TITLE OF THE INVENTION

HEAT-TREATABLE DUAL PHASE MARTENSITIC-FERRITIC STAINLESS STEEL ALLOY AND ITS PRODUCTION METHOD

FIELD OF THE INVENTION

The present invention in general is related to the field of metallurgy. More specifically the present invention belongs to the heat treatable dual phase martensitic-ferritic stainless steel alloy and its production method, where the alloy exhibits excellent toughness characteristics.

The.present invention can be used for all the applications where strength and toughness are desirable apart from the corrosion resistance.

BACKGROUND OF THE INVENTION

In the current marketing world, stainless steel processing provides remarkable solutions for a wide variety of industries and hundreds of applications where corrosion, oxidation and toughness are the prime factors for their application. The stainless steel family of alloys has enabled the advancement and growth of industries and has allowed society to evolve.

Stainless steels are commonly classified into four groups namely austenitic, martensitic, duplex and ferritic depending on the specific amounts of alloying elements, which control the micro structure of the alloy.

Austenitic stainless steels are the most weldable of the stainless and can be divided rather loosely into three groups: common Chromium-Nickel, Manganese-Chromium-Nickel-Nitrogen and specialty alloys. Austenitic is the most popular stainless steel group and is used for numerous industrial and consumer applications. Austenitic stainless steels have a face-centered cubic structure. Though generally weldable, some grades can be prone to sensitization of the weld heat-affected zone and weld metal hot cracking. The austenitic stainless steels, because of their high chromium and nickel content, are the most corrosion resistant of the stainless group providing moderate mechanical properties. They cannot be hardened by heat treatment, but can be hardened significantly by cold-working.

Martensitic stainless steels are similar in composition to the ferrite group, but contain a balance of Cobalt and Nickel versus Chromium and Molybdenum. Hence, austenite at high temperatures transforms to martensite at low temperatures. Like ferrite, they also have a body-centered cubic crystal structure in the hardened condition. The Carbon content of these hardenable steels affects forming and welding. To obtain useful properties and prevent cracking, the weldable martensitics usually require preheating and post-weld heat treatment.

Duplex stainless steels are developing rapidly today and have a microstructure of approximately equal amounts of ferrite and austenite. Although duplex and some austenitic have similar alloying elements, duplexes have higher yield strength and greater stress corrosion cracking resistance to chloride than austenitic stainless steels.

Ferritic stainless steel consists of Iron-Chromium alloys with body-centered cubic crystal structures. They can have good ductility and formability, but high-temperature strengths are relatively poor when compared to austenitic grades. Present available ferritic stainless steels are characterized by moderate strength and good corrosion resistance. However, these steels are not heat treatable and are characterized by very low toughness. Also, as ferritic stainless steel is essentially single phase material, micro structural control cannot be achieved by heat treatment practices.

This problem has been addressed to some extent by controlling the hot working schedule. However, such controls can only be applied to cold rolled products and cannot be implemented on thick section forgings.

So there is still a need to modify the composition suitably so as to change the microstructure in such a way that the ferritic stainless steel responds to heat treatment so that the strength as well as toughness can be enhanced, thereby making the material suitable for all the applications where strength as well toughness is of prime importance.

OBJECT OF THE INVENTION

One object of the invention is to overcome the disadvantages/drawbacks of the prior art.

Another object of the present invention is to produce a novel dual phase martensitic-ferritic stainless steel alloy which is heat treatable; as a result its strength and toughness can be enhanced.
It is a further object of the present invention to change the microstructure of the ferritic stainless steel alloy by intentionally adding austenite stabilizing elements such as Nitrogen and Nickel in a controlled environment.

SUMMARY OF THE INVENTION

The present invention discloses a novel heat treatable dual phase martensitic-ferritic stainless steel alloy having Carbon(C), Manganese (Mn), Silicon (Si), Chromium (Cr), Nickel (Ni), Nitrogen (N) and balance being Iron (Fe) as the alloying elements.

Further, the present invention discloses that the novel stainless steel alloy is produced by adding austenite stabilizing elements such as Nickel and Nitrogen to the parent ferritic steel to obtain a dual phase ferrite plus austenite structure at high temperature solution treatment of the production process.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Figure 1 shows a flowchart illustrating the production process of heat treatable dual phase martensitic-ferritic stainless steel alloy.

DETAILED DESCRIPTION OF THE INVENTION

The following description with reference to the accompanying flowchart is provided to assist in a comprehensive understanding of exemplary embodiments of the invention as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary.

The terms and words used in the following description and claims are not limited to the bibliographical meanings, but are merely used by the inventor to enable a clear and consistent understanding of the invention. Accordingly, it should be apparent to those skilled in the art that the following description of exemplary embodiments of the present invention are provided for illustration purpose only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.

It is to be understood that the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise.

Features that are described and/or illustrated with respect to one embodiment may . be used in the same way or in a similar way in one or more other embodiments and/or in combination with or instead of the features of the other embodiments. The present invention will be more clearly described with reference to the flowchart showing embodiments thereof.

According to the present invention, the novel heat treatable dual phase martensitic-ferritic stainless steel alloy comprises of Carbon(C), Manganese (Mn), Silicon (Si), Chromium (Cr), Nickel (Ni), Nitrogen (N) and balance being Iron (Fe).

The alloy according to the present invention contains Carbon at most about 0.09% and preferably < 0.09%; Carbon contributes to the high strength, hardness capability and temper resistance provided to the alloy. Too much Carbon adversely affects the toughness provided by the alloy. Therefore, Carbon is restricted to not more than about 0.09%.

At most about 0.3% - 0.5% Manganese is present in this alloy primarily to deoxidize the alloy. It has been found that Manganese also benefits the high strength provided to the alloy.

Silicon benefits the hardenability and temper resistance of this alloy. Therefore the alloy contains at least about 0.35% of Silicon but not more than 0.85%. Too much Silicon adversely affects the hardness, strength, and ductility of the alloy, so in order to avoid such adverse effects Silicon is restricted to not more than about 0.85%.

The alloy according to this invention contains at least about 15.5% of Chromium because Chromium contributes to the good hardenability, high strength, and corrosion resistance provided by the alloy. Preferably, the alloy contains in between 15.5% to 17.5% of Chromium.

The alloy according to this invention contains Nickel ranges from 0.75% to 1.25% and Nitrogen ranges from 0.03% to 0.05%. The main novel process of this invention is introduction of Nickel and Nitrogen to the composition to obtain the desired alloy which is heat treatable. The remaining composition of the alloy is iron (Fe).

According to the present invention, parent ferritic steel is intentionally alloyed with controlled addition of austenite stabilizing elements such as Nickel and Nitrogen so as to obtain a dual phase ferrite plus austenite structure during high temperature solution treatment.

The addition of Nickel and Nitrogen to the base ferritic steel modifies the structure to a dual phase ferrite plus tempered martensitic structure after heat treatment. The high temperature austenite phase being metastable readily transforms to martensitic during subsequent cooling to room temperature. The resultant dual phase martensitic ferritic microstructure not only has high strength by virtue of presence of tempered martensite but also excellent toughness.

According the present invention, the production process involves melting of compatible steel scraps and virgin raw material in an air induction melting furnace. The composition of molten steel is monitored continuously during melting process by drawing samples from the molten metal and analyzing using spectrographic / chemical methods. Nitrogen and Nickel are introduced into the melt in the form of nitrided ferro-alloy and Nickel metal pellets respectively. Once the targeted chemical composition is attained, the molten steel is tapped into ingot moulds to obtain a primary melted ingot. The quality of the primary melted material is further enhanced by subjecting the steel to re-melting especially to Electro Slag Re-melting (ESR). The Nitrogen added is easily retained by the alloy in interstitial sites during primary as well as secondary solidification. During the Electro Slag Re-melting, unwanted and harmful non-metallic sulphide and oxide inclusions are removed and hence aids in retention of Nitrogen uniformly throughout the solidified material. The re-melted ingot is forged to required sizes and subsequently subjected to heat treatment after pre-machining.

The heat treatment comprises of solution treatment and tempering, resulting in a dual phase ferrite - tempered martensitic structure with enhanced properties as presented below.

Typical mechanical properties of the Conventional AISI 430 standard in comparison with the resultant dual phase martensitic-ferritic stainless steel alloy

According to the invention, virgin raw material and steel scrap are processed through induction melting furnace, where the virgin raw material includes Nickel metal pellets, low Carbon ferro-chrome, low Carbon nitrided ferro-chrome, electrolytic Manganese metal and ferro-Silicon.

During the melting process, the resultant composition of molten steel is monitored continuously by drawing samples from the molten metal and analyzing using spectrograph!c / chemical methods to observe accurate quantitative characteristics.

Spectrographic / Chemical methods include an Optical Emission Spectrometer for analyzing the principal alloying elements, and a LECO Carbon-Sulphur Analyzer for analyzing the Carbon and Sulphur. Further, wet chemical analytical methods can also be used as alternate method to analyze principal alloying elements.

Optical Emission is used for direct analysis of solid metal samples and alloying elements; where solid steel sample is placed in the spectrometer. The LECO Carbon-Sulphur Analyzer is used to measure the total Carbon and Sulphur contents in a variety of materials by firstly combusting the sample followed by non-dispersive infrared detection at parts-per-million (ppm) levels.

The Wet chemical analytical methods include gravimetric and titrimetric techniques where the analysis is performed by dissolving the sample and performing a specific chemical reaction with a standardized reagent for each element of interest. Titrimetric procedures are typically based on acid-base reactions or complexing agents for metal ions.

Once the required standards of the molten steel are met after Spectrographic / Chemical analysis, the molten steel is then added with Nitrogen and Nickel in the form of nitrided ferro-alloy and Nickel metal pellets respectively to modify the structure to a dual phase ferrite plus tempered martensite structure. The presence of Nitrogen enhances the corrosion properties and weldability, although these aspects are not a part of this invention. Hence this material can be used for all application where strength and toughness are used for component design apart from corrosion resistance.

The resultant molten steel is solidified by pouring it into the mould; this process is termed as tapping. Molten steel is commonly tapped from melting furnace through a tapping spout in the side of the furnace. This process is carried out by gradually tilting the furnace in order to release the molten steel through the tapping spout into the mould. The tilting is stopped after almost all of the molten steel has flowed out and before the slag begins to flow out.

Next steel obtained from Induction Melting Furnace is further subjected to Electro Slag Re-melting (ESR) during which the quality of steel is further enhanced and also harmful non-metallic inclusions are preferentially removed; hence the solidification structure of the steel is controlled. In addition, the chemical homogeneity of the steel is also enhanced after Electro Slag Re-melting (ESR).

Electro-slag re-melting (ESR), also known as electro-flux re-melting, is a process of re-melting the special steels, super alloys and low alloy steels. The prime attribute of the Electro-Slag Re-melting (ESR) process differentiates it from other hosts of secondary refining processes is its capability to control both solidification structure and chemical homogeneity simultaneously. The Electro-Slag Re-melting (ESR) technology is of interest not only for the production of smaller weight ingots of tool steels and super alloys, but also of heavy forging ingots up to raw ingot weights of 165 tons.

In Electro-Slag Re-melting, the steel in form of electrode is first suspended from a mast assembly which can vertically move at a controlled rate. A reactive slag bath is contained in a water cooled copper crucible. The tip of the electrode is kept dipped in the slag pool which is heated and kept molten by passing a high ampere, low voltage current through the same. The temperature of the slag bath is higher than the melting point of the electrode material. As a result a thin film on the tip of the electrode melts. The liquid metal droplets travel through the slag to the bottom of the water-cooled mould and slowly freeze as the ingot is directionally solidified upwards from the bottom of the mould. The slag pool floats above the refined alloy, continuously floating upwards as the alloy solidifies. The molten metal is cleaned of impurities that chemically react with the slag or otherwise float to the top of the molten pool as the molten droplets pass through the slag.

During the Electro-Slag re-melting process, due to presence of an active slag which is essentially a mixture of CaF2, CaO and AI2O3, Sulphur removal from the liquid metal takes place rapidly. There is usually no change in chemical composition of the alloying elements but minor composition adjustments can be done during Electro-Slag Re-melting process. However, removal of Hydrogen is difficult during ESR melting. * Hydrogen has to be controlled by restricting Hydrogen content of the starting electrode. So Electro-Slag Re-melting process is a special process especially used for re-melting of special steels, super alloys and low alloy steels.

The re-melted steel is forged using localized compressive forces to get the required shape and size of steel.

The forged steel is then subjected to heat treatment after pre-machining in order to obtain the desired mechanical properties. The heat treatment comprises of two steps; solution treatment and tempering treatment. Solution treatment is a high temperature treatment where the steel product (forged or hot rolled bar, ring, etc.) is heated to 1050°C in order to produce a homogenous steel which ensures uniformity in properties at room temperature. The steel after high temperature treatment is air cooled to produce martensitic plus ferritic microstructure. In the tempering treatment, the steel is reheated to around 700°C in order to temper and produce a stable tempered martensitic plus ferritic microstructure. The process of tempering reduces the brittleness and relieves the internal stresses, and also for obtaining pre-determined mechanical properties of the steel.

The heat treatable dual phase martensitic-ferritic stainless steel alloy can be used for all the applications where strength and toughness are desirable apart from corrosion resistance. Since the resultant structure is essentially ferromagnetic in nature, this alloy can be used for making the armature of motors. Because of optimal combination of strength and toughness coupled with good corrosion resistance, heat treatable dual phase martensitic-ferritic stainless steel alloy can also be used in manufacture of structural components for chemical and petro-chemical industries.

The application of the heat treatable dual phase martensitic-ferritic stainless steel alloy include but are not limited to aerospace, power generation, nuclear, defense and other general engineering industries.

We Claim:

1. A heat treatable dual phase martensitic-ferritic stainless steel alloy having excellent toughness and strength in which the basic microstructure contains ferrite and metastable austenite after the solution treatment, where the metastable austenite capable of being transformed to unstable martensite after cooling, which can further transform to tempered martensite after tempering. The resultant micro structure comprises of ferrite and tempered martensite alloy essentially
consisting of, in percentage by weight:
Carbon (C) < 0.09 %
Manganese (Mn) 0.3 - 0.5 %
Silicon (Si) 0.35-0.85%
Chromium (Cr) 15.5 - 17.5 %
Nickel (Ni) 0.75-1.25%
Nitrogen (N) 0.03 - 0.05 %
The balance being Iron (Fe).

2. A production method of heat treatable dual phase martensitic-ferritic stainless steel alloy involves:

a. Processing the virgin raw materials and the compatible steel scrap
through the induction melting route;
b. Monitoring the composition of molten steel continuously during the melting process by drawing samples from the molten metal and analyzing through spectrographic / chemical methods;
c. Introducing the austenite stabilizing elements Nitrogen and Nickel into the melt just before the tapping;
d. Enhancing the quality of the primary melted material by subjecting the steel into re-melting, especially to Electro Slag Re-melting;
e. Forging the re-melted ingot into to required sizes and shapes; and
f. Pre-machining the forged bars and further carrying out heat
treatment.

3. The invention as claimed in claim 2, wherein the Virgin raw material includes Nickel metal pellets, low carbon ferro-chrome, low carbon nitrided ferro-chrome, electrolytic manganese metal and ferro-silicon.

4. The invention as claimed in claim 2, wherein spectrographs / chemical methods include an Optical Emission Spectrometer for analyzing the principal alloying elements, and a LECO Carbon Sulphur Analyzer for analyzing the Carbon and Sulphur, Also, Wet chemical analytical methods are used as alternate method to analyze principal alloying elements.

5. The invention as claimed in claim 2, wherein Nickel is introduced in the form of metal pellets.

6. The invention as claimed in claim 2, wherein Nitrogen is introduced in the form of low Carbon nitrided ferro-chrome.

7. The, invention as claimed in claim 2, wherein the heat treatment involves solution treatment followed by tempering treatment.

8. The invention as claimed in claim 7, wherein the solution treatment, involves:

a. Heating the steel product (forged or hot rolled bar, ring, etc.) at a temperature of around 1050°C in order to produce a homogenous steel to ensure the uniformity in properties at room temperature.

b. Air cooling the steel after high temperature treatment to
produce the martensitic plus ferritic microstructure.

9. The invention as claimed in claim 7, wherein the tempering treatment involves reheating the steel product at a temperature of around 700°C in order to temper and produce a stable tempered martensitic plus ferritic microstructure.

10. The invention as claimed in claim 2, wherein the heat treatment is performed to obtain the desired mechanical properties of the steel.