WO2008125700A1

WO2008125700A1 - Hardened and tempered steel and method for producing parts of said steel

Info

Publication number: WO2008125700A1
Application number: PCT/ES2007/000224
Authority: WO
Inventors: Maria Carmen Montero Pascual; Zuriñe IDOYAGA OLANO; Roberto Elvira Eguizabal; Jacinto José Albarran Sanz
Original assignee: Sidenor Investigación Y Desarrollo, S.A.
Priority date: 2007-04-13
Filing date: 2007-04-13
Publication date: 2008-10-23
Also published as: EP2159296B1; ES2576453T3; BRPI0721566A2; EP2159296A1; EP2159296A4

Abstract

Hardened and tempered steel with a specific composition of the following elements: 0.22 % ≤ C ≤ 0.30 %, 0.40 % ≤ Mn <1.00 %, 1.00 % ≤ Cr ≤ 2.50 %, 1.80, % ≤ Ni ≤ 4.00 %, 0.30 % ≤ Mo < 0.90 % and 0.01 % ≤ V < 0.50 %, which with a method for producing said steel achieves low P and S contents, contributes to increasing the mechanical resistance of the steel at values greater than 1200 N/mm2 and with high strength, low-temperature KV resilience, -20 °C, with values higher than 60 J, when a specific treatment of hardening and tempering is applied to a part for the manufacture of which said steel has been selected.

Description

STEEL OF TEMPLE AND KESVENIDO AND PROCEDURE TO OBTAIN

STEEL SAID PARTS

D E S C R I P C I Ó N

OBJECT OF THE INVENTION

The present invention relates to a tempering and tempering steel and a method for obtaining pieces of said steel, which has application in the field of the steel industry, allowing its use for metal structures in the construction sector, being especially suitable, said parts, in the shipping industry, for example for the manufacture of chains for ships and accessories of anchoring lines.

The invention allows to obtain a tempering and tempering steel, from a chemical composition and by means of a metallurgical process, which has a high mechanical resistance while also having a high tenacity at low temperature, in addition to having an optimum weldability.

BACKGROUND OF THE INVENTION

In the field of metallurgy, and more specifically in the case of steels, the industrial application of the elements or parts establishes minimum requirements as regards the mechanical behavior of steel.

In order to obtain certain mechanical properties in a steel, in addition to selecting its chemical composition, thermal treatments are carried out that allow the crystal structure of the steel to be modified without modifying its chemical composition, which allows Classify the steels according to the crystalline structure obtained after the heat treatment.

At present, a type of steel called tempering and tempering steels is known, which are used for the construction of metal structures and for the manufacture of elements and mechanical parts of responsibility, that is to say that they are subjected to loads during their work situation. , and for which a very small possibility of failure of said parts during their useful life is not admissible.

The fundamental mechanical characteristics that this type of parts has to have are high mechanical resistance, high toughness and an optimal relationship between the elastic limit and the mechanical resistance. In addition, high fatigue and elongation resistance is also required.

These characteristics are largely determined by the carbon content of the steel, which is usually between 0.03% and 0.70% by weight, as well as the content of other elements.

The high tensile strength values of quenching and tempering steels vary between 700 N / itun ² and 1700 N / mm ² , and are achieved with carbon weight contents ranging between 0.25% and 0.60%. In addition to improving other properties it is known to add varying amounts of alloying elements, such as Mn, Cr, Ni, Mo and V.

The increase of the carbon content in a steel produces on the one hand an increase in the tensile strength and in the Index of cold fragility of said steel, while on the other hand it produces a reduction of his tenacity and ductility.

Tenacity is the ability of a material to absorb energy without producing fissures, determined as an impact resistance, that is, the resistance offered by a material to a crack to propagate, or the energy absorption capacity of the material without producing fissures

The effect that each of the alloy elements has during the process of obtaining the steel, with respect to its response to heat treatments and in properties such as hardness and hardenability, is known by metallurgical technicians.

Among the multiple applications of tempering and tempering steels is the shipping industry, specifically in the manufacture of chains and other accessory elements and devices for anchoring lines, as well as in the liquefaction or gas transportation industries. In this area, it is essential that steels have high values of tensile strength and resistance to impact or toughness, which implies a combination of opposing mechanical properties, given that both characteristics are inversely proportional. In addition, these properties must be maintained even at low temperatures, of an order of magnitude of 20 ⁰ C below zero or even lower temperatures, considering the service conditions of these elements and parts given their field of application.

In addition, other requirements of tempering and tempering steels are that they have a good response to processes involved in the manufacturing, installation and assembly stages of these parts, such as for example welding or hot forming.

Tempering is a heat treatment whose objective is to harden and increase the resistance of steels at the cost of reducing ductility and resilience.

Resilience is the amount of energy that a material can absorb in the elastic field, that is, before the plastic deformation begins when it is under load. Resilience corresponds to the area under the stress-strain diagram of the tensile test of a material, between a zero strain value and a strain value corresponding to the creep stress, being an indicator of the fragility of the material .

The heat treatment of tempering consists in heating a steel to a temperature above its transformation point (Ac), also called higher critical temperature or transformation temperature, which depends on the chemical composition of the steel and can be for example between 800 ⁰ C and 950 ⁰ C, all in order to achieve an austenitic crystalline structure (Y). Subsequently, rapid cooling is carried out at a speed greater than the critical one in order to achieve a martensitic crystalline structure, or in any case if the cooling is not fast enough to achieve a bainitic crystalline structure, which provide steels with high resistance.

Thus, with tempering, a transformation of austenite (Y) into martensite and / or bainite of high resistance is achieved. The cooling speed depends on the dimensions of the piece or steel element to be tempered, water, oil, air or refrigerated means being used to perform said cooling, for example a refrigerated chamber.

In short, the factors that influence the hardening are the chemical composition of the steel, considering both the percentage of carbon and alloying elements, the temperature and the heating time and the cooling rate.

Once the tempering treatment has been carried out, it is very common to perform a tempering heat treatment to the steel in order to attenuate the effects and mechanical properties resulting from hardening, allowing to maintain to a large extent the strength and hardness values required while simultaneously manages to increase the toughness and elasticity of steel. Thus steels are obtained with an optimal combination of mechanical strength, elongation and elastic limit, steels with an elastic limit value of up to 75% being obtained the value of the breaking load. In addition to the combination between resistance and elongation, in the tempering and tempering steels the elastic limit is higher than the elastic limit of steels in which standardized or annealed heat treatments have been performed.

Tempering is a heat treatment which consists in heating to a temperature below the start of austenite transformation temperature, which usually occurs between 45O ⁰ C and 600 ° C. During this process the carbon contained in the martensite, in forced solution, precipitates in carbides and a transformation of the retained austenite (Y) occurs, while the martensite is it transforms into extremely small particles of cementite (Fe ₃ C) dispersed in a ferrite matrix (α), thereby eliminating the stresses created in the quenching of the quench treatment.

On the other hand, there is a thermal treatment called martempering, which is a particular case of the tempering and tempering treatment described above, in which the tempering stops before the martensitic transformation takes place, in order to homogenize the temperature of a piece of steel, prior to continuing to cool so that martensite is formed, proceeding below as in the case of a tempering treatment.

Therefore, for each type of application it is important to consider and define both the temperatures and the maintenance times at the tempering temperature, so that the final piece obtains the desired ratio of mechanical characteristics.

With regard to industrial applications that require higher levels of resistance, it is common to use steels alloyed with Mn, Cr, Ni, Mo and V, which achieves resistance values of up to 1000 N / mm ² and a toughness to low elevated temperature, with KV resilience values at -20 ° C around 60 J.

Currently, there are steels and procedures for obtaining them aimed at improving the service characteristics of the steels intended for the aforementioned applications, in which variable amounts of alloying elements are usually added, such as Mn, Cr, Ni, Mo , V or B, of which some examples are mentioned below. Korean patent no. KR 100320959-B describes a method for obtaining a steel with high tenacity at very low temperatures with high Mn content, which by weight is between 16% and 22%.

On the other hand, in the steel described in Korean patent no. KR 100325714-B, also manages to raise the tenacity at low temperature through a bainitic crystalline structure, however, the resistance values achieved are around 600 N / mm ² .

There are inventions related to steels with higher resistance values, which are intended to improve toughness at low temperatures, such as that described in European patent application no. EP 1697552, which refers to a steel wire rod product for cold forging and the process for its manufacture, comprising the addition of elements such as C, Si, Mn, Cr and B.

The Japanese patent no. JP 2000256783 describes a steel of high strength and toughness, with resistance to corrosion under stress, as well as its manufacturing method, where the elastic limit of said steel exceeds 960 N / mm ² (140 ksi), with contents by weight of C between 0.20% and 0.35%, of Cr between 0.20% and 0.70%, of Mo between 0.10% and 0.50%, and of V between 0.10% and 0.30% .

The development of the industries in which these steels are applied demands more and more superior values of tensile strength while maintaining toughness at low temperatures, without so far there is a known solution that combines tensile strength values greater than 1000 N / mm ² and high toughness, with KV resilience values at -20 ° C around at 60 J, while allowing the weldability of said steels.

Therefore, the properties of parts manufactured with tempering and tempering steels destined for these industries are likely to be optimized.

DESCRIPTION OF THE INVENTION

The present invention relates to a tempering and tempering steel and a method for obtaining pieces of said steel, in which as a result of various investigations an optimal combination of two opposing mechanical properties, a high tensile strength, with values has been achieved. of resistance of at least 1200 N / mm ² , and a high tenacity at low temperature, with KV resilience values at -20 ⁰ C of at least 60 J.

The invention makes it possible to obtain a tempering and tempering steel, from a novel chemical composition and a certain metallurgical process, which has a high mechanical resistance while also having a high tenacity at low temperature, in addition to having an optimum weldability, which It is important, for example, in the specific case of manufacturing chains for the shipping industry.

On the other hand, in addition to the chemical composition, the heat treatment performed on the steel has an important influence on the mechanical characteristics of the steel finally obtained, that is, the initial chemical composition is subjected to a certain tempering and tempering procedure, which is necessary to be done in specific time and temperature conditions.

For the manufacture of parts, it is necessary to apply a specific procedure in the manufacture of this steel in terms of the deoxidation process and with a process of decantation of inclusions under certain special conditions.

The inventors have proven a synergistic effect between a novel combination of chemical elements and a process for obtaining said steel, which contemplates a specific heat treatment, achieving a tempering and tempering steel of high strength and toughness, in addition to a good process aptitude Welding and forming.

The investigations carried out have resulted in a new quality of NiCrMoV alloy steel, which comprises the following chemical composition in percentage by weight:

0.22% <C ≤ 0.30% 0.40% <Mn <1.00% 1.00% <Cr <2.50% 1.80% <Ni <4.00%

0.30% <Mo <0.90% 0.001% ≤ V <0.50%

being the rest of the impurity elements that result from its obtaining.

These alloy elements are used in alloy steels to improve tensile strength, temper resistance, toughness or other characteristics, but not with the indicated weight concentrations, with the combination of elements that are proposes, neither to obtain the previously described properties that allow its use in the commented applications.

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

Manganese increases the hardenability and reduces the transformation temperature, which allows to obtain a crystalline structure of fine grains, allowing both to increase the resistance and improve the toughness.

The use of chromium allows a marked movement of the curves of the TTT, Temperature-Time-Transformation diagrams, to the right, which allows to greatly increase the hardenability in a less expensive way than other elements, as in the case of state of the art steels.

Nickel is a moderate temperability-enhancing agent and reduces the tendency to crack during hardening. The indicated nickel contents allow fine grain to be obtained, achieving greater resistance to shock, mainly at low temperatures.

Molybdenum has a strong temperability-enhancing effect, while being a strong carbide former, which provides a remarkable secondary hardening effect during tempering.

Finally, vanadium is a micro-alloying element that causes intense precipitation hardening and when it remains in solid solution It greatly increases the hardenability, also showing a strong secondary hardening effect during high temperature tempering, of an order of magnitude greater than 575 ° C.

In addition, the steel proposed by the invention may additionally comprise at least one of the following elements or a combination thereof, with a percentage by weight:

0.050% ≤ Yes ≤ 0.50%

P <0.015%

S <0.010%

Cu ≤ 0.350% 0.005% ≤ Al ≤ 0.050%

0.005% ≤ Ti ≤ 0.050%

0.004% ≤ N <0.020%

It is also contemplated that the steel of the invention comprises one or more of the following elements, with a percentage by weight:

Ca ≤ 0.005% Bi <0.15% Pb ≤ 0.20%

Te ≤ 0.02% Se ≤ 0.04%

the rest being residual elements that result from obtaining the steel.

Previously it has been proven that steels of similar composition to those that have been carried out a conventional tempering and tempering process did not reach the mechanical properties required and previously commented, because the degree of Cleaning was lower and the levels of S and P were not sufficiently reduced as in the quality that the steel of the invention presents.

In general, the presence of phosphorus and sulfur is detrimental for applications that require low temperature toughness, since they reduce the elongation and strength of steel, trying to eliminate these elements in the manufacturing processes. The general recommendation for ordinary steels of the state of the art is that the content of S, as well as that of P, does not exceed 0.060%, and in the case of quality steels, 0.030%.

A preferred composition of the steel proposed by the invention comprises a percentage by weight:

0.23% <C <0.28% 0.50% <Mn ≤ 0.90% 1.20% <Cr <2.0%

2.0% <Ni <3.50% 0.30% <Mo <0.70% 0.001% ≤ V <0.20%

For this preferred composition, additionally, the steel may comprise at least one of the following elements, or a combination thereof, by weight:

0.05% <If <0.50% P <0.015%

S <0.010%

Cu <0.350%

0.005% ≤ Al <0.050%

0.005% <Ti ≤ 0.050% 0.004% <N ≤ 0.020% Thus, after several experiments a rigorous procedure has been developed to obtain the steel following the following steps:

- Rigorously control the raw materials of the oven, that is, scrap metal and, especially, coke and lime.

- Use between 30% and 50% of high quality scrap. - Perform an oxidizing period in an electric oven, which is important for the defosforation of the steel, prior to the foamy slag.

- Once the foamy slag is finished, the oven is desescoriar until the furnace is practically left without slag, the objective being a phosphorus presence, in this step or stage, less than 0.007% by weight.

- Tilt to tilt with standard temperature and parts per million (ppm) of 0, according to standard clean steels, making sure that no slag passes from the oven to the spoon.

- Deoxidize with Al, to obtain very fluid white slag with lime-spate base.

- Strictly control refining raw materials, that is, ferromanganese, ferrochrome, nickel and lime.

- Make two gaps with intermediate H sample. Considering as a vacuum time that which is below 2 mbar and being 50% greater than the conventional vacuum time.

- Finish the second vacuum with a temperature sufficient to make a process of decantation of inclusions after it. During this settling time, the broth is stirred with argon without breaking the slag and without adding or heating of any kind. - Finally, a meticulous casting process must be followed with special jet protection.

All this steelmaking process allows to achieve low levels of sulfur, below 0.010% by weight, and phosphorus, below 0.015% in step, in addition to a low inclusion level.

The TTT (Temperature-Time-Transformation) diagrams allow to represent the thermal treatments for a specific chemical composition when the phase transformations occur in non-equilibrium conditions.

After several experimental tests, it has been found that after the steelmaking process that the invention proposes, with the chemical composition indicated above, adjusting the temperatures and maintenance times of tempering and tempering, a tensile strength steel is achieved above 1200 N / mm ² and a high toughness, resilience KV at -20 ⁰ C of 60 J. In addition, said steel has a good response to welding.

To obtain a piece of steel previously obtained, the invention contemplates the realization of a process by which said steel part is obtainable.

The procedure for obtaining parts of said steel comprises a hardening process which is performed with a higher austenitizing temperature to 800 ⁰ C, followed by further cooling, for example in water.

Next, the process comprises a tempering process that is carried out at a temperature higher than 550 ° C for about 2 hours, thus achieving the adjustment of the hardness and toughness of the material, in addition to avoiding decreases in resilience, which are associated with the phenomenon of brittleness of tempering.

Therefore, the procedure for obtaining steel parts comprises the following steps:

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

- Manufacture a piece of said steel, for example by forging or machining. - Carry out the previously defined tempering treatment on the piece.

- Carry out the tempering treatment defined above in the part.

When tempering and renewing a piece after machining, the work of lathe or milling cutter is easier to perform in an annealed state, being able to provide an excess in the final dimensions of the piece to eliminate deformations that occur during tempering and tempering, which can be removed later !, for example, by machining, in this case being small amounts of material, its removal is simple.

DESCRIPTION OF THE DRAWINGS

To complement the description that is being made and in order to help a better understanding of the features of the invention, according to a preferred example of practical realization thereof, accompanies as an integral part of said description, a set of drawings where, for illustrative and non-limiting purposes, the following has been represented:

Figure 1.- Shows a TTT diagram,

Temperature-Time-Transformation, of an eutectoid steel

(Y) with 0.77% C, in which a tempering and tempering heat treatment has been represented, where A is austenite, P is perlite, B is bainite and M is martensite.

Figure 2.- Shows a TTT diagram, Temperature-Time-Transformation, of an eutectoid steel like that of the previous figure, in which a martempering heat treatment has been represented.

Figure 3.- Shows a diagram representing the KV resilience values at -20 ⁰ C, in Joules, for each of the steel samples investigated in the present invention.

EXAMPLES OF EMBODIMENT OF THE INVENTION

Example 1

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, said samples are AE steels, steel F is the steel of the invention. Table 1 shows the chemical compositions in percentage by weight:

Table 1

All these steels have undergone tempering and tempering treatments under different conditions, with the aim of achieving the most optimal combination of mechanical resistance and low temperature toughness for each of them.

Thus, the most optimal results achieved are shown in table 2.

Table 2

As Table 2 shows, steels A, B, C, D and E do not reach a resistance of 1200 N / mm ² , maintaining a KV resilience at -20 ⁰ C of 60 J. Steels A and C have low carbon and vanadium contents, so that with the required toughness at low temperature, only resistance values around 1100 N / mm ² are achieved.

In turn, steels B, D and E, despite having a higher carbon content, do not achieve the desired resistance levels, since the combination of alloying elements is not adequate to achieve the required mechanical characteristics.

Figure 3 shows the resilience values KV at -20 ⁰ C obtained with a resistance of 1200 N / mm ² for the different AF steels.

However, for steel F, which has a chemical composition within the limits object of the invention, that is to say the steel that the invention proposes, it has been found that after being subjected to a bonus treatment it reaches the required mechanical characteristics and They also do not present any welding problem.

The invention has been described according to some preferred embodiments thereof, but it will be apparent to the person skilled in the art that multiple variations can be introduced in said preferred embodiments without exceeding the object of the claimed invention.

Claims

1.- Tempering and tempering steel, with high tensile strength and high impact resistance, characterized in that it comprises the following elements with a percentage by weight:

0.22% <C <0.30%

0.40% <Mn ≤ 1.0%

1.00% <Cr ≤ 2.50% 1.80% <Ni <4, 0%

0.30% <Mo <0.90%

0.001% ≤ V ≤ 0.50%.

2. Tempering and tempering steel according to claim 1, characterized in that it comprises at least one of the following elements with a percentage by weight:

0.050% ≤ Yes ≤ 0.50% P <0.015% S <0.010%

Cu <0.350% 0.005% ≤ Al ≤ 0.050% 0.005% <Ti <0.050% 0.004% ≤ N <0.020%.

3. Tempering and tempering steel according to claim 2, characterized in that it comprises at least one of the following elements with a percentage by weight: Ca <0.005%

Bi <0.15% Pb <0.20% Te <0.02% Se <0.04%.

4.- Tempering and tempering steel, according to the claim 1, characterized in that it comprises the following elements with a percentage by weight:

0.23% <C <0.28%

0.50% ≤ Mn ≤0.90% 1.20% ≤ Cr ≤ 2.0%

2.0% <Ni <3.50%

0.30% <Mo ≤ 0.70%

0.001% ≤ V ≤ 0.20%.

5. Tempering and tempering steel according to any of claims 2 and 4, characterized in that it comprises at least one of the following elements with weight percentage:

0.10% ≤ Yes ≤ 0.35% P ≤ 0.015%

S ≤ 0.010% Cu ≤ 0.350% 0.005% ≤ Al <0.035% 0.005% ≤ Ti <0.050% 0.004% ≤ N <0.020%.

6. Tempering and tempering steel, according to any of the preceding claims, characterized in that it comprises at least one of the following elements with weight percentage:

Ca ≤ 0.005% Bi ≤ 0.15% Pb <0.20% Te <0.02% Se <0.04%.

7. Tempering and tempering steel according to any of the preceding claims, characterized in that it has mechanical tensile strength greater than or equal to approximately 1200 N / mm ² and KV resilience at -20 ⁰ C greater than or equal to approximately 60 J.

8. Procedure for obtaining parts of a tempering and tempering steel according to any of claims 1 to 7, characterized in that it comprises obtaining said steel by the following steps: controlling the kiln raw materials, using between 30% and a 50% scrap of the highest quality, - perform an oxidizing period in an electric furnace, prior to the foamy slag, de-slag the foamy slag to a phosphorus presence of less than 0.007% by weight, tilt to the tilt with standard temperature and parts per million ( ppm) of 0, according to the standard of clean steels, making sure that no slag is passed from the oven to a spoon, deoxidize with Al, to obtain very fluid white slag with a lime-sparrow base, - check the refining raw materials, make two empty samples of intermediate H, considering as empty time that which is below 2 mbar, finish the second vacuum with a temperature sufficient to make a decanting process of inclusions after it and stir the broth with argon without breaking the slag and without making additions or heating, and - strain with special protection from the jet.

9. Method for obtaining hardened and tempered steel parts, according to claim 8, characterized in that after obtaining the steel it comprises the following steps: manufacturing a piece of said steel, perform a tempering heat treatment on the part that is performed with an austenization at a temperature higher than 800 ° C, followed by subsequent cooling, and perform a tempering treatment on the part that is carried out at a temperature greater than 550 ° C for about 2 hours.