WO2008082144A1

WO2008082144A1 - Ferritic stainless steel having superior corrosion resistance and stretchability and method of manufacturing the same

Info

Publication number: WO2008082144A1
Application number: PCT/KR2007/006862
Authority: WO
Inventors: Do Leal Yoo; Jeom Yong Choi; Jae Seok Park; Soo Chan Lee; Sung Yeun Won
Original assignee: Posco
Priority date: 2006-12-28
Filing date: 2007-12-27
Publication date: 2008-07-10
Also published as: JP5362582B2; JP2010514928A; CN101573465A; KR100821059B1

Abstract

The present invention provides a ferritic stainless steel with superior corrosion resistance, elongation, and strechablility, the ferritic stainless including 0.03 wt% or less of C, 0.5 wt% or less of Si, 0.5 wt% or less of Mn, 0.035 wt% or less of P, 0.01 wt% or less of S, 18 to 21 wt% of Cr, 0.5 wt% or less of Mo, 0.03 wt% or less of N, 0.5 wt% or less of Cu, 0.05 wt% or less of Al, 0.2 wt% or less of Ni, the balance Fe and inevitable impurities, so as to satisfy relationships between the compositions.

Description

FERRITIC STAINLESS STEEL HAVING SUPERIOR

CORROSION RESISTANCE AND STRETCHABILITY AND

METHOD OF MANUFACTURING THE SAME

Technical Field

[1] The present invention relates to a ferritic stainless steel having superior corrosion resistance and strechablility and a method of manufacturing the same, and more specifically to a ferritic stainless steel which is suitable for various pipes, mufflers and similar products used for a cold zone of an automobile exhaust system requiring superior corrosion resistance and stretchability and a method of manufacturing the same.

[2]

Background Art

[3] Generally, Mo is added to a ferritic stainless steel for improving corrosion resistance.

However, the cost of manufacturing the ferritic stainless steel increases when Mo is added. In particular, Mo makes the strectchability of the stainless steel degraded and the elongation of the stainless steel reduced. Thereby the rupture of the stainless steel frequently occurs when forming a muffler and similar products by stamping, and the rupture of a pipe frequently occurs when the pipe is expanded after TIG welding, like the end part of the exhaust system, in the low temperature.

[4] Reviewing conventional techniques suggested to solve such problems, EP 930375 discloses a manufacturing method of improving deep drawability and ridging resistance by a combination of component composition and hot rolling condition, Japanese Laid-Open Publication No. 2000-328197 discloses a method of improving surface bright and formability by adding a proper quantity of Al, and EP Patent 765741 discloses a method of improving ridging resistance and reducing plane anisotropy by optimizing composition and conditions of rolling and annealing.

[5] However, the conventional techniques do not disclose the composition and the conditions of manufacturing the stainless steel which has lower contents of Mo and superior corrosion resistance and formability. Therefore, the quality of a cold rolling product manufactured by the conventional techniques does not satisfy superior corrosion resistance and formability required for forming the muffler and the pipe which would be expanded in low temperature. Disclosure of Invention Technical Problem [6] Accordingly, the present invention is designed to solve such drawbacks of the prior art, and therefore an object of the present invention is to provide the following:

[7] It is an object of the present invention to solve the above-described problems, and to provide the high Cr ferritic stainless steel, which is a cold-rolled sheet, having superior corrosion resistance, elongation and strechablility, which would be suitably employed for a pipe, and also to provide a method for manufacturing the same.

[8] It is another object of the present invention to provide the stainless steel having larger amounts of Cr, which is useful for improving corrosion resistance, instead of adding Mo, which is more expensive than Cr, by using EL (Equation 1) and P.I. (Equation 2) equations which is an equation for computing elongation and fitting indexes. The stainless steel has optimally controlled values of C%+N%, C%/N%, and Ti%/(C%+N%) by mass. The added amounts of Ca, Mg, and Zr are controlled in order to lower impact transition temperature of TIG welding portion. A heating temperature, hot rolling temperature at final stage and hot annealing conditions of the slab are controlled in the present invention. Technical Solution

[9] In order to achieve these objects, according to one aspect of the present invention, a ferritic stainless steel with superior corrosion resistance and strechability comprises; a composition including 0.03 wt% or less of C, 0.5 wt% or less of Si, 0.5 wt% or less of Mn, 0.035 wt% or less of P, 0.01 wt% or less of S, 18 to 21 wt% of Cr, 0.5 wt% or less of Mo, 0.03 wt% or less of N, 0.5 wt% or less of Cu, 0.05 wt% or less of Al, 0.2 wt% or less of Ni, so as to satisfy the following equations (1) through (2),

[10] EL = -162.1x(C+N)-0.2xCr-l.lxMo-0.2xTi/(C+N)+42.2 > 31...(1)

[11] P.I.=Cr+3.3Mo >21 ...(2)

[12] wherein, the contents of C, Si, Mn, P, S, Cr, Mo, N, Cu, Al and Ni are in % by mass, and the remainder of the alloy consists of the balance Fe and inevitable impurities.

[13] The composition may further comprise one element or more than two elements selected from the group consisting of 0.005 wt% or less of Ca, 0.005 wt% or less of Mg and 0.005 wt% or less of Zr.

[14] Moreover, C/N ratio is 1.05 or less and Ti/(C+N) ratio is 18 to 25.

[15] According to another aspect of the present invention, with a manufacturing method of a ferritic stainless steel having superior corrosion resistance and strechablility, the slab for stainless steels is subjected to making a hot rolling at a heating temperature of 1230 to 128O⁰C and then being terminated at a temperature of 740 to 85O⁰C, and then making a hot annealing within a range of 900 to 1000⁰C, and then making a cold rolling at a reduction rate of 50% or more, and the making a cold annealing from a temperature of 900 to 1000⁰C, and then controlling a grain size within the range of 6.3 to 7.5 in the grain size number of the ASTM. [16]

Advantageous Effects

[17] As described above, the stainless steel of the present invention has superior corrosion resistance, elongation and strechablility, and the stainless steel is suitable for a muffler of an automobile exhaust system and the end part of an exhaust system. Moreover, the stainless steel has higher contents of Cr, which is useful for improving corrosion resistance, instead of adding Mo, which is more expensive than Cr, by using EL (Equation 1) and P.I. (Equation 2) equations which is an equation for computing elongation and fitting indexes. The stainless steel has optimally controlled values of C%+N%, C%/N%, and Ti%/(C%+N%) by mass. A heating temperature, hot rolling temperature at final stage and hot & cold annealing conditions of slab are controlled in the present invention. Brief Description of the Drawings

[18] FIG. 1 is a graph showing the change of elongation according to the change in the grain size number of the ASTM after a cold annealing in a 20Cr-0.3Mo-Ti steel;

[19] FIG. 2 is a graph showing a change of elongation according to C/N ratio in 20Cr-Ti steel;

[20] FIG. 3 is a graph showing a change of the grain size number of the ASTM after a cold annealing according to C/N ratio in 20Cr-Ti steel;

[21] FIG. 4 is a graph showing the change of elongation after a cold annealing according to the change of Ti/(C+N) ratio in 20Cr-Ti steel; and

[22] FIG. 5 is a graph showing the change of an impact transition temperature (DBTT) according to the added amounts of Ca, Mg, and Zr in 20Cr-Ti steel. Mode for the Invention

[23] Hereinafter, the present invention will be described in more detail.

[24] The stainless steel of the present invention has superior corrosion resistance, elongation and strechablility, and the stainless steel is suitable for a muffler of an automobile exhaust system and the end part of an exhaust system. Moreover, the stainless steel has higher contents of Cr, which is useful for improving corrosion resistance, instead of adding Mo, which is more expensive than Cr, by using EL (Equation 1) and P.I. (Equation 2) equations which is an equation for computing elongation and fitting indexes. The stainless steel has optimally controlled values of C%+N%, C%/N%, and Ti%/(C%+N%) by mass. A heating temperature, hot rolling temperature at final stage and hot & cold annealing conditions of slab are controlled in the present invention.

[25] In prior arts, STS436L steel is used for a muffler and various pipes for an end part of an automobile exhaust system. The STS436L steel contains expensive Mo from 1% to 1.2% and thus its manufacturing cost is high. Thereby the use of the STS436L steel is restricted.

[26] The stainless steel of this invention has superior corrosion resistance and strechability, and therefore the pipes which are made of the stainless are suitable for be expanded in the low temperature. Moreover, the stainless steel of this invention contains lower contents of Mo than the prior arts.

[27] In accordance with one aspect of the present invention, the above and other objects can be accomplished by the provision of a stainless steel comprising: 0.03 wt% or less of C, 0.5 wt% or less of Si, 0.5 wt% or less of Mn, 0.035 wt% or less of P, 0.01 wt% or less of S, 18 to 21 wt% of Cr, 0.5 wt% or less of Mo, 0.03 wt% or less of N, 0.5 wt% or less of Cu, 0.05 wt% or less of Al, 0.2 wt% or less of Ni, so as to satisfy the following equations (1) through (2),

[28] EL = -162.1x(C+N)-0.2xCr-l.lxMo-0.2xTi/(C+N)+42.2 > 31...(1)

[29] P.I.=Cr+3.3Mo >21 ...(2)

[30] wherein, the contents of C, Si, Mn, P, S, Cr, Mo, N, Cu, Al and Ni are in % by mass, and the remainder of the alloy consists of the balance Fe and inevitable impurities.

[31] The stainless steel may further comprise one element or more than two elements selected from the group consisting of 0.005 wt% or less of Ca, 0.005 wt% or less of Mg and 0.005 wt% or less of Zr. Moreover, C/N ratio is 1.05 or less and Ti/(C+N) ratio is 18 to 25.

[32] According to another aspect of the present invention, with a manufacturing method of a ferritic stainless steel having superior corrosion resistance and strechablility, the slab for stainless steels is subjected to making a hot rolling at a heating temperature of 1230 to 128O⁰C and then being terminated at a temperature of 740 to 85O⁰C, and then making a hot annealing within a range of 900 to 1000⁰C, and then making a cold rolling at a reduction rate of 50% or more, and the making a cold annealing from a temperature of 900 to 1000⁰C, and then controlling a grain size within the range of 6.3 to 7.5 in the grain size number of the ASTM.

[33] Now, the components and compositions of the stainless steel according to the present invention will be described in detail.

[34] C and N are interstitial elements which form a TiC and TiN. If the content of C and the content of N is increased, the excessive content of C do not form the TiC and the excessive content of N do not form the TiN. The excessive content of C and The excessive content of N reduce the elongation and strechablility of a material. Therefore, the content of C is limited to 0.03% or less and the content of N is limited to 0.03% or less. And, if the content of C+N is increased, the content of Ti should be increased. As a result, surface defects such as scab are likely to be generated due to the increase of inclusions, and a nozzle clogging is likely to happen during a continuous casting. Moreover the elongation is reduced due to the increase of dissolved the content of C and N. So the content of C+N should be limited to 0.014%.

[35] Si is a ferrite phase forming element. Si serves to somewhat stabilize the ferrite phase and improve oxidation resistance. However, if the content of Si exceeds 0.5%, the surface defects are likely to be generated due to the increase of Si inclusions. Moreover hardness, yield strength, and tensile strength is promoted and the elongation is reduced, thereby the workability being deteriorated. So, the content of Si should be limited to 0.5% or less.

[36] If the content of Mn is excessive, MnS is eluted to reduce pitting corrosion resistance. Therefore, the content of Mn should be limited to 0.5% or less.

[37] Ni is a gamma phase generating element. If the content of Ni is excessive, the gamma phase is increased and therefore the generation of martensite phase is promoted when a coil is air cooled after a hot rolling. Thereby the elongation is reduced due to the increase of strength and hardness. So, the content of Ni should be limited to 0.2% or less.

[38] P and S form inclusions such as MnS to degrade corrosion resistance and hot workability. They should be restricted as low as possible. Therefore, the content of P should be limited to 0.035% or less and the content of S should be limited to 0.01% or less.

[39] If the content of Cr is low, the corrosion resistance is reduced and if the content of Cr is high, the corrosion resistance is improved. And the strength is promoted but the elongation is deteriorated, thereby the workability being degraded. Therefore, the content of Cr should be limited to 18 to 21%.

[40] If the content of Mo is increased, the corrosion resistance is greatly improved. But the manufacturing cost is high. The strength is promoted but and the elongation is reduced, thereby the workability being deteriorated. Therefore, the content of Mo should be limited to 0.5% or less in consideration of the corrosion resistance and workability.

[41] Al is an added element as deoxidizer. If Al is added, the surface defect is likely to be generated. Therefore, the content of Al should be limited to 0.05% or less.

[42] Cu is a gamma phase generating element. If the content of Cu is excessive, the gamma phase is increased and therefore the generation of martensite phase is promoted when a coil is air cooled after a hot rolling. Thereby the elongation is reduced due to the increase of strength and hardness. So, the content of Cu should be limited to 0.5% or less.

[43] If the content of Ti is excessive, surface defects such as scab are likely to be generated due to the increase of inclusions, and a nozzle clogging is likely to happen during a continuous casting. Moreover the elongation is reduced due to the increase of the dissolved content of Ti. And, if Ti/(C+N) ratio is low considering the content of Ti as compared to the content of C+N, intergranular corrosion is likely to be generated, thereby the corrosion resistance being degrading. Therefore, the content of Ti should be limited to 0.4%. The value of Ti/C+N ratio should be limited to 18 to 25 in consideration of the corrosion resistance and the formability.

[44] One element or more than two elements selected from the group consisting of Ca,

Mg, and Zr is added, the grain size of heat-affected zone during a TIG welding is minuted to lower the impact transition temperature (DBTT), thereby the expandability of TIG welding pipe increasing at a low temperature like winter. However, if Ca, Mg, and Zr is added too much, the oxidation inclusions of Ca, Mg, and Zr are likely to be generated, thereby the corrosion resistance being degraded. Therefore, the contents of each element is limited as follows: the content of Ca is limited to 0.005% or less, the content of Mg is limited to 0.005% or less, and the content of Zr is limited to 0.01% or less.

[45] In the EL computation equation (equation 1) of the present invention for the improvement of elongation, if the EL value is less than 31, the rupture defect is likely to be generated due to the deteriorated elongation and strechability when stamping. So the value of EL is limited to 32 or more.

[46] EL = -162.1x(C+N)-0.2xCr-l.lxMo-0.2xTi/(C+N)+42.2...(l)

[47] Also, if the P.I(Pitting Index) value of equation 2 increases, the corrosion resistance is improved. Therefore, in order to increase this value, the content of Cr or Mo should be increased, thereby the elongation and strechability being degraded and the manufacturing cost being is high. Also, if the content of Cr or Mo content is too low, the corrosion resistance is deteriorated. Therefore, in order to have the corrosion resistance equivalent to the existing STS436L steel(steel to which 1% of Mo is added), the P.I. value of equation 2 is limited to 21 or more.

[48] P.I.= Cr+3.3Mo....(2)

[49] As to the C%/N% ratio, in the condition where the content of C% + N% content is the same, if the C%/N% value is 1.05 or more, that is, the C content is significantly over the N content, the elongation is degraded and the grain corrosion of the welding portion is likely to be generated. So, the C%/N% ratio is limited to 1.05 or less.

[50] As to the Ti%/(C%+N%), if the Ti%/(C%+N%) ratio is too low, the grain corrosion of the welding portion is likely to be generated after a welding. And, if the Ti%/(C%+N%) ratio is too high, the dissolved content of Ti is high, thereby the formability such as elongation being degraded. So, the Ti%/(C%+N%) is limited to the range of 18 to 25.

[51] The reason for limiting a manufacturing condition of the present invention is as follows.

[52] Recrystallization during the hot rolling is easily to be generated as the heating temperature becomes high. But if the heating temperature is too high, the surface defect is likely to be present. Therefore, the heating temperature of slab is limited to 1230 to 128O⁰C.

[53] As the temperature at final stage of the hot rolling is low, the deformation storage energy is high to help the recrystallization of annealing, thereby the elongation being improved. But if the temperature at final stage of the hot rolling is too low, the sticking defect, which is caused by adhering a roller to a material, is likely to be generated. So, the temperature at final stage of the hot rolling is limited to 740 to 85O⁰C.

[54] If the cold reduction rate of a material is too low, it is difficult to remove the surface defect and assure the surface characteristics. But, if the cold reduction rate of a material is too high, the formability is improved. Therefore, the cold reduction rate is limited to 50% or more when making the material.

[55] After the cold annealing, the elongation is the most excellent when the grain size number of ASTM is within the range of 6.3 to 7.5. So the grain size number is limited to this range.

[56] Hereinafter, the present invention will be described in more detail with reference to an example.

[57] (Example)

[58] An ingot with a thickness of 120mm is manufactured by melting the ferritic stainless steel composed as in the following table 1 in a vacuum melting facility of 50Kg. A hot- rolled sheet with a thickness of 3.0mm is manufactured by heating the manufactured ingot at 125O⁰C and then making the hot rolling within the temperature of 125O⁰C to 800⁰C. It is cleansed by acid after the hot annealing of 95O⁰C, then it is subject to a cold rolling at a thickness of 1.5mm and 0.6mm. It is cleansed by acid after the cold annealing of 95O⁰C. A grain size of the cold-rolled sheet is measured by using a tensile test, an Erichsen test, and an image analyzer. And, the pitting potential of the cold- rolled sheet is tested by a KSD 0238 method and is indicated by an average value by being measured five times with a value of Vc 10.

[59] The measurement of impact transition temperature (DBTT) is served by the steps below;

[60] V notch impacting samples are made from the cold-rolled sheet with a thickness of

1.5mm(the steel to which Ca, Zr, and Mg are added and the steel to which Ca, Zr, and Mg are not added) in a server size. And the impact tests of the samples are served within the temperature range of +20 to -70°at an interval of 1O⁰C.

[61] Hereinafter, the test results will be described in more detail with reference to the table 1. [62] The table 1 indicates chemical component of sample(by mass %), computation value of EL and P.I., corrosion resistance(pitting potential), and strechability(Erichsen value), or the like.

[63] The content of Cr and the content of Mo are controlled in the inventive steel by the

P.I. value of 21 or more. When the inventive steel is compared to a conventional comparison material(sample No.8) containing the content of Mo of 1%, it is found that the inventive steel has the corrosion resistance value(pitting potential≥277mV) of the same as the comparison material(sample No. 8).

[64] Also, it can be appreciated that the content of C, N, Cr, Mo, and Ti(C+N) being controlled to make the value of EL 31 or more by using equation 1 of EL value, the inventive steel has the superior corrosion resistance, the elongation measured of the inventive steel is 31.7% or more, and the Erichsen value indicating the strechability is 9mm or more. And, it can be appreciated that the inventive steel whose C/N ratio is low to 1.05 or less and Ti/(C+N) value is controlled within the range of 19 to 25 has the superior elongation and Erichsen value as compared to the comparison material beyond this range.

[65] FIG. 1 is a graph showing the change of elongation according to the change in the grain size number of the ASTM after a cold annealing in a 20Cr-0.3Mo-Ti steel(sample No. 7). As appreciated from FIG. 1, when the ASTM grain size number is within the range of 6.3 to 7.5 during the cold annealing, the elongation is the most excellent.

[66] FIG. 2 is a graph showing a change of elongation according to C/N ratio in 20Cr-Ti steel . In the condition where the content of C+N is 120ppm, The C/N content ratio is changed to 0.39, 1.05, and 2. If the C/N ratio is high to 1.05 or more, the elongation is degraded.

[67] FIG. 3 is a graph showing a change of the grain size number of the ASTM after a cold annealing according to C/N ratio in 20Cr-Ti steel. As the value of C/N ratio becomes high after the cold annealing in the same condition, the grain size is minuted, thereby the elongation being degraded. Therefore, in order to improve the formability such as the elongation, the C/N ratio should be controlled to 1.05 or less. As a result, it can be appreciated that the elongation and strechabililty are excellent.

[68] FIG. 4 is a graph showing the change of elongation after a cold annealing according to the change of Ti/(C+N) ratio in 20Cr-Ti steel. As the Ti(C+N) ratio becomes low, the elongation is excellent but if the Ti(C+N) ratio is 18 or less, the grain corrosion of the welding portion is likely to be generated. Therefore, it is necessary to add Ti by controlling the Ti/(C+N) ratio within the range of 18 to 25 considering the grain corrosion of the welding portion and the elongation.

[69] FIG. 5 is a graph showing the change of an impact transition temperature (DBTT) according to the added amounts of Ca, Mg, and Zr in 20Cr-Ti steel. If Ca is added or if Ca+Mg, Ca+Zr are added, the impact transition temperature is lowered to -5O⁰C. As a result, when the working temperature is low like winter, the expandability of the TIG pipe is excellent

[70] Table 1

[71] table indicating chemical component per sample, El, P.I., pitting potential and strechability (Erichsen value)

[72]

[73] [74]

[75]

[76]

Claims

[1] A ferritic stainless steel with superior corrosion resistance and strechability, comprising: 0.03 wt% or less of C; 0.5 wt% or less of Si; 0.5 wt% or less of Mn;

0.035 wt% or less of P; 0.01 wt% or less of S; 18 to 21 wt% of Cr; 0.5 wt% or less of Mo; 0.03 wt% or less of N; 0.5 wt% or less of Cu; 0.05 wt% or less of Al, and 0.2 wt% or less of Ni, so as to satisfy the following equations (1) through

(2),

EL = -162.1x(C+N)-0.2xCr-l.lxMo-0.2i/(C+N)+42.2≥31...(l)

P.I.=Cr+3.3Mo>21 ...(2) wherein, the percent of each element is by weight, and the remainder of the alloy consists of the balance Fe and inevitable impurities. [2] The ferritic stainless steel as set forth in claim 1, which further comprises one element or more than two elements selected from the group consisting of 0.005 wt% or less of Ca; 0.005 wt% or less of Mg, and 0.005 wt% or less of Zr. [3] The ferritic stainless steel as set forth in claim 1, wherein the contents of C, N and Ti meet the following formulas: C/N<1.05 and 18<Ti/(C+N)<25. [4] The ferritic stainless steel as set forth in claim 2, wherein the contents of C, N and Ti meet the following formulas: C/N<1.05 and 18<Ti/(C+N)<25. [5] A manufacturing method of a ferritic stainless steel having superior corrosion resistance and strechablility comprises the steps of: providing the ferritic stainless comprising: 0.03 wt% or less of C; 0.5 wt% or less of Si; 0.5 wt% or less of Mn; 0.035 wt% or less of P; 0.01 wt% or less of S; 18 to

21 wt% of Cr; 0.5 wt% or less of Mo; 0.03 wt% or less of N; 0.5 wt% or less of

Cu; 0.05 wt% or less of Al, and 0.2 wt% or less of Ni, so as to satisfy the following equations (1) through (2),

EL = -162.1x(C+N)-0.2xCr-l.lxMo-0.2xTi/(C+N)+42.2≥31...(l)

P.I.=Cr+3.3Mo>21 ...(2) subjecting the ferritic stainless steel to making a hot rolling at a heating temperature of 1230 to 128O⁰C and then being terminated at a temperature of 740 to 85O⁰C; annealing the ferritic stainless steel within a range of 900 to 1000⁰C; subjecting the ferritic stainless steel to making a cold rolling at a reduction rate of 50% or more; annealing the ferritic stainless steel from a temperature of 900 to 1000⁰C, and controlling the grain size of the ferritic stainless steel within the range of 6.3 to

7.5 in the grain size number of the ASTM, wherein, the percent of each element is by weight, and the remainder of the alloy consists of the balance Fe and inevitable impurities. [6] The method as set forth in claim 5, wherein the ferritic stainless steel comprises one element or more than two elements selected from the group consisting of

0.005 wt% or less of Ca; 0.005 wt% or less of Mg, and 0.005 wt% or less of Zr. [7] The method as set forth in claim 5, wherein the contents of C, N and Ti meet the following formulas: C/N<1.05 and 18<Ti/(C+N)<25. [8] The method as set forth in claim 6, wherein the contents of C, N and Ti meet the following formulas: C/N<1.05 and 18<Ti/(C+N)<25.