KR102178736B1 - High-grade structural steel with bainite structure, high-grade structural steel with bainite structure, and method for manufacturing forgings and forgings - Google Patents

Abstract

The present invention provides a steel with high strength and low warp tendency without the need to undergo complex heat treatment processes. To this end, the engineering steel according to the invention (in weight percent), up to 0.25% C, up to 1.5% Si, 0.20-2.00% Mn, up to 4.00% Cr, 0.6-3.0% Mo, 0.004-0.020 % N, max 0.40% S, 0.001-0.035% Al, 0.0005-0.0025% B, max 0.015% Nb, max 0.01% Ti, max 0.10% V, max 1.5% Ni, max 2.0 % Cu, the balance contains iron and unavoidable impurities, but the Al content of the engineering steel is %Al, the Nb content of %Nb, the Ti content of %Ti, the V content of %V and the N content of %N satisfy the following conditions.
%Al/27 + %Nb/45 + %Ti/48 + %V/25> %N/3.75.
The engineering steel according to the invention has a yield strength of at least 750 MPa, a tensile strength of at least 950 MPa and a structure consisting of at least 80% by volume of bainite and up to 20% by volume of retained austenite, ferrite, perlite and/or martensite as a whole. It is equipped with. Due to the characteristic profile of the steel according to the present invention, the engineering steel according to the present invention is suitable for manufacturing a product whose cross section varies greatly over the entire length of the product by a forging technique. The present invention discloses a method for manufacturing such a forged product.

Description

High-grade structural steel with bainite structure, high-grade structural steel with bainite structure, and method for manufacturing forgings and forgings

The present invention relates to an engineering steel of high strength and a structure comprising at least 80% by volume of bainite.

The present invention also relates to forged products made of such engineering steel.

Finally, the present invention relates to a method for producing a forged product from engineering steel according to the present invention.

In the following, detailed descriptions of alloys or other steel composition are given in "%", and in all cases, unless specifically indicated, "%" relates to weight.

All mechanical properties specified in the description of the steels according to the invention and all steels cited for comparison, unless otherwise indicated, have been measured according to DIN EN ISO 6892-1.

Diplom-engineer Christoph Keul et al in the Schmide-Journal created in September 2010 by the industrial band Massivem Pormung E. Be., especially in the forging industry, "For heavy-stressed forged parts. As reported in the "Development of High-Strength, Ductile, Bainite (HDB) Steel", there is a demand for a steel material that is high strength, has high ductility, and can shorten the process chain for production. In this regard, this article is also a material of bainite structure that combines excellent strength and ductile properties without additional heat treatment, with tensile strength greater than 1,200 MPa, yield strength greater than 850 MPa, and notched impact at room temperature. It states that it has demonstrated the potential to achieve this demand by a bainite material characterized by an energy of 27 J and an elongation at break of greater than 10%. As an example of an alloying concept that provides these properties, this article (by weight) is 0.18% C, 1.53% Si, 1.47% Mn, 0.007% S, 1.30% Cr, 0.07% Mo, 0.0020% B, 0.027% Nb, 0.026% Ti, 0.0080% N, the balance is steel consisting of iron and inevitable impurities, and 　 0.22% C, 1.47% Si, 1.50% Mn, 0.006% S, 1.31% Cr, 0.09% Mo, 0.0025% B, 0.035 % Nb, 0.026% Ti, 0.0108% N, the balance presents a steel consisting of iron and unavoidable impurities.

Another development work aimed at producing steel for the production of drop forgings, which can obtain high strength and high ductility without further heat treatment, is described in EP 1 546 426 B1. The steel known in this patent document contains (in weight percent) 0.12-0.45% C, 0.10-1.00% Si, 0.50-1.95% Mn, 0.005-0.060% S, and in all cases 0.004-0.050% Al And Ti, in all cases up to 0.60% Cr, Ni, Co, W, Mo and Cu, up to 0.01% B, up to 0.050% Nb, 0.10%-0.40% V, 0.015-0.04% N and the balance being iron and inevitable impurities But, when the product of V and N content in steel is 0.0021-0.0120, S-content %S, Al-content %Al, Nb-content %Nb, Ti-content %Ti, 1.6×%S + 1.5×%Al + 2.4× %Nb + 1.2× %Ti = 0.040-0.080%, and Mn-content %Mn, Cr-content %Cr, Ni-content %Ni, Cu-content %Cu and No-content %Mo 1.2 × %Mn + 1.4 × %Cr + 1.0 × %Ni + 1.1 × %Cu + 1.8 × %Mo = 1.00-3.50%.

It is considered fundamental here that the necessary improvement in ductility is achieved by reducing the carbon content in the steel. The inevitable loss of strength accompanying this prior art is compensated for by conventional alloying elements, and the content of these alloying elements is adjusted to an amount that can be strengthened through mixed crystal formation.

Also, from DE 697 28 076 T2 (EP 0 787 812 B1), (in weight percent) 0.1-0.4% C, 1-1.8% Mn, 0.15-1.7% Si, up to 1% Ni, up to 1.2% Cr, up to 0.3% Mo, 0.3% V max, 0.35% Cu max, and in each case as required 0.005-0.06% Al, 0.0005-0.01% B, 0.005-0.03% Ti, 0.005%-0.06% Nb, 0.005-0.1% S , Up to 0.006% calcium, up to 0.03% Te, up to 0.05% Se, up to 0.05% Bi and the balance cast steel consisting of iron and unavoidable impurities, forming a hot-forged semi-finished product to produce forgings in a conventional manner A process for producing a forged steel component is known. The forged product is subjected to a heat treatment including the step of cooling at a cooling rate Vr greater than 0.5°C/sec, from the temperature at which the steel is in austenite state to a temperature Tm between Ms+100°C and Ms-20°C. The forged product is then held for at least 2 minutes at a temperature between the temperature Tm and the temperature Tf (where Tf> Tm-100° C.). In this way, the invention obtains a steel part substantially consisting of a bainite structure comprising at least less than 15% and preferably at least 20% bainite formed between Tm and Tf.

Practical attempts made with steel materials of the above type have shown that such bainite steels are not suitable for products with a large number of cross-sectional changes due to the tendency of products to warp and fluctuations in mechanical properties.

Against this technical background, the problem to be solved by the present invention is that there is no need to perform a complex heat treatment process, and the tendency to bend is small. Is to provide a river of.

The present invention similarly proposes a forged product in which physical properties are optimally combined without performing a complicated heat treatment process.

Finally, the present invention proposes a method for manufacturing a forged product capable of manufacturing a forged product in which physical properties are optimally combined using a simple means.

With respect to these steels, the present invention solves the above-described problem with the engineering steels specified in claim 1.

With respect to the forged product, the solution according to the invention to the above-described problem consists in manufacturing a steel part from the steel according to the invention.

Finally, with regard to the method, the present invention solves the above-described problem by performing the process steps recited in claim 13 in the manufacture of a forged product.

Advantageous forms of the present invention are set forth in the dependent claims, and the advantageous forms of the present invention will be described in detail below together with the general idea of the present invention.

The yield strength of the engineering steel according to the invention is at least 750 MPa, the tensile strength is at least 950 MPa, at least 80% by volume of the structure is made of bainite structure, and the remaining 20% by volume of the structure is retained austenite, ferrite, pearlite. Or it may be martensite.

Here, the steel according to the invention is characterized in that the elongation at break A is at least 10%, in particular at least 12%, and it has been demonstrated that in practice the steel according to the invention achieves an elongation at break A of at least 15%.

According to the invention, engineering steels (by weight) are up to 0.25% C, up to 1.5% Si, in particular up to 1% Si or up to 0.45% Si, 0.20-2.00% Mn, up to 4.00% Cr, 0.7-3.0% Mo , 0.004-0.020% N, 0.40% S max, 0.001-0.035% Al, 0.0005-0.0025% B, 0.015% Nb max, 0.01% Ti max, 0.50% V max, 1.5% Ni max, 2.0% Cu max and cup The part contains iron and unavoidable impurities, and the following conditions are satisfied when the Al-content %Al, Nb-content %Nb, Ti-content %Ti, V-content %V and N-content %N.

%Al/27 + %Nb/45 + %Ti/48 + %V/25> %N/3.75

Inevitable impurities generated in the manufacturing process are present in an amount that does not affect the alloying process in relation to the properties of interest in the present invention, and the steel-making route or each selected starting material (scrap) enters the steel. Includes all elements present along the line. Unavoidable impurities, in particular, also contain up to 0.0035% by weight of the P component.

It is noted that the steel according to the present invention and the forgings made of the steel have a particularly uniform distribution of properties even when the cooling conditions are locally significantly different across the volume of the forged product while cooling the forging heat as the part dimensions change. It is characterized. Such insensitivity to cooling conditions is achieved by homogeneous holding of a bainite structure with entirely small hardness fluctuations, if possible, for the engineered steel according to the invention. This homogeneous microstructure at the same time has a low internal stress, which has a positive effect on the warping behaviour.

Consequently, the steel according to the invention is particularly suitable for producing forged parts in which sections of significantly different volume and diameter face each other. Examples of forged products that are particularly suitable for producing the steel according to the invention by the forging technique are crankshafts, piston rods and similar products used in particular in combustion engines.

In addition, parts in the chassis and wheel suspension regions having significantly different cross-sections can be reliably manufactured from the steel according to the present invention without undergoing a major post-treatment process by polishing while maintaining predetermined strength characteristics.

As can be seen from the time-temperature diagram of the steel according to the invention described in Figure 1, in terms of material technology, this is for bainite treatment when continuously cooled from the forging heat using the engineering steel according to the invention. In particular, wide windows can be used. In doing so, the alloying of the engineering steel according to the invention is selected so that none of the amounts of martensite or ferrite and/or pearlite affect the physical properties during cooling resulting in a structure. Accordingly, the engineering steel according to the invention mainly has a bainite structure of at most at least 80% by volume. Here, the content of the non-bainite structure components in the engineering steel according to the present invention is generally minimized so that the engineering steel according to the present invention completely has a bainite structure in the technical sense.

Here, when the engineering steel according to the present invention is used, the hardness is almost constant in the bainite regardless of the cooling rate. A constant hardness is preferably obtained as a result of the almost complete transformation of austenite to bainite in the bainite transformation step.

Limiting the C content to a maximum of 0.25% by weight means, on the one hand, that the engineering steel according to the present invention has excellent elongation and ductility properties, even at maximum strength. In the engineering steel according to the present invention, the low C content contributes to promoting bainite transformation, thereby preventing unwanted tissue components from developing.

At the same time, however, a certain amount of carbon in the engineering steel according to the present invention can also contribute to strength. For this purpose, a carbon content of at least 0.09% by weight can be expected in the steel. The optimized effect of the presence of C in the engineering steel according to the invention can be achieved if the C content is adjusted to 0.09-0.25% by weight.

If possible, the Si content of the engineering steel according to the invention is limited to 1.5% by weight, in particular 1% by weight or 0.75% by weight, so that bainite transformation can occur as early as possible. In order to particularly ensure the achievement of this effect, the Si content may be limited to a maximum of 0.45% by weight.

Mo is present in the engineering steel according to the present invention in an amount of 0.6-3.0% by weight to retard the transformation of the structure into ferrite or pearlite. This effect is obtained when Mo is present in the steel in particular at least 0.7% by weight, especially more than 0.70% by weight. If it is present more than 3.0% by weight, no economically viable increase in the positive effect of Mo occurs in the engineering steel according to the invention. In addition, if the Mo content exceeds 3.0% by weight, there is a risk of forming a molybdenum-rich carbide phase that may adversely affect ductile properties. When the Mo content is at least 0.7% by weight, the optimum effect by Mo can be expected in the engineering steel according to the invention. Here, it has been found to be particularly effective that the maximum content of the Mo component is 2.0% by weight.

In order to control the tensile strength and yield strength, manganese is present in 0.20%-2.00% by weight in the engineering steel according to the present invention. In order to increase the strength, it is essential that at least 0.20% by weight of Mn is added. In order to achieve this effect with special reliability, the Mn content can be provided with at least 0.35% by weight. When the Mn content is too high, bainite transformation is delayed and martensite transformation occurs mainly. Accordingly, the Mn content is limited to a maximum of 2.00% by weight, in particular 1.5% by weight.

The negative effects exhibited by the presence of Mn can be prevented particularly reliably by limiting the Mn content in the engineering steel according to the invention to a maximum of 1.1% by weight.

In order to support the machinability of the engineering steels according to the invention, the sulfur component can be up to 0.4% by weight, in particular up to 0.1% by weight or up to 0.05% by weight.

Fine control of the alloying technique with respect to the microstructure and mechanical properties of the engineering steel according to the present invention can be achieved by making the content of boron elements 0.0005-0.0025% by weight, nitrogen content 0.004-0.020% by weight, in particular at least 0.006% by weight N or up to 0.0150. The alloying concept according to the invention through micro-alloying of wt% N, aluminum content 0.001-0.035 wt% and niobium content up to 0.015 wt%, titanium content up to 0.01 wt% and vanadium content up to 0.10 wt% Is made according to.

Here, if the contents of Al, Nb, Ti, V, and N are set to %Al, %Nb, %Ti, %V and %N, these components are contained in the engineering steel when Al and essential components Nb, Ti and V are added. The nitrogen contained is completely bound, and thus boron is connected under the following conditions so that it can have a transformation-delaying effect. At the same time, the content and balance of the microelements provided according to the present invention and other elements and N components contribute to increasing the microcrystal stability and strength.

%Al/27 + %Nb/45 + %Ti/48 + %V/25> %N/3.75

The binding of N according to the present invention can make boron act effectively as an element dissolved in the matrix, and can suppress the formation of ferrite and/or pearlite.

In order to reliably utilize the advantages of fine alloying elements and aluminum, the Al content is at least 0.004% by weight, the Ti content is at least 0.001% by weight, the V content is at least 0.02% by weight, or the Nb content is at least 0.003% by weight. It may be appropriate to do. Here, the fine alloying elements V, Ti and Nb on the one hand and Al on the other hand are, in all cases, present in combination with one or more elements selected from the group "Al, V, Ti, Nb", or It can be present alone in one minimum content.

For Ti at a maximum content of 0.008% by weight, Nb at a maximum content of 0.01% by weight, V at a maximum content of 0.075% by weight or Al at a maximum content of 0.020% by weight, the action of these elements in the structural steel according to the invention is particularly good Can be used to represent At the same time, the formed nitride or carbon nitride increases the strength and contributes to the fine grain stability. Here, in order to achieve the optimum effect of the alloying element of interest in all cases, the upper limit of Ti, Nb, V or Al described above may be adhered to alone or in combination with each other.

Cr, which is present up to 4.00% by weight, in particular up to 3% by weight or up to 2.5% by weight, if necessary, contributes to the durability and corrosion resistance of the steel according to the invention. For this, as an example, Cr may be provided by at least 0.5% by weight or at least 0.8% by weight.

Similarly, Ni, which is present in up to 1.5% by weight as needed, can contribute to the hardenability of the steel as above.

Alloying elements present or intentionally added in the steel according to the invention via the starting material may include Cu. In this case, the content is limited to a maximum of 2.0% by weight in order to avoid negatively affecting the engineering steel according to the invention. The positive effects of copper present as needed in the alloying of engineering steels according to the present invention include finely forming residual austenite films and significantly increasing the ductility level. This effect can be achieved by the presence of at least 0.3% by weight of Cu, in particular more than 0.3% by weight, in the engineering steel according to the invention. By limiting the Cu content to a maximum of 0.9% by weight, the positive effect by the copper component can be optimally achieved.

When the engineering steel according to the invention is heated to a typical heating temperature at least 100° C. above the Ac3 temperature, it is heated to a temperature above 900° C., especially for hot working, and finally under a stationary or moving air stream after hot working. When cooled to temperatures below 200[deg.] C., particularly room temperature in a controlled or uncontrolled manner, a uniform bainite structure is obtained over a fairly wide range of cooling rates that follow the transformation. The temperature of the steel Ac3 can be determined in a known manner based on the composition of the steel. The upper limit of the temperature range to be heated is generally 1,300°C, in particular 1,250°C or 1,200°C.

As a cooling rate range, t8/5 hours, which is the time taken to cool each hot-worked product from 800°C to 500°C, can be used. Thus, in the case of the cooling of parts made of engineered steel according to the invention, this t8/5 time is intended to be 10-1,000 seconds.

The cooling time selected in all specific cases should be selected on the basis of each heating time. The effect of heating time can be understood from the time-temperature diagram shown in FIG. 2. In the diagram of FIG. 2, for heating temperatures of 900° C. (solid line), 1,100° C. (dashed line) and 1,300° C. (dotted line), each location of each bainite region is shown over the cooling time. Accordingly, in order to obtain a desired bainite structure, a short t8/5 time must be selected at a low heating temperature of 900°C, whereas a cooling rate may be reduced at a high heating temperature. When the engineering steel according to the present invention is heated in the range of 900-1,300°C and thus the t8/5 time is 100-800 seconds, the engineering steel according to the present invention is cooled to reach the bainite region regardless of each heating temperature. There is a high certainty of becoming.

The alloying concept according to the invention allows hot working at high temperatures in excess of 1,150° C., as a result of which the forming forces can be reduced without undesired grain growth during hot-working.

Forged products with a yield strength of at least 750 MPa, a tensile strength of at least 950 MPa and at least 80% by volume of bainite, and the rest of the structure up to 20% by volume may be retained austenite, ferrite, pearlite or martensite The method according to the present invention for producing a product comprises the following process steps.

a) providing a semi-finished product for forging comprising an engineered steel with a component composition according to the present invention as described above;

b) for forging, heating the semifinished product to a forging temperature at least 100° C. higher than the Ac3 temperature of each engineering steel, wherein the Ac3 temperature is determined in a conventional manner according to the composition of each component of the engineering steel;

c) forging the forging semi-finished product heated at the forging temperature into a forging product;

d) Cooling the forged product from the forging heat to a temperature below 500°C, but cooling t8/5-hours for cooling to 10-1,000 seconds.

In order to reduce the forming force during carrying out the method according to the invention with regard to minimization of the required forging force, it has proved advantageous if each semi-finished product representing the forging starting point is heated to a forging temperature above 1,150°C.

In order to further control the mechanical properties, especially strength and ductility of the hot-processed product of the engineering steel according to the present invention, especially the forged product, a tempering operation to keep each product at a temperature range of 180-375℃ for 0.5-2 hours This can be done.

Indeed, with the steel according to the invention, it is possible to obtain a tensile strength of at least 950 MPa, a yield strength of at least 750 MPa, and a judgment elongation A of at least 15%, where in practice the elongation at break A is a higher value of at least 17%. It turns out that can be achieved reliably. The combination of these features within forgings comprising the steel according to the invention is particularly advantageous if the forgings are produced in the manner according to the invention.

1 is a time-temperature diagram of a lecture according to the invention.
2 is a time-temperature diagram.

Hereinafter, the present invention will be described in more detail with exemplary embodiments.

Steel melts E1-E6 and comparative melts V1 according to the present invention with the composition shown in Table 1 were smelted and cast into semi-finished products containing blocks that can be used conventionally for further processing by forging techniques.

In order to forge the semi-finished product, the semi-finished product was heated to a heating temperature Tw, and then hot-processed in a conventional manner using a drop forging device to produce a forged product, and then cooled to room temperature in the atmosphere. Some of the obtained forged products were subjected to tempering treatment.

Table 2 shows the heating temperature Tw applied in this example, t8/5 hours, the tempering treatment temperature and residence time necessary to pass the critical temperature range of 800-500°C. This was actually carried out and shows the bainite fraction in the structure, the tensile strength Rm, the yield strength Re, the elongation A, and the notch impact energy W of the forged product obtained by forging.

The examples show that when the specifications according to the present invention are satisfied, the operating parameters set during the manufacture of the product can be varied over a wide range, thereby obtaining a hot-machined part with optimized mechanical properties. have.

Claims (15)

Engineered steel having a structure consisting of at least 80% by volume of bainite and up to 20% by volume of retained austenite, ferrite, pearlite and/or martensite as a whole, with a yield strength of at least 750 MPa, a tensile strength of at least 950 MPa. , The engineering steel (by weight),
C: 0.25% max (excluding 0%),
Si: 1.5% max (excluding 0%),
Mn: 0.20-2.00%,
Cr: 0-4.00%,
Mo: 0.6-3.0%,
N: 0.004-0.020%,
S: 0-0.40%,
Al: 0.001-0.035%,
B: 0.0005-0.0025%,
Nb: 0-0.015%,
Ti: 0-0.01%,
V: 0-0.10%,
Ni: 0-1.5%,
Cu: 0-2.0%,
The balance includes iron and unavoidable impurities,
Engineering steel, characterized in that the Al content %Al, Nb content%Nb, Ti content%Ti, V content%V and N content%N of the engineering steel in all cases satisfy the following conditions.
%Al/27 + %Nb/45 + %Ti/48 + %V/25> %N/3.75. The method of claim 1,
Engineering steel, characterized in that the C content is at least 0.09% by weight. The method of claim 1,
Engineering steel, characterized in that the Al content is at least 0.004% by weight. The method of claim 1,
Engineering steel characterized in that the Al content is up to 0.020% by weight. The method of claim 1,
Engineering steel, characterized in that the Nb content is at least 0.003% by weight. The method of claim 1,
Engineering steel, characterized in that the Nb content is up to 0.01% by weight. The method of claim 1,
Engineering steel, characterized in that the Ti content is at least 0.001% by weight. The method of claim 1,
Engineering steel characterized by a Ti content of up to 0.008% by weight. The method of claim 1,
Engineering steel, characterized in that the V content is at least 0.02% by weight. The method of claim 1,
Engineering steel, characterized in that the V content is up to 0.075% by weight. The method of claim 1,
Engineering steel, characterized in that the elongation A at the breaking point is at least 10%. A forged product comprising the steel obtained according to claim 1. Forged products with a yield strength of at least 750 MPa, a tensile strength of at least 950 MPa and at least 80% by volume of bainite, and the rest of the structure up to 20% by volume may be retained austenite, ferrite, pearlite or martensite As a manufacturing method, the method for manufacturing a forged product, characterized in that it comprises the following process steps.
a. Providing a semi-finished product for forging comprising an engineered steel having a composition according to any one of claims 1 to 10;
b. Heating the forging semi-finished product to a forging temperature at least 100° C. higher than the Ac3 temperature of engineering steel;
c. Forging a forging semi-finished product heated at a forging temperature into a forging product;
d. Cooling the forged product from the forging heat to a temperature below 200°C, but cooling the forged product from 800°C to 500°C for cooling takes 10 to 1,000 seconds. The method of claim 13,
A method for manufacturing a forging product, characterized in that the forging temperature is higher than 1,150°C. The method of claim 13,
After cooling the forged product, a method for manufacturing a forged product, characterized in that the forging product is subjected to a tempering treatment for 0.5-2 hours at a tempering temperature of 180-375°C.

Images