) than sulfur (S), so that (Mn + Cu) / S was less than 30. In addition, the precipitation strengthening effect was small due to the reduction of fine MnS and CuS-based precipitates, and the yield strength and tensile strength were also small. On the other hand, niobium (Nb) or titanium (Ti), etc. must be added to compensate for this, so the economic efficiency is deteriorated.

Comparative Example 8  To 10

In Comparative Examples 8 to 10, a large amount of boron (B) was included so that B was larger than 0.5N. In Comparative Examples 8 to 10, both yield strength and tensile strength were large, but workability was poor.

Comparative Example 11  And 12

Comparative Examples 11 and 12 contained less aluminum (Al), resulting in less than 10 Al / N. In Comparative Examples 11 and 12, both yield strength and tensile strength were large, while workability was bad.

It will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the following claims.

1 is a schematic flowchart illustrating a method of manufacturing a ferritic steel sheet according to an embodiment of the present invention.

Claims (12)

More than 0 wt% and less than 0.0055 wt% of carbon (C), 0.01 wt% to 0.35 wt% of silicon (Si), 0.15 wt% to 0.7 wt% manganese (Mn), 0.007 wt% to 0.013 wt% sulfur (S), 0.025 wt% to 0.065 wt% aluminum (Al), More than 0 wt% and less than 0.005 wt% nitrogen (N), 0.005 wt% to 0.015 wt% titanium (Ti), More than 0 wt% and less than 0.06 wt% copper (Cu), 0.01 wt% to 0.05 wt% niobium (Nb), 0.04 wt% to 0.09 wt% phosphorus (P), Boron (B) greater than 0.001 wt% and less than 0.005 wt% Remaining iron (Fe) and impurities As a ferritic steel sheet containing, The content of the titanium (Ti), the niobium (Nb), the carbon (C), the nitrogen (N) and the sulfur (S) satisfies the following formula, 4C + 48N / 14 <Ti * <4C + 48S / 32, Ti * = Ti + 48Nb / 93, The content of copper (Cu), manganese (Mn) and sulfur (S) satisfies the following formula,  (Mn + Cu) / S> 30, The content of the boron (B) and the nitrogen (N) satisfies the following formula, 0.001 <B <0.5 N, The content of the aluminum (Al) and nitrogen (N) is the following formula Al / N> 10, Ferrite steel plate to meet the requirements. delete delete delete The method of claim 1, Wherein said ferritic steel sheet comprises BN precipitates and NbC precipitates, wherein the amount of said NbC precipitates is greater than the amount of said BN precipitates. The method of claim 1, The ferritic steel sheet is a ferritic steel sheet containing AlN precipitates. The method of claim 6, Ferritic steel sheet does not have BN precipitate in the ferritic steel sheet. The method of claim 1, The ferrite steel sheet has a tensile strength of 340 MPa to 500 MPa. As a method for producing a ferritic steel sheet, More than 0 wt% and less than 0.0055 wt% carbon (C), 0.01 wt% to 0.35 wt% silicon (Si), 0.15 wt% to 0.7 wt% manganese (Mn), 0.007 wt% to 0.013 wt% sulfur ( S), 0.025 wt% to 0.065 wt% aluminum (Al), more than 0 wt% and less than 0.005 wt% nitrogen (N), 0.005 wt% to 0.015 wt% titanium (Ti), more than 0 wt% and 0.06 wt% Copper (Cu), 0.01 wt% to 0.05 wt% niobium (Nb), 0.04 wt% to 0.09 wt% phosphorus (P), 0.0002 wt% to 0.0015 wt% boron (B), and the remaining Fe and Contains impurities, The content of the titanium (Ti), the niobium (Nb), the carbon (C), the nitrogen (N) and the sulfur (S) satisfies the following formula, 4C + 48N / 14 <Ti * <4C + 48S / 32, Ti * = Ti + 48Nb / 93, The content of copper (Cu), manganese (Mn) and sulfur (S) satisfies the following formula,  (Mn + Cu) / S> 30, The content of the boron (B) and the nitrogen (N) satisfies the following formula, 0.001 <B <0.5 N, The content of aluminum (Al) and nitrogen (N) is Al / N> 10, Heating the slab to satisfy Hot rolling the heated slab to provide a hot rolled steel sheet, Winding the hot rolled steel sheet, Dissolving the wound hot rolled steel sheet and cold rolling to prepare a cold rolled steel sheet, and Annealing the cold rolled steel sheet at 700 ° C. to 850 ° C. to produce a ferritic steel sheet Method for producing a ferritic steel sheet comprising a. 10. The method of claim 9, The method of manufacturing a ferrite steel sheet further comprises the step of cooling the prepared ferritic steel sheet at a rate of 10 ° C / s to 100 ° C / s. 10. The method of claim 9, In the step of providing the cold rolled steel sheet, the slab is rolled at a cold rolling reduction rate of 50% to 85%. 10. The method of claim 9, In the step of manufacturing the ferrite steel sheet, the ferrite steel sheet includes a BN precipitate and NbC precipitates, the amount of the NbC precipitates is greater than the amount of the BN precipitates manufacturing method of ferrite steel sheet.

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