KR20140080350A - Ferritic stainless steel sheet and manufacturing method thereof - Google Patents

Abstract

The present invention relates to a ferritic stainless steel sheet and a method of manufacturing the same, and more particularly to a ferritic stainless steel sheet and a method of manufacturing the same, capable of improving high-temperature oxidation and intergranular corrosion characteristics by substituting a single Ti additive and a Ti+Nb complex additive, which serve as stabilization elements of materials for a vehicle exhaust system, with Si. The ferritic stainless steel sheet according to an embodiment of the present invention includes 0.02 wt% or less of C, 0.02 wt% or less of N, 0.5-1.5 wt% of Si, 1.0 wt% or less of Mn, 10.0-13 wt% of Cr, 0.1- 0.3 wt% of Ti, and the remaining amount of Fe and inevitable impurities. The value of C+N is 0.03 or less, the value of Si+2Ti is 0.8 or more, and the ratio of Ti/(C+N) is 8 or more.

Description

FIELD OF THE INVENTION The present invention relates to a ferritic stainless steel sheet and a manufacturing method thereof,

The present invention relates to a ferritic stainless steel and a method of manufacturing the same, and more particularly, to a ferritic stainless steel and a method of manufacturing the ferritic stainless steel, And a ferrite stainless steel having improved properties.

Generally, stainless steel is classified according to chemical composition or metal structure. According to the metal structure, the stainless steel is classified into an austenitic system (300 system), a ferrite system (400 system), a martensitic system, and an ideal system.

Among these stainless steels, ferritic stainless steel is excellent in corrosion resistance and is mainly used for various kitchen appliances, automobile exhaust system parts, building materials, and household appliances.

On the other hand, the muffler for an automobile exhaust system is heated to a temperature of 400 DEG C or more depending on the conditions of the automobile, etc., and a phenomenon referred to as "grain boundary sensitization" For example, when ferric stainless steels are exposed for a long time in the region of 400 ° C or higher, the Cr component in the base material bonds with C and Cr carbide such as Cr ₂₃ C ₆ precipitates near the grain boundary, and the Cr concentration in the grain boundary decreases And corrosion of intergranular corrosion occurs when exposed to corrosive elements such as condensed water components remaining in the automobile exhaust system.

In ferritic stainless steels for automotive exhaust systems, manufacturing costs are increased because the VOD process is performed after the AOD process, which is a steelmaking process, to lower the C and N contents for corrosion resistance and intercalation corrosion resistance.

Also, when passing through only the AOD process, the content of C and N is high, and in order to prevent intergranular corrosion, the Ti content is added so as to catch C and N in solid solution. However, when the Ti content is increased to a high level, Ti oxide inclusions are increased due to an increase in Ti oxide production, resulting in a large amount of surface defects of the coil, thereby making it difficult to produce a normal product.

When the C and N content and the Ti content are increased, the low-temperature impact characteristic of the welded portion of the TIG welded pipe is lowered. Therefore, when the temperature of the pipe is low, end pipe, and the like, the fracture failure occurs frequently, and the high Ti content has a problem that the cleanliness of the steel due to the reoxidization of Ti during the steelmaking refining process is greatly deteriorated, and surface defects are frequently generated.

In order to solve such a problem, conventionally known technologies will be described as follows.

Japanese Patent Application Laid-Open No. 2006-077300 (Patent Document 1) discloses a steel sheet comprising 10 to 20% of Cr, 0.001 to 0.02% of C, 0.01 to 1.0% of Si, 0.01 to 1.0% of Mn, 0.01 to 0.04% of P, : 0.001 to 0.020%, Nb: 0.1 to 0.6%, and the balance Fe and the ferrite-based stainless steel cold-rolled sheet which becomes inevitable impurities are annealed at 1000 to 1080 캜 and then cooled at 800 to 975 캜 For 5 to 300 sec. However, it is a steel containing Nb alone.

Japanese Unexamined Patent Application Publication No. 2006-063380 (Patent Document 2) discloses a ferritic stainless steel comprising 0.025% or less of C, 0.025% or less of N, 0.03% or less of C + N, 12 to 30% 0.3 to 0.7, Ti: 0.02 to 0.2% or less, and Zr: 0.02 to 0.2%, and the balance of Fe and inevitable impurities, wherein the metal structure is a non-recrystallized structure, And the difference in average coefficient of linear expansion from the crystallized glass for a weight detecting sensor substrate to 900 캜 is less than 10%. It is a steel which is subjected to a sintering heat treatment with a glass layer at a recrystallization temperature of 850 ° C or less and 1150 ° C or less and at a temperature of 800 to 900 ° C for 20 to 120 minutes and is used as an application for a weight detection sensor substrate. It is a river.

Japanese Unexamined Patent Application Publication No. 2005-314740 discloses a steel sheet comprising 0.015% or less of C, 0.10 to 0.25% of Si, 0.10 to 0.3% of Mn, 0.04% or less of P, 0.01% or less of S Ti: 3X (C + N), Ti: 3X (C + N), Ti: To 0.25%, Nb: 0.3 to 1%, and C + N: 0.02% or less, and the balance of Fe and inevitable impurities. The steel has a plate thickness of 1 to 3 mm, a 0.2% proof stress at 950 캜 of 15 MPa or more, an average expansion value at room temperature of 30% or more, and an average Steel plates with an r value of 1.3 or higher. The production method is characterized in that the ingot or slab is cast and the temperature of the hot-rolled sheet annealing is set to 800 to 950 占 폚 in the step of hot rolling, hot rolling annealing, acid washing, cold rolling, final annealing, pickling, And is a steel to which Mo is added.

Japanese Patent Application Laid-Open No. 2005-200746 discloses a steel sheet comprising 0.001 to 0.02% of C, 0.001 to 0.02% of N, 0.002 to 0.03% of C + N, 0.03 to 0.5% of Si, 0.05 to 0.5% of Mn, 0.1 to 0.8% of Cu, 11 to 15% of Cr, 0.2 to 0.5% of Nb, 0.02 to 0.3% of Ti, 0 to 0.2% of Al, 0 to 1% of Ni, , And at least one of Mo and W, and the balance of Fe and inevitable impurities is a ferritic stainless steel for an automotive exhaust system member. It is a steel containing at least one of Ca: 0.0002 to 0.005%, REM: 0.0001 to 0.01% and Y: 0.0001 to 0.01%.

However, the techniques described in the above-mentioned Patent Literature disclose a method of controlling C + N, adding stabilizing elements such as Ti and Nb, deriving equations related to addition amounts of stabilizing elements, and adding rare earths such as REM There was a problem such as a high cost increase at the top of the manufacturing process.

Japanese Patent Laid-Open No. 2006-077300 (Mar. 23, 2006) Japanese Patent Application Laid-Open No. 2006-063380 (Mar. 09, 2006) Japanese Laid-Open Patent Application No. 2005-314740 (November 10, 2005) Japanese Patent Laid-Open No. 2005-200746 (2005. 07. 28)

The present invention provides a ferritic stainless steel for an automobile exhaust system excellent in high temperature oxidation characteristics and welded portion corrosion resistance, and a method of manufacturing the ferritic stainless steel.

Particularly, a ferritic stainless steel for an automobile exhaust system excellent in corrosion resistance, low temperature impact resistance and corrosion resistance while lowering the manufacturing cost by replacing Ti alone and Ti + Nb composite additives used as stabilizing elements of automotive exhaust system materials in the past, Steel and a method of manufacturing the same.

The ferritic stainless steel according to one embodiment of the present invention contains 0.02 wt% or less of C, 0.02 wt% or less of N, 0.5 to 1.5 wt% of Si, 1.0 wt% or less of Mn, 10.0 to 13 wt% : 0.1 to 0.3 wt%, the balance of Fe and other unavoidable impurities, and has a C + N value of 0.03 or less, a Si + 2Ti value of 0.8 or more, and a Ti / (C + N) ratio of 8 or more .

C, N, Si and Ti in the above C + N, Si + 2Ti and Ti / (C + N) mean the content (wt%) of each component.

The stainless steel is characterized by containing at least one of Ca: 0.0005 to 0.002 wt% and Zr: 0.0002 to 0.01 wt%.

The stainless steel has an average potential value of 50 mV or more.

The stainless steel has an elongation of 33% or more.

A ferritic stainless steel manufacturing method according to an embodiment of the present invention comprises the steps of: C: not more than 0.02 wt%, N: not more than 0.02 wt%, Si: 0.5 to 1.5 wt%, Mn: A slab having a Si + 2Ti value of 0.8 or more and a ratio of Ti / (C + N) of 8 or more is prepared, and the slab is hot- Hot rolling, hot rolling, annealing and cold rolling. The cold rolling is characterized by rolling at a reduction ratio of 70 to 80%.

Si, Ti, C and N in the above Si + 2 Ti and Ti / (C + N) mean the content (wt%) of each component.

The hot finish rolling is performed at a temperature range of 740 to 880 캜.

According to the embodiment of the present invention, by replacing Ti alone and Ti + Nb composite additives used as stabilizing elements of automotive exhaust system materials with Si, the content of expensive stabilizing elements Ti and Nb can be reduced, There is an effect that can be lowered.

The manufacture of ferritic stainless steel having the advantages of excellent resistance to intergranular corrosion resistance, low-temperature impact resistance and corrosion resistance by appropriately adjusting the content of Si and Ti / (C + N) There is a possible effect. In addition, since the ferritic stainless steel which is excellent in the corrosion resistance of the intergranular corrosion resistance, the low temperature impact property and the corrosion resistance is applied to the muffler parts of the automobile exhaust system, the quality of the muffler is improved,

FIG. 1 is a graph (TTS) showing the degree of grain boundary corrosion after heat treatment of ferritic stainless steels according to Examples and Comparative Examples according to the present invention in a temperature range of 400 to 800 ° C.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. It will be apparent to those skilled in the art that the present invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, It is provided to let you know.

The present invention relates to a steel comprising: C: not more than 0.02 wt% (excluding 0 wt%), N: not more than 0.02 wt% (excluding 0 wt%), Si: 0.5 to 1.5 wt% 0.1 to 0.3 wt% of Ti, 0.1 to 0.3 wt% of other elements of Fe and other inevitable impurities, and optionally 0.0005 to 0.002 wt% of Ca and 0.0002 to 0.001 wt% of Zr, The river is targeted.

The content of C and N is preferably 0.02 wt% or less (excluding 0 wt%). C and N are present in the steel as Ti and Nb (C, N) carbonitride-forming elements and intrusion-type, and solid C, N, When N is used for a long time at 600 ° C or lower after welding, the elongation and low-temperature impact characteristics of the material deteriorate. Since Cr ₂₃ C ₆ carbide is generated and grain boundary corrosion occurs, the content of C and N is limited to 0.02% or less . In particular, when the content of Ti is high in the state where the C + N content is high, an increase in rigid inclusions causes a lot of surface defects such as scabs, nozzle clogging during performance, The elongation and impact properties are deteriorated, so that the C + N content is limited to 0.03% or less. In C + N, C and N mean the content (wt%) of each component.

The content of Si is preferably 0.5 to 1.5 wt%. Si is an indispensable element for improving the high-temperature oxidation property and the corrosion resistance of the weld portion and preventing deterioration, which is a problem in the present invention. At least 0.5 wt% And the content thereof is preferably limited to 0.5 to 1.5 wt% by limiting the production conditions.

The content of Mn is preferably 1.0 wt% or less (excluding 0 wt%). When Mn content is high, precipitates such as MnS are formed to reduce the pitting resistance. However, the addition of Mn in an appropriate amount increases the adhesion of oxide scale such as Cr and Fe oxide formed at high temperature, It is preferable to limit the content to 1 wt% or less in consideration of the adhesiveness of the pitting corrosion resistance and the oxidation scale.

The Cr content is preferably 10.0 to 13 wt%. When Cr is less than 10 wt%, the corrosion resistance, which is the basic property of stainless steel, is insufficient, so the Cr content is set to 10 wt% or more. If the Cr content is high, the elongation, the impact properties, and the toughness of the welded portion are weakened. Therefore, the content of Cr is preferably limited to 10.0 to 13 wt%.

The content of Ti is preferably 0.1 to 0.3 wt%. If the added amount of Ti is too large, the inclusion of rigid inclusions increases and surface defects such as scabs are generated a lot, and nozzle clogging phenomenon occurs at the time of playing. If the added amount of Ti is too large, the elongation and the low-temperature impact properties are deteriorated by the increase of the solid solution Ti content.

(Ti) / (C + N) ratio is too low as the amount of Ti added to the C + N content is too low, C + N) of 8 or more. In Ti / (C + N), Ti, C and N mean the content (wt%) of each component.

The value of Si + 2Ti is preferably kept at 0.8 or more.

The content of Ca is preferably 0.0005 to 0.002 wt%, if necessary. Ca is an essential element for improving the weldability. In order to improve the weldability, it is necessary to add 0.0005% or more. However, when it is added in an amount exceeding 0.002%, the size of the oxidized inclusions increases and the corrosion resistance is adversely affected, so the upper limit is 0.002%.

The content of Zr is preferably 0.0002 to 0.01 wt%, if necessary. Zr is an essential element for improving the weldability like Ca. Zr also serves to minimize the size of the oxide particles and to improve the corrosion resistance of the steel. In order to improve the weldability and the corrosion resistance, an addition of 0.0002% or more is required. However, when 0.01% or more is added, the upper limit is 0.01% due to the increase of the cost due to the increase of the addition amount and the clogging of the nozzle during the steel making process

Meanwhile, the present invention can improve the high-temperature oxidation characteristic and the intergranular corrosion property of the weld portion by replacing Ti alone and Ti + Nb complex additive used as a stabilizing element in a ferritic stainless steel for automobile exhaust system, It is preferable to maintain the value of Si + 2Ti at 0.8 or more so that the content of Si is kept higher than that of Ti. In Si + 2Ti, Si and Ti mean the content (wt%) of each component.

In order to produce stainless steel, the steel of the above-mentioned composition is produced by a conventional method to produce a slab, followed by hot rolling, hot rolling, hot rolling, cold rolling and cold rolling.

The manufacturing conditions of each step will be described below.

First, the higher the slab heating temperature before the hot rolling, the more favorable for recrystallization during hot rolling. However, if the heating temperature is too high, the surface heating is limited to 1180 ° C.

Since the deformation accumulation energy generated during hot rough rolling is high, the lower the temperature during the hot rolling, the more favorable the recrystallization during annealing. Therefore, the lower the temperature during the hot rolling, the better the elongation. However, if the finish rolling temperature is too low, the hot rolling temperature is limited to 1020 ° C, and the hot finish rolling temperature range is limited to 740 to 880 ° C, because sticking defects generated by the rolling rolls and the material are generated.

If the cold rolling reduction rate of the material during cold rolling is too low, it is difficult to remove surface defects and secure the surface characteristics. If the cold rolling reduction rate is high, the Lankford value (r-bar) The cold reduction ratio is preferably limited to 70 to 80%.

Further, since the crystal grain size number in the annealed sheet after cold rolling annealing is the most excellent in the range of 6.0 to 8.0, it is preferable that the grain size is limited within this range.

[Example]

The following examples illustrate the present invention.

First, an experiment was conducted to investigate the effect of improving the grain boundary corrosion characteristics with increasing Si content.

FIG. 1 is a graph (TTS) showing the degree of grain boundary corrosion after heat treatment of ferritic stainless steels according to Examples and Comparative Examples according to the present invention in a temperature range of 400 to 800 ° C.

Muffler material for automobile exhaust system according to the invention material and comparative material was produced with the composition as shown in the following [Table 1]. In particular, the inventive material (a) has an Si content of 0.5% or more and the comparative material (b) has a Si content of 0.5% or less. The inventive material having the composition as shown in the following [Table 1] and the comparative material were subjected to homogenization heat treatment at 1,200 ° C. for 1 h and then subjected to heat treatment at 400 to 800 ° C. for 0.1 h, 0.3 h, 3 h, 30 h, The degree of grain boundary corrosion was tested by the Modified Strauss test method. At this time, 6% CuSO ₄ + 0.5% H ₂ SO ₄ aqueous solution was used as the test solution, Cu balls were immersed in the lower part of the aqueous solution, then immersed in a boiling water solution for 24 hours and then weight loss and corrosion depth were measured. And the results are shown in Fig.

division
Content (wt%) C N Mn Si P S Ti TI (C + N) Inventive material (a) 0.0053 0.0059 0.186 0.569 0.0201 0.0021 0.209 18.6 The comparative material (b) 0.0042 0.0045 0.291 0.413 0.0231 0.0009 0.195 22.4

As can be seen from Fig. 1, it can be seen that sensitization of the grain boundary occurs at 400 to 600 占 폚, which is the temperature range for use in the automobile exhaust system, both of the inventive material and the comparative material. However, in the case of the inventive material (a), it can be seen that the degree of grain boundary corrosion is weaker than that of the comparative material (b). In the case of the comparative material (b), the ratio of Ti / (C + N) should have an excellent resistance to intergranular corrosion of 8 or more, but the intergranular corrosion degree is lower than that of the inventive material (a) You can see something serious.

From the above results, it was confirmed that the Si component is a useful component for preventing intergranular corrosion. From these results, it can be confirmed that it is possible to reduce the amount of Ti added to prevent intergranular corrosion.

Next, experiments were conducted on the formal dislocation, elongation, grain size, intergranular corrosion and high-temperature oxidation characteristics according to the respective component contents of the ferritic stainless steels.

Ferritic stainless steels as shown in Table 2 below were dissolved in a 50 Kg vacuum dissolving facility to produce 120 mm thick ingots. Nos. 1 to 5 are inventive materials according to the present invention. 6 to 10 indicate the components of the comparative material. Nos. 1 to 3 are steels in which the content of Si is increased to not less than 0.6%. 4 is a steel to which Ca is added; 5 is a steel to which Zr is added. And No. 10 is a component of STS 409 steel which is used as muffler material for general automobile exhaust system.

No C N Si Mn P S Cr Ni Ti Zr Ca (C + N) Ti /
(C + N) One
dog
foot
River 0.008 0.006 0.65 0.31 0.01 0.002 11.3 0.09 0.15 0.014 10.7 2 0.009 0.009 0.75 0.31 0.01 0.002 11.3 0.09 0.2 0.018 11.1 3 0.006 0.008 0.85 0.3 0.01 0.002 11.3 0.09 0.13 0.014 9.3 4 0.01 0.007 0.65 0.31 0.01 0.002 11.3 0.09 0.15 0.0009 0.017 8.8 5 0.004 0.0067 0.518 0.326 0.01 <0.002 11.25 0.093 0.21 0.0018 0.011 19.6 6
ratio
School
River 0.008 0.01 0.25 0.303 0.01 <0.002 11.31 0.092 0.198 0.018 11.0 7 0.009 0.0088 0.45 0.295 0.01 <0.002 11.32 0.09 0.214 0.018 12.0 8 0.008 0.006 0.47 0.3 0.01 0.002 11.3 0.09 0.19 0.014 13.5 9 0.01 0.008 0.34 0.3 0.01 0.002 11.3 0.09 0.35 0.018 19.4 10 0.006 0.006 0.45 0.3 0.01 0.002 11.3 0.09 0.19 0.012 15.8

The thus-produced ingot was heated at 1250 占 폚 and hot-rolled at a finish rolling temperature of 800 占 폚 to produce a hot-rolled sheet having a thickness of 5.0 mm. Hot rolled sheet was hot-rolled at 980 占 폚, pickled, and cold rolled to a thickness of 1.5 mm. Annealing temperature after changing the cold annealing temperature up to &lt; RTI ID = 0.0 &gt; 20 C &lt; / RTI &gt;

The cold rolled steel sheet was subjected to tensile test, intergranular corrosion test, formal dislocation, and grain size.

As a method of welding, TIG welding and GMA welding which can simulate the welding method of automobile muffler were performed in parallel.

GTA (TIG) welding was carried out with DC type welding machine (maximum welding current 350A) and bead on plate. Welding conditions were welding current 110A, welding speed 0.32m / min, tungsten electrode diameter 2.5mm, electrode tip angle 100o, arc length 1.5mm and protective gas Ar (15l / min).

GMA welding was carried out using an AC pulse welder in a shielding gas atmosphere containing a welding current of 71 A and a welding voltage of 16.7 V at a welding rate of 0.72 m / min and a protective gas of 2% O ₂ in an Ar atmosphere, Was welded using Y ₃ 0 ₈ wire (1.2 mmΦ).

The grain size was measured using an optical microscope. GTA (TIG) welding and GMA welding specimens were annealed at 500 ℃ for 10 hours in air to simulate the temperature atmosphere of automotive exhaust system. Thereafter, the test solution was immersed in an aqueous solution of boiling water for 24 hours in a solution of 6% Cu 2 _{S 4} O ₄ + 0.5% H ₂ SO ₄ solution, followed by observation of the cross-sectional structure of the test piece and U- To evaluate intergranular corrosion, and the results are shown in Table 3 below.

No
Official potential
(mV) Elongation
(%) Crystal grain size
Presence or absence of intergranular corrosion
(?, X) High temperature oxidation characteristic
(?,?) One dog
foot
River 70 35 7.2 ◎ ◎ 2 75 34 7.2 ◎ ◎ 3 84 33 7.5 ◎ ◎ 4 55 35 7.2 ◎ ○ 5 50 36 7.3 ◎ ○ 6 ratio
School
River 43 35 7.4 X ○ 7 51 36 6.8 X ○ 8 48 35 7.1 X ○ 9 48 34 7.4 ◎ △ 10 55 36 7.3 X ○

Here, "⊚" or "⊚" indicates the occurrence of intergranular corrosion, "X" means occurrence, "⊚" of high temperature oxidation characteristic is superior to STS 409L, "○" is equivalent to STS 409L, "△" refers to the thermal fatigue level compared to STS 409L.

As can be seen from Table 3, when the ratio of Si + 2Ti is 0.8 or more and the ratio of Ti / (C + N) is 8 or more while maintaining the Si content at 0.5 to 1.5 wt% It was confirmed that the formaldehyde value was 50 mV or more and the elongation was 33% or more.

In addition, it was confirmed that when the above-mentioned conditions of the present invention are satisfied, the high-temperature oxidation characteristics are equivalent to or superior to STS 409L, without causing grain boundary corrosion.

Although the present invention has been described with reference to the accompanying drawings and the preferred embodiments described above, the present invention is not limited thereto but is limited by the following claims. Accordingly, those skilled in the art will appreciate that various modifications and changes may be made thereto without departing from the spirit of the following claims.

Claims (6)

C: not more than 0.02 wt%, N: not more than 0.02 wt%, Si: 0.5 to 1.5 wt%, Mn: not more than 1.0 wt%, Cr: 10.0 to 13 wt%, Ti: 0.1 to 0.3 wt% Lt; / RTI &gt;
The C + N value is 0.03 or less, the Si + 2Ti value is 0.8 or more, and the ratio of Ti / (C + N) is 8 or more.
C, N, Si and Ti in the above C + N, Si + 2Ti and Ti / (C + N) mean the content (wt%) of each component.
The method according to claim 1,
Wherein the stainless steel contains 0.0005 to 0.002 wt% of Ca and 0.0002 to 0.01 wt% of Zr.
The method according to claim 1 or 2,
Wherein the stainless steel has an average potential value of 50 mV or more.
The method according to claim 1 or 2,
Wherein the stainless steel has an elongation of 33% or more.
C: not more than 0.02 wt%, N: not more than 0.02 wt%, Si: 0.5 to 1.5 wt%, Mn: not more than 1.0 wt%, Cr: 10.0 to 13 wt%, Ti: 0.1 to 0.3 wt% A slab having a Si + 2Ti value of 0.8 or more and a ratio of Ti / (C + N) of 8 or more is prepared,
The slab is subjected to hot rolling, hot rolling, annealing and cold rolling, and the cold rolling is performed at a rolling reduction of 70 to 80%.
Si, Ti, C and N in the above Si + 2 Ti and Ti / (C + N) mean the content (wt%) of each component.
The method of claim 5,
Wherein the hot rolling is performed in a temperature range of 740 to 880 占 폚.

Images