KR101950596B1 - Ultra high strength steel and methods of fabricating the same - Google Patents

Abstract

The present invention relates to an ultrahigh strength steel with excellent λ-EI characteristic and a manufacturing method thereof. According to one embodiment of the present invention, the manufacturing method of the ultrahigh strength steel comprises: (a) a step of providing a casting piece including 0.15 to 0.3 weight percent of C, 0.5 to 1.5 weight percent of Si, 1.5 to 3.0 weight percent of Mn, 0.2 to 0.5 weight percent of Al, 0.015 weight percent or less of P (excluding 0 weight percent), 0.003 weight percent or less of S (Excluding 0 weight percent), and the balance of Fe and inevitable impurities; (b) a step of winding a hot-rolled material of the casting piece; (c) a step of cold-rolling the wound rolled material; (d) a step of primarily annealing the cold rolled material at 760 to 820°C; (e) a step of quenching the primarily annealed rolled material at a martensite transition temperature or less; and (f) a step of secondarily annealing the quenched rolled material at 680 to 720°C. Accordingly, the final microstructure of the manufactured ultrahigh strength steel has ferrite and tempered martensite phases, wherein the remaining austenite is more formed in a tempered martensite grain.

Description

Ultra high strength steels and methods of making same

The present invention relates to an ultra-high strength steel and a manufacturing method thereof, and more particularly to an ultra high strength steel having excellent formability and a manufacturing method thereof.

Recently, as the automobile crash laws and environmental laws have been strengthened, the needs for material development for increasing the strength of automobile body and lightening weight are increasing. In general, the first-generation automotive steel sheet has drawbacks due to the increase of the material strength, which limits the molding of the drawing parts.

Related Prior Art Korean Patent Publication No. 2010-0076441 (published on July 6, 2010, entitled " High Strength Steel Having Uniform Material and Its Manufacturing Method ") is available.

It is an object of the present invention to provide an ultrahigh strength steel which is easy to produce and manufacture, and has excellent? -El characteristics and a method for producing the same.

(A) 0.15 to 0.3% by weight of C, 0.5 to 1.5% by weight of Si, 1.5 to 3.0% by weight of Mn, 0.2 to 0.2% by weight of Al, To 0.5% by weight, P: 0.015% by weight or less (excluding 0% by weight), S: 0.003% by weight or less (excluding 0% by weight), and the balance of Fe and inevitable impurities; (b) winding the rolled material hot-rolled relative to the cast steel; (c) cold rolling the wound rolled material; (d) firstly annealing the cold rolled rolled material at 760 to 820 캜; (e) quenching the first annealed rolled material to a martensitic transformation completion temperature or lower; And (f) secondarily annealing the quenched rolled material at a temperature of 680 to 720 DEG C, wherein the final microstructure of the ultra-high strength steel to be produced has a ferrite and a tempered martensite phase, Characterized in that residual austenite is further formed in the tempered martensite inlet.

(A) further contains 0.01 to 0.02% by weight of Nb and 0.01 to 0.05% by weight of V, and is characterized in that it further comprises a super-high strength steel produced by the steps (a) to (f) The final microstructure of the high strength steel may further comprise a VC precipitate located in the ferrite grain and a NbC precipitate located at the ferrite grain boundaries or the tempered martensite grain boundaries.

In the method of manufacturing ultra-high strength steel, the step (b) may include: reheating the cast steel at 1000 to 1300 ° C; Finishing the cast slab at 800 to 1000 占 폚; Cooling the cast steel at a cooling rate of 50 to 100 ° C / s and winding at 400 to 600 ° C.

In the method for producing an ultra high strength steel, the temperature of the second annealing in the step (f) may be set so that the ratio of transformation from martensite to austenite is 1 to 10%.

In order to achieve the above object, an ultra high strength steel according to an embodiment of the present invention includes 0.15 to 0.3 wt% of C, 0.5 to 1.5 wt% of Si, 1.5 to 3.0 wt% of Mn, 0.2 to 0.5 wt% of Al, P: not more than 0.015 wt% (excluding 0 wt%), S: not more than 0.003 wt% (excluding 0 wt%), the balance Fe and inevitable impurities, and the final microstructure comprises ferrite and tempered martensite and has a tempered martensite phase, wherein residual austenite is further formed in the tempered martensite inlet.

The ultrahigh strength steel further contains 0.01 to 0.02% by weight of Nb and 0.01 to 0.05% by weight of V, wherein the final microstructure contains the VC precipitates located in the ferrite grain and the NbC precipitates located at the ferrite grain boundaries or tempered martensite boundaries .

The ultra-high strength steel is characterized by having a yield strength of 800 MPa or more, a tensile strength of 1000 MPa or more, an elongation of 20% or more, and a hole expansion ratio (HER,?) Of 40 or more.

According to the embodiment of the present invention, an ultrahigh strength steel excellent in lambda-El characteristics while being easy to produce and manufacture and its manufacturing method can be realized. Of course, the scope of the present invention is not limited by these effects.

FIG. 1 is a flowchart illustrating a method of manufacturing an ultrahigh strength steel according to an embodiment of the present invention. Referring to FIG.
FIG. 2 is a graph showing a heat treatment temperature according to time after cold rolling in a method of manufacturing an ultrahigh strength steel according to an embodiment of the present invention.
3 is a conceptual diagram illustrating the ultimate microstructure of an ultra-high strength steel according to an embodiment of the present invention.
4 is a graph showing a heat treatment temperature according to time after cold rolling in a method of manufacturing an ultrahigh strength steel according to a comparative example of the present invention.
5 is a conceptual illustration of the final microstructure of an ultra-high strength steel according to an embodiment and a comparative example of the present invention.

Hereinafter, an ultrahigh strength steel according to an embodiment of the present invention and a method of manufacturing the same will be described in detail. The terms used below are appropriately selected terms in consideration of functions in the present invention, and definitions of these terms should be made based on the contents throughout this specification.

FIG. 1 is a flowchart illustrating a method of manufacturing an ultrahigh strength steel according to an embodiment of the present invention. Referring to FIG.

(A) 0.15 to 0.3% by weight of C, 0.5 to 1.5% by weight of Si, 1.5 to 3.0% by weight of Mn, 0.2 to 0.2% by weight of Al, To about 0.5 wt.%, P: not more than 0.015 wt.% (Excluding 0 wt.%), S: 0.003 wt.% Or less (excluding 0 wt.%) And the balance Fe and inevitable impurities ); (b) winding (S200) the rolled material hot-rolled and hot-rolled relative to the cast steel; (c) cold rolling the wound rolled material (S300); (d) firstly annealing the cold-rolled rolled material at a _first temperature between the Ac ₁ transformation temperature and the Ac ₃ transformation temperature (S400); (e) quenching the first annealed rolled material to a martensitic transformation completion temperature or less (S500); And (f) secondarily annealing the rolled material that has been quenched at a second temperature lower than the first temperature as a temperature between the Ac ₁ transformation temperature and the Ac ₃ transformation temperature (S600).

The final microstructure of the ultra-high strength steel produced by performing the steps (a) to (f) and cooling to room temperature has a ferrite and a tempered martensite phase, and the residual oyster Characterized in that a retained austenite is further formed.

Meanwhile, in the method of manufacturing an ultra high strength steel according to another embodiment of the present invention, the cast steel of step (a) may further contain 0.01 to 0.02% by weight of Nb and 0.01 to 0.05% by weight of V. In this case, the slab of the step (a) may contain 0.15 to 0.3 wt% of C, 0.5 to 1.5 wt% of Si, 1.5 to 3.0 wt% of Mn, 0.2 to 0.5 wt% of Al, 0.015 wt% (Excluding 0% by weight), S: 0.003% by weight or less (excluding 0% by weight), Nb: 0.01-0.02% by weight, and V: 0.01-0.05% by weight, the balance being Fe and inevitable impurities. The cast steel may further contain a trace amount of nitrogen (N) component (for example, 0.004% by weight).

The final microstructure of the ultra-high strength steel produced by performing the steps (a) to (f) above and having been cooled to room temperature has a ferrite and a tempered martensite phase, Retained austenite is further formed in the de martensite inlet. Further, it may further include a VC precipitate located in the ferrite grain and NbC precipitates located in the ferrite grain boundaries or the tempered martensite grain boundaries.

In the casting composition, carbon (C) is an austenite stabilizing element as an interstitial solid element. It is an element stabilizing the formation of retained austenite after cooling at room temperature by carbon enrichment in austenite. Carbon addition is recommended to be 0.15 wt% or more for promoting carbon concentration, and it is preferable to limit excess addition because of the problem of deterioration of spot weldability when it exceeds 0.3 wt%.

Silicon (Si) is a ferrite stabilizing element that suppresses the formation of low-temperature reverse carbides, which cleans the ferrite and strengthens the internal solidification of Si, thereby improving the elongation density and enhancing the elongation due to the ferrite strengthening effect and improving the stretch flange characteristics. However, by forming the surface SiO ₂ -based oxide, the plating wettability is inhibited, and the addition amount is preferably limited to the range of 0.5 to 1.5 wt%.

Manganese (Mn) is an element that stabilizes the austenite and is an element capable of increasing the martensitic transformation fraction during cooling by suppressing high-temperature inverse ferrite and low-temperature inverse bainite transformation during cooling. If the amount is less than 1.5% by weight, the strength is lowered due to uncertainty of the martensitic transformation. If the amount is more than 3.0% by weight (strictly, 2.5% by weight), the production process load due to the increase in hot-

Since the phosphorus (P) may cause embrittlement due to grain boundary segregation as a solid solution strengthening element, the addition amount is preferably limited to 0.015 wt% or less (excluding 0 wt%).

Since sulfur (S) forms an MnS inclusion in the steel material, the bendability decreases and can act as a hydrogen embrittlement generating portion. Therefore, the addition amount thereof is preferably limited to 0.003 wt% or less (excluding 0 wt%).

Aluminum (Al) is an element that stabilizes ferrite as an element to improve the ductility of ferrite and inhibits formation of low-temperature reverse carbide, thereby improving the degree of carbon concentration in austenite. When the aluminum content is less than 0.2% by weight, the effect of inhibiting low temperature reverse carbide formation is insignificant. The Ac ₃ transformation point is rapidly increased according to the addition amount, thereby causing a problem of annealing at a temperature higher than 860 ° C. in order to secure a stable γ fraction in the annealing of two phase regions, and the formation of slab embrittlement may be caused by promoting the formation of ferrite- This problem may occur if the aluminum content exceeds 0.5% by weight.

Niobium (Nb) is an element which induces the effect of micro-precipitation at the interfacial surface between ferrite / austenite phase and the effect of refining the ferrite grains. When the amount is less than 0.01% by weight, the effect of precipitation is insufficient. When the amount is more than 0.02% by weight, the addition amount is limited due to the decrease of the elongation.

Vanadium (V) is an element that induces the effect of precipitation of fine carbide in ferrite grain and ferrite strength upward effect. If it is added in an amount of less than 0.01% by weight, the effect of precipitation is insufficient, and when it is added in an amount exceeding 0.05% by weight, it is preferable to limit the amount of the additive added.

Meanwhile, in the method of manufacturing ultra-high strength steel, the step (b) may include: reheating the cast steel at 1000 to 1300 ° C; Finishing the cast slab at 800 to 1000 占 폚; Cooling the cast steel at a cooling rate of 50 to 100 ° C / s and winding at 400 to 600 ° C.

FIG. 2 is a graph showing a heat treatment temperature according to time after cold rolling in a method of manufacturing an ultrahigh strength steel according to an embodiment of the present invention.

2, a step (S400) of first annealing the cold-rolled rolled material at a _first temperature between the Ac ₁ transformation temperature and the Ac ₃ transformation temperature after cold rolling the rolled rolled material after hot rolling, (1 in Fig. 2). The first temperature between the Ac ₁ transformation temperature and the Ac ₃ transformation temperature may range from 760 to 820 ° C as the first annealing temperature of the rolled material that has been cold-rolled in step S400.

The A ₁ transformation temperature is a temperature at which a vacancy reaction occurs in which a ferrite and cementite (Fe ₃ C) are co-deposited when a y-chain austenite cools, and A ₃ transformation temperature is a y- The temperature at which the chain ferrite is mutually transformed. The A ₁ transformation temperature and the A ₃ transformation temperature are different from each other in the equilibrium state and the temperature at which the predetermined transformation occurs at the time of cooling when heated. The Ac ₁ transformation temperature and Ac ₃ transformation temperature described by adding the subscript c are A ₁ Which means the temperature at which transformation occurs when heating is performed between transformation temperature and A ₃ transformation temperature.

Subsequently, the step (S500) of quenching the first annealed rolled material to the martensite transformation completion temperature (Mf) or lower is performed (② in FIG. 2). The quenching rate can be set very quickly to the level of 100 ° C / s. In the step S500, the martensite transformation completion temperature Mf may be lower than or equal to 150 ° C. Below the martensitic transformation completion temperature (Mf), no phase transformation from austenite to martensite occurs.

Subsequently, the step (S600) of performing second annealing at a second temperature lower than the first temperature as the temperature between the Ac ₁ transformation temperature and the Ac ₃ transformation temperature is performed (step S600 in Fig. 2) . In the step (S500) as a temperature between the Ac ₁ transformation temperature and Ac ₃ transformation temperature lower than a second temperature above the first temperature may have a temperature range of 680 ~ 720 ℃. After the second annealing, room temperature cooling is performed. The temperature of the second annealing (680 to 720 ° C) may be set so that the ratio of transformation from martensite to austenite is 1 to 10%.

3 is a conceptual diagram illustrating the ultimate microstructure of an ultra-high strength steel according to an embodiment of the present invention.

Referring to FIG. 3, the final microstructure of the ultra-high strength steel according to an embodiment of the present invention is a final microstructure of ultra-high strength steel realized by performing the steps (a) to (f) And a tempered martensite phase, wherein a retained austenite is further formed in the tempered martensite inlet, further, a V-type precipitate (for example, a VC precipitate) located in the ferrite inlet, And Nb-based precipitates (for example, NbC precipitates) located at the ferrite grain boundaries and / or the tempered martensite grain boundaries.

In the above-mentioned final microstructure, the volume fraction of ferrite is 30 to 40 vol%, the volume fraction of tempered martensite is 55 to 60 vol%, the volume fraction of retained austenite is 5 to 10 vol% And the size of the precipitate is 2 to 20 nm, and the retained austenite may be a lath type having a carbon concentration of 0.5 wt% or more.

FIG. 4 is a graph showing a heat treatment temperature according to time after cold rolling in a method of manufacturing an ultrahigh strength steel according to a comparative example of the present invention, and FIG. 5 is a graph showing the final annealing temperature As shown in FIG.

Referring to FIG. 4, a method of manufacturing an ultrahigh strength steel according to a comparative example of the present invention includes performing steps (1), (2), and (4) after cold rolling. The steps 1 and 2 shown in FIG. 4 are the same as the steps 1 and 2 shown in FIG. In the method of manufacturing an ultrahigh strength steel according to a comparative example of the present invention, additional heat treatment is performed at a temperature between the martensitic transformation starting temperature (Ms) and the martensitic transformation completion temperature (Mf) (step 4).

The super high strength steel has a composition of 0.15 to 0.3 wt% of C, 0.5 to 1.5 wt% of Si, 1.5 to 3.0 wt% of Mn, 0.2 to 0.5 wt% of Al, 0.015 wt% , S: not more than 0.003 wt% (excluding 0 wt%), Nb: 0.01 to 0.02 wt%, V: 0.01 to 0.05 wt%, and the balance of Fe and inevitable impurities.

Referring to FIG. 2, FIG. 4 and FIG. 5 together, the microstructure of austenite (γ) and ferrite (F) appears when step (1) is performed. Nb forms a fine precipitate in a solid state to refine the crystal grains. Subsequently, the microstructure of the martensite (M) and ferrite (F) appears when the step (2) is carried out.

If the step (3) of FIG. 2 is performed after performing the steps (1) and (2), the ferrite (F) has a tempered martensite (TM) phase, and the retained austenite Further, the VC precipitates located in the ferrite (F) grain and the NbC precipitates located in the ferrite (F) grain boundary and / or the tempered martensite (TM) grain boundary appear.

Alternatively, if step (4) of FIG. 4 is performed after steps (1) and (2) are performed, a VC precipitate appears in the ferrite (F) mouth while having the ferrite (F) and the tempered martensite . However, no retained austenite (RA) is formed in the tempered martensite inlet, and NbC precipitates located in the ferrite (F) grain boundary and / or the tempered martensite (TM) grain boundary do not appear.

Hereinafter, preferred examples of the present invention will be described in order to facilitate understanding of the present invention. It should be understood, however, that the following examples are intended to aid in the understanding of the present invention and are not intended to limit the scope of the present invention.

Experimental Example

In the experimental examples of the present invention, first and second annealing were respectively applied to the rolled materials of the two compositions (composition 1 and composition 2 in Table 1) at several different temperatures (Table 2) after cold rolling . The composition 1 and the composition 2 may further contain a very small amount (for example, 0.004% by weight) of the nitrogen (N) component.

In this experimental example, the ingot produced at 50 mm for the composition 1 and the composition 2 was homogenized at 1220 캜 for 2 hours. The homogenized ingot was hot rolled at a temperature of 900 캜 to a thickness of 2.8 mm and then rolled at 500 캜. The purpose of winding at a coiling temperature of 560 ° C or less is to suppress the formation of low temperature Nb, V precipitates in the winding step and to promote the formation of fine precipitates after the cold-rolling heat treatment. The hot-rolled sheet was pickled to remove the surface scale and then cold-rolled to a thickness of 1.2 mm. The cold-rolled plate was firstly annealed at 780 ° C or 840 ° C, quenched to 150 ° C or lower, which is the martensitic transformation completion temperature, and subjected to secondary annealing at 200 ° C, 700 ° C, or 760 ° C for quenched specimens.

The objective of the secondary annealing in the present invention is to realize softening of light martensite generated during the primary cooling after the primary annealing and to form stable austenite in the grain of the tempered martensite and in the grain boundary, . Further, in order to control the ratio of the re-transformed austenite to martensite in the second annealing process within 30%, the temperature of the second annealing may have a range of AC ₁ to AC ₁ + 30 ° C. have. Subsequently, by performing the quenching step after the second high-temperature annealing, the ferrite grain refinement effect can be obtained. In addition, solid solution Nb in the primary annealing forms fine precipitates to make fine grains finer, and ferrite strengthening effect can be obtained by precipitating V in the ferrite grain at the time of secondary annealing.

Tables 3 and 4 are the results of evaluating the material, microstructure, hardness and hole expandability after the heat treatment as in the experiment example of the present invention described above.

In Table 3, SS item corresponds to the temperature of the first annealing, RH item corresponds to the temperature of the second annealing, YP corresponds to the yield strength, TS corresponds to the tensile strength, El corresponds to the elongation , Vp corresponds to the volume fraction of pearlite in the microstructure, VM corresponds to the volume fraction of martensite in the microstructure, VF corresponds to the volume fraction of ferrite in the microstructure, VTM is the tempered martensite Corresponds to the volume fraction of the site, and Vr corresponds to the volume fraction of retained austenite in the microstructure. In Table 4, HVp corresponds to hardness of pearlite in microstructure, HVM corresponds to hardness of martensite in microstructure, HVF corresponds to hardness of ferrite in microstructure, HVTM corresponds to hardness of tempered martensite And the HER item corresponds to the hole expansion ratio.

Referring to Tables 3 and 4, the results of the first and second annealing at 780 ° C in the compositions 1 and 2 show that the elongation and hole expansion characteristics are improved as compared to the first annealing at 840 ° C. This can be regarded as an effect of an increase in elongation due to an increase in the initial ferrite phase fraction and an inhibition of pearlite phase formation during the reheating process. The austenite fraction during the first annealing (SS) at 780 ° C and 840 ° C is 55% and 90%, respectively, and the carbon and manganese concentration in the austenite at 780 ° C is greatly increased compared to 840 ° _C (780A _C : 0.86 wt%, 780A _Mn : 2.71 wt%, 840A _C : 0.22 wt%, 840A _Mn : 2.10 wt%). Therefore, the martensite formed after the first quenching at 780 ° C during the reheating treatment suppresses the formation of carbides at high temperature, but the martensite formed at 840 ° C is transformed into pearlite carbide during the high temperature reheating process. The pearlite phase is a composite phase of soft ferrite and hard cementite, and void formation and crack propagation at the boundary portion are easy at the time of hole extension, thereby deteriorating the hole expanding property. It can be seen that the elongation and hole expansion characteristics are greatly improved compared with the case where the second reheating temperature is 700 ° C in the second heat treatment (RH) after the first heat treatment at 780 ° C, compared with the experiment at 760 ° C. This is related to the amount of initial austenite in the secondary annealing and the amount of carbon and manganese contained in the austenite. The temperature of 700 ℃ is stable within the tempered martensite with the Reverted Austenite content within 10% but the austenite is stable in the form of Lath, but at 760 ℃, the Reverted Austenite is 45% The tempered martensite is transformed into a reverted austenite and the unstable austenite is transformed into martensite again in the second cooling process and the difference in phase hardness between the ferrite and martensite is increased, Properties are visible. Therefore, in the present invention, it is proposed that the heat treatment is carried out so that the temperature at the time of the second annealing becomes 1 to 10% of the rate of transformation from martensite to austenite.

In order to improve the hole expandability of the DP steel, the present invention intends to improve the strength of the ferrite using micro precipitates. In order to improve the ferrite strength, Nb, V fine precipitates were used. In the hot rolling process, grain refinement and formation of Nb and V precipitates were suppressed through low temperature coiling, and fine precipitation in ferrite grains / grain boundaries during cold rolling annealing process was utilized as a ferrite strengthening mechanism. As shown in Table 3 and Table 4, the grain refinement effect of ferrite was observed at the addition of Nb and V, and the ferrite microhardness (Hv10) was greatly increased. As a result, the hole expansion characteristics were significantly improved .

It is to be understood that the invention includes various modifications and equivalent embodiments that can be derived from the disclosed embodiments as well as those of ordinary skill in the art to which the present invention pertains. Accordingly, the technical scope of the present invention should be defined by the following claims.

Claims (7)

(a) C: 0.15 to 0.3 wt%, Si: 0.5 to 1.5 wt%, Mn: 1.5 to 3.0 wt%, Al: 0.2 to 0.5 wt%, P: 0.015 wt% 0.003 wt% or less (excluding 0 wt%), the balance being Fe and inevitable impurities;
(b) winding the rolled material hot-rolled relative to the cast steel;
(c) cold rolling the wound rolled material;
(d) firstly annealing the cold rolled rolled material at 760 to 820 캜;
(e) quenching the first annealed rolled material to a martensitic transformation completion temperature or lower; And
(f) secondarily annealing the quenched rolled material at 680 to 720 占 폚;
Including,
Characterized in that the final microstructure of the ultra-high strength steel to be produced has a ferrite and a tempered martensite phase and further retained austenite is formed in the tempered martensite inlet.
Method of manufacturing super high strength steel. The method according to claim 1,
The cast steel of the step (a) further contains 0.01 to 0.02% by weight of Nb and 0.01 to 0.05% by weight of V, and the final microstructure of the ultra-high strength steel produced by the steps (a) to (f) And a NbC precipitate located at a ferrite grain boundary or a tempered martensite grain boundary. 4. The method according to any one of claims 1 to 3,
The step (b) may include reheating the cast steel at 1000 to 1300 ° C. Finishing the cast slab at 800 to 1000 占 폚; And cooling the cast steel at a cooling rate of 50 to 100 占 폚 / s and winding at 400 to 600 占 폚. 4. The method according to any one of claims 1 to 3,
Wherein the temperature of the second annealing in the step (f) is set so that the rate of transformation from martensite to austenite is 1 to 10%. delete delete delete

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