KR20220146646A - Steel plate for hot stamping and hot stamping body - Google Patents

Abstract

This steel sheet for hot stamping has a desired chemical composition, and in the metal structure, the pole density in the {112}<110> orientation of the central portion of the sheet thickness is more than 4.0, the area ratio is 50% or more of ferrite, and among all ferrites , the number ratio of the ferrite including the hard phase in the ferrite particles is 65% or more.

Description

Steel plate for hot stamping and hot stamping body

The present invention relates to a steel sheet for hot stamping and a hot stamped body.

This application claims priority based on Japanese Patent Application No. 2020-156558 for which it applied to Japan on September 17, 2020, and uses the content in this specification.

In recent years, high-strength steel sheets have been applied to body parts of automobiles in response to requests for weight reduction and improvement of collision safety. Since body parts are molded by press molding, improvement in press formability, particularly improvement in shape fixability, is a challenge. For this reason, the hot stamping method is attracting attention as a method of manufacturing high-strength body parts with excellent shape precision.

Also, in recent years, a technique for applying a tailored blank to a hot stamping method has been studied. A tailored blank is made by joining a plurality of steel sheets different in sheet thickness, chemical composition, metal structure, and the like by welding to form a single steel sheet. In a tailored blank, the properties in one joined steel sheet can be partially changed. For example, by giving a certain part high strength, the deformation|transformation in that part is suppressed, and by giving a low intensity|strength to another part, that part is deformed, and an impact can be absorbed. It is calculated|required that it is excellent in ductility so that the fracture|rupture at the time of deformation|transformation can be suppressed by the part with low intensity|strength.

As a technique for applying a tailored blank to the hot stamping method, there is a technique using a tailored blank in which a steel sheet having low strength after hot stamping and a steel sheet having high strength after hot stamping are joined by welding. As the steel sheet having high strength after hot stamping, for example, a steel sheet disclosed in Patent Document 1 can be used. As a steel sheet having low strength after hot stamping, the chemical composition of the steel may be adjusted so as to have low strength after cooling the die in hot stamping.

One of the steel types applied to tailored blanks is low-carbon steel. Since low-carbon steel has a low carbon content, it has a characteristic that it is difficult to increase strength even if it is rapidly cooled after heating. Patent Document 2 discloses that ultra-low carbon steel is used as a low-strength material for a hot stamping method. Patent Document 2 discloses a technique for improving local deformability by heating a steel sheet to a temperature of Ac ₃ or higher and then hot stamping to form a metal structure containing bainite and bainitic ferrite as a main phase. Patent Document 2 discloses that this technique makes it difficult to fracture when a vehicle body part is deformed in the bending mode at the time of a collision and is excellent in shock absorption ability due to plastic deformation.

In recent years, as a high-strength material with high impact performance, hot stamped articles having a tensile strength of less than 1500 MPa have been attracting attention. Such a hot-stamped article is required to have higher ductility after hot-stamping so that fracture upon deformation can be sufficiently suppressed after having desired strength.

Japanese Patent Laid-Open No. 2004-197213 International Publication No. 2012/157581

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a hot stamped article having high strength and excellent ductility, and a steel sheet for hot stamping capable of manufacturing the hot stamped article.

The present inventors studied a method for improving the ductility of a hot-stamped article. As a result, it was found that, in the metal structure of the hot stamped body, the ductility of the hot stamped body could be improved by increasing the number ratio of ferrite including a hard phase with high dislocation density in the ferrite particles.

Further, the present inventors have found that, in the steel sheet for hot stamping, the above hot stamped body can be obtained by preferably controlling the chemical composition and increasing the number ratio of ferrite containing a hard phase in the ferrite grains.

This invention was obtained based on the said knowledge, and the summary of this invention is as follows.

(1) The steel sheet for hot stamping according to one aspect of the present invention has a chemical composition in mass%,

C: 0.060 to 0.200%;

Si: 0.010 to 1.000%,

Mn: 0.80 to 2.00%,

Al: 0.010 to 0.500%,

Nb: 0.020 to 0.100%,

P: 0.100% or less;

S: 0.0100% or less;

N: 0.0100% or less;

Ti: 0 to 0.10%,

Cr: 0 to 0.50%,

B: 0 to 0.0100%;

Mo: 0 to 1.00%,

Co: 0 to 2.00%,

Ni: 0 to 0.50%;

V: 0 to 0.10%,

Ca: 0 to 0.0100%,

Mg: 0 to 0.0100%, and

REM: 0 to 0.0100%;

the balance contains Fe and impurities,

In the metal structure,

The pole density of the {112}<110> orientation of the central portion of the plate thickness is greater than 4.0,

In terms of area ratio, ferrite is 50% or more,

Among all the ferrites, the number ratio of the ferrites including the hard phase in the ferrite particles is 65% or more.

(2) In the steel sheet for hot stamping according to (1), the chemical composition is in mass%,

Ti: 0.01 to 0.10%,

Cr: 0.01 to 0.50%,

B: 0.0001 to 0.0100%;

Mo: 0.01 to 1.00%,

Co: 0.01 to 2.00%,

Ni: 0.01 to 0.50%,

V: 0.01 to 0.10%,

Ca: 0.0005 to 0.0100%,

Mg: 0.0005 to 0.0100%, and

REM: 0.0005 to 0.0100%

You may contain 1 type, or 2 or more types from the group which consists of.

(3) The hot-stamped article according to another aspect of the present invention has a chemical composition in mass%,

C: 0.060 to 0.200%;

Si: 0.010 to 1.000%,

Mn: 0.80 to 2.00%,

Al: 0.010 to 0.500%,

Nb: 0.020 to 0.100%,

P: 0.100% or less;

S: 0.0100% or less;

N: 0.0100% or less;

Ti: 0 to 0.10%,

Cr: 0 to 0.50%,

B: 0 to 0.0100%;

Mo: 0 to 1.00%,

Co: 0 to 2.00%,

Ni: 0 to 0.50%;

V: 0 to 0.10%,

Ca: 0 to 0.0100%,

Mg: 0 to 0.0100%, and

REM: 0 to 0.0100%;

the balance contains Fe and impurities,

In the metal structure,

In terms of area ratio, martensite is 20% or more,

Among all the ferrites, the number ratio of ferrites including a hard phase having a GAIQ value of 26000 or less in the ferrite particles is 50% or more.

(4) In the hot-stamped article according to (3), the chemical composition is in mass%,

Ti: 0.01 to 0.10%,

Cr: 0.01 to 0.50%,

B: 0.0001 to 0.0100%;

Mo: 0.01 to 1.00%,

Co: 0.01 to 2.00%,

Ni: 0.01 to 0.50%,

V: 0.01 to 0.10%,

Ca: 0.0005 to 0.0100%,

Mg: 0.0005 to 0.0100%, and

REM: 0.0005 to 0.0100%

You may contain 1 type, or 2 or more types from the group which consists of.

According to the above aspect of the present invention, it is possible to provide a hot stamped article having high strength and excellent ductility, and a steel sheet for hot stamping from which the hot stamped article can be manufactured.

Hereinafter, the steel sheet for hot stamping and the hot stamping body according to the present embodiment will be described in detail. First, the reason for limiting the chemical composition of the steel sheet for hot stamping according to the present embodiment will be described. In addition, the lower limit and the upper limit are included in the numerical limitation range described with "to" interposed therebetween. Numerical values expressed as “less than” and “exceed” are not included in the numerical range. In addition, all % regarding a chemical composition mean mass %.

The hot-stamped article according to the present embodiment has a chemical composition, in mass%, of C: 0.060 to 0.200%, Si: 0.010 to 1.000%, Mn: 0.80 to 2.00%, Al: 0.010 to 0.500%, Nb: 0.020 to 0.100%, P: 0.100% or less, S: 0.0100% or less, N: 0.0100% or less, and the balance: Fe and impurities. Hereinafter, each element is demonstrated.

C: 0.060 to 0.200%

C is an element that greatly affects the strength and ductility of the hot-stamped article. If the C content is too low, martensitic transformation is not promoted, the strength of the hot-stamped article is lowered, and fracture due to insufficient strength is likely to occur. Therefore, the C content is made 0.060% or more. Preferably, it is 0.080% or more, 0.100% or more, or 0.120% or more.

On the other hand, if the C content is too high, the transformation from austenite to ferrite is inhibited, so that a desired amount of ferrite cannot be obtained, and the ductility of the hot-stamped article is lowered. Therefore, the C content is made 0.200% or less. Preferably, it is 0.170% or less or 0.150% or less.

Si: 0.010 to 1.000%

Si is an element having a solid solution strengthening ability, and is an element necessary for obtaining strength of a hot stamped article. If the Si content is too low, the desired strength cannot be obtained in the hot stamped body. Therefore, the Si content is made 0.010% or more. Preferably, it is 0.100% or more, 0.200% or more, 0.250% or more, 0.300% or more, 0.400% or more, or 0.500% or more.

On the other hand, if the Si content is too high, the ferrite transformation proceeds excessively, and it is impossible to obtain a desired amount of martensite in the hot stamped body. Therefore, the Si content is made 1.000% or less. Preferably, it is 0.900% or less or 0.800% or less.

Mn: 0.80 to 2.00%

Mn is an element having a solid solution strengthening ability, and is contained in order to obtain strength of a hot-stamped article. If the Mn content is too low, the ferrite transformation proceeds too much, making it difficult to form martensite, and the desired strength cannot be obtained in the hot stamped body. Therefore, the Mn content is made 0.80% or more. Preferably, it is 1.00% or more or 1.20% or more.

On the other hand, if the Mn content is too high, the hardenability of the steel increases, the formation of ferrite in the air after heating during hot stamping is suppressed, and the ductility of the hot stamped body decreases. Therefore, the Mn content is set to 2.00% or less. Preferably, it is 1.80% or less or 1.60% or less.

Al: 0.010 to 0.500%

Al is an important element for promoting ferrite transformation. If the Al content is too low, ferrite transformation becomes difficult to proceed, and a desired amount of ferrite cannot be obtained in the hot stamped body. Therefore, the Al content is made 0.010% or more. Preferably, it is 0.020% or more or 0.030% or more.

On the other hand, if the Al content is too high, the transformation to ferrite proceeds excessively, and a desired amount of martensite cannot be obtained in the hot stamped body. Therefore, the Al content is made 0.500% or less. Preferably, it is 0.450% or less or 0.400% or less.

Nb: 0.020 to 0.100%

Nb is an element which suppresses grain growth of austenite, refines austenite grains, and promotes transformation into ferrite. If the Nb content is too low, a desired amount of ferrite cannot be obtained in the hot stamped body. Therefore, the Nb content is made 0.020% or more. Preferably, it is 0.030% or more or 0.040% or more.

On the other hand, when the Nb content is too high, the effect is saturated and the cost increases. Therefore, the Nb content is made 0.100% or less. Preferably, it is 0.090% or less or 0.080% or less.

P: 0.100% or less

P is an element that has a solid solution strengthening ability and is effective for obtaining a desired strength in a hot-stamped article. However, if the P content is too high, the ductility of the hot stamped article is deteriorated. Therefore, the P content is made 0.100% or less. Preferably, it is 0.080% or less, 0.060% or less, or 0.050% or less.

Although the lower limit of P content is not specifically prescribed|regulated, It is good also considering P content as 0.001 % or more or 0.005 % or more from a viewpoint of securing the intensity|strength by P.

S: 0.0100% or less

S is an element which is contained as an impurity in steel and embrittles steel. Therefore, it is so preferable that there is little S content. The S content is made 0.0100% or less. Preferably, it is 0.0080% or less, 0.0060% or less, or 0.0040% or less.

Although the lower limit of S content is not specifically prescribed|regulated, Since the cost in a desulfurization process will increase when S content is reduced excessively, it is good also considering S content as 0.0005 % or more or 0.0010 % or more.

N: 0.0100% or less

N is an impurity element and is an element that forms nitride in steel and deteriorates the ductility of the hot stamped body. If the N content is too high, the nitride in the steel coarsens, and the ductility of the hot stamped body deteriorates. Therefore, the N content is made 0.0100% or less. Preferably, it is 0.0080% or less or 0.0060% or less.

Although the lower limit in particular of N content is not prescribed|regulated, since cost in a steelmaking process increases when N content is reduced excessively, it is good also considering N content as 0.0010 % or more.

The steel sheet for hot stamping according to the present embodiment contains the above elements, and the balance may contain Fe and impurities. Examples of the impurity include those that are unavoidably mixed from steel raw materials or scrap and/or in the steelmaking process, or elements that are allowed in a range that does not impair the properties of the hot stamped body according to the present embodiment.

In the steel sheet for hot stamping according to the present embodiment, in order to improve various properties, an arbitrary element shown below may be contained in place of a part of Fe. Since it is not necessary to intentionally contain these arbitrary elements in steel for reduction of alloy cost, the lower limit of content of these arbitrary elements is all 0%.

Ti: 0.01 to 0.10%

Ti is an element which suppresses grain growth of austenite, refines austenite grains, and promotes transformation into ferrite. In order to acquire this effect reliably, it is preferable to make Ti content into 0.01 % or more.

On the other hand, when Ti content is too high, coarse Ti sulfide, Ti nitride, and Ti oxide are formed, and the formability of a steel plate deteriorates. Therefore, the Ti content is made 0.10% or less.

Cr: 0.01 to 0.50%

Cr is an effective element in order to increase the hardenability of steel, promote the formation of martensite, and increase the strength of the hot stamped body. In order to acquire this effect reliably, it is preferable to make Cr content into 0.01 % or more.

On the other hand, if the Cr content is too high, a large amount of coarse Cr carbides that can become a starting point of destruction are formed. Therefore, the Cr content is made 0.50% or less.

B: 0.0001 to 0.0100%

B is an element that segregates at the old austenite grain boundary, has an effect of suppressing ferrite transformation, and contributes to the improvement of the strength of the hot stamped body. In order to acquire this effect reliably, it is preferable to make B content into 0.0001 % or more.

On the other hand, if the B content is too high, the ductility of the hot stamped article is reduced. Therefore, the B content is made 0.0100% or less.

Mo: 0.01 to 1.00%

Mo forms carbides in steel and improves the strength of the hot stamped body by precipitation strengthening. In order to acquire this effect reliably, it is preferable to make Mo content into 0.01 % or more.

On the other hand, if the Mo content is too high, the ductility of the hot-stamped article is lowered. Therefore, Mo content shall be 1.00 % or less.

Co: 0.01 to 2.00%

Co improves the strength of the hot stamped article by solid solution strengthening. In order to reliably acquire this effect, it is preferable to make Co content into 0.01 % or more.

On the other hand, when the Co content is too high, the effect by the above action is saturated and the cost increases. Therefore, the Co content is set to 2.00% or less.

Ni: 0.01 to 0.50%

Ni improves the strength of the hot stamped article. In order to acquire this effect reliably, it is preferable to make Ni content into 0.01 % or more.

On the other hand, when Ni content is too high, castability may fall. Therefore, the Ni content is made 0.50% or less.

V: 0.01 to 0.10%

V improves the strength of the hot-stamped article by strengthening by the precipitate and refining the ferrite particles. In order to acquire this effect reliably, it is preferable to make V content into 0.01 % or more.

On the other hand, when V content is too high, carbonitride will precipitate abundantly and the formability of a steel plate will fall. Therefore, the V content is made 0.10% or less.

Ca: 0.0005 to 0.0100%

Ca is an element having an action of deoxidizing the molten steel to make the steel sound (it suppresses the occurrence of defects such as blowholes in the steel). In order to acquire this effect|action reliably, it is preferable to make Ca content into 0.0005 % or more.

On the other hand, since the said effect is saturated even if Ca content is too high, it is preferable to make Ca content into 0.0100 % or less.

Mg: 0.0005 to 0.0100%

Mg is an element which has the effect|action which deoxidizes molten steel and makes steel sound. In order to acquire this effect reliably, it is preferable to make Mg content into 0.0005 % or more.

On the other hand, even if the Mg content is too high, the above effect is saturated and causes an increase in cost. Therefore, it is preferable to make Mg content into 0.0100 % or less.

REM: 0.0005 to 0.0100%

REM is an element which has the effect|action which deoxidizes molten steel and makes steel sound. In order to reliably obtain this effect, it is preferable that the REM content be 0.0005% or more.

On the other hand, since the effect is saturated even if the REM content is too high, the REM content is preferably 0.0100% or less.

In the present embodiment, REM refers to a total of 17 elements including Sc, Y, and lanthanoids. In this embodiment, the content of REM refers to the total content of these elements.

What is necessary is just to measure the above-mentioned chemical composition by a general analysis method. For example, it may be measured using Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES). In addition, C and S may be measured using a combustion-infrared absorption method, and N may be measured using an inert gas melting-thermal conductivity method. When the steel sheet for hot stamping or the hot stamping body has a plating layer on the surface, the plating layer on the surface is removed by mechanical grinding, and then the chemical composition may be analyzed.

Next, the metal structure of the steel sheet for hot stamping according to the present embodiment will be described.

In the steel sheet for hot stamping according to the present embodiment, in the metal structure, the pole density of the {112}<110> orientation of the central portion of the sheet thickness is more than 4.0, the area ratio is 50% or more of ferrite, and among all ferrites , the number ratio of the ferrite including the hard phase in the ferrite particles is 65% or more. Hereinafter, each regulation will be described in detail.

In addition, in this embodiment, the area ratio of the said ferrite in the plate|board thickness 1/4 position from the surface (region of 1/8 depth of plate|board thickness from the surface to 3/8 depth of plate|board thickness from the surface) and the said ferrite Defines the number ratio.

Pole density in the {112}<110> orientation at the center of the plate thickness: > 4.0

If the pole density in the {112}<110> orientation of the central portion of the plate thickness is 4.0 or less, the desired metal structure cannot be obtained in the hot stamped body. Therefore, the pole density of the {112}<110> orientation of the central portion of the plate thickness is set to be greater than 4.0. Preferably, it is 4.5 or more or 5.0 or more. Although an upper limit is not specifically limited, It is good also as 10.0 or less.

In addition, in this embodiment, the plate|board thickness center part means the area|region of 1/4 depth of plate|board thickness from the surface - 3/4 depth of plate|board thickness from the surface.

The pole density of the {112}<110> orientation of the central portion of the plate thickness is obtained by the following method.

For the measurement, a combination of a scanning electron microscope and an EBSD analyzer and OIM Analysis (registered trademark) manufactured by TSL are used. {112}<110 > Find the pole density of the orientation. A measurement range is made into the area|region of 1/4 depth of plate|board thickness from the surface - 3/4 depth of plate|board thickness from the surface. The measurement pitch is 5 μm/step.

In addition, {hkl} denotes a crystal plane parallel to the rolling plane, and <uvw> denotes a crystal direction parallel to the rolling direction. That is, {hkl}<uvw> indicates a crystal in which {hkl} is directed in the normal direction of the plate and <uvw> is directed in the rolling direction.

Area ratio of ferrite: 50% or more

If the area ratio of ferrite is less than 50%, a desired metal structure cannot be obtained in the hot stamped body, and as a result, desired ductility cannot be obtained. Therefore, the area ratio of ferrite is made into 50% or more. Preferably it is 60 % or more, 70 % or more, or 80 % or more.

Although the upper limit of the area ratio of ferrite is not specifically limited, It is good also as 97 % or less, 95 % or less, or 90 % or less.

remaining tissue

The remainder structure other than ferrite is a hard phase containing 1 type(s) or 2 or more types of martensite, bainite, and pearlite. It is preferable that the area ratio of a hard phase shall be 5 % or more in total. Preferably it is 10 % or more. Although the upper limit of the area ratio of a hard phase is not specifically limited, It is good also as 50 % or less, 40 % or less, 30 % or less, or 20 % or less in total.

Measuring method of area ratio of metal structure

A sample is taken from a position spaced apart by 10 mm or more from the end face of the steel sheet for hot stamping so that the plate thickness section perpendicular to the surface becomes the observation face. After the observation surface was polished, nitrate was etched, and using an optical microscope and a scanning electron microscope (SEM), the plate thickness was 1/4 position from the surface (from a depth of 1/8 of the plate thickness from the surface to the plate thickness from the surface) At least 3 areas of 30 µm×30 µm in the 3/8 depth region) are observed. The respective area ratios of ferrite, pearlite, and bainite are obtained by performing image analysis on the structure photograph obtained by this structure observation. Thereafter, after performing repera corrosion at the same observation position, tissue observation is performed using an optical microscope and a scanning electron microscope, and image analysis is performed on the obtained tissue photograph to calculate the area ratio of martensite.

In the tissue observation described above, each tissue is identified by the following method.

Since martensite is a tissue having a high dislocation density and substructure such as blocks and packets in particles, it is possible to distinguish it from other metal structures according to an electron channeling contrast image using a scanning electron microscope.

It is a set of crystal grains of the lath phase, and contains a non-martensite structure among structures that do not contain Fe-based carbides with a long diameter of 20 nm or more inside the structure, and Fe-based carbides with a long diameter of 20 nm or more inside the structure, and the Fe A structure in which the carbide-based carbide has a single valence, that is, an Fe-based carbide elongated in the same direction, is regarded as bainite. Here, the Fe-based carbide elongated in the same direction means that the difference in the elongation direction of the Fe-based carbide is within 5°.

A structure that is a lumpy crystal grain and does not contain a substructure such as lath inside the structure is regarded as ferrite.

A structure in which plate-like ferrite and Fe-based carbide are layered on top of each other is considered as pearlite.

Number ratio of ferrite including hard phase in ferrite particles: 65% or more

If the number ratio of ferrite containing a hard phase in the ferrite grains among all ferrites is less than 65%, the number ratio of ferrite particles containing a hard phase in the metal structure of the hot stamped body is lowered, and as a result, excellent ductility is obtained. can't Therefore, the number ratio of the ferrite including the hard phase in the ferrite particles is 65% or more. Preferably it is 70 % or more, 75 % or more, or 80 % or more.

Although the upper limit of the number ratio of the ferrite containing a hard phase in a ferrite particle is not specifically limited, It is good also as 100 % or less, 90 % or less, or 85 % or less.

In addition, the hard phase here is the remainder structure mentioned above, and means 1 type, or 2 or more types of martensite, bainite, and a pearlite.

Method for measuring the number ratio of ferrite containing a hard phase in ferrite particles

The number of total ferrites and the number of ferrites including hard phases (martensite, bainite, and pearlite) inside the ferrite particles are measured using the structure photograph used for measuring the area ratio of the metal structure described above. By calculating the number of ferrites including a hard phase in the ferrite particles to the total number of ferrites, the ratio of the number of ferrites including a hard phase in the ferrite particles ((of ferrite including a hard phase inside the ferrite particles) number/total number of ferrites) x 100) is obtained.

The steel sheet for hot stamping according to the present embodiment may have a plating layer on one surface or both surfaces. Having a plating layer on the surface is preferable because the corrosion resistance of the hot-stamped article after hot-stamping is improved.

Examples of the plating to be applied include aluminum plating, aluminum-zinc plating, aluminum-silicon plating, hot-dip galvanizing, electro-galvanizing, alloyed hot-dip galvanizing, and the like.

Although the plate|board thickness of the steel plate for hot stamping is not specifically limited, From a viewpoint of body weight reduction, it is preferable to set it as 0.5-3.5 mm.

Next, a hot-stamped article according to the present embodiment obtained by hot-stamping the above-described steel sheet for hot-stamping will be described. Since the chemical composition of the hot stamping article according to the present embodiment can be regarded as the same as the chemical composition of the steel sheet for hot stamping described above, a description of the chemical composition is omitted.

In the hot stamped article according to the present embodiment, in the metal structure, martensite is 20% or more in terms of area ratio, and among all ferrites, the number ratio of ferrite containing a hard phase having a GAIQ value of 26000 or less in the ferrite particles is 50% or more. Hereinafter, each regulation is demonstrated.

In addition, in this embodiment, the area ratio of the said martensite and the said ferrite in the plate|board thickness 1/4 position (region of 1/8 depth of plate|board thickness from the surface to 3/8 depth of plate|board thickness from the surface) from the surface. Specifies the ratio of the number of

Area ratio of martensite: 20% or more

When the area ratio of martensite is less than 20%, the desired strength cannot be obtained in the hot stamped body. Therefore, the area ratio of martensite is made into 20% or more. Preferably, it is 30% or more, 40% or more, or 50% or more. Although the upper limit of the area ratio of martensite is not specifically limited, It is good also as 95 % or less, 90 % or less, 85 % or less, or 80 % or less.

remaining tissue

The remaining structure other than martensite is 1 type, or 2 or more types of ferrite, bainite, and pearlite. When the area ratio of ferrite is less than 5%, excellent ductility may not be obtained. Therefore, the area ratio of the ferrite may be 5% or more. More preferably, it is 10 % or more, 20 % or more, or 30 % or more.

The sum of the area ratios of bainite and pearlite may be 50% or less, 40% or less, or 30% or less.

The number ratio of ferrite including a hard phase having a GAIQ value of 26000 or less in the ferrite particles: 50% or more

A higher GAIQ value indicates a lower dislocation density, and a lower GAIQ value indicates a higher dislocation density. Therefore, a GAIQ value is a parameter which can reflect the dislocation density of a crystal grain. The ductility of the hot stamped article can be improved by increasing the number ratio of ferrite having a hard phase having a GAIQ value of 26000 or less in the ferrite particles, that is, a hard phase having a high dislocation density.

If the number ratio of ferrites containing a hard phase having a GAIQ value of 26000 or less in the ferrite particles among all ferrites is less than 50%, excellent ductility cannot be obtained. Therefore, the ratio of the number of ferrites containing a hard phase having a GAIQ value of 26000 or less in the ferrite particles is 50% or more. Preferably it is 55 % or more, 60 % or more, or 70 % or more.

Although the upper limit of the number ratio of the ferrite containing the hard phase whose GAIQ value is 26000 or less in a ferrite particle is not specifically limited, It is good also as 100 % or less or 95 % or less.

Moreover, martensite and bainite are contained in the hard phase whose GAIQ value is 26000 or less. In this embodiment, either or both of martensite and bainite may be contained as a hard phase whose GAIQ value is 26000 or less.

Method for measuring the area ratio of the metal structure and the number ratio of ferrite including a hard phase having a GAIQ value of 26000 or less in the ferrite particles

A sample is cut out so that the plate thickness cross section can be observed from a position (or a position avoiding the end) spaced 10 mm or more from the end face of the hot stamping body. The plate thickness section of this sample is polished using #600 to #1500 silicon carbide paper, and then a diamond powder having a particle size of 1 to 6 μm is dispersed in a diluted solution such as alcohol or pure water to finish it to a mirror finish. Then, it is polished for 8 minutes using colloidal silica containing no alkaline solution at room temperature to remove the strain introduced into the surface layer of the sample.

At an arbitrary position in the longitudinal direction of the plate thickness cross section of the sample, a region of 50 μm in length and 1/8 of the plate thickness from the surface to 3/8 of the plate thickness from the surface is formed at a measurement interval of 0.1 μm. Crystal orientation information is obtained by backscattering diffraction method. For the measurement, an EBSD device composed of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL) is used. At this time, the vacuum degree in the EBSD device is 9.6×10 ^-5 Pa or less, the acceleration voltage is 15 kV, the irradiation current level is 13, and the irradiation level of the electron beam is 62.

For the obtained crystal orientation information, using the "OIM Data Collection" function included with the EBSD device and the "Grain Average Misorientation" function installed in "OIM Analysis (registered trademark)", a Grain Average Image Quality map (GAIQ map) ) to get In the obtained GAIQ map, a region surrounded by grain boundaries with a crystal orientation difference of 5° or more is defined as a crystal grain. The area ratio of ferrite is obtained by considering the area|region in which the average GAIQ value in a unit crystal grain is 42000 or more as ferrite, and calculating the area ratio of the area|region.

In the obtained GAIQ map, the total number of ferrites and the number of ferrites containing a hard phase having a GAIQ value of 26000 or less inside the ferrite particles are measured. By calculating the number of ferrites including a hard phase having a GAIQ value of 26000 or less in the ferrite particles with respect to the total number of ferrites, the number ratio of ferrite including a hard phase having a GAIQ value of 26000 or less in the ferrite particles ((ferrite particles) The number of ferrites containing a hard phase having a GAIQ value of 26000 or less in the inside/the number of total ferrites) x 100) is obtained.

Next, a sample is taken from a position spaced apart from the end face of the hot-stamped body by 10 mm or more (or a position avoiding the end) so that the plate thickness section perpendicular to the surface becomes the observation face. After the observation surface was polished, nitrate was etched, and using an optical microscope and a scanning electron microscope (SEM), the plate thickness was 1/4 position from the surface (from a depth of 1/8 of the plate thickness from the surface to the plate thickness from the surface) At least 3 areas of 30 µm×30 µm in the 3/8 depth region) are observed. The respective area ratios of pearlite and bainite are obtained by performing image analysis on the structure photograph obtained by this structure observation. Thereafter, after performing repera corrosion at the same observation position, tissue observation is performed using an optical microscope and a scanning electron microscope, and image analysis is performed on the obtained tissue photograph to calculate the area ratio of martensite.

In the observation of the structure, each structure is identified by the same method as in the case of the steel sheet for hot stamping.

The hot-stamped article according to the present embodiment may have a plating layer on one or both surfaces. Having a plating layer on the surface is preferable because the corrosion resistance of the hot stamped article is improved.

Examples of the plating to be applied include aluminum plating, aluminum-zinc plating, aluminum-silicon plating, hot-dip galvanizing, electro-galvanizing, alloyed hot-dip galvanizing, and the like.

The thickness of the hot stamping body is not particularly limited, but it is preferably set to 0.5 to 3.5 mm from the viewpoint of weight reduction of the vehicle body.

The tensile (maximum) strength of the hot-stamped article according to the present embodiment may be 590 to 980 MPa. In addition, the total elongation of the hot-stamped article according to the present embodiment may be 10.0% or more. In the hot-stamped article according to the present embodiment, the product of tensile strength and total elongation (TS×El) may be 12000 MPa·% or more.

The tensile strength and total elongation are obtained by taking a JIS 5 test piece from a hot stamped body and performing a tensile test in accordance with JIS Z 2241:2011.

Next, a preferred method of manufacturing the steel sheet for hot stamping according to the present embodiment will be described. A preferred method for manufacturing a steel sheet for hot stamping according to the present embodiment includes the following steps.

A slab is obtained by setting the casting speed to 0.80 m/min or more.

A hot-rolled steel sheet is obtained by performing hot rolling by making a coiling|winding temperature into the temperature range of 500-700 degreeC.

After obtaining a cold-rolled steel sheet by cold rolling, the cold-rolled steel sheet is heated and held in a temperature range of 750 to Ac ₃ points (first holding), and thereafter, the average cooling rate in the temperature range of 600 to 700° C. is 15° C. Cool it down to /s or less. Then, it is rapidly cooled to a temperature range of 100°C or lower, or rapidly cooled to a temperature range of 300 to 500°C, held in that temperature range (second holding), and then rapidly cooled to a temperature range of 100°C or lower.

In addition, the rapid cooling here means cooling whose average cooling rate is more than 15 degreeC/s.

Hereinafter, each process is demonstrated.

Casting speed: 0.80m/min or more

By manufacturing the slab at a casting speed of 0.80 m/min or more, Mn segregation in steel can be promoted. The casting speed may be set to 3.00 m/min or less from the viewpoint of suppressing cracking of the slab.

Coiling temperature: 500 to 700°C

Mn can be concentrated in a carbide|carbonized_material by carrying out hot rolling by making a coiling|winding temperature into the temperature range of 500-700 degreeC. The other conditions of hot rolling are not specifically limited, What is necessary is just to set it as general conditions. In addition, the conditions of cold rolling may also be general, and what is necessary is just to make the cumulative rolling reduction into 30 to 70 %.

After the first holding, it is cooled so that the average cooling rate is 15°C/s or less.

After cold rolling, the cold-rolled steel sheet is heated and maintained in a two-phase region, that is, a temperature range of 750 to Ac ₃ points (first hold), and then the average cooling rate in a temperature range of 600 to 700° C. is 15° C./s or less. Cool as much as possible. Thereby, the carbide which Mn-concentrated can remain in the inside of the ferrite particle. By holding in the above temperature range, carbides not enriched with Mn are transformed into ferrite, but carbides enriched with Mn are not transformed into ferrite and remain as carbides because the transformation point is lowered.

In addition, what is necessary is just to set the holding time in 1st holding|maintenance to 10 to 300 second. In addition, in this embodiment, the average cooling rate is the value obtained by dividing the temperature difference between the surface temperature at the time of cooling start and the surface temperature at the time of cooling stop by the time difference from the time of cooling start to the time of cooling stop.

In addition, the Ac ₃ point can be calculated|required by the following formula.

Ac ₃ (℃)=910-203×C ^0.5 +66×Si-25×Mn+700×P-11×Cr+109×Al+400×Ti-15.2×Ni+104×V+31.5×Mo

The element symbol in the said formula shows content by mass % of each element, When the said element is not contained, 0 is substituted.

After quenching, hold for the second time, and further quench

After cooling so that the average cooling rate in a temperature range of 600 to 700°C is 15°C/s or less, it is rapidly cooled to a temperature range of 100°C or less. At this time, it may be rapidly cooled to a temperature range of 300 to 500°C, held in the temperature range (second holding), and then rapidly cooled to a temperature range of 100°C or less. Thereby, the carbide remaining in the ferrite particles can be transformed into a hard phase. As a result, the number ratio of the ferrite including the hard phase in the ferrite particles can be increased.

In addition, what is necessary is just to set the holding time in the 2nd holding|maintenance to 10 to 600 second.

By the manufacturing method described above, the steel sheet for hot stamping according to the present embodiment can be stably manufactured. Further, in addition to the above-described manufacturing method, a step of forming a plating layer on one side or both sides of the steel sheet for hot stamping may be provided.

Next, a preferred method for manufacturing a hot-stamped article according to the present embodiment will be described. A method for manufacturing a hot-stamped article according to the present embodiment includes the following steps.

The steel sheet for hot stamping is heated and maintained to a temperature range of Ac ₃ or higher.

After holding, it is cooled to 700°C so that the average cooling rate is 5 to 20°C/s.

It cools so that the average cooling rate becomes 30 degreeC/s or more to a temperature range of 100 degrees C or less.

Hereinafter, each process is demonstrated.

Heating temperature and holding temperature: Ac ₃ points or more

The above-described steel sheet for hot stamping can be sufficiently austenitized by heating and holding it in a temperature range of Ac ₃ or higher. Although the holding time in the temperature range of Ac ₃ point or more is not specifically limited, What is necessary is just to set it as 10-300 second, for example.

Average cooling rate to 700°C: 5 to 20°C/s

After the above holding, cooling is performed so that the average cooling rate to 700°C is 5 to 20°C/s, whereby a desired amount of ferrite can be obtained. What is necessary is just to perform cooling whose average cooling rate is 5-20 degreeC/s by air cooling. After cooling to 700°C, hot stamping is performed.

Average cooling rate to temperature range below 100°C: 30°C/s or more

A desired amount of hard phase can be obtained by cooling so that the average cooling rate to a temperature range of 100°C or less is 30°C/s or more. As a result, the ratio of the number of ferrites including a hard phase having a GAIQ value of 26000 or less in the ferrite particles can be increased. What is necessary is just to perform cooling to the temperature range of 100 degrees C or less by contact with a metal mold|die.

By the method described above, the hot-stamped article according to the present embodiment can be obtained. Since the steel sheet for hot stamping according to the present embodiment has relatively low strength, it is joined to a steel sheet having high strength after hot stamping to form a tailored blank, and is hot stamped to form a vehicle body part. This body part has a low-strength part and a high-strength part because it is manufactured by hot-stamping a tailored blank comprising a low-strength material and a high-strength material.

Although various methods, such as laser welding, seam welding, arc welding, and plasma welding, can be considered as the welding method at the time of manufacturing a tailored blank, it is not specifically limited. Moreover, the high strength material (steel sheet which becomes high strength after hot stamping) used together with the low strength material (the steel plate for hot stamping concerning this embodiment) is also not specifically limited. What is necessary is just to select an appropriate thing for each body part to be manufactured.

Without applying the steel sheet for hot stamping according to the present embodiment to the tailored blank, there is no problem at all even if a body part or the like is manufactured using only the steel sheet. There is no problem at all in producing overlapping blanks by joining steel plates such as patchwork by spot welding, and hot stamping the blanks.

Example

Next, although the Example of this invention is demonstrated, the conditions in an Example are one condition example employ|adopted in order to confirm the practicability and effect of this invention, and this invention is not limited to this one condition example. Various conditions can be employ|adopted for this invention, as long as the objective of this invention is achieved without deviating from the summary of this invention.

Using slabs having chemical compositions shown in Tables 1A and 1B, under the conditions shown in Tables 2A and 2B, steel sheets for hot stamping shown in Tables 2A and 2B were manufactured. Then, under the conditions shown in Tables 3A and 3B, hot-stamped articles shown in Tables 3A and 3B were obtained.

In addition, slabs were prepared by the casting rates listed in Tables 2A and 2B. In the cold rolling after winding, the cumulative reduction ratio was set to 30 to 70%. The holding time in the 1st holding|maintenance was 10 to 300 second, and the holding time in the 2nd holding|maintenance was made into 10-600 second. In addition, after cooling so that the average cooling rate in the temperature range of 600 to 700°C became the average cooling rate shown in Tables 2A and 2B, it was rapidly cooled to the second holding temperature. After the second holding, it was rapidly cooled to a temperature range of 100° C. or less. In addition, after cooling so that the average cooling rate in a temperature range of 600-700 degreeC might become the average cooling rate shown in Table 2B, steel plate No. 56 was rapidly cooled to 25 degreeC without performing a second holding|maintenance.

In addition, in the heating at the time of hot stamping, the holding time was made into 10-300 second.

By the method described above, the metal structure of the steel sheet for hot stamping, the metal structure of the hot stamping body, and mechanical properties (tensile strength and total elongation) were measured.

The example in which the tensile strength was 590-980 MPa was judged as having a high intensity|strength as a pass. On the other hand, the example in which the tensile strength was less than 590 MPa or more than 980 MPa was judged as disqualified.

In addition, the example in which total elongation was 10.0 % or more, and the product (TSxEl) of tensile strength and total elongation was 12000 MPa*% or more was judged as passing as the thing excellent in ductility. On the other hand, an example in which the total elongation was less than 10.0% or/and an example in which the product of tensile strength and total elongation (TS×El) was less than 12000 MPa·% was judged to be unacceptable because of poor ductility.

[Table 1A]

[Table 1B]

[Table 2A]

[Table 2B]

[Table 3A]

[Table 3B]

According to Tables 1A to 3B, it can be seen that the hot-stamped article according to the example of the present invention has high strength and excellent ductility.

On the other hand, it can be seen that the hot-stamped article according to the comparative example does not have high strength and/or excellent ductility.

According to the above aspect of the present invention, it is possible to provide a hot stamped article having high strength and excellent ductility, and a steel sheet for hot stamping from which the hot stamped article can be manufactured.

Claims (4)

The chemical composition, in mass %,
C: 0.060 to 0.200%;
Si: 0.010 to 1.000%,
Mn: 0.80 to 2.00%,
Al: 0.010 to 0.500%,
Nb: 0.020 to 0.100%,
P: 0.100% or less;
S: 0.0100% or less;
N: 0.0100% or less;
Ti: 0 to 0.10%,
Cr: 0 to 0.50%,
B: 0 to 0.0100%;
Mo: 0 to 1.00%,
Co: 0 to 2.00%,
Ni: 0 to 0.50%;
V: 0 to 0.10%,
Ca: 0 to 0.0100%,
Mg: 0 to 0.0100%, and
REM: 0 to 0.0100%;
the balance contains Fe and impurities,
In the metal structure,
The pole density of the {112}<110> orientation of the central part of the plate thickness is greater than 4.0,
In terms of area ratio, ferrite is 50% or more,
Among all the ferrites, the number ratio of the ferrite including the hard phase in the ferrite particles is 65% or more
Steel plate for hot stamping, characterized in that. According to claim 1, wherein the chemical composition, in mass%,
Ti: 0.01 to 0.10%,
Cr: 0.01 to 0.50%,
B: 0.0001 to 0.0100%;
Mo: 0.01 to 1.00%,
Co: 0.01 to 2.00%,
Ni: 0.01 to 0.50%,
V: 0.01 to 0.10%,
Ca: 0.0005 to 0.0100%,
Mg: 0.0005 to 0.0100%, and
REM: 0.0005 to 0.0100%
containing one or two or more of the group consisting of
Steel plate for hot stamping, characterized in that. The chemical composition, in mass %,
C: 0.060 to 0.200%;
Si: 0.010 to 1.000%,
Mn: 0.80 to 2.00%,
Al: 0.010 to 0.500%,
Nb: 0.020 to 0.100%,
P: 0.100% or less;
S: 0.0100% or less;
N: 0.0100% or less;
Ti: 0 to 0.10%,
Cr: 0 to 0.50%,
B: 0 to 0.0100%;
Mo: 0 to 1.00%,
Co: 0 to 2.00%,
Ni: 0 to 0.50%;
V: 0 to 0.10%,
Ca: 0 to 0.0100%,
Mg: 0 to 0.0100%, and
REM: 0 to 0.0100%;
the balance contains Fe and impurities,
In the metal structure,
In terms of area ratio, martensite is 20% or more,
Among all ferrites, the number ratio of ferrites including a hard phase having a GAIQ value of 26000 or less in the ferrite particles is 50% or more
A hot-stamped article, characterized in that. The method according to claim 3, wherein the chemical composition is:
Ti: 0.01 to 0.10%,
Cr: 0.01 to 0.50%,
B: 0.0001 to 0.0100%;
Mo: 0.01 to 1.00%,
Co: 0.01 to 2.00%,
Ni: 0.01 to 0.50%,
V: 0.01 to 0.10%,
Ca: 0.0005 to 0.0100%,
Mg: 0.0005 to 0.0100%, and
REM: 0.0005 to 0.0100%
containing one or two or more of the group consisting of
A hot-stamped article, characterized in that.