KR101903174B1 - Low alloy steel sheet with excellent strength and ductility - Google Patents

Abstract

The present invention minimizes the addition of alloying elements to reduce surface cracking during hot rolling, ensures corrosion resistance by constituting Cr as one of the main components, and realizes TRIP or TWIP phenomenon through high Mn design, thereby providing excellent hot workability, high strength And a low alloy steel sheet having high ductility. A low alloy steel sheet excellent in strength and ductility according to an embodiment of the present invention comprises 0.05 to 0.3% of C, 0.7 to 2.5% of Si, 8 to 12% of Mn, 13 to 15.5% of Cr, Wherein at least one of a ferrite phase and a martensite phase is contained in a microstructure in an amount of 0.5 to 3.0% of Cu, 0.1 to 0.2% of N, 0.25% or less of Al, 0.25% or less of Sn and the balance of Fe and other unavoidable impurities To 5% or less in volume fraction, and the remainder contains austenite phase.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a low alloy steel sheet having excellent strength and ductility,

The present invention relates to a high-strength high-ductility steel sheet suitable for structural members such as automobiles and railroads, and more particularly, to a steel sheet which minimizes alloying elements such as Ni and which has excellent strength and ductility by controlling microstructure with Cr and Mn as main components. Alloy steel sheet and a manufacturing method thereof.

In order to reduce the weight of the automobile body, high strength and high ductility steel sheets have been continuously used. Recently, a transformation steel structure having excellent processability compared to the conventional precipitation strengthening or solid solution strengthening steels has been developed and used. Transformational texture steel is represented by DP (Dual Phase) steel, TRIP (TRANSformation Induced Plasticity) steel, CP (Complex Phase) steel, etc. These transformed structure steels have mechanical properties, That is, the tensile strength and the elongation level are different.

TRIP steel, which is one of the transformed structure steel, can control both the cooling rate and the cooling termination temperature during the cooling process after forming the austenite during the annealing process, thereby partially improving the strength and ductility by partially retaining the austenite at the room temperature. The metastable retained austenite is transformed into martensite by deformation, thereby increasing elongation by delaying local stress concentration relaxation and necking with increasing strength. Therefore, it is important that TRIP steels retain austenite more than a certain fraction at room temperature. For this purpose, austenite stabilizing element should be added together with a large amount of Mn to maintain a certain percentage of retained austenite at room temperature.

On the other hand, TWIP (Twinning Induced Plasticity) steel which constitutes austenite single phase by adding C and Mn in a large amount of steel in addition to the above-mentioned transformed structure steel is also available. In the case of the TWIP steel, the material exhibits excellent tensile strength and elongation. However, in general, in order to produce TWIP steel, when the content of C is 0.4% by weight, the content of Mn is about 25% by weight or more and the content of C is 0.6% by weight, the content of Mn is about 20% , The austenite causing twinning in the mother phase can not be stably secured and martensite (? ') Of the BCT structure and the epsilon martensite (?) Of the HCP structure which are extremely harmful to the workability are formed, A large amount of austenite stabilizing element should be added so that austenite can be stably present.

PCT Published Patent Application No. 2012/077150 is a high Mn-containing TWIP steel having excellent mechanical properties and moldability, cold-rolled steel is subjected to cold-rolling annealing for recrystallization. In this patent document, alloying elements such as C, Al and Si are additionally added to stabilize the austenite phase or to control the stacking defect energy (SFE).

As described above, the TRIP steel and the TWIP steel to which a large amount of the alloy component is added are solidified into austenite single phase at the time of manufacture, and the hot workability is weakened, and a defect caused by inclusions such as Al easily occurs in hot rolling. There is a disadvantage that manufacturing technology such as casting and rolling process is very difficult due to a problem, and a manufacturing cost is high due to a large increase of alloy cost.

PCT Published Patent Publication No. 2012-077150 (June 14, 2012)

The present invention minimizes the addition of alloying elements to reduce surface cracking during hot rolling, ensures corrosion resistance by constituting Cr as one of the main components, and realizes TRIP or TWIP phenomenon through high Mn design, thereby providing excellent hot workability, high strength And a low alloy steel sheet having high ductility.

A low alloy steel sheet excellent in strength and ductility according to an embodiment of the present invention comprises 0.05 to 0.3% of C, 0.7 to 2.5% of Si, 8 to 12% of Mn, 13 to 15.5% of Cr, Wherein at least one of a ferrite phase and a martensite phase is contained in a microstructure in an amount of 0.5 to 3.0% of Cu, 0.1 to 0.2% of N, 0.25% or less of Al, 0.25% or less of Sn and the balance of Fe and other unavoidable impurities To 5% or less in volume fraction, and the remainder contains austenite phase.

Also, according to an embodiment of the present invention, the steel sheet may contain 0.2% or less of Ni by weight%.

According to an embodiment of the present invention, the steel sheet may contain 0.2% or less of Mo by weight.

According to an embodiment of the present invention, the steel sheet may have an elongation of 40% or more.

According to an embodiment of the present invention, the steel sheet may have a tensile strength of 650 MPa or more.

The low alloy steel sheet excellent in strength and ductility according to the embodiment of the present invention has a tensile strength of 650 MPa or more and an elongation of 40% or more by realizing TRIP or TWIP development, and can be used for automobile parts or other structural materials .

In addition, Cr is included as one of the main components, so that it has excellent corrosion resistance, minimizes addition of alloying elements, and can be excellent in hot workability.

1 to 4 are photographs showing the degree of occurrence of edge cracks during hot rolling of invention steels and comparative steels according to an embodiment of the present invention.
5 to 10 are optical micrographs of microstructures of the inventive steel and the comparative steel after hot-annealing according to an embodiment of the present invention.
11 to 14 are optical micrographs showing microstructure changes according to the hot-rolled annealing temperature of invention steel 9 according to an embodiment of the present invention.
15 is a graph showing tensile test results of inventive steels and comparative steels according to an embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following embodiments are provided to fully convey the spirit of the present invention to a person having ordinary skill in the art to which the present invention belongs. The present invention is not limited to the embodiments shown herein but may be embodied in other forms. For the sake of clarity, the drawings are not drawn to scale, and the size of the elements may be slightly exaggerated to facilitate understanding.

A low alloy steel sheet excellent in strength and ductility according to an embodiment of the present invention comprises 0.05 to 0.3% of C, 0.7 to 2.5% of Si, 8 to 12% of Mn, 13 to 15.5% of Cr, Cu: 0.5 to 3.0%, N: 0.1 to 0.2%, Al: 0.25% or less, Sn: 0.25% or less, and the balance includes Fe and other unavoidable impurities.

The role and content of each component included in the low alloy steel sheet excellent in strength and ductility according to the present invention will be described below. % ≪ / RTI > by weight refers to weight percent.

The content of C is not less than 0.05% and not more than 0.3%.

C is an austenite forming element and is an effective element for increasing the strength of a material by solid solution strengthening. Although it is advantageous to add a large amount of C in order to secure high strength, the corrosion resistance is lowered when it is added in excess, and the upper limit is limited to 0.3% or less. On the other hand, in the case of lower limit, the lower limit is limited to 0.05% in order to take the load of decarburization during smelting and to obtain the effect of increasing the strength by the minimum C. It is preferable to add C in the range of 0.05 to 0.3% in order to ensure stable production and strength by C.

The content of Si is not less than 0.7% and not more than 2.5%.

Si is partially added because it acts as a deoxidizing effect and a ferrite stabilizing element. However, if it is excessive, the mechanical properties related to corrosion resistance and impact toughness will be deteriorated. When a large amount of Si is added, the ferrite content as a ferrite forming element is increased to cause surface cracking in the hot rolling, and the load is caused at the time of production, so that the upper limit is limited to 2.5%. On the other hand, the lower limit is limited to 0.7% in order to control the stability of the austenite phase by Si addition, control of fired organic martensite formation, and ease of production. For controlling the phase fraction by Si addition and controlling the deformation mechanism during the austenite phase transformation, it is preferable to limit the range of Si to 0.7 to 2.5%.

The content of Mn is 8% or more and 12% or less.

Mn is an austenite-forming element, and is a major element constituting the austenite phase in the Cr-added steel. In particular, the same effect as Ni is utilized as a substitute for Ni. When Mn is contained in a large amount at the time of production, oxide-based inclusions cause defects in production or deterioration in corrosion resistance. An additional technique for reducing the employment oxygen of the special refining or the like is required for the inclusion reduction, and the manufacturing cost is increased. Therefore, we limit the upper limit to 12%. The minimum amount for Ni addition and the minimum amount for securing the austenite single phase or some ferrite or martensite structure is about 8%. Therefore, the range of Mn is preferably limited to 8 to 12%.

The content of Cr is 13.0% or more and 15.5% or less.

Cr is a representative ferrite-forming element and is an element that increases corrosion resistance. In particular, it is an element that greatly affects N employment. In order to minimize surface cracking during hot rolling, it is preferable to control the initial phase at the time of solidification by ferrite so as not to control the trace elements, particularly S and P which are intergranular segregated elements, at a very low level. When the amount of a certain amount of ferrite exceeds a certain amount, it is present in two phases of ferrite and austenite at a high temperature, resulting in deterioration of hot workability and a large amount of cracks are generated in hot rolling. In addition, some of the ferrite phases are present more than necessary in the final product manufacture, resulting in deterioration of mechanical properties. Therefore, the upper limit of Cr is limited to 15.5% or less. On the other hand, if the content of Cr is too low, it will solidify at the austenite phase during solidification at a high temperature, and control a trace amount of P and S which are intergranular segregation factors. In case of insufficient control, Occurs. In addition, at least 13.0% is required to have minimum corrosion resistance and superior corrosion resistance over carbon steel. Therefore, it is preferable that Cr is limited to 13.0 to 15.5% in order to solidify the ferrite as ferrite within the range of the desired alloying element and maintain the corrosion resistance of the minimum stainless steel level.

The content of Cu is 0.5% or more and 3.0% or less.

Cu is an austenite-forming element similar to Mn and Ni. As an element to be added in place of Ni, when it is added in excess, it is precipitated as metal Cu in excess of solubility and causes grain boundary embrittlement upon heating. Therefore, the maximum amount that can control the stability of austenite without exceeding the solubility is 3.0%. On the other hand, the minimum amount is 0.5%. When the amount is less than that, there is no meaning of Cu addition, and it does not affect the austenite stability and formation. Therefore, it is preferable that Cu is limited to 0.5 to 3.0%.

The content of N is 0.1% or more and 0.2% or less.

N is a representative austenite forming element along with Ni, and it is an element which improves the corrosion resistance of the material together with Cr and Mo. The effect of N addition is shown, and the minimum amount that improves the strength of the material with interstitial elements along with C is 0.1%. Most of the pressure is applied to increase the solubility of N in order to employ a large amount of N in the material. Even if Cr and Mn, which are representative elements for increasing the solubility of N, are present in a large amount, the amount that can maximally employ N without applying atmospheric pressure is 0.2%. Therefore, it is preferable that the appropriate amount of N is limited within the range of 0.1 to 0.2%.

The content of Al is 0% or more and 0.25% or less.

Al is a ferrite-forming element in Cr-added stainless steels. It is a useful element for deoxidation in steelmaking and simultaneously increases the energy of lamination defects of austenite phase to form fired organic martensite or mechanical twinning. It is known that it improves the delayed fracture resistance, which is a crack which is caused by cracking. If the content exceeds 0.25%, large-sized Al inclusions are generated and cause surface defects. Further, when it is added excessively, it contains a large amount of ferrite phase at a high temperature and causes cracking in hot rolling. Therefore, the content of Al is limited to 0.25% or less.

The content of Sn is 0% or more and 0.05% or less.

Sn is known as an element improving the corrosion resistance of the material and improving the pickling property by controlling the thickness of the annealing scale at annealing. That is, when a large amount of Si is added, the effect of suppressing the formation of SiO ₂ oxide on the scale surface layer generated in the cold rolling or hot-rolling annealing process can be increased and the efficiency of the cold rolling annealing process can be increased. However, excessive addition of Sn causes a decrease in hot workability and a decrease in the top of the production process, so that the upper limit is limited to 0.05%. In addition, in the case of the corrosion resistance, when Sn is added, Sn is added to the surface of the passivation layer of the stainless steel to increase the resistance of the coating. Therefore, the content of Sn is limited to a range of 0.05% or less.

According to an embodiment of the present invention, the steel sheet may further contain 0.2% or less of Ni and / or 0.2% or less of Mo in weight%.

Ni is an austenite-forming element and plays the same role as Mn. Most of Ni is replaced with Mn, and some of them are present as impurities by scrap or the like. The residual amount is limited to 0.2% or less.

 Mo is an expensive element that increases the corrosion resistance and forms ferrite. In the absence of addition, the amount is limited to 0.2% or less.

The steel sheet according to the present invention satisfying the alloy element composition range has at least one of a ferrite phase and a martensite phase in a microstructure at a volume fraction of 5% or less and the remainder includes an austenite phase. The ferrite phase and the martensite phase may contain inevitable precipitation phases, but the sum of the volume fractions thereof may be 5% or less.

The low alloy steel sheet according to the present invention can be manufactured through reheating of the slab, hot rolling, hot rolling annealing, pickling or the like according to a conventional manufacturing method for the molten steel satisfying the above-mentioned component system.

For example, the slab can be hot-rolled at a normal rolling temperature, and the hot-rolled steel sheet can be annealed at 900 to 1,200 ° C for 10 to 60 minutes. Thereafter, the hot-rolled steel sheet can be cold-rolled by a conventional method to be made into a thin sheet. The low alloy steel sheet having high strength and high ductility according to the present invention can be used, for example, in a general product for molding, and can be used as a strip, a bar, a plate, a sheet, a pipe a pipe, or a tube.

Hereinafter, the present invention will be described in detail with reference to the following examples. However, the following examples are intended to illustrate the present invention in more detail, but the scope of the present invention is not limited to these examples.

Example

Specimens of steels corresponding to the composition range of the composition according to the present invention were prepared, and the phase fraction, elongation, and tensile strength of the material were measured after hot rolling and hot rolling. Table 1 below shows the alloy composition (wt%) for the experimental steel types.

division C Si Mn Cr Cu N Ni Mo Al Sn Inventive Steel 1 0.078 1.98 10.3 14.1 2.05 0.134 0 0.100 - - Invention river 2 0.080 2.02 8.2 13.9 2.03 0.144 0 0 - - Invention steel 3 0.080 1.03 8.9 13.9 2.00 0.157 0.1 0.1 - - Inventive Steel 4 0.078 1.00 9.0 14.1 1.00 0.149 0.06 0.1 - - Invention steel 5 0.079 2.01 9.0 14.0 0.51 0.146 0.12 0 - - Invention steel 6 0.12 1.79 9.8 14.4 2.0 0.14 0.1 0 0.11 0.003 Invention steel 7 0.082 2.36 11.8 15.3 2.48 0.133 0.1 0.1 - - Inventive Steel 8 0.082 0.198 9.7 14.1 2.00 0.140 0.05 0.05 - - Invention river 9 0.077 2.05 10.3 13.4 1.98 0.160 0.1 0.1 - - Invented Steel 10 0.21 1.99 10.0 14.0 2.00 0.145 0.1 0 0.002 0.003 Invention steel 11 0.1 1.23 9.2 14.1 2.03 0.167 0 0 0.2 0.04 Comparative River 1 0.086 2.01 4.1 15.9 2.02 0.140 0 0 - - Comparative River 2 0.085 0.197 4.1 15.8 1.96 0.130 0.1 0.06 - - Comparative Steel 3 0.071 2.04 4.0 15.9 0.27 0.210 0.12 0 - - Comparative Steel 4 0.087 0.200 6.0 17.9 2.00 0.180 0 0 - - Comparative Steel 5 0.019 1.88 10.0 14.1 0.82 0.066 0 0 0.002 0.003

The hot-rolled steel sheet obtained by hot-rolling the material prepared as in the above composition at ordinary rolling temperature was subjected to hot-rolling annealing to evaluate the microstructure and phase fraction and the related strength and elongation.

1 to 4 are photographs showing the hot workability of the inventive steel and the comparative steel. Fig. 1 shows the invention steel 8, Fig. 2 shows the invention steel 9, and Fig. 3 shows the invention steel 11. Referring to FIGS. 1 to 3, it was confirmed that there is almost no edge crack at the time of hot rolling due to the presence of austenite single phase structure or some martensite after cooling. In the case of the inventive steel 11 in which some ferrite phases were present at 5% or less, cracks occurred at some edges, but this was not a problem. However, in the case of Comparative Steel 4 shown in FIG. 4, a considerable amount of ferrite was present, which caused severe cracking during hot rolling, causing problems in hot rolling.

On the other hand, the hot rolled steel sheet obtained by hot rolling the manufactured material at a normal rolling temperature was subjected to hot-rolling annealing to determine the microstructure and the phase fraction, and the results are shown in Table 2 below. After being subjected to hot-rolling annealing at 1,100 ° C for about 30 minutes, water cooling was performed, and the structure was observed under an optical microscope. The phase fraction was measured by using a ferrite scope. When the martensite phase was present as an observation result of the optical microscope, the ferrite fraction and the martensite fraction were separated by comparing with the ferrite scope data using an image analyzer .

division Residual tissue ferrite(%) Martensite (%) Inventive Steel 1 - - Invention river 2 - 3 Invention steel 3 - - Inventive Steel 4 - - Invention steel 5 - - Invention steel 6 4 - Invention steel 7 - - Inventive Steel 8 - 2 Invention river 9 - - Invented Steel 10 - - Invention steel 11 4 - Comparative River 1 20 - Comparative River 2 30 - Comparative Steel 3 37 - Comparative Steel 4 42 - Comparative Steel 5 15 62

Referring to Table 2, it can be seen that, in the case of most major invention steels, the parent phase is an austenite phase and the residual structure varies slightly depending on the component system, but a ferrite or martensite phase remains. Further, it was confirmed that the residual amount was 5% or less. However, in the case of the comparative steel with severe edge cracking during hot rolling, it was found that most of the steel was a two phase structure composed of austenite and ferrite phase. Particularly, in the case of the comparative steel 5, most of the microstructure after the hot-rolling annealing is transformed into a martensite phase from the austenite, and a part thereof remains in the retained austenite phase. It was confirmed that a considerable amount of elongated ferrite phase in the rolling direction existed. In the case of the inventive steel at the annealing temperature range, it was confirmed that there was almost no difference in the microstructure except for the difference of crystal grains, the presence of some ferrite phase or martensite phase.

Figs. 5 to 10 are optical microscope photographs showing the structures of inventive steel and comparative steel which were heat-treated at a hot-rolled annealing temperature of 1,100 DEG C for 30 minutes and then water-cooled.

Fig. 5 is a photograph showing the microstructure of the inventive steel 2, in which an austenite phase and a residual martensite phase were observed.

Fig. 6 is a photograph showing the microstructure of Inventive Steel 8, in which an austenite phase and a residual ferrite phase were observed.

7 is a photograph showing the microstructure of the inventive steel 9, in which only austenite phase was observed.

8 is a photograph showing the microstructure of the comparative steel 1, and a ferrite phase elongated in the rolling direction together with the austenite phase was observed.

9 is a photograph showing the microstructure of the comparative steel 4, in which austenite phase and ferrite phase were observed.

10 is a photograph showing the microstructure of the comparative steel 5, in which both a ferrite and a martensite phase were observed together with an austenite phase.

5 to 10 are typical tissue photographs corresponding to Table 2. Actually, the measurement results and the observation results are in good agreement.

11 to 14 are photographs showing changes in microstructure with respect to a specimen having an alloy composition range of Inventive Steel 9 according to a hot-rolled annealing temperature of 900 to 1,200 ° C.

Even when the hot-rolled annealing temperature rises to 900 DEG C or higher, the crystal grains are coarsened, but it has been found that a sufficiently desired structure can be secured.

15 is a graph showing tensile test results of inventive steels and comparative steels according to an embodiment of the present invention.

Referring to FIG. 15, in the case of the inventive steel, the elongation percentage was more than 40% and the tensile strength was 650 MPa or more. In addition, when the material is deformed or stressed, the values of the stress-strain curve and the tensile strength largely vary according to the deformation behavior of the main phase, austenite phase. It was found that in the case of inventive steels 5 and 8, in which firing organic martensite transformation occurs well, it is possible to secure a high strength of not less than 40% and tensile strength of 1,200 MPa.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited thereto. Those skilled in the art will recognize that other embodiments may occur to those skilled in the art without departing from the scope and spirit of the following claims. It will be understood that various changes and modifications may be made.

Claims (5)

0.1 to 0.2% of N, 0.1 to 0.2% of Al, and more than 0% of Al in terms of% by weight, C: 0.05 to 0.3%, Si: 0.7 to 2.5%, Mn: 8 to 12% 0.25% or less, Sn: more than 0 and 0.25% or less, Ni: not more than 0.2%, the balance being Fe and other unavoidable impurities,
At least one of a ferrite phase and a martensite phase is contained in a microstructure at a volume fraction of 5% or less and the remainder contains an austenite phase,
A low alloy steel sheet having an elongation of 40% or more and a tensile strength of 650 MPa or more and excellent strength and ductility. delete The method according to claim 1,
The steel sheet contains 0.2% or less of Mo by weight, and has excellent strength and ductility. delete delete

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