KR100840287B1 - Composite steel of retained austenite and hcp martensite, and method for heat treatment thereof - Google Patents

Abstract

A composite steel of residual austenite and hexagonal close packed martensite and a method for treating the same with heat are provided to improve toughness and strength by lowering transformation temperature of martensite while restraining components other than manganese. A composite steel of residual austenite and hexagonal close packed martensite comprises less than 0.5 weight percent of C, not less than 15 and not more than 35 weight percent of Mn, equal to or less than 3 weight percent of at least one Co, Ti, Al and W, equal to or less than 10 of at least one of Cr, Cu, Mo, Ni, Nb Si and V, balance of Fe, and unavoidable impurities. The contents of components in the alloy satisfy an MS equation described below. The value of MS equation is 50 to 120. MS=260-330C+2Al+7Co-14Cr-13Cu-19(Mn-15)-5Mo-4Nb-13Ni-7Si+3Ti-4V.

Description

Composite Steel of Retained Austenite and HCP Martensite, and Method for Heat Treatment Thereof}

1 is a photograph showing the final microstructure of the composite tissue steel according to an embodiment of the present invention.

Figure 2 is a photograph showing the final microstructure of the composite tissue steel according to Comparative Example 1.

3 is a photograph showing the final microstructure of the composite tissue steel according to Comparative Example 2.

4 is a photograph showing the final microstructure of the composite steel according to Comparative Example 3.

The present invention relates to a composite tissue steel in which residual austenite and HCP martensite structures are mixed, and a method for heat treatment thereof. More specifically, the present invention relates to a composite tissue steel in which retained austenite and HCP martensite structure are mixed, in which austenite remains at room temperature and the martensite plate is fine to improve toughness and strength, and a method for heat treatment thereof.

Representative steel using the residual austenite at room temperature are known TRIP steel (TR ansformation I nduced lasticity P).

TRIP steels are obtained by a method of obtaining stable retained austenite at room temperature, mainly by concentrating carbon dissolved in the known austenite by residual ferrite and bainite phase transformation in the low carbon steel region.

On the other hand, according to the Quenching & Partitioning method recently attracted attention, the interior of the martensite phase formed by isothermally holding in one or two stages after quenching between the martensite transformation start temperature (M _s ) and the transformation end temperature (M _f ). By moving the carbon present in the residual austenite to concentrate the carbon in a stable state at room temperature it can be produced a steel having excellent strength and toughness at room temperature.

However, in this case, partitioning should be performed at a temperature as high as room temperature or higher to promote the movement of carbon. Therefore, a facility for heating and maintaining the steel during the manufacturing process of the steel is required. And there is a problem that the manufacturing cost increases.

On the other hand, there are many researches on the production of steel containing residual austenite at room temperature using high alloy steels in which nickel (Ni) or chromium (Cr) is added in a large amount, but in practice, in order to leave residual austenite at room temperature Since a large amount of expensive additional elements must be added, the manufacturing cost is high and there is a problem of low economic efficiency.

The present invention has been invented to solve this problem, instead of limiting the amount of the remaining components except for manganese (Mn) by using the martensite reverse transformation to reduce the start temperature of martensite transformation, austenite remains at room temperature and martensite It is an object of the present invention to provide a composite tissue steel in which the size of the plate is minute so that toughness and strength are always present, and a method of heat treatment thereof.

The present invention, based on the weight percent, C: 0.5 or less, Mn: more than 15 and 35 or less, the sum of any one or more of Co, Ti, Al and W: 3 or less, Cr, Cu, Mo, Ni, Nb, Si and It provides a composite tissue steel in which the sum of any one or more of V is 10 or less and a mixture of residual austenite and HCP martensite structure composed of the remaining Fe and other inevitably added impurities.

In the composite tissue steel, the content of the alloy components satisfies the following MS equation, and the value of the MS equation may be 50 to 120. MS = 260-330C + 2Al + 7Co-14Cr-13Cu-19 (Mn-15)-5Mo-4Nb-13Ni-7Si + 3Ti-4V.

In addition, the present invention, after hot-rolling the composite tissue steel to produce a steel material, the primary quenching step of heating and quenching the steel material to quench to have a martensite structure; A second quenching step of heating the quenched steel in an austenite reverse transformation start temperature (A _s ) to 800 ° C. or lower and then quenching to refine the size of the reverse austenite grains; A quenching step of quenching the steel to a refrigerant of room temperature (25 ° C.) or lower after the second quenching step; And it provides a heat treatment method of a composite tissue steel mixed with the retained austenite and HCP martensite structure, characterized in that the steel is maintained at room temperature after the quenching step.

In the above method, the composite tissue steel, the content of the alloy components satisfy the following MS formula, the value of the MS formula may be 50 to 120. MS = 260-330C + 2Al + 7Co-14Cr-13Cu-19 (Mn-15)-5Mo-4Nb-13Ni-7Si + 3Ti-4V.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. First of all, in adding reference numerals to the components of each drawing, it should be noted that the same reference numerals are used as much as possible even if displayed on different drawings. In describing the present invention, when it is determined that the detailed description of the related well-known configuration or function may obscure the gist of the present invention, the detailed description thereof will be omitted.

Composite tissue steel according to an embodiment of the present invention, C, Mn, Co, Ti, Al, W, Mo, Ni, Nb, Si, V, the remaining Fe and alloy components consisting of impurities are inevitably added.

More specifically, the composite tissue steel according to the embodiment of the present invention, based on the weight percent, C: 0.5 or less, Mn: more than 15 and 35 or less, the sum of any one or more of Co, Ti, Al and W: 3 or less, The sum of any one or more of Cr, Cu, Mo, Ni, Nb, Si, and V is 10 or less, and may be composed of the remaining Fe and other inevitably added impurities.

In particular, in the composite steel according to the embodiment of the present invention, the MS value calculated by using the content of the alloying components as a variable is represented by the following formula (1).

MS = 260-330C + 2Al + 7Co-14Cr-13Cu-19 (Mn-15)-5Mo-4Nb-13Ni-7Si + 3Ti-4V (Equation 1)

Equation 1 is derived from investigating and correcting the influence of manganese (Mn) based on the martensite production temperature equation proposed by Ishida et al.

On the other hand, the martensite transformation start temperature (M _s ) is influenced not only by the composition but also by the size of the austenite grains.

In other words, the smaller the size of the austenite grains, the lower the start temperature of martensite transformation. This is because driving force required for transformation is additionally required.

In addition, in the Example of this invention, Formula 1 was used only as a parameter for restricting a composition.

In the embodiment of the present invention, the composition was limited so that the MS value was in the range of 50 to 120, and the actual martensite transformation start temperature was reduced to about 10 to 40 ° C from Equation 1 through the refinement of the austenite grains through the martensite reverse transformation. This is to obtain an appropriate fraction of HCP residual austenite in a situation where the degree is lowered.

If the MS value is 50 or less, a sufficient amount of martensite cannot be obtained, so the strength of the steel is lowered.

On the other hand, if the MS value exceeds 120, even if the actual martensite transformation start temperature (M _s ) is reduced through the refinement of the grain, the amount of retained austenite obtained at room temperature is very small, and thus sufficient toughness increase effect cannot be obtained. .

In the case of carbon (C), it may be regulated to about 0.5% by weight or less, and in the case of containing more than that, even if the content of other alloying elements is adjusted, it is difficult to obtain the composite structure steel which is the object of the present invention. In addition, when the carbon content is high, stacking fault energy (Stacking Fault Energy) is increased, there is a fear that the BCT martensite instead of HCP martensite is generated after the heat treatment.

On the other hand, the reason for limiting the content of manganese (Mn) to more than 15% by weight is to obtain martensite of HCP structure at room temperature. Meanwhile, the content of manganese is preferably 35% by weight or less.

In other words, HCP martensite is a metastable phase, which transforms into a BCC or BCT structure due to stress, which greatly contributes to toughness improvement. Considering the refinement of crystal grains, the content of manganese (Mn) is 15% by weight. The reason for this is that martensite of the HCP structure is not produced in a sufficient amount so that the effect of increasing toughness is insignificant. On the other hand, the above values are estimated from the experimental results for the Fe-Mn-C alloy and Fe-Mn-C- (a small amount of Ni, Cr) alloy.

The reason for limiting the total content of the total content of the alloy components Cr, Cu, Mo, Ni, Nb, Si and V to 10 wt% or less is as follows.

The martensite transformation start temperature (M _s ) is considerably raised from room temperature in general steels other than high alloy steels containing a large amount of alloying components such as nickel (Ni) and chromium (Cr). Therefore, in order to obtain a composite steel with mixed austenite and martensite structure at room temperature, an alloying element should be added to lower the martensite transformation start temperature (M _s ). In other words, if a large amount of the above alloying elements are used, the range of MS values proposed in the present invention can be adjusted, and residual austenite can be obtained at room temperature. Because you can't get it.

In addition, since the above alloy elements are mostly expensive, a large amount of these alloys cause additional economic problems.

Next, a heat treatment method of the composite structure steel according to the embodiment of the present invention will be described.

The room temperature residual austenite and HCP martensite composite tissue steel according to the embodiment of the present invention are melted and made of molten steel, continuously cast and cast into slabs, and then reheated in a heating furnace to a sheet metal through hot rolling. Are manufactured. In addition, thereafter, cold rolling, annealing heat treatment, and sheet metal processing are used to manufacture the product shape as necessary.

First, the steel is maintained in a quenching heat treatment furnace, and then quenched by using a coolant (cooling water, cooling oil, etc.) to quench heat treatment. In this process, a martensitic structure is obtained. The steel thus treated is quenched again above the austenite reverse transformation start temperature (A _s ) and below 800 ° C. The reverse transformation after quenching and quenching is for miniaturizing the size of the grains of austenite reversely transformed through reverse transformation in the martensite state.

Martensitic tissue is a tissue containing a large number of plate boundaries and dislocations inside, and the grain size becomes smaller when reverse transformation due to the large number of nucleation sites corresponding to the sub-grain boundary. .

In addition, the reason for maintaining the re-heating temperature at the time of reverse transformation above 800 ° C above the reverse transformation start temperature (A _s ) is to obtain reversed austenite structure, but to prevent the growth of fine grains during quenching and to maintain the fine size. To do this.

On the other hand, the reverse transformation of the steel by quenching using a refrigerant such as ice water at room temperature (25 ° C. or less) and then maintaining it at room temperature is for obtaining an appropriate amount of HCP martensite and preserving residual austenite at room temperature. On the other hand, the temperature of the refrigerant during quenching can be adjusted according to the desired intensity level. Of course, the elongation at this time decreases with increasing strength, so the temperature of the refrigerant should be adjusted in consideration of the characteristics of the final product.

Hereinafter, the present invention will be described in more detail with reference to Examples.

Example

After melting and casting the room temperature residual austenite and HCP martensite composite steel having a composition range satisfying Equation 1, hot rolled steel was manufactured. Using the hot rolled steel sheet thus prepared, a 100 mm wide, 230 mm long, 1 mm thick specimen was fabricated, held for 10 minutes in a quenching heat treatment furnace maintained at 950 ° C, and then quenched using a coolant (cooling oil). Quenching heat treatment was performed. Then, after the martensite structure was obtained by quenching, the steel material was quenched again above the austenite reverse transformation start temperature (A _s ) and below 800 ° C. On the other hand, the austenite reverse transformation start temperature (A _s ) was measured and determined because it is affected by the composition and heating rate of the steel.

On the other hand, in the present Example, it was maintained at the same temperature of 780 degreeC and re-quenched for 20 minutes so that the comparison with a comparative example might be easy.

Table 1 lists the chemical compositions and MS values of the examples of the present invention and each of the comparative examples.

division C Mn Si Cr Ni P S Remainder MS value Example 0.30 20.50 0.18 0.11 0.02 0.0030 0.0017 Balance Fe and Other Unavoidable Impurities 53.44 Comparative Example 1 0.86 0.65 0.28 0.22 0.32 0.0009 0.0015 239.65 Comparative Example 2 0.36 1.76 0.20 0.40 30.56 0.0012 0.0021 -11.52 Comparative Example 3 0.15 10.15 0.28 3.20 15.30 0.0015 0.0020 56.99

Meanwhile, the river thus formed is quenched in ice and salt (salt) mixed water, and then maintained at room temperature to observe the microstructure and the physical properties are measured through a tensile test. And Table 2 are shown.

1 is a photograph showing the final microstructure of the composite tissue steel according to an embodiment of the present invention, Figures 2 to 4 is a photograph showing the final microstructure of the composite tissue steel according to Comparative Examples 1 to 3. On the other hand, Table 2 describes the tensile test results and the microstructure of the composite tissue steel according to Examples and Comparative Examples 1 to 3 of the present invention.

division Tensile Strength TS (MPa) Elongation T-El (%) Microstructure Example 802 38.5 Residual Austenitic + HCP Martensite Comparative Example 1 2045 0.7 BCT Martensite (Plate) Comparative Example 2 660 16.8 Austenite Comparative Example 3 742 22.9 Residual Austenite + BCT Martensite (Lath)

Hereinafter, with reference to Figures 1, 2 and Table 2 will be described with respect to the difference between the composite tissue steel according to the embodiment of the present invention and Comparative Examples 1 to 3.

First, in Comparative Example 1, due to the high carbon composition, the strength immediately after the heat treatment appears very high, but since it is composed of martensitic single-phase structure of BCT structure at room temperature after heat treatment, it has a brittle property. Therefore, the composite tissue steel according to Comparative Example 1 shows low elongation and low toughness.

Next, in the case of Comparative Example 2, the austenitic structure intended for use at room temperature was included using high alloy steel using a large amount of nickel (Ni). However, in this case, the microstructure is composed of austenite single phase structure, and since the solid solution strengthening effect is large due to the addition of a large amount of nickel (Ni), the strength is higher than that of other austenitic single phase structures, but the martensite structure is combined. The intensity is relatively low compared to the phase.

In addition, in Comparative Example 2, the elongation is not so high as compared with the steel (Example and Comparative Example 3) where the elongation is improved by the stress induced transformation of the retained austenite into martensite when the stress is applied.

On the other hand, Comparative Example 3 has a composite structure of austenite and martensite, which were intended at room temperature, by appropriate alloy selection. In addition, the value obtained by multiplying the strength and elongation of Comparative Example 3 is better than Comparative Examples 1 and 2.

However, as in Comparative Example 3, when the content of manganese (Mn) is insufficient compared to the composition range of the present invention, or when alloying elements other than manganese (Mn) are added beyond the composition range of the present invention, martens existing at room temperature The site is BCC (or BCT) martensite, and the composite structure of residual austenite and BCC (or BCT) martensite shows a lower elongation than the composite structure of residual austenite and HCP martensite as in the embodiment of the present invention.

In the case of HCP martensite shown in the embodiment of the present invention, it shows relatively high elongation and high toughness because it transforms into martensite of BCC or BCT structure when stress is applied. In Comparative Example 3, BCC (or BCT) was initially shown. Since it is formed of martensite of the structure, it is difficult to have such an effect, and therefore cannot have the effect shown in the embodiment of the present invention.

On the other hand, referring to Figure 1, the microstructure of the composite structure steel according to an embodiment of the present invention is composed of a very fine composite plate of thin plate martensite and austenite. Therefore, the composite tissue steel according to the embodiment of the present invention exhibits high strength due to such a fine structure.

In addition, in the grain boundary or the boundary boundary, twin boundary, stacking fault boundary, and the like within the phase boundary or martensite plate, the stress energy is moved when the stress is applied. By absorbing serves to increase the elongation.

The above description is merely illustrative of the present invention, and those skilled in the art may make various modifications and changes without departing from the essential characteristics of the present invention. In addition, the protection scope of the present invention should be interpreted by the claims, and all technical ideas within the equivalent scope should be construed as being included in the scope of the present invention.

According to the present invention, austenite remains at room temperature and the size of the martensite plate is fine, so that the toughness and strength of the composite tissue steel are always effective.

Claims (4)

In weight percent, C: 0.5 or less, Mn: more than 15 and less than 35, The sum of any one of Co, Ti, Al, and W: 3 or less, The sum of any one or more of Cr, Cu, Mo, Ni, Nb, Si, and V is 10 or less, Residual austenite consisting of the remaining Fe and other inevitably added impurities and the HCP martensite structure are mixed, The content of the alloy components satisfy the following MS formula, the value of the MS formula is a composite tissue steel mixed with the residual austenite and HCP martensite structure of 50 to 120. MS = 260-330C + 2Al + 7Co-14Cr-13Cu-19 (Mn-15)-5Mo-4Nb-13Ni-7Si + 3Ti-4V delete C: 0.5 or less, Mn: more than 15 and 35 or less, sum of any one or more of Co, Ti, Al, and W: 3 or less, based on weight percent, any one of Cr, Cu, Mo, Ni, Nb, Si, and V The sum of the above is 10 or less, and the remaining austenite and HCP martensite structure of the remaining Fe and other unavoidable impurities are mixed, and the content of the alloying components is "MS = 260-330C + 2Al + 7Co-14Cr-13Cu -19 (Mn-15)-5Mo-4Nb-13Ni-7Si + 3Ti-4V ", which satisfies the MS equation, wherein the value of the MS equation is a mixture of residual austenite and HCP martensite structure having a value of 50 to 120. After hot-rolling the composite tissue steel to produce steel A first quench step of heating and quenching the steel to quench the martensite structure; A second quenching step of heating the quenched steel in an austenite reverse transformation start temperature (A _s ) to 800 ° C. or lower and then quenching to refine the size of the reverse austenite grains; A quenching step of quenching the steel to a refrigerant of room temperature (25 ° C.) or lower after the second quenching step; And After the quenching step, the steel material is maintained at room temperature, characterized in that the heat treatment method of a composite tissue steel mixed with austenite and HCP martensite structure. delete

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