KR101673342B1

KR101673342B1 - Method for hot stamping and hot stamping steel

Info

Publication number: KR101673342B1
Application number: KR1020150139188A
Authority: KR
Inventors: 김두영; 윤경일; 홍승현
Original assignee: 현대자동차 주식회사
Priority date: 2015-10-02
Filing date: 2015-10-02
Publication date: 2016-11-16

Abstract

A hot stamping method according to an embodiment of the present invention includes: heating a blank; Pressing the blank to form the blank; And cooling the blank, wherein the blank comprises 0.22 to 0.32 wt% of C, 0.18 to 0.25 wt% of Si, 1.1 to 1.3 wt% of Mn, 0.0015 0.001 to 0.0015% by weight of B, 2 to 6% by weight of Ni, 0.05 to 0.10% by weight of Nb, 0.1 to 0.25% by weight of Cr, 0.18 to 0.22% , And Mo: 0.05 to 0.10 wt%, the balance being Fe and impurities.

Description

METHOD FOR HOT STAMPING AND HOT STAMPING STEEL [0001]

Hot stamping method and hot stamping steel.

Recently, in the automobile industry, ultra high strength steels have been applied to improve the weight of the vehicle body and the safety of collision. For example, hot stamping (also referred to as "hot press forming") technology is one of the high-strength steel forming techniques of 1500 MPa class.

The hot stamping molding technique is a method in which a blank is heated and press-formed, and quenched in a mold to produce an ultra-high strength body part through a phase change.

However, the present hot stamping material has been subjected to localized softening due to low elongation after molding. The problem of brittle fracture can be solved by the local softening method, but the process is complicated and the cost is increasing.

Therefore, it is necessary to develop hot stamping material and hot stamping method to reduce cost and strengthen impact toughness.

One embodiment of the present invention is to provide a hot stamping method.

Another embodiment of the present invention is to provide a hot stamping steel.

A hot stamping method according to an embodiment of the present invention includes: heating a blank; Pressing the blank to form the blank; And cooling the blank,

The blank comprises 0.22 to 0.32 wt% of C, 0.18 to 0.25 wt% of Si, 1.1 to 1.3 wt% of Mn, 0.0015 wt% or less of P, 0.0015 wt% or less of S, 0.18 to 0.22 wt% of Cr, 0.018 to 0.025 wt% of Ti, 0.0010 to 0.0015 wt% of B, 2 to 6 wt% of Ni, 0.05 to 0.10 wt% of Nb and 0.05 to 0.10 wt% of Mo And the remainder may contain Fe and impurities.

In the step of cooling the blank, the average cooling rate may be 30 to 40 DEG C / sec.

After the step of cooling the blank, the structure of the blank may include 70 to 80 area% of martensite, 10 to 20 area% of residual austenite, and the balance may be bainite.

The step of heating the blank may be to heat until the entire structure of the blank's blank becomes austenite.

In the step of heating the blank, the blank heating temperature may be 950 DEG C or higher.

The hot stamped steel according to one embodiment of the present invention comprises 0.22 to 0.32 wt% of C, 0.18 to 0.25 wt% of Si, 1.1 to 1.3 wt% of Mn, 0.0015 wt% of P, 0.0015 to 0.0015% by weight of B, 2 to 6% by weight of Ni, 0.05 to 0.10% by weight of Nb, and 0.1 to 0.25% by weight of B, Mo: 0.05 to 0.10 wt%, and the remainder may contain Fe and impurities.

The structure of the hot stamping steel may include martensite: 70 to 80 area%, residual austenite: 10 to 20 area%, and the balance may be bainite.

According to an embodiment of the present invention, it is possible to provide a hot stamping steel having an improved elongation and an improved crash performance.

According to one embodiment of the present invention, the local softening method can be omitted, thereby reducing the manufacturing cost.

According to one embodiment of the present invention, the continuous cooling transformation (CCT) curve is shifted to the left side by adjusting the alloy component of the hot stamping steel, thereby causing the bainite and retained austenite to be added to the martensite structure in the conventional hot stamping process. The strength can be improved by improving the material ductility, grain refinement and precipitation strengthening effect.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an SEM photograph showing the structure of the inventive material 1; Fig.
Fig. 2 is a SEM photograph showing the structure of Inventive Material 2; Fig.
3 is a continuous cooling transformation (CCT) curve according to a hot stamping method and hot stamping steel according to an embodiment of the present invention.
4 is a schematic microstructure view according to a hot stamping method and hot stamping steel according to an embodiment of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS The advantages and features of the present invention, and the manner of achieving them, will be apparent from and elucidated with reference to the embodiments described hereinafter in conjunction with the accompanying drawings. However, it is to be understood that the present invention is not limited to the disclosed embodiments, but may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. It is intended that the disclosure of the present invention be limited only by the terms of the appended claims. Like reference numerals refer to like elements throughout the specification.

Thus, in some embodiments, well-known techniques are not specifically described to avoid an undesirable interpretation of the present invention. Unless defined otherwise, all terms (including technical and scientific terms) used herein may be used in a sense commonly understood by one of ordinary skill in the art to which this invention belongs. Whenever a component is referred to as "including" an element throughout the specification, it is to be understood that the element may include other elements, not the exclusion of any other element, unless the context clearly dictates otherwise. Also, singular forms include plural forms unless the context clearly dictates otherwise.

A hot stamping method according to an embodiment of the present invention will be described.

First, prepare the blank.

The blank comprises 0.22 to 0.32 wt% of C, 0.18 to 0.25 wt% of Si, 1.1 to 1.3 wt% of Mn, 0.0015 wt% or less of P, 0.0015 wt% or less of S, 0.18 to 0.22 wt% of Cr, 0.018 to 0.025 wt% of Ti, 0.0010 to 0.0015 wt% of B, 2 to 6 wt% of Ni, 0.05 to 0.10 wt% of Nb and 0.05 to 0.10 wt% of Mo , And the remainder may contain Fe and impurities.

The reason for limiting the composition of the blank is as follows.

Carbon (C) is added in an amount of 0.22 to 0.32 wt% to serve as an interstitial solid solution, and it is possible to increase the strength by forming fine carbides with Nb, Mo, Ti and the like. If the content of carbon is less than 0.22 wt%, the strength may be lowered or the percentage of austenite may be decreased, and the elongation may be lowered. If the content of carbon is more than 0.32 wt%, the weldability may be deteriorated.

Silicon (Si) is added in an amount of 0.18 to 0.25% by weight and can act as a deoxidizing agent to inhibit the formation of an oxide which is an impurity.

Manganese (Mn) may be added in an amount of 1.10 to 1.30 wt% to increase the austenite fraction and increase the elongation. If the content of manganese is less than 1.10 wt%, the elongation may be lowered. If the content of manganese is more than 1.30 wt%, cracking and distortion may occur during quenching.

Phosphorus (P) and sulfur (S) are impurities present in the steel, which can increase the embrittlement in the presence of the steel, so that the content of each of them can be controlled to be 0.0015 wt% or less.

Molybdenum (Mo) is added in an amount of 0.05% by weight or more so that the bainite region is made gentle in the state of continuous cooling to facilitate securing bainite and improve the hardenability. In hot stamping process, high temperature temper embrittlement can be prevented and grain refinement can be made. If the molybdenum content exceeds 0.1% by weight, the fraction of martensite may increase and the elongation percentage may decrease.

Niobium (Nb) is added in an amount of 0.05 wt% or more to make the bainite region gentle in the state of continuous cooling, thereby facilitating the securing of bainite. Further, Cr carbide formation can be suppressed and stable Nb carbide can be formed. It can also delay softening during the heat treatment. If the niobium content exceeds 0.1% by weight, the fraction of martensite may increase and the elongation percentage may decrease.

2% by weight or more of nickel (Ni) is added to stabilize the austenite and make the structure of the steel finer and improve the ductility and strength at the same time. If the nickel content exceeds 6% by weight, more specifically, exceeds 5% by weight, the percentage of austenite and bainite increases and tensile strength may be lowered.

In case of Cr, one of the three key elements of this steel type is to stabilize (Mn, Ni, Cr) austenite to ensure retained austenite. (Austenite stabilization effect C >> Mn> Ni> Cr)

When Cr is added in an amount less than 0.18% by weight, it is difficult to secure 10% of retained austenite. When Cr is added in an amount of 0.22% by weight or more, another low temperature phase (martensite) is formed.

Ti is used to control the shape of microstructure. TiC nano-precipitates can be formed and a grain boundary pinning effect can be obtained. When the pinning effect is obtained, the growth of grains can be suppressed and the grain size can be kept small. The microstructure having a small grain has characteristics favoring crack propagation and elongation (large grains in the same microstructure, ). However, if Ti is added in an amount smaller than 0.018 wt%, it is difficult to obtain a target grain refinement effect, and when Ti is added in an amount of 0.025 wt% or more, the precipitation phase becomes large and acts as a crack initiation point and becomes defective.

In the case of B, the most commonly used ingenious hardening element is to move the nose of the continuous cooling transformation (CCT) curve to the right to form a hard texture (martensite). Though 30ppm is used in general hot stamping steel, this steel is lowered to half because it needs different tough texture.

When B is less than 10 ppm (0.0010 wt%), it is difficult to secure a desired martensite fraction. When B is added in an amount of 0.0015 wt% or more, it is difficult to secure bainite and austenite.

The blank of the substrate is heated. At this time, the blank may be heated so as to have an austenite phase in the entire region. In addition, the blank heating temperature may be higher than or equal to 950 占 폚 so as to have an oscenite phase in the entire region of the blank.

The blank is then loaded into the hot stamping apparatus. When the blank is loaded, the mold of the hot stamping apparatus presses the blank to form a predetermined part shape.

Further, when the mold of the hot stamping apparatus and the blank meet, the blank is cooled while the heat is lost from the blank to the mold.

At this time, the die of the hot stamping apparatus is provided with a cooling channel, so that the cooling rate of the blank can be adjusted.

The average cooling rate of the blank is maintained at 30 to 40 ° C / sec to contain martensite: 70 to 80% by area and retained austenite: 10 to 20% by area, and the remainder can be a bainite-structured steel.

As described above, according to the hot stamping method, the continuous cooling transformation nose (NOSE) is moved to the left as shown in FIG. 3 and FIG. 4 through the adjustment of the alloy component under the same cooling condition, An alloy system capable of securing bainite and retained austenite in the site structure can be implemented to improve the strength by improving material ductility, grain refinement and precipitation strengthening effect.

When the martensite content is less than 70% by area, it is difficult to secure the target tensile strength.

If the martensite exceeds 80% by area, it is difficult to secure the target elongation.

If the retained austenite is 10% by area or less, it is difficult to secure the target elongation percentage.

When the retained austenite exceeds 20% by area, it is difficult to secure the target tensile strength.

Hereinafter, a hot stamping steel manufactured by a hot stamping method according to an embodiment of the present invention will be described.

The hot stamped steel according to one embodiment of the present invention comprises 0.22 to 0.32% by weight of C, 0.18 to 0.25% by weight of Si, 1.1 to 1.3% by weight of Mn, 0.0015% by weight of P, 0.001 to 0.0015% by weight of B, 2 to 6% by weight of Ni, 0.05 to 0.10% by weight of Nb, 0.1 to 0.25% by weight of Cr, 0.18 to 0.22% And Mo: 0.05 to 0.10% by weight, the balance being Fe and impurities.

The reason for the composition limitation in the hot stamping steel is described for the reason given for the blank in the above, and the detailed explanation will be omitted.

Further, the structure of the hot stamping steel may include 70 to 80 area% of martensite, 10 to 20 area% of residual austenite, and the balance may be bainite.

The reason for limiting the area fraction for the structure of the hot stamped steel is described in the hot stamping method, and a detailed description thereof will be omitted.

In the hot stamping steel, the amount of Nb carbide, Mo carbide, and Ti carbide is extremely fine because the amount of carbide of Nb, Mo, and Ti is the effect of pinning the grain boundaries in a small nano-sized precipitate form .

Hereinafter, the embodiment will be described in detail. The following examples are illustrative of the present invention only and are not intended to limit the scope of the present invention.

&Lt; Example 1 >

A blank having the same composition as in Table 1 was heated at 950 占 폚 to have an austenite phase throughout the blank area. Thereafter, it was loaded on a hot stamping apparatus, followed by press molding and cooling to room temperature.

The average cooling rate was maintained at 35 [deg.] C / sec.

element C Si Mn P S Cr Ti B Ni Nb Mo Tensile Strength (MPa) Elongation (% EL) Inventory 1 0.221 0.15 1.11 0.0015 0.0015 0.186 0.024 0.0013 2.7 0.057 0.058 1433.68 11.24 Inventory 2 0.229 0.25 1.27 0.0015 0.0015 0.216 0.018 0.001 5.8 0.052 0.059 1290.46 14.46 Comparison 1 0.231 0.22 1.15 0.0015 0.0015 0.185 0.021 0.0012 - 0.055 0.052 1511.42 5.71 Comparative material 2 0.223 0.2 1.24 0.0015 0.0015 0.187 0.026 0.0011 0.8 0.054 0.051 1495.1 6.21 Comparative material 3 0.206 0.24 1.29 0.0015 0.0015 0.199 0.021 0.0013 7.4 0.051 0.056 962.64 14.12 Comparison 4 0.232 0.25 1.18 0.0015 0.0015 0.189 0.022 0.00014 8.5 0.049 0.051 911.72 15.23 Comparative material 5 0.231 0.21 1.21 0.0015 0.0015 0.192 0.019 0.0015 5.1 0.01 0.01 1120.33 12.34 Comparative material 6 0.228 0.24 1.24 0.0015 0.0015 0.221 0.025 0.0009 5.4 0.12 0.12 1531.25 6.4

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an SEM photograph showing the structure of the inventive material 1; Fig. Referring to FIG. 1, the area fraction of the inventive material 1 was found to be 70% by area of martensite, 10% by area of bainite, and 20% by area of retained austenite.

Fig. 2 is a SEM photograph showing the structure of Inventive Material 2; Fig.

Referring to FIG. 2, the area fraction of Inventive Material 2 was found to be 80% by area of martensite, 10% by area of bainite, and 10% by area of retained austenite.

In general, it is very difficult to distinguish bainite and martensite from microscopic photographs. This is because both are non-planar. In this experiment, austenite: martensite + bainite = 2: 8 was derived.

The comparative materials 1 and 2 did not satisfy the amount of Ni added, and the required percentages of austenite and bainite were not ensured and the elongation percentage was decreased.

In Comparative Examples 3 and 4, the addition amount of Ni was excessive, and the austenite and bainite fractions were high and it was difficult to ensure the tensile strength.

In the case of comparative material 5, the addition amount of Nb and Mo did not satisfy the addition amount, so that the martensite fraction was small and the crystal grains were not miniaturized and the tensile strength was decreased.

In the case of the comparative material 6, Nb and Mo were excessive, and the elongation percentage was decreased due to the high martensite fraction.

In the automobile, the final performance shows that the performance of the parts can be determined, so that the microstructure fraction is predominantly estimated in terms of final tensile performance.

&Lt; Example 2 >

A blank having the same composition as Inventive Material 2 in Table 1 was heated at 950 占 폚 to have an austenite phase in the entire area of the blank. Thereafter, it was loaded on a hot stamping apparatus, followed by press molding and cooling to room temperature.

The average cooling rate was varied from 25 to 45 DEG C / sec to produce a hot stamped steel.

While the present invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, You will understand.

It is therefore to be understood that the above-described embodiments are illustrative in all aspects and not restrictive. The scope of the present invention is defined by the appended claims rather than the detailed description, and all changes or modifications derived from the meaning and scope of the claims and their equivalents should be interpreted as being included in the scope of the present invention .

Claims

Heating the blank;
Pressing the blank to form the blank; And
And cooling the blank,
The blank comprises 0.22 to 0.32 wt% of C, 0.18 to 0.25 wt% of Si, 1.1 to 1.3 wt% of Mn, 0.0015 wt% or less of P, 0.0015 wt% or less of S, 0.18 to 0.22 wt% of Cr, 0.018 to 0.025 wt% of Ti, 0.0010 to 0.0015 wt% of B, 2 to 6 wt% of Ni, 0.05 to 0.10 wt% of Nb and 0.05 to 0.10 wt% of Mo And the remainder comprises Fe and impurities.

The method according to claim 1,
Wherein the average cooling rate in the step of cooling the blank is 30 to 40 占 폚 / sec.

3. The method according to claim 1 or 2,
After the step of cooling the blank,
Martensite: 70 to 80 area%, residual austenite: 10 to 20 area%, and the balance being bainite.

The method of claim 3,
Wherein the heating of the blank is performed until the entire structure of the blank becomes austenite.

5. The method of claim 4,
Wherein the blank heating temperature at the step of heating the blank is 950 DEG C or higher.

0.2 to 0.32 wt% of C, 0.15 to 0.25 wt% of Si, 1.1 to 1.3 wt% of Mn, 0.0015 wt% or less of P, 0.0015 wt% or less of S, 0.0015 wt% or less of Cr, 0.18 wt% or less of Cr, By weight of Ti, 0.05 to 0.10% by weight of Mo, 0.05 to 0.10% by weight of Mo, and 0.05 to 0.10% by weight of Mo, Hot stamping steel containing Fe and impurities.

The method according to claim 6,
The structure of the hot stamping steel includes martensite: 70 to 80 area%, residual austenite: 10 to 20 area%, and the balance is bainite.