KR20090052954A

KR20090052954A - Low chrome ferritic stainless steel with high corrosion resistance and stretchability and method of manufacturing the same

Info

Publication number: KR20090052954A
Application number: KR1020070119529A
Authority: KR
Inventors: 유도열; 이수찬
Original assignee: 주식회사 포스코
Priority date: 2007-11-22
Filing date: 2007-11-22
Publication date: 2009-05-27
Also published as: WO2009066868A1; CN101874126A; EP2220260A1; EP2220260A4

Abstract

The present invention relates to a low chromium ferritic stainless steel having excellent corrosion resistance and elongation resistance and a method for manufacturing the same, in terms of weight%, C: 0.03 or less, Si: 0.5 or less, Mn: 0.5 or less, P: 0.035 or less, S: 0.01 or less, Cr: 14 to 16, Mo: 0.2 or less, N: 0.030 or less, Cu: 0.5 or less, Al: 0.05 or less, Ni: 0.2 or less, C + N: 0.040 or less, Ti: 0.5 or less Remaining Fe and inevitably Corrosion resistance and elongation resistance to control the EL value defined by Equation (1) to 33 or more, and to adjust the PI value defined by Equation (2) to 14 to 16 in a stainless steel composed of impurity added The low chromium ferritic stainless steel excellent in formation is made into a summary.

EL = -162.1x (C + N) -0.2xCr-1.1xMo-0.2xTi / (C + N) +42.2 (1)

P.I. = Cr + 3.3Mo (2)

Corrosion Resistance, Elongation Resistance Formation, Low Chrome, Ferritic, Stainless Steel

Description

Low chrome ferritic stainless steel with high corrosion resistance and stretchability and method of manufacturing the same

The present invention relates to a low chromium ferritic stainless steel having excellent corrosion resistance and elongation resistance and a method for manufacturing the same. More particularly, various pipes and mufflers of automotive exhaust zone cold zones requiring high corrosion resistance and high molding characteristics are provided. The present invention relates to a low chromium ferritic stainless steel having excellent corrosion resistance and elongation resistance used in (Muffler) and the like and a method of manufacturing the same.

In general, ferritic stainless steel is added with Cr and Mo to improve the corrosion resistance. However, when expensive Cr and Mo are added, manufacturing cost increases, and elongation property is lowered during molding of a stamping type muffler due to a decrease in elongation. In addition, when the temperature is low, such as winter, there is a problem that the failure failure frequently occurs when the pipe is expanded after the TIG welding, such as the end pipe (end pipe).

In order to solve such a problem, looking at the known techniques, European Patent No. 0930375 discloses a manufacturing method for improving the deep drawing and the ridging property by combining the composition of the composition and hot rolling conditions. Japanese Laid-Open Patent Publication No. 2000-328197 discloses a method of improving the excellent surface gloss and formability by adding an appropriate amount of Al. In addition, European Patent No. 0765741 discloses a method for improving the leachability and in-plane anisotropy by optimizing the composition, rolling conditions and annealing conditions. In addition, Japanese Laid-Open Patent Publication No. 1995-032997 has suggested the composition and standard of the composition of low-cost ferritic stainless steel having excellent corrosion resistance in the air environment, but it is specified in the range of 17 to 32% Cr: higher than the Cr content of the present patent.

However, the above patents are not limited to components and manufacturing conditions for satisfying customers' requirements for low cost while satisfying corrosion resistance and formability at the same time. Accordingly, when the ferritic stainless steel is used for muffler and pipe expansion which require high corrosion resistance and high formability, there is a problem in that the material of the cold rolled product cannot be satisfied.

An object of the present invention is to improve the low temperature impact characteristics (DBTT) by controlling the amount of Ca, Mg, Zr added, EL (elongation and fitting index calculation formula for controlling the heating temperature and finish rolling temperature, hot rolling and cold rolling annealing conditions of the slab) Reduced production cost of Cr and Mo, which are expensive raw materials, by using Formula 1) and PI (Formula 2), and reduced production costs and added alloying elements (C, N, Si, Mn, Cr, Low chromium ferritic stainless steel with excellent corrosion resistance and elongation resistance that can increase the expandability of TIG welded pipe by optimally adjusting Mo, Ti) content and Ti% / (C% + N%) content ratio and its manufacture To provide a method.

The low chromium ferritic stainless steel having excellent corrosion resistance and elongation resistance according to the present invention is in weight%, C; 0.03 or less, Si: 0.5 or less, Mn: 0.5 or less, P: 0.035 or less, S: 0.01 or less, Cr: 14 ~ 16, Mo: 0.2% or less, N: 0.030% or less, Cu: 0.5% or less, Al: 0.05% or less, Ni: 0.2% or less, C + N: 0.040 or less, Ti: 0.5 or less Remaining Fe and inevitably added The EL value defined by the following equation (1) is controlled to 33 or more, and the PI value defined by the following equation (2) is adjusted to the range of 14-16.

EL = -162.1x (C + N) -0.2xCr-1.1xMo-0.2xTi / (C + N) +42.2 (1)

P.I. = Cr + 3.3Mo (2)

In addition, in the present invention, it may contain at least one component selected from the group consisting of Ca: 0.005 or less, Mg: 0.005 or less, Zr: 0.01 or less by weight.

In addition, in the present invention, the Ti / (C + N) ratio is preferably 15 to 20.

In addition, the method for producing a low chromium ferritic stainless steel having excellent corrosion resistance and elongation resistance formation according to the present invention, the heating temperature of the ferritic stainless steel slab prepared as described above is 1230 ~ 1280 ℃, the finish rolling temperature is 740 ~ 850 ℃ After hot rolling and hot-rolled annealing at 900 ~ 1000 ℃ cold rolling annealing at 50% or more to 50% or more, after cold-rolled annealing at 900 ~ 1000 ℃, including adjusting the particle size of the material in the range of ASTM grain size number 6.0 ~ 7.0 do.

As described above, according to the present invention, the elongation and fitting index to improve the low temperature impact characteristics (DBTT) by controlling the amount of Ca, Mg, Zr added, and to control the heating temperature and finish rolling temperature, hot rolling and cold rolling annealing conditions of the slab By reducing the amount of Cr and Mo added as expensive raw materials using the formulas EL (1 formula) and PI (2 formulas), the manufacturing cost is reduced and the added alloy elements (C, N, Si, By optimally adjusting the Mn, Cr, Mo, Ti) content and Ti% / (C% + N%) content ratio, it is possible to increase the expandability of the TIG welded pipe, thereby providing corrosion resistance, elongation, elongation resistance and low temperature. Low cost low chromium ferritic stainless steel cold rolled sheet with excellent pipe expandability can be manufactured. Therefore, the material which can be used for automobile exhaust system muffler, exhaust system end part, etc. can be ensured.

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

In the present invention, 409L steel, which is 11% Cr steel used for various pipes and mufflers for automobile exhaust system end parts, has a low corrosion resistance and can solve a lot of problems in condensate corrosion when used as an exhaust system muffler. In addition, there is no problem in corrosion resistance, but 439 steel, which is 17.5% Cr steel, satisfies the corrosion resistance, but the manufacturing cost is increased due to the increase of Cr content, and the problem of being limited in the expansion of materials due to the problem of poor formability and pipe expandability. I can solve it.

To this end, the low chromium ferritic stainless steel having low Cr and Mo contents and excellent pipe resistance at low temperature, corrosion resistance, elongation formation and low temperature, has an EL value defined by Equation (1) below 33 and Equation (2). Under the condition that the defined PI value is adjusted in the range of 14-16, in weight% C: 0.03% or less, Si: 0.5% or less, Mn: 0.5% or less, P: 0.035 or less, S: 0.01 or less, Cr: 14-16 %, Mo: 0.2% or less, N: 0.030% or less, Cu: 0.5% or less, Al: 0.05% or less, Ni: 0.4% or less, C + N: 0.04% or less, Ti: 0.05% or less, remaining Fe and inevitable It is composed of impurities that are added.

EL = -162.1x (C + N) -0.2xCr-1.1xMo-0.2xTi / (C + N) +42.2 (1)

P.I. = Cr + 3.3Mo (2)

In addition, the method of manufacturing ferritic stainless steel includes at least one component selected from the group consisting of the above-described composition and other alloy compositions of Ca: 0.005% or less, Mg: 0.005% or less, Zr: 0.01% or less. While being a ferritic stainless steel sheet, a low-cost low chromium ferritic stainless steel sheet having a condition that the Ti / (C + N) ratio is 15 to 20 is prepared. After hot-rolling the steel slab's heating temperature to 1230 ~ 1280 ℃ and finishing rolling temperature to 740 ~ 850 ℃, hot-rolled annealing at 900 ~ 1000 ℃, cold-rolling annealing with the cold reduction rate of 50% or more, and then The particle size of is adjusted to the range of ASTM grain size number 6.0 ~ 7.0.

Hereinafter, the composition range of the present invention and the reason for limitation thereof will be described in more detail.

C and N are present as an invasive Ti (C, N) carbonitride forming element. As the C and N content increases, solid solution C and N, which are not formed of Ti (C, N) carbonitride, lower the elongation and elongation of the material. Accordingly, the content of C is 0.03% or less, and N is 0.03% or less, and at the same time, when the C + N content is increased, the Ti content is added to increase the steelmaking inclusions, resulting in many surface defects such as scab. Occurs. In addition, C + N content is limited to 0.04% or less because the problem of nozzle clogging during playing and the elongation of solid solution C and N decrease.

Si is a ferrite phase forming element, the content of the ferrite phase is increased and the oxidation resistance is improved when the content is increased, but when it is added more than 0.5%, surface defects are likely to occur due to the increase in steelmaking Si inclusions. In addition, since hardness, yield strength, and tensile strength are increased and the elongation is lowered, the workability is limited to 0.5% or less.

Mn is limited to 0.5% or less because the content of Mn elutes MnS and lowers pitting resistance.

Ni is a gamma phase generating element. When a large amount is added, the gamma phase increases, and when the coil is air-cooled after hot rolling, the martensite phase is promoted to increase the strength and hardness, so that the elongation is lowered. Therefore, Ni addition amount is limited to 0.2% or less.

Since P and S form inclusions such as MnS and inhibit corrosion resistance and hot workability, P and S are preferably controlled as low as possible, so that P is limited to 0.035% or less and S to 0.01% or less.

If Cr is low, the corrosion resistance is lowered. If the content is too high, the corrosion resistance is improved, but the strength is high and the elongation is low, which lowers the workability, so the content is limited to 14 to 16%.

Increasing the high Mo content significantly improves the corrosion resistance, but worsens the workability due to the problem that the manufacturing cost of the material increases and the elongation is reduced by increasing the strength. Therefore, considering the corrosion resistance and processability, the Mo content is limited to 0.2% or less.

Al is limited to 0.05% or less because a large amount of element is added as a deoxidizer, and causes surface defects.

Cu is a gamma phase generating element like Ni. When a large amount is added, the gamma phase increases, and when the coil is air-cooled after hot rolling, the martensite phase is promoted and the strength and hardness are increased, thereby reducing the elongation. Therefore, Cu addition amount is limited to 0.5% or less.

If the amount of Ti is added too high, the steelmaking inclusions increase, causing a lot of surface defects such as scab. In addition, the elongation is lowered due to the problem of clogging of the nozzle during the play and the increase in the solid solution Ti content, the amount of Ti added is very low compared to the C + N content. Accordingly, when the Ti / (C + N) ratio is lowered, grain boundary corrosion occurs and corrosion resistance is lowered. Therefore, the Ti addition amount is limited to 0.5% or less, and the Ti / C + N ratio value is considered to be corrosion resistance and formability. To 15-20.

When Ca, Mg, and Zr are added alone or in combination of two, the crystal grain size of the heat affected zone is refined by TIG welding, thereby lowering the impact transition temperature (DBTT), thereby increasing the expandability of the TIG welding pipe at low temperatures such as winter. However, if the added amount is too large, the corrosion resistance decreases due to the increase in the amount of Ca, Mg, and Zr oxidative inclusions generated. Therefore, the added amount of each element is less than 0.005% of Ca, less than 0.005% of Mg, and less than 0.01% of Zr. It is limited.

When the EL value is less than 33 in the EL calculation formula (1) obtained for improving the elongation in the present invention, the material for the stamping type muffler lacks elongation and elongation property formation. As a result, breakage failure occurs during molding, so the EL value is limited to 33 or more.

EL = -162.1x (C + N) -0.2xCr-1.1xMo-0.2xTi / (C + N) +42.2 (1)

In addition, when P.I. (Pitting Index) value of Formula 2 becomes high, corrosion resistance improves. Therefore, in order to increase the P.I. value, the Cr or Mo content, which is an expensive element, may be increased. However, when the P.I. value is excessively high, the elongation and elongation-forming properties are lowered and the manufacturing cost increases. In addition, if too low, the corrosion resistance is deteriorated, so that the P.I. The P.I. value of the formula (2) is limited to the range of 14-16.

P.I. = Cr + 3.3Mo (2)

As for the Ti% / (C% + N%) ratio, if the Ti% / (C% + N%) ratio is too low, the intergranular corrosion of the weld zone will occur after welding, whereas the Ti% / (C% + N%) ratio is too high. The higher the content of solid solution Ti, the lower the formability such as elongation, so the Ti% / (C% + N%) ratio is limited to 15-20.

The following describes the manufacturing conditions of the present invention and the reason for the limitation thereof.

The higher slab heating temperature in hot rolling conditions is advantageous for recrystallization during hot rolling operations. However, if the heating temperature is too high, the surface defects will occur frequently, so the slab heating temperature is limited to 1230 ~ 1280 ℃.

The lower the finish rolling temperature during hot rolling, the higher the strain accumulation energy during hot rolling, which helps recrystallization during annealing, which is advantageous for improving elongation. However, if the finishing rolling temperature is too low, there is a sticking surface defect that occurs due to the sticking of the rolling roll and the material, the finishing rolling temperature range is limited to 740 ~ 850 ℃.

In addition, if the cold reduction rate of the material is too low, it is difficult to remove the surface defects and to secure the surface properties, and conversely, if the cold reduction rate is high, it is advantageous to improve the formability, so the cold reduction rate when manufacturing the material is limited to 50% or more.

The ASTM grain size number in the annealing plate after cold annealing is limited within this range because the elongation is the best in the range of 6.0 to 7.0.

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

(Example)

Table 1 shows the chemical composition, EL calculated values, and P.I. calculated values for each specimen. In addition, Table 2 shows the measured elongation for each specimen, the formula potential, the presence of grain boundary corrosion, the impact transition temperature and the Eric value of the TIG pipe weld.

The ferritic stainless steel composition as shown in Table 1 was dissolved in a 50Kg vacuum melting equipment to prepare a 120 mm thick ingot. The ingot thus prepared is heated at 1250 ° C. and hot rolled at 800 ° C. finish rolling temperature to produce a 3.0 mm thick hot rolled sheet. And after hot-rolled annealing at 960 ℃, cold rolled to 1.5mmt and 0.6mmt thickness, cold-rolled annealing at 960 ℃ and then pickled, the crystal grain size of cold-rolled annealing plate using a tensile test, an elimination test and an image analyzer Was measured.

In addition, the official potential of the cold-rolled annealing plate was tested by the KS D 0238 method and measured five times with the value at V'c10 and expressed as an average value.

The measurement of impact transition temperature (DBTT) was performed by processing 1.5mm thick cold rolled annealing plates (Ca, Zr, Mg-added steel and unadded steel) into V-notch impact specimens of server size. The impact transition temperature was determined by measuring at intervals of 10 ° C. in the range.

division No division C Si Mn P S Cr Ni Cu Mo Al Invention steel One 15% Cr 0.0069 0.4 0.16 0.022 0.001 14.9 0.09 0.02 0.0001 0.003 2 15% Cr 0.0065 0.4 0.15 0.02 0.001 15 0.1 0.02 0.0001 0.003 3 16% Cr 0.0061 0.35 0.25 0.02 0.001 16 0.009 0.02 0.0003 0.003 4 Ca 0.0069 0.4 0.16 0.022 0.001 15 0.09 0.02 0.0001 0.003 5 Zr 0.0069 0.4 0.16 0.022 0.001 15 0.09 0.02 0.0001 0.003 6 Mg 0.0069 0.4 0.16 0.022 0.001 15 0.09 0.02 0.0001 0.003 Comparative steel 7 13% Cr 0.0075 0.35 0.3 0.025 0.002 13 0.02 0.02 0.25 0.003 8 16.5% Cr 0.0075 0.35 0.3 0.025 0.002 16.5 0.02 0.02 0.0001 0.003 9 Low Ti 0.01 0.45 0.4 0.025 0.002 15 0.02 0.02 0.0001 0.003 10 Low Ti 0.011 0.45 0.4 0.025 0.002 15 0.02 0.02 0.0001 0.003 11 High Ti 0.0065 0.45 0.4 0.025 0.002 15 0.02 0.02 0.0001 0.003 12 High Ti 0.0063 0.45 0.4 0.025 0.002 15 0.02 0.02 0.0001 0.003 13 409L 0.007 0.541 0.17 0.021 0.002 11.3 0.08 0.027 0.02 0.005 14 439 0.01 0.14 0.15 0.02 0.002 17.6 0.13 0.03 0.02 0.032

division No Ti N Ca Zr Mg C + N Ti / C + N El (calculated value) P.I. (calculated value) Invention steel One 0.27 0.0086 0 0 0 0.0155 17.4 33.2 14.9 2 0.25 0.008 0 0 0 0.0145 17.2 33.4 15.0 3 0.25 0.0078 0 0 0 0.0139 18.0 33.1 16.0 4 0.27 0.0086 0.002 0 0 0.0155 17.4 33.2 15.0 5 0.27 0.0086 0.0012 0.003 0 0.0155 17.4 33.2 15.0 6 0.27 0.0086 0.0011 0 0.002 0.0155 17.4 33.2 15.0 Comparative steel 7 0.28 0.0095 0 0 0 0.017 16.5 33.3 13.8 8 0.3 0.0095 0 0 0 0.017 17.6 32.6 16.5 9 0.2 0.015 0 0 0 0.025 8.0 33.5 15.0 10 0.2 0.017 0 0 0 0.028 7.1 33.2 15.0 11 0.4 0.009 0 0 0 0.0155 25.8 31.5 15.0 12 0.4 0.0088 0 0 0 0.0151 26.5 31.5 15.0 13 0.21 0.0079 0 0 0 0.0149 14.0 34.7 11.3 14 0.3 0.0092 0 0 0 0.0192 15.7 32.4 17.7

division No El (%, found) Formula potential (mV) Presence of intergranular corrosion Impact Transition Temperature of TIG Pipe Weldment (℃) Emission value (mm) Invention steel One 34 142.7 O -20 9.32 2 34.2 145 O -20 9.34 3 34.1 152 O -20 9.33 4 34 145 O -50 9.31 5 34 145 O -50 9.32 6 34 144 O -50 9.32 Comparative steel 7 34.2 120 O -20 9.43 8 32 150 O -20 9.12 9 32.6 144 X -20 9.22 10 32.5 144 X -20 9.21 11 30.5 144 O -20 9.05 12 30.5 144 O -20 9.01 13 32.7 50 O -30 9.2 14 31.6 189 O -20 9

Referring to the test results below, Tables 1 and 2 show chemical compositions, EL and P.I. Calculated values, corrosion resistance (formula potential) and elongation resistance formation (emission value) are shown. In the present invention, the Cr and Mo contents were adjusted so that the PI value was in the range of 14 to 16 using the equation (2), which is the PI value calculation formula, and the middle of the conventional steel (409L: No. 13, 439 steel: No. 14). The degree of product characteristics is shown. In addition, the present invention steel was appropriately adjusted to the C, N, Cr, Mo, Ti / (C + N) content so that the EL value is 33 or more by using the formula (1) which is the EL value calculation formula. Accordingly, it can be seen that the elongation measured at the same time as being excellent in corrosion resistance and high is 34% or more, and the erythema value indicating elongation formation is also high by 9.3 mm or more. And it can be seen that the steel of the present invention in which the Ti / (C + N) ratio value is adjusted in the range of 15 to 20 does not generate weld boundary grain corrosion compared to the comparative material outside this range.

1 is a view showing the change in elongation according to the change of ASTM grain size of the annealing plate after cold rolling annealing in 15Cr-Ti (Sample No. 1) steel. As can be seen in Figure 1, when cold-rolled annealing can be seen that the elongation is the most excellent within the ASTM grain size number range 6.0 ~ 7.0.

FIG. 2 is a view illustrating a change in elongation after cold rolling annealing according to a Ti / (C + N) ratio change in 15Cr-Ti-added steel. The lower the Ti / (C + N) ratio, the better the elongation. However, when the Ti / (C + N) ratio is less than 15, as shown in the result of Table 1, weld boundary grain corrosion occurs, and when the Ti / (C + N) ratio exceeds 20, the elongation is lowered. In consideration of the Ti / (C + N) ratio needs to be added to adjust the Ti in the range 15 to 20.

3 is a diagram showing the change in impact transition temperature according to Ca, Mg, Zr addition and non-addition in 15Cr-Ti addition steel, the impact transition temperature is increased when the composite addition of Ca, Ca + Mg or the composite addition of Ca + Zr Low to -50 ℃ makes the TIG pipe expandable at low working temperatures such as winter.

As described above, the preferred embodiment of the present invention has been disclosed through the detailed description and the drawings. The terms are used only for the purpose of describing the present invention and are not used to limit the scope of the present invention as defined in the meaning or claims. Therefore, those skilled in the art will understand that various modifications and equivalent other embodiments are possible from this. Therefore, the true technical protection scope of the present invention will be defined by the technical spirit of the appended claims.

1 is a view showing a change in elongation according to the ASTM grain size number change after cold rolling annealing in 15Cr-Ti steel (Sample No. 1).

2 is a view showing the change in elongation according to the Ti / (C + N) ratio in 15Cr-Ti steel.

3 is a view showing a change in impact transition temperature (DBTT) according to the addition amount of Ca, Mg, Zr in 15Cr-Ti steel.

Claims

By weight%, C; 0.03 or less, Si: 0.5 or less, Mn: 0.5 or less, P: 0.035 or less, S: 0.01 or less, Cr: 14-16, Mo: 0.2 or less, N: 0.030 or less, Cu: 0.5 or less, Al: 0.05 or less, Ni: 0.2 or less, C + N: 0.040 or less, Ti: 0.5 or less The EL value defined by the following Equation (1) is 33 or more for a stainless steel composed of remaining Fe and inevitable addition of impurities. A low chromium ferritic stainless steel having excellent corrosion resistance and elongation resistance, characterized by controlling and adjusting the PI value defined by the following equation (2) to a range of 14 to 16.

EL = -162.1x (C + N) -0.2xCr-1.1xMo-0.2xTi / (C + N) +42.2 (1)

P.I. = Cr + 3.3Mo (2)

The method of claim 1,

A low chromium ferritic stainless steel having excellent corrosion resistance and elongation resistance, characterized in that, by weight%, contains one or two or more from the group consisting of Ca: 0.005 or less, Mg: 0.005 or less and Zr: 0.01 or less.

The method according to claim 1 or 2,

Low chromium ferritic stainless steel excellent in corrosion resistance and elongation formation, characterized in that the Ti / (C + N) ratio is 15 to 20.

After the hot rolling of the ferritic stainless steel slab according to any one of claims 1 to 3 in the heating temperature of 1230 ~ 1280 ℃, the finish rolling temperature of 740 ~ 850 ℃ hot-rolled annealing at 900 ~ 1000 ℃ to reduce the cold rolling rate 50 Cold rolling annealing at 900 ~ 1000 ℃ by more than%, and the cold-rolling annealing ferrite-based stainless steel manufacturing method excellent in corrosion resistance and elongation-forming formation comprising the step of adjusting the particle size of the ASTM crystal grain number in the range of 6.0 ~ 7.0.