KR20190066734A - High hardness austenitic stainless steel with excellent corrosion resistance - Google Patents

Abstract

Discloses a high hardness austenitic stainless steel which realizes high hardness but does not deteriorate corrosion resistance after cold forming.
The high hardness austenitic stainless steel excellent in corrosion resistance according to one embodiment of the present invention is characterized in that it contains 0.05% or less of C, 0.5 to 2.0% of Si, 3.0% or less of Mn, 20 to 24% of Cr, (Hv) value represented by the following formula (1) satisfies the range of 250 to 410, and the Ni content is 10 to 15%, the Cu content is not more than 2.0%, the N content is 0.1 to 0.3%, and the balance of Fe and unavoidable impurities .
(1) 122 - 23939 * C + 376 * Si + 44 * Cr + 2.62 * Mn - 36 * Ni + 54 * N - 43 * Cu + 2.85 * CRR
Here, C, Si, Cr, Mn, Ni, N and Cu mean the content (weight%) of each element, and CRR means the cold rolling reduction (%).

Description

[0001] HIGH HARDNESS AUSTENITIC STAINLESS STEEL WITH EXCELLENT CORROSION RESISTANCE [0002]

The present invention relates to austenitic stainless steels excellent in hardness and corrosion resistance, and more particularly to high hardness austenitic stainless steels which do not deteriorate in corrosion resistance after cold working while realizing high hardness.

Austenitic stainless steels typified by STS304 and STS316L are steel grades that are widely used in various applications due to their excellent formability and corrosion resistance. These steels have a hardness of 150 to 160 Hv in terms of Vickers hardness when they are produced from a sheet material, and in the case of STS304, when the sheet is cold-formed, the austenite phase is transformed into martensite phase and the hardness due to the martensite phase can be expected have. However, in general, austenitic stainless steel is not a steel grade used to utilize high hardness characteristics, and it causes a deterioration in corrosion resistance due to transformation into a martensite phase.

In recent years, stainless steel has been increasingly employed in order to obtain the advantageous surface properties of stainless steel in applications such as household appliances. In order to secure the resistance against scratches occurring during use, there is a demand for high hardness austenitic stainless steels .

Patent Document 1 discloses a ferrite comprising 0.2% or less of C, 4.0% or less of Si, 5.0% or less of Mn, 4.0 to 12.0% of Ni, 12.0 to 20.0% of Cr, 0.24 to 5.0% And an inevitable impurity, and describes a high-hardness austenitic stainless steel having a Vickers hardness of 400 Hv or higher through the Md (N) formula.

However, Patent Document 1 uses a phase transformation to a martensite phase to obtain high hardness, and does not mention corrosion resistance.

Stainless steels, which are mostly used for applications requiring high hardness, have a hardness by applying a steel composed of martensite to the base structure.

US Patent Publication No. 6764555 (July 20, 2004)

Embodiments of the present invention solve the above problems and provide a high hardness austenitic stainless steel having high hardness and excellent corrosion resistance without causing deterioration in corrosion resistance even after cold working.

The high hardness austenitic stainless steel excellent in corrosion resistance according to one embodiment of the present invention is characterized in that it contains 0.05% or less of C, 0.5 to 2.0% of Si, 3.0% or less of Mn, 20 to 24% of Cr, (Hv) expressed by the following formula (1) satisfies the range of 250 to 410, and the balance of Fe and unavoidable impurities.

(1) 122 - 23939 * C + 376 * Si + 44 * Cr + 2.62 * Mn - 36 * Ni + 54 * N - 43 * Cu + 2.85 * CRR

Here, C, Si, Cr, Mn, Ni, N and Cu mean the content (weight%) of each element, and CRR means the cold rolling reduction (%).

Further, according to an embodiment of the present invention, the Md30 value represented by the following formula (2) may satisfy -150 or less.

(2) Md30 = 551 - 462 * (C + N) - 9.2 * Si - 8.1 * Mn - 13.7 * Cr - 29 * (Ni + Cu) - 18.5 * Mo + 1.42

Here, C, N, Si, Mn, Cr, Ni, Cu, and Mo mean the content (weight%) of each element.

Also, according to an embodiment of the present invention, the stainless steel may have a martensite phase fraction of less than 0.5% by volume in the microstructure.

Further, according to an embodiment of the present invention, the inner equation index (PREN) represented by the following formula (3) may satisfy the range of 23 to 28. [

(3) PREN = Cr + 3.3 * Mo + 16 * N

Here, Cr, Mo, and N mean the content (weight%) of each element.

The high corrosion resistant austenitic stainless steel according to the embodiment of the present invention can be applied to various external appliances used for various home appliances or devices.

By improving hardness of austenitic stainless steel, resistance to scratches or surface scratches during use can be improved, and the beautiful surface of the applied parts can be maintained for a long time, and excellent corrosion resistance can be maintained regardless of cold forming. You can contribute.

1 is a graph showing the relationship between the estimated hardness value and the measured hardness value according to the formula (1).
2 is a photograph showing whether or not edge cracks have occurred after the hot rolling of the inventive example and the comparative example according to the embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following embodiments are provided to fully convey the spirit of the present invention to a person having ordinary skill in the art to which the present invention belongs. The present invention is not limited to the embodiments shown herein but may be embodied in other forms. For the sake of clarity, the drawings are not drawn to scale, and the size of the elements may be slightly exaggerated to facilitate understanding.

Also, when an element is referred to as "comprising ", it means that it can include other elements as well, without departing from the other elements unless specifically stated otherwise.

The singular forms " a " include plural referents unless the context clearly dictates otherwise.

The hardness of the stainless steel depends on the constituent elements constituting the stainless steel and the processing history. The constituent elements contained in the iron (Fe) alloy affect the mechanical properties such as the strength and hardness of the steel due to the solid solution strengthening phenomenon. Particularly in the case of interstitial elements having small particle sizes such as carbon (C) and nitrogen (N) Which plays a major role in increasing the hardness of stainless steel. In addition, the processing history of stainless steel also affects the hardness, and cold forming such as cold forging, cold rolling and the like serves as a main factor for increasing hardness by forming microstructure transformation or dislocation.

In addition, stainless steel is used as it is without pretreatment such as coating or plating due to its beautiful surface characteristics. In such cases, scratches or friction scratches on the surface during use can damage the appearance of the material. In order to prevent this, it is important to improve the hardness of the material surface so as to have scratch resistance.

In order to improve the hardness of the austenitic stainless steel according to the present invention, the content of the element N is controlled, and at the same time, the content of Cr and N is controlled to secure the corrosion resistance. In the case of hardness, the component system and the cold rolling condition which can have high hardness in comparison with the ordinary resistant STS316L steel grade are derived, and in the case of the hot rolling which can be caused by deriving the component system for realizing both hardness and high corrosion resistance, The problem of lowering the workability is also considered.

The high hardness austenitic stainless steel excellent in corrosion resistance according to one embodiment of the present invention is characterized in that it contains 0.05% or less of C, 0.5 to 2.0% of Si, 3.0% or less of Mn, 20 to 24% of Cr, (Hv) value represented by the following formula (1) satisfies the range of 250 to 410, and the Ni content is 10 to 15%, the Cu content is not more than 2.0%, the N content is 0.1 to 0.3%, and the balance of Fe and unavoidable impurities .

(1) 122 - 23939 * C + 376 * Si + 44 * Cr + 2.62 * Mn - 36 * Ni + 54 * N - 43 * Cu + 2.85 * CRR

Here, CRR means a cold rolling reduction (%).

Hereinafter, the reasons for limiting the numerical values of the alloy element content in the examples of the present invention will be described. Unless otherwise stated, the unit is wt%.

The content of C is 0.05% or less.

C is a strong austenite phase stabilizing element and is an effective element for increasing the strength of a material by solid solution strengthening. However, the content of C is limited to 0.05% or less because it easily bonds with a carbide forming element such as Cr which is effective for corrosion resistance at the ferrite-austenite phase boundary at a higher content to lower the Cr content around grain boundaries to reduce the corrosion resistance. In order to minimize the risk of precipitation of carbide which may hinder corrosion resistance, it is preferable to limit the content of C to 0.03% or less.

The content of Si is 0.5 to 2.0%.

Si, which is also used as a ferrite phase stabilizing element, is effective in improving corrosion resistance, and it plays a role as a main deoxidizer, so that it is necessary to add Si at 0.5% or more. However, if it is excessive, it induces precipitation of intermetallic compounds such as σ-phase to lower mechanical properties and corrosion resistance related to impact toughness, and causes cracking in hot rolling, so it is limited to 2.0% or less.

The content of Mn is more than 0 and not more than 3.0%.

Mn is an austenite phase stabilizing element such as C and Ni, and can improve N solubility. However, since the increase of Mn content is not preferable when the corrosion resistance is required due to the involvement of MnS and the like, it is preferable to limit the Mn content to 3.0% or less in order to secure corrosion resistance.

The Cr content is 20 to 24%.

Cr is a basic element that is the most abundant among the elements for improving corrosion resistance of stainless steel. In order to exhibit corrosion resistance above STS316L in the present invention, at least 20% of Cr should be contained. However, Cr is a ferrite stabilizing element. When the Cr content is increased, the ferrite fraction is increased to lower the hot workability of the steel, which promotes the formation of a sigma phase, which causes deterioration of mechanical properties and corrosion resistance. Therefore, the Cr content is preferably limited to 24% or less.

The content of Ni is 10 to 15%.

Ni is the most powerful element among the austenite phase stabilizing elements and should be contained in an amount of 10% or more in order to maintain the austenite phase. However, the increase in Ni content is directly related to the rise in raw material prices, so it is desirable to limit the Ni content to 15% or less.

The content of Cu is 2.0% or less (including 0).

Cu is an austenite stabilizing element and has an advantage of suppressing phase transformation into a martensite phase upon cold deformation and improving corrosion resistance in a sulfuric acid atmosphere. However, in the chlorine atmosphere, the Cu content is limited to 2.0% or less because it has a disadvantage in reducing the formal resistance and lowering the hot workability.

The content of N is 0.1 to 0.3%.

N is a useful element for stabilizing the austenite phase as well as improving the corrosion resistance in the chlorine atmosphere. Particularly, when Mo is low, it is necessary to add 0.1% or more in order to secure the corrosion resistance of the steel. However, when added in large amounts, the hot workability is reduced to lower the real water yield of the steel, so that it is preferable to limit the steel to 0.3% or less.

In order to impart resistance against scratches on the surface of stainless steel, surface hardness must be improved. In order to have excellent scratch resistance, the surface hardness is required to satisfy the Vickers hardness of 250 to 410 Hv. In case of hardness lower than 250Hv, component adjustment is possible regardless of cold rolling reduction and there is no resistance to scratches. On the other hand, a material having a hardness higher than 410 Hv generates a springback when the coil is wound, resulting in a very high manufacturing difficulty, thereby significantly reducing the manufacturing error rate.

As described above, the hardness of the stainless steel is greatly influenced by the constituent elements and the cold working history. In the present invention, the relationship between the component elements and the cold reduction ratio for obtaining the hardness having scratch resistance is evaluated by the following equation ), Respectively. In the formula (1), hardness (Hv) is Vickers hardness, C, Si, Cr, Mn, Ni, N and Cu mean the content ) Means the reduction ratio (%) in cold rolling.

(1) 122 - 23939 * C + 376 * Si + 44 * Cr + 2.62 * Mn - 36 * Ni + 54 * N - 43 * Cu + 2.85 * CRR

On the other hand, STS304 steel, which is widely used as a typical austenitic stainless steel for aquaculture, building materials, chemical containers, etc., is austenitic stainless steel or a quasi-stable austenitic microstructure that transforms austenite phase into martensite phase during cold working I have. Such phase transformation occurring during cold working has a detrimental effect on the corrosion resistance due to the characteristics of the martensite phase.

Therefore, in the case of a high-hardness austenitic stainless steel excellent in corrosion resistance to be provided by the present invention, there is no phase transformation to a martensite phase during cold working. To this end, in the present invention, the value of Md30 expressed by the following formula (2) satisfies -150 or less as the content (weight%) of each component element.

(2) Md30 = 551 - 462 * (C + N) - 9.2 * Si - 8.1 * Mn - 13.7 * Cr - 29 * (Ni + Cu) - 18.5 * Mo + 1.42

That is, when the Md30 value is -150 or less, it means that the austenite phase is stable and no phase transformation occurs to the martensite phase even during the cold working, thereby causing no decrease in corrosion resistance due to the phase transformation.

Further, since the martensite phase transformation by cold working does not occur, the martensite phase fraction may be less than 0.5% by volume after the cold working. The stainless steel according to the present invention, which has a quasi-square structure and has a non-magnetic austenite phase of 99.5% by volume or more, may exhibit a non-magnetic property.

In general, the index of corrosion resistance of austenitic stainless steels will be based on the formula of PREN calculated by the combination of Cr, Mo and N contents. The value of STS304 is 20 or less and the value of STS316L is 22 to 24. Therefore, it can be understood that the corrosion resistance of the high hardness austenitic stainless steel to be provided in the present invention should be equal to or higher than 23 on the basis of the internal equivalent index, and thus it can have corrosion resistance equal to or higher than that of STS316L.

On the other hand, in order to increase the official equivalence index, it is necessary to increase Cr, Mo and N contents. However, when the content of Cr and Mo is increased, precipitation of intermetallic compounds such as σ-phase is promoted to cause brittleness, which may cause deterioration of corrosion resistance. When N content is high, Resulting in a drop in the manufacturing error rate. Therefore, it is important to ensure that the value of the official equivalence index does not exceed 28 when these considerations are taken into consideration.

Therefore, in the high hardness austenitic stainless steel excellent in corrosion resistance according to the embodiment of the present invention, the inner formal equivalent index (PREN) value represented by the following formula (3) may satisfy the range of 23 to 28.

(3) PREN = Cr + 3.3 * Mo + 16 * N

Hereinafter, preferred embodiments of the present invention will be described in detail.

Example

The steel having the alloy composition shown in Table 1 was dissolved in a vacuum induction melting furnace, hot rolling was carried out, and a solution heat treatment was carried out at 1,100 to 1,150 ° C to prepare a hot rolled plate having a thickness of 5 mm.

STS316L was used as a comparative example.

division Composition (% by weight) C Si Mn Cr Ni Mo Cu N foot
persons
Yes A 0.031 1.00 1.0 20.3 12.1 0.0 0.0 0.177 B 0.030 1.00 1.0 20.7 11.1 0.0 2.0 0.178 C 0.031 0.99 2.0 20.3 10.9 0.0 1.0 0.180 D 0.032 1.01 2.9 20.7 10.0 0.0 2.0 0.172 E 0.031 0.97 3.0 21.0 10.9 0.0 2.0 0.133 F 0.030 1.00 2.0 21.6 13.7 0.0 1.0 0.125 G 0.045 1.95 1.5 23.8 15.0 0.0 1.5 0.223 H 0.032 1.35 1.5 21.7 14.5 0.0 1.5 0.287 ratio
School
Yes I 0.031 2.50 2.5 21.5 13.7 0.0 1.0 0.178 J 0.028 1.00 2.0 21.3 13.5 0.0 2.5 0.214 K 0.030 0.80 1.8 21.5 12.9 0.0 1.5 0.312 L 0.020 0.50 1.2 16.5 10.1 2.0 0.3 0.030

After cold rolling at cold reduction rates of 50% and 60%, Vickers hardness was measured without annealing. The sticky hardness was obtained by polishing the cross section of the specimen and pressing it with a diamond tip, and the inner diameter of the indentation defect observed with a microscope was converted into hardness.

Table 2 shows the Md30 value, the internal formula index (PREN), the hardness (Hv) according to the cold rolling reduction ratio and the martensitic phase fraction before and after the cold rolling at a reduction ratio of 60%.

division Md30 PREN Hardness (Hv) Hot rolling
Edge crack
Occurrence Martensite
Phase fraction (%) Reduction rate
0% Reduction rate
50% Reduction rate
60% Cold rolling
I'm Cold rolling
after foot
persons
Yes A -190.0 23.1 201 368 382 × 0.0 0.0 B -224.5 23.5 200 360 366 × 0.0 0.0 C -193.3 23.2 206 364 380 × 0.0 0.0 D -206.0 23.5 207 350 371 × 0.0 0.0 E -219.2 23.1 200 339 353 × 0.0 0.0 F -266.1 23.6 205 337 356 × 0.0 0.0 G -406.0 27.4 224 355 396 × 0.0 0.0 H -380.8 26.3 237 382 403 × 0.0 0.0 ratio
School
Yes I -308.2 24.3 209 342 372 ○ 0.0 0.0 J -340.6 24.7 210 378 399 ○ 0.0 0.0 K -339.7 26.5 248 392 412 ○ 0.0 0.0 L -49.7 23.6 149 353 364 × 0.2 11.1

As shown in Table 2, it can be seen that the inventive products A to H satisfy the hardness of 250 to 410 Hv. 1 is a graph showing the relationship between the estimated hardness value and the measured hardness value according to the formula (1). As can be seen in FIG. 1, the predicted hardness value (Equation (1)) predicts the actual hardness of the material well and is statistically confirmed to show a hit rate of 98.8%.

In the inventive examples A to H, the Md30 value calculated by the formula (2) satisfies -150 or less, and the reduction ratio is 60%, which means that there is no increase in the fraction of the martensite phase before and after the cold rolling.

The 316L steel grades used in Comparative Example L were 50% and 60% in reduction ratio, and the hardness of the material after cold rolling satisfied the range of the present invention. However, the composition of 316L grades did not satisfy the range of Md30 in Equation (2) And the martensite phase fraction increased to 11.1% due to the 60% reduction in cold rolling.

On the other hand, the inventive examples A to H exhibited the same or higher equivalent formal equivalence index (PREN) of 316L of Comparative Example L, so that corrosion resistance was not considered to be deteriorated.

2 is a photograph showing whether or not edge cracks have occurred after the hot rolling of the inventive example and the comparative example according to the embodiment of the present invention.

As shown in FIG. 2, Comparative Examples I, J and K satisfy the Md30, PREN and hardness range of the present invention, respectively, but the Si content is more than 2.0%, the Cu content is more than 2.0%, the N content is more than 0.3% As a result of the occurrence of many edge cracks during hot rolling, it was confirmed that it was difficult to manufacture the plate material due to the deterioration of hot workability.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited thereto. Those skilled in the art will recognize that other embodiments may occur to those skilled in the art without departing from the scope and spirit of the following claims. It will be understood that various changes and modifications may be made.

Claims (4)

The steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains, by weight%, 0.05% or less of C, 0.5 to 2.0% of Si, 3.0% or less of Mn, 20 to 24% of Cr, 10 to 15% of Ni, The balance of Fe and unavoidable impurities,
A hardness-austenitic stainless steel excellent in corrosion resistance having a hardness (Hv) represented by the following formula (1) satisfying the range of 250 to 410.
(1) 122 - 23939 * C + 376 * Si + 44 * Cr + 2.62 * Mn - 36 * Ni + 54 * N - 43 * Cu + 2.85 * CRR
(%) Of each element (CRR means a cold rolling reduction rate (%)), C, Si, Cr, Mn, Ni, The method according to claim 1,
A high hardness austenitic stainless steel excellent in corrosion resistance, wherein an Md30 value represented by the following formula (2) satisfies -150 or less.
(2) Md30 = 551 - 462 * (C + N) - 9.2 * Si - 8.1 * Mn - 13.7 * Cr - 29 * (Ni + Cu) - 18.5 * Mo + 1.42
(Here, C, N, Si, Mn, Cr, Ni, Cu and Mo mean the content (% by weight) 3. The method of claim 2,
In the stainless steel,
A high-hardness austenitic stainless steel excellent in corrosion resistance having a martensitic phase fraction of less than 0.5% by volume. The method according to claim 1,
A high hardness austenitic stainless steel excellent in corrosion resistance, the inner formal equivalence index (PREN) expressed by the following formula (3) satisfying the range of 23 to 28:
(3) PREN = Cr + 3.3 * Mo + 16 * N
(Where Cr, Mo, and N mean the content (weight%) of each element)

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