ES2860953T3 - Micro-alloyed steel for hot forming of high strength and high elastic limit parts - Google Patents

Abstract

Microalloyed steel for hot forming of high resistance and high elastic limit pieces and procedure to obtain components of said steel, with a specific composition of the following elements: 0.30% <= C <= 0.50%, 0.50% <= Si <= 1.50%, 0.50% <= Mn <= 1.50%, 0.05% <= V <0.50%, 0.001% <= Ti <0.050%, than with a procedure suitable for obtaining said steel, it achieves low P contents and good inclusionary cleaning. In turn, with a controlled cooling process, tensile strength values between 1050-1200 MPa are achieved.

Description

DESCRIPTION

Micro-alloyed steel for hot forming of high strength and high elastic limit parts

Object of the invention

The present invention relates to a high-strength, high-elastic microalloyed steel.

The invention makes it possible to produce a microalloyed steel, with high mechanical resistance and high elastic limit, in addition to having a good response to machining operations, based on a certain chemical composition, a specific metallurgical procedure and a controlled subsequent cooling.

Background of the invention

A microalloyed steel is basically a classic construction steel to which small amounts of alloying elements are added, such as, V, Al, Ti, Nb, Zr, B, etc. capable of forming micro-precipitates.

The main characteristic of microalloyed steels is based on the appearance of micro-precipitates that are produced both during the rolling process and during the forging process. For this reason, it is necessary to know the formation and evolution of micro-precipitates as a function of thermal cycles, hot deformations and cooling rates in order to be able to master the process and obtain a steel with the desired mechanical properties.

One of the main applications of microalloyed steels are certain components of the automobile engine, such as the crankshaft, connecting rods, common-rail rail, etc. All these elements are components subjected to high working pressures and stresses, which is why it is essential that they have very high tensile strength and elastic limit values.

The advantage over the quenched and tempered steels they replace is based on the fact that these microalloyed steels acquire characteristics directly through thermomechanical treatment and subsequent controlled cooling.

The microalloyed steels of the present patent have a ferrite-pearlite structure and their final application is hot forming.

On the other hand, the machinability of a microalloyed steel after a hot forming process is comparable to that of a quenched and tempered steel, as long as both have similar levels of hardness and sulfur content.

The main advantage derived from the use of microalloyed steels is the direct reduction in the cost of the shaped components up to 10-20%, mainly due to the simplification of the manufacturing process due to the elimination of the heat treatment of quenching and tempering, as well as the subsequent straightening operations, stress relief operations, as well as hardening cracks and their controls to detect them.

At present, different qualities of microalloyed steels are used according to the resistance to be achieved in the components for which they are to be used. The characteristics are largely determined by the carbon content of the steel, which can range between 0.15% and 0.50% by weight, as well as the content of other alloying elements such as Mn and Si, which help to solid solution hardening of ferrite and other microalloying elements such as Ti, V and in certain cases also Nb, which form micro-precipitates of carbides and nitrides, acting as controllers of grain size and hardening by precipitation.

The different qualities of microalloyed steels that are manufactured today range from 550 MPa to 1000 MPa of resistance. These values depend remarkably on the thickness of the piece and the process of shaping and subsequent cooling to which the piece is subjected.

Currently there are different patents related to microalloyed steels. Here are some examples.

Patent application No. CN102776439 (A) 2012 "Niobium microalloying Si-Mn-B series hot forming steel plate and rolling technology" describes a microalloyed steel with Nb but for flat product with a mechanical resistance between 700-900 MPa and a elastic limit of 500-600 MPa.

Patent application No. CN101629268 (A) 2010 "Microalloying high-strength and anti-crack flat steel for automobile girder and production process thereof" describes a microalloyed V-shaped steel producing a rolled product between 750-850 MPa and a limit elastic between 640-700 MPa.

Patent application No. CN101307413 (A) 2008 "Microalloying steel for ultra-high-strength sucker rod" describes a microalloyed steel with Nb, Ti and V for rods, with a tensile strength greater than 965 MPa.

Patent application No. EP2816131 refers to a hot forging rolled steel bar suitable as a starting material for a common rail used for a diesel engine fuel injection system, a hot forged section material produced By forming the rolled steel bar, the common rail and the common rail production procedure. Nb is not present in the described rolled steel bar.

Patent application no. EP2159296 refers to a steel hardened by quenching and tempering and to a process for obtaining parts of said steel. Nb is not present in the composition of the steel. JPH10121200 discloses a high strength steel material for shear reinforcement excellent in pliability and weldability having a specific composition containing specific amounts of Nb and V and also having a composite structure of ferrite and pearlite.

Thus, in the state of the art there is no microalloyed steel whose tensile strength exceeds 1000 MPa.

Description of the invention

The present invention refers to a microalloyed steel for the forming of high-strength and high-elastic parts as defined in claim 1. The steel of the invention consists of a ferrite-pearlite structure with mechanical characteristics in which the resistance tensile strength is 1050-1200 MPa, depending on the diameter of the bar and the elastic limit is greater than 750 MPa.

The invention makes it possible to produce a microalloyed steel from a novel chemical composition and a certain metallurgical process, which has a high mechanical resistance as well as a high elastic limit, something fundamental for all automotive components.

The thermomechanical treatment to which the component formed with the steel in question is subjected has an important influence on the microstructure and therefore on the mechanical characteristics of the final component, that is, the steel with the initial chemical composition must be formed under certain conditions. and be subjected to a controlled cooling under certain conditions to achieve the optimized mechanical characteristics of the final component.

In order to guarantee high mechanical characteristics which in turn carry a high fatigue life, it is necessary to apply a specific procedure for deoxidation and settling of inclusions in the manufacturing process of this steel under certain special conditions.

A synergistic effect between a novel combination of chemical elements and a controlled cooling procedure has been proven to obtain the characteristics required for said steel.

In turn, as most of the automotive components for which this steel has been designed require significant machining operations after the forging procedure, the chemical composition of this steel is designed so that its machinability after the forming procedure is comparable to or even better than that of classic quenched and tempered steel, when both have similar levels of hardening and sulfur content.

The investigations carried out have resulted in a new quality of steel alloyed to SiMnV, with the chemical composition disclosed in claim 1.

These alloying elements are used in microalloyed steels to improve tensile strength, and other characteristics, which allow their use in the applications described.

Each of the alloying elements, in the proportions indicated above, influences certain parameters and properties of the steel finally produced.

Carbon is an essential element to obtain high strength and hardness, as well as high fatigue life. In microalloyed steels, carbon tends to form carbides that harden and give resistance to said steels. Silicon, in addition to being a powerful deoxidizer, provides solid solution hardening by dissolving in the ferritic matrix.

Manganese avoids the detrimental effect of sulfur, combining with it to form MnS and thus improve machinability. On the other hand, manganese, like silicon, dissolves in the crystalline lattice of the ferrite, replacing the iron atoms and causing a solid solution hardening.

Vanadium is a microalloying element that helps to refine the grain size and that when combined with carbon, forms vanadium carbides that cause intense precipitation hardening. Vanadium precipitates are hydrogen nucleators, so in corrosive environments they fix it and improve resistance to hydrogen. hydrogen-induced delayed fracture. However, with very high vanadium contents the precipitates coalesce and their effect can become detrimental. Therefore, the optimum vanadium content is between 0.05% and 0.50%.

Niobium is a microalloying element with similar effects to vanadium in controlling the grain size and in the hardening of steel by precipitation, in such a way that it contributes to increase the mechanical resistance and to improve the toughness. In addition, niobium precipitates bind hydrogen that attacks steel in corrosive environments, improving resistance to delayed fracture. Above 0.050%, however, there is a thickening of the precipitates which is detrimental to the mechanical properties in addition to increasing the risk of the appearance of bainites. The optimum niobium content is set between 0.001% and 0.050%.

Furthermore, it is contemplated that the steel proposed by the invention may additionally comprise at least one of the following elements or a combination of them, with a percentage by weight:

P <0.015%

S <0.10%

Cr <0.50%

Ni <0.50%

Mo <0.10%

Cu <0.25%

0.001% <Al <0.050%

0.001% <Nb <0.050%

0.004% <N <0.020%

The phosphorus hardens the steel and segregates at the austenite grain boundaries drastically reducing the toughness of the steel. In addition, it favors hydrogen embrittlement and delayed fracture. To limit its adverse effect, the phosphorus content is limited to less than 0.015%.

Sulfur embrittles steel in a similar way to phosphorous. Although this effect is counteracted by combination with manganese, manganese sulphides form inclusions that deform longitudinally in the forging or rolling direction and considerably deteriorate the transverse mechanical properties and fatigue behavior. However, sulfur must be added in small quantities as it causes improvements in the machinability of microalloyed steel, hence its content is limited to 0.10%.

Chromium is an essential element to ensure the hardenability of steels; However, as we want a ferrite-pearlite structure for our microalloyed steel, chromium is an element that we must control, hence we limit it to a maximum of 0.50%.

Nickel is an element that in high concentrations inhibits the formation of ferrite and also favors hardenability, hence, like chromium, it is an element that must be controlled in microalloyed steels, for that reason it must be limited to a maximum of 0.50%.

Molybdenum has a strong effect to favor hardenability, as does chromium and more than nickel, hence for a microalloyed steel with a ferrite-pearlite structure it is limited to a maximum of 0.10%.

The addition of copper prevents decarburization of steel and improves corrosion resistance in a similar way to nickel by inhibiting the growth of corrosion pits. However, high copper content worsens the hot ductility of steel, so the upper limit for copper is set at 0.25%.

Aluminum is an element that acts as a powerful deoxidizer during the steelmaking process. Aluminum forms aluminum nitrides that help control austenitic grain size during heating prior to hot forming procedures. However, it forms high hardness oxides that are very detrimental to fatigue life, so that its upper limit is set at less than 0.050%.

Nitrogen combines with Ti, Nb, Al and V to form nitrides, the precipitation temperatures of which depend on the respective content of the different elements and on constant characteristics. With a suitable size, these nitrides exert an anchoring effect on the austenitic grain, controlling its size at high temperature and preventing its coalescence and growth. However, if the content of nitrogen or microalloying elements is very high, precipitation occurs at high temperature and the precipitates thicken, becoming ineffective in controlling the grain and detrimental to fatigue life. Therefore, the nitrogen content in the steel is limited to 0.004% to 0.020%.

Thus, after various experiments, a rigorous procedure has been developed to produce steel according to the following steps:

- Strictly control the raw materials of the furnace.

- Carry out an oxidizing period in an electric furnace, which is important for the dephosphorization of the steel, prior to foamy slag.

- Once the foamy slag has finished, the slag is removed until the furnace is practically free of slag, the objective being a presence of phosphorus, in this step, of less than 0.007% by weight.

- Tilt to overturn with standard temperature and parts per million (ppm) of oxygen, making sure that no slag passes from the oven to the ladle.

- Carry out a prolonged vacuum, considering the vacuum time that is below 1 mbar.

- Finish the vacuum treatment at a temperature sufficient to carry out a decantation procedure for inclusions after it of at least fifteen minutes, without making additions or heating of any kind.

- Finally, a meticulous casting procedure must be followed with special protection of the liquid steel jet.

- If the solidification process is in continuous casting, the conditions of casting speed, cooling and stirring will be adjusted to produce a homogeneous solidification microstructure.

- Provide an adequate design of secondary cooling, as well as a slow tertiary cooling, in order to avoid thermomechanical stresses that can lead to the appearance of cracks.

- Reaching the straightening at a temperature above the hot ductility drop temperatures for these steels.

This entire steelmaking process makes it possible to achieve the desired sulfur levels, and phosphorous levels below 0.015% by weight, in addition to a low level of inclusion.

The CCT (continuous cooling transformation) diagrams allow to know the adequate cooling rates to obtain a ferrite-pearlite microstructure in the steel according to the invention.

The solidification products are subsequently transformed under hot conditions, through a procedure that consists of heating to a temperature between 900-1300 ° C and a series of consecutive deformations by forging or hot rolling until an intermediate product of the section is produced, form and suitable microstructure.

After various experimental tests, it has been found that, after the steel manufacturing process disclosed in the present document, with the above-mentioned chemical composition and adjusting the cooling rates after the forging or rolling process, a steel of ferrite-pearlite structure with tensile strength between 1050-1200 MPa, depending on the section of the bar or component. In order to produce a component of the steel previously produced with the aforementioned characteristics, the disclosure contemplates the possibility of carrying out a process by which said piece of steel is obtainable. This possibility includes, after the steel production, the following steps:

- manufacture a component made from said steel,

- carry out a controlled cooling of the component immediately after hot forming, - machining of the component with its final geometry.

Therefore, according to the disclosure, the process for obtaining pieces of said steel comprises a hot forming process, with a prior heating to a temperature between 900-1300 ° C that allows to provide the steel with sufficient hot ductility. , to give the steel part a shape similar to that of the final component. After putting it into shape, the piece is subjected to a controlled cooling that will allow it to achieve the desired mechanical characteristics with a 100% ferrite-pearlite structure.

Subsequently, it is contemplated that said disclosure of the manufacturing procedure of the micro-alloyed steel part comprises a machining procedure to obtain the final geometry of the component, hence, in order to improve the response of the steel to machinability, the steel may eventually have elements such as sulfur and other elements to improve machinability.

Therefore, it is contemplated that an embodiment of the procedure for obtaining steel parts comprises the following steps:

- Manufacture of the steel of the invention, previously described, in which the selected steel comprises the general composition or the preferred composition previously defined.

- Procedure for rolling the steel in question.

- Manufacture of a piece of said steel by means of a hot forming process.

- Controlled cooling of the piece after hot forming.

- Machining operations of the shaped part.

Description of the drawings

To complement the description that is being made and in order to help a better understanding of the characteristics of the invention, according to a reference example thereof, it is attached as part an integral part of said description, a set of drawings in which, for illustrative and non-limiting purposes, the following has been represented:

Figure 1 shows a table, divided into figures 1a and 1b, comprising photographs of the microstructures obtained after hot forming for the same part in the samples of the A-G steels defined in table 1.

Figure 2 shows photographs of the microstructures in which details of the bainites found in casts B, C, D and E can be seen.

Reference examples not according to the invention

By way of example, the tests carried out with samples of steels with compositions other than the chemical composition of the steel of the invention are described below. Table 1 shows the chemical compositions in percentage by weight, the rest being iron and impurities:

Table 1

Table 2 shows the values of the mechanical characteristics obtained on the same component already formed with all the castings in table 1.

Table 2

It can be seen that steels B-C-D and E have exceeded 1050 MPa of resistance and 750 MPa of elastic limit; however, these steels have shown traces of bainites in their microstructure after hot forming.

The following table shows the different percentages of phases found in the different components with the washes from Table 1.

Table 3

Figure 1 shows the photographs of the microstructures produced after hot forming for the same part.

Figure 2 shows the details of the bainites found in streams B, C, D and E.

As shown in table 2, steels A and G do not reach 1000 MPa of resistance. Steel A has a low percentage of carbon and manganese, so that it does not reach 1000 MPa of resistance or 700 MPa of elastic limit. In turn, steel G, although it has a higher percentage of carbon and manganese than A, has a low percentage of silicon, as well as the percentage of vanadium, so that the hardening by precipitation is lower than in the case of steel A.

Steel F has a tensile strength greater than 1000 MPa and an elastic limit of 700 MPa, somewhat higher than steels A and G. However, the mechanical characteristics that are the object of this patent have not been achieved due to the percentage of carbon. a little lower than the proper level.

The compositions of steels B and C were designed based on the composition of steel A but increasing the percentages of manganese in the case of steel B and manganese and silicon in the case of steel C. Thus, although higher strengths were achieved than 1050 MPa and elastic limits around 800 MPa, the bainite percentages obtained have not been acceptable for a microalloyed steel with a ferrite-pearlite microstructure.

Subsequently, the chemical compositions of steels D and E were designed, in which, in addition to the carbon percentage, the manganese percentage of casting B was maintained and the silicon percentages were increased. However, despite obtaining good mechanical characteristics with strengths greater than 1100 MPa and elastic limits greater than 800 MPa, residual percentages of bainite have continued to appear in the different components.

However, for H steel, it has been found that 1100 MPa of strength can be achieved for 30 mm diameter bar configuration parts.

Claims (1)

1. Microalloyed ferrite-pearlite steel with high tensile strength between 1050 and 1200 MPa and high elastic limit greater than 750 MPa, which comprises the following elements in percentage by weight: 0.35% <C <0.48% 0.55% <Yes <1.40% 0.55% <Mn <1.40% 0.05% <V <0.40% 0.005% <Ti <0.040%; 0.001% <Nb <0.050% the microalloyed steel optionally further comprising at least one of the following elements in percentage by weight: P <0.015% S <0.10% Cr <0.50% Ni <0.50% Mo <0.10% Cu <0.25% 0.001% <Al <0.050% 0.004% <N <0.020% Bi <0.15% Pb <0.20% Te <0.02% Se <0.04% the remainder being iron and impurities.