阅读说明：本技术 焊接部的低温韧性优异的添加ti和nb的铁素体不锈钢 (Ti-and nb-added ferritic stainless steel excellent in low-temperature toughness in weld parts ) 是由 金锺哲 金完伊 郑壹酂 金镇锡 安德灿 于 2018-11-02 设计创作，主要内容包括：根据本发明的一个实施方案的焊接部的低温韧性优异的添加Ti和NB的铁素体不锈钢以重量计包含：0.004％至0.015％的C、0.004％至0.015％的N、0.01％至0.7％的Si、0.01％至0.7％的Mn、0.0001％至0.04％的P、0.0001％至0.005％的S、10％至30％的Cr、0.005％至0.04％的Al、0.1％至0.5％的Ti、0.1％至0.6％的Nb、0.0001％至0.003％的Ca,以及余量的铁(Fe)和其他不可避免的杂质,铁素体不锈钢满足下式(1),其中基于Al-Ca-Ti-Mg-O的氧化物和包含该氧化物的Ti-Nb-C-N碳氮化物具有3μm至10μm的平均直径为和4/mm～2或更大的分布密度。(1){(Ti+0.5*Nb)*(C+N)}/Al＞0.25。(The Ti and NB added ferritic stainless steel excellent in low temperature toughness of a welded part according to one embodiment of the present invention includes by weight: 0.004-0.015% of C, 0.004-0.015% of N and 0.01-0.7%0.01 to 0.7% of Mn, 0.0001 to 0.04% of P, 0.0001 to 0.005% of S, 10 to 30% of Cr, 0.005 to 0.04% of Al, 0.1 to 0.5% of Ti, 0.1 to 0.6% of Nb, 0.0001 to 0.003% of Ca, and the balance of iron (Fe) and other unavoidable impurities, and a ferritic stainless steel satisfying the following formula (1) in which an oxide based on Al-Ca-Ti-Mg-O and a Ti-Nb-C-N carbonitride comprising the oxide have an average diameter of 3 to 10 μm and 4/mm 2 Or greater distribution density. (1) { (Ti + 0.5. multidot. Nb) (C + N) }/Al > 0.25.)

1. A Ti, Nb-added ferritic stainless steel having excellent low-temperature toughness of a weld zone, comprising in weight percent (%) of the total composition: c: 0.004% to 0.015%, N: 0.004% to 0.015%, Si: 0.01 to 0.7%, Mn: 0.01% to 0.7%, P: 0.0001% to 0.04%, S: 0.0001 to 0.005%, Cr: 10 to 30%, Al: 0.005% to 0.04%, Ti: 0.1 to 0.5%, Nb: 0.1% to 0.6%, Ca: 0.0001% to 0.003%, iron (Fe) and other inevitable impurities in the remainder, and

satisfying the following formula (1),

oxides based on Al-Ca-Ti-Mg-O and Ti-Nb-C-N carbonitrides comprising said oxides having a mean diameter of between 3 μm and 10 μm and 4/mm²Or a greater density of distribution of the particles,

(1) {(Ti+0.5*Nb)*(C+N)}/A1＞0.25

here, Ti, Nb, C, N and Al mean contents (wt%) of the respective elements.

2. The ferritic stainless steel of claim 1, wherein the distribution density is 4/mm²Above 15/mm²The following.

3. The ferritic stainless steel of claim 1, further comprising any one or more selected from the group consisting of: mo: 0.1 to 2.0%, Ni: 0.1% to 2.0% and Cu: 0.1% to 2.0%.

4. The ferritic stainless steel according to claim 1, wherein the Al-Ca-Ti-Mg-O based oxide satisfies the following formulas (2) to (4),

(2) ％(TiO₂)+％(CaO)+％(Al₂O₃)≥80

(3) {％(TiO₂)+％(CaO)}/{％(TiO₂)+％(CaO)+％(Al₂O₃)}≥0.3

(4) 0.3≤％(CaO)/％(TiO₂)≤0.8。

5. the ferritic stainless steel of claim 1 or claim 4, wherein the Ti-Nb-C-N carbonitride has the Al-Ca-Ti-Mg-O based oxide as a core and is formed to surround the Al-Ca-Ti-Mg-O based oxide.

6. The ferritic stainless steel of claim 1, wherein an average grain size of a solidification structure of the weld zone is less than 150 μ ι η.

7. The ferritic stainless steel of claim 1, wherein the impact energy of the weld zone is 90J/cm at-30 ℃²Or larger.

8. The ferritic stainless steel of claim 1, wherein the DBTT of the weld zone is-25 ℃ or less.

Technical Field

The present disclosure relates to ferritic stainless steels, and more particularly to Ti, Nb-added ferritic stainless steels having excellent weld zone low temperature toughness.

Background

The primary use of ferritic stainless steels is for automotive exhaust system components. Mainly, the final product is manufactured by forming through press working and welding these worked products, or by expanding and shaping a welded pipe. Therefore, as an important requirement of ferritic stainless steel for automobile exhaust systems, the processing characteristics of the welded zone are mentioned.

The welding process of ferritic stainless steel generally melts a base metal using arc heat and rapidly cools the molten metal to form a solidification structure, and the grain size and shape of the solidification structure have an important influence on the workability of a welded zone.

In particular, the welding method for automobile exhaust systems has a large heat input and a wide range, which increases the possibility of cracking during subsequent machining due to grain coarsening in the weld zone. Further, coarsening of weld zone grains has a characteristic of impairing low-temperature toughness characteristics, and in particular, there is a problem in that the incidence of weld zone cracks rapidly increases during winter processing of products.

Therefore, it was found that it is necessary to refine the solidification structure of the melted portion so as to satisfy the weld zone characteristics of the automobile exhaust equipment component.

As solidification structure refinement techniques, a low-temperature casting method and electromagnetic stirring are used, and these techniques can refine the solidification structure of the base material, but have no effect on refinement of the solidification structure of the molten portion during welding.

In particular, the solidification condition of the welded zone has a feature that the solidification structure becomes coarsened because the cooling rate is faster than that of the normal solidification condition, and thus it is advantageous to grow into columnar crystals. Therefore, the solidification structure of the welded zone can be refined by promoting non-uniform nucleation. When the re-dissolved melted portion re-solidifies during welding, non-uniform nucleation occurs due to the remaining oxide, which promotes the nucleation and growth of equiaxed crystals and is expected to refine the solidification structure.

As an example of the non-uniform nucleation using oxides of ferritic stainless steel, prior document 1 discloses a technique of refining the texture of a base material using Al — Mg-based inclusions. Prior document 2 discloses a technique of manufacturing stainless steel mainly using a composite oxide containing Ti and Ca. Further, prior document 3 discloses that MgO and MgO-Al can be produced₂O₃To ensure the base material organization.

However, the above prior documents 1 to 3 focus on refinement of the solidification structure of the base material without considering the composition of the oxide or the number of sizes of the oxide used for the solidification structure of the weld zone. In particular, in the case of a welded zone, unlike a general cast structure, the melting temperature is high so that the effect may be lost due to re-dissolution of oxides, and the cooling rate is fast so that size control of oxides for refinement is required. Therefore, in the case of the prior art document, it cannot be said that it is a preferable method for refining the solidification structure of the weld zone.

(Prior Art document 1) Korean patent application laid-open No. 10-2011-

(Prior art document 2) Japanese patent application laid-open No. 2000-001715 (published in 1/7/2000)

(Prior Art document 3) Japanese patent application laid-open No. 2001-254153 (published in 2001, 9/18)

Disclosure of Invention

Technical problem

Embodiments of the present disclosure are directed to providing a ferritic stainless steel capable of improving low-temperature toughness of a welded zone through refinement of a base material structure of the stainless steel and a solidification structure of the welded zone.

Technical scheme

According to one aspect of the present disclosure, a Ti, Nb-added ferritic stainless steel having excellent weld zone low-temperature toughness includes, in weight percent (%) of the entire composition: c: 0.004% to 0.015%, N: 0.004% to 0.015%, Si: 0.01 to 0.7%, Mn: 0.01% to 0.7%, P: 0.0001% to 0.04%, S: 0.0001 to 0.005%, Cr: 10 to 30%, Al: 0.005% to 0.04%, Ti: 0.1 to 0.5%, Nb: 0.1% to 0.6%, Ca: 0.0001 to 0.003%, the remainder being iron (Fe) and other unavoidable impurities, satisfying the following formula (1), and the oxides based on Al-Ca-Ti-Mg-O and Ti-Nb-C-N carbonitrides comprising the same having an average diameter of 3 to 10 μm and 4/mm²Or greater distribution density.

(1){(Ti+0.5*Nb)*(C+N)}/Al＞0.25

Here, Ti, Nb, C, N and Al mean contents (wt%) of the respective elements.

The distribution density can be 4/mm²Or greater and 15/mm²Or smaller.

The ferritic stainless steel may further comprise any one or more selected from the group consisting of: mo: 0.1 to 2.0%, Ni: 0.1% to 2.0% and Cu: 0.1% to 2.0%.

The oxides based on Al-Ca-Ti-Mg-O may satisfy the following formulas (2) to (4).

(2)％(TiO₂)+％(CaO)+％(Al₂O₃)≥80

(3){％(TiO₂)+％(CaO)}/{％(TiO₂)+％(CaO)+％(Al₂O₃)}≥0.3

(4)0.3≤％(CaO)/％(TiO₂)≤0.8

The Ti-Nb-C-N carbonitride may have an Al-Ca-Ti-Mg-O based oxide as a core and may be formed to surround the Al-Ca-Ti-Mg-O based oxide.

The average grain size of the solidification structure of the welded zone may be less than 150 μm.

The impact energy of the weld zone may be 90J/cm at-30 DEG C²Or larger.

The DBTT of the weld zone may be-25 ℃ or less.

Advantageous effects

The examples of the present disclosure can control the size and distribution density of effective nucleation products in the base metal of the stainless steel by controlling the composition of the ferritic stainless steel to which Ti, Nb are added, and thus, the solidification structure of the weld zone can be refined and the low temperature toughness of the weld zone can be improved.

Drawings

Fig. 1 is a photograph showing a solidification structure of a Ti, Nb-added ferritic stainless steel weld zone according to one embodiment of the present disclosure.

Fig. 2 is a photograph showing a solidification structure of a welded zone of Ti, Nb-added ferritic stainless steel according to a comparative example.

Fig. 3 is a graph illustrating the analysis result of the nucleation inclusions in the center of the grains of the weld zone solidification structure of the Ti, Nb-added ferritic stainless steel according to one embodiment of the present disclosure.

Fig. 4 is a graph showing the distribution of the number of effective nucleation products according to size according to the embodiments and comparative examples of the present disclosure.

Fig. 5 is a graph showing the number of effective nucleation products having a size of 3 μm to 10 μm per unit area in examples and comparative examples according to the present disclosure.

Fig. 6 is a graph illustrating the results of measuring the average grain size of the welded zone solidification structure according to the embodiment and the comparative example of the present disclosure.

Fig. 7 is a graph illustrating a result of measuring impact energy of a welded zone according to an embodiment and a comparative example of the present disclosure.

Fig. 8 is a graph showing the results of measuring the land DBTT according to the embodiment and the comparative example of the present disclosure.

Fig. 9 is a graph showing the correlation between the value of equation (1) and the average grain size of the welded area according to the embodiment and the comparative example of the present disclosure.

Detailed Description

The Ti, Nb-added ferritic stainless steel having excellent weld zone low-temperature toughness according to one embodiment of the present disclosure includes, in weight percent (%) of the entire composition: c: 0.004% to 0.015%, N: 0.004% to 0.015%, Si: 0.01 to 0.7%, Mn: 0.01% to 0.7%, P: 0.0001% to 0.04%, S: 0.0001 to 0.005%, Cr: 10 to 30%, Al: 0.005% to 0.04%, Ti: 0.1 to 0.5%, Nb: 0.1% to 0.6%, Ca: 0.0001 to 0.003%, the remainder being iron (Fe) and other unavoidable impurities, satisfying the following formula (1), and the oxides based on Al-Ca-Ti-Mg-O and Ti-Nb-C-N carbonitrides comprising the same having an average diameter of 3 to 10 μm and 4/mm²Or greater distribution density.

(1)((Ti+0.5*Nb)*(C+N)}/Al＞0.25

Here, Ti, Nb, C, N and Al mean contents (wt%) of the respective elements.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. The following embodiments are provided to convey the technical concept of the present disclosure to those of ordinary skill in the art. However, the present disclosure is not limited to these embodiments, and may be presented in another form. In the drawings, portions irrelevant to the description may not be shown in order to clarify the present disclosure, and the sizes of components are more or less exaggeratedly shown in order to facilitate understanding.

The inventors of the present disclosure had to control the size and number of oxide-containing Ti-Nb-C-N carbonitrides that promote δ -ferrite nucleation to serve as effective nucleation products. Thus, efficient nucleation product formation conditions can be obtained.

In the present disclosure, effective nucleation products mean Al-Ca-Ti-Mg-O based oxides and Ti-Nb-C-N carbonitrides comprising the same.

The Ti, Nb-added ferritic stainless steel having excellent weld zone low-temperature toughness according to one embodiment of the present disclosure includes, in weight percent (%) of the entire composition: c: 0.004% to 0.015%, N: 0.004% to 0.015%, Si: 0.01 to 0.7%, Mn: 0.01% to 0.7%, P: 0.0001% to 0.04%, S: 0.0001 to 0.005%, Cr: 10 to 30%, Al: 0.005% to 0.04%, Ti: 0.1 to 0.5%, Nb: 0.1% to 0.6%, Ca: 0.0001% to 0.003%, iron (Fe) and other inevitable impurities in the remaining part.

Hereinafter, the reason for the numerical limitation on the contents of the alloy component elements in the embodiments of the present disclosure will be described. Hereinafter, unless otherwise specified, the unit is weight%.

The contents of C and N are respectively 0.004% to 0.015%.

It was confirmed that the effects of C and N in the Ti-Nb composite material-added steel had an important influence on the refinement of the weld zone structure. That is, after the oxide is formed, Ti — Nb composite carbonitride is formed, and δ ferrite nucleation is generated based on the carbonitride. Here, the contents of C and N are related to the crystallization temperature of Ti-Nb-C-N carbonitride, and must have a minimum value to have an effective influence on the nucleation of δ ferrite. Therefore, each content is limited to at least 0.004% or more, and the influence of the content value will be described in detail later when the Ti and Nb contents are set. Further, in the case of C and N, both elements are interstitial elements (interstitial elements), and when the addition amount is increased, workability during molding is decreased due to a lower elongation, and corrosion resistance is lower due to formation of grain boundary Cr carbonitride, thus limiting each maximum value to 0.015%.

The contents of Si and Mn are 0.01 to 0.7%, respectively.

For Si and Mn, corrosion resistance and formability must be considered at the same time, and it is limited to 0.01% or more in terms of corrosion resistance and 0.7% or less in terms of workability.

Si is an element added in terms of corrosion resistance, and if it is less than 0.01%, it is difficult to obtain sufficient corrosion resistance. When Si exceeds 0.7%, impurities in the material increase, elongation and work hardening index (n value) decrease, and silicon-based inclusions increase, resulting in poor workability. Therefore, the content range thereof is preferably 0.01% to 0.7%.

Mn is an element added in terms of corrosion resistance, and if it is less than 0.01%, it is difficult to obtain sufficient corrosion resistance, but if it exceeds 0.7%, there is a problem in that elongation and corrosion resistance are reduced due to increased impurities in the material. Therefore, the content range thereof is preferably 0.01% to 0.7%.

It is preferable that both P and S are low, but in view of manufacturing cost, P is limited to 0.0001% to 0.04% and S is limited to 0.0001% to 0.005%.

The content of P is preferably low in terms of corrosion resistance. Preferably, the lower limit of the content is 0.0001% in consideration of the cost in the steel-making process. Therefore, the content range thereof is preferably 0.0001% to 0.04%.

The content of S is preferably low in terms of corrosion resistance. Preferably, the lower limit of the content is 0.0001% in consideration of the cost in the steel-making process. Therefore, the preferable content range is 0.0001% to 0.005%.

The content of Cr is 10 to 30%.

When the content of Cr is less than 10%, corrosion resistance as stainless steel is insufficient, and when the content of Cr is more than 30%, formability is reduced, and the content range thereof is preferably 10% to 30%.

The Al content is 0.005% to 0.04%.

In the case of Al, it is absolutely necessary as a deoxidizing element, but when added in a large amount, it is difficult to improve low-temperature toughness because coarsening of weld grains cannot be suppressed due to the formation of ineffective oxides. Therefore, while at least 0.005% is included in consideration of the deoxidation effect, the maximum value is limited to 0.04% for grain refinement of the weld zone.

The content of Ti is 0.1 to 0.5 percent.

Ti is the most important element in determining the effective nucleation products of the present disclosure, and through a series of experiments, the lower limit of Ti is limited to 0.1% to satisfy the composition, size and distribution of the effective nucleation products proposed in the present disclosure. Further, when added in a large amount exceeding 0.5%, linear defects due to inclusions in the final product due to high melting point nitrides (e.g., TiN) frequently occur, so the upper limit is limited to 0.5%.

The content of Nb is 0.1 to 0.6%.

Nb is an essential element for ensuring the high-temperature strength of high-temperature exhaust system components and at the same time has an effect on the formation of effective nucleation products. In particular, in order to ensure the characteristics as a component of a high-temperature exhaust system of 660 ℃ or more, it must contain at least 0.1%, whereas if it is added in excess of 0.6%, the cost of raw materials is higher than the increase in high-temperature strength, so the upper limit is limited to 0.6%.

The content of Ca is 0.0001-0.003%.

In the case of Ca, as the deoxidizing element, it is an important element in forming the effective oxide in the present disclosure. However, when contained in a large amount, the formation of an effective oxide is suppressed and the corrosion resistance is also adversely affected, so the maximum value is limited to 0.003%, and the minimum value is 0.0001% which is the minimum value for the formation of an effective oxide.

Further, the Ti, Nb-added ferritic stainless steel having excellent weld zone low temperature toughness according to one embodiment of the present disclosure may further include any one or more selected from the following in weight percent (%): mo: 0.1 to 2.0%, Ni: 0.1% to 2.0% and Cu: 0.1% to 2.0%.

The amount of Mo is 0.1 to 2.0%. Mo may be additionally added as a composition to improve corrosion resistance of the stainless steel, and if excessively added, impact characteristics are deteriorated, thereby increasing the risk of cracking during machining and increasing the cost of materials. Therefore, in view of this point in the present disclosure, it is preferable to limit the content of Mo to 0.1% to 2.0%.

The amount of Ni is 0.1% to 2.0%. Ni is an element that improves corrosion resistance, and if added in a large amount, not only hardens, but stress corrosion cracking may also occur, so it is preferably 2.0% or less.

The amount of Cu is 0.1% to 2.0%. Preferably, Cu is included in an amount of 0.1% to 1.0% to improve corrosion resistance. However, when it exceeds 1.0%, there is a problem that workability is deteriorated.

In addition to the aforementioned alloying elements, the remainder of the ferritic stainless steel consists of iron and other unavoidable impurities.

In the case of a high Cr ferritic stainless steel to which a Ti — Nb composite material is added, impact energy according to temperature change does not rapidly change. Therefore, the impact energy value (90J/cm) reduced by 50% compared with the impact energy at room temperature will be exhibited²) Is defined as Ductile Brittle Transition Temperature (DBTT) and is shown in Table 8. Based on the DBTT temperature, the fracture behavior changes from ductile fracture to brittle fracture, which is the main cause of cracking during weld zone processing under low temperature conditions. Therefore, the DBTT is expected to be low.

Based on the apparent difference in the weld zone solidification structure and the change in the DBTT value even under the same steel grade and similar composition conditions, the solidification structure refinement mechanism was determined, and based on this, the present disclosure that can improve the low-temperature toughness of the weld zone was proposed.

In addition to the limitation of the range of the components of the molten steel described above, as a result of studying the mutual influence of the composition of the molten steel and the refinement of the solidification structure of the weld zone, the following formula (1) can be obtained.

(1)((Ti+0.5*Nb)*(C+N)}/Al＞0.25

When the calculated value of ((Ti +0.5 × Nb) ((C + N) }/Al exceeds 0.25) in the case of the content range of Ti, Nb, C, N, and Al of the above composition, an oxide based on Al-Ca-Ti-Mg-O and a Ti-Nb-C-N carbonitride containing the same are easily formed, the weld zone solidification structure is refined by forming such an effective nucleation product, and excellent DBTT characteristics can be obtained. And the DBTT value is also increased to-20 ℃ or more, which deteriorates low-temperature processing characteristics.

Fig. 1 is a photograph showing a solidification structure of a Ti, Nb-added ferritic stainless steel weld zone according to one embodiment of the present disclosure. Fig. 2 is a photograph showing a solidification structure of a welded zone of Ti, Nb-added ferritic stainless steel according to a comparative example.

When comparing the solidification structures of the welded zones of the embodiment of fig. 1 and the comparative example of fig. 2, it can be seen that most of the columnar crystals are formed and equiaxed crystals are formed in some centers in the case of the comparative example. However, in the case of the embodiment, columnar crystals exist, but it can be seen that a part of fine equiaxed crystals is widely formed.

Fig. 3 is a graph illustrating the analysis result of the nucleation inclusions in the center of the grains of the weld zone solidification structure of the Ti, Nb-added ferritic stainless steel according to one embodiment of the present disclosure.

To determine the cause of the solidification structure difference in the weld zone region for the examples and comparative examples of fig. 1 and 2, fig. 3 shows the results of a careful observation of a nucleating inclusion in the center of an equiaxed crystal with an electron microscope. In the case of examples, spherical oxides and carbonitrides of Ti-Nb-C-N surrounding them were observed, and most of the carbonitrides of Ti-Nb-C-N of 3 μm or more contained spherical oxides therein. When the spherical oxides were carefully observed by a transmission electron microscope, crystalline CaO-TiO could be seen₂Phase and Al₂O₃-MgO is present in the same way. On the other hand, in the case of comparative example, the Ti-Nb-C-N carbonitrides were small in size and small in number, and the oxide composition in the Ti-Nb-C-N carbonitrides was determined to be single Al₂O₃MgO phase, Al₂O₃-MgO and MgO composite phase, or Al₂O₃MgO and Al₂O₃And (4) compounding phases. Therefore, from the above results, the catalyst was prepared by including CaO-TiO₂The refinement of the solidification structure of the welded zone can be determined by oxides composed of a plurality of oxide crystal phases of the phases and Ti-Nb-C-N carbonitride formed by using these oxides as nuclei.

In particular, it was determined that Ti-Nb-C-N carbo-nitrides have high crystallization temperatures compared to TiN nitrides found in conventional Ti-alone added steels. That is, in the case of Ti-Nb composite steel, it was found through experiments and thermodynamic analysis that Ti-Nb-C-N carbonitride crystallizes at a higher temperature than that of steel to which Ti alone is added under the same Ti composition conditions. Therefore, Ti-Nb-C-N carbonitride is easily formed around the effective oxide formed in the present disclosure, and thus delta ferrite nucleation is easily generated below the liquidus temperature, thereby increasing the equiaxed crystal ratio of the weld zone.

As described above, in order to determine that Ti-Nb-C-N carbonitride causes a difference in the solidification structure in the weld zone region, the size and number distributions of Ti-Nb-C-N carbonitride present in the base materials of the examples and comparative examples were compared and analyzed, and are shown in fig. 4.

Fig. 4 is a graph showing the distribution of the number of effective nucleation products according to size according to the embodiments and comparative examples of the present disclosure. Fig. 5 is a graph showing the number of effective nucleation products having a size of 3 μm to 10 μm per unit area in examples and comparative examples according to the present disclosure.

Referring to FIG. 4, in the case of the comparative example, a large amount of Ti-Nb-C-N carbonitrides smaller than 3 μm were distributed, but the number of Ti-Nb-C-N carbonitrides of 3 μm or more rapidly decreased. In the case of the examples, it can be seen that a large amount of Ti-Nb-C-N carbonitride having a size of 3 μm or more is distributed. Based on these results, it was confirmed that the solidification structure of the weld zone was refined to Ti-Nb-C-N carbonitride of 3 μm or more. On the other hand, when the size of Ti-Nb-C-N carbonitride exceeds 10 μm, float separation (float separation) is easily performed on the surface of the molten portion, and thus cannot play a role of δ ferrite nucleation.

Further, as shown in FIG. 5, as a result of comparing the number per unit area, it was determined that the distribution density should be 4/mm²Or larger. However, when the number of Ti-Nb-C-N carbonitrides exceeds 15/mm²When it is formed into an aggregate, and this becomes a main factor of surface defects, it is desirable that the distribution density is 15/mm²Or smaller.

According to an embodiment of the present disclosure, as a method for refinement of a weld zone solidification structure, it should include an Al-Ca-Ti-Mg-O-based oxide that does not re-dissolve in molten steel and remains in a solid state even under high welding heat. Which provides nucleation sites for Ti-Nb-C-N carbonitride upon solidification of the molten metal in the weld zone, and therefore, the amount of equiaxed crystal formation increases.

Typically, the oxide observed under Al deoxidation conditions is Al-Ca-Ti-Mg-O. The Al-Ca-Ti-Mg-O based oxide comprises TiO₂、CaO、Al₂O₃MgO, and the like, and may be made of Al₂O₃-TiO₂CaO ternary diagram prediction for simultaneous formation of CaO-TiO as a favorable oxide for ferrite nucleation₂Phase and Al₂O₃-conditions of the MgO phase. As a result of accurate analysis and statistics of oxides present in the base materials of examples and comparative examples, the average oxide composition of the base metal having improved low-temperature toughness of the weld zone should satisfy the following formulas (2) to (4).

According to one embodiment of the present disclosure, the Al-Ca-Ti-Mg-O-based oxide may satisfy the following formulas (2) to (4).

(2)％(TiO₂)+％(CaO)+％(Al₂O₃)≥80

(3){％(TiO₂)+％(CaO)}/{％(TiO₂)+％(CaO)+％(Al₂O₃)}≥0.3

(4)0.3≤％(CaO)/％(TiO₂)≤0.8

According to the formula (2), the inclusions during the deoxidation of Al are Al-Ca-Ti-Mg-O, and% (TiO)₂) -% (CaO) and% (Al)₂O₃) Should be 80% orAnd is larger. When% (TiO)₂) -% (CaO) and% (Al)₂O₃) Is less than 80%, since it is in the form of MgO-rich oxide or Al₂O₃MgO oxides are stable and it is therefore difficult to form CaO-TiO effective for nucleation₂And (4) phase(s). Due to the high crystallization temperature, it is difficult to remain in the liquid phase because it is easily coarsened during the cooling process.

The CaO-TiO content is set according to the formula (3)₂(CaO) and (TiO) of origin₂) (TiO) of the total ratio₂) -% (CaO) and% (Al)₂O₃) In order to ensure a large amount of CaO-TiO₂The phase, which is advantageous as an equiaxed crystal nucleation site for the solidification structure of the weld zone. If the ratio is less than 0.3, CaO-TiO₂The ratio of the phases is reduced, so that sufficient refinement of the average grain diameter of the solidification structure of the welded zone becomes difficult.

According to formula (4), even if formulas (2) and (3) are satisfied, when% (CaO)/% (TiO)₂) At a ratio of less than 0.3, the oxide composition may not sufficiently ensure CaO-TiO favorable for nucleation₂And (4) phase(s). When% (CaO)/% (TiO)₂) When the ratio of (B) exceeds 0.8, the oxide composition is converted into CaO-Al₂O₃And converted to an oxide that is not effective for nucleation.

Fig. 6 is a graph illustrating the results of measuring the average grain size of the welded zone solidification structure according to the embodiment and the comparative example of the present disclosure. As a result of comparing the sizes of the equiaxed crystals of the lands of the examples and comparative examples, it can be seen that the sizes of the equiaxed crystals are finer by about 40% in the case of the examples than in the comparative examples. Specifically, the average grain diameter of the welded zone solidified structure of the ferritic stainless steel according to the example was 97.5 μm, which was 110 μm or less, but the average grain diameter of the welded zone solidified structure of the ferritic stainless steel according to the comparative example was 167.1 μm, which was 150 μm or more.

That is, the average grain size of the weld zone solidification structure of the Ti, Nb-added ferritic stainless steel having excellent weld zone low temperature toughness according to one embodiment of the present disclosure may be less than 150 μm.

Fig. 7 is a graph illustrating a result of measuring impact energy of a welded zone according to an embodiment and a comparative example of the present disclosure. Fig. 8 is a graph showing the results of measuring the land DBTT according to the embodiment and the comparative example of the present disclosure.

Referring to fig. 7 and 8, Ductile Brittle Transition Temperature (DBTT) can be obtained from the weld zone impact energy diagram of fig. 6, and it is evaluated as-35.8 ℃ in the examples and-17.8 ℃ in the comparative examples. It can be seen that the DBTT of the examples was about 20 ℃ lower than the DBTT of the comparative examples.

That is, the Ti, Nb-added ferritic stainless steel having excellent weld low-temperature toughness according to one embodiment of the present disclosure may have a weld impact energy of 90J/cm at-30 ℃²Or greater, and the weld zone DBTT may be-25 ℃ or less.

The evaluation results of the low-temperature toughness of the weld zone and the weld zone microstructure results are summarized below. When the welded zone has a fine coagulated structure, it has a low DBTT property. For this reason, refinement of the solidification structure of the weld zone confirms that the Al-Ca-Ti-Mg-O-based oxide satisfying formula (1) and satisfying all of formulae (2) to (4) and Ti-Nb-C-N carbonitride having an average diameter of 3 μm to 10 μm containing the same should have a thickness of 4/mm²Or greater distribution density.

On the other hand, oxides present in the test specimens may appear in a mixture of plural types, wherein, in the case of specimens in which the distribution of Ti-Nb-C-N carbonitrides containing oxides satisfying formulas (2) to (4) does not satisfy the above conditions, the weld zone solidification structure is also coarsened and the DBTT value is also high.

Therefore, in the case of the Ti, Nb-added ferritic stainless steel according to the present disclosure, the oxide whose composition satisfies formula (1) and satisfies all of formulas (2) to (4), and Ti-Nb-C-N carbonitride having an average diameter of 3 μm to 10 μm including the same should have 4/mm²Or greater distribution density.

Hereinafter, the present disclosure will be described in more detail by examples.

Examples 1 to 8

After manufacturing cast steel by a process of an electric furnace (EAF) -refining furnace (AOD) -composition adjustment (LT) -tundish-continuous casting process for stainless steel including the composition of base materials according to examples 1 to 8 of table 1 below, a cold rolled coil having a final thickness of 2.0mm was manufactured by hot rolling and annealing and cold rolling and annealing.

Comparative examples 1 to 7

After manufacturing cast steel by a process of an electric furnace (EAF) -refining furnace (AOD) -composition adjustment (LT) -tundish-continuous casting process for stainless steel including the compositions of base materials according to comparative examples 1 to 7 of table 1 below, a cold rolled coil having a final thickness of 2.0mm was manufactured by hot rolling and annealing and cold rolling and annealing.

< Table 1>

Thereafter, in order to evaluate the welding characteristics of the steel sheets manufactured according to the examples and comparative examples, grain size of the welded zone, section and surface analysis of the welded zone, hardness analysis, Ericsson test, and impact energy of the welded zone were studied after welding by the GTA process. The molten steel components, which are the main influencing factors, and the types and size distributions of internal oxides according thereto were studied and are shown in table 2 below.

< Table 2>

By effective nucleation product is meant an Al-Ca-Ti-Mg-O based oxide with an average diameter of 3 to 10 μm and Ti-Nb-C-N carbonitrides comprising this oxide.

Referring to tables 1 and 2, examples 1 to 6 satisfy Al-Ca-Ti-Mg-O oxide compositions of formulae (2) to (4) by satisfying the conditions of formula (1). At the same time, the distribution density of Ti-Nb-C-N carbon nitride (effective nucleation product) containing the same is also 4/mm²Or larger. Specifically, weldingThe average grain size of the solidification structure at the area is 30 to 60 μm smaller than that of comparative examples 1 to 5, and the DBTT temperature is also reduced by about 15 ℃ compared to that of comparative examples.

Further, in the case of examples 7 and 8, if the condition of formula (1) is satisfied in the same manner for the high Cr ferritic stainless steel to which Mo is added, it is determined that the average grain size is small and the DBTT temperature is also low as compared with comparative examples 6 and 7 of the same steel type.

Fig. 9 is a graph showing the correlation between the value of equation (1) and the average grain size of the welded area according to the embodiment and the comparative example of the present disclosure.

In summary, as can be seen from the examples of table 2 and fig. 9, in order to ensure the low-temperature toughness of the weld zone, even if the composition is within the range of the present disclosure, if formula (1) is not satisfied, it can be seen that the average grain size of the weld zone solidification structure is coarsened because the distribution density of 3 μm to 10 μm of the effective nucleation product cannot be ensured.

In the foregoing, exemplary embodiments of the present disclosure have been described, but the present disclosure is not limited thereto, and those of ordinary skill in the related art will not depart from the concept and scope of the following claims. It will be understood that various changes and modifications are possible.

INDUSTRIAL APPLICABILITY

The ferritic stainless steel according to the present disclosure can refine the grain size of the weld zone solidification structure, thereby ensuring excellent weld zone low temperature toughness.