CN116547402A

CN116547402A - High strength austenitic stainless steel with excellent hot workability

Info

Publication number: CN116547402A
Application number: CN202180077816.2A
Authority: CN
Inventors: 金冏勋; 金鹤; 河万琎; 金志洙
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2020-11-23
Filing date: 2021-10-29
Publication date: 2023-08-04
Also published as: EP4249623A1; US20230407443A1; KR20220071005A; KR102463015B1; WO2022108170A1; JP2023550424A

Abstract

Disclosed is a high strength austenitic stainless steel having excellent hot workability. The high strength austenitic stainless steel having excellent hot workability according to the present disclosure comprises, in weight percent (wt.%): 0.01 to 0.035% of C, 0.5% or less of Si, 0.5 to 1.5% of Mn, 17 to 22% of Cr, 6 to 11% of Ni, 1% or less of Mo, 1% or less of Cu, 0.1 to 0.22% of N, and Fe and unavoidable impurities in the balance, wherein the value of the following formula (1) is 1.9 or more, or the precipitation temperature of chromium nitride satisfies the value represented by the following formula (2) or less. (1) 3× (Cr+Mo) +5×Si-2×Ni-Mn-70× (C+N) -27 (2) 1180+36×C+12×Mo+17×Cu+411×N-35×Mn.

Description

High strength austenitic stainless steel with excellent hot workability

Technical Field

The present disclosure relates to a high strength austenitic stainless steel having excellent hot workability, and more particularly, to a high strength austenitic stainless steel having excellent hot workability so as to have excellent surface quality and high hardness.

Background

As price competition for products has recently increased, efforts are continually being made to reduce the cost of materials used in components. The method of reducing the amount of material used is effective in reducing the cost of the material used in the component. For this reason, high-strength materials have been studied, and the thickness of the member can be reduced by using the high-strength materials.

Among materials for manufacturing parts, austenitic stainless steel is a steel grade widely used in various fields because austenitic stainless steel is easy to form a complicated shape due to excellent ductility and has excellent work hardening capacity. The strength of austenitic stainless steel can be increased by using interstitial elements that hinder dislocation migration when stress is applied thereto.

Among the interstitial elements, carbon and nitrogen, which are low-priced elements, are very useful elements for improving strength without increasing costs. However, since carbon and nitrogen significantly improve the stability of the austenite phase, the formation of delta ferrite is reduced during solidification and thus hot workability is deteriorated during hot rolling.

Disclosure of Invention

Technical problem

Provided is a high strength austenitic stainless steel having high hardness while preventing deterioration of hot workability.

Technical proposal

According to one aspect of the present disclosure, a high strength austenitic stainless steel having excellent hot workability comprises, in weight percent (wt.%): 0.01 to 0.035% of C, 0.5% or less of Si, 0.5 to 1.5% of Mn, 17 to 22% of Cr, 6 to 11% of Ni, 1% or less of Mo, 1% or less of Cu, 0.1 to 0.22% of N, and Fe and unavoidable impurities in the balance, wherein the value of the following formula (1) is 1.9 or more, or the precipitation temperature of chromium nitride satisfies the value represented by the following formula (2) or less.

(1)3×(Cr+Mo)+5×Si-2×Ni-Mn-70×(C+N)-27

(2)1180+36×C+12×Mo+17×Cu+411×N-35×Mn

(wherein Cr, mo, si, ni, mn, C, N and Cu represent weight% of the corresponding elements.)

Further, the number of surface cracks may be 0.3 pieces per meter (m) or less.

Further, the high strength austenitic stainless steel having excellent hot workability may have a hardness of 190Hv or more.

The high strength austenitic stainless steel having excellent hot workability may further comprise, in weight percent (wt.%): 0.05% or less of P, 0.01% or less of S, 0.1% or less of Al, 0.01% or less of Ti, and 0.005% or less of B.

Advantageous effects

The high strength austenitic stainless steel having excellent hot workability according to one embodiment of the present disclosure can achieve high strength without deteriorating surface quality by forming ferrite during solidification by using interstitial elements.

According to the high strength austenitic stainless steel having excellent hot workability of one embodiment of the present disclosure, occurrence of cracks caused during hot working can be suppressed by controlling the precipitation temperature of CrN phase, and manufacturing cost can be reduced by omitting a subsequent surface treatment process for obtaining surface quality.

Detailed Description

A high strength austenitic stainless steel having excellent hot workability according to one embodiment of the present disclosure comprises, in weight percent (wt.%): 0.01 to 0.035% of C, 0.5% or less of Si, 0.5 to 1.5% of Mn, 17 to 22% of Cr, 6 to 11% of Ni, 1% or less of Mo, 1% or less of Cu, 0.1 to 0.22% of N, and Fe and unavoidable impurities in the balance, wherein the value of the following formula (1) is 1.9 or more, or the precipitation temperature of chromium nitride satisfies the value represented by the following formula (2) or less.

(1)3×(Cr+Mo)+5×Si-2×Ni-Mn-70×(C+N)-27

(2)1180+36×C+12×Mo+17×Cu+411×N-35×Mn

(wherein Cr, mo, si, ni, mn, C, N and Cu represent weight% of the corresponding elements) embodiments of the present invention

Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. The following embodiments are provided to fully convey the spirit of the disclosure to those of ordinary skill in the art to which the disclosure pertains. The present disclosure is not limited to the embodiments shown herein, but may be presented in other forms. In the drawings, portions irrelevant to the description are omitted for clarity of description of the present disclosure, and the sizes of elements may be exaggerated for clarity.

A high strength austenitic stainless steel having excellent hot workability according to one embodiment of the present disclosure comprises, in weight percent (wt.%): 0.01 to 0.035% of C, 0.5% or less of Si, 0.5 to 1.5% of Mn, 17 to 22% of Cr, 6 to 11% of Ni, 1% or less of Mo, 1% or less of Cu, 0.1 to 0.22% of N, and the balance of Fe and unavoidable impurities.

Hereinafter, the reasons for numerical limitation concerning the content of the alloy element in the embodiments of the present disclosure will be described. Hereinafter, unless otherwise indicated, units are% by weight.

The content of carbon (C) is 0.01% to 0.035%.

C is a representative interstitial element effective to improve strength. In order to improve strength, C needs to be added in an amount of 0.01% or more. However, when the C content is excessive, the formation of delta ferrite is suppressed during solidification due to the austenite stabilizing effect, resulting in deterioration of hot workability. Therefore, in order to obtain hot workability, the C content may be controlled to 0.035% or less.

The content of silicon (Si) is 0.5% or less.

Si as a ferrite stabilizing element effectively prevents reduction of ferrite phase formation due to addition of C and N. However, excessive Si may promote precipitation of intermetallic compounds such as sigma (σ) phase, resulting in deterioration of mechanical properties and corrosion resistance, and thus Si content may be controlled to 0.5% or less.

The content of manganese (Mn) is 0.5% to 1.5%.

Since Mn increases the solid solubility of N to prevent defects caused by N gas, mn is preferably added in an amount of 0.5% or more. However, as the element Mn which acts similarly to Ni, austenite phase stabilization element like C and N, so that in order to obtain hot workability, the upper limit thereof is preferably controlled to 1.5% or less.

The content of chromium (Cr) is 17% to 22%.

Cr is the most important element for improving the corrosion resistance of stainless steel. Further, cr, which is an important element for increasing strength, may be added in an amount of 17% or more. However, an excessive amount of Cr as a ferrite phase stabilizing element decreases the stability of the austenite phase, resulting in an increase in Ni content and thus an increase in manufacturing cost. Further, intermetallic compounds such as sigma (σ) phase precipitate, and thus, problems of deterioration in mechanical properties and corrosion resistance may occur. Therefore, the upper limit thereof can be controlled to 22% in the present disclosure.

The content of nickel (Ni) is 6% to 11%.

Ni is the strongest austenite phase stabilizing element, and should be added in an amount of 6% or more in order to obtain sufficient stability of the austenite phase in austenitic stainless steel. However, since the increase in Ni content is directly related to the increase in raw material cost, the Ni content can be controlled to 11% or less.

The content of molybdenum (Mo) is 1% or less.

Mo is an element effective for improving corrosion resistance, but since Mo is a high-priced element, it causes an increase in manufacturing cost when added in a large amount. Further, since intermetallic compounds such as sigma (σ) phase are thereby precipitated, there arises a problem that mechanical characteristics and corrosion resistance are deteriorated. Therefore, the Mo content can be controlled to 1% or less in the present disclosure.

The content of copper (Cu) is 1% or less.

Cu, which is an element effective for stabilizing the austenite phase, can be used as a substitute for Ni, a high-priced element. However, excessive Cu causes formation of a phase having a low melting point to deteriorate hot workability, thereby deteriorating surface quality. Therefore, the Cu content can be controlled to 1% or less.

The nitrogen (N) content is 0.1% to 0.22%.

N, which is a low-priced element useful for increasing strength, is an essential element in high-strength austenitic stainless steel. Therefore, N should be added in an amount of 0.1% or more. However, an excessive amount of N promotes formation of chromium nitride (CrN) to deteriorate hot workability, thereby deteriorating surface quality. Therefore, the N content can be controlled to 0.22% or less.

The austenitic stainless steel according to one embodiment of the present disclosure may contain, in addition to the above-mentioned alloying elements: 0.05% or less of P, 0.01% or less of S, 0.1% or less of Al, 0.01% or less of Ti, and 0.005% or less of B, more preferably comprising: 0.035% or less of P, 0.0035% or less of S, 0.04% or less of Al, 0.003% or less of Ti, and 0.0025% or less of B.

The content of phosphorus (P) is 0.05% or less, and the content of sulfur (S) is 0.01% or less.

P and S are impurities inevitably contained in steel, and when the contents of P and S exceed 0.05% and 0.01%, respectively, they segregate in steel, resulting in surface cracks. Accordingly, the P content and the S content can be controlled to 0.05% or less and 0.01% or less, respectively, and more preferably to 0.035% or less and 0.0035% or less, respectively.

The content of aluminum (Al) is 0.1% or less.

Al improves resistance to high temperature oxidation. However, excessive Al deteriorates surface quality due to the formation of Al inclusions. Therefore, the Al content can be controlled to 0.1% or less, more preferably to 0.04% or less.

The content of titanium (Ti) is 0.01% or less.

Ti prevents high temperature corrosion of the slab when the slab is heated, thereby preventing surface cracks from occurring during hot rolling. However, the addition of a large amount of Ti may cause the formation of coarse precipitates, resulting in the problem of deterioration of impact toughness. Therefore, the Ti content can be controlled to 0.01% or less, more preferably to 0.003% or less.

The content of boron (B) is 0.005% or less.

B segregates in the grain boundaries of austenite to inhibit grain boundary fracture and improve ductility. However, an excessive amount of B may deteriorate the toughness of the steel sheet. Accordingly, the B content can be controlled to 0.005% or less, more preferably, to 0.0025% or less.

The remaining alloying elements of stainless steel, in addition to the above-mentioned alloying elements, are Fe and other unavoidable impurities.

While the austenitic stainless steel is solidified, delta ferrite is formed in a small amount, and thus occurrence of thermal cracking is prevented. However, since C and N reduce the amount of δ ferrite, the occurrence of thermal cracking caused by the reduction of δ ferrite tends to increase in the case of adding C and N.

In the present disclosure, the strength of austenitic stainless steel is increased by adding C and N. In this case, the reduction of hot workability due to the reduction of delta ferrite formation during solidification is prevented by ensuring the amount of ferrite or controlling the chromium nitride (CrN) phase.

In the present disclosure, an appropriate composition range of the alloy element shown by the following formula (1) is obtained. Although the contents of C and N are increased, an appropriate amount of ferrite can be obtained during solidification so that hot workability is not deteriorated by the composition range.

(1)3×(Cr+Mo)+5×Si-2×Ni-Mn-70×(C+N)-27

(in the formula (1), cr, mo, si, ni, mn, C and N represent% by weight of the corresponding elements.)

When the value of the formula (1) is less than 1.9, the hot workability deteriorates to cause cracks to appear on the surface. In contrast, when the value of formula (1) is 1.9 or more, the number of surface cracks after thermal annealing is 0.3 pieces/meter (m) or less.

Although the surface quality can be obtained by improving the hot workability by using the formula (1) as described above, the composition range of the alloy element is limited. Therefore, factors other than delta ferrite have been studied, and thus a correlation between the surface quality and the precipitation temperature of CrN phase has been obtained.

The addition of N to obtain high strength increases the precipitation temperature of the CrN phase, and the increased precipitation temperature causes the CrN phase to remain during hot rolling, thereby deteriorating hot workability.

The precipitation temperature of the CrN phase can be experimentally evaluated by using a thermal evaluation device (e.g., TGA and DSC) or calculated numerically by using a phase change analysis program.

In the present disclosure, the decomposition limit temperature of the CrN phase shown by the following formula (2) is obtained, and it is determined that the hot workability is improved in the case where the precipitation temperature of the CrN phase is the decomposition limit temperature or lower.

(2)1180+36×C+12×Mo+17×Cu+411×N-35×Mn

(in the formula (2), C, mo, cu, N and Mn represent weight% of the corresponding elements.)

When the value of the formula (2) is lower than the precipitation temperature (c) of chromium nitride (CrN), surface cracks occur due to deterioration of hot workability. In contrast, when the value of formula (2) is the precipitation temperature (c) of chromium nitride (CrN) or higher, a hot-rolled annealed material having the number of surface cracks of 0.3 pieces/meter or less can be provided.

According to one embodiment of the present disclosure, the austenitic stainless steel may have a hardness of 190Hv or more and a number of surface cracks of 0.3 per meter (m) or less after thermal annealing.

Hereinafter, a method of manufacturing a high strength austenitic stainless steel having excellent hot workability according to one embodiment of the present disclosure will be described.

The high strength austenitic stainless steel having excellent hot workability according to one embodiment of the present disclosure may be manufactured by any method commonly used in the art for manufacturing austenitic stainless steel, and the method includes hot rolling a slab comprising, in weight percent (wt.%): 0.01 to 0.035% of C, 0.5% or less of Si, 0.5 to 1.5% of Mn, 17 to 22% of Cr, 6 to 11% of Ni, 1% or less of Mo, 1% or less of Cu, 0.1 to 0.22% of N, and the balance of Fe and unavoidable impurities; and annealing the hot rolled steel sheet prepared by hot rolling.

By satisfying at least one of the following formulas (1) and (2), the slab may have improved hot workability, and more specifically, the number of surface cracks after annealing may be 0.3 pieces/meter (m) or less. Further, the hardness of the hot rolled annealed material may be 190Hv or more.

(1)3×(Cr+Mo)+5×Si-2×Ni-Mn-70×(C+N)-27≥1.9

(2) 1180+36 XC+12 XMo+17 XCu+411 XN-35 XMn ≡chromium nitride (CrN) precipitation temperature (. Degree.C.)

The reason for the limitation of the formulae (1) and (2) is as described above, and thus a detailed description thereof will be omitted.

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

Examples

Steels having chemical compositions shown in tables 1 and 2 were cast into 200mmt slabs by continuous casting, and hot rolled and annealed to prepare hot rolled steel sheets. After heating at 1250 ℃ for 2 hours, hot rolling is performed to a thickness of 4mmt to 8mmt, and annealing heat treatment is performed at 1150 ℃ after hot rolling.

TABLE 1

TABLE 2

The values of formulas (1) and (2) of the steel grades shown in tables 1 and 2 and the precipitation temperature of the CrN phase are shown in table 3 below. The precipitation temperature of the CrN phase can be experimentally evaluated by using a thermal evaluation device (e.g., TGA and DSC) or calculated numerically by using a phase change analysis program. Table 3 shows the values using the phase change analysis program.

After the hot rolled coil having the above composition and a thickness of 4mmt to 8mmt was prepared, the number of surface defects and hardness of the hot rolled coil were evaluated, and the evaluation results thereof are shown in table 3 below. The number of defects is the number of defects per meter obtained by dividing the total number of surface defects of the hot rolled coil after annealing and pickling by the length of the coil. In general, in the case of the number of 0.3 or less, the coil stock was evaluated as having excellent quality. Hv hardness was used as the hardness, and the test was performed using a load of 10kgf, a measurement interval of 1mm, and a reduction time of 10 seconds.

TABLE 3 Table 3

Referring to tables 1 to 3, in the hot rolled annealed steel sheets of steel grade nos. 1 to 10, the value of formula (1) was 1.9 or more and the number of surface cracks was 0.24 number/m or less, indicating excellent surface quality.

In steel grade nos. 11 to 29, the value of formula (1) was less than 1.9, but the precipitation temperature of CrN was the value of formula (2) or less, and the number of surface cracks was 0.25 number/m or less, indicating excellent surface quality.

In contrast, in steel grade numbers 30 to 37, the value of formula (1) was less than 1.9, and the precipitation temperature of CrN exceeded the value of formula (2), indicating an increase in the number of cracks.

As described above, when the value of formula (1) is 1.9 or more or the precipitation temperature of CrN is the value of formula (2) or less, it is determined that the number of surface defects is reduced due to the improvement of the hot workability of austenitic stainless steel.

Further, the hardness of the heat annealed steel sheets of steel grade nos. 1 to 29, which satisfy the chemical composition of the alloy element according to the present disclosure, was determined to be 190Hv or more.

While the present disclosure has been particularly described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the disclosure.

[ Industrial applicability ]

The austenitic stainless steel according to the present disclosure can have high strength without deteriorating surface quality by forming ferrite during solidification, can suppress occurrence of cracks caused during hot working, and can be manufactured at reduced cost by omitting a subsequent surface treatment process for obtaining surface quality, and thus is industrially applicable.

Claims

1. A high strength austenitic stainless steel having excellent hot workability, comprising in weight percent (wt.%): 0.01 to 0.035% of C, 0.5% or less of Si, 0.5 to 1.5% of Mn, 17 to 22% of Cr, 6 to 11% of Ni, 1% or less of Mo, 1% or less of Cu, 0.1 to 0.22% of N, and the balance of Fe and unavoidable impurities,

wherein the value of the following formula (1) is 1.9 or more, or the precipitation temperature of chromium nitride satisfies the value represented by the following formula (2) or less:

(1)3×(Cr+Mo)+5×Si-2×Ni-Mn-70×(C+N)-27

(2)1180+36×C+12×Mo+17×Cu+411×N-35×Mn

wherein Cr, mo, si, ni, mn, C, N and Cu represent weight% of the corresponding elements.

2. The high strength austenitic stainless steel of claim 1, wherein the number of surface cracks is 0.3 pieces per meter (m) or less.

3. The high strength austenitic stainless steel of claim 1, wherein the hardness is 190Hv or greater.

4. The high strength austenitic stainless steel of claim 1, further comprising, in weight percent (wt.%): 0.05% or less of P, 0.01% or less of S, 0.1% or less of Al, 0.01% or less of Ti, and 0.005% or less of B.