STAINLESS STEEL RESISTANT TO DELAYED CRACKING AND A METHOD FOR ITS PRODUCTION

Abstract

The invention relates to a stainless steel exhibiting transformation-induced plasticity (TRIP) effect resistant to delayed cracking and to the method for producing the stainless steel. The resistance to delayed cracking in the stainless steel is achieved limiting the total hydrogen content of the stainless steel measured by inert gas fusion method below 4 weight ppm, preferably below 3 weight ppm by a heat treatment performed at the temperature range between 100° C. and 700° C. for 0.1-300 hours, preferably at 200-600° C. for 1-100 hours, and more preferably at 250-500° C. for 1-100 hours.

Description

This invention relates to an unstable stainless steel exhibiting transformation-induced plasticity (TRIP) effect, which is highly resistant to so-called delayed cracking phenomenon. The stainless steel is an unstable austenitic stainless steel or an unstable austenitic-ferritic duplex stainless steel. The invention also relates to a method for producing the stainless steel in order to improve the resistance to delayed cracking, which method leads to a lower hydrogen content compared to conventional stainless steel production method.

Low-alloyed and unstable austenitic and duplex stainless steels exhibiting so-called transformation induced plasticity (TRIP) effect due to transformation of unstable austenite phase to strain-induced ε and/or α′-martensite phases during plastic deformation are susceptible to delayed cracking phenomenon, sometimes also called as season cracking. Delayed cracking manifests itself by cracking of formed metal parts immediately after the forming process or after a certain period of time after the forming. According to the prior art, sensitivity of the ε and/or α′-martensite, most probably that of α′-martensite phase, to the hydrogen inherently contained by the steel is the main cause of the cracking phenomenon.

Delayed cracking is a serious problem, as it limits the use of unstable stainless steel in a wide range of application areas, where severe forming operations are needed. Particularly problematic are forming methods which result in high residual tensile stresses in the formed component. Deep drawing is an example of such forming process. Therefore, it is of high importance to be able to control and avoid the delayed cracking phenomenon.

In particular, steels having a low nickel content to improve the cost efficiency of the steel are highly susceptible to delayed cracking. Stainless steels susceptible to the delayed cracking phenomenon cover a wide range of chemical compositions, contents of main alloying elements ranging typically as follows: chromium 15-20 weight %, nickel content 0-8 weight %, manganese content 0-10 weight %, nitrogen content 0-0.3 weight %, carbon content 0-0.1 weight %, copper content 0-3 weight %. Examples of such commercially available steels are, for instance, steel grades AISI 301, AISI 301 LN, AISI 201, AISI 201 LN and AISI 204Cu.

New low-nickel austenitic stainless steels have also been disclosed in several patent documents. Examples of these patent documents include, for instance, EP 0694626, U.S. Pat. No. 3,893,850 and EP 0593158. However, none of these documents reveal means to avoid the delayed cracking.

Delayed cracking is related to the formation and presence of strain-induced martensite phases during the plastic deformation. Thus, delayed cracking can be prevented by careful fine tuning of the chemical composition of the stainless steel so that the formation of martensite during plastic deformation is prevented, i.e., the stainless steel is made stable against the martensite formation. Such an austenitic stainless steel is described in WO publication 2011/138503. However, the problem of such an approach is that the mechanical properties of the steel must be compromised. Formation of strain-induced martensite phases during deformation enhances the tensile strength and elongation due to the TRIP (TRansformation Induced Plasticity) effect, which results in a superior combination of strength and elongation compared to a stable stainless steel. Such property combination is particularly useful in lightweight structures where high crash resistance and energy absorption capacity is required. Stainless steels according to WO publication 2011/138503 are stable and do not exhibit the TRIP effect. Thus, their combination of tensile strength and elongation is inferior to stainless steels exhibiting strain-induced martensite formation and the TRIP effect.

According to the prior art, low-nickel austenitic stainless steels exhibiting the TRIP effect and thus desirable mechanical properties without the susceptibility to delayed cracking cannot be produced.

Conventional austenitic-ferritic duplex stainless steels consist of ferrite phase and stable austenite phase, which does not transform to martensite during plastic deformation. Thus, conventional austenitic-ferritic stainless steels are not susceptible to delayed cracking phenomenon. However, a recently developed novel austenitic-ferritic duplex steel contains unstable austenite phase transforming to strain-induced martensite phase during plastic deformation, i.e. the steel exhibits the TRIP effect. This feature makes the combination of strength and elongation of the novel austenitic-ferritic stainless steel superior compared to conventional austenitic-ferritic stainless steels. However, due to the TRIP effect, the steel is also susceptible to delayed cracking phenomenon, which limits its applicability. The steel is described in the publications WO 2012/143610 and WO 2011/135170, but in these publications there is no means to avoid delayed cracking in such austenitic-ferritic stainless steel exhibiting the TRIP effect.

It is known that the susceptibility of austenitic stainless steel to hydrogen embrittlement can be reduced by controlling the hydrogen content of the steels. However, no means for avoiding delayed cracking of unstable austenitic or austenitic-ferritic duplex stainless steel exhibiting the TRIP effect have been disclosed.

The EP patent application 2 108 710 discloses a method to remove hydrogen from an austenitic stainless steel. However, this EP patent application does cover only austenitic stainless steels containing more than 8% nickel, i.e., does not consider low-nickel austenitic stainless steels or austenitic-ferritic stainless steels. The steels of this EP patent application do not exhibit TRIP effect enhancing mechanical properties. Furthermore, the steels of this EP patent application are known to be practically stable against the strain-induced martensite transformation and thus resistant to delayed cracking. This EP patent application does not provide means for avoiding delayed cracking phenomenon in unstable austenitic stainless steels, but focuses on reduction of fatigue crack growth rate by control of hydrogen content. Furthermore, the method of this EP patent application aims to reduce the hydrogen content to unnecessarily low level, and suggests that the heat treatment should be carried out in a very low pressure (vacuum), which is not practical in industrial scale production.

The JP patent applications 1998-121208 and 2005-298932 relate to an unstable austenitic stainless steel wire. In these JP patent applications in order to avoid so-called longitudinal cracking phenomenon in drawn wire, a heat treatment method to reduce hydrogen content of the steel is proposed. However, these JP patent publications do not consider delayed cracking phenomenon in flat stainless steel products, and do not provide means to avoid the delayed cracking phenomenon in austenitic or austenitic-ferritic stainless steels.

The object of the present invention is to prevent drawbacks of the prior art and to produce a stainless steel exhibiting transformation-induced plasticity (TRIP) effect with improved resistance to delayed cracking by limiting the hydrogen content, which stainless steel is an unstable austenitic stainless steel or an unstable austenitic-ferritic duplex stainless steel. The present invention relates also to a production method of such a stainless steel. The essential features of the present invention are enlisted in the appended claims.

The present invention relates to a stainless steel, an unstable low-nickel austenitic stainless steel or an unstable austenitic-ferritic duplex stainless steel particularly in the form of a flat product, which stainless steel exhibits formation on strain-induced martensite during deformation (TRIP effect) enhancing their mechanical properties, but which is resistant to delayed cracking. The resistance to delayed cracking is achieved limiting the hydrogen content of the steel below 4 weight ppm (parts per million), preferably below 3 weight ppm measured by inert gas fusion method. The steel according to the invention combines desired features, such as low nickel content, excellent combination of strength and elongation due to formation of strain-induced martensite phase during plastic deformation (TRIP effect), and low susceptibility to the delayed cracking phenomenon. In the method of the invention the material is heat treated at the temperature range of 100-700° C. to control the hydrogen content of the stainless steel and to improve the resistance of the stainless steel to delayed cracking. The improved resistance for delayed cracking in the stainless steel of the invention is shown by deep drawing, and in this deep drawing a drawing ratio up to 2.0 or even higher is achieved without occurrence of delayed cracking.

In accordance with one embodiment the stainless steel of the invention is an austenitic stainless steel containing in weight % 0-0.15% C, 0-3% Si, 0-15% Mn, 10-30% Cr, 0-8% Ni, 0-3% Mo, 0-3% Cu, 0-0.5% N, 0-0.5% Nb, 0-0.5% Ti, 0-0.5% V, the balance of Fe and inevitable impurities including hydrogen.

In accordance with another embodiment the stainless steel of the invention is a duplex austenitic ferritic stainless steel which microstructure contains 10-95%, preferably 30-90% ferrite phase and which contains in weight % 0-0.10% C, 0-2% Si, 0-10% Mn, 10-30% Cr, 0-8% Ni, 0-3% Mo, 0-3% Cu, 0-0.4% N, 0-0.5% Nb, 0-0.5% Ti, 0-0.5% V, the balance of Fe and inevitable impurities including hydrogen.

The stainless steel exhibiting transformation-induced plasticity (TRIP) effect according to the invention is advantageously in the form of a flat product such as a plate, a sheet, a strip, a coil.

The stainless steel and its production method according to the invention is based on the reduction and the control of the hydrogen content of the stainless steel by a heat treatment. The heat treatment shall be carried out at a temperature so that the microstructure and other properties of the stainless steel are not significantly affected, but to enable sufficient rapid effusion of hydrogen from the material. The duration of the heat treatment is determined by reaching sufficient reduction in the hydrogen content, so that a desired improvement in cracking resistance is achieved.

According to invention the resistance to delayed cracking is improved by a heat treatment performed at temperature ranging between 100° C. and 700° C. for 0.1-300 hours, preferably at 200-600° C. for 1-100 hours, and more preferably at 250-500 ° C. for 1-100 hours.

The stainless steel according to the invention is produced via the conventional stainless steel process route including among others melting in electric arc furnace, AOD (Argon Oxygen Decarburization) converter and ladle treatments, continuous casting, hot rolling, cold rolling, annealing and pickling. After the conventional processing of the stainless steel to a cold rolled flat product, the material is heat treated according to the invention to control the hydrogen content of the stainless steel and improve the resistance of the steel to delayed cracking. According to the invention, this heat treatment can be carried out in air atmosphere, in atmosphere containing at least partly protective gas or in vacuum. Either continuous or batch process may be used. The stainless steel according to the invention may also be strengthened by temper rolling, i.e. by subjecting the steel to desired cold rolling reduction of 0.1-60% either before or after the heat treatment according to the invention.

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

FIG. 1 shows cup samples deep drawn to drawing ratio of 2.12 from austenitic stainless steel of the invention in cold-rolled, annealed and pickled condition (as supplied) and after heat treatment of cold-rolled, annealed and pickled material at 400° C. for 3 (400° C./3 h), 24 (400° C./24 h) and 72 hours (400° C./72 h) in air atmosphere,

FIG. 2 shows cup samples deep drawn to drawing ratio of 2.0 from austenitic 5 stainless steel of the invention in cold-rolled, annealed and pickled condition (as supplied) and after heat treatment of cold-rolled, annealed and pickled material at 400° C. for 3 (400° C./3 h), 24 (400° C./24 h), and 72 hours (400° C./72 h) in air atmosphere,

FIG. 3 shows cup samples deep drawn from austenitic-ferritic duplex stainless steel of the invention in cold-rolled, annealed and pickled condition (as supplied) and after heat treatment of cold-rolled, annealed and pickled material at 300° C. for 24 (300° C./24 h) and 72 (300° C./72 h), hours and at 400° C. for 24 (400° C./24 h) and 72 hours (400° C./72 h) in air atmosphere,

FIG. 4 shows the influence of heat treatment at 400° C. on total hydrogen content of austenitic stainless steel of the invention measured by inert gas fusion method with Leco TCH 600 analyser, and

FIG. 5 shows the influence of heat treatment at 300° C. and 400° C. on total hydrogen content of the austenitic-ferritic duplex stainless steel of the invention measured by inert gas fusion method with Leco TCH 600 analyser.

The stainless steel of the invention was tested by deep drawing, and a drawing ratio up to 2.0 or even higher is achieved without occurrence of delayed cracking. The drawing ratio is defined as the ratio of the diameters of a circular blank having a varying diameter and a punch with a constant diameter used in the deep drawing operation.

FIG. 1 shows the effect of heat treatment at 400° C. on delayed cracking of austenitic stainless steel of the invention containing 17% chromium, 4% nickel and 7% manganese as the main alloying elements and deep drawn to drawing ratio of 2.12. The as-supplied steel was in cold-rolled, annealed and pickled condition and 0.8 mm thick. The results show that the as-supplied material was susceptible to the cracking, whereas the heat treated steel was completely immune to the cracking.

FIG. 2 shows the effect of the heat treatment at 400° C. on delayed cracking of austenitic stainless steel of the invention containing 15% chromium, 1% nickel, 9% manganese and 2% copper as the main alloying elements and deep drawn to drawing ratio of 2.0. The as-supplied steel was in cold-rolled, annealed and pickled condition and 1.0 mm thick. The results show that the extent of cracking was substantially reduced by the heat treatment in this 1% nickel containing austenitic steel, which is inherently very prone to delayed cracking. Although the cracking could not be fully avoided, the substantially reduced number of cracks in the very severe cup forming operation indicates much improved material performance in practical applications.

FIG. 3 shows the effect of the heat treatment at 300 and 400° C. on delayed cracking of an unstable austenitic-ferritic duplex stainless steel of the invention containing 20% chromium, 1% nickel, 3% manganese and 0.2% nitrogen as the main alloying elements and exhibiting TRIP effect deep drawn to drawing ratio of 2.12. The as-supplied stainless steel was in cold-rolled, annealed and pickled condition and 1.0 mm thick. According to the results, the as-supplied material was susceptible to delayed cracking, whereas the cracking was fully avoided in the material heat treated according to the invention.

FIG. 4 shows the influence of heat treatment at 400° C. on total hydrogen content of austenitic stainless steel of the invention. FIG. 5 shows the influence of heat treatment at 300° C. and 400° C. on total hydrogen content of the austenitic-ferritic duplex stainless steel of the invention. According to the results, the hydrogen content of the material was clearly reduced by the heat treatment. However, it is apparent that still moderate hydrogen content, around 2 ppm, can be accepted. This is an important because reaching very low hydrogen content may require impractically long heat treatment time and use of vacuum, which both are not desired from the industrial applicability viewpoint. When assessing the influence of hydrogen content on delayed cracking phenomenon it is important to bear in mind that the accurate measurement of low hydrogen contents is rather difficult. There is scatter between individual measurements and measurements performed with different instruments can show inconsistency.

According to the invention the delayed cracking resistance of austenitic stainless steels or austenitic-ferritic stainless steel exhibiting the TRIP effect is improved by reducing the hydrogen content to level of about 2 ppm by performing a proper heat treatment for the material. The temperature and the time for the heat treatment are selected so that enough hydrogen is effused from the material. At temperatures lower than 300° C. too slow hydrogen diffusion would lead to impractically long holding times. At temperatures higher than 400° C. there is a risk of precipitation of carbides and nitrides and other undesired changes in the microstructure of the steel. The heat treatment according to the invention was carried out in air atmosphere, which at the studied temperatures leads to oxidation of the surfaces. This can be avoided by carrying out annealing in a protective atmosphere, like in nitrogen or argon, or most preferably, in vacuum. Minimization of hydrogen partial pressure of the atmosphere will also facilitate effusion of the hydrogen from the material and enable reaching lower hydrogen contents.

In industrial scale the heat treatment according to the invention can be realized by utilizing a batch furnace, such as a bell furnace and the gas atmosphere which contains at least partly inert protecting gas such as nitrogen or argon. Also the utilization of a continuous annealing line is possible if the atmosphere, temperature and holding time are properly chosen to enable sufficient removal of hydrogen from the material.

Claims

1. Stainless steel exhibiting transformation-induced plasticity (TRIP) effect resistant to delayed cracking, characterized in that the resistance to delayed cracking for a flat stainless steel product is achieved limiting the total hydrogen content of the steel measured by inert gas fusion method below 4 weight ppm, by a heat treatment performed at the temperature range between 100° C. and 700° C. for 0.1-300 hours, preferably at 200-600° C. for 1-100 hours.

2. Stainless steel exhibiting transformation-induced plasticity (TRIP) effect according to the claim 1, characterized in that the stainless steel is an austenitic stainless steel containing in weight % 0-0.15% C, 0-3% Si, 0-15% Mn, 10-30% Cr, 0-8% Ni, 0-3% Mo, 0-3% Cu, 0-0.5% N, 0-0.5% Nb, 0-0.5% Ti, 0-0.5% V, the balance of Fe and inevitable impurities including hydrogen.

3. Stainless steel exhibiting transformation-induced plasticity (TRIP) effect according to the claim 1, characterized in that the stainless stainless steel is a duplex austenitic-ferritic stainless steel whose microstructure contains 10-95% ferrite phase and which contains in weight % 0-0.10% C, 0-2% Si, 0-10% Mn, 10-30% Cr, 0-8% Ni, 0-3% Mo, 0-3% Cu, 0-0.4% N, 0-0.5% Nb, 0-0.5% Ti, 0-0.5% V, the balance of Fe and inevitable impurities including hydrogen.

4. Stainless steel exhibiting transformation-induced plasticity (TRIP) effect according to claim 1, characterized in that the stainless steel is in the form of a flat product such as a plate, a sheet, a strip, a coil.

5. Stainless steel exhibiting transformation-induced plasticity (TRIP) effect according to claim 1, characterized in that for the stainless steel after deep drawing a drawing ratio up to 2.0 or even higher is achieved without occurrence of delayed cracking.

6. Method for producing a stainless steel exhibiting transformation-induced plasticity (TRIP) effect and resistant to delayed cracking, characterized in that for the resistance to delayed cracking the steel is heat treated at the temperature range between 100C and 700° C. for 0.1-300 hours.

7. Method according to the claim 6, characterized in that the steel is heat treated in a batch furnace, to reduce hydrogen content of the steel and improve the resistance to delayed cracking.

8. Method according to the claim 6, characterized in that the steel is heat treated in a continuous annealing line to reduce hydrogen content and improve the resistance to delayed cracking.

9. Method according to the claim 6, characterized in that the heat treatment is performed in atmosphere containing at least partly protective gas to enhance effusion of hydrogen from the steel.

10. Method according to the claim 6, characterized in that the heat treatment is performed in vacuum to enhance effusion of hydrogen from the steel.

11. Method according to the claim 6, characterized in that the heat treatment is performed in air atmosphere to enhance effusion of hydrogen from the steel.

12. Method according to claim 6, characterized in that the steel is strengthened by cold-rolling before the heat treatment.

13. Method according to claim 6, characterized in that the steel is strengthened by cold-rolling after the heat treatment.