WO2024118030A2 - Low carbon bainitic steel for the machinery manufacturing industry - Google Patents

Abstract

The invention relates to a new steel alloy that shows similar mechanical properties without heat treatment as the tempered 42CrMo4 steel alloy frequently preferred in the machinery manufacturing industry, and is also more economical and better weldable.

Description

LOW CARBON BAINITIC STEEL FOR THE MACHINERY MANUFACTURING INDUSTRY

TECHNICAL FIELD

The invention is about obtaining a low carbon bainitic (DKB) steel alloy that does not require heat treatment after hot rolling.

STATE OF THE ART

Today, high strength and high toughness properties as well as good weldability and machinability performance are expected from the engineering steels used in the machinery manufacturing industry. A very good balance must be maintained between the strength, toughness and weldability properties of the steel alloys to be used in the mechanical design of machines and components. Tempered steels are a group of steels that are widely used in this field to meet specified engineering requirements. However, they gain the mechanical properties they offer after a cost-increasing additional heat treatment process (+QT = Quenching + Tempering) applied after the production of semi-finished or final parts.

The fact that the carbon content in tempered steels, which can provide high yield and tensile strength as well as good impact toughness by means of their tempered martensite structure, is generally more than 0.30% weakens weldability, which is another important material feature in the machinery manufacturing sector. The high carbon content in the chemical composition of tempered steels causes the formation of a hard, brittle untempered martensite phase due to rapid cooling in the heat affected zone (HAZ) during and after welding, and this phase structure can cause crack formation starting from the welding area of the material. The weldability of materials is determined by their carbon equivalent (CE), and as the CE value of the material increases, its weldability decreases.

The following basic process sequence is applied to obtain tempered steels to be produced in the form of long semi-finished products.

• First of all, the materials are heated in annealing furnaces up to an average temperature of 1200 °C for the shaping process. Conventionally, steels rolled in the temperature range of 1000-1150 °C are left to cool in still air on cooling grids.

• In order to harden the rolled materials, a martensitic phase structure is obtained by first heating the austenite phase region and then applying rapid cooling (quenching) in cooling media such as water, oil or polymer.

• After quenching, the very hard martensitic structure is tempered at average temperatures of 450-550 °C, reducing the hardness of the steel and increasing its ductility.

• Warping may occur in thin diameter materials after quenching. If such a situation exists, the materials are put through the straightening process. Stress relief annealing is applied to relieve the internal stress that occurs in the material after the straightening process.

As can be seen, the long production chain of semi-finished products to be produced using tempered steels begins with the annealing of the material to hot rolling temperature. The material shaped by the rolling process is then heated to the austenitising temperature again for the quenching process and kept in this region for a certain period of time, depending on the material cross-section. After the austenitising process, in order to increase the hardness of the material, rapid cooling by quenching is applied to ensure that the microstructure becomes martensitic. However, since this structure is a very hard and brittle structure, a tempered martensite structure is obtained by the tempering process applied again at temperatures of 450-550 ° C, and by reducing the hardness of the material, its toughness is increased and final mechanical properties are gained. In addition, in the production of long semi-finished products, warpage may occur in thin-section materials after quenching and a straightening operation is applied. Finally, stress relief annealing can be applied to relieve the stresses that occur during straightening.

Apart from tempered steels, another steel group that stands out as having high strength values and lower costs due to the production process is microalloyed steels. Although high strength values are obtained from such steels with the contribution of microalloying elements such as Ti, V, Nb, the disadvantage of such steels compared to tempered steels is that their properties such as impact toughness, fracture toughness and weldability, which are critical for the machinery manufacturing industry, are inadequate. Despite this long production chain of tempered steels, the process cycle of bainitic steels, which can show the same mechanical properties, is much shorter. Bainitic steels can be cooled in a controlled manner in still air after hot rolling to obtain the targeted final mechanical values. This type of steel can also offer different mechanical properties by rapid cooling after rolling.

Although tempered steels provide high mechanical values and are widely used in the machinery manufacturing industry, constantly increasing raw material, energy and natural gas prices, and poor weldability properties direct manufacturers to search for new steels that are lower cost, better weldable and whose production process is less harmful to the environment, provided that the relevant engineering requirements are met, due to manufacturers' expectations of faster production cycles and CO2 emissions occurring in the reclamation process.

Provided that it provides the mechanical properties of 42CrMo4 quality, which is the most widely used among tempering steels, when negativities such as the poor weldability mentioned for tempered steels, the additional heat treatment process after forming and the resulting extra electricity and natural gas consumption, and the presence of high-cost alloying elements such as Molybdenum and the studies carried out to find a solution to these negativities with a single steel quality are examined, it is understood that no permanent solution has been found in the relevant technical field. In order to eliminate these negativities, it has become necessary to make an innovation in the relevant technical field.

THE AIM OF THE INVENTION

The present invention aims to eliminate the above-mentioned problems and make a technical innovation in the relevant field.

The main aim of the invention is to present an economical, better weldable steel material that can provide the mechanical properties obtained by the reclamation process of 42CrMo4 (16-40 mm diameter) tempered steel quality by cooling only in still air, without the need for additional heat treatment process after hot rolling through obtaining a steel with a low carbon bainitic structure by means of a unique chemical composition and process relationship. The aim of the invention is to introduce a new steel quality with lower production costs that can be used as an alternative to tempered steels with high production costs in the production of long semi-finished products.

The purpose of the invention is to prevent additional CO2 emissions occurring in production by eliminating additional heat treatment.

The aim of the invention is to eliminate the cost of natural gas and electricity consumption used in the reclamation process, as there will be no need for additional heat treatment.

The aim of the invention is to reduce the cost of steel materials by not using high-cost alloying elements such as Mo and Ni, provided that the mechanical properties of 42CrMo4 tempered steel quality are achieved through the tempering process.

CONTEXT OF THE INVENTION

The invention is a low carbon bainitic steel alloy that does not require heat treatment after hot rolling.

In an alternative embodiment of the invention, there is 0.10% to 0.15% carbon (C) by weight to increase the yield and tensile strength of the steel and reduce the percent elongation, formability and weldability.

In another alternative embodiment of the invention, 0.80% to 1.00% silicon (Si) by weight is used to increase the yield, tensile strength and elasticity of the steel and to prevent electrical current loss.

Another alternative embodiment of the invention comprises 2.9% to 3.1 % manganese by weight to increase the strength, hardenability and weldability of steel and to form manganese sulphur compound by preventing the formation of the iron sulphur compound that causes brittleness.

In a preferred embodiment of the invention, there is 0.08% to 0.12% vanadium by weight in order to increase the yield and tensile strengths and hardenability of steels by having a grain-reducing effect on the steel, as well as to provide tempering and second hardening. In another preferred embodiment of the invention, there is 0.020% to 0.050% aluminium by weight in order to remove oxygen in the structure of the steel, increase the yield strength and impact strength and reduce the grain size.

Another preferred embodiment of the invention comprises 0.040% to 0.060% sulphur by weight to reduce the toughness and ductility of the steel.

In a possible embodiment of the invention, there is 0.0250 to 0.400% titanium by weight to reduce the grain structure of the steel and to eliminate the negative effect of chromium carbide in stainless steels.

Another possible embodiment of the invention comprises 0.0050% to 0.0080% nitrogen by weight to increase the surface hardness and wear resistance of the steel material.

In another possible embodiment of the invention, there is 0.0300% to 0.0500% by weight of niobium, which has superconducting properties, to increase the strength of steel at low temperatures.

In another preferred embodiment of the invention, there is 0.0010% to 0.0030% boron by weight to improve the hardenability of steels and increase their strength in the quenched and tempered state.

Another alternative embodiment of the invention comprises iron as the raw material of steel in an amount that will complete the weight to 100% to obtain steel.

An accepted embodiment of the invention has a yield strength of at least 800 megapascals (MPa).

Another accepted embodiment of the invention has a tensile strength of at least 1150 MPa.

An accepted embodiment of the invention features at least 15% elongation.

Another accepted embodiment of the invention has a cross-sectional reduction of at least 45%.

An accepted embodiment of the invention comprises a notch impact toughness of at least 60 Joules at room temperature. The invention is the production method of a low carbon bainitic steel alloy that does not require heat treatment after hot rolling.

In an alternative embodiment of the invention, the alloying is performed by melting 0.10-0.15% Carbon (C), 0.80-1.00% Silicon (Si), 2.9-3.1 % Manganese (Mn), 0.08- 0.12% Vanadium (V), 0.020-0.050% Aluminium (Al), 0.040-0.060% Sulphur (S), 0.0250-0.0400% Titanium (Ti), 0.0010-0.0030% Boron (B), 0.0300-0.0500% Niobium (Nb), and 0.0050-0.0080% Nitrogen (N) by weight, and adding iron at a rate to amount to 100% by weight.

Another preferred embodiment of the invention is achieved by solidifying the resulting liquid steel by ingot casting or continuous casting.

Another preferred embodiment of the invention is obtained by heating the resulting ingots or billets to 1200 °C and performing a thermomechanical rolling process.

An accepted embodiment of the invention comprises the process step of controlled cooling.

Another accepted embodiment of the invention is characterised in that one or any combination selected from the group comprising at most 0.20% chromium by weight to ensure the corrosion and oxidation resistance of steel and to increase its hardening ability, nickel at most 0.20% by weight to increase the impact resistance of steel and to provide high formability to the austenitic stainless steel form, and molybdenum at most 0.03% by weight to prevent the grain size of steel from increasing, to increase its hardening ability, to eliminate temper brittleness and to increase corrosion resistance are used in the chemical composition determination process.

ABBREVIATIONS TO BE USED IN THE DETAILED DESCRIPTION

• Tdet is the deformation temperature.

• TNR is the recrystallisation temperature.

• °C is degrees Celsius.

• MPa is megapascal.

• Si is silicon.

• Mn is manganese. • C is carbon.

• V is vanadium.

• B is boron.

• Ti is titanium.

• Nb is niobium.

• Al is aluminium.

• S is sulphur.

• N is nitrogen.

• Cr is chromium.

• Mo is molybdenum.

• Ni is nickel.

• Fe is iron.

• LCS is low carbon steel.

• TMRP is the thermomechanical rolling process.

• CE is carbon equivalent.

DETAILED DESCRIPTION OF THE INVENTION

In this detailed description, the invention is described with non-limiting examples only for better understanding of the subject.

The invention, described in detail below, relates to a low carbon bainitic steel alloy that is designed for the production of long semi-finished products in the diameter range of 16-40 mm, comprises lower bainite, granular bainite and martensite phases in its microstructure under thermomechanical rolling (Tdet < TNR temperatures) and still air cooling (0.5-1 °C/second cooling rates), has a yield Strength (Rp) of minimum 800 MPa, Tensile strength (Rm) of minimum 1 150 MPa, % Elongation (L) of minimum 15%, % Section Reduction (A) of minimum 45%, and notch impact toughness of minimum 60 Joules at room temperature and comprises 0.05-0.15% C, %0,50-1 Si, %2,5-3 Mn, %0, 06-0, 12 V, %0, 0010-0,0020 B, %0, 0200-0, 0350 Ti, %0, 0300-0, 0500 Nb, %0,015- 0,0100 Al, %0,040 - 0,070 S and at most %0,0080 N by weight, with a complementary Fe element but without the addition of Cr, Mo, and Ni.

Below, the functions and weight ratios of the alloying elements used to obtain low carbon bainitic steel are given. • Carbon is used to increase the yield and tensile strength of steel. However, the carbon content is limited to low rates in order to increase percent elongation, low temperature toughness and weldability. The amount of carbon used in the composition of low carbon bainitic steel is between 0.05% and 0.15% by weight.

• Silicon is used to increase the yield and tensile strength of steel and to reduce the precipitation of the cementite phase into ferrite. The amount of silicon used in the composition of low carbon bainitic steel is between 0.80% and 1 .00% by weight.

• Manganese is used to increase the strength and hardening ability of steel. The most important feature of manganese is that it will achieve the high strength and toughness properties required in a low carbon steel through the solid solution hardening mechanism. Additionally, Mn reduces the initial temperature of bainite. In the TTT diagram, it shifts ferrite and pearlite noses to the right. The amount of manganese used in obtaining low carbon bainitic steel is between 2.50% and 3.00% by weight.

• Chromium is not used because it reduces toughness values. The maximum amount of chromium allowed in the chemical composition of low carbon bainitic steel is 0.20% by weight.

• Nickel is not used because it is a high-cost alloying element. The maximum amount of nickel allowed in the chemical composition of low carbon bainitic steel is 0.20% by weight.

• Molybdenum is not used because it is a high-cost alloying element. The maximum amount of molybdenum allowed in the chemical composition in obtaining low carbon bainitic steel is 0.03% by weight.

• Vanadium is used to increase the yield and tensile strength and toughness of steel by having a grain-reducing effect on steel. The amount of vanadium used in the chemical composition of low carbon bainitic steel is between 0.05% and 0.15% by weight.

• Aluminium is used to remove oxygen in the structure of steel, increase its yield strength and impact strength, and reduce the grain size. The amount of aluminium used in the chemical composition of low carbon bainitic steel is between 0.020% and 0.100% by weight. • Sulphur is used to increase the machinability of steel. The amount of sulphur used in the chemical composition of low carbon bainitic steel is between 0.040% and 0.070% by weight.

• Titanium is used to refine the grain structure of steel with TiN precipitates. The amount of titanium used in obtaining low carbon bainitic steel is between 0.025% and 0.040% by weight.

• Nitrogen is used to form TiN precipitate in steel material. The amount of nitrogen used in the chemical composition of low carbon bainitic steel is between 0.005% and 0.008% by weight.

• Niobium is used to increase the recrystallization temperature of steel. The amount of niobium used in the chemical composition of low carbon bainitic steel is between 0.030% and 0.050% by weight.

• Boron is used to obtain bainitic structure by increasing hardenability in steels. The amount of boron used in the chemical composition of low carbon bainitic steel is between 0.001 % and 0.003% by weight.

The low carbon bainitic steel alloy production process begins with the refining and chemical composition determination process in the ladle furnaces after melting the scrap in the electric arc furnace in the steel mill to obtain liquid steel. Liquid steel, whose composition is determined, is solidified by different casting techniques (ingot casting or continuous casting) and turned into ingots or billets.

These ingots or billets are then heated to 1200 °C for the hot rolling operation and rolled to the final diameter measurement. The production process of the invention is completed after hot rolling. The resulting long semi-finished product can be processed into desired geometries and used in the machine-manufacturing sector.

In order to further improve the toughness and weldability properties of bainitic steels, which are similar to tempered steels and can reach higher strength values, this steel group has been developed over time and a new bainitic steel group has been developed under the name of low carbon bainitic (LCB) steels. In this type of steel, carbon is no longer the most important element that provides strength and to compensate for the strength values lost by reducing the carbon content, new steel alloys are designed by adding main alloying elements such as Si, Mn, Cr, Mo, Ni and microalloying elements such as Ti, B, Nb, V to the steel in different proportions.

On the other hand, it is stated that the high strength and toughness values of LCB steels depend significantly not only on the chemical composition but also on the rolling conditions. The low deformation temperature and high deformation rate applied during hot forming ensure that the material has a fine grain structure.

This process, which provides a finer microstructure, is based on the deformation process applied below the temperature at which the austenite phase recrystallises (TNR) during rolling, in addition to preventing the grain growth of microalloy precipitates precipitated on the austenite grain boundary, and this process is called the thermomechanical rolling process (TMRP). The cooling rate of semi-finished or final parts produced with TMRP application after shaping with TMRP is also one of the determining factors for the formation of bainitic structure and the final mechanical properties of the material.

One of the difficulties in the production of this type of steel as semi-finished products is the control of the cooling regime after rolling. Since the cooling rate of long semifinished products, especially those with a diameter greater than 40 mm, slows down towards the central region of the material, it becomes difficult to obtain the entire structure as bainitic.

As a result, the differences of LCB steel, which is the subject of the invention, compared to high-cost tempering steels showing similar properties are stated below: a. Avoid adding Nickel and Molybdenum, which are high-cost alloying elements, b. Carbon equivalent being lower than that of 42CrMo4 steel, which is an indicator of the decrease in weldability level as its value increases, c. the microstructure becoming bainitic under cooling conditions in still air after hot forming under thermomechanical rolling conditions, and d. Ensuring the mechanical properties of tempered 42CrMo4 steel under all these conditions.

The scope of protection of the invention is stated in the attached claims and cannot be limited to what is explained in this detailed description for exemplary purposes. Because it is obvious that a person skilled in the art can produce similar structures in the light of those described above, without deviating from the main theme of the invention.

Claims

1. A low carbon bainitic steel alloy that does not require heat treatment after hot rolling, wherein the chemical comprising comprises i. 0,05-0,15% carbon (C) ii. 0.80-1 .00% Silicon (Si) iii. 2.50-3.00% Manganese (Mn) iv. 0.05-0.15% Vanadium (V) v. 0.020-0.100% Aluminium (Al) vi. 0.040-0.070% Sulphur (S) vii. 0.025-0.040% Titanium (Ti) viii. 0.001 -0.003% Boron (B) ix. 0.030-0.050% Niobium (Nb) x. 0.005-0.008% Nitrogen (N).

2. Low carbon bainitic steel alloy according to Claim 1 , having a yield strength (Rp) of at least 800 megapascals (MPa) under conditions of rolling temperature (Tdet) < TNR and post-rolling cooling rate at 0.5-1 °C/s.

3. Low carbon bainitic steel alloy according to Claim 1 , having a tensile strength (Rm) of at least 1150 MPa under conditions of rolling temperature (Tdet) < TNR and post-rolling cooling rate at 0.5-1 °C/s.

4. Low carbon bainitic steel alloy according to Claim 1 , having an elongation (%L) of at least 15% under conditions of rolling temperature (Tdet) < TNR and postrolling cooling rate at 0.5-1 °C/s.

5. Low carbon bainitic steel alloy according to Claim 1 , having a cross-sectional narrowing (%A) of at least 45% under conditions of rolling temperature (Tdet) < TNR and post-rolling cooling rate at 0.5-1 °C/s.

6. Low carbon bainitic steel alloy according to Claim 1 , having a notch impact toughness of at least 60 Joules under conditions of rolling temperature (Tdet) < TNR and post-rolling cooling rate at 0.5-1 °C/s.