KR20180098827A - Ferritic Stainless Steel for with Excellent Formability of Welded Zone - Google Patents

Abstract

The present invention provides ferritic stainless steel containing, in weight percentage: 0.01% or less of C (excluding 0%); 0.1% or less of Si (excluding 0%); 0.4% or less of Mn (excluding 0%); 0.05% or less of P; 0.02% or less of S; 16 to 18% of Cr; 0.1 to 0.3% of Ni; 0.04% or less of Al (excluding 0%); 0.35% or less of Ti (excluding 0%); 0.02% or less of N (excluding 0%); the balance consisting of Fe and other inevitable impurities. In this case, the value of C + N can be controlled to be equal to or less than 0.015% and, by extension, the value of Ti/(C + N) can be controlled to be equal to or greater than 20.4. According to the present invention, by controlling the alloy elements of the ferritic stainless steel, excellent weldability and corrosion resistance can be obtained even by using the existing TIG welding method, such that high quality products can be expected by the cheap and simple TIG welding.

Description

[0001] Ferritic stainless steel for excellent workability [0002]

TECHNICAL FIELD The present invention relates to a ferritic stainless steel having excellent weldability, and more particularly, to a ferritic stainless steel excellent in workability of a welded portion after TIG welding by controlling the amount of alloying elements.

In recent years, in order to meet the high environmental regulations of various countries such as Europe, the Americas, and Japan, materials used in automobile exhaust systems have been used in order to reduce the weight of automobile materials and to improve the fuel efficiency by raising the temperature of the exhaust gas. There is an increasing demand for high-alloy ferritic stainless steels superior in strength, heat resistance and corrosion resistance.

In particular, ferritic stainless steels based on 17 ~ 18Cr and containing Ti as a stabilizing element are available in all parts of the exhaust system because they are inexpensive and have excellent corrosion resistance.

 Since the automobile exhaust system is mostly molded in a complicated shape, the degree of processing applied to the steel plate or the steel pipe is very severe. Particularly, ferritic stainless steel welded steel pipes welded by high frequency welding, TIG welding, laser welding, etc., can be welded to welded metal or heat-affected zone (HAZ) when secondary processing such as bending or expansion is applied Welding cracks can occur, which is pointed out as an important problem that may appear in the secondary processing. This phenomenon is more conspicuous when the processing temperature is low in winter or when the processing speed is high, and the frequency and degree of occurrence may vary depending on the type of steel.

18Cr-Ti ferritic stainless steel strength among various ferritic stainless steels is different according to various welding methods. When 18Cr-Ti ferritic stainless steel is welded by high frequency welding or laser welding, the room temperature and low temperature processability of the welded steel pipe And exhibited sufficiently satisfactory characteristics. However, in TIG welded steel pipe, workability at low temperature as well as at room temperature was remarkably lowered. This is because the weld heat input of the TIG welding method is much higher than that of the high-frequency welding method or the laser welding method, so that the grain growth of the weld metal is accelerated and the width of the bead portion affected by the welding heat is about 5 times wider than the other welding method .

Therefore, when the steel tube is manufactured by the TIG welding method and then the second process is performed, the stress is concentrated on the weld metal portion, which is remarkably lower in workability than the base metal, so that cracks may occur. It can lead to defects and is very dangerous.

However, since TIG welding method is inexpensive and low in power consumption and is still used as a main welding method in China and Southeast Asia, there is a need to improve the material itself rather than improving the welding method. In particular, high alloy ferritic stainless steels containing not less than 17% by weight of Cr are more susceptible to degradation in the workability of such weld metal parts and are more difficult to secure in secondary workability than low alloy ferritic stainless steels. There are many cases and an increase in manufacturing cost can not be avoided.

Conventionally, various researches and efforts have been made to secure the processability of high-alloy ferritic stainless steel pipes. For example, in the conventional art for securing the processability of the TIG welded portion by refining the weld metal solidification crystal grains, Ti, Nb, etc. as stabilizing elements are added and oxide generating elements Mg, Ca, Zr and the like for securing the solidification nucleus nucleation site Or a technique of securing the workability of the welded part through an action of electromagnetic stirring or the like during welding is provided. Another example is a method of using TiN or an oxide as a technique of refining the solidification structure using an inclusion.

However, when the temperature of the molten metal is difficult to control, it is difficult to apply TiN to the molten metal to refine the solidification structure of the steel casting. However, since a large amount of Ti and TiN impairs the toughness of the steel, There is a problem in that it is not preferable from the viewpoint of brittle fracture of the steel.

Further, when an oxide-forming element such as Mg, Y or Ca is added, it is difficult to predict the recovery rate due to the characteristics of Mg, Y, Ca and the like, which are excellent in oxidation reactivity, There is still a problem that the surface quality and corrosion resistance may not be good.

The present invention solves the problems of the prior arts, and in order to improve the physical properties of the welded parts of high alloy ferritic stainless steels, the present invention focuses mainly on the constituent ranges of the main elements causing work hardening in the steel, The present invention provides a ferritic stainless steel which can improve the workability of the welded portion during TIG welding by improving the low temperature processability of the TIG welded portion.

The steel sheet according to the present invention is characterized by containing, by weight%, C: not more than 0.01% (excluding 0%), Si: not more than 0.1% (excluding 0%), Mn: not more than 0.4% S: not more than 0.02%, Cr: 16 to 18%, Ni: 0.1 to 0.3%, Al: not more than 0.04% (excluding 0%), Ti: not more than 0.35% 0% is excluded), the balance Fe and other unavoidable impurities. In this case, the C + N value can be controlled to 0.015% or less, and furthermore, the Ti / (C + N) value can be controlled to 20.4 or more. The ferritic stainless steel of the present invention may have an DBTT of -60 캜 or less and an impact energy value of -20 캜 of 17 J or more.

According to the present invention, by controlling the alloying element of the ferritic stainless steel, superior weldability and corrosion resistance can be obtained even when the conventional TIG welding method is used, so that a high quality product can be expected by an inexpensive and simple TIG welding.

The present inventors investigated the effect of addition of various alloying elements on the low-temperature processability of a welded portion by a statistical method (Correlation / Regression) when a high alloy ferritic stainless steel was welded by the TIG welding method. As a result, it was found that Si acts as a main factor in addition to C, N, C + N and Ti / (C + N).

Particularly, if the range of Si addition is controlled precisely, the degree of hardening of the weld portion is reduced, and the workability of the steel can be improved and the low-temperature processability of the weld portion can be remarkably improved without addition of a large amount of oxide-generating element.

Hereinafter, a component system constituting the ferritic stainless steel of the present invention will be described in more detail. However,% of the following components means% by weight.

C: not more than 0.01% (excluding 0%) C is an element that increases the strength of steel, but ductility and workability may decrease with increasing strength. In the present invention, C is not more than 0.01% , Preferably 0.005% or less.

Si: 0.1% or less (excluding 0%)

 Si is an element that acts as a deoxidizing agent in the molten steel and is an element necessary for steelmaking. It plays a role of improving arc stability during welding and has a function of controlling the amount of spatter by improving fluidity. Even if the amount of Si added is high, there is no problem in hot workability, but the hardness can be increased due to the solubility enhancement phenomenon, which is not preferable from the viewpoint of the cold workability, so the upper limit of Si is limited to 0.1%. Further, the lower limit may be limited to 0.05% in order to sufficiently guarantee the workability of the welded portion and to prevent the drop in the weldability due to the lowered fluidity of the welded portion.

 Mn: 0.4% or less (excluding 0%)

 Mn is necessary for the improvement of the strength. However, in the present invention, MnS is formed and added in an amount limited to 0.4% or less as an element capable of adversely affecting the pitting resistance

 P: not more than 0.05%

 P exhibits a function of increasing the strength, but in general, the workability is deteriorated, so that it is mostly processed as an impurity, and it is preferable to reduce it as much as possible. However, since extremely low P is not preferable in view of refining costs and productivity, it is limited to 0.05% or less, preferably 0.025% or less.

S: not more than 0.02%

S is present as an inclusion in the steel and is an impurity element which lowers the corrosion resistance. However, it is preferable to lower the S as much as possible from the viewpoint of corrosion resistance. However, since the S is extremely poor in terms of cost and time, do.

Cr: 16 to 18%

Cr is an alloying element which must be added to improve corrosion resistance in stainless steel. It is difficult to obtain sufficient corrosion resistance when the Cr content is low, so in the present invention, at least 16% is added. However, when Cr exceeding 18% is added excessively, corrosion resistance is improved but Cr is controlled at a level of 16 ~ 18% because the strength becomes too high and thus the elongation and impact characteristics may be abruptly lowered.

 Ni: 0.1 to 0.3%

Ni acts as an austenite stabilizing element. That is, it is necessary to add an appropriate amount of Ni because it controls the austenite fraction in the tissue to a certain range, and it also affects the improvement of toughness. Therefore, in the present invention, 0.1% or more is added. However, when it is over 0.3%, the steel is hardened and the workability is lowered, so that it becomes difficult to form a complicated device used in the exhaust system. Therefore, Ni is added in an amount of 0.1 to 0.3%.

Al: 0.04% or less (excluding 0%)

Al, like Si, is an element that acts as a deoxidizer in molten steel and is useful in the steelmaking process. However, when Al is added in an excessively large amount, Al may be added in an amount of 0.04% or less because surface defects of the stainless steel may be caused and the merchantability may be deteriorated.

Ti: 0.35% or less (excluding 0%)

Ti is an alloying element added for strength and corrosion resistance, and in the present invention, 0.35% or less is added. If Ti is added in an amount exceeding 0.35%, too much inclusions are liable to cause surface defects such as scabs, and nozzle clogging may occur during performance. In addition, the elongation and the low-temperature impact characteristics may be lowered by increasing the solid solution Ti. On the other hand, since the amount of Ti added to the content of C + N is so low that the Ti / (C + N) ratio is low, intergranular corrosion occurs, which is not good for corrosion resistance. Therefore, Ti content is limited to 0.35% + N) to a certain level or more.

N: 0.02% or less (excluding 0%)

N, like C, plays a role of increasing the strength of the steel, but it can lower ductility and workability and is reduced as much as possible in the present invention. Particularly, N is 0.02% or less, preferably 0.007% or less, in order to ensure sufficient weldability.

C + N: 0.015% or less

The present invention is intended to ensure workability of a welded portion in TIG welding of a ferritic stainless steel. When the total of C + N exceeds 0.015%, the strength is increased, so that sufficient workability is hardly exhibited. Therefore, the sum of C + N can be limited to 0.015% or less, preferably 0.01% or less.

Ti / (C + N) > = 20.4

In the present invention, the relationship between Ti and C + N is experimentally measured. In order to obtain sufficient corrosion resistance at the welded portion and low-temperature processability of the welded portion, Ti is used as a Ti / (C + N) > = 20.4.

Subsequently, the ductile brittle transition temperature (DBTT), the impact energy at -20 DEG C, the elongation of the base material and the workability of the welded part were measured for each steel material, and the results are shown in Table 2 below. For reference, TIG welding was performed using a DCtype welder (maximum welding current: 350 A) to test the workability of the welded part, using a bead on plate. Welding conditions were: welding current of 90 ~ 110A, welding speed of 0.3m / min, tungsten electrode diameter of 2.4mm, electrode tip angle of 60 °, arc length of 1.5mm and protective gas of Ar (15l / min).

As a result of correlation analysis, DBTT showed statistical significance and difference with the change of Si, C + N, Si and Ti / C + N as follows. In the following equation, the P value is a value indicating whether each independent variable (alloying element) is a factor (a significant factor) affecting the change of the dependent variable (DBTT). If the value is less than 0.05, the P value is statistically significant. The regression equation can be said to be closer to true if the regression equation combined with a significant independent variable explains the actual value and the larger the value representing the probability of true.

(Provided that the corrected correlation coefficient R2adj. 95.3 and P = 0.005)

(Provided that the corrected correlation coefficient (R2 adj.) 85.2 and P = 0.027)

(Provided that the corrected correlation coefficient (R2 adj.) 84 and P = 0.03)

The low temperature impact energy of DBTT and -20 ℃ showed that the lower the Si and C + N values were, the better the elongation of base metal was. Particularly, D

And F were controlled to 0.09% or less, 0.0135% or less and 20.7 or more, respectively, in addition to the addition amounts of Si, C + N and Ti / (C + N) The impact energy at -20 ℃ was more than 17.7J and the elongation rate of the base material was excellent at 33% or more.

The size of the grain of the welded part was obtained by polishing the cross section of the welded part using sandpaper and abrasive, electrolytically etching with Nital solution, and observing with an optical microscope. The hardness distribution of the welds was measured using a micro Vickers hardness tester with a load of 200 g and a holding time of 10 sec at intervals of 0.2 mm. Further, the elongation of the welded portion was measured using a conventional JIS No. 15 test piece at a rate of 10 mm / min by performing notch at a depth of 2 mm on both sides of the welded portion in order to measure the elongation of the welded portion.

Generally, the size of the grain is coarsened due to the heat input from the welding heat, thereby increasing the degree of hardening of the weld and deteriorating the workability. However, when the content of Si and C + N is controlled to be low as in the present invention, as shown in Table 3, even if the grain size of the welded portion is somewhat large, the degree of hardening of the welded portion is decreased and the elongation rate of the welded portion is increased. It can be seen that the characteristics of the welded portion can be improved.

Claims (1)

C: not more than 0.01% (excluding 0%), Si: not more than 0.1% (excluding 0%), Mn: not more than 0.4% (excluding 0%), P: not more than 0.05%, S: not more than 0.02% Ti: not more than 0.35% (excluding 0%), N: not more than 0.02% (excluding 0%), Ti: not more than 0.03% ), The balance Fe, and other unavoidable impurities.
Claim 2
The ferritic stainless steel according to claim 1, wherein C and N have a C + N value of 0.015% or less.
Claim 3
3. The ferritic stainless steel according to claim 1 or 2, wherein the Ti, C and N have a Ti / (C + N) value of 20.4 or more.