High Strength Austenitic Stainless Steel and Production Method Thereof

Abstract

An austenitic stainless steel including in weight % 0-0.4% C, 0-3% Si, 3-20% Mn, 10-30% Cr, 0-4.5% Ni, 0-3% Mo, 0-3% Cu, 0.05-0.5% N, 0-0.5% Nb, 0-0.5% Ti, 0-0.5% V, the balance of Fe and inevitable impurities. The content of at least one of the elements in the group of niobium (Nb), titanium (Ti) or vanadium (V) is more than 0.05% so that the total amount of niobium (Nb), titanium (Ti) and vanadium (V) contents is in the range of 0.05-0.5%. The grain size of the steel is less than 10 micrometer after annealing the cold deformed product and the difference between the yield strengths of the steel measured in transverse and parallel directions to the rolling direction is lower than 5%. Also, a method for producing the austenitic stainless steel.

Description

This invention relates to a high strength austenitic stainless steel exhibiting good combination of strength and elongation and high isotropy of the mechanical properties. The invention relates also to the production method of the steel.

Yield strength of austenitic stainless steel in annealed condition is relatively low. A conventional method for increasing the yield strength of austenitic stainless steels strip is temper rolling, i.e., strengthening of the steel strip by cold-rolling. Temper rolling, however, has an important disadvantage: mechanical properties of temper-rolled steel tend to be highly anisotropic. For instance, yield strength of temper-rolled austenitic stainless steel may be up to 20% higher in transverse direction compared to direction parallel to the rolling direction. The anisotropy is a drawback that, for instance, makes the forming of the austenitic stainless steel more difficult.

Furthermore, temper rolling increases the strength at the expense of elongation. For some austenitic stainless steel grades, remaining elongation and formability after the temper rolling process may be too low.

The refinement of grain size of steel is a well-known and efficient method to increase yield strength of austenitic stainless steels. The method can be utilized instead of temper rolling. Yield strength of the steel increases with decreasing grain size according to the well-known Hall-fetch relationship. The refinement of grain size compared to temper rolling has also the advantage that the anisotropy of the mechanical properties is substantially lower. However, the production of fine grained steel is difficult, because the grain growth is very fast at its initial stages, and thus, the process window, i.e. the allowable time and temperature range to reach a certain small grain size and strength level, may be too small. If the process window is too small, the mechanical properties may vary too much along the steel strip. In the case the target mechanical properties cannot be reached, substantial yield losses may occur.

It is well known that grain growth can be restricted by addition of carbide and nitride forming elements to the austenitic stainless steel. These elements form carbides and nitrides, which limit the grain growth due to so called Zener pinning effect. For instance, the JP publication 2010215953 discloses an austenitic stainless steel containing niobium (Nb), titanium (Ti) or vanadium (V). However, a drawback of this steel is that it contains at least 4.5 weight % nickel (Ni). The JP publication 2014001422 relates to an austenitic stainless steel plate, with an average crystal grain size in the parent phase 10 μm or less, and to its manufacturing method, which steel contains in weight % C: 0.02 to 0.30%, Cr: 10.0 to 25.0%, Ni: 3.5 to 10.0%, Si: 3.0% or less, Mn: 0.5% to 5.0%, N: 0.10 to 0.40%, C+3×N: 0.4% or more and Fe and impurities as the balance, and further optionally Mo: <3%, Cu: <3%, Nb: <0.5%, Ti: <0.1% and V: <1 so that the sum of Nb+Ti+V is 0-1.6%. According to this JP publication 2014001422 when using Nb, Ti and V as the alloying components the nickel content is at the range of 5.0-6.6 weight %. Due to the high and fluctuating nickel price, such austenitic stainless steel is not enough cost efficient. There is market demand for more cost efficient low-nickel high strength austenitic stainless steels.

The object of the present invention is to prevent drawbacks of the prior art and to produce a cost efficient high strength austenitic stainless steel exhibiting small grain size, high strength and isotropic mechanical properties. The invention relates also to the method of processing of the steel, and on the alloying of the steel with carbide and nitride forming elements in order to restrict grain growth and thus improve the processability of the steel. The essential features of the present invention are enlisted in the appended claims.

According to invention an austenitic stainless steel is alloyed with carbide and nitride forming elements, such as niobium (Nb), titanium (Ti) and vanadium (V). These elements for carbide and nitride precipitates effectively restrict grain growth. Thus, during the annealing process carried out to produce a fine grain size for a cold deformed product made of the austenitic stainless steel of the invention, the presence of these carbide precipitates and nitride precipitates enables a larger process window and processability. In order to provide a sufficiently strong effect, more than 0.05 weight % of at least one of the elements in the group of niobium (Nb), titanium (Ti) or vanadium (V) shall be added. In order to keep the austenitic stainless steel cost efficient, the total amount of niobium (Nb), titanium (Ti) and vanadium (V) is lower than 0.5 weight %.

The austenitic stainless steel according to the invention is made cost efficient by the reduction of the nickel content compared to conventional nickel-containing austenitic stainless steels. Therefore, the steel according to the invention does not contain more than 4.5 weight % nickel.

The stainless steel of the invention is an austenitic stainless steel containing in weight % 0-0.4% C, 0-3% Si, 3-20% Mn, 10-30% Cr, 0-4.5% Ni, 0-0.5% Mo, 0-3% Cu, 0.05-0.5% N, 0-0.5% Nb, 0-0.5% Ti, 0-0.5% V. the total amount of the niobium (Nb), titanium (Ti) and vanadium (V) contents being at the range of 0.05-0.5% so that the content of at least one of the elements in the group of niobium (Nb), titanium (Ti) or vanadium (V) is more than 0.05%, the balance of Fe and inevitable impurities, such as phosphorus, sulphur and oxygen. In order to ensure desirable mechanical properties, the grain size after annealing for a cold deformed product is lower than 10 micrometers, preferably lower than 7 micrometers, and more preferably lower than 5 micrometers. The difference between the yield strengths of the stainless steel measured in transverse and parallel directions to the rolling direction is less than 5%.

The high strength austenitic stainless steel according to the invention is produced via the conventional stainless steel process route including among others melting, AOD (Argon Oxygen Decarburization) converter and ladle treatments, continuous casting, hot rolling, cold rolling, annealing and pickling. However, the austenitic stainless steel according to the invention is annealed below the temperature of 1050° C., which temperature is lower than in a conventional production process. Lowering of the annealing temperature slows the grain growth, and thus smaller grain size and higher yield strength can be achieved. However, in order to avoid harmful sensitization phenomenon, the annealing temperature shall be higher than 700° C. The desired annealing temperature range is thus 700-1050° C., and the annealing time is 1-400 seconds, preferably 1-200 seconds. The cold deformation reduction, such as the cold rolling reduction, before the annealing process shall be high enough to enable formation of fine grain size. The deformation reduction degree, such as cold rolling reduction degree shall be at least 50%.

The present invention is described in more details referring to the following drawings, in which

FIG. 1 shows influence of annealing time and temperature on grain size of a reference alloy containing no niobium,

FIG. 2 shows influence of annealing time and temperature on grain size of a test alloy according to the invention containing 0.05% niobium,

FIG. 3 shows influence of annealing time and temperature on grain size of a test alloy according to the invention containing 0.11% niobium,

FIG. 4 shows influence of annealing time and temperature on grain size of a test alloy according to the invention containing 0.28% niobium,

FIG. 5 shows influence of annealing time and temperature on grain size of a test alloy according to the invention containing 0.45% niobium and

FIG. 6 shows the annealing window, i.e., combinations of annealing time and temperature, corresponding to reaching 2-3 micrometer (μm) grain size in test alloys containing no niobium and 0.11% niobium.

Five austenitic test alloys 1-5 with varying amounts of niobium were studied. The chemical compositions of the test alloys are shown in Table 1.

TABLE 1
Chemical compositions of the test alloys 1-5
	C	Si	Mn	P	S	Cr	Ni	Nb	Cu	N
	(%)	(%)	(%)	(%)	(%)	(%)	(%)	(%)	(%)	(%)
1	0.079	0.40	9.0	0.032	0.004	15.2	1.1	0	1.7	0.115
2	0.070	0.30	9.1	0.006	0.005	15.2	1.1	0.05	1.7	0.165
3	0.072	0.28	9.2	0.006	0.005	15.2	1.1	0.11	1.7	0.130
4	0.083	0.28	9.2	0.006	0.004	15.1	1.1	0.28	1.7	0.160
5	0.100	0.30	8.9	0.008	0.006	15.2	1.1	0.45	1.7	0.160

The alloy 1 was produced in full-scale production and the alloys 2-5 in a pilot scale production unit. After melting, casting and hot rolling, the materials were subjected to a 60% cold rolling reduction. Annealing tests were performed on the cold rolled materials at different temperatures and for varying annealing times with a Gleeble 1500 thermomechanical simulator. The heating rate was 200° C./s and the cooling rate 200° C./s down to 400° C. before natural air cooling.

FIGS. 1-5 show the influence of the annealing time and the annealing temperature on the resulting grain size for alloys 1, 2, 3, 4 and 5 with different niobium (Nb) contents, respectively. From the figures it can be observed, that grain growth was substantially restricted by niobium alloying, because the area of for instance under 5 micrometer (μm) in the time-temperature coordinate system of the FIGS. 1-5 will increase in accordance with the increase of the niobium content. Correspondingly, the contour lines corresponding to different grain sizes were shifted to the top right direction, indicating that the allowable range of annealing temperatures and times became larger when niobium (Nb) was added to the austenitic stainless steel according to the invention. Furthermore, it can be observed that relatively large effect was achieved already with 0.11 weight % niobium (Nb) alloying. Further increase in the niobium (Nb) content did not have a strong further effect on the grain growth.

FIG. 6 further demonstrates the beneficial effect of the niobium (Nb) content. FIG. 6 presents the annealing window, i.e., the allowable combinations of the annealing temperature and the annealing time for reaching the grain size of 2-3 micrometers defined based on the experimental results. It is obvious that the annealing window is much larger for the alloy 3 with 0.11 weight % niobium (Nb). For instance, at the temperature range around 900° C. the allowable annealing time range for the alloy 1 without niobium (Nb) was only about 1-10 s, whereas for the alloy 3 with 0.11 weight % niobium (Nb) the allowable annealing time range was 1-100 s. Such difference makes processing of the alloy 3 more feasible, resulting in more uniform product quality and better yield and efficiency.

In order to study the effect of the production method according to the present invention on mechanical properties of stainless steels, the two more alloys were tested. The chemical compositions of these alloys are shown in Table 2.

TABLE 2
Chemical compositions of the test alloys 6 and 7
	C	Si	Mn	P	S	Cr	Ni	Mo	Nb	Cu	N
	(%)	(%)	(%)	(%)	(%)	(%)	(%)	(%)	(%)	(%)	(%)
6	0.77	0.28	9.11	0.007	0.006	16.0	2.0	1.1	0.17	2.0	0.212
7	0.10	0.27	9.14	0.007	0.004	17.0	1.0	0	0.16	2.1	0.241

The alloys 6 and 7 were produced in a pilot scale production unit. As the alloys 1-5 after melting, casting and hot rolling, the alloys 6 and 7 were subjected to a 60% cold rolling reduction. Tensile test samples were cut from the cold rolled sheets in the angles 0°, 45° and 90° to the rolling direction. The tensile test samples were subsequently annealed in a laboratory furnace at temperatures of 900° C. and 950° C. for 300 seconds and water quenched.

Table 3 presents test results of these samples measured in the tensile test directions having the angles of 0°, 45° and 90° to the rolling direction. Also the grain sizes of the materials are shown. It can be observed that the measured yield strength values measured in different directions are close to each other, i.e., the properties do not exhibit high anisotropy. The difference between the yield strengths of the alloys 6 and 7 measured in transverse and parallel directions to the rolling direction is less than 5%. Furthermore, the grain size of the alloys 6 and 7 has remained at low levels despite the rather long annealing time due to the beneficial effect of the Nb alloying, which has resulted in attractive mechanical properties.

TABLE 3
Results for mechanical properties for the alloys 6 and 7
				Yield	Tensile	Elongation
		Grain	Tensile	strength	strength	to fracture
	Annealing	size	test	Rp0.2	Rm	A80 mm
Alloy	temperature	μm	direction	MPa	MPa	%
6	900° C.	4.1	0	522	842	38
6	900° C.	4.1	45	536	816	31
6	900° C.	4.1	90	518	816	39
6	950° C.	3.7	0	478	833	39
6	950° C.	3.7	45	477	802	34
6	950° C.	3.7	90	481	802	40
7	900° C.	2.9	0	566	886	38
7	900° C.	2.9	45	539	859	39
7	900° C.	2.9	90	544	864	33
7	950° C.	3.2	0	523	862	38
7	950° C.	3.2	45	522	837	39
7	950° C.	3.2	90	504	827	25

Claims

1. An austenitic stainless steel, comprising, in weight 0-0.4% C, 0-3% Si, 3-20% Mn, 10-30% Cr, 0-4.5% Ni, 0-3% Mo, 0-3% Cu, 0.05-0.5% N, 0-0.5% Nb, 0-0.5% Ti, 0-0.5% V, the balance Fe and inevitable impurities, wherein the content of at least one of the elements in the group consisting of niobium (Nb), titanium (Ti) and vanadium (V) is more than 0.05% so that the total amount of the niobium (Nb), titanium (Ti) and vanadium (V) contents is in the range of 0.05-0.5%, and after annealing of the cold deformed steel, the grain size is less than 10 micrometers and a difference between the yield strengths of the steel measured in transverse and parallel directions to the rolling direction is less than 5%.

2. The austenitic stainless steel according to claim 1, wherein the grain size of the steel is less than 7 micrometers.

3. The austenitic stainless steel according to claim 1, wherein the steel comprises 0-1.5% molybdenum.

4. The austenitic stainless steel according to claim 1, wherein the steel comprises 0.05-0.30% Nb.

5. The austenitic stainless steel according to claim 1, wherein the steel comprises 0.05-0.30% Ti.

6. The austenitic stainless steel according to claim 1, wherein the steel comprises 0.05-0.30% V.

7. A method for producing an austenitic stainless steel, comprising, in weight %, 0-0.4% C, 0-3% Si, 3-20% Mn, 10-30% Cr, 0-4.5% Ni, 0-3% Mo, 0-3% Cu, 0.05-0.5% N, 0-0.5% Nb, 0-0.5% Ti, 0-0.5% V, the balance of Fe and inevitable impurities, wherein the content of at least one of the elements in the group consisting of niobium (Nb), titanium (Ti) and vanadium (V) is more than 0.05% so that the total amount of niobium (Nb), titanium (Ti) and vanadium (V) is in the range of 0.05-0.5%, the method comprising cold deforming the steel with a reduction degree of at least 50% and annealing the steel, wherein after annealing, the steel has a grain size of less than 10 micrometers, and the difference between the yield strengths of the steel measured in transverse and parallel directions to the rolling direction is lower than 5%.

8. The method according to claim 7, wherein the steel is annealed at a temperature of 700-1050° C. for 1-400 seconds.

9. The method according to the claim 7 or 8, wherein the deformation is cold rolling.

10. The austenitic stainless steel according to claim 1, wherein the grain size of the steel is less than 5 micrometers.

11. The austenitic stainless steel according to claim 1, wherein the steel comprises 0-0.5% molybdenum.

12. The austenitic stainless steel according to claim 1, wherein the steel comprises 0.05-0.20% Nb.

13. The austenitic stainless steel according to claim 1, wherein the steel comprises 0.05-0.20% Ti.

14. The austenitic stainless steel according to claim 1, wherein the steel comprises 0.05-0.20% V.

15. The method according to claim 8, wherein the steel is annealed for 1-200 seconds.