IRON-BASED ALLOY COMPOSITION, PARTS PRODUCED FROM THIS COMPOSITION AND PRODUCTION METHOD

Abstract

An iron-based alloy composition includes 0.28-0.34% C, max 0.25% Si, max 0.8% Mn, 0.85-0.95% Cr, 1.10-1.50% Ni, 0.41-0.50% Mo, 0.001-0.007% B, 0.002-0.03% Nb, and balanced amount of Fe and inevitable impurities. Moreover, parts with a hardness of at least 480 HB, a tensile strength of at least 1700 MPa, a total elongation of at least 7% and an impact strength of at least 16 J are obtained by the iron-based alloy composition and a production method of the parts.

Description

CROSS REFERENCE TO THE RELATED APPLICATIONS

This application is a continuation application of International Application No. PCT/TR2021/050790, filed on Aug. 11, 2021, which is based upon and claims priority to Turkish Patent Application No. 2020/18497, filed on Nov. 18, 2020, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The invention relates to an iron-based alloy for hot forming process, the parts obtained from this composition and production method.

The invention relates to an armor steel chemical composition and production method to be used for production of armored vehicles, armored parts and armored buildings in order to be protected against various ammunitions. This production method particularly relates to hot forming process and heat treatment of armor parts in three-dimensional geometry.

BACKGROUND

Armor steels are produced via heat treatment after hot rolling process. Ballistic protection feature is achieved after this heat treatment process. Armor steels are defined in MIL-DTL-46100E standard, one of the standards specifying armor steels, and their chemical composition percentage by weight are 0.32% maximum carbon, 0.03% maximum boron, 0.010% maximum sulfur, and 0.02% maximum phosphorus; and it is stated that the manganese, nickel, chromium, and molybdenum elements are not compulsory. Specifically designated armor steels such as Armox 500 and Secure 500 have been produced for years. These alloys include Mn, Cr, Ni, and Mo. Chemical composition and production methods have been studied in different studies metallurgically.

For example; in the patent numbered EP1052296B1, the use of a steel containing 0.15-0.20% carbon, 0.10-0.20% silicon, 0.70-1.70% manganese, <0.02% phosphorus, <0.005% sulfur <0.01% nitrogen, 0.009-0.10% aluminum, 0.50-1.00% chromium, 0.20-0.70% molybdenum 1.00-2.50% nickel, 0.05-0.25% vanadium, 0.0050% boron, and iron and inevitable impurities for the manufacture of an armor plate is described.

For example; an armor steel with ballistic protection feature was developed in the patent numbered RU2236482C1 by providing a different element distribution (0.46-0.54% carbon, 0.17-0.37% silicon, max 0.5% manganese, 2.80-3.20% chromium, 1.50-2.00% nickel, 1.70-2.20% molybdenum, 0.25-0.35% vanadium, 0.01-0.03% aluminum, max 0.012% sulfur, max 0.012% phosphorus). This steel was produced by hot forging, surface course removing, and hot rolling. Heat treatment was applied after hot rolling.

Armor steel with a different chemical composition (0.29-0.38% carbon, 0.15-0.37% silicon, 0.30-0.60% manganese, 1.20-2.00% chromium, 1.20-2.20% nickel, 0.72-0.90% molybdenum, 0.06-0.20% vanadium, 0.01-0.05% aluminum, 0.005-0.020% nitrogen, max 0.50% copper, max 0.05% niobium, max 0.012% sulfur, max 0.015% phosphorus and iron in the rest) was developed in the patent numbered RU2341583C2.

Armor steel with a different chemical composition (0.28-0.40% carbon, 0.80-1.40% silicon, 0.50-0.80% manganese, 0.10-0.70% chromium, 1.50-2.20% nickel, 0.30-0.80% molybdenum, 0.005-0.05% aluminum, max 0.30% copper, max 0.012% sulfur, max 0.015% phosphorus, and iron and 0.8-2.0% molybdenum/carbon in the rest) was developed in the patent numbered RU2520247C1.

Armor steel with a different chemical composition (0.12% <0.20 carbon, 0.8-2.5% manganese, 0.01-0.05% aluminum, ≤1.0% silicon, and preferably <1.0% chromium, <0.009% nickel, 0.015-0.18% titanium, 0.0020-0.0040 boron, and iron and inevitable impurities in the rest) was developed in the patent numbered DE10220476B9. The hardness of this steel is below 400 HB. Its tensile strength is over 800 MPa.

A different chemical composition (0.20-0.40% carbon, 0.05-0.50% silicon, 0.50-1.50% manganese, max 0.015% phosphorus, 0.003-0.10% niobium, 0.0003-0.010% boron, 0.003-0.30% aluminum, 0.0005-0.010% nickel, 0.05-1.50% copper, 0.05-2.00%, nickel, 0.10-2.00% chromium, 0.05-1.50% molybdenum, 0.003-0.20% vanadium, 0.003-0.10% titanium, and Fe and inevitable impurities in the rest) was developed in the patent numbered JPH09118950A. The steel is produced after the plate with relevant composition is heated to 1250° C. or to a lower temperature, hot rolled and cooled, and is reheated to a temperature above Ac3, and cooled at a rate of 1.5° C./s subsequently.

High hardened steel with a new chemical composition (0.25-0.45% carbon, 0.01-1.5%, silicon, 0.35%-3.0% manganese, 0.5-4.0% nickel, 0.01-1.2% aluminum, max 2.0% chromium, max 1.0% molybdenum, max 1.5% copper, max 0.5% vanadium, max 0.2% niobium, max 0.2% titanium, max 0.01% boron, max 0.01% calcium) was developed in the patent numbered EP2789699A1. The hardness of this steel is above 450 HB and the previous austenite grains are oriented in the rolling direction to have a minimum aspect ratio of 1.2.

High-hard steels exemplified above can be used as armor steel. Although having different compositions, these steels are generally produced by massive forming methods such as plate forging and hot rolling. Ballistic features such as high hardness are acquired via subsequent austenitization, rapid cooling, and tempering heat treatment.

Patent numbered U.S. Pat. No. 9,121,088B2, registered by ATI Properties LLC, has completely changed the chemical composition of typical armor steel and provides the steel with ballistic protection via air cooling without the need for quenching and tempering processes after austenitization. Patent numbered U.S. Pat. No. 9,657,363B2 provides ballistic protection by tempering after air cooling. Chemical compositions used in armor steels produced with air cooling are as follows:

U.S. Pat. No. 9,121,088B2 (0.48-0.52% carbon; 0.15-1.00% manganese; 0.15-0.45% silicon; 0.95-1.70% chromium; 3.30-4.30% nickel; 0.35-0.65% molybdenum; 0.0008-0.0030% boron; 0.001-0.015% cerium; 0.001-0.015% lanthanum; max 0.002% sulfur; max 0.015% phosphorus; 0.10% nitrogen; iron and inevitable impurities in the rest)

U.S. Pat. No. 9,657,363B2 (0.18-0.26% carbon; 3.50-4.00% nickel; 1.60-2.00% chromium; max 0.50% molybdenum; 0.80-1.20% manganese; 0.25-0.45% silicon; 0.005% titanium; 0.020% phosphorus; max % 0.005 boron; max 0.003% sulfur; iron; and inevitable impurities in the rest).

As is seen, both of these patents include a high amount of Nickel. Nickel amplifies the hardenability of the steels and facilitates the conversion into martensite. The use of nickel is limited due to its high cost.

All of the above-mentioned steels are produced after a process of hot rolling or plate forging. Forming the steels mentioned in the current technique into a three-dimensional geometry implies great challenges due to their high hardness. In general, parts are cut from these armor steel plates via water jet, laser, plasma, etc. methods and weld bonded afterward. The ballistic protection feature is lost due to heat input in the weld zones after weld bonding. For this reason, additional armor steels are adjoined to the back of these zones. This causes an increase in the weight of the parts. Some parts are cold formed to a certain extent. Cold forming, which increases energy and initial investment costs, is generally avoided as high press forces are required.

In the patent numbered U.S. Pat. No. 9,671,199B1, an innovative method was developed by Premier Body Armor LLC so as to avoid the above-mentioned problems. In this method, armor steel plates are cut in desired geometries, annealed at austenitizing temperature, placed in previously produced forming tools, and formed between the tools by applying press force. The next step involves re-annealing of the three-dimensional product at austenitizing temperature and rapid cooling. Tempering is the last step of the process. Cooling is applied during tool forming in a way that is slower than air cooling. The mentioned method is a long process and increases energy costs, while resolving the aforementioned forming problems. Heating the steel for forming purposes and reheating it to gain ballistic feature can cause many problems. The first of these problems is the decarburization. Double austenitization may cause excessive decarburization on the surface. Double heating and cooling, on the other hand, increases thermal stress. Additionally, heat treatment of the product on the outside of the tool after forming process may cause distortions in the structure and cause deformation. One of the most crucial issues is that double heat treatment increases the production cost.

Hot formed steel is mentioned in the patent numbered EP2341156B1. Its chemical composition includes 0.15-0.35% carbon, 0.8-2.5% manganese, 1.5-2.5% silicon, max 0.4% chromium, max 0.1% aluminum, max 0.3% nickel, 0.0008-0.1% boron, 0.005-0.1% titanium, max 0.1% niobium, and iron and inevitable impurities in the rest. The amount of manganese and silicon is high, while the amount of nickel and chromium is lower than the desired amounts to provide steel with the ballistic feature.

A specific steel type is developed for the production of tube-formed steels in the patent numbered EP1961832B1, and its chemical composition includes high amount of silicon and carbon. Its composition is insufficient in terms of armor steel production. Its chemical composition includes 0.40-0.44% carbon, 1.5-2.2% silicon, 0.3-0.8% manganese, 1.1-1.5% chromium, 0.004-0.015% nitrogen, 0.02-0.04% niobium, 0.01-0.015% vanadium, 0.002-0.004% boron, and iron in the rest, and includes 0.015% phosphorus, max 0.01% sulfur, max 0.2% nickel, max 0.1% copper, max 0.02% tin, max 0.015% aluminum, max 0.01% titanium, max 0.08% molybdenum as conventional impurities.

A steel type that can be coated with nitride before forming was developed in the patent numbered U.S. Pat. No. 9,200,358B2 in order to prevent decarburization during hot forming. Its chemical composition includes 0.22-0.25% carbon; 0.10-0.50% silicon; 1.00-2.50% manganese; max 0.025% phosphorus; max 0.010% sulfur; 0.010-0.060% aluminum; 0.0015-0.005% boron; 0.10-0.80% chromium; 0.020-0.050% titanium; max 0.50% molybdenum; max 0.10% copper; max 0.30% nickel; and iron and post-production elements in the rest. The manganese amount of this steel is high, while the carbon amount is low. Manganese segregation occurs in the production of steels with high manganese content and makes it hard to have a homogenous structure in this regard. For this reason, it is hard to produce armor steel in high manganese content. The carbon amount of this steel is not sufficient to provide required hardness for the armor property. The amount of nickel and chromium is lower than the amounts required for ballistic protection features.

Steel produced by hot forming is mentioned in the patent numbered ES2336967T3. Its chemical composition includes 0.18-0.30% carbon, 0.1-0.7% silicon, 1.0-2.5% manganese, 0.025% phosphorus, max 0.01% sulfur, 0.1-0.8% chromium, 0.1-0.5% molybdenum, 0.02-0.05% titanium, % 0.002-0.005 boron, 0.01-0.06% aluminum. Composition, however, does not include nickel required to provide the expected high hardness and toughness relevance. Its manganese ratio is high.

Hot formed steel is mentioned in the patent numbered DE102005014298B4. Its chemical composition includes 0.2-0.4% carbon, 0.3-0.8% silicon, 1.0-2.5% manganese, max 0.020% phosphorus, max 0.05% sulfur, 0.1-0.5% chromium, 0.1-1.0% molybdenum, max 2% copper, % 0.1-1.0 nickel, 0.001-0.01% molybdenum, 0.001-0.01% boron, max 0.05% aluminum, 0.01-1% tungsten, max 0.005% nitrogen. The amount of silicon is high, while the amount of chromium is low. Therefore, similar to the patent mentioned above, said chemical composition is not suitable for the production of desired armor steel.

The production of hardened steel parts by hot forming is mentioned in the patent numbered DE102008010168B4. Its chemical composition includes 0.35-0.55% carbon, 0.1-2.5% silicon, 0.3-2.5% manganese, max 0.05% phosphorus, max 0.01% sulfur, max 0.08% aluminum, max 0.5% copper, 0.1-2.0% chromium, max 3.0% nickel, max 1.0% molybdenum, max 2.0% cobalt, 0.001-0.005% boron, 0.01-0.08% niobium, max 0.4% vanadium, max 0.02% nitrogen, max 0.2% titanium. This composition is distinctively high in carbon. High carbon amount decreases weldability. It also increases distortion formation due to heat treatment during cooling process.

The production of hardened steel parts by hot forming is mentioned in the patent numbered DE102012109693B4. Its chemical composition includes 0.29-0.32% carbon, 0.35-0.45% silicon, 0.8-0.9% manganese, max 0.015% phosphorus, max 0.003% sulfur, 0.01-0.03% aluminum, 0.8-0.95% chromium, 0.3-0.4% molybdenum, % 1.0-1.65 nickel, max 0.15% copper, max 0.1% titanium, 0.002-0.003% boron, 0.02-0.03% niobium, max 0.012% nitrogen, 0.002-0.55% cobalt. Weldability of the steel is sufficient owing to its carbon amount. Its silicon amount, however, is high. Silicon oxide formation occurs on the surface after heat treatment is applied to the steels with high carbon content at elevated temperatures. These oxides, known as red scale in the industry, cannot be removed after heat treatment. Therefore, it both reduces the commercial value of the steel and decreases the paint coating workability by causing surface defects. Furthermore, these oxides damage the tool itself during the cooling process. Problems related to high amount of silicon in armor steel production under normal conditions can therefore be eliminated while it is difficult to avoid the negative effects of silicon if it is aimed to produce parts by hot forming. Another issue is that the manganese amount of the alloy is above the desired values in order to prevent segregation. Molybdenum amount is also within the limits of adequate hardenability.

The methods mentioned in the abovementioned patents numbered EP1052296B1, RU2236482C1, RU2341583C2, RU2520247C1, DE10220476B9, JPH09118950A, EP2789699A1 are related to developing armor steel. Armor steels produced with relevant methods, however, are produced with conventional production methods. These production methods are hot forging, hot rolling and heat treatment after casting to gain armor feature to the steel.

Methods elaborated in the patents numbered U.S. Pat. No. 9,121,088B2 and U.S. Pat. No. 9,657,363B2 similarly eliminate the heat treatment stage after production. Ballistic protection feature is achieved during air cooling process. Notwithstanding, this process is costly because it includes high-alloy elements.

Armor steels produced with the abovementioned methods are produced as plates with post-heat treatment ballistic protection feature. Armored vehicles are cut and welded to the desired dimensions by the manufacturers, and 3-dimensional parts are produced thereafter. This is because their forming is limited due to their high hardness. Armor feature is lost during welding process due to temperature action. Additional armored parts are adjoined behind the weld zones herewith. This increases the weight. Furthermore, it also prevents armored vehicle or armored equipment designers from freely devising against possible threats. Hereby, even if a design is made in complex geometry, production of the relevant design will be impossible. Regardless, geometrical properties of the armor materials have great importance for protection against threats. For example, parts with different geometrical properties against the incoming threat show different ballistic resistance depending on the counterbalancing angle.

Production of armor steel parts via hot forming might offer a proper solution for the abovementioned problems. Designers will be able to design parts with different geometries. Thus, armored vehicles with improved aerodynamic structure will be able to be produced. In this case, three-dimensional parts of the desired geometry can be produced directly, instead of welding many different armor steels; thereby, vehicle weights can be reduced, and maneuverability can be increased.

Nevertheless, the production of armor steel by hot forming is not an easy process as it appears to be. Patents numbered EP2341156B1, EP1961832B1, U.S. Pat. No. 9,200,358B2, ES2336967T3, DE102005014298B4 relates to part production with hot forming. Alloys developed for hot forming are insufficient in terms of ballistic resistance after forming process. The reason is that alloying elements must be selected meticulously in order to gain armor feature to steel. Armor feature requires a complete optimization of the material's hardness, yielding strength, tensile strength, impact toughness. It is also preferred that the armor steels do not contain segregation and anisotropy. Chemical compositions developed in the patents numbered EP2341156B1, EP1961832B1, U.S. Pat. No. 9,200,358B2, ES2336967T3, DE102005014298B4 are not sufficient to provide the threat resistance properties expected of an armor material with effective thickness.

Armor steel production with hot forming is mentioned in the patent numbered DE102008010168B4. Nevertheless, carbon amount is between 0.35-0.55 in this patent. Welding the armor steels with this amount of carbon with other parts in the vehicle production causes defects in the weld zone. For this reason, this process is not desired by armored vehicle manufacturers. In addition to this, cracks may occur in the tool due to thermal stress during cooling process.

Manganese amount of the developed armor steel in the patent numbered DE102012109693B4 is between 0.8-0.9%. It is known that steels containing manganese in these intervals transform into an inhomogeneous structure due to manganese segregation after hot rolling. Even though manganese increases hardenability, it creates difficulties during production process. Silicon content is high in the mentioned patent and many other patents relate to armor steel (U.S. Pat. No. 9,121,088B2, U.S. Pat. No. 9,657,363B2, DE102005014298B4, ES2336967T3, EP2341156B1). This causes oxide formation on the surface after hot rolling or during heat treatment, depending on the silicon content in the steel. This oxidation cannot be removed by acid and sanding. It, therefore, constitutes a problem for later coating or painting. Fayalite (Fe₂SO₄) is formed during hot rolling of steels containing silicon, and bonds with FeO. Since this is a strong bond, it makes it difficult to remove the oxide and causes the formation of red scale. The scale cannot be removed by traditional oxidizing methods, causing defects in these areas and decreasing the paint coating workability. In a study evaluating the surfaces of the hot rolled steels with different silicon contents via heat treatment, it was observed that if the silicon amount is below 0.25%, Fe₂SO₄ compound does not form, which causes the formation of red scales (Fukagawa, T., Okada, H., & Maehara, Y. (1994). Mechanism of red scale defect formation in Si-added hot-rolled steel sheets. ISIJ international, 34(11), 906-911.)

As a result, due to abovementioned drawbacks and the insufficiency of present solutions in the art, it is necessary to make an improvement in the related technical field.

SUMMARY

The present invention relates to hot-formed armor steel composition and production method that provides the above-mentioned requirements, eliminates all disadvantages and implies certain additional advantages.

The object of the invention is to develop an alloy composition that will make it possible to produce armor steel by hot forming and to demonstrate the production method using this composition. It is aimed that the developed alloy can be able to martensitic transformation at limited cooling rates that can be applied during hot forming, be a material with paint coating workability by minimizing the oxide layers on its surface, and be easily cut into desired geometries before hot forming. Hence, solid armor steel parts are designed in desired geometries, armored vehicles are produced with enhanced aerodynamics, and costs of energy and production are reduced.

The invention involves iron-based alloy composition for obtaining hot-formed armor steel, the use of this composition, hot-formed armor steel parts obtained with the use of this composition, and the production management of these parts for the fulfillment of the objectives explained above.

Composition subject to invention basically includes 0.28-0.34% carbon, max 0.25% silicon, max 0.8% manganese, 0.85-0.95% chromium, 1.10-1.50% nickel, 0.41-0.50% molybdenum, 0.001-0.007% boron, 0.002-0.03% niobium and iron and inevitable impurities in balanced amount.

Embodiments of the invention include one or more elements selected from the group containing trace amounts of phosphorus, sulfur, copper, aluminum, tungsten, cobalt, titanium, oxygen, hydrogen, nitrogen as inevitable impurities.

Armor steel obtained from the combination of the invention has a hardness of at least 480 HB, tensile strength of at least 1700 MPa, total elongation of at least 7% and/or impact strength of 16 J, and includes at least 90% martensite in its microstructure.

The invention is the production method of hot formed armor steel with an aim of achieving the abovementioned objectives, and

involves the foregoing process steps: ingot or slab casting of the alloy including 0.28-0.34% carbon, max 0.25% silicon, max 0.8% manganese, 0.85-0.95% chromium, 1.10-1.50% nickel, 0.41-0.50% molybdenum, 0.001-0.007% boron, 0.002-0.03% niobium, and balanced amounts of iron and inevitable impurities,

hot rolling the slab or ingot into a plate,

plate cooling and cutting,

applying primary heat treatment to cut plates,

Forming the heated plates by pressing in the cooled tool,

applying secondary heat treatment to formed steel parts

An embodiment of the invention involves one or more elements selected from the group containing trace amounts of phosphorus, sulfur, copper, aluminum, tungsten, cobalt, titanium, oxygen, hydrogen, nitrogen (i) as impurities unsolicited alloy in the process steps.

An embodiment of the invention involves (ii) heating the slab or ingot to above 1050° C. for at least 4 hours in the foregoing process step.

An embodiment of the invention involves (iii) the cooling of the plates down to 2° C./s or slower, microstructure of ferrite+perlite, bainite, or a mixture of these phases and obtaining a plate with a hardness scale below 300 HB, a heating process of the plate above 300° C. and transforming its microstructure into tempered martensite, provided that the process is performed faster without cooling.

An embodiment of the invention involves (iv) primary heat treatment of plates cut in the foregoing process step by heating them to a temperature below 1000° C. and above AC3 for at least 10 minutes.

An embodiment of the invention involves (v) forming the plates by cooling them to a temperature of 300° C. or less at a rate of over 4° C./s in the foregoing process step.

An embodiment of the invention involves (vi) tempering of steel parts formed in the foregoing process step by applying a secondary heat treatment at a temperature of 250° C. or less, and obtaining at least 90% martensitic microstructure.

An embodiment of the invention is (vi) the tempering of the formed steel parts at a temperature between 140° C.-200° C. by applying a secondary heat treatment for 2-8 hours and obtaining at least 90% martensitic microstructure in the foregoing process step.

In an embodiment of the invention, (vi) three-dimensional steel parts obtained after the foregoing process step have a hardness of at least 480 HB, a tensile strength of at least 1700 MPa, a total elongation of at least 7% and/or an impact strength of at least 16 J.

An embodiment of the invention includes (vi) cleaning the surface of the formed steel part before or after the processing step.

The structural and characteristic features and all advantages of the invention will be understood more precisely by means of the detailed explanations and figures hereinbelow; accordingly, the evaluation should be in line with this detailed explanation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1F The phase transformation graphs obtained in the method according to the invention during cooling process at the cooling rates of the C-001 material produced as hot rolled plate (Ms: Martensite starting temperature, Mf: Martensite final temperature, Bs: Bainite starting temperature)

FIG. 2 Electron microscope image of the C-001 sample after hot forming and tempering processes

DETAILED DESCRIPTION OF THE EMBODIMENTS

Alloy composition and production method for the production of hot-formed armor steel according to the invention is explained in this detailed description in order to better understand the subject with its preferred embodiments, and in a way that does not have any restrictive effect.

The invention is predicated on the development of an iron-based alloy composition for obtaining three-dimensional parts from armor steel hard enough to endure hot forming and which has a ballistic protection feature, and the optimization of the hot forming method using this composition.

Iron-based alloy composition according to the invention basically includes;

0.28-0.34% carbon (C),

max 0.25% silicon (Si),

max 0.8% manganese (Mn),

0.85-0.95% chromium (Cr),

1.10-1.50% nickel (Ni),

0.41-0.50% molibden (Mo),

0.001-0.007% boron (B),

0.002-0.03% niobium (Nb), and

balanced amounts of iron and inevitable impurities.

According to one embodiment of the invention, the iron-based alloy composition also may contain one or more unenviable elements selected from the group containing phosphorus (P), sulfur (S), copper (Cu), aluminum (Al), tungsten (W), cobalt (Co), titanium (Ti), oxygen (O), hydrogen (H), nitrogen (N).

Within the scope of the invention, the alloy formed with the chemical composition described above is transformed from liquid steel form to solid steel form by ingot casting or continuous casting, thus, casting process is performed and the steel is casted into ingot or slab in three-dimensional armor steel hot forming method. The slab or ingot is heated above 1050° C.—preferably to 1200° C.—for at least 4 hours and then hot rolled into a plate.

Hot rolled plate is cooled down to 2° C./s or slower; therefore, its microstructure includes ferrite+pearlite, bainite, or a mixture of these phases. The plate is heated above 300° C. and its microstructure is transformed into tempered martensite, provided that the process is performed faster without cooling process. In conclusion, if the desired slow cooling rates are achieved, a plate with a hardness scale of 300 HB is obtained.

Hot rolled plate is cut into desired forms by means of CNC, flame, water jet, laser, saw, etc., and the cut plates are subjected to primary heat treatment for at least 10 minutes by heating to a temperature below 1000° C., and above Ac₃. The heated plate, thereafter, is placed in a water-cooled tool in a press while it is still hot.

The hot plate is shaped by cooling it to a temperature of 300° C. or below so that a martensitic microstructure is obtained at a speed above 4° C./s by means of the force applied by the press and the water-cooled tool in the press.

Three-dimensionally formed steel part is removed from the tool and tempered by applying a secondary heat treatment at a temperature of 250° C. or below, and tempered martensitic microstructure is obtained. The part surface is cleaned by sandblasting, polishing, etc.

Three-dimensional steel parts produced by means of the method described above provide hardness of at least 480 HB, tensile strength of at least 1700 MPa, total elongation of at least 7% and/or impact strength of at least 16 J, and can be used as armored parts with ballistic resistance.

By means of the recommended method according to the invention, the plate in the chemical composition developed for the steel alloy is produced with a microstructure consisting of ferrite+perlite, bainite or a mixture of these phases, after a cooling process of 2° C./s or slower upon the hot rolling. The microstructure of plate is transformed into tempered martensite when performed faster by heating the plate up to a temperature of 300° C. and above, without the need for a cooling process. The plate produced in this way has a hardness scale below 300 HB and does not yet have a ballistic resistance, making it easier to be cut in the desired form. The microstructure of the plates cut in desired sizes is transformed into martensite by means of being heated to a temperature below 1000° C. and austenitized and then placed in the tool for the forming process, and obtaining a three-dimensional form in the tool with the help of a press and cooling the tool from outside with water. The austenite begins to transform into martensite at a temperature slightly above 300° C. The part with the desired form can be removed from the tool below this temperature. The surface of the three-dimensional steel part can be flattened via sanding and polishing, after a cooling process at room temperature. Surface cleaning process refers to the cleaning of the surface up to a depth of 100 microns. The part is re-heated and tempered by applying heat treatment at a temperature below 250° C. for at least 1 hour at the final phase of the method. Surface cleaning can be applied after tempering as well.

In the recommended method according to the invention, the chemical composition of the developed steel has been designed in such a way that its microstructure can transform into martensite at cooling rates of 4° C./s or higher during cooling, and thus martensitic structure can be obtained at relatively low cooling rates observed in thick-sectioned parts. Produced steel includes at least 90% martensite microstructurally after hot forming and press hardening processes. The carbon amount in the designed iron-based alloy composition is between 0.28%-0.34%, providing a high weldability in the produced steel. Its manganese amount is below 0.8% in order to prevent segregation. The chromium amount is limited between 0.85%-0.95% so as to delay the perlite formation during cooling process, and to ensure high hardenability. The nickel amount is optimized between 1.10%-1.50% and the molybdenum content between 0.41% and 0.50% in order to increase the hardening depth and hardenability features to provide ballistic properties. Silicon content is limited below 0.25% in order to prevent the formation of silicon oxide in hot rolling and heat treatment processes.

Welding the armor steels including high amount of carbon with other parts in the vehicle production causes defects in the weld zone. For this reason, this process is not desired by armored vehicle manufacturers. Furthermore, cracks may occur due to thermal stress that occurs during tool cooling in high carbon steels and stresses caused by Bain strains occurring with martensite transformation. The present invention provides the necessary armor feature with a carbon amount of 0.28%-0.34% to eliminate these problems.

It is known that steels with high manganese content convert into an inhomogeneous structure due to manganese segregation after hot rolling. Even though manganese intensifies the hardenability of the steel, crack formation in continuous casting during steel production causes hardships such as routing, etc. For this reason, manganese amount was limited below 0.8% in the present invention. Furthermore, high silicon content causes silicon-based oxide formation on the surface of the steel after hot rolling or during heat treatment. This oxidation cannot be removed by acid and sanding. It, therefore, constitutes a problem for later coating or painting. Fayalite (Fe₂SO₄) is formed during hot rolling of steels containing silicon, and bonds with FeO. Since this is a strong bond, it makes it difficult to remove the oxide and causes the formation of red scale. The scale cannot be removed by traditional oxide removal methods, causing defects in these areas and decreasing the paint coating workability. It is known in the present art that there is no Fe₂SO₄ compound formation, which causes red scale formation when below 0.25%. Hard oxide layers formed on the surface during hot forming damage and reduce the lifetime of the tool, besides the paint coating workability. Therefore, the silicon amount of the alloy developed in compliance with hot forming was kept below 0.25%. Hereby, its surface properties are also improved. However, silicon affects solid solution hardening positively and raises hardenability. Silicon can be used to prevent carbide formation in steels. Decreasing of hardenability dramatically affects the ballistic properties by enabling formation of unwanted phases during cooling. Hence, 0.41% molybdenum alloying is also employed to increase hardening. Hence, the alloy according to the invention is unique on this sense.

Iron-based alloy composition developed according to the invention has a structure that can provide ballistic properties with hot forming and tool cooling, and subsequent tempering. Due to low amount of silicon, no red scale formation is observed on the surface of the steel, and the oxidation layer can be, thereby, easily removed. Therefore, it is a material with high paint coating workability. The hardness scale of the final product is at least 480 HB, its tensile strength is typically at least 1700 MPa, its total elongation value is at least 7%, and the notched impact toughness value is at least 16 J at the room temperature.

Tests and analyzes were carried out with hot formed armor steel part samples obtained within the scope of the invention, and comparative results were recorded and presented in tables below.

Compositions of the hot formed armor steel samples developed within the scope of the invention are presented in Table 1. Phase transformation at different cooling rates for C-001 alloy is demonstrated in FIGS. 1A-1F. It is evident that bainite is formed prior to martensite transformation when the alloy is cooled down to 2° C./s or slower. Therefore, before hot forming process, the material should be cooled down to 2° C./s or slower after hot rolling in order to be easily cut to the desired dimensions. The hardness scale of the material produced by cooling in this manner is below 300 HB. Mechanical properties of armor steels produced by heating at 900° C. for 10 minutes and via cooling in the tool and subsequent tempering are presented in Table 2. It is evident that the hardness scale above 500 HV are obtained after hot forming and tempering processes. Ballistic performance values of different alloys produced by hot forming and tempering are given in Table 3 and Table 4 after being tested with different ammunitions. The scanning electron microscope image of the C-001 sample after hot forming and tempering processes is given in FIG. 2. It is evident that a martensitic structure has been obtained. It was observed that the ballistic performance of the H009 alloy—whose composition is presented in Table 1—is not sufficient, even though it has the highest impact toughness value. C and Mo contents of the H009 alloy are lower than the other alloys, while its Mn content is slightly higher. The H010 alloy, which is very similar to the H009 alloy and has only a slightly higher C amount, has shown a high ballistic performance against the 7.62×51 Nato Ball ammunition, though having a lower thickness when compared to H009 alloy. Therefore, H009 alloy is excluded from the patent scope. H010 alloy with a slightly higher carbon amount did not display the desired performance on the ballistic tests performed with the 5.56×45 mm SS109 ammunition. Hence, the C-001 alloy is developed by increasing the Cr, Mo, B and Nb amounts of this alloy and decreasing the Mn amount albeit. This alloy is produced via vacuum melting method, differing from other alloys being produced by melting under Ar protection under atmospheric conditions. Therefore, it is ensured that amounts of N, O, H elements and the relevant inclusions are reduced in steel, and the casting cavities are largely eliminated. The ballistic performance of the C-001 alloy produced in different thicknesses are demonstrated in the Table 4 after ballistic tests performed with 7.62×51 Nato Ball ammunition. It is evident that this alloy provides ballistic strength even at lower thicknesses compared to other alloys. The ballistic performance of the C-001 alloy is demonstrated in the Table 5 after ballistic tests performed with 5.56×45 SS109 ammunition, and the desired protection level is reached. For this reason, C-001 alloy is a patented chemical composition. Nonetheless, H-009.5 alloy is also patented as it provides the desired protection level in 7 mm thickness. Due to similar production methods, it is regarded that the N, O, H amounts of the H009.5 alloy are similar to the H010 alloy. C-001 alloy is produced by vacuum melting method. Therefore, the N, O, H amounts are lower. The restricted amounts of N, O, H elements are, thus, considered to be ineffective regarding the ballistic performance. However, due to phenomena such as hydrogen embrittlement and grain boundary corrosion that may arise problems over time, it is aimed to keep the amount of these elements low even if no effect on ballistic performance is observed. No significant effect of Al, S, P elements have been observed against ballistic performance. In the steels produced at the present time, boron addition is limited to 10-20 ppm (Sharma, M., Ortlepp, I., & Bleck, W. (2019). Boron in Heat-Treatable Steels: A Review steel research international, 90(11), 1900133). Nevertheless, it is possible to achieve a higher amount of boron addition with necessary precautions. There are different reasons for this limitation. Considering the manufacturability, in the boron addition phase of the steel, nitrogen forming elements should be bound with nitrogen and B₂O₃ formation should be prevented by keeping the amount of BN and oxygen low. This problem was tried to be avoided with the addition of high amount of Nb (246 ppm) and Al (160 ppm) in C-001 steel and limiting the N (59 ppm) and O (45 ppm) amounts. Otherwise, it is inevitable for free B atoms to form BN. Boron carbides can be formed in boron added steels to some extend even when deemed protection is applied, but their stability is low and they dissolve at temperatures above 800° C. Excessive boron addition (>80 ppm) causes hot shortness. It is possible to work with lower amounts of boron in terms of manufacturability. Furthermore, boron element can be diverged from steel in the phenomenon called “boron fade” when being heated above 900° C. The hardenability is also affected in such a case. This patent relates to thick-sectioned parts; thus, this risk may occur at near-surface, just as decarbonization. According to thermodynamic calculations, up to 41.9 ppm B can dissolve in austenite. This amount may rise up to 97.4 ppm in delta ferrite during solidification. Interstitially or substitutionally dissolving boron generally segregates to grain boundaries and near regions. It increases the hardenability by delaying ferrite or perlite nucleation in these regions. Regarding the hardenability, the boron being in dissolved form or in fine precipitates is considered suitable for a high hardenability. Hardenability decreases with excessive boron and coarse boron-based carbide formation. Nevertheless; in the present case, boron carbides are regarded as dissolved due to quenching process performed in approximately 1000° C. There are publications in terms of toughness regarding that the boron element at the grain boundary decreases the toughness, while there are also papers claiming that it has no effect at all Toughness is generally dependent on steel alloy and is highly related to the toughness level expected from steel. For example, if there is no Al, Ti, Nb and similar elements that can form nitride, BN precipitates can cause austenite grain coarsening and reduce toughness. Therefore, the minimum-maximum amounts of these elements have been determined based on the alloys demonstrated in the Table 1. W, Co, Cu, Ti, Al, S, P elements were observed in trace amounts in 4 different alloys, and no effect on ballistic resistance have been observed. The relevant values per alloy are also given in Table 1.

TABLE 1
Hot formed armor steel compositions measured by OES before hot stamping process
	Amount of Element
Sample	(by weight - %)
Code	C	Si	Mn	P	S	Cr	Mo	Ni	W	Co	Cu
H009	0.26	0.11	0.88	0.080	0.020	0.79	0.39	1.27	0.025	0.01	0.045
H009.5	0.30	0.22	0.73	0.080	0.020	0.88	0.41	1.38	0.034	<0.005	0.083
H010	0.34	0.19	0.78	0.070	0.020	0.77	0.37	1.25	0.018	<0.005	0.08
C-001	0.33	0.18	0.70	0.007	0.001	0.91	0.43	1.25	0.029	<0.005	0.064
		Amount of Element
	Sample	(by weight - %)
	Code	Ti	Al	B	Nb	N	O	H	Fe
	H009	0.003	0.0045	0.0013	0.0141	n/a	n/a	n/a	Rest
	H009.5	<0.002	0.0060	0.0023	0.0095	n/a	n/a	n/a	Rest
	H010	0.003	0.0038	0.0012	0.0156	0.0114	0.0144	0.000058	Rest
	C-001	<0.002	0.0160	0.0064	0.0246	0.0059	0.0045	0.000088	Rest

TABLE 2
Mechanical properties of developed armor steels after hot forming and tempering.
	Impact	Tensile	Total	Micro
	Toughness	Strength	Elongation	Hardness
	[J]	[MPa]	[%]	[HV]
Sample		Std		Std		Std		Std
Code*	Mean	Deviation	Mean	Deviation	Mean	Deviation	Mean	Deviation
H009	64	14	1606	4.5	10.9	1.2	512	14
H010	41.1	0.70	2074	23.1	11.30	1.2	584	21
C-001	41.8	0.2	1838.6	37.1	11.1	3.0	553	11

TABLE 3
Ballistic test results of the H009, H009.5 and H010
materials produced by hot forming and tempering.
		Measured	Measured	7.62 mm × 51	5.56 mm × 45
		Sample	Sample	Nato Ball	Nato SS109
	Sample	Size	Thickness	Speed	Speed	Penetration
Firing	Code	[mm]	[mm]	[m/s]	[m/s]	after Test
1	H010	100 × 200	6.0	845.74	—	NONE
2				—	912.82	NONE
3				—	959.76	OBSERVED
1	H009.5	100 × 200	7.0	842.71	—	NONE
2				—	904.53	NONE
3				—	956.99	NONE
1	H009	100 × 195	6.4	839.72	—	OBSERVED
2				857.02	—	OBSERVED
3				847.08	—	OBSERVED

TABLE 4
Ballistic test results of C-001 material produced by hot forming and
tempering in different thickness.
Sample	Thickness	Rate of	7.62 mm × 51 Nato Ball
code	[mm]	fire	Speed [m/s]	Test result
C-001	5.7	1	840	No
				penetration
		2	843	No
				penetration
C-001	6.0	1	830	No
				penetration

TABLE 5
Ballistic test results of the C-001 material produced
by hot forming and tempering in 6.5 mm thickness.
		Measured	Measured	7.62 mm × 51	5.56 mm × 45
		Sample	Sample	Nato Ball	Nato SS109
	Sample	Size	Thickness	Speed	Speed	Penetration
Firing	Code	[mm]	[mm]	[m/s]	[m/s]	after Test
1	C-001	100 × 200	6.5		965	NONE
2				—	956	NONE
3				—	955	NONE
4					962	NONE
5					963	NONE
6					971	OBSERVED
7				842		NONE

Claims

1. An iron-based alloy composition developed for producing a hot formed armor steel, comprising 0.28-0.34% carbon, max 0.25% silicon, max 0.8% manganese, 0.85-0.95% chromium, 1.10-1.50% nickel, 0.41-0.50% molybdenum, 0.001-0.007% boron, 0.002-0.03% niobium by weight and a balanced amount of iron and inevitable impurities.

2. The iron-based alloy composition according to claim 1, comprising one or more elements selected from the group containing trace amounts of phosphorus, sulfur, copper, aluminum, tungsten, cobalt, titanium, oxygen, hydrogen, and nitrogen.

3. A hot formed armor steel, comprising 0.28-0.34% carbon, max 0.25% silicon, max 0.8% manganese, 0.85-0.95% chromium, 1.10-1.50% nickel, 0.41-0.50% molybdenum, 0.001-0.007% boron, 0.002-0.03% niobium by weight and a balanced amount of iron and inevitable impurities in a composition of the hot formed steel.

4. The hot formed armor steel according to claim 3, comprising one or more elements selected from the group containing trace amounts of phosphorus, sulfur, copper, aluminum, tungsten, cobalt, titanium, oxygen, hydrogen, and nitrogen.

5. The hot formed armor steel according to claim 3, wherein the hot formed armor steel has a hardness of at least 480 HB, a tensile strength of at least 1700 MPa, a total elongation of at least 7%, and/or an impact strength of at least 16 J.

6. The hot formed armor steel according to claim 3, comprising at least 90% martensite in a microstructure of the hot formed armor steel.

7. A hot formed armor steel production method, comprising the following steps:

i. an ingot or slab casting of an alloy comprising 0.28-0.34% carbon, max 0.25% silicon, max 0.8% manganese, 0.85-0.95% chromium, 1.10-1.50% nickel, 0.41-0.50% molybdenum, 0.001-0.007% boron, 0.002-0.03% niobium by weight, and balanced amounts of iron and inevitable impurities to obtain an ingot or a slab,

ii. hot rolling the slab or the ingot into a plate,

iii. plate cooling and cutting,

iv. applying a primary heat treatment to cut the plate,

v. forming the plate by pressing in a cooled tool,

vi. applying a secondary heat treatment to formed steel parts.

8. The hot formed armor steel production method according to claim 7, comprising one or more elements selected from the group containing trace amounts of phosphorus, sulfur, copper, aluminum, tungsten, cobalt, titanium, oxygen, hydrogen, and nitrogen in the step i.

9. The hot formed armor steel production method according to claim 7, comprising heating the slab or the ingot to above 1050° C. for at least 4 hours.

10. The hot formed armor steel production method according to claim 7, comprising cooling the plate down to 2° C./s or slower, a microstructure of ferrite+perlite, bainite, or a mixture of phases and obtaining a plate with a hardness scale below 300 HB, a heating process of the plate above 300° C. and transforming a microstructure of the plate into a tempered martensite, provided that the heating process is performed faster without cooling in the step iii.

11. The hot formed armor steel production method according to claim 7, wherein the primary heat treatment for cutting the plate is performed by heating the plate to a temperature below 1000° C. and above AC3 for at least 10 minutes in the step iv.

12. The hot formed armor steel production method according to claim 7, comprising forming the plate by cooling the plate to a temperature of 300° C. or less at a rate of over 4° C./s in the step v.

13. The hot formed armor steel production method according to claim 7, comprising tempering of the formed steel parts in the step vi by applying the secondary heat treatment at a temperature of 250° C. or less, and obtaining at least 90% martensitic microstructure.

14. The hot formed armor steel production method according to claim 7, comprising tempering of the formed steel parts at a temperature between 140° C.-200° C. by applying the secondary heat treatment for 2-8 hours and obtaining at least 90% martensitic microstructure in the step vi.

15. The hot formed armor steel production method according to claim 7, wherein three-dimensional steel parts obtained after the step vi has a hardness of at least 480 HB, a tensile strength of at least 1700 MPa, a total elongation of at least 7%, and/or an impact strength of at least 16 J.

16. The hot formed armor steel production method according to claim 7, comprising cleaning surfaces of the formed steel parts before or after the step vi.