KR101185269B1 - High hardness cold-rolled steel with excellent burring workability and method of manufacturing the same - Google Patents

Abstract

Disclosed is a method for producing a high strength cold rolled steel sheet having excellent burring and workability having a tensile strength (TS) of 780 MPa or more, an elongation (EL) of 22% or more, and a hole expansion ratio (HER) of 70% or more.
High strength cold rolled steel sheet manufacturing method according to the present invention (a) carbon (C): 0.04 ~ 0.15% by weight, silicon (Si): 0.1 ~ 0.5% by weight, manganese (Mn): 1.4 ~ 2.0% by weight, titanium (Ti) Hot rolling a slab steel consisting of: 0.05 to 0.15% by weight, tungsten (W): 0.1 to 0.3% by weight, nitrogen (N): 60 ppm or less and the remaining Fe and other unavoidable impurities; (b) cooling and winding the hot rolled steel; (c) pickling the wound steel; (d) cold rolling the pickled steel; And (e) annealing the cold rolled steel.

Description

High-strength cold-rolled steel sheet of 780MPa grade with excellent burring processability and manufacturing method thereof {HIGH HARDNESS COLD-ROLLED STEEL WITH EXCELLENT BURRING WORKABILITY AND METHOD OF MANUFACTURING THE SAME}

The present invention relates to a 780MPa-class high strength cold-rolled steel sheet and a method for manufacturing the same, and more particularly, to control the alloy components and processing conditions to reduce the elongation and hole expandability due to non-uniform formation of the ferrite structure burring workability It relates to an excellent 780MPa class high strength cold rolled steel sheet and a method of manufacturing the same.

Cold rolled steel is usually manufactured through a hot rolling process, a winding process, a pickling process, a cold rolling process and an annealing process.

In the hot rolling process, the slab steel in the semi-finished state is hot rolled at a predetermined rolling rate using a rolling roll.

In the winding process, hot rolled steel is wound at a specific winding temperature.

In the pickling process, pickling is used to modify the surface of the wound steel.

In the cold rolling process, the pickled steel is cold rolled at a predetermined rolling rate.

In the annealing process, the cold rolled steel is heat-treated to a specific temperature.

An object of the present invention is to control the alloy components and process conditions to maintain the precipitated carbides (Ti-WC, Ti-W-Nb-C) precipitated after cold rolling annealing to a fine size, the grain size of the final ferrite after cold rolling annealing 6㎛ The present invention provides a method for producing a high strength cold rolled steel sheet of 780 MPa class, which has excellent burring workability that can improve elongation and hole expandability by encouraging the microstructure below.

Another object of the present invention is to provide a 780MPa class high strength cold rolled steel sheet having excellent burring processability produced by the above production method.

780MPa grade high strength cold rolled steel sheet manufacturing method excellent in burring workability according to an embodiment of the present invention for achieving one of the above object is carbon (C): 0.04 ~ 0.15% by weight, silicon (Si): 0.1 ~ 0.5% by weight, Manganese (Mn): 1.4 to 2.0% by weight, Titanium (Ti): 0.05 to 0.15% by weight, tungsten (W): 0.1 to 0.3% by weight, nitrogen (N): 60 ppm or less and the slab made of the remaining Fe and other unavoidable impurities Hot rolling the steel; (b) cooling and winding the hot rolled steel; (c) pickling the wound steel; (d) cold rolling the pickled steel; And (e) annealing the cold rolled steel.

In addition, the slab steel may further include at least one of niobium (Nb): 0.03 to 0.20% by weight and vanadium (V): 0.03 to 0.20% by weight.

780MPa grade high strength cold rolled steel sheet having excellent burring processability according to an embodiment of the present invention for achieving the above another object is carbon (C): 0.04 ~ 0.15% by weight, silicon (Si): 0.1 ~ 0.5% by weight, manganese (Mn) ): 1.4 ~ 2.0% by weight, titanium (Ti): 0.05 ~ 0.15% by weight, tungsten (W): 0.1 ~ 0.3% by weight, nitrogen (N): 60ppm or less and the remaining Fe and other unavoidable impurities, the final ferrite The grain size of the microstructure is characterized in that 6㎛ or less.

In addition, the steel sheet may further include at least one of niobium (Nb): 0.03 to 0.20 wt% and vanadium (V): 0.03 to 0.20 wt%.

The 780MPa class high strength cold rolled steel sheet manufacturing method having excellent burring processability according to the present invention is a 780MPa class high strength cold rolled steel sheet, but also has excellent elongation and hole expandability.

Therefore, the 780MPa class high strength cold rolled steel sheet having excellent burring processability according to the present invention can be used for automobile parts requiring high processability since it can not only have excellent cutting property when drilling or die cutting, but also can minimize burring amount.

1 is a flow chart schematically showing a method of manufacturing a high strength cold rolled steel sheet having excellent burring processability according to an embodiment of the present invention.

Advantages and features of the present invention and methods for achieving them will be apparent with reference to the embodiments described below in detail with the accompanying drawings. It should be understood, however, that the invention is not limited to the disclosed embodiments, but is capable of many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, To fully disclose the scope of the invention to those skilled in the art, and the invention is only defined by the scope of the claims. Like reference numerals refer to like elements throughout the specification.

Hereinafter, a high strength cold rolled steel sheet having excellent burring processability and a method of manufacturing the same according to preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

Cold rolled steel sheet

780MPa grade high strength cold rolled steel sheet having excellent burring process according to the present invention is carbon (C): 0.04 ~ 0.15% by weight, silicon (Si): 0.1 ~ 0.5% by weight, manganese (Mn): 1.4 ~ 2.0% by weight, titanium (Ti ): 0.05 ~ 0.15% by weight, tungsten (W): 0.1 ~ 0.3% by weight, nitrogen (N): 60ppm or less and the rest of Fe and other unavoidable impurities, the crystal grain size of the final ferrite microstructure is less than 6㎛ It is done.

In addition, the steel sheet may further include at least one of niobium (Nb): 0.03 to 0.20 wt% and vanadium (V): 0.03 to 0.20 wt%.

Hereinafter, the role and content of each component included in the 780MPa class high strength cold rolled steel sheet excellent in burring processability according to the present invention will be described.

Carbon (C)

In the present invention, carbon (C) is added to secure the strength of the steel sheet. In addition, carbon serves to stabilize the austenite phase depending on the amount of thickening in the austenite phase.

The content of carbon is preferably 0.04 to 0.15% by weight of the total weight of the steel sheet.

The degree of stabilization of the austenite phase is different depending on the concentration of carbon in the austenite phase. When the carbon content is less than 0.04% by weight, the austenite phase is transformed into a ferrite phase, and thus it is difficult to obtain a desired martensite phase fraction. At least 0.04% by weight of carbon content is required. On the other hand, when the content of carbon exceeds 0.15% by weight, weldability is lowered, and the strength-ductility balance decreases with increasing strength.

In the case of the present invention, setting the content of carbon (C) to a low carbon region of 0.04 to 0.15% by weight is to secure the amount of solid carbon to facilitate the aging resistance. In this case, there is an advantage that does not need to narrowly manage the carbon (C), nitrogen (N) content.

Silicon (Si)

In the present invention, silicon (Si) is a reinforcing element capable of high strength without significantly lowering the ductility of the steel sheet, and also suppresses the formation of carbides when the austenite phase is transformed into the bainite phase, thereby improving the stability of the unaffected austenite phase. It is good to add it properly because it has an effect to make it. Also, in the steel to which the appropriate Mn is added, the flowability of the molten metal during welding is improved to minimize the residue of inclusions in the weld.

The silicon is preferably included in the content ratio of 0.1 to 0.5% by weight of the total weight of the steel sheet. When the content of silicon is less than 0.1% by weight, the silicon-containing effect may not be properly obtained.

In the present invention, the coating property can be improved by setting the content of silicon to 0.5% by weight or less, and even if the content of silicon is 0.5% by weight or less, the stability of the unmodified austenite phase can be maintained high, and an appropriate amount of retained austenite phase is obtained. It can be secured.

Manganese (Mn)

In the present invention, since manganese (Mn) is an effective element for preventing hot cracking by sulfur (S), it is preferable to contain an appropriate amount according to the amount of sulfur (S) present in the steel. In addition, manganese (Mn) is an element that stabilizes the retained austenite phase by concentrating the austenite phase as a solid solution strengthening element, and has an effect of greatly improving the hardenability and greatly increasing the steel sheet strength.

The content of manganese (Mn) is preferably 1.4 to 2.0% by weight of the total weight of the steel sheet. When the content of manganese is less than 1.4% by weight, the effect of adding manganese is insignificant, and when the content of manganese (Mn) is more than 2.0% by weight, weldability is remarkably decreased, and manganese band (Mn Band) is developed at the center of the material thickness. As a result, bending workability is lowered.

Titanium (Ti)

Titanium (Ti) is a powerful carbonitride-forming element. Titanium (Ti) is combined with nitrogen (N) in the ratio of 3.4: 1 in the steel sheet to reduce the solid solution nitrogen.

The addition amount of titanium (Ti) in the steel sheet is determined according to the amount of nitrogen dissolved, but preferably in the present invention may present 0.05 to 0.15% by weight of the total weight of the steel sheet. When the addition amount of titanium is less than 0.05% by weight, the above titanium addition effect cannot be properly exhibited. In addition, when the addition amount of titanium (Ti) exceeds 0.15% by weight, there is a problem in that the yield ratio is excessively increased by combining with carbon (C) in the steel sheet.

Niobium (Nb)

Cold rolled steel sheet according to the present invention may further include niobium (Nb) in addition to the above components. Niobium precipitates the solid solution carbon (C) as a composite precipitate during hot rolling, thereby increasing the strength of the refined ferrite and improving moldability.

Such niobium exerts a sufficient effect when added at 0.03% by weight or more. However, when niobium exceeds 0.20% by weight, the yield ratio is increased and the ductility of the cold rolled steel sheet is drastically reduced, so niobium is preferably added at 0.03 to 0.20% by weight.

Vanadium (V)

Vanadium (V) contributes to strength enhancement through solid solution strengthening and complex precipitate formation with niobium (Nb) at low temperatures.

The vanadium (V) is preferably added at 0.03 to 0.20% by weight of the total weight of the hot rolled steel sheet according to the present invention. If the added amount of vanadium (V) is less than 0.03% by weight, the effect of adding vanadium (V) is insufficient. On the contrary, when the amount of vanadium (V) added exceeds 0.20% by weight, the weldability may be reduced, and problems may occur during winding due to excessive precipitation at low temperatures.

Tungsten (W)

Tungsten (W) is a precipitate-forming element that is stable even at a high temperature of 760 ° C. or higher, and contributes to the high strength of the steel through a single or complex precipitation hardening phenomenon.

The tungsten is preferably added at 0.1 to 0.3% by weight of the total weight of the steel according to the present invention. If the content of tungsten is less than 0.1% by weight, the effect of addition is insufficient, and the precipitation effect may be insignificant. On the contrary, if the content of tungsten is added more than 0.3% by weight, the final ferrite grain size may be coarsened after cold rolling annealing to increase elongation and There is a problem of reducing the hole expandability.

Nitrogen (N)

Nitrogen (N) is also an unavoidable impurity, and there is a problem in that solid solution nitrogen increases when a large amount is added, thereby reducing elongation and formability of steel. Therefore, the content of nitrogen in the total steel content according to the present invention is preferably limited to 60 ppm or less.

Cold Rolled Steel Sheet Manufacturing Method

1 is a flow chart schematically showing a method of manufacturing a high strength cold rolled steel sheet having excellent burring processability according to an embodiment of the present invention.

Referring to FIG. 1, the illustrated high strength cold rolled steel sheet manufacturing method includes a hot rolling step (S110), a winding step (S120), a pickling step (S130), a cold rolling step (S140), and an annealing step (S150).

Hot rolling

In the hot rolling step (S110), carbon (C): 0.04 to 0.15% by weight, silicon (Si): 0.1 to 0.5% by weight, manganese (Mn): 1.4 to 2.0% by weight, titanium (Ti): 0.05 to 0.15% by weight , Tungsten (W): 0.1 to 0.3% by weight, nitrogen (N): 60ppm or less and hot-rolled slab steel composed of the remaining Fe and other unavoidable impurities.

In addition, the slab steel may further include at least one of niobium (Nb): 0.03 to 0.20% by weight and vanadium (V): 0.03 to 0.20% by weight.

At this time, it is preferable that the finish hot rolling temperature (FDT) is Ar3 point-Ar3 point + 100 degreeC so that the structure of the steel before cooling after hot rolling may have a structure of an austenite phase. If the finish hot rolling temperature is less than the Ar3 point, abnormal reverse rolling occurs, resulting in the presence of elongated ferrite and pearlite and the formation of a pearlite band, which may lower the ductility.In contrast, if the finish hot rolling temperature exceeds the Ar3 point + 100 ° C. There is a problem that can not sufficiently secure the strength of the steel produced.

Although not shown in the drawings, a slab reheating step (not shown) may be further performed before hot rolling the slab steel.

In the slab reheating step, the slab steel having the composition is reheated.

Slavic steel can be obtained through continuous casting process after obtaining molten steel of desired composition through steelmaking process. By reheating the slab steel, the segregated components are cast again.

At this time, the slab reheating temperature (SRT) is preferably carried out in a temperature range of 1150 ~ 1250 ℃. If the slab reheating temperature (SRT) is less than 1150 ° C, segregated components cannot be reclaimed during casting, and if the slab reheating temperature (SRT) is above 1250 ° C, the austenite grain size increases and the final ferrite grain size is coarsened and the strength is increased. Decreases and also increases the steel manufacturing cost due to excessive heating process.

Winding

Winding step (S120) is subjected to shear quenching in order to prevent the surface thickening of manganese and silicon and the coarsening of carbide, and then wound and coiled at 620 ~ 660 ℃ to induce stable growth of ferrite.

Pickling

In the pickling step 130, pickling is performed on the wound hot rolled steel to remove scale of the hot rolled steel.

Cold rolled

In the cold rolling step 140, the pickled hot rolled steel is cold rolled at a rolling reduction of 50 to 70%. If the cold reduction rate is less than 50%, the target thickness (0.65mm or less) is difficult to secure and the shape of the steel sheet is difficult to correct, while if the cold reduction rate exceeds 70%, cracks in the steel sheet edges may occur. This can lead to high and cold rolled loads. Therefore, the cold reduction rate is preferably limited to 50 to 70%.

Annealed

In the annealing step 150, the cold rolled steel manufactured as described above is heat-treated at Ac1 to Ac3, specifically 760 to 840 ° C. The annealing step is for forming ferrite austenite and distributing carbon at the same time as the recrystallization. When the annealing temperature is less than Ac 1 point, not only does not sufficiently recrystallize, but also it is difficult to form sufficient austenite. Difficult to secure On the other hand, when it exceeds Ac3, since the productivity decreases and excessive austenite is formed and the ductility decreases, the annealing temperature is preferably limited to Ac1 to Ac3 points.

Although not shown in the drawings, it may further include a cooling step of cooling the annealing cold rolled steel after the annealing step.

In the present annealing step, in the present invention, instead of molybdenum, by adding tungsten (W), which is a thermally stable precipitate-forming element at a high temperature of 760 ° C or higher, precipitated carbides precipitated after cold rolling annealing (Ti-WC, Ti-W-Nb- By maintaining C) at a fine size, a uniform and fine structure having a final grain size of 6 µm or less can be obtained.

In the high-strength cold-rolled steel sheet according to the present invention manufactured as described above, the addition of niobium and vanadium, as well as tungsten can be further fine and uniform in the final ferrite structure to further improve the elongation and hole expandability.

At this time, the high strength cold rolled steel sheet according to the present invention has a tensile strength (TS): 780MPa or more and elongation (EL): 22% or more.

In addition, the high strength cold rolled steel sheet according to the present invention prepared as described above has a hole expansion ratio (HER): 70% or more.

Example

Hereinafter, the configuration and operation of the present invention through the preferred embodiment of the present invention will be described in more detail. It is to be understood, however, that the same is by way of illustration and example only and is not to be construed in a limiting sense.

Details that are not described herein will be omitted since those skilled in the art can sufficiently infer technically.

1. Manufacture of cold rolled steel

Table 1 shows the component ratios of Examples 1,2,3 and Comparative Example 1 of the present invention.

The slab steel having the composition shown in Table 1 above was reheated for 3 hours or longer at Ar3 or higher, and then subjected to finish hot rolling at a temperature of Ar3 + 100 ° C or lower, and then cooled by forced cooling.

Next, it was wound at 650 ℃ to produce a fine hot rolled steel sheet. Then, the hot rolled steel sheet produced by winding as described above was pickled and cold rolled to produce a cold rolled steel sheet.

The specimen of the cold rolled steel sheet was subjected to a tensile test using a universal tensile tester to obtain an experimental result as shown in Table 2 below.

 [Table 1] (unit:% by weight)

2. Mechanical Properties

Table 2 shows the comparison between the grain size and the mechanical properties of the microstructure according to Examples 1, 2, 3 and Comparative Example 1.

 [Table 2]

Referring to Table 1 and Table 2, in Examples 1 and 2, it was confirmed that the tensile strength (TS): 780MPa or more, elongation (EL): 22% or more and hole expansion ratio (HER): 70% or more Can be.

On the other hand, in Comparative Example 1 it can be seen that the tensile strength (TS): 795 MPa and elongation (EL): 20% and hole expansion (HER): 40% does not reach the target value.

Compared with Example 1, in Examples 2 and 3, the final microstructure has a grain size (FSG) of 2 to 4 μm. As in Examples 2 and 3, when niobium (Nb) and vanadium (V) are further added to the turn-stack (W) -added steel, it can be confirmed that the final ferrite structure is more uniform and finely formed.

As a result of the above experiment, in the case of Comparative Example 1, the addition of molybdenum (Mo) is represented by 5 ~ 15㎛ by Ti-Mo-C composite carbide can secure the strength of 780MPa grade, but due to the non-uniformity of the ferrite structure It can be seen that (EL) and hole expandability (HER) are significantly reduced.

On the contrary, in Examples 1 and 2 and 3, when tungsten (W) is added instead of molybdenum (Mo), the final ferrite grain size (FSG) becomes fine to 6 μm or less, and niobium (W) The addition of Nb) and vanadium (V) can confirm that the final ferrite grain size (FSG) can have a finer and more uniform structure of less than 4㎛.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments. Such changes and modifications are intended to fall within the scope of the present invention unless they depart from the scope of the present invention. Accordingly, the scope of the present invention should be determined by the following claims.

S110: hot rolling stage
S120: winding step
S130: Pickling Step
S140: cold rolling stage
S150: Annealing Step

Claims (11)

(a) Carbon (C): 0.04 to 0.15 wt%, Silicon (Si): 0.1 to 0.5 wt%, Manganese (Mn): 1.4 to 2.0 wt%, Titanium (Ti): 0.05 to 0.15 wt%, Tungsten (W) ): 0.1 to 0.3% by weight, nitrogen (N): 60 ppm or less and hot rolling the slab steel composed of the remaining Fe and other unavoidable impurities;
(b) cooling and winding the hot rolled steel;
(c) pickling the wound steel;
(d) cold rolling the pickled steel; And
(e) annealing the cold rolled steel;
After the step (e), the steel has a grain size of 6 ㎛ or less of the final ferrite microstructure, tensile strength (TS): 780MPa or more and elongation (EL): characterized in that the production of high strength cold rolled steel sheet Way.
The method of claim 1,
The slab steel
Niobium (Nb): 0.03 to 0.20% by weight and vanadium (V): 0.03 to 0.20% by weight of at least one of the production method of high strength cold rolled steel sheet characterized in that it further comprises.
The method of claim 1,
Before step (a)
The slab reheating step of reheating the slab steel to 1150 ~ 1250 ℃ further comprising the step of producing a high strength cold rolled steel sheet.
The method of claim 1,
Finishing hot rolling temperature in the step (a) is
Ar3 point ~ Ar3 point + 100 ℃ high strength cold rolled steel sheet manufacturing method characterized in that.
The method of claim 1,
In step (b), the winding temperature is
High strength cold rolled steel sheet manufacturing method characterized in that 600 ~ 700 ℃.
The method of claim 1,
In the step (e) the annealing temperature is
760 to 840 ℃ high strength cold rolled steel sheet manufacturing method characterized in that.
Carbon (C): 0.04 to 0.15 wt%, Silicon (Si): 0.1 to 0.5 wt%, Manganese (Mn): 1.4 to 2.0 wt%, Titanium (Ti): 0.05 to 0.15 wt%, Tungsten (W): 0.1 ~ 0.3% by weight, nitrogen (N): 60ppm or less and the remaining Fe and other unavoidable impurities,
A high-strength cold-rolled steel sheet having a final grain size of 6 microns or less and having tensile strength (TS) of 780 MPa or more and elongation (EL) of 22% or more.
The method of claim 7, wherein
The steel sheet
Niobium (Nb): 0.03 to 0.20% by weight and vanadium (V): 0.03 to 0.20% by weight of the high strength cold rolled steel sheet further comprises.
The method of claim 8,
The grain size of the final ferrite microstructure is
High strength cold rolled steel sheet, characterized in that 2 ~ 4㎛.
delete The method of claim 7, wherein
The steel sheet
Hole expansion ratio (HER): High strength cold rolled steel sheet characterized in that it has a 70% or more.

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