EP4249623A1 - High-strength austenitic stainless steel having excellent hot workability - Google Patents

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 includes, in percent by weight (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 inevitable impurities, wherein a value of Formula (1) below 1.9 or more, or a precipitation temperature of chromium nitride satisfies a value represented by Formula (2) below or less. 3×Cr+Mo+5×Si−2×Ni−Mn−70×C+N−271180+36×C+12×Mo+17×Cu+411×N−35×Mn

Description

[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, thereby having excellent surface quality and high hardness.
[Background Art]
• Due to recent intensified price competition of products, efforts to reduce costs of materials used in parts are increasing. Methods of lowering amounts of the materials used therefor are effective for reducing costs of the materials used in parts. To this end, research into high-strength materials has been conducted, and thickness of parts may be reduced by using the high-strength materials.
• Among the materials used to manufacture parts, austenitic stainless steel is a steel type widely used in various fields because the austenitic stainless steel is easy to form complex shapes due to excellent elongation and has excellent work hardening. Strength of austenitic stainless steel may be improved by using interstitial elements that hinder migration of dislocation when a stress is applied thereto.
• Among the interstitial elements, carbon and nitrogen, which are low-priced elements, are very useful element to improve strength without increasing costs. However, because carbon and nitrogen significantly improve stability of an austenite phase, formation of delta ferrite is reduced during solidification and hot workability deteriorates during hot rolling.
[Disclosure] [Technical ProblemI
• Provided is a high-strength austenitic stainless steel having high hardness while preventing hot workability from deteriorating.
[Technical Solution]
• In accordance with an aspect of the present disclosure, a high-strength austenitic stainless steel having excellent hot workability includes, in percent by weight (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 inevitable impurities, wherein a value of Formula (1) below 1.9 or more, or a precipitation temperature of a chromium nitride satisfies a value represented by Formula (2) below or less. $$\begin{array}{l}3\times \left(\mathrm{Cr}+\mathrm{Mo}\right)+5\times \mathrm{Si}-2\times \mathrm{Ni}-\mathrm{Mn}-70\times \left(\mathrm{C}+\mathrm{N}\right)\\ -27\end{array}$$

  

  $$\begin{array}{l}1180+36\times \mathrm{C}+12\times \mathrm{Mo}+17\times \mathrm{Cu}+411\times \mathrm{N}-\\ 35\times \mathrm{Mn}\end{array}$$

  

  
  (wherein Cr, Mo, Si, Ni, Mn, C, N, and Cu denote wt% of the respective elements.)
• In addition, the number of surface cracks may be 0.3 or less per meter (m).
• In addition, the high-strength austenitic stainless steel having excellent hot workability may have a hardness of 190 Hv or more.
• The high-strength austenitic stainless steel having excellent hot workability may further include, in percent by weight (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 an embodiment of the present disclosure may high strength by using interstitial elements without deteriorating surface quality by forming ferrite during solidification.
• According to the high-strength austenitic stainless steel having excellent hot workability according to an embodiment of the present disclosure, occurrence of cracks caused during hot working may be inhibited by controlling the precipitation temperature of the CrN phase and manufacturing costs may be reduced by omitting a subsequent surface treatment process for obtaining surface quality.
[Best Mode]
• A high-strength austenitic stainless steel having excellent hot workability according to an embodiment of the present disclosure includes, in percent by weight (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 inevitable impurities, wherein a value of Formula (1) below 1.9 or more, or a precipitation temperature of a chromium nitride satisfies a value represented by Formula (2) below or less. $$\begin{array}{l}3\times \left(\mathrm{Cr}+\mathrm{Mo}\right)+5\times \mathrm{Si}-2\times \mathrm{Ni}-\mathrm{Mn}-70\times \left(\mathrm{C}+\mathrm{N}\right)\\ -27\end{array}$$

  

  $$\begin{array}{l}1180+36\times \mathrm{C}+12\times \mathrm{Mo}+17\times \mathrm{Cu}+411\times \mathrm{N}-\\ 35\times \mathrm{Mn}\end{array}$$

  

  
  (wherein Cr, Mo, Si, Ni, Mn, C, N, and Cu denote wt% of the respective elements.)
[Modes of 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 fully convey the spirit of the present disclosure to a person having ordinary skill in the art to which the present disclosure belongs. The present disclosure is not limited to the embodiments shown herein but may be embodied in other forms. In the drawings, parts unrelated to the descriptions are omitted for clear description of the disclosure and sizes of elements may be exaggerated for clarity.
• A high-strength austenitic stainless steel having excellent hot workability according to an embodiment of the present disclosure includes, in percent by weight (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 inevitable impurities.
• Hereinafter, reasons for numerical limitations on the contents of alloying elements in the embodiment of the present disclosure will be described. Hereinafter, the unit is wt% unless otherwise stated.
• The content of carbon (C) is from 0.01 to 0.035%.
• C is a representative interstitial element effective on improving 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, formation of delta ferrite is inhibited during solidification due to austenite stabilizing effect, resulting in deterioration of hot workability. Therefore, the C content may be controlled to 0.035% or less to obtain hot workability.
• The content of silicon (Si) is 0.5% or less.
• Si, as a ferrite-stabilizing element, is effective on preventing reduction in formation of a ferrite phase caused by adding C and N. However, an excess of Si may promote precipitation of intermetallic compounds such as sigma (σ) phase, resulting in deterioration of mechanical properties and corrosion resistance, and therefore the Si content may be controlled to 0.5% or less.
• The content of manganese (Mn) is from 0.5 to 1.5%.
• Because Mn increases solid solubility of N to prevent defects caused by N gases, it is preferable to add Mn in an amount of 0.5% or more. However, Mn, as an element acting similar to Ni, is an austenite phase-stabilizing element like C and N, and thus it is preferable to control the upper limit thereof to 1.5 or less to obtain hot workability.
• The content of chromium (Cr) is from 17 to 22%.
• Cr is the most important element to improve corrosion resistance of a stainless steel. Also, Cr, as an important element to increase strength, may be added in an amount of 17% or more. However, an excess of Cr, as a ferrite phase-stabilizing element, reduces stability of an austenite phase to involve an increase in the Ni content, resulting in an increase in manufacturing costs. In addition, intermetallic compounds such as sigma (σ) phase are precipitated and thus a problem of deteriorating mechanical properties and corrosion resistance may occur. Therefore, an upper limit thereof may be controlled to 22% in the present disclosure.
• The content of nickel (Ni) is from 6 to 11%.
• Ni is the strongest austenite phase-stabilizing element and should be added in an amount of 6% or more to obtain enough stability of an austenite phase in an austenitic stainless steel. However, since the increase in the Ni content is directly related to the increase in costs of raw materials, the Ni content may be controlled to 11% or less.
• The content of molybdenum (Mo) is 1% or less.
• Mo is an element effective on improving corrosion resistance but causes an increase in manufacturing costs in the case of being added in a large quantity since Mo is a high-priced element. In addition, a problem of deteriorating mechanical properties and corrosion resistance occurs since intermetallic compounds such as sigma (σ) phase are precipitated thereby. Therefore, the Mo content may be controlled to 1% or less in the present disclosure.
• The content of copper (Cu) is 1% or less.
• Cu, as an element effective on stabilizing an austenite phase, may be used as a substitute of the high-priced element Ni. However, an excess of Cu causes formation of a phase having a low melting point to deteriorate hot workability, thereby deteriorating surface quality. Therefore, the Cu content may be controlled to 1% or less.
• The content of nitrogen (N) is from 0.1 to 0.22%.
• N, as a low-priced element useful to increase strength, is an essential element in a high-strength austenitic stainless steel. Therefore, N should be added in an amount of 0.1% or more. However, an excess of N promotes formation of a chromium nitride (CrN) to deteriorate hot workability, thereby deteriorating surface quality. Therefore, the N content may be controlled to 0.22% or less.
• The austenitic stainless steel according to an embodiment of the present disclosure may further include, in addition to the above-described 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, include 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 steels, and when the contents of P and S exceed 0.05% and 0.01%, respectively, they are segregated into steels causing surface cracks. Therefore, the P content and the S content may be controlled to 0.05% or less and 0.01% or less, respectively, more preferably, 0.035% or less and 0.0035% or less, respectively.
• The content of aluminum (Al) is 0.1% or less.
• Al improves high-temperature oxidation resistance. However, an excess of Al deteriorates surface quality due to formation of Al inclusions. Therefore, the Al content may 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 slabs while heating the slabs, thereby preventing occurrence of surface cracks during hot rolling. However, addition of a large amount of Ti may cause formation of coarse precipitates to cause a problem of deteriorating impact toughness. Therefore, the Ti content may 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 is segregated in grain boundaries of austenite to inhibit fracture of grain boundaries and improve ductility. However, an excess of B may deteriorate toughness of a steel sheet. However, the B content may be controlled to 0.005% or less, more preferably, to 0.0025% or less.
• The rest of the alloying elements of the stainless steel, except for the above-described alloying element, are Fe and other inevitable impurities.
• While an austenitic stainless steel is solidified, delta ferrite is formed in a small amount and thus occurrence of hot cracks is prevented. However, since C and N reduce the amount of delta ferrite, occurrence of hot cracks caused by reduced in delta ferrite tends to increase in the case of adding C and N.
• In the present disclosure, strength of the austenitic stainless steel is increased by adding C and N. In this case, reduction in hot workability caused by decreased formation of delta ferrite during solidification is prevented by securing the amount of ferrite or controlling a chromium nitride (CrN) phase.
• In the present disclosure, an appropriate composition range of the alloying elements is derived as shown Formula (1) below. Although the contents of C and N increase, the appropriate amount of ferrite may be obtained during solidification, so that hot workability does not deteriorate by the composition range. $$\begin{array}{l}3\times \left(\mathrm{Cr}+\mathrm{Mo}\right)+5\times \mathrm{Si}-2\times \mathrm{Ni}-\mathrm{Mn}-70\times \left(\mathrm{C}+\mathrm{N}\right)\\ -27\end{array}$$

  

  
  (In Formula (1), Cr, Mo, Si, Ni, Mn, C, and N denote wt% of the respective elements.)
• When the value of Formula (1) is less than 1.9, hot workability deteriorates causing occurrence of cracks on the surface. On the contrary, when the value of Formula (1) is 1.9 or more, the number of surface cracks is 0.3 or less per meter (m) after hot annealing.
• Although surface quality may be obtained by improving hot workability using Formula (1) as described above, the composition range of the alloying elements is limited. Therefore, studies on factors other than delta ferrite have been conducted, and thus relevance between surface quality and a precipitation temperature of the CrN phase has been derived.
• Addition of N to obtain high strength raises the precipitation temperature of the CrN phase, and the raised precipitation temperature induces residues of the CrN phase during hot rolling, thereby deteriorating hot workability.
• The precipitation temperature of the CrN phase may be experimentally evaluated by using a calorimetric evaluation device such as TGA and DSC or derived by numerical calculation using a phase transformation analysis program.
• In the present disclosure, a decomposition limit temperature of the CrN phase is derived as shown in Formula (2) below, and it was confirmed that hot workability is improved in the case where the precipitation temperature of the CrN phase is the decomposition limit temperature or below. $$\begin{array}{l}1180+36\times \mathrm{C}+12\times \mathrm{Mo}+17\times \mathrm{Cu}+411\times \mathrm{N}-\\ 35\times \mathrm{Mn}\end{array}$$

  

  
  (In Formula (1), C, Mo, Cu, N, and Mn denote wt% of the respective elements.)
• When the value of Formula (2) is below the precipitation temperature (°C) of the chromium nitride (CrN), surface cracks occur due to deteriorated hot workability. On the contrary, when the value of Formula (2) is the precipitation temperature (°C) of the chromium nitride (CrN) or more, a hot-rolled, annealed material having the number of surface cracks of 0.3 or less per meter may be provided.
• According to an embodiment of the present disclosure, the austenitic stainless steel may have a hardness of 190 Hv or more and the number of surface cracks of 0.3 or less per meter (m) after hot annealing.
• Hereinafter, a method of manufacturing a high-strength austenitic stainless steel having excellent hot workability according to an embodiment of the present disclosure will be described.
• The high-strength austenitic stainless steel having excellent hot workability according to an embodiment of the present disclosure may be manufacturing any method commonly used in the art to manufacture austenitic stainless steels, and the method includes hot-rolling a slab including, in percent by weight (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 inevitable impurities and annealing a hot-rolled steel sheet prepared by the hot rolling.
• The slab may have improved hot workability by satisfying at least one of Formulae (1) and (2) below, and more particularly, the number of surface cracks after annealing may be 0.3 or less per meter (m). In addition, the hot-rolled, annealed material may have a hardness of 190 Hv or more. $$\begin{array}{l}3\times \left(\mathrm{Cr}+\mathrm{Mo}\right)+5\times \mathrm{Si}-2\times \mathrm{Ni}-\mathrm{Mn}-70\times \left(\mathrm{C}+\mathrm{N}\right)\\ -27\ge 1.9\end{array}$$

  

  
  (In Formula (1), Cr, Mo, Si, Ni, Mn, C, and N denote wt% of the respective elements.) $$\begin{array}{l}1180+36\times \mathrm{C}+12\times \mathrm{Mo}+17\times \mathrm{Cu}+411\times \mathrm{N}-\\ 35\times \mathrm{Mn}\ge \mathrm{precipitation}\mathrm{temperature}\left(°\mathrm{C}\right)\mathrm{of}\mathrm{chromium}\mathrm{nitride}\left(\mathrm{CrN}\right)\end{array}$$

  

  
  (In Formula (2), C, Mo, Cu, N, and Mn denote wt% of the respective elements.)
• The reasons for limitations of Formulae (1) and (2) are as described above and thus detailed descriptions thereof will be omitted.
• Hereinafter, the present disclosure will be described in more detail through examples.
Examples
• Steels having the chemical compositions shown in Tables 1 and 2 were cast into 200 mmt slabs by continuous casting and hot-rolled and annealed to prepare hot-rolled steel sheets. After heating at 1250°C for 2 hours, the hot rolling was performed to a thickness of 4 to 8 mmt, and annealing heat treatment was performed at 1150°C after the hot rolling. Table 1

  Example	Steel type No.	C	Si	Mn	P	S	Cr	Ni	Mo	Cu	N	Al	Ti	B
  Inventive Example	1	0.020	0.48	0.70	0.030	0.0011	21.4	10.6	0.64	0.95	0.198	0.003	0.002	0.0023
  	2	0.016	0.48	0.99	0.032	0.0010	21.6	10.9	0.52	0.83	0.195	0.003	0.002	0.0023
  	3	0.013	0.48	1.36	0.028	0.0010	17.0	6.8	0.50	0.49	0.131	0.003	0.002	0.0023
  	4	0.010	0.32	1.24	0.008	0.0035	17.4	6.7	0.32	0.29	0.137	0.004	0.002	0.0025
  	5	0.030	0.50	0.94	0.010	0.0005	21.5	10.8	0.59	0.85	0.190	0.003	0.002	0.0023
  	6	0.016	0.30	0.86	0.034	0.0011	21.2	10.6	0.57	0.95	0.198	0.003	0.002	0.0023
  	7	0.021	0.41	1.49	0.030	0.0007	17.4	6.5	0.00	0.23	0.132	0.003	0.002	0.0023
  	8	0.023	0.48	1.42	0.030	0.0008	17.4	6.7	0.07	0.05	0.136	0.003	0.002	0.0023
  	9	0.019	0.32	0.90	0.029	0.0012	21.4	10.7	0.53	0.70	0.202	0.003	0.002	0.0023
  	10	0.024	0.48	0.88	0.025	0.0009	21.2	10.7	0.59	0.99	0.206	0.003	0.002	0.0023
  	11	0.011	0.32	0.75	0.022	0.0012	21.2	10.7	0.61	0.71	0.219	0.003	0.002	0.0023
  	12	0.034	0.32	0.84	0.032	0.0011	21.2	10.4	0.75	0.71	0.211	0.003	0.002	0.0023
  	13	0.035	0.33	0.72	0.030	0.0010	21.6	10.9	0.61	0.88	0.207	0.003	0.002	0.0023
  	14	0.032	0.35	0.75	0.032	0.0007	21.5	10.8	0.72	0.73	0.213	0.004	0.003	0.0019

  Table 2

  Example	Steel type No.	C	Si	Mn	P	S	Cr	Ni	Mo	Cu	N	Al	Ti	B
  Inventive Example	15	0.023	0.37	0.70	0.029	0.0011	21.3	10.9	0.71	0.79	0.211	0.004	0.003	0.0019
  	16	0.024	0.31	1.49	0.032	0.0009	17.2	6.4	0.02	0.07	0.131	0.004	0.003	0.0020
  	17	0.022	0.38	1.21	0.032	0.0011	17.3	7.0	0.36	0.34	0.146	0.002	0.003	0.0020
  	18	0.018	0.41	1.24	0.034	0.0011	17.2	6.6	0.01	0.17	0.141	0.003	0.002	0.0021
  	19	0.010	0.30	1.24	0.034	0.0011	17.0	6.6	0.40	0.25	0.155	0.004	0.002	0.0019
  	20	0.018	0.34	1.31	0.030	0.0008	17.2	7.0	0.20	0.49	0.132	0.004	0.002	0.0019
  	21	0.033	0.33	0.71	0.025	0.0006	21.2	10.8	0.53	0.95	0.194	0.003	0.002	0.0019
  	22	0.019	0.32	0.72	0.030	0.0012	21.3	10.7	0.50	0.81	0.213	0.003	0.003	0.0019
  	23	0.030	0.38	0.71	0.029	0.0011	21.2	10.8	0.57	0.71	0.206	0.003	0.003	0.0020
  	24	0.017	0.32	0.83	0.034	0.0021	21.2	10.8	0.74	0.71	0.220	0.004	0.003	0.0020
  	25	0.025	0.50	1.27	0.032	0.0007	17.2	6.8	0.46	0.02	0.159	0.004	0.003	0.0021
  	26	0.030	0.31	0.71	0.033	0.0012	21.3	11.0	0.66	0.90	0.207	0.002	0.002	0.0023
  	27	0.030	0.39	1.30	0.031	0.0011	17.5	6.8	0.08	0.01	0.145	0.003	0.002	0.0023
  	28	0.029	0.32	1.26	0.020	0.0018	17.1	6.8	0.49	0.39	0.147	0.003	0.002	0.0019
  	29	0.024	0.33	1.28	0.018	0.0011	17.1	7.0	0.02	0.16	0.159	0.003	0.003	0.0019
  Compara tive Example	30	0.021	0.42	1.40	0.034	0.0028	17.0	6.6	0.09	0.26	0.130	0.004	0.003	0.0019
  	31	0.035	0.35	0.96	0.024	0.0016	21.2	10.6	0.55	0.93	0.196	0.003	0.002	0.0023
  	32	0.023	0.45	0.89	0.029	0.0024	21.3	11.0	0.79	0.75	0.220	0.004	0.002	0.0022
  	33	0.033	0.33	0.97	0.030	0.0015	21.6	10.5	0.55	0.82	0.217	0.004	0.002	0.0022
  	34	0.020	0.50	1.36	0.032	0.0017	17.1	6.7	0.37	0.09	0.152	0.003	0.003	0.0022
  	35	0.011	0.31	1.50	0.029	0.0008	17.3	6.5	0.06	0.23	0.150	0.003	0.003	0.0022
  	36	0.025	0.41	1.40	0.020	0.0024	17.4	6.8	0.00	0.13	0.147	0.003	0.003	0.0022
  	37	0.026	0.40	1.43	0.010	0.0011	17.2	7.0	0.06	0.01	0.153	0.004	0.002	0.0025

• The values of Formulae (1) and (2) of the steel types shown in Tables 1 and 2 and the precipitation temperatures of the CrN phase are shown in Table 3 below. The precipitation temperature of the CrN phase may be experimentally evaluated by using a calorimetric evaluation device such as TGA and DSC or derived by numerical calculation using a phase transformation analysis program. Table 3 shows values using a phase transformation analysis program.
• After preparing hot-rolled coils having the above-described compositions and a thickness of 4 to 8 mmt, numbers of surface defects and hardnesses of the hot-rolled coils were evaluated and 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 by the length of the coil after annealing and acid pickling in the hot-rolled coil. In general, in the case where the number is 0.3 or less, the coil is evaluated as having excellent quality. Hv hardness was used as the hardness, and the test was performed using a load of 10 kgf, a measurement interval of 1 mm, and a reduction time of 10 sec. Table 3

  Example	Steel type No.	Formula (1)	Precipitation temperature of CrN	Formula (2)	No. of defects	Hardness (Hv)
  Inventive Example	1	4.5	1266	1261	0.17	198
  	2	4.2	1261	1246	0.18	199
  	3	3.0	1202	1201	0.14	208
  	4	2.8	1188	1202	0.12	192
  	5	3.6	1261	1248	0.22	202
  	6	2.8	1257	1255	0.24	199
  	7	2.0	1193	1187	0.02	208
  	8	2.0	1191	1189	0	201
  	9	2.7	1251	1250	0.13	195
  	10	2.5	1277	1259	0.17	202
  	11	1.8	1260	1263	0.03	199
  	12	1.7	1258	1260	0.12	199
  	13	1.8	1263	1263	0.17	213
  	14	1.8	1261	1263	0.08	198
  	15	1.8	1261	1265	0.07	199
  	16	1.0	1180	1184	0.02	212
  	17	1.0	1201	1208	0.15	211
  	18	1.0	1193	1198	0.03	213
  	19	0.9	1198	1210	0.03	211
  	20	1.0	1197	1200	0.23	198
  	21	1.6	1256	1258	0.05	199
  	22	1.5	1262	1263	0.16	211
  	23	1.5	1256	1260	0.2	202
  	24	1.4	1262	1263	0.25	209
  	25	0.7	1204	1207	0.12	197
  	26	1.2	1262	1265	0	203
  	27	0.6	1190	1196	0	211
  	28	0.2	1203	1210	0.12	212
  	29	-2.1	1203	1204	0.18	211
  Comparative Example	30	1.2	1193	1191	0.45	198
  	31	1.8	1262	1251	0.47	195
  	32	1.7	1273	1262	0.83	198
  	33	1.5	1271	1257	0.78	201
  	34	0.9	1203	1202	0.43	194
  	35	0.8	1200	1194	0.78	201
  	36	0.1	1199	1195	0.83	199
  	37	-1.1	1198	1195	0.65	211

• Referring to Tables 1 to 3, in the hot-rolled, annealed steel sheets of the Steel Type Nos. 1 to 10, the value of Formula (1) was 1.9 or more and the number of surface cracks was 0.24 count/m or less indicating excellent surface quality.
• In Steel Type 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 below, and the number of surface cracks was 0.25 count/m or less, indicating excellent surface quality.
• On the contrary, in Steel Type Nos. 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 that increases 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 below, it was confirmed that the number of surface defects decreased due to improved hot workability of austenitic stainless steels.
• In addition, it was confirmed that the hot-annealed steel sheets of Steel Type Nos. 1 to 29 satisfying the chemical composition of the alloying elements according to the present disclosure had a hardness of 190 Hv or more.
• While the present disclosure has been particularly described with reference to exemplary embodiments, it should be understood by those of skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the present disclosure.
[Industrial Applicability]
• The austenitic stainless steel according to the present disclosure may have high strength without deteriorating surface quality by forming ferrite during solidification, occurrence of cracks caused during hot working may be inhibited, and may be manufactured with lowered costs by omitting a subsequent surface treatment process for obtaining surface quality, and thus the austenitic stainless steel is industrially applicable.

Claims (4)

1. A high-strength austenitic stainless steel having excellent hot workability comprising, in percent by weight (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 inevitable impurities,

   wherein a value of Formula (1) below 1.9 or more, or a precipitation temperature of a chromium nitride satisfies a value represented by Formula (2) below or less: $$3\times \left(\mathrm{Cr}+\mathrm{Mo}\right)+5\times \mathrm{Si}-2\times \mathrm{Ni}-\mathrm{Mn}-70\times \left(\mathrm{C}+\mathrm{N}\right)-27$$

   

   $$1180+36\times \mathrm{C}+12\times \mathrm{Mo}+17\times \mathrm{Cu}+411\times \mathrm{N}-35\times \mathrm{Mn}$$

   

   (wherein Cr, Mo, Si, Ni, Mn, C, N, and Cu denote wt% of the respective elements.)
2. The high-strength austenitic stainless steel according to claim 1, wherein the number of surface cracks is 0.3 or less per meter (m).
3. The high-strength austenitic stainless steel according to claim 1, wherein a hardness is 190 Hv or more.
4. The high-strength austenitic stainless steel according to claim 1, further comprising, in percent by weight (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.