WO2008082096A1

WO2008082096A1 - Ferritic stainless steel with execellent corrosion resistnace and excellent discoloration resistance

Info

Publication number: WO2008082096A1
Application number: PCT/KR2007/006456
Authority: WO
Inventors: Yong Deuk Lee; Yong Heon Lee; Kwang Tae Kim; Jong Seog Lee
Original assignee: Posco
Priority date: 2006-12-28
Filing date: 2007-12-12
Publication date: 2008-07-10
Also published as: JP2010514924A; KR100821060B1; CN101573466A; JP5372775B2; CN101573466B

Abstract

The present invention relates to a high Cr ferritic stainless steel substituting Cr, Cu, Ti, Nb, etc, for an austenitic steel containing expensive Ni high corrosion resistance ferritic stainless steel and to a high corrosion resistance ferritic stainless steel with the same or more corrosion resistance as compared to the type 304 steel. The ferritic stainless steel is made of C of 0.01% or less, Si of 0.2 to 1.0%, Mn of 0.3% or less, Cr of 20 to 23%, Ni of 0.2 to 0.4%, N of 0.01% or less, Al of 0.03 to 0.10%, S 0.002% or less, Cu of 0.3 to 0.5%, Zr of 0.02 to 0.06%, Ti of 0.2 to 0.4% (Ti/(C+N) > 20), and Nb of 0.06 to 0.45% (Nb/(C+N)>28)), and Fe and inevitable impurities as the remaining components, wherein a unit is mass %. Also, a manufacturing method of performing a cold rolling and a surface polishing on the ferritic stainless steel is provided.

Description

FERRITIC STAINLESS STEEL WITH EXECELLENT CORROSION RESISTNACE AND EXCELLENT DISCOLORATION RESISTANCE

Technical Field

[1] The present invention relates to a ferritic stainless steel with excellent corrosion resistance and excellent discoloration resistance. More specifically, the present invention relates to a high chromium ferritic stainless steel containing corrosion resistance improving elements capable of substituting Ni so as to substitute an austenitic stainless steel containing an expensive Ni element and to ferritic stainless steel with excellent corrosion resistance and excellent discoloration resistance controlling Ti, Nb, Cu, Ni, Si, Zr components so as to have the same corrosion resistance as the type 304 material used for building materials. Background Art

[2] The existing type 304 austenitic stainless steel, which is made of an alloy of 18% Cr-

8% Ni as main compoments, has a problem of low price competitiveness because expensive Ni content is high.

[3] Also, since the existing type 436L ferritic stainless steel having the same corrosion resistance as the type 304 austenitic stainless steel, and so on contains an expensive Mo element, it has also a problem of low price competitiveness due to increased manufacturing cost.

[4] In order to substitute the type 304 steel or the type 436L steel whose manufacturing cost is increased due to a large amount of expensive Ni or Mo component, there have been proposed several methods to reduce Ni or Mo. However, according to such methods, its manufacturing cost can be substantially reduced through the reduction of Ni or Mo, it has a problem that it is difficult to obtain a user desired corrosion resistance.

Disclosure of Invention Technical Problem

[5] The present invention proposes to solve the problems. It is an object of the present invention to provide a high chromium ferritic stainless steel with excellent corrosion resistance and excellent discoloration resistance by the addition of Cr, Cu, Si, Ni, Zr elements, etc, so as to substitute expensive Ni and Mo in the existing type 304 (18Cr-8Ni) steel and the type 436L (18Cr-l.2Mo-0.2Ti) steel and the complex addition of Ti and Nb components so that Ti(C+N) is 20 or more or Nb(C+N) is 28 or more, so as to stabilize C and N component.

[6] Also, with the present invention, it is another object of the present invention to provide a steel having composition of

21%Cr-0.3%Ti-0.06%Nb-0.5%Si-0.4%Cu-0.3%Ni-0.02%Zr-0.007%C-0.008%N as representative alloy components and to provide a cold rolling method and a surface polishing method so as to improve the corrosion resistance and the high temperature discoloration resistance of the steel.

[7] The objects of the present invention are not limited to the aforementioned objects and other objection will be apparently understood by those skilled in the art from the description below. Technical Solution

[8] In order to accomplish the objects, a ferritic stainless steel with excellent corrosion resistance and excellent discoloration resistance according to one embodiment of the present invention is made of C of 0.01% or less, N of 0.01% or less, Ti of 0.2 to 0.4%, Nb of 0.05 to 0.45%, Si of 0.2 to 1.0%, Mn of 0.3% or less, Cr of 20 to 23%, Ni of 0.2 to 0.4%, Cu of 0.3 to 0.5%, Al of 0.03 to 0.10%, S 0.002% or less, Zr of 0.02 to 0.06 %, Ti/(C+N) of 20 or more or Nb/(C+N) of 28 or more, and Fe and inevitable impurities as the remaining components, wherein a unit is mass %.

[9] Also, with the present invention, a ferritic stainless steel with excellent corrosion resistance and excellent discoloration resistance existing in a area where a* value is smaller than 11 and b* value is larger than 11 after heating for 90 minutes at 500⁰C, upon measuring colors by using a colorimeter meeting ASTM D2244 is provided.

[10] Further, with the present invention, a ferritic stainless steel with excellent corrosion resistance and excellent discoloration resistance where the Nb is 0.1% or less, Si is 0.45% to 0.5%, and an annealing crystalline grain size is ASTM No. 7.5 or more is provided.

[11] In addition, with the present invention, a ferritic stainless steel with excellent corrosion resistance and excellent discoloration resistance where a hair line polishing is performed on the ferritic stainless steel in a brush roll type of 320 mesh or more and cold rolled surface roughness Ra and Rz values each is limited to 0.25 and 1.70 is provided.

Advantageous Effects [12] As described above, the present invention, a ferritic stainless steel with improved corrosion resistance to the same level as the type 304 steel by adding inexpensive chromium (Cr), copper (Cu), silicon (Si), zirconium (Zr), titanium (Ti) or niobium (Nb), etc, instead of expensive Ni is provided so as to substitute the type 304 steel that is an austenitic steel. Also, a manufacturing method of performing a proper cold rolling and a roll type of surface polishing on the steel to have excellent corrosion resistance is provided. Brief Description of the Drawings

[13] FIGS. 1 (a) and 1 (b) are an optical microscope photographs according to surface polishing conditions of steel of the present invention, wherein l(a) is a surface photograph showing an increase of polishing discontinuous length after using a polishing belt and l(b) is a surface photograph showing a reduction of polishing discontinuous length after using a brush roll. Best Mode for Carrying Out the Invention

[14] Hereinafter, features and actions of present invention will be described. First, the composition range and limitation reason of the present invention will be described.

[15] C is an austenite forming element. When the C is added, high temperature strength is improved. However, when the C is excessively added, it reacts with Cr to generate chromium carbide, thereby degrading the corrosion resistance, elongation, weldability of ferritic steel. Therefore, it is exemplarily limited to 0.01% or less, which is possibly low content.

[16] N is an austenite phase stabilizing element. It has an advantage of substituting a Ni element and improving strength and pitting corrosion resistance and has a disadvantage of degrading elongation and workability. Therefore, in the present invention it is limited to 0.01% or less.

[17] Ti stabilizes C and N elements using Ti (C, N) precipitate, suppresses the precipitation of chromium carbide at high temperature to improve corrosion resistance, and controls a recrystallized texture upon performing an annealing heat treatment to improve workability. However, when the Ti is excessively added, it causes a problem of workability degradation and surface defect. Therefore, it is limited to 0.2% to 0.4%, and in particular Ti/(C+N) becomes 20 or more.

[18] Nb stabilizes C and N elements using Nb (C, N) precipitate, suppresses the precipitation of chromium carbide at high temperature to improve corrosion resistance, controls a recrystallized texture upon performing an annealing heat treatment, and minutes a crystalline grain to prevent an occurrence of an orange peel phenomenon upon processing it as a material for an elevator. However, since workability degradation and surface defect such as ridging occur when the Nb is excessively added, it is controlled to 0.05% to 0.45% and in particular Nb/(C+N) becomes 28 or more.

[19] Mn is an austenite phase stabilizing element likewise nitrogen and is an element substituting Ni. The Mn quasi-stabilizes the austenite phase and increase strength when it is added to the ferrictic steel, however, degrades the workability. Therefore, it is limited to 0.3% or less.

[20] Cr is an oxidation film formation promoting element and increases corrosion resistance to the same level as the type 304 steel. It is necessary to add Cr of 21 % or more in order to substitute a Mo element of the type 436L steel. However, when the Cr is excessively added, a sticking defect is increased due to a generation of densified oxidation scale when a hot rolling is performed. Therefore, it is limited to 23%.

[21] Ni is an austenite phase stabilizing element likewise C and N and is an element reducing corrosion speed to improve corrosion resistance. However, it is an expensive element and is therefore limited to 0.2% to 0.4% when considering economical efficiency.

[22] Si becomes 0.2% or more so as to improve high temperature oxidation resistance and to improve corrosion resistance by reinforcing a passivation film in a stainless steel. However, when the Si is excessively added, it degrades elongation and is therefore limited to 1.0% or less.

[23] S is a very small amount of impurity element. It is a main element that is segregated in a crystalline grain boundary to lead to a processing crack when a hot rolling is performing. Therefore, it is limited to 0.002% or less, which is possibly low content.

[24] Cu is an element reducing corrosion propagation speed to improve corrosion resistance. However, when the Cu is excessively added, it is an element to degrade high temperature oxidation resistance and hot workability. Therefore, it is limited to 0.3% to 0.5%.

[25] Al is a deoxidizer element controlling a crystalline grain size. When the Al is excessively added, a nozzle can be stopped due to inclusions in a continuous casting process. Therefore, it is limited to 0.10% or less.

[26] Zr is an element improving impact toughness at a welding portion. When the Zr is excessively added, a nozzle can be stopped due to inclusions in a continuous casting process. Therefore, it is limited to 0.06% or less. [27] In the following table 1, in order to change Ti/(C+N) fraction in basic components of

21%Cr-0.3%Ti-0.006%C-0.006%N-0.3%Ni-0.5%Si-0.4%Cu and obtain the corrosion resistance of the same level as the type 304 steel and the type 436L steel, a high chromium ferritic stainless steel, which substitutes elements of Cr, Cu, etc, for Ni and Mo is compared with the type 304 steel and the compared example is listed in the table 1.

[28] Each of the steel according to one embodiment of the present invention and the compared steel is melted and is cast in an ingot. Thereafter, a hot rolling, a cold rolling, an annealing and pickling are performed on them to manufacture a cold rolled strip with a thickness of 1.0mm. The corrosion resistance test results performed on the cold rolled strip are listed in the table 1.

[29] Table 1

[Table 1]

[Table ]

Physical properties of steel according to one embodiment of the present invention and compared steel

[30] CPT: Critical Pitting Temperature

[31] The critical pitting temperature (CPT) means a temperature where the current density of a test piece reaches 100 μk/cm by putting the test piece into lM(mol) NaCl solution and applying +30OmV to the test piece, compared with a calomel electrode. Reviewing the results of the table 1, it can be appreciated that the CPT of very expensive austenitic type 304 steel due to the addition of nickel of 8% is 29.4⁰C. The austenitic type 304 steel is used as general building interior and exterior materials.

[32] Typically, when the austenitic type 304 steel is used as the general building interior and exterior materials, the corrosion can be caused by the attachment of foreign materials to its surface due to external corrosion environments. Therefore, the steel should be periodically wahsed. In order to have sufficient resistance against detergent, rust should not be caused in a repetitive salt water spray test based on ISO 14993. In order to meet such criteria, the CPT should exceed 32⁰C.

[33] The present invention provides the ferritic stainless steel with excellent corrosion resistance capable of being used in the environment that the general type 304 steel is used, by controlling a very small amount of element while adding chromium of 21% or more without adding expensive nickel.

[34] As can be appreciated from the table 1, in the case of the compared steel 1 with chromium of 19% or less, the CPT is 20.1⁰C. When it compares with the type 304 steel, it can be appreciated that the CPT is very low. When the Cr content is increased to 21% as in the compared steels 2 and 3, the CPT is significantly increased to 25⁰C.

[35] The compared steels 4 and 5 are stabilization elements and are added with Ti and Nb, respectively. When comparing them with the compared steel 2, they are added with a very small amount of Cu and Ni so that their CPTs are increased to 28.1⁰C and 29.8⁰C, respectively. However, since the Si content is lowered to a 0.1% level, it is insufficient to obtain the corrosion resistance higher than that of the type 304 steel. Therefore, it can be appreciated that the corrosion resistance should be improved by increasing the Si content simultaneously with adding a very small amount of Cu and Ni.

[36] The compared steel 6 with Si of 0.45% is added with a small amount of Ni to increase its CPT to 30.2⁰C, as compared to the compared steel 3. However, its corrosion resistance is still insufficient as compared to the type 304 steel.

[37] On the other hand, the steel according to one embodiment of the present invention is a stabilization element and is steel to which Ti and Nb are complex-added. It contains Ti of 0.3% and Nb of 0.06%, and the Si content is increased to 0.45%. In the case of the steel according to one embodiment of the present invention, it can be appreciated that the critical pitting temperature (CPT) is about 34 to 35⁰C and the corrosion resistance much higher than that of the type 304 steel is obtained. This means that the Si content in the Cr steel of 21% is increased to 0.45% and when Cu and Ni are added, the corrosion resistance is significantly improved.

[38] In the case of the building interior and exterior materials, a bending work is generally performed At this time, when the crystalline grain is large, the orange peel phenomenon occurs so that the damage of the designs formed on the surface occurs. The present invention investigates the surface after bending the steels with different crystalline grain sizes to investigate whether the orange peel phenomenon occurs. The results thereof are shown in a table 2.

[39] Table 2 [Table 2] [Table ] Degree of orange peel occurrence according to crystalline grain size of steel

[40] O: orange peel non-occurrence, Δ: orange peel unclear, X: orange peel occurrence [41] Reviewing the results of the table 2, it can be appreciated that there is the difference of the orange peel occurrence according to the crystalline grain size. When the crystalline grain size is 7.5 or more based on American Society of Testing Materials (ASTM) No, it can be appreciated that the orange peel does not occur. Therefore, when Nb of 0.06% or more is added, NbC segregates are more stabilized at a high temperature as compared to TiC to suppress the growth of the crystalline grain so that a fine crystalline grain can be obtained when an annealing is perfomed. Also, in the steel according to one embodiment of the present invention the Si content is 0.45% higher than the existing steel (the compared steel 4: Si of 0.10%). Since the equiaxed grain of the slab is about 40% when performing the continuous casting, the workability such as the bending processing, etc, is significantly improved.

[42] When the steel is used as the building materials, a hair line polishing on the surface is positively necessary. In the following table 3, the corrosion resistance of the steel according to one embodiment of the present invention varied according to the surface polishing method is compared and evaluated.

[43] With the table 3, the surface polishing is performed by means of a brush roll to allow the surface roughness Ra value to be 0.25 or less so that excellent corrosion resistance can be obtained Herein, the measured surface defect length is a value summing the surface defect lengths existing in the respective polishing surfaces after randomly photographing three places using an optical microscope that has a magnification of 1000 times.

[44] Table 3 [Table 3] [Table ]

Change in corrosion resistance due to surface polishing conditions of steel according to one embodiment of the present invention

[45] When the polishing belt is used, the reason why the surface roughness is rough and the defect length is increased is that many surface defects capable of inducing the corrosion occur during the polishing process when using the polishing belt. For this reason, the surface polishing of the ferritic steel should be softened by means of the brush roll.

[46] FIG. 1 shows the surface photographs after polishing the surface using the polishing belt and after polishing the surface using the brush roll. FIGS. l(a) and l(b) are an optical microscope photographs according to surface polishing conditions of steel of the present invention. Herein, it can be appreciated from FIG. l(a) that the polishing discontinuous length after using the polishing belt is increased and it can be appreciated from FIG. l(b) that the polishing discontinuous length after using the brush roll is reduced.

[47] When using the polishing belt, there is much tearing phenomenon on the surface. In other words, when polishing the surface using the polishing belt, the defect length exceeds 2.58mm. However, it can be appreciated that the defect length is significantly reduced to 0.67mm when using the brush roll and when a mesh is 320 (the number of apertures of sieve per one inch), the corrosion resistance is very improved

[48] Also, the present inventors found through the experimental results several times that there are defects as shown in FIG. l(b) when cold rolled reduction rate is 70% or less in performing the cold rolling process. If there are such defects in the steel, the corrosion resistance is degraded. Therefore, in order to improve the corrosion resistance, it is found that a thickness of a hot rolled strip should initially be thick and the reduction rate of 70% or more should be applied.

[49] Table 4

[Table 4]

[Table ]

Zr element effect influencing on impact toughness about welding portion of steel according to one embodiment of the present invention

[50] Where the erichsen value means a value obtained by performing an erichsen experiment on the respective steel. The erichsen experiment means an experiment measuring the variation value of steel by applying a load to a thin plate after making the steel, which is an object of experiment, into the thin plate.

[51] When the steel according to one embodiment of the present invention is manufactured and used into a pipe in a cold area, a welding should be performed, wherein the steel is the Zr containing steel. Therefore, the impact toughness of the welding portion at a low temperature should be excellent when the welding is performed.

[52] Since the steel according to one embodiment of the present invention contains Zr element of 0.02%, it shows excellent impact toughness energy value as compared to the Zr non-containing steel as in the results of the table 4. When the welding is performed, the equiaxed crystal nucleation is increased by means of the Zr precipitate for a solidification structure so that the crystalline grain is fine and the impact toughness is excellent.

[53] Table 5 [Table 5]

[Table ]

Cr, Si elements effect influencing on steel according one embodiment of the present invention

[54] O: discoloration non-occurrence, X: discoloration occurrence

[55] When using as a gas grill, the surface discoloration of the ferritic type 439 steel, etc, is fester than that of the type 304 steel in a high temperature heating. However, in the case of the steel according to one embodiment of the present invention Cr and Si are increased as in the table 5 so that it can be appreciated that it has the excellent discoloration resistance. Therefore, when the steel according to one embodiment of the present invention (21Cr-Ti, Nb-0.45Si) is used as the gas grill that does not excessively require the processing, it can obtain the same discoloration resistance as the type 304 steel.

[56] With the steel according to one embodiment of the present invention, when the colors are measured by means of the colorimeter meeting the ASTM D2244 irrespective of the surface finish shape, the ferritic stainless steel with excellent discoloration resistance existing in an area where the a* value is smaller than 11 and the b* value is larger than 11 after heating for 90 minutes at 500⁰C can be obtained.

[57] Although exemplary embodiments of the present invention have been shown and described, it would be appreciated by those skilled in the art that changes might be made in this embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.

Claims

[1] A ferritic stainless steel with excellent corrosion resistance and excellent discoloration resistance is made of C of 0.01% or less, N of 0.01% or less, Ti of 0.2 to 0.4%, Nb of 0.05 to 0.45%, Si of 0.2 to 1.0%, Mn of 0.3% or less, Cr of 21 to 23%, Ni of 0.2 to 0.4%, Cu of 0.3 to 0.5%, Al of 0.03 to 0.10%, S 0.002% or less, Zr of 0.02 to 0.06%, Ti/(C+N) of 20 or more or Nb/(C+N) of 28 or more, and Fe and inevitable impurities as the remaining components, wherein a unit is mass %.

[2] The ferritic stainless steel as claimed in claim 1, wherein the Si is 0.45 to 0.5%.

[3] The ferritic stainless steel as claimed in claim 1, wherein the Cr is 21% and the

Si is 0.45 to 0.5%, and it exists in a area where a* value is smaller than 11 and b* value is larger than 11 after heating for 90 minutes at 500⁰C, upon measuring colors by using a colorimeter meeting ASTM D2244.

[4] The ferritic stainless steel as claimed in claim 1, wherein the Nb is 0.1% or less and the Si is 0.45% to 0.5%, and an annealing crystalline grain size is ASTM No. 7.5 or more.

[5] A manufacturing method of a ferritic stainless steel by performing a cold rolling process and a surface polishing process on the ferritic stainless steel made of A ferritic stainless steel with excellent corrosion resistance and excellent discoloration resistance is made of C of 0.01% or less, N of 0.01% or less, Ti of 0.2 to 0.4%, Nb of 0.05 to 0.45%, Si of 0.2 to 1.0%, Mn of 0.3% or less, Cr of 21 to 23%, Ni of 0.2 to 0.4%, Cu of 0.3 to 0.5%, Al of 0.03 to 0.10%, S 0.002% or less, Zr of 0.02 to 0.06%, Ti/(C+N) of 20 or more or Nb/(C+N) of 28 or more, and Fe and inevitable impurities as the remaining components, wherein a unit is mass %, the method comprising the steps of: performing the cold rolling process at cold rolled reduction rate of 70% or more; performing the surface polishing process by means of a hair line surface polishing in a brush roll type of 320 mesh or more to limit cold rolled surface roughness Ra and Rz values each to 0.25 and 1.70.

[6] The manufacturing method as claimed in claim 5, wherein the Si is 0.45 to 0.5%.

[7] The manufacturing method as claimed in claim 5, wherein the Cr is 21% and the

Si is 0.45 to 0.5%, and it exists in a area where a* value is smaller than 11 and b* value is larger than 11 after heating for 90 minutes at 500⁰C, upon measuring colors by using a colorimeter meeting ASTM D2244. [8] The manufacturing method as claimed in claim 5, wherein the Nb is 0.1% or less and the Si is 0.45% to 0.5%, and an annealing crystalline grain size is ASTM No. 7.5 or more.