KR20150074668A - Austenite Stainless Hot-rolled Steel Sheet and Manufacturing Method Thereof - Google Patents

Abstract

The present invention relates to a method to manufacture an austenite-based stainless hot-rolled steel sheet having a shape and a surface quality improved by reducing cracks on the surface of the steel, and the method comprises the steps of: manufacturing a slab including 0.01-0.02 wt% of carbon (C), 0.1-0.3 wt% of silicon (Si), 0.2-0.4 wt% of manganese (Mn), 0.01-0.03 wt% of phosphorus (P), 0.0001-0.0005 wt% of sulfur (S), 22-24 wt% of chromium (Cr), 20-24 wt% of nickel (Ni), 5.0-7.0 wt% of molybdenum (Mo), 0.1-0.4 wt% of nitrogen (N), and the remainder consisting of iron (Fe) and inevitable impurities; re-heating the manufactured slab in the air; roughing rolling the re-heated slab; and conducting strip rolling after roughing rolling. According to the manufacturing method, the austenite-based stainless hot-rolled steel sheet manufactured improves a quality of a hot-rolled steel sheet and has excellent strength and corrosion resistance with a pitting resistance equivalent number of 45-55 as a surface defect occurrence index caused by rapid gain growth can be reduced by re-heating the slab at temperatures of 1250-1290°C.

Description

Austenite Stainless Hot-rolled Steel Sheet and Manufacturing Method Thereof "

The present invention relates to an austenitic stainless steel hot-rolled steel sheet and a method of manufacturing the same, and more particularly, to an austenitic stainless steel hot-rolled steel sheet with reduced surface cracks in steel and improved in shape and surface quality.

Generally, super-austenitic stainless steels contain 22 to 24% by weight of chromium (Cr) relative to the total weight percent of steel. It has a high formal resistance index because it contains 20 to 23 wt% of nickel (Ni), 6.0 to 6.8 wt% of molybdenum (Mo), and 0.21 to 0.32 wt% of nitrogen (N) .

As described above, super-austenitic stainless steels excellent in corrosion resistance have a high content of chromium, molybdenum, and nitrogen, so that hot workability is poor, and surface cracking occurs easily during hot rolling. In order to secure hot workability, sulfur (S) is controlled to 0.0005% (5ppm) or less, and most high alloy stainless steels are rolled by using a steckel mill.

In addition, since the sigma (s) phase generated at the center of the slab during continuous casting is not completely decomposed during the heating operation, additional sigma (s) phase is easily generated when exposed to a temperature range of 900 ° C to 1050 ° C. Therefore, hot rolling control conditions are required to avoid surface cracking during hot rolling and to suppress the generation of additional intermetallic compounds.

Therefore, the production method of super-austenitic stainless steel mainly concentrates on the generation of sigma (s) phase production, and uses a casting method in which the equiaxed rate in the slab is controlled to 20% or less in order to suppress the precipitation on the sigma (s) during continuous casting. However, when super-austenitic stainless steel contains more than 0.5% of nitrogen, it is difficult to produce slab under atmospheric pressure due to the generation of pores, and the hot workability is poor due to high alloy element content. In order to perform the hot rolling of the slab, the content of sulfur is controlled to 0.0005% or less. However, there is a problem that the refining time becomes long, and it is difficult to control the shape of the hot- to be.

SUMMARY OF THE INVENTION An object of the present invention is to provide a method of manufacturing an austenitic stainless steel hot-rolled steel sheet capable of preventing the occurrence of a sigma (s) phase.

Another object of the present invention is to provide a method of manufacturing an austenitic stainless steel hot-rolled steel sheet capable of reducing the amount of upward bending.

Still another object of the present invention is to provide an austenitic stainless steel hot-rolled steel sheet with improved surface quality.

In order to accomplish the above object, the present invention provides a method of manufacturing an austenitic stainless steel hot-rolled steel sheet, comprising: 0.01 to 0.02 carbon (C), 0.1 to 0.3 silicon (Si), and 0.2 to 0.4 manganese (Mn) (P): 0.01 to 0.03, sulfur (S): 0.0001 to 0.0005, chromium (Cr): 22 to 24, nickel (Ni): 20 to 24, molybdenum (Mo) : 0.1 to 0.4, the balance iron (Fe) and unavoidable impurities, and having an official resistance index of 45 to 55, calculated by the formula of resistance index (PREN) =% Cr + 3.3% Mo + 16% Rolling the prepared slab in the air, rough rolling the reheated slab, and subjecting the slab to rough rolling after the rough rolling, wherein the reheating is performed at a temperature of 1250 to 1290 DEG C, A first temperature for heating the upper portion of the slab, and a second temperature for heating the lower portion of the slab, wherein the first temperature is higher than the second temperature.

In addition, the reheating step may be controlled in the range of 0.8 to 1 by volume ratio of water / carbon dioxide in the air.

Also, the first temperature may be 1270 to 1290 ° C, the second temperature may be 1250 to 1270 ° C, and the difference between the first temperature and the second temperature may be 18 to 22 ° C.

The step of rough rolling may be performed at a reduction ratio of 10% or less.

In addition, the finish rolling may be performed at a reduction rate of 15 to 25%.

In order to attain the above object, the austenitic hot-rolled steel sheet according to the present invention comprises 0.01 to 0.02 carbon (C), 0.1 to 0.3 silicon (Si), 0.2 to 0.4 manganese (Mn) ): 0.01 to 0.03, sulfur (S): 0.0001 to 0.0005, chromium (Cr): 22 to 24, nickel (Ni): 20 to 24, molybdenum (Mo) , Residual iron (Fe) and unavoidable impurities, and the formula resistance index calculated by the following formula (1) is 45 to 55.

Official Resistance Index (PREN) =% Cr + 3.3% Mo + 16% N ... ... (One)

The austenitic hot rolled steel sheet may have an upward bending amount of 200 mm or less.

The austenitic hot-rolled steel sheet may have a thickness of 26 to 29 mm.

As described above, according to the method of manufacturing an austenitic stainless steel hot-rolled steel sheet according to the present invention, since the slab is reheated at a temperature of 1250 to 1290 ° C, the surface defect occurrence index due to rapid grain growth can be reduced, The effect of improving the quality of the stainless steel hot-rolled steel sheet is obtained.

Further, according to the method of manufacturing an austenitic stainless steel hot-rolled steel sheet according to the present invention, it is possible to reduce the amount of upward bending by reducing the reduction rate of rough rolling, An excellent effect is obtained.

Further, according to the austenitic stainless steel hot-rolled steel sheet according to the present invention, the strength and the corrosion resistance are improved with an official resistance index of 46 or more.

FIG. 1 is a process diagram for explaining a method of manufacturing an austenitic stainless steel hot-rolled steel sheet according to the present invention,
Fig. 2 is a view showing the upward bending amount of A1 and A2,
Fig. 3 is a diagram showing a deformation resistance curve of A1 and A2. Fig.
4 is a view showing a crack occurrence index according to a reduction rate per pass,
Fig. 5 is a view showing the surface of the hot-rolled steel sheet E of the present invention and the prior art F. Fig.

Hereinafter, the present invention will be described in detail with reference to the embodiments shown in the accompanying drawings, but the present invention is not limited to the illustrated embodiments.

These and other objects and novel features of the present invention will become more apparent from the description of the present specification and the accompanying drawings.

Hereinafter, the steel composition component of the present invention and the reason for its limitation will be described.

The steel composition component of the present invention is composed of carbon, silicon, manganese, phosphorus, boron, sulfur, chromium, nickel, molybdenum, ), Nitrogen (N), iron (Fe), impurities and the like.

The carbon (C) is an essential element for increasing the strength of the hot-rolled steel sheet, and contains 0.01% by weight to 0.02% by weight based on the total weight%. If the carbon content is less than 0.01% by weight, the refining price for producing a high purity product becomes high. If the carbon content is more than 0.02% by weight, the impurities of the material increase and the elongation becomes low.

The silicon (Si) is an element which improves the hardenability of the steel sheet by solid solution strengthening, and it contains 0.1 to 0.3% by weight based on the total weight%. If the content of silicon is less than 0.1% by weight, the refining price becomes high. If the content of silicon is more than 0.3% by weight, the material is hardened and the ductility is deteriorated.

The manganese (Mn) is an impurity inevitably contained in the steel, but is an element effective for solid solution strengthening and transformation strengthening, and contains 0.2 to 0.4% by weight based on the total weight%. If the manganese content is less than 0.2 wt%, the roping and ridging can not be inhibited. If the manganese content is more than 0.4 wt%, the elongation is decreased due to the coarsening of the precipitate.

The phosphorus (P) is an impurity inevitably contained in the steel, but contributes a part to the improvement in strength. The phosphorus is contained in an amount of 0.01 to 0.03% by weight based on the total weight%. If phosphorus is less than 0.01% by weight, the refining price becomes high, and if it exceeds 0.03% by weight, grain boundary corrosion occurs or pickling performance is deteriorated.

The sulfur (S) is an impurity inevitably contained in the steel, but is an element that forms a nonmetallic inclusion such as manganese sulfide (MnS) by binding with manganese and contains 0.0001 to 0.0005 wt% based on the total weight%. If the sulfur content is less than 0.0001% by weight, the strength reduction is affected. If the sulfur content exceeds 0.0005% by weight, emulsion type inclusions such as manganese sulfide are formed during the rolling process, which causes cracks and the like, which hinders hot workability.

The chromium (Cr) is an element which improves hardenability and generates complex carbide to improve hardness, strength, softening resistance and abrasion resistance, and contains 22 to 24% by weight based on the total weight%. If the content of chromium is less than 22 wt%, the effect of improving the hardenability is decreased, so that it is difficult to obtain a uniform cross-sectional hardness, and the generation of molybdenum and complex carbides is reduced, and the resistance to temper softening is reduced and the effect of improving the strength and oxidation resistance is reduced. If it exceeds 24% by weight, the corrosion resistance increases sharply and the workability becomes worse.

The nickel (Ni) is an element which improves toughness and improves stability at high temperature, and contains 20 to 24% by weight based on the total weight%. If the content of nickel is less than 20% by weight, the effect of improving the toughness, which can prevent the decrease in toughness required, can be reduced in accordance with the increase in hardness and strength. If the content exceeds 24% by weight, residual austenite is formed, Moderate deformation can occur, cutting workability is reduced, and it is uneconomical.

The molybdenum (Mo) is an element which generates carbide to improve hardness and strength, and contains 5 to 7% by weight based on the total weight%. If the molybdenum content is less than 5% by weight, the hardening phenomenon is lowered and the hardness and strength at high temperature are decreased. If the content is more than 7% by weight, the effect of molybdenum is reduced and also economical.

The nitrogen (N) serves to refine the crystal grains by forming carbonitride, and it contains 0.1 to 0.4% by weight based on the total weight%. This is because if the nitrogen content is less than 0.1 wt%, the equiaxed crystal ratio of the slab is lowered, and if the nitrogen content is more than 0.4 wt%, the elongation and formability of steel are deteriorated due to an increase in solid nitrogen.

Hydrogen (H), oxygen (O) and the like are further included as the unavoidable impurities.

In addition, except for the above-mentioned composition components, the remainder consists essentially of iron (Fe).

The fact that the remainder consists essentially of iron (Fe) means that even if it contains other trace elements, including unavoidable impurities, it can be included in the scope of the present invention, as long as it does not hinder the effect of the present invention.

Also, the steel composition component has a pitting resistance equivalent number (PREN) of 45 to 55 calculated by the following equation (1).

Hereinafter, a method of manufacturing an austenitic stainless steel hot-rolled steel sheet using the steel composition component as described above will be described.

1 is a process diagram for explaining a method of manufacturing an austenitic stainless steel hot-rolled steel sheet according to the present invention.

As shown in Fig. 1, first, a slab is manufactured (S10).

(C), silicon (Si), manganese (Mn), phosphorus (P), sulfur (S), chromium (Cr), nickel (Ni), molybdenum (Mo), nitrogen (N) (C): 0.01 to 0.02, silicon (Si): 0.1 to 0.3, manganese (Mn): 0.2 to 0.4, phosphorus (P): 0.01 to 0.03, sulfur (Fe), and inevitable impurities (Fe), in the range of 0.0001 to 0.0005, in terms of chromium (Cr), in the range of 22 to 24, in the nickel (Ni), in the molybdenum To produce a slab.

Next, the slab is reheated (S20).

The slab produced in the step S10 is controlled to have a moisture / carbon dioxide volume ratio of 0.8 to 1.0 in the air, which means the flow rate control of the heating furnace fuel, to 1250 to 1290 ° C, preferably 1270 to 1290 ° C, 1290 占 폚 and the lower 1250 to 1270 占 폚, the temperature difference between the upper and lower portions is maintained at 18 to 22 占 폚, more preferably 19 to 21 占 폚. When the moisture / carbon dioxide volume ratio in the air is less than 0.8 or more than 1.0, it is impossible to ensure a desired shape due to a lack of a sufficient degree of heating of the slab, due to an upward bending amount of 300 mm or more during the rough rolling.

If the temperature is less than 1250 DEG C, the upward bending of the slab increases, and it is difficult to secure the throughput and shape. When the temperature exceeds 1290 DEG C, rapid surface growth increases due to rapid grain growth, There arises a problem that the manufacturing cost is increased due to the excessive heating process. Therefore, in the rough rolling stage, the up-bending amount of the slab is 200 mm or less, preferably 100 to 200 mm, so that it is possible to operate the slab. In order to improve the shape and the throughput, And the lower temperature difference is maintained at 18 to 22 占 폚.

Thereafter, the reheated slab is rough-rolled (S30).

In step S20, the reheated slab is hot-rolled at a reduction ratio of 10% or less at a temperature of 1000 to 1200 DEG C, preferably 1100 DEG C, using a tandem mill to form a rough-rolled bar.

Next, the rough rolling bar is subjected to finish rolling (S40).

The rough-rolled bar subjected to the rough rolling in the step S30 is subjected to finish rolling at a reduction rate of 15 to 25% at a temperature of 1000 to 1200 ° C, preferably 1100 ° C. If the reduction rate is less than 15%, upward bending will not occur, but it will be difficult to ensure surface quality that satisfies the normal index of surface cracking index. If the reduction rate exceeds 25%, rough rolling with a high reduction rate per pass The upward bending amount of the bar is greatly increased and the surface cracking index is high, which is not preferable.

After completion of the finishing step, at least one of cooling, hot rolling, winding, annealing and pickling may be further carried out.

When the finishing step as described above is completed, an austenitic stainless steel hot-rolled steel sheet or an austenitic stainless steel hot-rolled coil (S50) having a thickness of 26 to 29 mm, preferably 27 to 28 mm, is obtained. The austenitic stainless steel hot-rolled steel sheet or the austenitic stainless steel hot-rolled coil produced by the above-described manufacturing method can reduce the surface defect occurrence index due to rapid grain growth due to reheating the slab at a temperature of 1250 to 1290 ° C, It can improve the quality of the steel sheet and has an average resistance index of 45 to 55, which is excellent in strength and corrosion resistance.

Test Example 1 Bending amount  Measure

In order to confirm the effect of the temperature setting during the reheating step on the reduction of the bending amount in the steps of rough rolling and finish rolling in the manufacturing method of the present invention, comparative tests were carried out as follows.

First, after the slabs were prepared with Al and A2 containing the components listed in Table 1 below, the upper and lower temperatures were set at 1290 캜 and 1270 캜, respectively, Reheated, and A2 was reheated at 1280 [deg.] C without setting the upper and lower temperatures. After the reheating was completed, the amount of bending was measured in each of rough rolling and finish rolling.

2 is a view showing the amount of upward bending of A1 and A2.

As a result, as shown in Fig. 2, A2 was 445 mm and A1 (hot-rolled steel sheet of the present invention) was 150 mm, and it was confirmed that the bending amount of A1 of the hot-rolled steel sheet of the present invention was smaller than that of A2 .

Table 1 below shows the contents of the composition components of A1 and A2.

River C Si Mn P S Cr Ni Mo N A1 and A2 0.015 0.2 0.3 0.02 0.0003 23.5 22 6.25 0.28

Test example 2. Strength measurement

In order to confirm the effect of improving the strength of the hot-rolled steel sheet produced by the production method of the present invention, the following test was conducted.

First, after the slabs were prepared with A1 and A2 containing the contents of the components listed in Table 1, A1 was set to 1290 DEG C and 1270 DEG C at the upper and lower temperatures, respectively, Reheated, rough-rolled at a reduction ratio of 10% or less, and finish-rolled at a reduction ratio of 15 to 25%. The A2 was reheated at 1280 DEG C and then subjected to rough rolling and finish rolling at a reduction ratio of 25 to 35%. The strain rate was measured by applying compressive load under the temperature of 1100 캜 to A1 and A2 where the finish rolling was completed.

Fig. 3 is a diagram showing a deformation resistance curve of A1 and A2. Fig.

As a result, as shown in FIG. 3, it was confirmed that the hot rolled steel sheet A1 of the present invention exhibited a high deformation resistance of about 160 to 180% as compared with the conventional steel sheet A2.

Test example 3. Crack generation index measurement

In order to confirm the effect of the reduction rate per pass on the surface crack, the following test was carried out.

The reduction rate of 10%, 20%, 30%, and 40% per one pass was applied to the hot-rolled steel sheet of the present invention produced by the manufacturing method of the above example Were measured.

4 is a view showing a crack occurrence index according to a reduction rate per pass.

As a result, as shown in FIG. 4, it was confirmed that the cracking index increased sharply as the reduction rate per pass increased. As a result, if the reduction rate is 30% or more, the surface cracks of the hot-rolled steel sheet are found to be numerous.

Test Example 4. Quality improvement effect

In order to confirm the effect of improving the quality of the hot rolled steel sheet produced by the production method of the present invention and the production method thereof, the following tests were conducted.

First, carbon (C): 0.015, silicon (Si): 0.2, manganese (Mn): 0.3, phosphorus (P): 0.02, sulfur (S): 0.0003, chromium (Cr): 23.5, nickel A to G containing molybdenum (Mo): 6.25, nitrogen (N): 0.28, residual iron (Fe) and unavoidable impurities were applied as shown in Table 2, And surface cracking index were measured.

As a result, as shown in the following Table 2, A is the upward generation and surface cracking index 4, B is the upward generation and surface cracking index 3, C is the upward generation and surface cracking occurrence index is 2 to 3, D is the upward generation Surface cracking occurrence index 2 to 3, E (present invention) shows upward occurrence and surface crack occurrence index 2, F is upward occurrence and surface crack occurrence index 4, G is upward occurrence and surface crack occurrence index is 2 to 3 And it was confirmed that the quality of the hot-rolled steel sheet produced by the manufacturing method of the present invention was remarkably improved.

Fig. 5 is a view showing the surface of the hot-rolled steel sheet E of the present invention and the prior art F. Fig.

As shown in FIG. 5, it was found that F produced according to the conventional manufacturing conditions of Tables 2 and 3 below produced more cracks than E produced by the manufacturing conditions of the present invention.

Table 2 below shows the quality characteristics according to the heating furnace and the rolling conditions in the steel making method.

division Heating furnace and rolling conditions Quality characteristics heating
Temperature Water / carbon dioxide ratio Slab temperature difference
(Upper-lower) Per pass
Reduction rate Slab -> Bending bar Surface crack
Occurrence index A 1270 ~ 1290 ℃ 0.8 to 1.0 20 ℃ 25 to 35% Upward 4
(Very high) B 1270 ~ 1290 ℃ 0.8 to 1.0 20 ℃ 20 to 30% Not raised upward 3
(usually) C 1270 ~ 1290 ℃ 0.8 to 1.0 Not set 15 to 25% Upward 2 to 3
(Low / Medium) D 1270 ~ 1290 ℃ 0.6 to 0.7 20 ℃ 15 to 25% Upward
(Lack of lodging) 2 to 3
(Low / Medium) E
(Invention) 1270 ~ 1290 ℃ 0.8 to 1.0 20 ℃ 15 to 25% Not raised upward 2
(lowness) F 1280 ° C 1.3-1.7 Not set 25-35% Upward 4
(Very high) G 1270 ~ 1290 ℃ 1.3-1.7 Not set 15-25% Upward 2 to 3
(Low / Medium)

Although the present invention has been described in detail with reference to the above embodiments, it is needless to say that the present invention is not limited to the above-described embodiments, and various modifications may be made without departing from the spirit of the present invention.

Claims (8)

(C): 0.01 to 0.02, silicon (Si): 0.1 to 0.3, manganese (Mn): 0.2 to 0.4, phosphorus (P): 0.01 to 0.03, sulfur (S): 0.0001 to 0.0005, (Fe) and unavoidable impurities, and has an average resistance index (Cr) of 22 to 24, a nickel (Ni) of 20 to 24, a molybdenum (Mo) of 5.0 to 7.0, (PREN) =% Cr + 3.3% Mo + 16% N,
Reheating the prepared slab in air,
Subjecting the reheated slab to rough rolling,
And subjecting to rough rolling after rough rolling,
Wherein the reheating step is performed at a first temperature for heating an upper portion of the slab and a second temperature for heating a lower portion of the slab under a temperature of 1250 to 1290 ° C, wherein the first temperature is higher than the second temperature Wherein the austenitic stainless steel hot-rolled steel sheet is produced by the method.
The method according to claim 1,
Wherein the step of reheating is performed such that the volume ratio of water to carbon dioxide in the air is controlled to fall within a range of 0.8 to 1. The austenitic stainless steel hot-
The method according to claim 1,
The first temperature is 1270 to 1290 ° C, the second temperature is 1250 to 1270 ° C,
Wherein the difference between the first temperature and the second temperature is 18 to 22 占 폚.
The method according to claim 1,
Wherein the step of rough rolling is performed at a reduction ratio of 10% or less. ≪ RTI ID = 0.0 > 11. < / RTI >
The method according to claim 1,
Wherein the finishing step is performed at a reduction ratio of 15 to 25%. ≪ RTI ID = 0.0 > 18. < / RTI >
(C): 0.01 to 0.02, silicon (Si): 0.1 to 0.3, manganese (Mn): 0.2 to 0.4, phosphorus (P): 0.01 to 0.03, sulfur (S): 0.0001 to 0.0005, (Fe) and unavoidable impurities, and the following (1) to (3) are satisfied: Cr (1): Cr: 22 to 24; Ni: 20 to 24; molybdenum ) ≪ / RTI > of 45 to 55. The austenitic stainless steel hot-rolled steel sheet according to claim 1,
Official Resistance Index (PREN) =% Cr + 3.3% Mo + 16% N ... ... (One)
The method according to claim 6,
Wherein the austenitic stainless steel hot-rolled steel sheet has an upward bending amount of 200 mm or less.
The method according to claim 6,
The austenitic stainless steel hot-rolled steel sheet has a thickness of 26 to 29 mm.

Images