WO2024136287A1 - Cold-rolled steel sheet for hot forming with excellent bendability, hot-formed member, and manufacturing methods therefor - Google Patents

Abstract

An aspect of the present invention aims to provide a steel material for hot forming, a hot-formed member using same, and a manufacturing method therefor, wherein the steel material is suitable for automotive parts requiring impact resistance and possesses a strength of as high as or higher than 1,800 MPa on the basis of tensile strength standard while securing high bendability.

Description

Cold rolled steel sheets for hot forming with excellent bendability, hot forming members, and their manufacturing methods

The present invention relates to cold rolled steel sheets for hot forming with excellent bendability, hot forming members, and methods for manufacturing them.

Hot-formed ultra-high-strength members have recently been widely used in structural members of automobiles for the purposes of improving fuel efficiency and protecting passengers by reducing the weight of automobiles.

Patent Document 1 is proposed as a representative technology for such hot forming. Patent Document 1 proposes a technology for securing ultra-high strength with a tensile strength exceeding 1600MPa by heating a steel sheet to 850°C or higher and then forming the structure of the member into martensite through hot forming and rapid cooling using a press. In the case of the technology proposed in Patent Document 1, complex shapes can be easily formed because they are molded at high temperatures, and a weight reduction effect due to high strength can be expected through an increase in strength due to rapid cooling in the mold.

The representative index for evaluating the crash resistance characteristics of HPF forming members used for purposes such as passenger protection is considered to be bendability. For example, in the case of an automobile B-pillar, when the HPF molded member is bent when subjected to a side collision of the vehicle, characteristics (bendability) that can withstand over a certain distance (angle) without fracture are required.

Therefore, as shown in Patent Document 2, various studies have been conducted to improve the collision resistance characteristics of HPF steel materials and members, such as improving the collision energy absorption ability through partial bendability improvement by forming HPF steel types of different strengths through TWB (Tailor Welded Blank). progressed.

However, even in improving crash resistance through TWB, there have been limitations in improving the properties of parts that require crash resistance, such as bendability becoming inferior due to deterioration of the weld zone.

(Prior art literature)

(Patent Document 1) U.S. Reexamination Certificate No. 6296805

(Patent Document 2) Korean Patent Publication No. 10-2021-0080239

One aspect of the present invention is to provide a steel material for hot forming, a hot forming member, and a manufacturing method thereof that can provide the member with high strength and excellent bendability.

The object of the present invention is not limited to the above-described content. Anyone skilled in the art to which the present invention pertains will have no difficulty in understanding the additional problems of the present invention from the overall details of the present invention specification.

One aspect of the present invention is, in weight percent, C: 0.25-0.45%, Si: 0.01-3.0%, Mn: 0.01-4.0%, Al: 0.001-0.4%, P: 0.001-0.05%, S: 0.0001- 0.02%, Cr: 0.1% or more but less than 5.0%, N: 0.001-0.02%, including the remaining Fe and other inevitable impurities;

Provided is a cold-rolled steel sheet for hot forming having a structure ratio value of 0.2 or more and 1.3 or less, expressed by [Relational Equation 1] below.

[Relation 1] Organizational ratio =

(of the above formula

and

represents the area ratio of pearlite and cementite in the surface layer, respectively,

and

represents the area ratio of pearlite and cementite in the center, respectively.)

The cold rolled steel sheet for hot forming may further include one or more selected from a) and f) below.

a) Sum of Ti, Nb, Zr and V contents: 0.001% to 0.4% by weight,

b) B: 0.0001~0.01% by weight,

c) Sum of Mo and W contents: 0.001 to 1.0% by weight;

d) Sum of Cu and Ni contents: 0.005~2.0% by weight,

e) Sum of Sb and Sn contents: 0.001~1.0% by weight

f) REM: 0.0001~0.02% by weight

Another aspect of the present invention includes heating a steel slab having the above-described alloy composition to 1000-1300°C; Obtaining a hot rolled steel sheet by hot rolling the heated slab at a finish rolling temperature of Ar3 to 1000°C; Cooling the hot rolled hot rolled steel sheet at a cooling rate of 400°C/s or more and 750°C/s or less; Winding the hot-rolled steel sheet at a temperature range above Ms and below 750°C; Obtaining a cold rolled steel sheet by cold rolling the coiled hot rolled steel sheet; And it provides a method of manufacturing a cold-rolled steel sheet for hot forming, including the step of continuously annealing the cold-rolled steel sheet.

The steel slab may further include one or more selected from a) to f) below.

a) Sum of Ti, Nb, Zr and V contents: 0.001% to 0.4% by weight,

b) B: 0.0001~0.01% by weight,

c) Sum of Mo and W contents: 0.001 to 1.0% by weight;

d) Sum of Cu and Ni contents: 0.005~2.0% by weight,

e) Sum of Sb and Sn contents: 0.001~1.0% by weight

f) REM: 0.0001~0.02% by weight

The cold rolling can be performed at a cumulative reduction rate of 30 to 80%.

The continuous annealing can be performed for 1 to 1000 seconds at a temperature range of 700 to 900°C.

Another aspect of the present invention includes manufacturing a cold-rolled steel sheet according to the method for manufacturing a cold-rolled steel sheet for hot forming; Heating the cold rolled steel sheet at a temperature increase rate of 1 to 1000°C/sec to a temperature of 700°C or higher; Hot forming the heated cold rolled steel sheet; and cooling the hot-formed steel sheet at a cooling rate of 10 to 1000° C./sec.

The cooling can be performed by setting the cooling stop temperature to Mf (martensite transformation end temperature) or lower. However, the cooling may be performed by setting the cooling stop temperature to Mf (martensite transformation end temperature) or more or Ms (martensite transformation start temperature) or less. In this case, after cooling, the temperature is maintained at the cooling end temperature or returned to Ac1 or below. A heating step may be further included.

Another aspect of the present invention is, in weight percent, C: 0.25-0.45%, Si: 0.01-3.0%, Mn: 0.01-4.0%, Al: 0.001-0.4%, P: 0.001-0.05%, S: 0.0001~0.02%, Cr: 0.1% or more but less than 5.0%, N: 0.001~0.02%, including the remaining Fe and other inevitable impurities;

Provided is a hot-formed member having a hardness ratio value of 0.1 or more and 10 or less, expressed by [Relational Equation 2] below.

[Relationship 2] Hardness ratio =

(In equation 1 above,

represents the standard deviation of the surface hardness,

is the standard deviation of the hardness of the center.)

The hot forming member may further include one or more selected from a) and f) below.

a) Sum of Ti, Nb, Zr and V contents: 0.001% to 0.4% by weight,

b) B: 0.0001~0.01% by weight,

c) Sum of Mo and W contents: 0.001 to 1.0% by weight;

d) Sum of Cu and Ni contents: 0.005~2.0% by weight,

e) Sum of Sb and Sn contents: 0.001~1.0% by weight

f) REM: 0.0001~0.02% by weight

The hot formed member may have a tensile strength of 1800 MPa or more and a yield strength of 1200 MPa or more.

In addition, the means for solving the above problems do not enumerate all the features of the present invention. The various features of the present invention and the resulting advantages and effects can be understood in more detail by referring to the specific embodiments below.

According to one aspect of the present invention, a steel material for hot forming that can secure high bendability while having high strength of 1800 MPa or more based on tensile strength, a hot forming member using the same, and a method of manufacturing the same can be provided.

The various and beneficial advantages and effects of the present invention are not limited to the above-described content, and may be more easily understood through description of specific embodiments of the present invention.

Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention may be modified into various other forms, and the scope of the present invention is not limited to the embodiments described below. Additionally, the embodiments of the present invention are provided to more completely explain the present invention to those with average knowledge in the relevant technical field.

In addition, unless otherwise specified in the specification of the present invention, the unit of content of each element is based on weight, and the unit of tissue ratio is based on area.

The present inventor recognized that in the case of non-plated ultra-high-strength cold-rolled steel sheets for hot forming, the bendability decreases after the hot forming process under normal hot rolling conditions, making it difficult to secure excellent bendability, and conducted in-depth research to solve this problem. .

As a result, by controlling the cooling rate of the cooling section during the hot rolling process, the area ratio of pearlite/cementite in the surface layer and center of the cold rolled steel sheet after annealing can be adjusted, which reduces the hardness deviation between the surface layer and martensite in the center after hot forming. It was confirmed that it was possible to secure excellent bending properties by reducing the size, and the present invention was completed.

Hereinafter, a cold rolled steel sheet for hot forming according to one aspect of the present invention will be described in detail.

The cold-rolled steel sheet for hot forming with excellent surface quality according to one aspect of the present invention has the following weight percent: C: 0.25-0.45%, Si: 0.01-3.0%, Mn: 0.01-4.0%, Al: 0.001-0.4%, P : 0.001~0.05%, S: 0.0001~0.02%, Cr: 0.1% to less than 5.0%, N: 0.001~0.02%, may contain remaining Fe and other unavoidable impurities.

First, the alloy composition of the cold rolled steel sheet for hot forming with excellent surface quality according to one aspect of the present invention will be described in detail.

C: 0.25~0.45%

C is an essential element to increase the strength of heat-treated members and must be added appropriately.

If the C content is less than 0.25%, it is difficult to secure sufficient strength, so it is preferable to add 0.25% or more. A more preferable lower limit is 0.26%, and an even more preferable lower limit is 0.27%. On the other hand, if the content exceeds 0.45%, the strength of the hot rolled material is too high when cold rolling the hot rolled material, which not only greatly reduces cold rolling properties, but also significantly reduces spot weldability, so it is preferable to be 0.45% or less. A more preferable upper limit is 0.42%, and an even more preferable upper limit is 0.40%.

Si: 0.01~3.0%

Si not only plays an important role in forming a Si-based amorphous oxide layer by concentrating on the surface when cold-rolled steel sheets are annealed in a continuous annealing line, but also suppresses the formation of (Fe, Mn, Cr) oxide layers during the hot forming process, thereby improving the spot weldability of the member. It plays a role in securing.

If the Si content is less than 0.01%, the above-described effect is insufficient, so the lower limit is preferably 0.01%. A more preferable lower limit is 0.1%. On the other hand, if the content is more than 3.0%, there is a problem in that a too thick Si-based amorphous oxide layer is formed, which reduces spot weldability. A more preferable upper limit is 2.8%, and an even more preferable upper limit is 2.5%.

Cr: 0.1% or more but less than 5.0%

Cr not only improves the hardenability of steel sheets, but can also play a role in stably forming a surface Si-based amorphous oxide layer through an appropriate reaction with Si.

If the Cr content is less than 0.1%, the above-mentioned effect is insufficient. A more preferable lower limit is 0.15%, and an even more preferable lower limit is 0.2%. On the other hand, when the Cr content is 5.0% or more, the effect is saturated and the manufacturing cost increases. A more preferable upper limit is 4.5%, and an even more preferable upper limit is 4.0%.

Mn: 0.01~4.0%

Mn needs to be added not only to secure the solid solution strengthening effect, but also to lower the critical cooling rate to secure martensite in hot formed members.

If the Mn content is less than 0.01%, the above-mentioned effect is insufficient. A more preferable lower limit is 0.05%, and an even more preferable lower limit is 0.1%. On the other hand, if the Mn content exceeds 4.0%, the strength of the steel sheet before the hot forming process increases too high, which not only makes blanking work difficult, but also increases the cost due to the addition of excessive iron alloy and deteriorates spot weldability. A more preferable upper limit is 3.0%, and an even more preferable upper limit is 2.5%.

Al: 0.001~0.4%

Al, along with Si, can increase the cleanliness of steel by acting as a deoxidizer in steelmaking.

If the Al content is less than 0.001%, the above-mentioned effect is insufficient. A more preferable lower limit is 0.002%, and an even more preferable lower limit is 0.003%. If the content is more than 0.4%, there is a problem that the Ac3 temperature rises excessively and the heating temperature must be increased. A more preferable upper limit is 0.3%, and an even more preferable upper limit is 0.2%.

P: 0.001~0.05%

P is an impurity, and controlling its content to less than 0.001% requires a lot of manufacturing costs, and if its content exceeds 0.05%, it can significantly reduce the weldability of hot-formed members. A more preferable upper limit is 0.03%.

S: 0.0001~0.02%

S is an impurity, and it costs a lot of manufacturing cost to control its content to less than 0.0001%, and if its content exceeds 0.02%, it impairs the ductility, impact properties, and weldability of the member. A more preferable upper limit is 0.01%.

N: 0.001~0.02%

N is an impurity, and it costs a lot of manufacturing cost to control its content to less than 0.001%. If the content is more than 0.02%, not only does it become susceptible to cracks when playing the slab, but its impact characteristics may deteriorate. A more preferable upper limit is 0.01%.

The remaining component of the present invention is iron (Fe). However, in the normal manufacturing process, unintended impurities from raw materials or the surrounding environment may inevitably be mixed, so this cannot be ruled out. Since these impurities are known to anyone skilled in the ordinary manufacturing process, all of them are not specifically mentioned in this specification.

In addition to the above component composition, the present invention may further include one or more selected from a) to f) below. By arbitrarily adding these elements, properties such as surface quality and hot formability can be further improved.

a) Sum of Ti, Nb, Zr and V contents: 0.001%~0.4%

Ti, Nb, Zr, and V are effective in improving the strength of heat-treated members by forming fine precipitates, stabilizing retained austenite, and improving impact toughness by refining grains. If the content (meaning the total of two or more types when added) is less than 0.001%, the above-mentioned effect may be insufficient, and the more preferable lower limit is 0.005%, and the more preferable lower limit is 0.008%. If the content exceeds 0.4%, the effect is not only saturated but also may result in an increase in cost due to excessive addition of ferroalloy. A more preferable upper limit is 0.38%, and an even more preferable upper limit is 0.35%.

b) B: 0.0001~0.01%

B is an element that can not only improve hardenability even with a small amount of addition, but can also suppress the embrittlement of hot-formed members due to grain boundary segregation of P and/or S by segregating at prior austenite grain boundaries.

If the B content is less than 0.0001%, the above-mentioned effect is insufficient. A more preferable lower limit is 0.00012%, and an even more preferable lower limit is 0.00015%. If it exceeds 0.01%, not only will the effect be saturated, but it may also cause hot embrittlement during hot rolling. A more preferable upper limit is 0.005%.

c) Sum of Mo and W contents: 0.001~1.0% by weight

Mo and W can be added to improve hardenability, improve strength through precipitation strengthening effect, and refine grains. If the content (meaning the total when both Mo and W are added) is less than 0.001%, the above-mentioned effect is insufficient, and the more preferable lower limit is 0.0015%, and the more preferable lower limit is 0.002%. If it exceeds 1.0%, not only is the effect saturated, but there is also a problem of increased costs. A more preferable upper limit is 0.95%, and an even more preferable upper limit is 0.9%.

d) Sum of Cu and Ni contents: 0.005~2.0% by weight

Cu can be added as an element to improve strength by forming fine precipitates. Additionally, Ni may cause hot embrittlement when added alone to Cu, so it is added as needed. However, if the sum of these components is less than 0.005%, the above-described effect may be insufficient, and a more preferable lower limit is 0.006%, and an even more preferable lower limit is 0.007%. Exceeding 2.0% may result in excessive cost increases. A more preferable upper limit is 1.95%, and an even more preferable upper limit is 1.9%.

e) Sum of Sb and Sn contents: 0.001~1.0% by weight

The Sb and Sn have the effect of suppressing the formation of oxides that may be generated at the surface grain boundaries of hot rolled steels to which Si is added, and can suppress dent defects caused by surface grain boundary falloff during annealing of cold rolled steels. To achieve this effect, it is desirable to add 0.001% or more. A more preferable lower limit is 0.002%, and an even more preferable lower limit is 0.03%.

On the other hand, if the content (meaning the total when both Sb and Sn are added) exceeds 1.0%, not only may the cost increase excessively, but it may also be dissolved in the grain boundaries of the slab and form a coil edge during hot rolling. Cracks may occur. A more preferable upper limit is 0.95%, and an even more preferable upper limit is 0.9%.

f) REM: 0.0001~0.02%

The REM element can control the formation thickness of surface Fe scale during hot forming by controlling the activity of Fe in the steel. To obtain this effect, addition of 0.0001% or more of REM element is required. A more preferable lower limit is 0.00015%, and an even more preferable lower limit is 0.0002%. On the other hand, if it exceeds 0.02%, control of Fe activity may be lost and surface quality may be deteriorated. Therefore, it is preferable to control it to 0.02% or less, and more preferably to 0.01% or less.

The cold rolled steel sheet for hot forming according to one aspect of the present invention not only satisfies the above-described alloy composition, but also may have a structure ratio value of 0.2 or more and 1.3 or less, as expressed by [Relational Equation 1] below.

[Relation 1] Organizational ratio =

(of the above formula

and

represents the area ratio of pearlite and cementite in the surface layer, respectively,

and

represents the area ratio of pearlite and cementite in the center, respectively.)

In the present invention, the surface layer part may refer to an area within 100 μm in the thickness direction from the surface, and the central part in the present invention is an area of 1/2t ± 50 μm in the thickness direction from the surface (where t is the steel thickness (mm)). can mean).

If the structure ratio expressed in [Relational Equation 1] exceeds 1.3, the hardness deviation between martensite formed in the surface layer after hot forming increases, so the stress due to bending may be concentrated in the stronger martensite, and the hardness deviation in the thickness direction may increase. Due to the difference, stress unevenness in the thickness direction may increase and bendability may deteriorate. More preferably, the tissue ratio may be 1.15 or less, and even more preferably 0.95 or less. On the other hand, if the tissue ratio is less than 0.2 as suggested by this patent, the strength after hot forming cannot be secured and the tensile strength may be less than 1800MPa. A more preferable lower limit is 0.25, and an even more preferable lower limit is 0.3.

Additionally, the microstructure of the cold rolled steel sheet according to the present invention may include ferrite and cementite. There is no need to specifically limit the area ratio, but for example, the total of ferrite and cementite may be 5% or more in area ratio.

When making a blank from a cold-rolled steel sheet to manufacture a hot-formed member, if the strength is excessive, mold wear can easily occur, so the above-described microstructure must be secured. If this is not taken into consideration, bainite, martensite, etc. may be included and are not excluded.

Below, the elements of the present invention will be described in detail.

The member of the present invention may satisfy a hardness ratio value of 0.1 or more and 10 or less, as expressed by [Relational Equation 2] below.

[Relationship 2] Hardness ratio =

(In equation 1 above,

represents the standard deviation of the surface hardness,

is the standard deviation of the hardness of the center.)

If the standard deviation of the hardness of the surface layer increases and the hardness ratio expressed in [Relational Equation 2] exceeds 10, the bendability may be deteriorated due to the stress concentration phenomenon and stress imbalance in the thickness direction due to the hardness deviation between martensite. In the case below, the hardness deviation is good and bendability can be improved. However, if the hardness ratio is less than 0.1, the hardness deviation in the center of the thickness direction becomes relatively large, which may result in poor bendability.

Since the composition of the base steel sheet of the member according to the present invention is the same as the composition of the cold rolled steel sheet described above, it will not be described separately.

Below, the microstructure of the member of the present invention will be described in detail.

The hot formed member according to one aspect of the present invention may include martensite or bainite as the main phase to ensure high strength. In the present invention, the main phase may refer to the phase with the largest area ratio among several phases that make up the microstructure. The area ratio is not particularly limited, but more preferably, the area ratio may be 50% or more.

Hereinafter, a method for manufacturing a cold rolled steel sheet for hot forming, which is another aspect of the present invention, will be described in detail.

Another aspect of the present invention, a method of manufacturing a cold rolled steel sheet for hot forming, includes heating a slab satisfying the above-described alloy composition to 1000-1300°C; Obtaining a hot rolled steel sheet by hot rolling the heated slab at a finish rolling temperature of Ar3 to 1000°C; Cooling the hot rolled hot rolled steel sheet at a cooling rate of 400°C/s or more and 750°C/s or less; Winding the hot-rolled steel sheet at a temperature range above Ms and below 750°C; Obtaining a cold-rolled steel sheet by cold-rolling the coiled hot-rolled steel sheet at a cumulative reduction rate of 30 to 80%; And it may include the step of continuously annealing the cold rolled steel sheet for 1 to 1000 seconds at a temperature range of 700 to 900°C.

Slab heating stage

A slab satisfying the above-described alloy composition is heated to 1000-1300°C.

If the heating temperature is less than 1000°C, it is difficult to homogenize the slab structure, and if it exceeds 1300°C, excessive oxide formation and increased manufacturing costs may occur.

hot rolling stage

The heated slab is hot rolled at a finish rolling temperature of Ar3 to 1000°C to obtain a hot rolled steel sheet.

If the finish rolling temperature is lower than the Ar3 temperature, abnormal rolling is likely to occur, a mixed structure is generated in the surface layer, and there is difficulty in controlling the shape of the hot rolled steel sheet. When the finish rolling temperature exceeds 1000°C, the crystal grains of the hot rolled steel sheet tend to become coarse.

Hot rolling cooling stage

The hot-rolled hot-rolled steel sheet is cooled at a cooling rate of 400°C/s or more and 750°C/s or less.

If cooling exceeds the upper limit of the cooling rate during hot rolling, an excessive area ratio of pearlite and cementite is formed in the surface layer after annealing, and the hardness deviation between surface layer martensite increases after hot forming, thereby failing to secure excellent bendability. . If the steel sheet is cooled below the lower cooling rate limit, the formation of surface pearlite and cementite is minimal, making it impossible to secure sufficient strength after hot forming of the cold steel sheet.

winding step

The hot-rolled steel sheet is wound in a temperature range above Ms and below 750°C.

When the coiling temperature is below Ms (martensite transformation onset temperature), the strength of the hot rolled steel sheet becomes too high, which reduces cold rolling properties. If the coiling temperature exceeds 750°C, the thickness of the oxide layer increases and oxidation of surface grain boundaries occurs, which not only deteriorates pickling properties, but also causes the surface grain boundaries to fall off during annealing in a continuous annealing furnace.

cold rolling stage

The coiled hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. This is to control the thickness of the steel sheet more precisely, and pickling can be performed before cold rolling.

At this time, the reduction rate of the cold rolling does not need to be specifically limited, but may be performed at a reduction rate of 30 to 80% in order to secure a predetermined target thickness.

Continuous annealing stage

The continuous annealing of the cold rolled steel sheet is performed in a temperature range of 700 to 900°C.

If the annealing temperature is less than 700℃, it is difficult for the rolling structure created by cold rolling to recover and recrystallize, and if it exceeds 900℃, the annealing equipment may deteriorate, which increases process costs due to frequent replacement of equipment. It can be.

Additionally, the annealing time may be 1 to 1000 seconds. If the annealing time is less than 1 second, it is difficult to obtain an annealing effect, and if the annealing time is more than 1000 seconds, productivity may decrease.

Hereinafter, a method for manufacturing a hot formed member, which is another aspect of the present invention, will be described in detail.

Another aspect of the present invention, a method for manufacturing a hot-formed member, includes heating the cold-rolled steel sheet manufactured by the method for manufacturing a cold-rolled steel sheet according to the present invention described above at a temperature increase rate of 1 to 1000°C/sec to a temperature of 700°C or higher. ; Hot forming the heated cold rolled steel sheet; And cooling the hot formed steel sheet at a cooling rate of 10 to 1000°C/sec.

heating step

The cold-rolled steel sheet manufactured by the cold-rolled steel sheet manufacturing method according to the present invention described above is heated at a temperature increase rate of 1 to 1000°C/sec to a temperature of 700°C or higher.

If the heating temperature is less than 700°C, recrystallization of ferrite may not be sufficient, and there may be a problem of increased anisotropy in bending after hot forming.

If the temperature increase rate is less than 1°C/sec, it is difficult to secure sufficient productivity, and if the temperature increase rate is more than 1000°C/sec, excessively expensive equipment is required.

Hot forming and cooling steps

After hot forming the heated cold rolled steel sheet, it is cooled at a cooling rate of 10 to 1000°C/sec.

If the cooling rate is less than 10°C/sec, it is difficult to secure tensile strength because unwanted ferrite and pearlite are formed. On the other hand, controlling the cooling rate to exceed 1000°C/sec requires expensive special cooling equipment.

At this time, the cooling stop temperature of the cooling step may be below M _f (martensite transformation end temperature). This is because if cooling is stopped above M _f and then cooled to room temperature again, it may be difficult to secure the shape freezing of the hot formed member.

However, in order to secure better elongation and impact properties in hot forming members, cooling is stopped between M _f (martensite transformation end temperature) and M _s (martensite transformation start temperature) and then maintained at the cooling end temperature or Ac1. It is possible to temper the martensite and stabilize the retained austenite by heating again below the temperature.

Additionally, the hot formed member may have martensite or bainite as the main phase to ensure high strength. Here, the main phase refers to the phase with the largest area ratio among the various phases that make up the microstructure. There is no need to specifically limit the area ratio, but for example, the area ratio may be 50% or more.

Meanwhile, the hot formed member may have a tensile strength of 1800 MPa or more. By securing a high strength of 1800 MPa or more, it can be preferably applied to automobile structural members or reinforcement materials that require crash resistance.

Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are only for illustrating and explaining the present invention in more detail, and are not intended to limit the scope of the present invention. This is because the scope of rights of the present invention is determined by matters stated in the patent claims and matters reasonably inferred therefrom.

(Example)

A 40mm-thick slab with the composition shown in Table 1 below was vacuum melted, heated in a furnace at 1200°C for 1 hour, and then hot-rolled at a final rolling temperature of 930°C to produce a hot-rolled steel sheet with a final thickness of 3mm. The hot rolled steel sheet was cooled at the cooling rate shown in Table 2 below and then coiled at 640°C. Next, the hot rolled steel sheet was pickled and then cold rolled at a cold rolling reduction rate of 50%. In addition, a cold rolled steel sheet for hot forming was manufactured by performing continuous annealing at 800°C for 80 seconds after the cold rolling.

Thereafter, the manufactured cold-rolled steel sheet was heated at a temperature increase rate of 20°C/sec, then heat-treated at 900°C for 6 minutes, and the heated cold-rolled steel sheet was hot formed. Then, the hot-formed steel sheet was cooled to room temperature at a cooling rate of 20°C/sec to manufacture a hot-formed member.

Steel grade	C	Si	Mn	Cr	Mo	Ti	B	Sb
A	0.423	0.22	1.3	0.15	0.1	0.03	0.0025	-
B	0.29	1.54	0.8	0.7	-	0.015	0.0025	0.03
C	0.254	1.49	0.791	2.01	-	0.03	0.002	-
D	0.255	1.55	0.83	1.53	-	0.031	0.0028	-
E	0.31	0.6	0.9	0.2	0.015	0.03	0.0025	-

The area ratio of the surface layer and the central structure of the manufactured cold rolled steel sheet for hot forming and the tissue ratio of [Relational Equation 1] are shown in Table 2. In order to measure the area ratio of the tissue at each location in the thickness direction, a cross-section of the tissue after nital etching was observed at 500x magnification using an optical microscope (OM). After optical photo measurement, the area ratio of the superficial and central tissues was measured three times each using CLEMEX Vision PE software, and the average values are shown in Table 2.

In addition, the hardness ratio, which is the ratio of the standard deviation of the hardness of the surface layer and the center of the hot formed member manufactured after hot forming, is shown in Table 2 based on [Relational Equation 2]. Tensile strength and maximum bending angle were also shown. Hardness was measured at intervals of 1 mm at least 10 points by applying a load of 10 kgf using a Vikckers Hardness tester (Dura Scan 80G5), and tensile strength was measured at room temperature according to the ISO6892 standard using a JIS-5 specimen. It was measured through. The maximum bending angle was described as the value of the bending outer angle converted from the maximum bending strength specified in the standard according to the bendability evaluation method according to the VDA238-100 standard. In addition, the bending angle change rate represents the bending angle deviation ratio between the bending angle of a specimen manufactured under the manufacturing conditions proposed in the present invention and the specimen manufactured outside the proposed manufacturing conditions.

Steel grade	note	hot rolling Cooling speed	Pearlite + Cementite			YS	TS	Hardness ratio	maximum bending angle	bending angle rate of change
		℃/s	center	surface layer	organization ratio	MPa	MPa		o	%
A	Invention Example 1	607.6	0.322	0.318	0.99	1382	2010	5.3	59.7	-
A	Comparative Example 1	880.7	0.251	0.415	1.65	1373	2010	16.4	50.6	-15
A	Comparative example 2	312.2	0.321	0.05	0.16	1180	1670	6.6	65.3	+9
B	Invention Example 2	506.1	0.344	0.331	0.96	1377	2009	6.4	56.7	-
B	Comparative example 3	861.0	0.296	0.434	1.47	1377	2011	10.7	50.8	-10
C	Invention Example 3	633.4	0.351	0.415	1.18	1338	1990	0.6	59.5	-
C	Comparative Example 4	805.2	0.318	0.467	1.47	1345	1994	27.2	54.1	-9
D	Invention Example 4	685.2	0.337	0.325	0.96	1403	2070	4.4	50.4	-
D	Comparative Example 5	767.2	0.318	0.418	1.32	1396	2079	24.4	43.9	-13
E	Invention Example 5	421.8	0.363	0.365	1.01	1294	1900	9.5	61.6	-
E	Comparative Example 6	990.7	0.306	0.425	1.39	1366	1972	12.8	53.3	-13
E	Comparative example 7	385.7	0.460	0.08	0.17	1080	1720	2.1	66.2	+7

As shown in Table 2, in the case of Comparative Examples 1 and 3 to 6 in which hot rolling was performed exceeding the upper limit of the cooling rate limited in the present invention, the structure ratio expressed by [Relational Equation 1] after annealing exceeded 1.3, and hot rolling After molding, the standard deviation of the surface hardness increased compared to the standard deviation of the central hardness, and as a result, the hardness ratio expressed in [Relational Equation 2] exceeded 10, resulting in an inferior bending angle.

In Comparative Examples 2 and 7, in which the cooling rate during hot rolling did not meet the range suggested by this patent and did not reach the lower limit, the maximum bending angle was improved after hot forming of the cold rolled steel sheet, but soft martensite was excessive in the surface layer. It was formed and did not secure sufficient strength.

In the case of Invention Examples 1 to 5, the cooling rate after hot rolling was controlled within the range limited by the present invention, so that the structure ratio of the cold rolled steel sheet satisfied the range of 0.2 to 1.3 and at the same time, the hardness ratio was 10 or less. The hot formed member showed good bendability.

Claims (12)

1. In weight percent, C: 0.25-0.45%, Si: 0.01-3.0%, Mn: 0.01-4.0%, Al: 0.001-0.4%, P: 0.001-0.05%, S: 0.0001-0.02%, Cr: 0.1% Not less than 5.0%, N: 0.001~0.02%, including the remaining Fe and other inevitable impurities;

   A cold-rolled steel sheet for hot forming with a tissue ratio value of 0.2 or more and 1.3 or less, as expressed by [Relational Equation 1] below.

   [Relation 1] Organizational ratio =

   

   (of the above formula

   

   and

   

   represents the area ratio of pearlite and cementite in the surface layer, respectively,

   

   and

   

   represents the area ratio of pearlite and cementite in the center, respectively.)
2. According to paragraph 1,

   A cold rolled steel sheet for hot forming further comprising one or more selected from a) and f) below.

   a) Sum of Ti, Nb, Zr and V contents: 0.001% to 0.4% by weight,

   b) B: 0.0001~0.01% by weight,

   c) Sum of Mo and W contents: 0.001 to 1.0% by weight;

   d) Sum of Cu and Ni contents: 0.005~2.0% by weight,

   e) Sum of Sb and Sn contents: 0.001~1.0% by weight

   f) REM: 0.0001~0.02% by weight
3. In weight percent, C: 0.25-0.45%, Si: 0.01-3.0%, Mn: 0.01-4.0%, Al: 0.001-0.4%, P: 0.001-0.05%, S: 0.0001-0.02%, Cr: 0.1% Heating the slab containing more than 5.0%, N: 0.001-0.02%, the remainder Fe and other inevitable impurities to 1000-1300°C;

   Obtaining a hot rolled steel sheet by hot rolling the heated slab at a finish rolling temperature of Ar3 to 1000°C;

   Cooling the hot rolled hot rolled steel sheet at a cooling rate of 400°C/s or more and 750°C/s or less;

   Winding the hot-rolled steel sheet at a temperature ranging from Ms to 750°C;

   Obtaining a cold rolled steel sheet by cold rolling the coiled hot rolled steel sheet; and

   A method of manufacturing a cold-rolled steel sheet for hot forming, comprising the step of continuously annealing the cold-rolled steel sheet.
4. According to clause 3,

   The steel slab is a method of manufacturing a cold rolled steel sheet for hot forming, further comprising one or more selected from a) and f) below.

   a) Sum of Ti, Nb, Zr and V contents: 0.001% to 0.4% by weight,

   b) B: 0.0001~0.01% by weight,

   c) Sum of Mo and W contents: 0.001 to 1.0% by weight;

   d) Sum of Cu and Ni contents: 0.005~2.0% by weight,

   e) Sum of Sb and Sn contents: 0.001~1.0% by weight

   f) REM: 0.0001~0.02% by weight
5. According to clause 3 or 4,

   A method of manufacturing a cold rolled steel sheet for hot forming, wherein the cold rolling is performed at a cumulative reduction rate of 30 to 80%.
6. According to clause 3 or 4,

   A method of manufacturing a cold-rolled steel sheet for hot forming, wherein the continuous annealing is performed for 1 to 1000 seconds in a temperature range of 700 to 900 ° C.
7. Manufacturing a cold rolled steel sheet according to claim 3 or 4;

   Heating the cold rolled steel sheet at a temperature increase rate of 1 to 1000°C/sec to a temperature of 700°C or higher;

   Hot forming the heated cold rolled steel sheet; and

   A method of manufacturing a hot-formed member comprising cooling the hot-formed steel sheet at a cooling rate of 10 to 1000° C./sec.
8. In clause 7,

   A method of manufacturing a hot formed member wherein the cooling is performed by setting the cooling stop temperature to Mf (martensite transformation end temperature) or lower.
9. In clause 7,

   The cooling is performed by setting the cooling stop temperature to Mf (martensite transformation end temperature) or more and Ms (martensite transformation start temperature) or less.

   After the cooling, the method of manufacturing a hot formed member further includes the step of maintaining the temperature at the cooling end temperature or heating again to Ac1 or lower.
10. In weight percent, C: 0.25-0.45%, Si: 0.01-3.0%, Mn: 0.01-4.0%, Al: 0.001-0.4%, P: 0.001-0.05%, S: 0.0001-0.02%, Cr: 0.1% Not less than 5.0%, N: 0.001~0.02%, including the remaining Fe and other inevitable impurities;

    A hot formed member with a hardness ratio value of 0.1 or more and 10 or less, expressed by [Relational Equation 2] below.

    [Relationship 2] Hardness ratio =

    

    (In equation 1 above,

    

    represents the standard deviation of the surface hardness,

    

    is the standard deviation of the hardness of the center.)
11. According to clause 10,

    A hot forming member further comprising one or more selected from a) and f) below.

    a) Sum of Ti, Nb, Zr and V contents: 0.001% to 0.4% by weight,

    b) B: 0.0001~0.01% by weight,

    c) Sum of Mo and W contents: 0.001 to 1.0% by weight;

    d) Sum of Cu and Ni contents: 0.005~2.0% by weight,

    e) Sum of Sb and Sn contents: 0.001~1.0% by weight

    f) REM: 0.0001~0.02% by weight
12. The method of claim 10 or 11,

    A hot formed member with a tensile strength of 1800 MPa or more and a yield strength of 1200 MPa or more.