WO2023101642A2 - High-strength, low-alloy steel composition and related production method - Google Patents

High-strength, low-alloy steel composition and related production method Download PDF

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WO2023101642A2
WO2023101642A2 PCT/TR2022/051238 TR2022051238W WO2023101642A2 WO 2023101642 A2 WO2023101642 A2 WO 2023101642A2 TR 2022051238 W TR2022051238 W TR 2022051238W WO 2023101642 A2 WO2023101642 A2 WO 2023101642A2
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weight
ratio
strength
alloyed
micro
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PCT/TR2022/051238
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WO2023101642A3 (en
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Fulya EYÇİN
Osman ÇULHA
Ferit SİMSAROĞLU
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Ti̇rsan Kardan Sanayi̇ Ve Ti̇caret Anoni̇m Şi̇rketi̇
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Application filed by Ti̇rsan Kardan Sanayi̇ Ve Ti̇caret Anoni̇m Şi̇rketi̇ filed Critical Ti̇rsan Kardan Sanayi̇ Ve Ti̇caret Anoni̇m Şi̇rketi̇
Publication of WO2023101642A2 publication Critical patent/WO2023101642A2/en
Publication of WO2023101642A3 publication Critical patent/WO2023101642A3/en

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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/02—Hardening articles or materials formed by forging or rolling, with no further heating beyond that required for the formation
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D7/00—Modifying the physical properties of iron or steel by deformation
    • C21D7/13—Modifying the physical properties of iron or steel by deformation by hot working
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/0075—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for rods of limited length
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/002—Bainite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/004—Dispersions; Precipitations
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/009—Pearlite

Definitions

  • the present invention relates to a high strength, low alloy steel developed for use as long product raw material in all areas where hot forging processes can be applied.
  • the invention particularly relates to a micro alloy steel composition
  • a micro alloy steel composition comprising; carbon (C) in a ratio of 0.430-0.470% by weight, silicon (Si) in a ratio of 0.250-0.300% maximum by weight, manganese (Mn) in a ratio of 1.600-1.650% by weight, chromium (Cr) in a ratio of 0-0.120% maximum by weight, molybdenum (Mo) in a ratio of 0-0.050% by weight, nickel (Ni) in a ratio of 0-0.120% by weight, aluminum (Al) in a ratio of 0-0.015% by weight, vanadium (V) in a ratio of 0,035-0,045% by weight, phosphorous (P) in a ratio of 0-0.010% by weight, sulfur (S) in a ratio of 0-0.090% by weight, copper (Cu) in a ratio of 0-0.0150 by weight and nitrogen (N) in 70-140 ppm steel composition (S) consisting 5-7%
  • Steels constitute the most important group of engineering materials. There are continuous developments in the process and physical metallurgy of steels to meet all kinds of demands and changes.
  • micro-alloy steels have been seen as one of the most important metallurgical achievements. It can be said that this development is the result of a clear understanding of the structure-feature relationships in low carbon steels. Certainly, the final product is the result of a successful combination of physical, mechanical and process metallurgy. Micro-alloyed steels are successfully replacing mild steels as basic construction materials.
  • micro-alloyed steels For high-strength low-alloy, micro-alloyed steels, the total alloy content usually do not exceed 2%. While most of these alloying elements are niobium (Nb), titanium (Ti), and vanadium (V), nanometer-sized precipitates formed by these alloying elements with carbon (C) and nitrogen (N) atoms provides high yield strength to steel. In addition, mechanical properties such as high corrosion resistance, ductility and toughness increase the use of micro-alloyed steels. Specific development characteristics of micro alloy steels are steels with perlite and without perlite. Features such as formability, thickness, and weldability etc. are significantly increased by substantially reducing the carbon ratio. Said features are generally required in the production of high strength and lightweight parts by forming the same.
  • the yield limit of these steels can only reach 500 N/mm 2 with controlled rolling with the grain refinement and hardening effects of micro alloy elements as aluminum (Al), niobium (Nb), titanium (Ti), vanadium (V).
  • Carbides, nitrides and carbonitrides formed by micro alloy elements remain insoluble in the austenite phase in case the dissolving temperatures are not exceeded during hot forming processes. These insoluble hard structures provide to obtain both a small-grain steel structure and an increase in the toughness of the material by preventing austenite grain growth.
  • the invention which is the subject of the patent document number EP1929053B1 found in the literature, is related to the production process of a multi-phase microstructured steel part.
  • Chemical composition of the steel subject to the invention 0.01 % ⁇ C ⁇ 0.50, 50% ⁇ Mn ⁇ 3.0%, Cr ⁇ 1.5%, 0.01% ⁇ Si ⁇ 3.0%, 0.005% ⁇ AI ⁇ 3.0%, Mo ⁇ 1.0%, P ⁇ 0.10%, Ti ⁇ 0.20%, V ⁇ 0.1% and optionally Ni ⁇ 2.0%, Cu ⁇ 2.0% , S ⁇ 0.05%, Nb ⁇ O.15%, the rest contains iron and impurities from the preparation process.
  • the multiphase microstructure of the steel contains 25 to 75% superficial ferrite and 25 to 75% superficial martenist and/or bainite.
  • Patent application numbered EP1228253B1 relates to 0.25-0.35 % C, 24-28 % Cr, 10-15% Ni, 3-6% Mn, 1.75-2.50% Nb, 0.50-0.70% steel composition by weight.
  • the invention also relates to a method for making said compositions and valves manufactured from said compositions or using said method and exhibiting excellent mechanical strength and oxidation resistance at temperatures between 800 and 900°C.
  • Patent application numbered EP1070153B1 relates to a steel composition containing; 0.6- 0.65% carbon (C) by weight, max. 0.4% silicon (Si), 0.6-0.9% manganese (Mn), 0.03-0.07% phosphorus (P), 0.07-0.11% sulfur (S), max. 0.5% chromium (Cr), max. 0.1% molybdenum (Mo), max. 0.5% nickel (Ni), max. 0.5% copper (Cu), max. 0.5% aluminum (Al), max. 0.03% nitrogen (N), vanadium and iron.
  • the present invention relates to a high-strength, low-alloy steel composition and related production method that meets the abovementioned requirements, eliminates all disadvantages and brings some additional advantages.
  • the aim of the invention is to introduce high strength, low alloy steel developed for use in all areas that can be used in hot forging processes as a long product raw material.
  • the aim of the invention is to obtain a steel with a yield strength of 660-680 Mpa, a tensile strength of 900-950 MPa and a hardness of 280-290 HV after controlled forging.
  • the aim of the invention is to activate the strength increase mechanism by grain refinement and to increase the strength with toughness, thanks to the use of vanadium as the sole carbide builder, aluminum in deoxidation, nitrogen in terms of forming nitrides in controlled cooling with casting and forming temperatures.
  • the aim of the invention is to provide grain refining elements to reveal their effect by the fact that, in the primary cooling, the heat transfer coefficient is 80-120 W/m 2 K for 100-130. in the secondary cooling, the heat transfer coefficient is 40-60 W/m 2 K for 1200-1500 seconds required for forging temperature and cooling environment after forging.
  • the aim of the invention is to apply the mechanism of solid solution hardening and grain size reduction and strength increase together.
  • An aim of the invention is to present an alloy composition and a controlled cooling process that together increase the strength mechanism, toughness and strength.
  • An aim of the invention is to provide an alloy composition that does not adversely affect weldability.
  • An aim of the invention is to provide the increase in strength with the ferrite and bainite phase structure.
  • the invention is a high-strength, low-alloyed micro-alloyed steel developed for use in all areas that can be used in hot forging processes as a long product raw material.
  • carbon carbon
  • Si silicon
  • Mn manganese
  • Mn manganese
  • Cr chromium
  • Mo molybdenum
  • Ni nickel
  • Al aluminum
  • V vanadium
  • Sn tin
  • micro alloy steel has 600-615 MPa yield strength, 800-860 MPa tensile strength, 250-265 HV hardness and 0.650-0.665 Ceq equivalent carbon value.
  • micro alloy steel has 600-615 MPa yield strength, 900-950 MPa tensile strength, 280-290 HV hardness and 0.704-0.806 Ceq equivalent carbon value.
  • the invention is a method for the production of micro alloy steel comprising of the following process steps; • obtaining the steel produced by micro alloying in the form of billets by continuous casting method,
  • said forging temperature is at 1200°C
  • the heat transfer coefficient of the primary cooling after forging is 80-120 W/m 2 K for 100-130 seconds
  • the heat transfer coefficient is 40-60 W/m2K for 1200-1500 seconds for secondary cooling.
  • the invention is a high-strength, low-alloyed micro-alloyed steel developed for use in all areas that can be used in hot forging processes as a long product raw material, characterized in that, it comprises of the following; carbon (C) in a ratio of 0.430-0.470% by weight, silicon (Si) in a ratio of 0.250-0.300% maximum by weight, manganese (Mn) in a ratio of 1.600-1.650% by weight, chromium (Cr) in a ratio of 0-0.120% maximum by weight, molybdenum (Mo) in a ratio of 0-0.050% by weight, nickel (Ni) in a ratio of 0-0.120% by weight, aluminum (Al) in a ratio of 0-0.015% by weight, vanadium (V) in a ratio of 0,035- 0,045% by weight, phosphorous (P) in a ratio of 0-0.010% by weight, sulfur (S) in a ratio of 0- 0.090% by weight, copper (
  • alloy elements are determined so as to obtain the inventive formulation of the high-strength low-alloy steel.
  • the amount and diversity of alloy elements in the steel composition are important parameters for developing the mechanical features.
  • intermediate atoms such as carbon (C), nitrogen (N) and vanadium (V) are used so as to make nitride, carbide and carbonitride, in order to make different strength increasing mechanisms to occur in combination.
  • TTT isothermal transformation diagram
  • OCT continuous cooling transformation diagram
  • hot forging methodology is formed.
  • the temperature and deformation rates in the hot forging process are rearranged with respect to the original alloy.
  • a cooling process is applied.
  • the cooling regime for the target microstructure is determined according to the TTT (isothermal conversion diagram) and CCT (continuous cooling conversion diagram) diagrams. Cooling is provided in such a way that the ferrite and bainite phase ratios can be changed with the TTT and CCT-based controlled two-stage functional cooling method.
  • Martensite and bainite conversion in the iron-carbon phase diagram has an important position in the hardening mechanism of the alloy, in the iron-carbon phase diagram.
  • cooling rates are significantly higher than balance conditions due to process conditions in industrial steel production.
  • TTT The iron-carbon phase diagram used in determining the phase conversion with the increase of the cooling rate is not used. The most important reason for this is that said diagram is formed under very slow cooling conditions. Thus, diagrams which are called TTT and indicate the change of phase conversion due to temperature and time are used at high cooling rates. In fast-cooled steels, the aspects as when the austenite will begin conversion, how long the conversion will be completed and what products will be formed as a result are determined by means of the isothermal transformation diagrams. Thus, TTT diagrams are preferred in determining the phase transformations that will occur in the alloy depending on the supercooling conditions as a function of temperature and time.
  • CCT continuous cooling conversion curves
  • vanadium is used as the sole carbide builder and aluminum, which is also used in deoxidation.
  • Nitrogen is in the steel composition in terms of forming nitrides at casting and forming temperatures and controlled cooling.
  • Production method of the high-strength, low-alloyed steel composition is as follows
  • composition of steel produced by micro-alloying is obtained in the form of billets by continuous casting method by using electric arc furnace, ladle furnace, vacuum furnace and tundish dipping closed ceramic tube respectively, • Converting the produced billets into cylindrical semi-finished products in the round long group by hot rolling,
  • Said forging temperature used in the inventive production method occurs at 1200°C
  • the heat transfer coefficient of the primary cooling after forging is 80-120 W/m2K for 100-130 seconds
  • the heat transfer coefficient of secondary cooling occurs 40-60 W/m 2 K for 1200-1500 seconds under atmospheric conditions.
  • Convection is a type of heat transfer between a solid surface and an adjacent fluid (liquid or gas) in motion. The faster the fluid movement, the greater the convection heat transfer. If the bulk or mass fluid movement disappears, the heat transfer between the solid surface and the adjacent fluid occurs only by the random movement of molecules, that is, by conduction.
  • Heat transfer coefficient (K) is defined as the amount of heat carried from unit surface area, unit temperature difference and unit time. This coefficient varies depending on various factors. Some of these elements are the material and roughness of the fluid contact surface, flow pattern, flow rate, hydraulic diameter, viscosity and density of the fluid.
  • the heat convection coefficient varies according to the heat convection type.
  • thermal insulation materials The most decisive feature in the selection of thermal insulation materials is the heat transfer coefficient. Because the lower the thermal conductivity coefficient, the higher the thermal insulation resistance of the systems. Knowing the thermal properties of materials is very important for achieving optimum performance where the material is used. Many measurement methods developed for this purpose have been used for many years. The thermal properties of materials (thermal conductivity coefficient, specific heat, thermal permeability) can be measured with existing methods. In particular, there are complexes in the micro and macro level of the materials developed recently, and it becomes difficult to make accurate measurements under this condition. The smaller the thermal conductivity coefficient is, the less the heat loss. The effective use of this energy is available with thermal insulation. The selection of thermal insulation material or the manufacture of a new material is very important in this regard. In order to make calculations after the manufacture of a new material or material selection, the heat transmission coefficient of that material must be known.
  • the alloy composition used does not have a negative effect on weldability.
  • Equivalent carbon value is 0.650-0.665 Ceq.
  • the long product mentioned includes all hot rolled, square, round and flat steel products.
  • the equivalent carbon value of the inventive micro-alloy steel is 0.650-0.665 Ceq. Carbon equivalent is a measure that defines weldability, and calculations are performed with the amount of alloying elements in the steel. The values of the specified alloys are equivalent to the existing steel to be substituted and provide an advantage in terms of creating higher strength. The formula used in calculating Ceq within the scope of the invention is given below.

Abstract

The invention particularly relates to a steel composition comprising of the following; carbon (C) in a ratio of 0.430-0.470% by weight, silicon (Si) in a ratio of 0.250-0.300% maximum by weight, manganese (Mn) in a ratio of 1.600-1.650% by weight, chromium (Cr) in a ratio of 0- 0.120% maximum by weight, molybdenum (Mo) in a ratio of 0-0.050% by weight, nickel (Ni) in a ratio of 0-0.120% by weight, aluminum (Al) in a ratio of 0-0.015% by weight, vanadium (V) in a ratio of 0,035-0,045% by weight, phosphorous (P) in a ratio of 0-0.010% by weight, sulfur (S) in a ratio of 0-0.090% by weight, copper (Cu) in a ratio of 0-0.0150 by weight and nitrogen (N) in 70-140 ppm steel composition (S) consisting 5-7% ferrite phase, 54-58% pearlite phase, 30-39% bainite phase with 660-680 MPa yield strength, 900-950 MPa tensile strength.

Description

High-strength, low-alloy steel composition and related production method
Field of the Invention
The present invention relates to a high strength, low alloy steel developed for use as long product raw material in all areas where hot forging processes can be applied.
The invention particularly relates to a micro alloy steel composition comprising; carbon (C) in a ratio of 0.430-0.470% by weight, silicon (Si) in a ratio of 0.250-0.300% maximum by weight, manganese (Mn) in a ratio of 1.600-1.650% by weight, chromium (Cr) in a ratio of 0-0.120% maximum by weight, molybdenum (Mo) in a ratio of 0-0.050% by weight, nickel (Ni) in a ratio of 0-0.120% by weight, aluminum (Al) in a ratio of 0-0.015% by weight, vanadium (V) in a ratio of 0,035-0,045% by weight, phosphorous (P) in a ratio of 0-0.010% by weight, sulfur (S) in a ratio of 0-0.090% by weight, copper (Cu) in a ratio of 0-0.0150 by weight and nitrogen (N) in 70-140 ppm steel composition (S) consisting 5-7% ferrite phase, 54-58% pearlite phase, 30-39% bainite phase with 660-680 MPa yield strength, 900-950 MPa tensile strength.
State of the Art
Steels constitute the most important group of engineering materials. There are continuous developments in the process and physical metallurgy of steels to meet all kinds of demands and changes.
In recent years, the development of micro-alloy steels has been seen as one of the most important metallurgical achievements. It can be said that this development is the result of a clear understanding of the structure-feature relationships in low carbon steels. Certainly, the final product is the result of a successful combination of physical, mechanical and process metallurgy. Micro-alloyed steels are successfully replacing mild steels as basic construction materials.
For high-strength low-alloy, micro-alloyed steels, the total alloy content usually do not exceed 2%. While most of these alloying elements are niobium (Nb), titanium (Ti), and vanadium (V), nanometer-sized precipitates formed by these alloying elements with carbon (C) and nitrogen (N) atoms provides high yield strength to steel. In addition, mechanical properties such as high corrosion resistance, ductility and toughness increase the use of micro-alloyed steels. Specific development characteristics of micro alloy steels are steels with perlite and without perlite. Features such as formability, thickness, and weldability etc. are significantly increased by substantially reducing the carbon ratio. Said features are generally required in the production of high strength and lightweight parts by forming the same. Although the carbon ratio is low, the yield limit of these steels can only reach 500 N/mm2 with controlled rolling with the grain refinement and hardening effects of micro alloy elements as aluminum (Al), niobium (Nb), titanium (Ti), vanadium (V).
Niobium (Nb), titanium (Ti), vanadium (V) and aluminum (Al), which are used as alloy elements in micro alloy steels, have a direct effect on the mechanical properties of the material and form carbide, nitride or carbonitride. Carbides, nitrides and carbonitrides formed by micro alloy elements remain insoluble in the austenite phase in case the dissolving temperatures are not exceeded during hot forming processes. These insoluble hard structures provide to obtain both a small-grain steel structure and an increase in the toughness of the material by preventing austenite grain growth.
In the state of the art, feature of increasing the amount of carbon in the alloy and grain refinement of the micro alloy elements are used so as to obtain high strength. The high carbon steel alloy is exposed to heat treatment after hot forging and a secondary treatment is required so as to increase strength. On the other hand, in the state of the art, micro alloy steels cannot increase the strength of the forged material as much as the heat treated high carbon alloy. While the yield strength of the steel alloy in the high carbon C45E standard is 500 MPa and the tensile strength is 750-850 MPa, the yield strength is 560 MPa and the tensile strength is between 580-790 MPa in the micro alloy steel. As mentioned before, the increase in strength of high carbon steel (C45E) as a result of a high cost application with secondary process and low yield strength in micro alloy steel shows that there is a need for developing a new steel alloy.
The invention, which is the subject of the patent document number EP1929053B1 found in the literature, is related to the production process of a multi-phase microstructured steel part. Chemical composition of the steel subject to the invention, 0.01 %^C^0.50, 50%^Mn^3.0%, Cr^1.5%, 0.01%^Si^3.0%, 0.005%^AI ^3.0%, Mo^1.0%, P<0.10%, Ti^0.20%, V^0.1% and optionally Ni^2.0%, Cu^2.0% , S^0.05%, Nb^O.15%, the rest contains iron and impurities from the preparation process. The multiphase microstructure of the steel contains 25 to 75% superficial ferrite and 25 to 75% superficial martenist and/or bainite. Patent application numbered EP1228253B1 relates to 0.25-0.35 % C, 24-28 % Cr, 10-15% Ni, 3-6% Mn, 1.75-2.50% Nb, 0.50-0.70% steel composition by weight. N is 0-0.30% Si, and the remainder relates to a steel composition composed largely of iron and unavoidable impurities, where C+N>=0.8%. The invention also relates to a method for making said compositions and valves manufactured from said compositions or using said method and exhibiting excellent mechanical strength and oxidation resistance at temperatures between 800 and 900°C.
Patent application numbered EP1070153B1 relates to a steel composition containing; 0.6- 0.65% carbon (C) by weight, max. 0.4% silicon (Si), 0.6-0.9% manganese (Mn), 0.03-0.07% phosphorus (P), 0.07-0.11% sulfur (S), max. 0.5% chromium (Cr), max. 0.1% molybdenum (Mo), max. 0.5% nickel (Ni), max. 0.5% copper (Cu), max. 0.5% aluminum (Al), max. 0.03% nitrogen (N), vanadium and iron.
As can be seen in said applications, there are many steel compositions in the state of the art. However, the requirement of micro alloy steel alloys with high strength and high yield strength is increasing day by day.
As a result, due to the abovementioned disadvantages, deficiencies, there is a requirement to make an innovation in the relevant technical field.
Aim of the Invention
The present invention relates to a high-strength, low-alloy steel composition and related production method that meets the abovementioned requirements, eliminates all disadvantages and brings some additional advantages.
The aim of the invention is to introduce high strength, low alloy steel developed for use in all areas that can be used in hot forging processes as a long product raw material.
The aim of the invention is to obtain a steel with a yield strength of 660-680 Mpa, a tensile strength of 900-950 MPa and a hardness of 280-290 HV after controlled forging.
The aim of the invention is to activate the strength increase mechanism by grain refinement and to increase the strength with toughness, thanks to the use of vanadium as the sole carbide builder, aluminum in deoxidation, nitrogen in terms of forming nitrides in controlled cooling with casting and forming temperatures. The aim of the invention is to provide grain refining elements to reveal their effect by the fact that, in the primary cooling, the heat transfer coefficient is 80-120 W/m2K for 100-130. in the secondary cooling, the heat transfer coefficient is 40-60 W/m2K for 1200-1500 seconds required for forging temperature and cooling environment after forging.
The aim of the invention is to apply the mechanism of solid solution hardening and grain size reduction and strength increase together.
An aim of the invention is to present an alloy composition and a controlled cooling process that together increase the strength mechanism, toughness and strength.
An aim of the invention is to provide an alloy composition that does not adversely affect weldability.
An aim of the invention is to provide the increase in strength with the ferrite and bainite phase structure.
In order to fulfill the above-mentioned purposes, the invention is a high-strength, low-alloyed micro-alloyed steel developed for use in all areas that can be used in hot forging processes as a long product raw material. 0.430-0.470% by weight carbon (C), 0.250-0.300% by weight silicon (Si), 1.600-1.650% by weight manganese (Mn), 0-0.120% by weight chromium (Cr), 0- 0.050% by weight molybdenum (Mo), 0-0.120% by weight nickel (Ni), 0-0.015% by weight aluminum (Al), 0.035-0.045% by weight vanadium (V), 0-0.005% by weight tin (Sn), 0% by weight - Steel composition containing 0.010% phosphorus (P), 0-0.090% by weight sulfur (S) and 0-0.150% by weight copper (Cu) and 70-140 ppm nitrogen (N) and 5-7% ferrite phase, 54% Contains -58 perlite phase, 30-39% bainite phase.
In order to achieve the aims of the invention, micro alloy steel has 600-615 MPa yield strength, 800-860 MPa tensile strength, 250-265 HV hardness and 0.650-0.665 Ceq equivalent carbon value.
In order to achieve the aims of the invention, micro alloy steel has 600-615 MPa yield strength, 900-950 MPa tensile strength, 280-290 HV hardness and 0.704-0.806 Ceq equivalent carbon value.
In order to fulfill the above-described aims, the invention is a method for the production of micro alloy steel comprising of the following process steps; • obtaining the steel produced by micro alloying in the form of billets by continuous casting method,
• converting the produced billets into cylindrical semi-finished products in the round long group by hot rolling,
• forming precipitation by subjecting the long semi-finished products to the controlled hot forging and cooling phases, characterized in that;
• said forging temperature is at 1200°C, the heat transfer coefficient of the primary cooling after forging is 80-120 W/m2K for 100-130 seconds and the heat transfer coefficient is 40-60 W/m2K for 1200-1500 seconds for secondary cooling.
The structural and characteristic features of the present invention will be understood clearly by the following detailed description and therefore the evaluation shall be made by taking the detailed description into consideration.
Detailed Description of the Invention
In this detailed description, the inventive new micro-alloy steel, is described only for clarifying the subject matter in a manner such that no limiting effect is created.
The invention is a high-strength, low-alloyed micro-alloyed steel developed for use in all areas that can be used in hot forging processes as a long product raw material, characterized in that, it comprises of the following; carbon (C) in a ratio of 0.430-0.470% by weight, silicon (Si) in a ratio of 0.250-0.300% maximum by weight, manganese (Mn) in a ratio of 1.600-1.650% by weight, chromium (Cr) in a ratio of 0-0.120% maximum by weight, molybdenum (Mo) in a ratio of 0-0.050% by weight, nickel (Ni) in a ratio of 0-0.120% by weight, aluminum (Al) in a ratio of 0-0.015% by weight, vanadium (V) in a ratio of 0,035- 0,045% by weight, phosphorous (P) in a ratio of 0-0.010% by weight, sulfur (S) in a ratio of 0- 0.090% by weight, copper (Cu) in a ratio of 0-0.0150 by weight and 70-140 ppm nitrogen (N), iron (Fe) and impurities in equilibrium with other elements and 5-7% ferrite phase, 54-58% pearlite phase, 30-39% bainite phase. The inventive formulation of the micro alloy steel;
Figure imgf000007_0001
First of all the alloy elements are determined so as to obtain the inventive formulation of the high-strength low-alloy steel. The amount and diversity of alloy elements in the steel composition are important parameters for developing the mechanical features.
The features of intermediate atoms such as such as carbon (C), nitrogen (N) and vanadium (V) are used so as to make nitride, carbide and carbonitride, in order to make different strength increasing mechanisms to occur in combination. On the other hand, by making use of the characteristic that the TTT (isothermal transformation diagram) and OCT (continuous cooling transformation diagram) diagrams vary according to the alloy element, fine grained structure is obtained by solid solution hardening mechanism. In the developed alloy structure, it is ensured that the bainite phase is revealed together with the ferrite phase.
Secondly, hot forging methodology is formed. The temperature and deformation rates in the hot forging process are rearranged with respect to the original alloy. Finally, a cooling process is applied. The cooling regime for the target microstructure (ferrite and bainite) is determined according to the TTT (isothermal conversion diagram) and CCT (continuous cooling conversion diagram) diagrams. Cooling is provided in such a way that the ferrite and bainite phase ratios can be changed with the TTT and CCT-based controlled two-stage functional cooling method. Martensite and bainite conversion in the iron-carbon phase diagram has an important position in the hardening mechanism of the alloy, in the iron-carbon phase diagram. However, cooling rates are significantly higher than balance conditions due to process conditions in industrial steel production. The iron-carbon phase diagram used in determining the phase conversion with the increase of the cooling rate is not used. The most important reason for this is that said diagram is formed under very slow cooling conditions. Thus, diagrams which are called TTT and indicate the change of phase conversion due to temperature and time are used at high cooling rates. In fast-cooled steels, the aspects as when the austenite will begin conversion, how long the conversion will be completed and what products will be formed as a result are determined by means of the isothermal transformation diagrams. Thus, TTT diagrams are preferred in determining the phase transformations that will occur in the alloy depending on the supercooling conditions as a function of temperature and time.
It is required to change the conversion curve so as to see the time and temperature effects separately in the conversion reaction. The curves showing this situation are called continuous cooling conversion curves (CCT). CCT diagrams can be used for all heat treatments that also include continuous cooling. The main aim of CCT diagrams is to foresee which structure elements will be obtained and thus which hardness can be obtained by using the cooling curve. These diagrams ensure determining the phase or phases contained in the final microstructures to be obtained after both isothermal heat treatments where the temperature is kept constant and the conversion in continuous cooling.
In the high-strength, low-alloyed steel composition, which is the subject of the invention, vanadium is used as the sole carbide builder and aluminum, which is also used in deoxidation. Nitrogen, on the other hand, is in the steel composition in terms of forming nitrides at casting and forming temperatures and controlled cooling. With these aforementioned uses, the mechanism of increasing strength by grain refinement is activated, and an increase in strength is provided with toughness. This is necessary for the forging temperature and the cooling environment after forging in order for the grain refining elements to exert their effect.
Production method of the high-strength, low-alloyed steel composition is as follows
• The composition of steel produced by micro-alloying is obtained in the form of billets by continuous casting method by using electric arc furnace, ladle furnace, vacuum furnace and tundish dipping closed ceramic tube respectively, • Converting the produced billets into cylindrical semi-finished products in the round long group by hot rolling,
• Exposing semi-finished products to controlled hot forging and cooling phases,
• Obtaining a steel alloy with desired mechanical values with the precipitate hardening mechanism,
Said forging temperature used in the inventive production method occurs at 1200°C, the heat transfer coefficient of the primary cooling after forging is 80-120 W/m2K for 100-130 seconds and the heat transfer coefficient of secondary cooling occurs 40-60 W/m2K for 1200-1500 seconds under atmospheric conditions.
Convection is a type of heat transfer between a solid surface and an adjacent fluid (liquid or gas) in motion. The faster the fluid movement, the greater the convection heat transfer. If the bulk or mass fluid movement disappears, the heat transfer between the solid surface and the adjacent fluid occurs only by the random movement of molecules, that is, by conduction.
Heat transfer coefficient (K) is defined as the amount of heat carried from unit surface area, unit temperature difference and unit time. This coefficient varies depending on various factors. Some of these elements are the material and roughness of the fluid contact surface, flow pattern, flow rate, hydraulic diameter, viscosity and density of the fluid.
In addition, the heat convection coefficient varies according to the heat convection type. There are two types of heat transfer. The first one is the forced heat convection where the fluid moves due to the pressure applied to the system. The second one is the natural convection of heat when the fluid moves in the system due to the density difference.
The most decisive feature in the selection of thermal insulation materials is the heat transfer coefficient. Because the lower the thermal conductivity coefficient, the higher the thermal insulation resistance of the systems. Knowing the thermal properties of materials is very important for achieving optimum performance where the material is used. Many measurement methods developed for this purpose have been used for many years. The thermal properties of materials (thermal conductivity coefficient, specific heat, thermal permeability) can be measured with existing methods. In particular, there are complexes in the micro and macro level of the materials developed recently, and it becomes difficult to make accurate measurements under this condition. The smaller the thermal conductivity coefficient is, the less the heat loss. The effective use of this energy is available with thermal insulation. The selection of thermal insulation material or the manufacture of a new material is very important in this regard. In order to make calculations after the manufacture of a new material or material selection, the heat transmission coefficient of that material must be known.
Accurate calculation of the heat transfer coefficient is of great importance in engineering problems. It is inevitable In the calculation of the heat transfer coefficient that the experimental and theoretical errors will occur, as well as the loss of time and cost. The heat transfer coefficient is towards the side where the temperature decreases. The following unitless characteristic values are used in the calculation of the heat convection coefficient:
Reynold number;
Figure imgf000010_0001
Prandtl number;
Figure imgf000010_0002
Nusselt number;
Figure imgf000010_0003
These are; v: Velocity of the fluid,
I: It is the characteristic length related to flow and heat transfer; wall height on walls and pipe diameter on pipes. v: Kinematic viscosity, v=g/p, p: Density, cp: Specific heat at constant pressure,
A: Heat transfer coefficient g: Gravitational acceleration, y: Coefficient of volumetric expansion, t: Temperature difference a: Temperature diffusion coefficient, (a=A/p. cp)
Newton's law of cooling (Q) is used to calculate the amount of heat transferred per unit time by convection, and the formula used in its calculation is given below. In Table-1, typical values of the convection heat transfer coefficient are given.
Q = hAs[Ts - T^
Table- 1: Typical values of the convection heat transfer coefficient
Figure imgf000011_0001
With the alloy composition of the invention and the controlled cooling process applied to this composition;
• Solid solution hardening and grain size reduction and strength increase mechanism are applied together.
• The increase in strength is obtained with the ferrite and bainite phase structure.
• The alloy composition used does not have a negative effect on weldability.
5-7% ferrite phase, 54-58% pearlite phase, 55-60% bainite phase and V-C precipitates are formed with the inventive steel composition and production method.
Mechanical features of the High-strength, low-alloy steel;
• Yield strength 600-615 MPa,
• Tensile strength 800-860 MPa,
• Hardness 250-265 HV,
• Equivalent carbon value is 0.650-0.665 Ceq. In the preferred embodiment of the invention, the long product mentioned includes all hot rolled, square, round and flat steel products.
The equivalent carbon value of the inventive micro-alloy steel is 0.650-0.665 Ceq. Carbon equivalent is a measure that defines weldability, and calculations are performed with the amount of alloying elements in the steel. The values of the specified alloys are equivalent to the existing steel to be substituted and provide an advantage in terms of creating higher strength. The formula used in calculating Ceq within the scope of the invention is given below.
International Institute of Welding (IIW):
Ce (I I W)=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15

Claims

1. The present invention is a high-strength, low-alloyed micro-alloyed steel developed for use in all areas that can be used in hot forging processes as a long product raw material, characterized in that, it has a steel composition comprising of the following; carbon (C) in a ratio of 0.430-0.470% by weight, silicon (Si) in a ratio of 0.250-0.300% maximum by weight, manganese (Mn) in a ratio of 1.600-1.650% by weight, chromium (Cr) in a ratio of 0-0.120% maximum by weight, molybdenum (Mo) in a ratio of 0-0.050% by weight, nickel (Ni) in a ratio of 0-0.120% by weight, aluminum (Al) in a ratio of 0-0.015% by weight, vanadium (V) in a ratio of 0,035-0,045% by weight, phosphorous (P) in a ratio of 0-0.010% by weight, sulfur (S) in a ratio of 0-0.090% by weight, copper (Cu) in a ratio of 0-0.0150 by weight and 70-140 ppm nitrogen (N) and 5-7% ferrite phase, 54-58 % pearlite phase, 30-39% bainite phase.
2. High-strength, low-alloyed micro-alloyed steel composition according to claim 1, characterized in that; it has yield strength of 660-680 MPa.
3. High-strength, low-alloyed micro-alloyed steel composition according to claim 1, characterized in that; it has tensile strength of 900-950 MPa.
4. High-strength, low-alloyed micro-alloyed steel composition according to claim 1, characterized in that; it has hardness of 280-290 MPa.
5. High-strength, low-alloyed micro-alloyed steel composition according to claim 1, characterized in that; it has 0.704-0.806 Ceq equivalent carbon value.
6. A production method for high-strength, low-alloyed micro-alloyed steel comprising the process steps of,
• obtaining the steel produced by micro alloying in the form of billets by continuous casting method,
• converting the produced billets into cylindrical semi-finished products and long semifinished products in the round long group by hot rolling,
• forming precipitation by subjecting the long semi-finished products to the controlled hot forging and cooling phases, characterized in that; said forging temperature is at 1200°C, the heat transfer coefficient of the primary cooling after forging is 80-120 W/m2K for 100-130 seconds and the heat transfer coefficient is 40-60 W/m2K for 1200-1500 seconds for secondary cooling.
7. High-strength, low-alloyed micro-alloyed steel production method according to claim 6, characterized in that; it has a steel composition comprising of the following; carbon (C) in a ratio of 0.320-0.400% by weight, nitrogen (N) in a ratio of 0.007% maximum by weight, silicon (Si) in a ratio of 0.250-0.300% maximum by weight, manganese (Mn) in a ratio of 1.550-1.700% maximum by weight, chromium (Cr) in a ratio of 0-0.120% maximum by weight, molybdenum (Mo) in a ratio of 0-0.020% maximum by weight, nickel (Ni) in a ratio of 0-0.050% maximum by weight, aluminum (Al) in a ratio of 0-0.005% by weight, vanadium (V) in a ratio of 0-0.030% maximum by weight, phosphorous (P) in a ratio of 0-0.005% maximum by weight, sulfur (S) in a ratio of 0-0.010% maximum by weight, copper (Cu) in a ratio of 0- 0.100% maximum by weight and 15-20% ferrite phase, 20-28% pearlite phase, 55-60%.
8. High-strength, low-alloyed micro-alloyed steel production method according to any of the claims 6-8, characterized in that; the precipitate formed is based on VC and VN.
9. High-strength, low-alloyed micro-alloyed steel production method according to any of the claims 6-8, characterized in that; it has yield strength of 660-680 MPa.
10. High-strength, low-alloyed micro-alloyed steel production method according to any of the claims 6-8, characterized in that; it has tensile strength of 900-950 MPa.
11. High-strength, low-alloyed micro-alloyed steel production method according to any of the claims 6-8, characterized in that; it has hardness of 280-290 HV.
12. High-strength, low-alloyed micro-alloyed steel production method according to any of the claims 6-8, characterized in that; it has 0.704-0.806 Ceq equivalent carbon value.
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