Technical Field

The invention relates to high-strength, low-alloy steel developed to be used in all areas that can be used in hot forging processes as a long product raw material.

The invention particularly relates to a micro-alloyed steel composition comprising; carbon (C) in the ratio of 0.400-0.430% by weight, silicon (Si) in a ratio of 0.250-0.300% by weight, manganese (Mn) in a ratio of 1.600-1.650% by weight, chromium (Cr) in a ratio of 0-0.120% 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 0.015-0.025% by weight, tin (Sn) in a ratio of 0-0.005 % by weight, phosphorus (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- 0.150% by weight and nitrogen (N) with an amount of 70-140 ppm and 10-15% ferrite phase, 52-58% perlite phase, 30-35% bainite phase with yield strength of 615-630 MPa, tensile strength of 850-880 MPa.

Prior Art

The most important group of engineering materials is made up of steels. There are continuous improvements in the process and physical metallurgy of steels to meet all kinds of demands and changes. In recent years, the development of micro-alloyed steels has been seen as one of the most important metallurgical successes. It can be said that this development is the result of a clear understanding of the structure-feature relations in low- carbon steels. The final product is the result of a successful combination of physical, mechanical, and process metallurgy. Micro-alloyed steels successfully replace mild steels as basic building materials.

In high-strength, low-alloy, micro-alloyed steels, the total amount of alloy usually does not exceed 2%. While most of these alloy elements are formed by niobium (Nb), titanium (Ti), and vanadium (V), the nanometer-sized precipitates formed by these alloy elements with carbon (C) and nitrogen (N) atoms provide high yield strength to steel. In addition, its mechanical properties such as high corrosion resistance, ductility, and toughness also increase the use of micro-alloyed steels. The specific development aspects of micro-alloyed steels consist of low-perlite and nonperlite steels. Features such as formability, toughness, and weldability are significantly increased by significantly lowering the carbon ratio. These features are generally desired in the production of high-strength and lightweight parts by shaping. Despite the low carbon ratio, these steels can only reach the yield limit of 500 N/mm ² with controlled rolling with the thinning and hardening effects of aluminum (Al), niobium (Nb), and titanium (Ti), vanadium (V), which are micro-alloy elements.

Niobium (Nb), titanium (Ti), vanadium (V), and aluminum (Al), which are used as alloy elements in micro-alloyed steels, have significant direct effects on the mechanical properties of the material and form carbide, nitride, or carbonite. Carbide, nitride, and carbonitrides formed by micro-alloy elements remain undissolved in the austenite phase if the dissolution temperatures are not exceeded during hot forming processes. These insoluble hard structures prevent austenite grain growth and provide both a small-grained steel structure and increase the toughness of the material.

In the prior art, to obtain high strength, increasing the amount of carbon in the alloy or grain thinning feature of micro-alloy elements is exploited. The high carbon steel alloy is heat- treated after hot forging and secondary treatment is required to increase strength. On the other hand, Micro-alloyed steels are not able to increase the strength of the forged material as much as heat-treated high carbon alloy. While the yield strength of the steel alloy in the high carbon C45E standard is 500 MPa, the tensile strength is 750-850 MPa, the yield strength is 560 MPa and the tensile strength is between 580-790 MPa in micro-alloyed steel. As mentioned, increasing the strength of high carbon steel (C45E) as a result of a costly application with secondary treatment and low yield strength in micro-alloyed steel reveals the need to develop 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 EP1070153B1 relates to a steel composition containing 0.6-0.65% carbon (C) by weight, maximum 0.4% silicon (Si), 0.6-0.9% manganese (Mn), 0.03-0.07% phosphorus (P), 0.07-0.11% sulfur (S), maximum 0.5% chromium (Cr), maximum 0.1% molybdenum (Mo), maximum 0.5% nickel (Ni), 0.5% copper (Cu), maximum 0.5% aluminum (Al), maximum 0.03% nitrogen (N), vanadium and iron.

As can be seen in the aforementioned applications, there are many steel compositions in the prior art. However, the need for micro-alloyed steel alloys with high strength and high yield strength is increasing day by day.

Consequently, due to the aforementioned problems and deficiencies, there has been a need to make an innovation in the related technical field.

Object of the Invention

The present invention relates to high-strength micro-alloyed steel and related production method that meets the aforementioned needs and eliminates all the disadvantages and provides advantages thereof.

The object of the invention is to provide high-strength, micro-alloyed steel developed to be used in all areas that can be used in hot forging processes as a long product raw material.

The object of the invention is to obtain steel with a| yield strength of 615-630 Mpa, a tensile strength of 850-880 MPa, and a hardness of 260-275 HV Rafter controlled forging.

The object of the invention is to to activate the strength increase mechanism by grain refinement and to increase the strength with toughness, by using vanadium as the sole carbide builder, aluminum in deoxidation, nitrogen in terms of forming nitrides in casting and forming temperatures and controlled cooling.

The object of the invention is to provide the effect of the grain thinning elements because in the primary cooling the heat convection coefficient is 80-120 W/m ² K for 100-130 seconds, in secondary cooling, the heat convection coefficient is 40-60 W/m ² K for 1200-1500 seconds, which are required for the forging temperature and post-forging cooling environment, i

The object of the invention is to apply the strength enhancement mechanism together with solid melt hardening and grain size reduction.

The object of the invention is to present an alloy composition and a controlled cooling process that together increase the strengthening mechanism, toughness and strength. The object of the invention is to provide an alloy composition that does not adversely affect weldability.

The object of the invention is to achieve an increase in strength with ferrite and bainite phase structure, i

To fulfill the above-described purposes, the invention is a high-strength, low-alloy microalloyed steel developed to be used in all areas that can be used in hot forging processes as long product raw material, characterized in that; it comprises a steel composition containing ^ carbon (C) in the ratio of 0.400-0.430% by weight, silicon (Si) in a ratio of 0.250-0.300% by weight, manganese (Mn) in a ratio of 1.600-1.650% by weight, chromium (Cr) in a ratio of 0- 0.120% 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 0.015-0.025% by weight, tin (Sn) in a ratio of 0-0.005 % by weight, phosphorus (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 0-0.150% by weight and nitrogen (N) with an amount of 70-140 ppm and 10-15% ferrite phase, 52-58% perlite phase, 30-35% bainite phase.

High-strength Micro-alloyed steel has a yield strength of 615-630 MPa, a tensile strength of 850-880 MPa, a hardness of 260-275 HV, and an equivalent carbon value of 0.660-0.770 Ceq to achieve the objects of the invention.

To fulfill the above-described objects, the invention is a high-strenght micro-alloyed steel production method, comprising the following process steps:

• obtaining the steel produced by micro-alloying in the form of billets by continuous casting method,

• making the produced billets into cylindrical semi-finished products and long semifinished product in the round long group by hot rolling,

• formation of precipitates by passing long semi-finished products through controlled hot forging and cooling stages, i characterized in that;

• the said forging temperature is 1200°C, the primary cooling after forging is for 100- 130 seconds with a heat convection coefficient of 80-120 W/m ² K, and the secondary cooling is for 1200-1500 seconds with a heat convection coefficient of 40-60 W/m ² K under atmospheric conditions, i

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 considering the detailed description, i

Detailed Description of the Invention

The high-strength micro-alloyed steel is merely described for a better understanding of the subject matter and without any limiting effect in this detailed description.

The invention is a high-strength, low-alloy micro-alloyed steel developed to be used in all areas that can be used in hot forging processes as long product raw material, characterized in that; it comprises a steel composition containing carbon (C) in the ratio of 0.400-0.430% by weight, silicon (Si) in a ratio of 0.250-0.300% by weight, manganese (Mn) in a ratio of 1.600- 1.650% by weight, chromium (Cr) in a ratio of 0-0.120% 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 0.015-0.025% by weight, tin (Sn) in a ratio of 0-0.005 % by weight, phosphorus (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 0-0.150% by weight and nitrogen (N) with an amount of 70-140 ppm and 10-15% ferrite phase, 52-58% perlite phase, 30-35% bainite phase.

Formulation of the high-strength micro-alloyed steel composition according to the invention;

To obtain the high-strength micro-alloyed steel according to the invention, the alloy elements are first determined. The amount and variety of alloy elements in the steel composition are important parameters in the development of mechanical properties.

To ensure the coexistence of different strength enhancement mechanisms, the nitride-, carbide-, and carbonitride-making properties of vanadium (V) with interstitial atoms such as carbon (C), and nitrogen (N) are used. On the other hand, by utilizing the fact that TTT (isothermal conversion diagram) and OCT (continuous cooling conversion diagram) diagrams vary according to the alloy element, a fine-grained structure is obtained with the solid melt hardening mechanism. In the developed alloy structure, it is ensured to reveal the bainite phase together with the ferrite phase, j

Secondly, a hot forging methodology is created. The temperature and deformation rates in the hot forging process are rearranged according to the original alloy. Finally, the cooling process is applied. According to the TTT (isothermal conversion diagram) and CCT (continuous cooling conversion diagram) diagrams, the cooling regime for the target microstructure (ferrite and bainite) is determined. With TTT- and CCT-based controlled dualstage functional cooling methods, cooling is provided in a way that ferrite and bainite phase ratios can be changed;

In the iron-carbon phase diagram, martensite and bainite conversion have an important place in the hardening mechanism of the alloy. However, due to the process conditions in industrial steel production, the cooling rates are considerably higher than the equilibrium conditions. With the increase in the cooling rate, the iron-carbon phase diagram used in the determination of phase conversions is not used. The most important reason for this is that the diagram in question is formed under very slow cooling conditions. For this reason, diagrams called TTT, which show the change of phase conversion depending on temperature and time, are used at high cooling rates. With isothermal conversion diagrams, subjects are determined such as when austenite will start to be converted in fast-cooled steels, how long the conversion will take, and which products will eventually form. Therefore, TTT diagrams are preferred in determining the phase conversions that will occur in the alloy under extreme cooling conditions as a function of temperature and time, i In the conversion reaction, it is necessary to change the conversion curve to see the time and temperature effects separately. Curves showing this situation are called continuous cooling conversion curves (CCT). CCT diagrams can be used for all thermal processes involving continuous cooling. The main purpose of CCT diagrams is to know in advance which structural elements can be obtained and which hardness can be obtained by using the cooling curve. These diagrams allow the determination of the phase or phases contained in the final microstructures to be obtained after the conversion in both isothermal heat treatments where the temperature is kept constant and continuous cooling, i

Vanadium is used as the sole carbide builder in the steel composition, and aluminum, which is also used in deoxidation, is used. 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. Forging temperature and post-forging cooling environment are required for these grain refining elements to exert their effect. |

The high-strength micro-alloyed steel production method according to the invention;

• The steel composition produced by micro-alloying is obtained in the form of billet by continuous casting method using electric arc furnace, crucible furnace, vacuum furnace, and tundish immersion closed ceramic tube, respectively,

• The produced billets are made into cylindrical semi-finished and long semi-finished products in the round long group by hot rolling,

• The said semi-finished product is subjected to controlled hot forging and cooling stages,

• With the resulting precipitate hardening mechanism, steel alloy is obtained at the desired mechanical values.

The hot forging temperature used in the inventive production method is 1200°C, the primary cooling after forging is carried out under atmospheric conditions with a heat convection coefficient of 80-120 W/m ² K for 100-130 seconds, and the secondary cooling is carried out with a heat convection coefficient of 40-60 W/m ² K for 1200-1500 seconds.

Convection is a type of heat transfer between a solid surface and the fluid (liquid or gas) in motion adjacent to it. The faster the fluid movement, the greater the heat transfer through convection. If the mass or mass fluid movement disappears, the heat transfer between the solid surface and the adjacent fluid occurs only by random movement of the molecules, i.e. by conduction.

Heat convection coefficient is defined as the amount of heat carried from the unit surface area in the unit temperature difference and in the unit time. This coefficient differs depending on various elements. Some of these elements are the material and roughness of the surface with which the fluid contacts, flow pattern, flow rate, hydraulic diameter, viscosity, and density of the fluid.

In addition, the heat convection coefficient is different according to the type of heat convection. There are two types of heat convection. The first is the forced heat convection where the fluid moves due to the pressure applied to the system. The second is the natural heat convection where the fluid moves in the system due to the density difference.

The most determining feature in the selection of thermal insulation materials is the heat transmission coefficient. Because the lower the heat transmission coefficient, the higher the thermal insulation resistance of the systems. Knowing the thermal properties of the materials is very important in terms of achieving optimum performance where the material is used. Many measurement techniques have been used for this purpose for many years. Thermal properties of the materials (thermal conductivity coefficient, specific heat, thermal permeability) can be measured with the current techniques. Especially recently, there are complexities in the micro and macro level internal structure of the materials developed, and it is difficult to make accurate measurements under this condition, i

The smaller the coefficient of thermal conductivity, the less heat loss occurs. Effective use of this energy is possible with thermal insulation. The choice of thermal insulation material or the production of new material is very important in this regard. It is necessary to know the heat transmission coefficient of that material to make the calculations after the production of new material or material selection.

Correct calculation of the heat transmission coefficient is of great importance in engineering problems. In addition to the occurrence of experimental and theoretical errors in the calculation of the heat transmission coefficient, inevitably, the time and cost loss spent will also occur. The heat transmission coefficient is towards the side where the temperature decreases. In the calculation of the heat convection coefficient, the following unitless characteristic identification values are used: i

Reynolds number;

Prandtl number;

Nusselt number;

These are; v: Speed of fluid,

I: It is the characteristic length related to flow and heat transfer; it is the wall height in the walls and the pipe diameter in the pipes. v: Kinematic viscosity, v=g/p, p: Density, c _p : Specific heat at constant pressure,

A: Heat transmission coefficient g: Gravitational acceleration, y: Volumetric expansion coefficient, t: Temperature difference a: Temperature dissipation coefficient, (a=A/.c _p )

Newton’s Law of Cooling (Q) is used to calculate the amount of heat transferred in unit time by convection and the formula used in its calculation is given below. The typical values of the convection heat transfer coefficient are given in Table- 1.

Q = hA _s (T, - .'

Table- 1: Typical values of convection heat transfer coefficient

With the alloy composition according to the invention and the controlled cooling process applied to this composition;

• Solid melt hardening and grain size reduction and strength enhancement mechanisms are applied together.

• Strength increase is obtained with ferrite and bainite phase structure.

• The alloy composition used has no negative impact on weldability.

With the steel composition and production method of the invention, V-C precipitates are formed with a i10-15% ferrite phase, 52-58% perlite phase, and 30-35% bainite phase.

The mechanical properties of the micro-alloyed steel of the invention are as follows:

• Yield strength of 615-630 MPa,

• Tensile strength of 850-880 MPa,

• Hardness of 260-275 HV,

• The equivalent carbon value of 0.660-0.770 Ceq.

The long product mentioned in the preferred embodiment of the invention covers all hot- rolled, square, round, and flat steel products^

The equivalent carbon value of the micro-alloyed steel of the invention is 0.660-0.770 Ceq. Carbon equivalent is a measure that defines weldability, and calculation is made with the amounts of alloy elements in steel. The values of the specified alloys are equivalent to the steel to be substituted and provide an advantage in terms of creating higher strength. The formula used to calculate the Ceq within the scope of the invention is given below.

International Institute of Welding (IIW):

Ce (IIW)=C+Mn/6+(Cr+Mo+V)/5+(Ni+Cu)/15 |