CN116144895A

CN116144895A - High strength stainless steel, heat treatment process and formed member

Info

Publication number: CN116144895A
Application number: CN202310149350.3A
Authority: CN
Inventors: 易红亮; 邓彪; 王国栋; 熊小川
Original assignee: Ironovation Materials Technology Co Ltd
Current assignee: Ironovation Materials Technology Co Ltd
Priority date: 2018-11-14
Filing date: 2018-11-14
Publication date: 2023-05-23
Also published as: CN109594022A; WO2020098448A1

Abstract

The invention relates to high-strength stainless steel, a heat treatment process and a formed member. The high strength stainless steel comprises, by weight, 0.2-0.35% of C, 0.7-3.50% of Mn, 0.6-2.0% of Si, 11-17% of Cr, 0.20% or less of Nb, 0.20% or less of Ti, and the balance of Fe and impurities. The heat treatment process comprises the following steps: step a: heating the high-strength stainless steel to 1000-1200 ℃ and then preserving heat for 1-3600 s; step b: c, cooling the high-strength stainless steel treated in the step a to below 100 ℃; and step c, heating the high-strength stainless steel treated in the step b to 150-400 ℃, preserving heat for 10-10000 s, and then cooling to room temperature in any cooling mode. The formed member is made of the high strength stainless steel, and the microstructure of the formed member is composed of 5% -30% of retained austenite, 70% -95% of martensite, less than 1% of Nb, ti carbides, and less than 2% of Cr carbides by area.

Description

High strength stainless steel, heat treatment process and formed member

The present application is a divisional application of the invention patent application with the application number 201811350469.2, the application date 2018, 11 and 14, and the invention name of "high-strength stainless steel, heat treatment process and formed member".

Technical Field

The invention relates to high-strength stainless steel, a heat treatment process and a formed member.

Background

The conventional martensitic chromium stainless steel (Cr 13, cr13Mo, etc.) is widely used for cutters, turbine blades, bearings, valves, structural members, wear-resistant members, etc. because of its excellent corrosion resistance and high strength. The hardness of the stainless steel is still high after the quenching and tempering treatment, the toughness is improved, and the toughness and the corrosion resistance are gradually reduced along with the gradual increase of the hardness with the increase of the carbon content.

The most common and cheapest way of high strength stainless steel is to increase the hardness obviously by increasing the carbon content in the components, but the toughness is also reduced sharply, and chromium carbide is difficult to dissolve completely during high temperature solution treatment, so that the corrosion resistance of chromium-poor pitting at the grain boundary is reduced, which is the difficult problem faced by the development of the current high strength stainless steel.

The microstructure of the common high-strength stainless steel is basically of a full martensitic structure, and austenite reduces hardness, so that the austenitic structure is often eliminated, but the effect of improving toughness is ignored. The quenching-tempering distribution (Q & P) process utilizes diffusion enrichment of carbon atoms into residual austenite in the tempering process to stabilize the austenite, thereby obviously improving the strong plasticity of the material. In recent years, many researches on improving the plasticity of martensitic stainless steel by utilizing a quenching-partitioning process are also sequentially published, and a proper amount of metastable austenite at room temperature plays an obvious role in improving the shaping and toughness of the high-strength stainless steel.

For example, in the academic paper "tissue and performance study of 3Cr13 martensitic stainless steel" in 2013 (university of northeast, zhang Chengpan), 3Cr13 martensitic stainless steel is taken as a research object, a plurality of different Q & P heat treatment processes are designed for 3 parameters of quenching temperature, partitioning temperature and partitioning time in the Q & P process, the microstructure of a sample is observed, the rule of influence of the Q & P heat treatment process on the tissue transformation and mechanical properties of the 3Cr13 steel is analyzed by comparing the mechanical properties of the sample, the possibility of the Q & P heat treatment process applied to the traditional martensitic stainless steel is evaluated, the optimal tensile strength of the Q & P heat treatment process reaches 1700MPa, and the elongation rate of the Q & P heat treatment process reaches 18% (round bar sample).

However, the paper is only an attempted application of the Q & P process to 3Cr13 steel, and does not recognize the problem of corrosion resistance degradation caused by a large amount of undissolved chromium carbide at the time of high-temperature solution treatment and the problem of stability of the properties thereof, and at the same time, the quenching temperature has a large influence on the properties, and the process window is narrow, which is difficult to be industrially applied.

Disclosure of Invention

In view of the above problems of the prior art, an object of the present invention is to provide a high strength high toughness stainless steel having a low martensitic transformation start temperature (Ms) so that the quenching temperature can be lowered to room temperature at the time of heat treatment, so that the heat treatment process window is large, while ensuring that a formed member having ultra high toughness is obtained.

Another object of the present invention is to provide a process for heat-treating the high-strength and high-toughness stainless steel, which has a large process window and is simple and reliable in temperature control, and which can ensure that a formed member with ultra-high toughness is obtained.

It is still another object of the present invention to provide a formed member which can significantly improve the plasticity and toughness while ensuring higher strength and hardness.

A first aspect of the present invention relates to a high strength stainless steel comprising, in terms of weight content, 0.2 to 0.35% of C, 0.7 to 3.5% of Mn, 0.6 to 2.0% of Si, 11.0 to 17.0% of Cr, 0.20% or less of Nb, 0.20% or less of Ti, and the balance of Fe and impurities. According to the invention, by adjusting the proportion of Si and Mn content to C, cr, the martensite transformation starting temperature (Ms) is lower than 250 ℃, so that tempering heat treatment can be performed after quenching to room temperature for any time, and the heat treatment process window is enlarged. Meanwhile, cr carbide is basically completely solid-dissolved in solution treatment, a proper amount of residual austenite can be reserved when quenching to room temperature, during the subsequent tempering process, the addition of Si inhibits the formation of cementite, austenite is not decomposed, and the residual austenite can stably exist under the room temperature condition due to the diffusion enrichment of carbon atoms, so that the plasticity and toughness of a formed member are increased.

Preferably, the content of C is 0.2 to 0.3%, the content of Cr is 12.0 to 14%, the content of Nb is not more than 0.10%, and the content of Ti is not more than 0.10%.

In this case, the Ms point can be further controlled within the range of 120 to 200 ℃, thereby obtaining a larger process window and making it easier to obtain a molded member having high toughness.

The high strength stainless steel may further comprise one or more of the group consisting of: mo less than 2.0%; w1.0% or less; v1.0% or less; 3.0% or less of Cu;4.0% or less of Ni; zr 0.4% or less.

A second aspect of the present invention relates to a heat treatment process of high strength stainless steel, the heat treatment process comprising the steps of: step a: heating the high-strength stainless steel of the first aspect to 1000-1200 ℃ and then preserving heat for 1-3600 s; step b: cooling the high-strength stainless steel treated in the step a to below 100 ℃; and c, heating the high-strength stainless steel treated in the step b to 150-400 ℃, preserving heat for 10-10000 s, and then cooling to room temperature in any cooling mode.

According to the invention, the solution treatment temperature in the step a is between 1000 and 1200 ℃, and the carbide of Cr is basically completely dissolved, so that the pitting corrosion resistance of the matrix is improved; the martensite has a lower initial transformation point, a proper amount of residual austenite can be reserved after quenching to room temperature in the step b after solution treatment, and the strength and the hardness are higher but the plasticity and the toughness are extremely poor due to the fact that the internal stress of the quenched martensite is larger and the stability of the residual austenite is poor; and then, through low-temperature tempering treatment in the step c, eliminating internal stress of quenched martensite to obtain a tempered martensite structure so as to improve toughness of the martensite itself, and simultaneously, distributing carbon atoms from supersaturated martensite to residual austenite to stabilize austenite, thereby obtaining metastable plate-shaped or film-shaped austenite with reasonable stability at room temperature, wherein the residual austenite can greatly improve plasticity and toughness of steel materials due to TRIP effect, so that compared with the quenched structure in the step b, the tempered structure after the step c has obviously improved plasticity and toughness under the premise of slightly reducing strength and hardness.

The macroscopic hardness of the high-strength stainless steel treated by the heat treatment process reaches 48 HRC-55 HRC, the impact absorption power of a 2mm deep U-shaped notch standard sample is between 30J and 130J, the yield strength is 1100 MPa-1300 MPa, the tensile strength is 1600 MPa-1900 MPa, the elongation is 9.0% -15%, and the pitting potential in a NaCl solution with the mass fraction of 3.5% reaches more than 270mV, so that the high corrosion resistance, the high strength and the high toughness and the high plasticity are simultaneously realized. Furthermore, according to the present invention, the high strength stainless steel is quenched to room temperature in step b, so that the process window is large and the temperature control is more easily performed.

Preferably, in the step a, the temperature is maintained for 60 to 3600 seconds after heating to 1000 to 1100 ℃, and when the thickness of the high-strength stainless steel is less than 4mm, the temperature maintaining time can be set to 60 to 900 seconds, and when the thickness of the high-strength stainless steel is greater than or equal to 4mm, the temperature maintaining time can be set to 90 to 1200 seconds.

Or preferably, in the step a, the temperature is maintained for 1 to 600 seconds after the rapid heating to 1050 to 1200 ℃ at a heating rate of more than 10 ℃/s. More preferably, in the step a, the temperature is maintained for 1 to 30 seconds after the rapid heating to 1050 to 1200 ℃ at a heating rate of 20 ℃/s or more.

In the present invention, the solution treatment of step a involves long-term and rapid austenitization. When the heat preservation is carried out for a long time, the chromium carbide is basically completely dissolved in solid because of higher solution temperature, and crystal grains are easy to coarsen, so that the lower heating temperature is 1000-1100 ℃. On the other hand, because the growth of the crystal grains requires time, the rapid austenitizing can avoid the growth and coarsening of the crystal grains, reduce the size of the crystal grains, and adopt a higher heating temperature of 1050-1200 ℃ for rapidly realizing the austenitizing. Therefore, for the step a, the crystal grain size can be obviously reduced by heating to 1050-1200 ℃ rapidly at a heating rate of more than 10 ℃/s and preserving heat for 1-600 s. The grain size is further reduced by rapid heating to 1050-1200 ℃ at a heating rate of 20 ℃/s or more for 1-30 s.

Between step a and step b, the high strength stainless steel may be hot stamped into the desired component shape.

In a hot stamping line, the material is first solution austenitized and then hot stamped through a die and then rapidly cooled to room temperature. The process is similar to the steps a and b of the heat treatment process of the invention, and between the steps a and b, the workpiece can be hot stamped into a required component shape after being austenitized at a high temperature in the step a, and then is rapidly cooled in the step b, so that the material performance is not greatly influenced.

Preferably, in step b, the cooling is performed to room temperature at an average cooling rate of 10 ℃/s or more.

The high-strength stainless steel of the present invention has a high hardenability due to the addition of a large amount of alloying elements, but in order to avoid precipitation of Cr carbide, it is preferable to cool the steel to room temperature at an average cooling rate of 10 ℃/s or more in step b.

In the step c, when the thickness of the high-strength stainless steel is less than 4mm, the heat-retaining time may be set to 1 to 30 minutes, and when the thickness of the high-strength stainless steel is 4mm or more, the heat-retaining time may be set to 4 to 60 minutes.

A third aspect of the present invention relates to a formed member made of the high strength stainless steel of the first aspect, the microstructure of the formed member being constituted by 5% to 30% of retained austenite, 70% to 95% of martensite, less than 1% of Nb, ti carbides, and less than 2% of Cr carbides by area.

The above-mentioned formed member can be produced by the heat treatment process of the second aspect, and after the step b, a quenched structure of the high-strength stainless steel is obtained, the microstructure of which is constituted by, in terms of area, 5% -30% of retained austenite, 70% -95% of martensite, less than 1% of Nb, ti carbides, and less than 2% of Cr carbides, the retained austenite being present in the form of laths or films, the average thickness being less than 300 nm.

Preferably, after step b, a quenched structure of the high strength stainless steel is obtained, the microstructure of which consists, in terms of area, of 5% -20% of retained austenite, less than 0.3% of Nb, ti carbides, and less than 0.5% of Cr carbides, the remainder being martensite, the retained austenite being present in the form of laths or films, the thickness being less than 300 nm.

The tempered structure is obtained after the step c, internal stress of quenched martensite is greatly reduced after tempering treatment, toughness is remarkably improved, and meanwhile, carbon atoms are diffused and distributed into residual austenite from supersaturated martensite due to addition of Si element and high Cr element to inhibit precipitation of cementite, so that the residual austenite is more stable, and the residual austenite still exists in a large quantity after tempering. The microstructure of the high-strength stainless steel is still composed of 5% -30% of residual austenite, 70% -95% of martensite, less than 1% of Nb and Ti carbides and less than 2% of Cr carbides by area, wherein the residual austenite exists in a lath or film shape, and the average thickness is less than 300 nanometers.

Preferably, after step c a tempered structure of high strength stainless steel is obtained, the microstructure of which still consists, in terms of area, of 5% -20% of retained austenite, less than 0.3% of Nb, ti carbides, and less than 0.5% of Cr carbides, the remainder being martensite, the retained austenite being present in the form of laths or films, the thickness being less than 300 nm.

The macroscopic hardness of the formed component is between 48HRC and 55HRC, the impact absorption power of a standard sample with a U-shaped notch with the depth of 2mm is between 30J and 130J, and the toughness is obviously improved compared with the stainless steel with the same hardness; the pitting potential in the NaCl solution with the mass fraction of 3.5 percent reaches more than 270 mV; the yield strength is 1100 MPa-1300 MPa, the tensile strength is 1600 MPa-1900 MPa, and the elongation is 9.0% -15%. The high-strength stainless steel has better plasticity than the prior Cr13 martensitic stainless steel products, can be used for traditional high-strength stainless steel products such as medical equipment, springs, valves, general engineering components in steam environment and the like, meets the requirements of automobile collision safety structural members, can be used for hot stamping production lines, and is applied to the automobile collision safety structural members.

By adopting the technical scheme of the invention, the production cost is not greatly different from that of the traditional Cr13 martensitic stainless steel, but the toughness and the corrosion resistance of the steel are obviously improved. The heat treatment process is the same as the traditional stainless steel quenching and tempering process, the original stainless steel production line is not changed, but the microstructure is obviously different from the quenching and tempering process, and the microstructure mainly consists of tempered martensite and metastable austenite, and cementite is not precipitated, so that the plastic toughness is obviously improved on the premise of higher strength.

Drawings

FIG. 1 is a photograph of a microstructure of samples 3 to 7 according to the present invention, wherein the broken line is a crystal boundary.

FIG. 2 is a graph of the polarization of electrochemical corrosion for samples 3-7 according to the present invention.

Detailed Description

The high strength stainless steel of the present invention comprises, in weight: 0.2 to 0.35 percent of C, 0.7 to 3.50 percent of Mn, 0.6 to 2.0 percent of Si, 11.0 to 17.0 percent of Cr, less than or equal to 0.20 percent of Nb and less than or equal to 0.20 percent of Ti. Wherein the content of C is preferably 0.2-0.3%, the content of Cr is preferably 12.0-14%, the content of Nb is preferably 0.10% or less, and the content of Ti is preferably 0.10% or less. One or more of the group consisting of: mo less than 2.0%; w1.0% or less; v1.0% or less; 3.0% or less of Cu;4.0% or less of Ni; zr 0.4% or less. The balance being Fe and impurities.

The content ranges of the respective chemical components are described below.

C:0.20 to 0.35%, preferably 0.2 to 0.3%

In the steel material of the present invention, carbon is the most important strengthening element, and the strength is improved by solid solution strengthening and dispersion precipitation strengthening. Carbon is also an austenite stabilizing element in addition to being a strengthening element. The carbon atoms are smaller, the diffusion rate is high, and the carbon-supersaturated martensite can be quickly diffused into the residual austenite in the low-temperature tempering process, and the carbon-supersaturated martensite is enriched in the residual austenite, so that the stability of the residual austenite is improved. When the carbon content is less than 0.20%, the strength thereof is low; when the carbon content is higher than 0.35%, the temperature at which the carbide is completely dissolved in the matrix is too high, the prior austenite grains are coarse, and the toughness is extremely poor. In order to integrate good strength and toughness, the carbon content is controlled to be 0.20-0.35%. In this range, the carbon element can meet the requirement of stabilizing the retained austenite, and the toughness of the material is increased.

Cr:11.0 to 17.0%, preferably 12.0 to 14%

In the steel material of the present invention, chromium has a main effect of improving corrosion resistance, and it can react with oxygen to form a dense passivation film, thereby inhibiting further corrosion of the substrate. Chromium is also a strong carbide-forming element, which is extremely prone to form Cr with carbon ₂₃ C ₆ And carbide is precipitated at the grain boundary, so that the growth of crystal grains can be effectively inhibited, and meanwhile, the chromium content at the grain boundary can be reduced, and the corrosion resistance is reduced. The solution treatment temperature is appropriately increased to completely dissolve chromium carbide and increase the intergranular corrosion resistance, but on the other hand, the austenite grains are abnormally coarse. Since the chromium carbide in the product structure of the invention is completely dissolved in the matrix, the 17.0% chromium content meets the corrosion resistance requirement, and when the chromium content is too high, the chromium carbide complete solution temperature is increased, and the crystal grains are too large, so the upper limit of the chromium content is 17.0%. On the other hand, when the chromium content is less than 11.0%, a dense passivation film cannot be formed, and the corrosion resistance is drastically reduced, and thus the lower limit of the chromium content is 11.0%.

Si：0.6～2.0％

Silicon is an important element of the steel according to the invention. In the steel material of the present invention, silicon can suppress precipitation of cementite during low-temperature tempering, and promote diffusion of carbon into retained austenite in a free state. When the silicon content is less than 0.6%, cementite precipitation cannot be effectively suppressed; on the other hand, when the silicon content is too high, toughness and plasticity of the steel material can be significantly reduced. Therefore, the silicon content is controlled to be 0.6 to 2.0%.

Mn：0.7～3.50％

Manganese is another important element in the steel of the present invention. Manganese can increase hardenability, while manganese is also an austenite stabilizing element, and can reduce the onset temperature (Ms) of austenite to martensite. Under the premise of determining other alloy components influencing the Ms point, the Ms point can be adjusted only by controlling the manganese content, and further the residual austenite content when quenching to room temperature is controlled. The formed member of the present invention contains 5.0 to 30.0% by area of retained austenite, which is mainly austenite that is not transformed when quenched to room temperature. The manganese content is limited by the contents of carbon, chromium and silicon, and is controlled to be 0.7-3.50% in order to obtain a proper amount of residual austenite at room temperature.

Nb, ti: less than or equal to 0.20 percent, preferably less than or equal to 0.10 percent

Niobium and titanium are both strong carbide forming elements, and have a small solid solution carbide deposition compared to chromium carbides. In the steel of the invention, chromium carbide is completely solid-dissolved during high-temperature solid-solution treatment, while niobium and titanium carbide still exist to inhibit the growth of crystal grains. When niobium and titanium are not present, austenite grain growth is not inhibited during solution treatment, the grains are coarse, and the toughness is reduced. On the other hand, when the content of niobium and titanium is too high, more large-sized niobium and titanium oxycarbide are easily precipitated in the solidification process, and the toughness of the material is affected. Therefore, the content of niobium and titanium is controlled to 0.20% or less.

Mo:2.0% or less; w: less than 1.0%; v: less than 1.0%; zr: less than 0.4%

Molybdenum, tungsten, vanadium and zirconium are all carbide forming elements, which can refine grains and increase strength. In the steel material of the present invention, these elements mainly play a role in solid solution strengthening and precipitation strengthening, and the increase in the content is not significant for the improvement of these roles.

Ni:4.0% or less

Nickel improves the workability and corrosion resistance of the steel, and nickel is also an austenite stabilizing element. In the steel of the present invention, excessive nickel affects the retained austenite content and increases unnecessary costs.

Cu:3.0% or less

Copper can improve strength and corrosion resistance, but when the copper content exceeds 3.0%, it causes deterioration of workability and increases unnecessary costs.

It should be noted that molybdenum, tungsten, vanadium, zirconium, nickel and copper are not essential elements for the steel material of the present invention.

Table 1 below shows the composition of some steels according to the invention, the balance iron and impurities, all in weight percent.

TABLE 1 example Material chemical composition (wt%)

Sample preparation	C	Mn	Si	Cr	Nb	Ti
							1	0.21	3.1	1.55	11.4	-	-
2	0.30	2.1	1.55	11.7	-	-
							3	0.23	2.3	1.55	12.5	0.05	0.035
4	0.27	1.9	0.99	14.0	0.05	0.041

Wherein "-" means substantially free of the element.

A plurality of samples were prepared and heat-treated in each of tables 1, 2, 3 and 4.

Specifically, the heat treatment process comprises the following steps:

a) Carrying out solution treatment: heating to 1000-1200 deg.c, preferably 1000-1100 deg.c, and maintaining for 1-3600 s;

b) Cooling to below 100deg.C;

c) Tempering heat treatment: heating to 150-400 ℃, preserving heat for 10-10000 s, and then cooling to room temperature in any cooling mode.

The heat treatment process parameters of each sample and the mechanical properties of the samples after heat treatment are shown in tables 2, 3 and 4, wherein the temperatures of the stretching, impact and three-point bending test are all room temperature.

TABLE 2 Heat treatment Process, tissue and Strength (impact specimen notch 2mm deep U-notch)

TABLE 3 high plasticity (tensile specimen gauge length 50mm, thickness 2 mm)

TABLE 4 three-point bending property of partial samples (sample thickness 2.0 mm)

Sample preparation	Load force/kN	Bending angle/°
			3-1	24.5	62.3
3-2	23.1	56.9
			3-4	24.9	44.9
3-7	23.0	56.3

In this case, "-" indicates that the treatment was not performed, and "residual austenite amount" indicates the residual austenite content in terms of area.

As shown in the polarization curve of FIG. 2, the pitting potential of the steel of the present invention reaches 270mV. Wherein, the electrolyte solution is 3.5% NaCl physiological saline, the reference electrode is KCl saturated calomel electrode, and the counter electrode is platinum electrode.

From the above description, it is clear that the formed member obtained by the heat treatment method of the present invention has high hardness, high toughness and high intergranular corrosion resistance, and the production cost is not significantly increased, thus having a high industrial value.

While the invention has been described with reference to the preferred embodiments, various modifications can be made thereto without departing from the scope of the invention. In particular, the technical features mentioned in the respective embodiments may be combined in any manner as long as there is no conflict. The present invention is not limited to the specific embodiments disclosed herein, but encompasses all technical solutions falling within the scope of the claims.

Claims

1. A heat treatment process for high strength stainless steel, the heat treatment process comprising the steps of:

step a: heating high-strength stainless steel to 1000-1100 ℃, then preserving heat for 60-3600 seconds or rapidly heating to 1050-1200 ℃ at a heating rate of more than 10 ℃/s, and then preserving heat for 1-600 seconds, wherein the high-strength stainless steel comprises 0.2-0.35% of C, 0.7-3.50% of Mn, 1.55-2.0% of Si, 11-17% of Cr, less than or equal to 0.20% of Nb, less than or equal to 0.20% of Ti, and the balance of Fe and impurities by weight, so that the martensitic transformation starting temperature is lower than 250 ℃;

step b: c, cooling the high-strength stainless steel treated in the step a to room temperature at an average cooling rate of more than 10 ℃/s; and

and c, heating the high-strength stainless steel treated in the step b to 150-400 ℃, preserving heat for 10-10000 s, and then cooling to room temperature in any cooling mode, so that the yield strength of the high-strength stainless steel is 1100-1300 MPa, and the elongation is 9.0-15%.

2. The heat treatment process according to claim 1, wherein in the step a, the heat is rapidly heated to 1050 to 1200 ℃ at a heating rate of 20 ℃/s or more and then is maintained for 1 to 30 seconds.

3. The heat treatment process according to claim 1 or 2, wherein between the step a and the step b, the high-strength stainless steel is hot-stamped into a desired member shape.

4. A stainless steel product, characterized in that the stainless steel product is a high strength stainless steel or a formed member made of the high strength stainless steel, the high strength stainless steel comprising, in weight content, 0.2-0.35% C, 0.7-3.50% Mn, 1.55-2.0% Si, 11-17% Cr, 0.20% or less Nb, 0.20% or less Ti, the balance being Fe and impurities, the microstructure of the stainless steel product consisting, in area, of 5% -30% retained austenite, 70% -95% martensite, less than 1% Nb, ti carbides, and less than 2% Cr carbides, the stainless steel product being made by the heat treatment process of any one of claims 1 to 3, having a yield strength of 1100MPa to 1300MPa, an elongation of 9.0% -15%.

5. The stainless steel product according to claim 4, wherein the C content is 0.2-0.3%, the Cr content is 12.0-14%, the Nb content is 0.10% or less, and the Ti content is 0.10% or less.

6. The stainless steel product of claim 4 or 5, wherein the microstructure is composed of, by area, 5% -20% retained austenite, less than 0.3% Nb, ti carbides, and less than 0.5% Cr carbides, the remainder being martensite.

7. The stainless steel product of claim 4 or 5, wherein the retained austenite is present in the form of laths or films having an average thickness of less than 300 nanometers.

8. The stainless steel product of claim 4 or 5, wherein the microstructure is a tempered microstructure.