KR20010064844A - A ferritic stainless steel having improved formability, ridging resistance and a method for manufacturing it - Google Patents

Abstract

PURPOSE: A method for manufacturing ferritic stainless steel is provided to improve formability and ridging resistance thereof by lowering theoretical yield strength, raising maximum gamma phase percentage, and controlling slab casting temperature optimal. CONSTITUTION: The ferritic stainless steel is manufactured by casting a steel comprising C 0.07wt.% or less, Si 0.4wt.% or less, Mn 1.0wt.% or less, P 0.035wt.% or less, S 0.002wt.% or less, Cr 15-20wt.%, Mo 0.5wt.% or less, N 0.05wt.% or less, Cu 0.6wt.% or less, Ni 0.4wt.% or less, Al 0.01wt.% or less, Ti 0.15wt.% or less, a balance of Fe, and other inevitable impurities at a casting temperature of 1536deg.C or lower; reheating the obtained steel slab in the temperature range of 1170 to 1250deg.C; finish hot rolling it in the temperature range of 750 to 900deg.C; annealing the hot rolled steel sheet in the temperature range of 860 to 900deg.C for 3-10 min; furnace cooling and pickling; cold rolling; annealing the cold rolled steel sheet in the temperature range of 860 to 900deg.C for 3-5 min; and then air cooling. The obtained ferritic stainless steel has a theoretical yield strength of 33.20 kg/mm2 or less and the theoretical yield strength is calculated by the follow equation: theoretical yield strength (kg/mm2) = 4.857+9.43Si+11.12Mn+81.73P+0.967Cr+6.99£Ti|+ 3.58Al+95.82£C+N|wherein in the case of Ti/(C+N)< 10,£Ti|is 0 and£C+N|is equal to (C+N)-Ti/10, however in the case of Ti/(C+N) >10,£Ti|is equal to Ti-10(C+N) and£C+N|is zero. Also, the ferritic stainless steel has a maximum gamma phase of 45% or higher and the maximum gamma phase is calculated by the follow equation: maximum gamma phase (%)=420(C%)+470(N%)+9(Cu%)+10(Mn%)+23(Ni%)-11.5(Cr%)-11.5(Si%)-12(Mo%)-49(Ti% )-52(Al%)+180.

Description

Ferritic stainless steel with excellent moldability and stiffness resistance and manufacturing method thereof {A FERRITIC STAINLESS STEEL HAVING IMPROVED FORMABILITY, RIDGING RESISTANCE AND A METHOD FOR MANUFACTURING IT}

The present invention relates to a method for manufacturing ferritic stainless steel used for various kitchen appliances and washing tubs. More particularly, the present invention relates to a ferritic stainless steel and its manufacture which are not only soft and excellent in moldability but also excellent in rinsing resistance. It is about a method.

Generally, ferritic stainless steel is more expensive than STS304 austenitic stainless steel because it does not contain Ni, but it is used for spinning or deep processing because of its poor corrosion resistance, low firing rate, and high yield strength. If there is a problem that the use is limited.

Conventional techniques for solving such a problem include Japanese Patent 61-136621, Japanese Patent 61-127822, and the like, and these patents disclose a method of improving the etchability by adding Al. In addition, Japanese Patent Application No. 01-118341 discloses a method of improving the etching property by adding Ti. However, these methods have a disadvantage in that manufacturing cost increases due to the addition of a large amount of Al or Ti ferroalloy, and there are also problems such as the occurrence of surface defects and the improvement of morphology and formability, which are the biggest problems of ferritic stainless steel. .

Accordingly, the present inventors conducted research and experiments to solve the above problems, and proposed the present invention based on the results. The present invention lowers the theoretical yield strength in the ferritic stainless steel and increases the maximum gamma phase (%). By adjusting the components to increase) and by adjusting the slab casting temperature to the optimum temperature to increase the equiaxed crystallization rate in the slab, it is intended to provide a ferritic stainless steel and a method of manufacturing the same, which is soft and excellent in moldability and crush resistance. There is this.

1 is a graph showing the change of equiaxed constant in cast steel according to the casting temperature

In the present invention, C: 0.07% or less, Si: 0.4% or less, Mn: 1.0% or less, P: 0.035% or less, S: 0.002% or less, Cr: 15-20%, Mo: 0.5% or less, N : 0.05% or less, Cu: 0.6% or less, Ni: 0.4%, Ti: no addition or 0.15% or less, Al: 0.01% or less, remaining Fe and inevitably contained impurities, and yielding the theoretical yield obtained from the following relational formula 1 The maximum gamma phase obtained by the strength of 33.20 kg / mm ² or less and the following relation 2 relates to a ferritic stainless steel having excellent moldability and resistance to stiffness of 45% or more.

[Relationship 1]

Theoretical Yield Strength (kg / mm ² ) = 4.857 + 9.43Si + 11.12Mn + 81.73P + 0.967Cr + 6.99 [Ti]

+ 3.58Al + 95.82 [C + N]

However, when Ti / (C + N) <10, [Ti] = 0, [C + N] = (C + N) -Ti / 10

When Ti / (C + N)> 10, [Ti] = Ti-10 (C + N), [C + N] = 0

[Relationship 2]

Gamma phase (%) = 420 (C%) + 470 (N%) + 9 (Cu%) + 10 (Mn%) + 23 (Ni%)

11.5 (Cr%)-11.5 (Si%)-12 (Mo%)-49 (Ti%)-52 (Al%) + 180

In addition, the present invention is a method of manufacturing a ferritic stainless steel,

By weight% C: 0.07% or less, Si: 0.4% or less, Mn: 1% or less, P: 0.035% or less, S: 0.02% or less, Cr: 15-20%, Mo: 0.05% or less, N: 0.05% Or less, Cu: 0.6% or less, Ni: 0.04% or less, Ti: no addition or 0.15% or less, Al: 0.01% or less, remaining Fe and inevitably contained impurities, and the theoretical yield strength obtained in the above relation 1 is 33.20kg / mm ² or less, the maximum gamma phase obtained in the above equation 2 is to produce a steel slab by casting a steel satisfying 45% or more at a casting temperature of 1536 ℃ or less, and reheated at a temperature range of 1170 ~ 1250 ℃ After hot rolling under the finishing rolling temperature condition of 750-900 ° C, when Ti is added, hot-rolled sheet annealing is performed at a temperature of 860-900 ° C for 3 to 10 minutes, and when Ti is not added, it is 820-850 ° C. Hot-annealed for 4-6 hours at temperature, furnace-cooled and pickled, cold-rolled annealing for 3-10 minutes at a temperature of 860-900 ° C if Ti is added after cold rolling And, Ti is the temperature of 3-5 minutes moldability and rijjing ferritic stainless steel excellent in resistance to the manufacturing method characterized in that the cold-rolled sheet annealing, and air cooling is during the 820 ~ 850 ℃ case is not added.

Hereinafter, the present invention will be described in detail.

C and N are carbonitride-forming elements, and when present in an invasive form, the strength is increased and the impact resistance, corrosion resistance, and moldability are lowered. Therefore, the content thereof is preferably set to 0.07% or less for C and 0.05% or less for C.

The Si is a ferrite forming element, the content of the ferrite phase is increased and the oxidation resistance is improved as the content thereof is increased, but when it is added more than 0.4%, the hardness, yield strength, tensile strength and elongation are deteriorated. It is preferable to limit to the following.

When Mn is added in a large amount, MnS is eluted to lower pitting resistance, and therefore, the content thereof is preferably set to 1.0% or less.

Since P and S form inclusions such as MnS to inhibit corrosion resistance and hot workability, it is better to manage P and S as low as possible. Therefore, it is preferable to limit P to 0.035% or less and S to 0.002% or less.

When the Cr is added in a large amount, the corrosion resistance is improved, but when the content exceeds 20%, the strength is increased, the elongation is low, the moldability is lowered, and in particular, there is a high possibility of precipitating a sigma phase. On the other hand, when the content is less than 15%, since the corrosion resistance is lowered, the content is preferably limited to 15 to 20%.

Mo significantly improves the corrosion resistance but increases the strength to inhibit moldability. Therefore, in consideration of corrosion resistance and moldability, the content is preferably set to 0.5% or less.

Al is an element added as a deoxidizer, and when added in a large amount, Al is preferably limited to 0.01% or less.

Since Cu and Ni are gamma-phase generating elements, when a large amount is added, the production cost increases due to an increase in the input amount of ferroalloy. Therefore, Cu and Ni are preferably limited to 0.6% or less and Ni to 0.4% or less.

The Ti is an element selectively added in the present invention, and combines with C and N to form carbonitrides to improve leachability, and also inhibits precipitation of Cr carbides to improve corrosion resistance. However, excessive addition of Ti lowers the surface quality, so the content thereof is preferably limited to 0.15% or less.

Meanwhile, in the present invention, the theoretical yield strength value obtained by the following relational formula 1 is limited to 33.20 kg / mm ² when forming steel. That is, by adjusting the theoretical yield strength to 33.20 kg / mm ² or less, it is possible to soften the steel and lower the yield strength of the actual product, thereby improving the formability.

[Relationship 1]

Theoretical Yield Strength (kg / mm ² ) = 4.857 + 9.43Si + 11.12Mn + 81.73P + 0.967Cr + 6.99 [Ti]

+ 3.58Al + 95.82 [C + N]

However, when Ti / (C + N) <10, [Ti] = 0, [C + N] = (C + N) -Ti / 10

When Ti / (C + N)> 10, [Ti] = Ti-10 (C + N), [C + N] = 0

In addition, the present invention limits the maximum gamma phase (%) obtained by the following relational formula 2 to 45% or more in forming steel. If the maximum gamma image is 45% or more, the scratch resistance can be improved.

[Relationship 2]

Gamma phase (%) = 420 (C%) + 470 (N%) + 9 (Cu%) + 10 (Mn%) + 23 (Ni%)

11.5 (Cr%)-11.5 (Si%)-12 (Mo%)-49 (Ti%)-52 (Al%) + 180

In casting the steel formed as described above, if the equiaxed crystallinity increases, the resistivity is increased, so that the casting temperature is preferably set to 1536 ° C or lower in order to ensure at least 50% of the equiaxed crystallinity.

After reheating the hot-rolled steel slab, hot rolling and hot annealing, and then cold rolling and cold annealing to produce a ferritic stainless steel, the lower the reheating temperature, the finish rolling temperature and the coiling temperature, the more moldability and resistance Is improved. However, if the finishing rolling temperature is too low, sticking phenomenon, which is a surface defect of the slab surface layer buried on the rolling roll, occurs and the defects on the surface of the hot rolled coil occur frequently. Thus, the reheating temperature is preferably set to 1170 ~ 1250 ℃, the finish rolling temperature in the range of 750 ~ 900 ℃.

The hot-rolled annealing is carried out for 3 to 10 minutes in the temperature range of 860 ~ 900 ℃ in the case of Ti-added steel, 4 to 6 hours in the temperature range of 820 ~ 850 ℃ in the case of non-Ti steel, cold-rolled annealing after cold rolling In the case of additive steel, it is carried out for 3 ~ 5 minutes at 860 ~ 900 ℃ and for 3 ~ 5 minutes in the temperature range of 820 ~ 850 ℃ for Ti-free steel to prevent the problem of coarse grains or insufficient recrystallization. It is desirable to.

After the hot-rolling annealing, it is preferable to cool the furnace in order to prevent high temperature embrittlement and to obtain excellent mechanical properties, and to cool the air after the cold-rolling annealing.

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

(Example)

Melting the steel composition as shown in Table 1 below was made of a slab of 50kg, 148mm square. The slab thus produced was heated at 1250 ° C. for 150 minutes and hot rolled at a finish rolling temperature of 810 ° C. to produce a 4 mm thick hot rolled plate. Then, the resultant was wound at 700 ° C, the added steel was heat treated at 870 ° C for 5 hours at 830 ° C, and furnace cooled and pickled.

The hot rolled sheet was cold rolled at a rolling rate of 80% to produce a cold rolled sheet having a thickness of 0.8 mm, followed by cold rolling for 4 minutes at the same temperature as the hot rolled sheet temperature, followed by air cooling and pickling, followed by 1% cold rolling. Various specimens were prepared by temper rolling.

Then, in order to measure the yield strength of the specimen, the tensile specimen was processed by JIS138 and subjected to a tensile test to measure the yield strength and the results are shown in Table 1 below.

In addition, in order to evaluate the formability, the Erichsen test was carried out three times for each specimen by the JIS Z 22478 method and the average values are shown in Table 1 below.

Furthermore, for evaluation of the refrigerating properties of cold rolled annealing plates, the tensile specimens were machined in parallel with the rolling direction according to the JIS No. 5 standard to polish the parallel portions of the specimens, and then stretched by 15% to maximize the surface profile with the surface roughness. The height of the risch was measured by the Rt value, which is the height, and the results are shown in Table 1 below.

division Chemical composition (wt%) ① ② ③ Characteristics evaluation result C P S Si Mn Cr Mo Cu AI Ti Ni N ④ ⑤ ⑥ Invention Material A 0.05 0.01 0.001 0.20 0.8 16.15 0.01 0.5 0.003 0.04 0.4 0.03 31.76 46.54 1536 28.7 9.8 22 Invention Material B 0.065 0.01 0001 0.20 0.8 16.15 0.01 0.4 0.002 0 0.3 0.03 33.19 51.65 1530 30.6 9.9 20 Invention Material C 0.065 0.01 0.001 0.25 0.8 16.15 0.01 0.1 0.002 0.02 0.3 0.025 33.18 45.05 1528 30.5 10.0 15 Comparative Material 1 0.065 0.02 0.001 0.25 0.8 16.15 0.01 0.1 0.003 0 0.3 0.02 33.52 43.62 1540 31.7 9.5 30 Comparative Material 2 0.04 0.02 0.001 0.25 0.8 16.15 0.01 0.1 0.003 0 0.3 0.02 31.13 33.12 1550 29.5 9.5 30 Comparative Material 3 0.065 0.02 0.001 0.25 0.8 16.15 0.01 0.1 0.003 0 0.3 0.025 34.00 48.67 1560 32.8 9.4 28 Comparative Material 4 0.065 0.02 0.001 0.25 0.8 16.15 0.01 0.1 0.003 0 0.3 0.025 34.00 45.97 1590 32.8 9.4 37 Comparative Material 5 0.065 0.02 0.001 0.25 0.8 16.15 0.01 0.1 0.003 0.14 0.3 0.025 34.00 39.11 1570 32.8 9.1 35 Comparative Material 6 0.065 0.01 0.001 0.25 0.8 16.15 0.01 0.1 0.003 0 0.3 0.025 33.19 45.97 1558 32.5 9.2 29 Comparative Material7 0.05 0.02 0.001 0.4 0.45 16.25 0.01 0.03 0.003 0.02 0.3 0.04 35.12 36.44 1568 33.4 9.2 35 Comparative Material 8 0.063 0.021 0.001 0.28 0.78 16.2 0.01 0.03 0.003 0.009 0.039 0.0291 34.60 46.94 1560 33.1 9.2 28 * ①: Theoretical yield strength (kg / mm ² ), ②: Maximum gamma phase (%), ③: Casting temperature (℃), ④: Yield strength (kg / mm ² ) ⑤: Ericsen (mm), ⑥: Resizing Height (㎛)

As can be seen in Table 1, in the case of the invention materials (A), (B), and (C) satisfying the scope of the present invention, the yield strength is lower than that of the comparative materials (1) to (8), It can be seen that the moldability is excellent and the rhythm resistance is excellent at the same time.

On the other hand, Figure 1 shows the change in equiaxed crystallization according to the change in casting temperature, in the case of the present invention, all are manufactured at a casting temperature of 1536 ℃ or less, it can be seen that secures more than 50% equiaxed crystallization.

As described above, the present invention adjusts the composition of steel to have an appropriate theoretical yield strength and maximum gamma value, and also controls the manufacturing conditions such as casting temperature during performance, thereby making it soft and excellent in moldability and at the same time resisting rhythm. It is possible to produce excellent ferritic stainless steel.

Claims (4)

By weight% C: 0.07% or less, Si: 0.4% or less, Mn: 1.0% or less, P: 0.035% or less, S: 0.002% or less, Cr: 15-20%, Mo: 0.5% or less, N: 0.05% Cu: 0.6% or less, Ni: 0.4% or less, Al: 0.01% or less, remaining Fe, and inevitably contained impurities, and the theoretical yield strength obtained from the following relational formula 1 is 33.20 kg / mm ² or less, The maximum gamma phase obtained from relation (2) is ferritic stainless steel with excellent moldability and crush resistance satisfying 45% or more. [Relationship 1] Theoretical Yield Strength (kg / mm ² ) = 4.857 + 9.43Si + 11.12Mn + 81.73P + 0.967Cr + 6.99 [Ti] + 3.58Al + 95.82 [C + N] However, when Ti / (C + N) <10, [Ti] = 0, [C + N] = (C + N) -Ti / 10 When Ti / (C + N)> 10, [Ti] = Ti-10 (C + N), [C + N] = 0 [Relationship 2] Gamma phase (%) = 420 (C%) + 470 (N%) + 9 (Cu%) + 10 (Mn%) + 23 (Ni%) 11.5 (Cr%)-11.5 (Si%)-12 (Mo%)-49 (Ti%)-52 (Al%) + 180 The ferritic stainless steel of claim 1, wherein the steel contains 0.15% or less of Ti. In the manufacturing method of ferritic stainless steel, By weight% C: 0.07% or less, Si: 0.4% or less, Mn: 1.0% or less, P: 0.035% or less, S: 0.002% or less, Cr: 15-20%, Mo: 0.5% or less, N: 0.05% Or less, Cu: 0.6% or less, Ni: 0.4% or less, Al: 0.01% or less, remaining Fe and inevitably contained impurities, and the theoretical yield strength and maximum gamma image obtained by the following Equations 1 and 2, respectively, Steel slabs are manufactured by casting steels satisfying 33.20 kg / mm 2 or less and 45% or more at a casting temperature of 1536 ° C. or lower, and reheating at a temperature range of 1170 to 1250 ° C., and then at a temperature range of 750 to 900 ° C. After finish rolling and hot rolling, hot roll annealing for 4-6 hours at the temperature of 820 ~ 850 ℃, cold cooling and pickling, then cold roll annealing for 3 ~ 5 minutes at the temperature of 820 ~ 850 ℃ after cold rolling Method for producing ferritic stainless steel excellent in formability and resistance to rinsing, characterized in that The method of claim 3, wherein the Ti is added to the steel component and the hot rolled sheet annealing is performed for 3 to 10 minutes at a temperature range of 860 ~ 900 ℃, and the cold rolled sheet annealing is 3 to 5 at a temperature range of 860 ~ 900 ℃ Process for producing ferritic stainless steel excellent in formability and rhythm resistance, characterized in that performed for minutes