JP4735211B2 - Automotive member and manufacturing method thereof - Google Patents

Description

  The present invention relates to a member for an automobile having an excellent strength-ductility balance and having little softening of a heat affected zone, and a method for producing the same.

  In the present invention, “excellent balance between strength and ductility” means that TS × El, which is the product of tensile strength TS and total elongation El, is 12000 MPa ·% or more (tensile specimen size: JIS No. 5, plate thickness: 1.0 “When there is little softening of the weld heat-affected zone”, it means that the difference ΔHv between the average Vickers hardness of the base metal and the minimum Vickers hardness of the heat-affected zone is 50 or less.

In recent years, from the viewpoint of protecting the global environment, there has been a demand for improved fuel efficiency of automobiles, and from the viewpoint of protecting passengers in the event of a vehicle collision, there has also been a demand for improved safety of automobile bodies. For this reason, studies are being actively conducted to reduce the weight and strengthen the automobile body.
It is said that it is effective to increase the strength of component materials in order to achieve weight reduction and strengthening of the automobile body at the same time. Recently, a high-tensile steel sheet with a tensile strength (TS) of 980 MPa or more. However, it is actively used in automotive structural parts such as door impact beams, center pillars, and bumpers. That is, a high-tensile steel sheet is applied to reduce the thickness of the steel sheet used, thereby simultaneously reducing the weight and strengthening of the automobile body.

A member for an automobile that is formed by processing and forming a high-strength thin steel sheet is required to absorb impact energy at the time of collision by being deformed without breaking the member at the time of automobile collision. In this respect, in a member having a low strength-ductility balance or a member in which the heat-affected zone at the time of welding is significantly softened, the amount of energy absorbed by impact is remarkably reduced due to fracture at the time of collision.
For this reason, in order to prevent this, suppression of the softening of the heat affected zone at the time of welding is required with a high strength-ductility balance.

However, with regard to the balance between strength and ductility, many structural parts for automobile bodies made of thin steel sheets are formed by press working. Therefore, in high-tensile steel sheets with a tensile strength of 980 MPa or more, the ductility of the base material Inevitably lowers the ductility after pressing. That is, when the strength of the steel sheet is increased, there is a problem that the elongation is lowered, the press formability is deteriorated, and the strength-ductility balance of the member after press forming is lowered.
In addition, in a high-tensile steel sheet having a tensile strength of 980 MPa or more, it is difficult to avoid springback, and deterioration in dimensional accuracy after forming has also been a problem.

As a typical method for solving these problems, a steel sheet having a base metal strength of about 440 to 590 MPa is press-formed, heated to a temperature of the Ac ₃ transformation point or higher, and then rapidly cooled to obtain 980 MPa or more. The method of obtaining the steel plate strength of this.
For example, Patent Document 1 includes, in mass%, C: 0.05 to 0.20%, Mn: 0.3 to 2.5%, P: 0.02% or less, S: 0.02% or less, Al: 0.06% or less, Ti: 0.015% or less, A steel sheet for induction hardening including N: 0.010% or less and B: 0.0005 to 0.0040% and excellent in toughness of a hardened portion made of the remaining Fe and inevitable impurities has been proposed. In this steel sheet, the amount of Ti, N, B and the prior austenite grain size are controlled, so that the quenching effect by B can be exhibited even in short time heating such as high frequency heating, and high impact absorption energy without cracking at high speed deformation. However, since the microstructure after quenching is mainly composed of martensite and bainite obtained by controlling the cooling rate, there arises a problem that it is difficult to obtain a sufficient strength-ductility balance. There is a problem that the part softens, resulting in a decrease in shock absorption energy and a decrease in fatigue characteristics.

Further, in Patent Document 2, in mass%, C: 0.10 to 0.37%, Si: 1.0% or less, Mn: 2.5% or less, P: 0.10% or less, S: 0.03% or less, sol.Al: 0.01 to 0.10 %, N: 0.0005 to 0.0050%, Ti: 0.005 to 0.05% or less, B: 0.0003 to 0.0050%, B- (10.8 / 14) N ^* ≧ 0.0005% is satisfied, and TiN is a precipitate in steel A thin steel sheet having excellent impact characteristics after quenching, characterized in that the average grain size of 0.06 to 0.30 μm and the prior austenite grain size after quenching is 2 to 25 μm, has been proposed. In this steel sheet, excellent Charpy impact absorption energy is obtained by controlling the Ti, N, B amount, TiN grain size and prior austenite grain size, but in this case as well, in order to increase the strength after quenching When the microstructure is mainly composed of martensite, not only an excellent strength-ductility balance cannot be obtained, but also the heat-affected zone is softened during welding.

On the other hand, as a method for effectively avoiding the above-mentioned problems, that is, the decrease in strength-ductility balance after press molding and the occurrence of springback, a method using hot forming (hot press) instead of cold forming is attracting attention. Have been bathed.
In this method, the steel sheet is formed in a state heated to a temperature higher than the Ac ₃ transformation point, so there is no problem with formability, and a low temperature transformation phase mainly composed of martensite is obtained by controlling the cooling rate after forming. Thus, high strength exceeding 980 MPa can be obtained.

  For example, Patent Document 3 includes, in mass%, C: 0.18 to 0.25%, Si: 0.15 to 0.35%, Mn: 1.15 to 1.40%, Cr: 0.15 to 0.25%, Ti: 0.01 to 0.03%, and the balance Has been proposed a manufacturing technique for a vehicle collision reinforcement material, which comprises hot forming a thin steel plate made of Fe and inevitable impurities. With this technology, mainly by controlling the hot forming conditions, a tensile strength of about 1500 MPa has been obtained, and it has succeeded in avoiding springback, but for automobiles that require particularly excellent shock absorption characteristics. When applied to a member, there has been a problem that impact absorption energy is reduced due to breakage caused by softening of the heat affected zone.

Non-Patent Document 1 discloses a technique for hot forming a thin steel sheet containing C: 0.22%, Mn: 1.2%, Cr: 0.15%, and B: 0.002% by mass. In this technology, the strength of the base metal: 590 MPa class steel sheet is heated to a temperature above the Ac ₃ transformation point, and then rapidly cooled at the same time as press forming, so that the tensile strength is 1530 MPa and the strength-ductility balance is 12000 MPa ·%. (Tensile test specimen size: JIS No. 5, plate thickness: 1.4 mm), but in this case, the maximum Vickers hardness difference ΔHv between the base metal and the heat-affected zone is as large as 150. It was.

JP 2000-248338 A JP 2002-309345 A Japanese Patent Laid-Open No. 2002-102980 “Suehiro et al .; Nippon Steel Technical Report No. 378, P.15-20 (2003)”

Conventionally, in structural members for automobiles manufactured by the quenching method, in order to make the tensile strength: 980 MPa or more, the main phase of the microstructure needs to be martensite. It has been difficult to simultaneously obtain a good strength-ductility balance and softening resistance of the weld heat affected zone (hereinafter referred to as weld heat affected zone softening property).
Also, for automotive structural members molded by hot pressing, the main phase of the microstructure must be martensite in order to achieve a tensile strength of 980 MPa or more, and as described above, it is necessary as an automotive structural member. It has been difficult to obtain a good strength-ductility balance and weld heat-affected zone softening characteristics at the same time.

This is because when the main phase is martensite, high ductility is difficult to obtain, and even if it is attempted to improve ductility by tempering, the strength decreases due to coarsening of Fe ₃ C, but the ductility does not improve so much. is there. Further, in the microstructure mainly composed of martensite, when laser welding or the like is performed, there is a disadvantage that significant softening occurs in the heat-affected zone, leading to a significant decrease in strength of the weld zone and deterioration of fatigue characteristics.

  The present invention advantageously solves the above-mentioned problem, regardless of whether it is performed at room temperature or hot forming, the tensile strength after tempering is 980 MPa or more, the strength-ductility balance is excellent, and the weld heat affected zone It aims at proposing the member for motor vehicles with the softening of this with the advantageous manufacturing method.

Now, the inventors have intensively studied to solve the above-mentioned problems, and as a result, by specifically precipitating carbides containing V in the steel, specifically, the particles containing V having a particle size of 20 nm or less are included. Knowledge that the intended purpose can be advantageously achieved by precipitating 1000 or more carbides per unit volume of 1μm ³ and controlling the average particle size of carbide containing V with a particle size of 20nm or less to 10nm or less. Got.
The present invention is based on the above findings.

That is, the gist configuration of the present invention is as follows.
(1) By mass% C: 0.10 to 0.25%,
Si: 1.5% or less,
Mn: 1.0-3.0%
P: 0.10% or less,
S: 0.005% or less,
Al: 0.01 to 0.5%,
N: 0.010% or less and V: 0.10 to 1.0%
Include, and satisfied (10Mn + V) / C ≧ 50, the balance being Fe and unavoidable impurities, the volume ratio of the tempered martensite phase is 80% or more, a particle size: 20 nm carbide containing the following V An automobile member characterized in that 1000 / μm ³ or more is precipitated and the average particle size of the carbide containing V of 20 nm or less is 10 nm or less.

(2) In the above (1), the member is further in mass%.
Nb: 0.1% or less,
Ti: 0.1% or less and B: 0.0050% selected from among the following one or automotive parts, characterized in that a composition containing two or more.

(3) In the above (1) or (2), the member is further in mass%.
Cr: 0.005 to 1.0% and
Mo: 0.005 to 0.5%
An automotive member comprising a composition containing one or two selected from the above in a range satisfying (2Cr + Mo) /2V≦2.0.

(4) In any one of the above (1) to (3), the member is further in% by mass.
Cu: 0.5-5.0%
An automotive member comprising a composition containing

(5) In the above (4), the member is further in mass%.
Ni: 0.1-2.0%
An automotive member comprising a composition containing

(6) By mass% C: 0.10 to 0.25%,
Si: 1.5% or less,
Mn: 1.0-3.0%
P: 0.10% or less,
S: 0.005% or less,
Al: 0.01 to 0.5%,
N: 0.010% or less and V: 0.10 to 1.0%
Includes and satisfies (10Mn + V) / C ≧ 50, the balance being Fe and unavoidable impurities, the volume ratio of the ferrite phase is 60% or more, the particle size was determined for the following deposit 80 nm V After forming a steel plate having an average particle size of carbide containing 30 nm or less into a predetermined shape at room temperature, it is heated to a temperature not lower than the Ac ₃ transformation point and not higher than (Ac ₃ transformation point +200) ° C., and then 300 ° C. Cooling at an average cooling rate of 20 ° C./s or more to the following, and further tempering under conditions satisfying the following expression (1):
Record
11000−3000 [% V] ≦ Tb (20 + logt) ≦ 15000−1000 [% V] (1)
However, Tb ≦ Ac ₁ transformation point where [% V] is the V content (mass%)
Tb is the tempering temperature (℃)
t is the holding time at the tempering temperature (s)

(7) By mass% C: 0.10 to 0.25%,
Si: 1.5% or less,
Mn: 1.0-3.0%
P: 0.10% or less,
S: 0.005% or less,
Al: 0.01 to 0.5%,
N: 0.010% or less and V: 0.10 to 1.0%
Hints, and satisfied (10Mn + V) / C ≧ 50, the balance being Fe and unavoidable impurities, grain size with an average particle diameter of carbide containing V calculated for the following precipitates 80nm is 30nm or less A steel sheet is heated to a temperature not lower than the Ac ₃ transformation point and not higher than (Ac ₃ transformation point +200) ° C., formed into a predetermined shape, and then cooled to an average cooling rate of 300 ° C. or lower at a rate of 20 ° C./s or higher. A method for producing a member for an automobile, which is cooled and further subjected to tempering under conditions satisfying the following expression (1).
Record
11000−3000 [% V] ≦ Tb (20 + logt) ≦ 15000−1000 [% V] (1)
However, Tb ≦ Ac ₁ transformation point where [% V] is the V content (mass%)
Tb is the tempering temperature (℃)
t is the holding time at the tempering temperature (s)

(8) By mass% C: 0.10 to 0.25%,
Si: 1.5% or less,
Mn: 1.0-3.0%
P: 0.10% or less,
S: 0.005% or less,
Al: 0.01 to 0.5%,
N: 0.010% or less and V: 0.10 to 1.0%
It includes and satisfies (10Mn + V) / C ≧ 50, the balance being a steel slab having the composition of Fe and unavoidable impurities, was heated to above 1000 ° C., and a sheet bar by rough rolling and then finish rolling delivery temperature : After finish rolling under conditions of 800 ° C or higher, cool to 450 ° C or higher and to temperature Ta (° C) or lower expressed by the following formula (2), wind up, and then obtain the hot-rolled sheet obtained After forming into a specified shape at room temperature, heat it to a temperature of Ac ₃ transformation point or higher and (Ac ₃ transformation point +200) ° C. or lower, and then cool down to 300 ° C. or lower at an average cooling rate of 20 ° C./s or higher. And a method for producing a member for an automobile, further comprising tempering under conditions satisfying the following expression (1):
Record
11000−3000 [% V] ≦ Tb (20 + logt) ≦ 15000−1000 [% V] (1)
Ta (° C.) = [5500 / {6.7 + log ([% V] × [% C])}] − 400 (2)
However, Tb ≦ Ac ₁ transformation point Here, [% C] and [% V] are the contents of each element (mass%), respectively.
Tb is the tempering temperature (℃)
t is the holding time at the tempering temperature (s)

(9) By mass% C: 0.10 to 0.25%,
Si: 1.5% or less,
Mn: 1.0-3.0%
P: 0.10% or less,
S: 0.005% or less,
Al: 0.01 to 0.5%,
N: 0.010% or less and V: 0.10 to 1.0%
It includes and satisfies (10Mn + V) / C ≧ 50, the balance being a steel slab having the composition of Fe and unavoidable impurities, was heated to above 1000 ° C., and a sheet bar by rough rolling and then finish rolling delivery temperature : After finish rolling under conditions of 800 ° C or higher, the steel sheet is cooled to a temperature Ta (° C) or lower expressed by the following formula (2), wound up, and then rewound. After cold-rolled sheet by hot rolling, the cold-rolled sheet is heated to a temperature range not lower than Ac ₁ transformation point and not higher than (Ac ₃ transformation point +200) ° C., and maintained at this temperature range for 10 to 300 seconds, at least 500 ° C. The average cooling rate is 10 to 50 ° C./s until the cold-rolled annealed sheet is formed into a predetermined shape at room temperature, and then the Ac ₃ transformation point or higher (Ac ₃ transformation point +200). ) Heated to a temperature of ℃ or less, then cooled to 300 ℃ or less, average cooling rate: 20 ℃ / s or more, further down (1) A method of manufacturing automobile members and characterized by applying tempering treatment under conditions satisfying the equation.
Record
11000−3000 [% V] ≦ Tb (20 + logt) ≦ 15000−1000 [% V] (1)
Ta (° C.) = [5500 / {6.7 + log ([% V] × [% C])}] − 400 (2)
However, Tb ≦ Ac ₁ transformation point Here, [% C] and [% V] are the contents of each element (mass%), respectively.
Tb is the tempering temperature (℃)
t is the holding time at the tempering temperature (s)

(10) By mass% C: 0.10 to 0.25%,
Si: 1.5% or less,
Mn: 1.0-3.0%
P: 0.10% or less,
S: 0.005% or less,
Al: 0.01 to 0.5%,
N: 0.010% or less and V: 0.10 to 1.0%
It includes and satisfies (10Mn + V) / C ≧ 50, the balance being a steel slab having the composition of Fe and unavoidable impurities, was heated to above 1000 ° C., and a sheet bar by rough rolling and then finish rolling delivery temperature : After finish rolling under conditions of 800 ° C or higher, the steel sheet is cooled to a temperature Ta (° C) or lower expressed by the following formula (2), wound, and then rewound, and then the hot rolled sheet is moved to the Ac ₃ transformation point or higher. , and heated to (Ac ₃ transformation point +200) ° C. below the temperature, after formed into a predetermined shape, the average cooling rate to 300 ° C. or less: 20 ° C. / s cooled at speeds above, further the following equation (1) The manufacturing method of the member for motor vehicles characterized by performing a tempering process on the conditions which satisfy | fill.
Record
11000−3000 [% V] ≦ Tb (20 + logt) ≦ 15000−1000 [% V] (1)
Ta (° C.) = [5500 / {6.7 + log ([% V] × [% C])}] − 400 (2)
However, Tb ≦ Ac ₁ transformation point Here, [% C] and [% V] are the contents of each element (mass%), respectively.
Tb is the tempering temperature (℃)
t is the holding time at the tempering temperature (s)

(11) By mass% C: 0.10 to 0.25%,
Si: 1.5% or less,
Mn: 1.0-3.0%
P: 0.10% or less,
S: 0.005% or less,
Al: 0.01 to 0.5%,
N: 0.010% or less and V: 0.10 to 1.0%
It includes and satisfies (10Mn + V) / C ≧ 50, the balance being a steel slab having the composition of Fe and unavoidable impurities, was heated to above 1000 ° C., and a sheet bar by rough rolling and then finish rolling delivery temperature : After finish rolling under conditions of 800 ° C or higher, the steel sheet is cooled to a temperature Ta (° C) or lower expressed by the following formula (2), wound up, and then rewound. After cold-rolled sheet by hot rolling, the cold-rolled sheet is heated to a temperature range not lower than Ac ₁ transformation point and not higher than (Ac ₃ transformation point +200) ° C. Is cooled at an average cooling rate of 10 ° C / s or higher, and the obtained cold-rolled annealed sheet is then heated to a temperature not lower than Ac ₃ transformation point and not higher than (Ac ₃ transformation point +200) ° C. After cooling to 300 ° C or less, the product is cooled at an average cooling rate of 20 ° C / s or more to 300 ° C or less, and the following equation (1) is satisfied. Method for producing automotive parts, characterized in that performing the tempering treatment under the conditions of.
Record
11000−3000 [% V] ≦ Tb (20 + logt) ≦ 15000−1000 [% V] (1)
Ta (° C.) = [5500 / {6.7 + log ([% V] × [% C])}] − 400 (2)
However, Tb ≦ Ac ₁ transformation point Here, [% C] and [% V] are the contents of each element (mass%), respectively.
Tb is the tempering temperature (℃)
t is the holding time at the tempering temperature (s)

(12) In any of (6) to (11) above, the steel slab is
Nb: 0.1% or less,
Ti: 0.1% or less and B: 0.0050% 1 kind selected from among the following or manufacturing process of automotive parts, characterized in that a composition containing two or more.

(13) In any one of the above (6) to (12), the steel slab is further in mass%.
Cr: 0.005 to 1.0% and
Mo: 0.005 to 0.5%
A method for producing a member for automobile, comprising a composition containing one or two selected from the above in a range satisfying (2Cr + Mo) /2V≦2.0.

(14) In any one of the above (6) to (13), the steel slab is further in mass%.
Cu: 0.5-5.0%
The manufacturing method of the member for motor vehicles characterized by comprising the composition containing this.

(15) In (14) above, the steel slab is
Ni: 0.1-2.0%
The manufacturing method of the member for motor vehicles characterized by comprising the composition containing this.

  According to the present invention, by utilizing V, which has not been actively used so far for improving the performance of thin steel sheets for automobile structural members, it has excellent strength-ductility balance after hot forming, and An automotive member having a tensile strength of 980 MPa or more can be obtained with less softening of the heat affected zone.

Hereinafter, the experimental results that led to the present invention will be described.
The automotive member according to the present invention can be produced by molding at room temperature and then performing quenching and tempering treatment, or by performing tempering treatment after hot molding.
Therefore, first, the development process of a thin steel sheet that is assumed to be subjected to quenching and tempering treatment or hot forming and tempering treatment after room temperature forming will be described. Unless otherwise specified, “%” in relation to ingredients means mass%.
Si: 0.01%, P: 0.009%, S: 0.002%, Al: 0.03% and N: 0.0025% are the basic compositions, and C, Mn, and V are respectively C: 0.11 to 0.25%, Mn: 1.00 to 1.55% V: 0.15 to 0.82% In various ranges, the balance is changed to Fe and unavoidable impurities. The sheet bar is heated and soaked at 1250 ° C, and the finish rolling finish temperature is 900. Rolling of 3 passes was performed to obtain a hot rolled sheet having a thickness of 4.0 mm. After finishing rolling, a heat retention treatment of 550 ° C. and 1 h corresponding to the coil winding treatment was performed. Next, after cold rolling with a rolling reduction of 75% to obtain a cold-rolled sheet with a thickness of 1.0 mm, after holding at 750 ° C. between the Ac ₁ transformation point and the Ac ₃ transformation point for 60 s, up to 500 ° C. It cooled to room temperature so that an average cooling rate might be 20 degrees C / s, and it pickled. The holding time above the Ac ₁ transformation point was 90 s, and the average cooling rate from the Ac ₁ transformation point to 500 ° C. was 20 ° C./s.
Next, the thin steel sheet obtained as described above was held at 950 ° C above the Ac ₃ transformation point for 60 s and then quenched in ice water (corresponding to quenching conditions after forming or hot forming conditions) at 600 ° C. Tempering for 10 min was performed.
Here, the Ac ₁ transformation point was 670 to 700 ° C., and the Ac ₃ transformation point was 770 to 840 ° C.

Thus, the tensile properties (yield strength YS, tensile strength TS, elongation El) of the steel sheet obtained by giving a thermal history assuming a quenched and tempered material or a hot-formed material were determined. The tensile properties were determined by a tensile test in accordance with the provisions of JIS Z 2241 using a JIS No. 5 tensile test piece having a major axis in a direction perpendicular to the rolling direction.
In addition, the welded heat-affected zone softening characteristics of the obtained steel sheet were also investigated. Welding heat-affected zone softening characteristics were evaluated by CO ₂ laser welding, welding under the conditions of laser output: 3kW, welding speed: 4m / min, laser focus position: thin steel plate surface, shield gas: Ar. Measure Vickers hardness at the thickness 1/4 position of the thickness section from the base metal part that is not affected by welding and the heat-affected zone to the heat-affected zone at a load of 200 g at 0.1 mm intervals. The difference ΔHv between the average Vickers hardness and the maximum Vickers hardness of the heat-affected zone was obtained.
We examined the relationship between the tensile properties and welding heat-affected zone softening properties and the component composition, especially C, Mn and V, which are thought to affect the tensile properties through precipitation of carbides and softening resistance during quenching and tempering. It has been found that these characteristics can be accurately evaluated by using (10Mn + V) / C as a parameter.
In addition, (10Mn + V) / C is a regression equation obtained in the above examination, and Mn, V, and C in the equation are contents (mass%) of each element.
Moreover, details of the reason why the above characteristics can be accurately evaluated by using (10Mn + V) / C as a parameter are unknown, but the softening resistance during tempering and the size of carbides including Fe and V generated during tempering This is thought to be because the contribution to the interaction of C, Mn, and V, which is thought to affect the difference, differs for each element.

FIG. 1 shows the result of examining the effects of the amounts of C, Mn, and V on TS × EI in the relationship of (10Mn + V) / C.
FIG. 2 shows the result of examining the effects of C, Mn, and V amounts on the weld heat-affected zone softening characteristics (ΔHv) in a relationship of (10Mn + V) / C.
1 and 2, TS × El and ΔHv change greatly in the vicinity of (10Mn + V) / 50 = 50. If (10Mn + V) / C ≧ 50, TS × EI: Excellent strength of 12000MPa ·% or more -It is understood that not only a ductile balance is obtained, but also an excellent weld heat-affected zone softening property of ΔHv: 50 or less is obtained.

Furthermore, when the precipitates of the thin steel sheet as a raw material were also investigated, all the steel sheets that obtained good tensile properties and weld heat-affected zone softening properties obtained V obtained for the precipitates having a particle size of 80 nm or less. It was found that the average particle size of the contained carbide was 30 nm or less.
Here, the size of the precipitates to be investigated was limited to 80 nm or less. Coarse precipitates with a particle size exceeding 80 nm are precipitates that remained undissolved during slab heating, and were sized under the conditions of the subsequent process. This is because, unlike precipitates of 80 nm or less, in which the characteristics greatly change, the characteristics are not greatly affected.
Further, when the same investigation was performed on the quenched and tempered material, the steel material having good tensile properties and weld heat-affected zone softening properties was obtained by using a carbide containing V having a particle size of 20 nm or less in a unit volume of 1 μm ^3. It has been found that the average particle size of carbides containing 1000 or more particles per particle and V containing 20 nm or less of V is 10 nm or less.

The carbide containing V is a precipitate in which both V and C are detected by energy dispersive X-ray spectroscopy (EDX) using a transmission electron microscope (TEM). It is defined as a thing.
Moreover, the average particle diameter of the carbide containing V can be obtained by image processing based on the observation result with a transmission electron microscope. That is, the area of each precipitate is obtained by image processing, converted into an equivalent circle diameter, and the average particle diameter of 80 nm or less may be obtained.

As described above, the average particle size of carbides containing V obtained by adjusting the amount of C, Mn, and V in the steel component, particularly in the range of (10Mn + V) / C ≧ 50, as well as for precipitates having a particle size of 80 nm or less. Although the details of the mechanism by which the excellent strength-ductility balance and softening properties of the heat-affected zone of the weld are obtained after quenching and tempering by controlling the thickness to 30 nm or less have not yet been clearly clarified, It is done.
Since conventional steel sheets for quenching have a structure mainly composed of martensite, the strength-ductility balance is low, and martensite is tempered in the heat-affected zone during welding, so that it softens significantly. Furthermore, even if tempering is performed to improve the strength-ductility balance and weldability, the strength-ductility balance is greatly reduced due to the precipitation of coarse carbides near the grain boundaries, etc. The characteristics are not improved.

In this respect, in the present invention, by setting the average particle size of the carbide containing V in the quenched and tempered thin steel sheet to 30 μm or less, it is possible to ensure the solid solution V after quenching, and (10Mn + V) / C ≧ 50. By doing so, the carbide | carbonized_material containing Fe and V can be disperse | distributed finely and uniformly easily at the time of a tempering process, and the improvement of a strength-ductility balance and weldability can be aimed at.
First, for improving the strength-ductility balance,
(1) The carbide structure containing Fe and V generated by tempering due to the increase in carbide generation sites associated with the high dislocation density martensite structure after quenching,
(2) Fine homogenization of carbides containing Fe and V generated by tempering treatment due to decrease in C diffusion rate due to solute V and Mn.
(3) It is thought to be due to the suppression of TS decrease due to the increase in temper softening resistance due to the interaction of Mn, V and C and the securing of El, etc. Especially, the above effect is remarkable by controlling the particle size of carbide containing V. By setting the component range to (10Mn + V) / C ≧ 50, it is considered that the matters described in (2) and (3) above work effectively.

  Further, regarding the improvement of the welding heat-affected zone softening characteristics, the action of V seems to be particularly remarkable, and the inclusion of V within the scope of the present invention can suppress martensite softening in the vicinity of the heat-affected zone during welding. For the same reason as the above (1) to (3), martensite is reduced by making the carbide containing Fe and V fine in the vicinity of the heat affected zone during welding and suppressing the coarsening of the carbide containing Fe and V. This is thought to be due to the remarkable effect of suppressing the softening of the heat-affected zone of TS: 980MPa class mainly composed of sites.

Next, in the present invention, the reason why the component composition of the automobile member, that is, the component composition of the steel sheet used in the present invention is limited to the above range will be described. In addition, the component composition of a steel plate does not differ by the case where the thin steel plate for hardening and tempering is used, and the case where the thin steel plate for hot forming is used.
C: 0.10 to 0.25%
C is an important element from the viewpoint of increasing the strength of the steel sheet and generating carbides. In the present invention, in order to ensure the desired strength and desired amount of carbide after quenching and tempering or after hot forming-tempering, 0.10% or more. Of C. On the other hand, the content of C exceeding 0.25% significantly deteriorates the weldability. For this reason, the amount of C was limited to the range of 0.10 to 0.25%. In addition, More preferably, it is 0.10 to 0.20% of range.

Si: 1.5% or less
Si is a useful element that can increase the strength of a steel sheet without significantly reducing the ductility of the steel. However, especially in the case of steel sheets for automobiles that require high surface aesthetics and corrosion resistance, the inclusion of Si in excess of 1.5% adversely affects the surface properties and chemical conversion treatment properties, and eliminates these adverse effects. For this reason, a large increase in cost is inevitable due to the long time required for pickling the steel sheet surface. Therefore, Si is limited to 1.5% or less. In applications where more excellent surface aesthetics and corrosion resistance are required, the content is preferably 0.5% or less. Furthermore, in order to obtain a more excellent surface beauty and corrosion resistance, the content is preferably 0.25% or less.
Conventionally, a technique for improving the balance between strength and ductility by increasing Si has been disclosed. However, in the present invention, as described above, the Si amount is set to a small amount of about 0.01% in order to obtain excellent surface beauty and corrosion resistance. Also, a good balance between strength and ductility can be obtained by making the carbide fine and uniform.

Mn: 1.0 to 3.0% or less
Mn is an element that improves hardenability, and further contributes greatly to improving the strength-ductility balance and suppressing temper softening as described above. In order to obtain such a high-performance thin steel sheet after quenching and tempering, it is important to obtain a microstructure mainly composed of martensite after quenching. Also, in order to obtain a high-performance thin steel sheet after hot forming-tempering, it is important to obtain a microstructure mainly composed of martensite after hot forming. The thin steel sheets for quenching and tempering of the present invention are mainly used for quenching and tempering using induction quenching, etc., so the cooling rate after quenching may not be so high, and the hardenability can be controlled by controlling the steel composition. It is important to ensure. This is also the case with thin steel sheets for hot forming, and when forming and quenching by hot forming, the cooling rate may not be so high, and the hardenability can be controlled by controlling the steel composition. It is important to ensure. Mn is an element that is also effective in preventing hot cracking due to S.
The above effect is recognized when the Mn content is in the range of 1.0% or more. However, if the content exceeds 3.0%, the above effect is saturated, and the strength of the base material before quenching is remarkably increased, and the base material is molded. Not only is the property deteriorated, but the strength during hot forming increases.
For this reason, Mn was limited to the range of 1.0 to 3.0%. In addition, when more excellent moldability is required, it is desirable to set it to 1.0 to 1.8%.

P: 0.10% or less P has an effect of strengthening steel, and can contain a necessary amount according to desired strength, and is preferably contained 0.005% or more, but the P amount exceeds 0.10%. And weldability deteriorates. For this reason, the amount of P was limited to 0.10% or less. In addition, when more excellent weldability is required, the P content is preferably 0.05% or less.

S: 0.005% or less S is present as an inclusion in the steel sheet, not only causing deterioration of weldability, but also a coarse inclusion containing S serves as a starting point for the destruction of the steel sheet in the event of an automobile collision, and the impact of the collision is sufficient. Since there exists a possibility that a steel plate may fracture | rupture without absorbing, it is preferable to reduce inclusion of S as much as possible. Since these adverse effects can be ignored if the S amount is 0.005% or less, in the present invention, the S amount is allowed to be 0.005% as the upper limit. In addition, when more excellent weldability and impact absorption characteristics are required, the S content is preferably 0.003% or less.

Al: 0.01 to 0.5%
Al is added as a deoxidizing element for steel, is an element useful for improving the cleanliness of steel, and is also an element that is desirable to be added for refining the structure of steel. In addition, the aluminum killed steel to which Al in the proper range is added has better mechanical properties than the conventional rimmed steel to which Al is not added. Furthermore, like Si, it has the effect of improving the strength-ductility balance. For this reason, the lower limit of the Al amount is set to 0.01%. On the other hand, an increase in Al content leads to deterioration of the surface properties, so the upper limit was made 0.5%.

N: 0.010% or less N is an element that increases the strength of the steel sheet by solid solution strengthening, and is preferably contained by 0.001% or more. However, when adding B for the purpose of improving hardenability, N is combined with B to reduce the amount of free B in steel effective for improving hardenability. If it exceeds 0.010%, the hardenability deteriorates, so the upper limit was made 0.010%. When particularly excellent hardenability is required, for example, when the cooling rate at the time of quenching is low, it is more preferable to set the content to 0.008% or less.

V: 0.10 to 1.0%
V is the most important element in the present invention, and precipitates as ultrafine carbides during tempering, so that ductility can be recovered without reducing strength during high-temperature tempering.
Ti, Nb, V, Mo, Cr, etc. are known as elements that precipitate during tempering and contribute to precipitation strengthening, but carbides such as Ti, Nb do not dissolve easily, and sufficient precipitation strengthening during tempering. In order to obtain the amount, it is necessary to heat to a high temperature exceeding 1100 ° C., and not only when quenching and tempering is performed after molding, but also when quenching and tempering processing is performed after hot molding, it only causes deterioration of surface properties It is inappropriate because it leads to cost increase. Further, although carbides such as Mo and Cr are easier to dissolve than V carbides, in order to obtain a sufficient precipitation strengthening amount during tempering, it is necessary to contain more than several percent, leading to an increase in cost. For this reason, in the present invention aiming to obtain strength by precipitating fine carbides during molding, tempering after quenching or tempering after hot molding, it can be dissolved at a relatively low temperature in a short time. V, which shows a significant increase in strength during tempering without the need to add a large amount, is most suitable. In addition, the structure strengthened by precipitation with the ultrafine carbide containing V has very little softening of the heat-affected zone during welding, and V also has the effect of improving hardenability.
Such an effect becomes remarkable at 0.10% or more, but excessive addition exceeding 1.0% brings about an increase in cost and deterioration of workability during molding. Therefore, the V amount is limited to the range of 0.10 to 1.0%. In order to maximize the effect of V described above, the lower limit of the V amount is preferably 0.15%, and more preferably 0.20%.

Furthermore, in order to obtain the intended strength-ductility balance and softening resistance of the weld heat affected zone in the present invention, the amount of C, Mn, V is adjusted to (10Mn + V) after adjusting to the above-mentioned preferred component composition range. It is important to satisfy the condition of / C ≧ 50.
That is, by adjusting the amount of C, Mn, and V in the range of (10Mn + V) / C ≧ 50 in the steel composition, as shown in FIGS. Affected zone softening characteristics can be obtained.
Although details of this reason are unknown, it is considered that by setting (10 × Mn + V) / C to 50 or more, it becomes possible to finely and uniformly disperse carbides including Fe and V.

The basic components have been described above. However, in the present invention, other elements described below can be appropriately contained.
Nb: 0.1% or less
Nb has the effect of suppressing the coarsening of austenite by forming NbN, and can be added as necessary. Such a coarsening suppression effect becomes significant at 0.005% or more, but addition exceeding 0.1% also promotes precipitation of excessive NbC and reduces solid solution C. Therefore, the volume fraction of carbide containing V during tempering is reduced. Decrease. Therefore, Nb is contained at 0.1% or less. In order to obtain a quenched and tempered thin steel sheet having more excellent formability, Nb is preferably contained at 0.05% or less. In addition, in order to acquire the said effect, it is preferable to contain Nb 0.005% or more.

Ti: 0.1% or less
Ti has the effect of suppressing the coarsening of austenite by forming TiN. In addition, when B is added to improve hardenability by preferentially binding to N, there is an effect of suppressing the binding of B to N. Such an effect becomes prominent at 0.005% or more, but addition exceeding 0.1% also promotes precipitation of excessive TiC and reduces solute C, so that the volume fraction of carbides containing V decreases during tempering. Therefore, Ti is contained at 0.1% or less. In order to obtain a quenched and tempered thin steel sheet having more excellent formability, Ti is preferably contained at 0.05% or less. Moreover, in order to acquire the said effect, it is preferable to contain Ti 0.005% or more. Furthermore, when adding B for improving hardenability, it is preferable to add Ti according to the N content.

B: 0.0050% or less B has an effect of significantly increasing hardenability, stably producing martensite after quenching or hot forming, and is an important element for achieving fine homogenization of carbides during tempering. . Addition of B is unnecessary if cooling can be performed at a rate sufficient to obtain a structure mainly composed of martensite during quenching or after hot forming, but should be added if the cooling rate is not sufficiently high. Is preferred. In order to exert such effects, it is preferable to contain 0.0003% or more of B. More preferably, it is 0.0005% or more. However, if the content exceeds 0.0050%, the above effect is saturated, and rather the hot rolling resistance is increased and the workability is lowered. Therefore, the upper limit of the B content is set to 0.0050%.

One or two types selected from Cr: 0.005-1.0% and Mo: 0.005-1.0% (2Cr + Mo) /2V≦2.0
Cr and Mo improve the hardenability and have the effect of forming a structure mainly composed of martensite after quenching or after hot forming. Even if these elements are added alone, it is not possible to obtain an increase in strength commensurate with the amount added during tempering, but by adding them in combination with V, it is possible to further improve the strength-ductility balance after tempering. It became clear. Further, such an effect becomes significant when 0.005% or more of Cr and Mo are added, respectively, and it has become clear that it is extremely effective to contain in the range of (2Cr + Mo) /2V≦2.0.

After examining the relationship between Cr, Mo, and V content, which is considered to be related to precipitation of TS × El and V after tempering, the relationship can be accurately evaluated by using (2Cr + Mo) / 2V as a parameter. There was found. Note that (2Cr + Mo) / 2V is a regression equation obtained by conducting an experiment, and Cr, Mo, V in the equation is the content (% by mass) of each element.
FIG. 3 shows the relationship between (2Cr + Mo) / 2V and TS × EI after tempering.
As is clear from the figure, TS × El after tempering changes greatly in the vicinity of (2Cr + Mo) /2V=2.0, and Cr and / or Mo are contained in a range satisfying (2Cr + Mo) /2V≦2.0. It can be seen that an excellent strength-ductility balance is obtained.
The reason for this is not clear, but if (2Cr + Mo) / 2V exceeds 2.0, the composition of carbides containing V that precipitates during tempering becomes Mo and Cr rich, and as a result, the precipitates are likely to become coarser, resulting in strength-ductility. It is considered that the softening resistance of the balance and the heat affected zone is deteriorated.
In addition, excessive addition of Cr and Mo exceeding 1.0% respectively leads to cost increase and deterioration of workability during hot forming. Therefore, the preferred range for Cr and Mo is 0.005 to 1.0%, respectively.

Cu: 0.5-5.0%
Cu precipitates independently during tempering after quenching and contributes to increasing strength, promotes the formation of ultrafine carbides containing Fe and V, and makes ultrafine carbides containing Fe and V more uniform and finer. Thus, it has the effect of increasing the strengthening ability with respect to the added amount, and particularly by adding it in combination with V, the strength-ductility balance and the weld heat-affected zone softening property can be further improved.
The reason why such an effect is obtained is not necessarily clear, but the ultrafine Cu is precipitated prior to the carbide containing Fe and V, so that the ultrafine Cu contains a nucleation site of fine carbide containing Fe and V. It is thought that it is due to acting as.
The above effect is recognized when the Cu content is in the range of 0.5% or more. However, if the content exceeds 5.0%, not only the above effect is saturated but also the strength of the steel sheet is remarkably increased and the formability is deteriorated. .
For this reason, Cu was limited to the range of 0.5 to 5.0%. In addition, since said effect becomes especially remarkable when the amount of Cu is 1.0% or more, it is preferable to add 1.0% or more. Moreover, when more excellent moldability is required, 4.0% or less is desirable.

Ni: 0.1-2.0%
Ni is effective in preventing surface defects generated on the surface of the steel sheet when Cu is added, and can be contained as necessary when Cu is added. In that case, the Ni content depends on the Cu content, and is preferably about half of the Cu content, that is, about 30 to 80% of the Cu content. However, if the Ni content exceeds 2.0%, the effect will be saturated, and not only will it be impossible to expect an effect commensurate with the increase in content, it will be economically disadvantageous, but also the steel sheet strength will increase markedly and formability will deteriorate Invite. For this reason, the amount of Ni was limited to the range of 0.1 to 2.0%.

In the present invention, the components other than those described above are not particularly limited, but there is no problem even if Ca, Zr, REM, etc. are contained within the range of the normal steel composition.
The balance other than the above components is Fe and inevitable impurities. Inevitable impurities include, for example, Sb, Sn, Zn, Co, etc. The acceptable ranges of these contents are Sb: 0.01% or less, Sn: 0.1% or less, Zn: 0.01% or less, Co: 0.1 % Or less.

Next, the microstructure of the automotive member of the present invention will be described.
In the present invention, after quenching the thin steel sheet for quenching and tempering, and after hot forming for the thin steel sheet for hot forming, tempering below the Ac ₁ transformation point and depositing carbides containing Fe and V uniformly and finely Further, an automotive member having a tensile strength of 980 MPa or more and excellent strength-ductility balance and resistance to welding heat-affected zone softening can be obtained.
In order to precipitate carbides containing Fe and V uniformly and finely, it is necessary to obtain martensite or bainite having a high dislocation density by quenching or cooling after hot forming. In order to achieve this, it is necessary to make the martensite phase 80% or more, and temper the Ac ₁ transformation point or less to make the tempered martensite phase 80% or more. More preferably, it is 90% or more.
The remaining structure is not particularly limited and may be any of a ferrite phase, a bainite phase, a retained austenite phase, a pearlite phase, and the like.

Particle size: 1000 carbides / μm ³ or more containing V having a particle size of 20 nm or less, and an average particle size of carbides containing V having a particle size of 20 nm or less is 10 nm or less Size of carbides containing V observed after tempering The distribution is such that a particle size: 1000 or more carbides containing V having a particle size of 20 nm or less are deposited per unit volume: 1 μm ³ , and the particle size of the carbide containing V having a particle size of 20 nm or less is 10 nm or less. .
In the present invention, by depositing fine carbide containing V after tempering, an automotive member having a tensile strength of 980 MPa or more and excellent strength-ductility balance and weld heat-affected zone softening characteristics can be obtained. However, when the number of precipitates of carbide containing V having a particle size of 20 nm or less is less than 1000 per unit volume of 1 μm ³ or the average particle size of carbide containing V having a particle size of 20 nm or less exceeds 10 nm. Does not have a sufficient strength increase with respect to the amount of V added, and a good balance between strength and ductility cannot be obtained. In addition, good welding heat-affected zone softening characteristics cannot be obtained. More preferably, the particle size: carbide containing V of 20 nm or less: 2000 particles / μm ³ or more, and the average particle size of carbide containing V having a particle size of 20 nm or less: 5 nm or less.
Here, the reason why the particle size of the carbide containing V as defined above is set to 20 nm or less is that the carbide exceeding 20 nm has no effect on the strength.

The average particle size of carbides containing V was observed with a transmission electron microscope at a magnification of 200,000 and over 10 fields of view, and V and C were detected by elemental analysis by EDX (energy dispersive X-ray spectroscopy). For the precipitates, the area of each precipitate was determined using an image analysis device, converted into a circle equivalent diameter, and considered to have remained undissolved during quenching, excluding those with a diameter exceeding 20 nm, and precipitates of 20 nm or less Was averaged to obtain an average particle size.
The density of the carbide containing V is selected by counting the number of carbides containing V having a particle size of 20 nm or less by the same method as described above, and divided by the total volume (area x thin film thickness) of the observation field. It was set as the density of the precipitate. The thickness of the thin film of the transmission electron microscope was measured by EELS (Electron Energy Loss Spectroscopy).

The above-described automotive member has two types of materials, that is, a case of manufacturing using a quenching and tempering thin steel plate and a case of manufacturing using a hot forming thin steel plate. The microstructure will be described.
Here, the thin steel plate means a steel plate having a thickness of about 0.5 to 4.0 mm, such as a hot rolled steel plate and a cold rolled steel plate.

First, the microstructure of the quenched steel sheet for tempering will be described.
The volume fraction of the ferrite phase is 60% or more. In the case of a quenched and tempered thin steel sheet, the ferrite phase needs to have a ferrite phase of 60% or more in terms of the volume fraction of the entire structure. The reason is that if the ferrite phase is less than 60% in volume ratio to the entire structure, the high ductility required for a thin steel sheet for automobiles that requires high workability as in the case of quenching after press forming should be secured. This is because it becomes difficult. In applications where even better ductility is required, the ferrite phase is preferably 70% or more in terms of the area ratio relative to the entire structure. Here, the ferrite phase includes ferrite such as polygonal ferrite, bainitic ferrite, acicular ferrite and the like.
In addition, it is not specifically limited except the above-mentioned ferrite phase, What is necessary is just to set it as a bainite phase, a martensite phase, a pearlite phase, a retained austenite phase, etc.

The average particle size of carbides including V obtained for precipitates having a particle size of 80 nm or less is 30 nm or less. In the present invention, V that has not been actively used to improve the performance of steel sheets for quenching and tempering is conventionally used. By utilizing it, the microstructure after quenching and tempering becomes a structure mainly composed of tempered martensite, and by precipitating fine carbides containing V after tempering, it is excellent in strength-ductility balance and softening resistance of the weld heat affected zone. Automotive parts with a tensile strength of 980 MPa or more can be obtained. When quenching and tempering are performed after forming as in the present invention, the steel sheet is heated to a temperature equal to or higher than the Ac ₃ transformation point after forming. This suppresses cost increase required for the heating process and oxidation of the steel sheet surface during heating. It is preferable to make the heating temperature and holding time as low and short as possible, and for that purpose, it is necessary to make the carbide containing V in the steel sheet for quenching and tempering finer and dissolve it as low and as short as possible. .

In the present invention, in order to make maximum use of the performance enhancement effect of the thin steel sheet due to the V, the grain size of the carbide containing V in the thin steel sheet for quenching and tempering needs to be 30 nm or less. This is because the carbide containing V is dissolved at a low temperature in a short time during heating during quenching, and during tempering.
(1) Precipitating a necessary amount of fine carbide containing V.
(2) This is because it is easy to ensure a sufficient amount of solute V in order to exert effects such as ensuring the temper softening resistance by V.
In addition, when it is necessary to dissolve at a lower temperature and in a shorter time, the average particle size of the carbide containing V is preferably 20 nm or less. However, if the amount of carbide containing extremely fine V is increased, the strength of the thin steel plate for quenching and tempering is rapidly increased, and the formability is deteriorated.
Here, the average particle size of carbides containing V is observed by 10 or more fields at a magnification of 100,000 using a transmission electron microscope, and V and C are detected by elemental analysis by EDX (energy dispersive X-ray spectroscopy). Obtain the area of each precipitate using an image analysis device, and convert it into an equivalent circle diameter, which remains undissolved during slab heating, or in which V is dissolved in a coarse precipitate such as TiN The average particle diameter was determined by excluding those considered to have a diameter exceeding 80 nm and averaging the precipitates of 80 nm or less. In addition, in this invention, since it is going to dissolve the carbide | carbonized_material containing V at the time of the heating in a hardening process, the case where the carbide | carbonized_material containing V does not precipitate at all is also included about the steel plate before a hardening process. Of course.
In the present invention, it is preferable that the steel sheet for quenching and tempering having the above-described structure and components is formed into a predetermined shape at room temperature and then subjected to a predetermined quenching and tempering treatment to obtain a member for an automobile.

Next, the microstructure of the hot forming thin steel sheet will be described.
In this thin steel sheet for hot forming, since press forming is performed at a high temperature, workability such as ductility as a steel sheet is not particularly problematic, and the volume fraction of the ferrite phase is not particularly limited.

The average particle size of carbide containing V obtained for precipitates having a particle size of 80 nm or less is 30 nm or less. In the present invention, conventionally, it has not been used so actively for improving the performance of hot forming thin steel sheets. By using V, the microstructure after hot forming and tempering becomes a structure mainly composed of tempered martensite, and fine carbides containing V are precipitated after tempering, so that the strength-ductility balance and the weld heat affected zone. This makes it possible to produce a hot-formed thin steel sheet from which an automotive member having an excellent softening resistance and a tensile strength of 980 MPa or more can be obtained. When hot forming is performed as in the present invention, the steel sheet is heated to a temperature equal to or higher than the Ac ₃ transformation point. In order to suppress the cost increase required for the heating process and oxidation of the steel sheet surface during heating, the heating temperature It is preferable that the holding time be as low as possible and as short as possible. For that purpose, it is necessary to make the carbide containing V in the thin steel sheet for hot forming fine and dissolve it as low as possible in the shortest time.

In the present invention, in order to make maximum use of the performance enhancement effect of the thin steel sheet due to V, it is necessary that the average grain size of carbides containing V of the hot forming thin steel sheet be 30 nm or less. The reason for this is that, as in the case of the steel for quenching and tempering, carbides containing V are dissolved in a short time at a low temperature during heating during hot forming, and during tempering.
(1) Precipitating a necessary amount of fine carbide containing V.
(2) This is because it is easy to ensure a sufficient amount of solute V in order to exert effects such as ensuring the temper softening resistance by V.
In addition, when it is necessary to dissolve at a lower temperature and in a shorter time, the average particle size of the carbide containing V is preferably 20 nm or less.
In the present invention, it is preferable that the above-described thin steel sheet for hot forming is hot formed under predetermined conditions, and then subjected to a predetermined tempering treatment to obtain a member for an automobile.

  Incidentally, the quenched and tempered steel sheet and hot-formed steel sheet targeted by the present invention may be either hot-rolled steel sheets or cold-rolled steel sheets, and these are used as original sheets for hot-dip galvanized steel sheets. You can also

Next, the preferable manufacturing conditions for the automotive member of the present invention will be described.
A steel slab adjusted to the above suitable component composition range is used as a material, and the material is hot-rolled to obtain a hot-rolled steel plate having a predetermined thickness. The steel slab to be used is preferably produced by a continuous casting method in order to prevent macro segregation of components, but can also be produced by an ingot casting method or a thin slab casting method. In addition to the conventional method in which the slab is manufactured and then cooled to room temperature and then heated again, without being cooled, it is inserted into a heating furnace as it is, or after a little heat retention, it is immediately rolled. Energy saving processes such as direct feed rolling and direct rolling can be applied without problems.

The hot rolling conditions are defined as follows.
Slab heating temperature: 1000 ° C. or more When the heating temperature is less than 1000 ° C., the rolling load increases, and the risk of trouble during hot rolling increases. Moreover, if it is less than 1000 ° C., there are many carbides containing undissolved coarse V, which cannot be sufficiently dissolved during heating during hot forming, and sufficient due to precipitation during tempering after hot forming. In some cases, the strength cannot be increased.
Therefore, the slab heating temperature is set to 1000 ° C. or higher. However, if the heating temperature is too high, the slab heating temperature is desirably 1300 ° C. or lower because it leads to an increase in scale loss accompanying an increase in oxidized weight.
Moreover, it goes without saying that it is effective to use a so-called sheet bar heater that heats the sheet bar from the viewpoint of lowering the slab heating temperature and preventing troubles during hot rolling.

Finishing rolling delivery temperature: 800 ° C. or higher After heating to the slab heating temperature described above, a sheet bar is formed by rough rolling, and then finish rolling is performed. By setting the finish rolling exit temperature (hereinafter also referred to as the finish rolling temperature) to 800 ° C. or more, a uniform hot-rolled base metal structure can be obtained, and it can be used without any problem in use. However, when the finish rolling temperature is lower than 800 ° C., the structure of the steel sheet becomes non-uniform, and the risk of causing various problems during forming increases. Even if a higher coiling temperature is used to avoid the remaining of the processed structure at a rolling temperature lower than the above, in this case, the same problem associated with the generation of coarse grains occurs.
Therefore, the finish rolling temperature is set to 800 ° C. or higher. Although the upper limit is not particularly restricted, it is preferable to set the temperature to about 1000 ° C. or lower because rolling at an excessively high temperature causes scale wrinkles.

The steps up to the finish rolling process are the same for both the quenched and tempered thin steel sheet and the hot-formed thin steel sheet. However, since the subsequent winding processes differ from each other, each will be described individually.
First, in the case of a thin steel plate for quenching and tempering, it is necessary that the coiling temperature is 450 ° C. or higher and the temperature Ta (° C.) or lower expressed by the following formula (2).
Ta (° C.) = [5500 / {6.7 + log ([% V] × [% C])}] − 400 (2)
Here, [% C] and [% V] are the contents of each element (% by mass).
Control of the coiling temperature after the finish rolling is extremely important in controlling the average particle size of the carbide containing V targeted in the present invention to 30 nm or less.
The inventors consider that the particle size of the carbide containing V depends on the contents of C and V, which are considered to affect the precipitation rate and growth rate of the carbide, and the inclusion of C and V on the carbide particle size. The effect of quantity and winding temperature was investigated.
As a result, [5500 / {6.7 + log ([% V] × [% C])}]-400 (° C.) or less is cooled and wound to control the average particle size of carbides containing V to 30 nm or less. It became clear that we could do it.

Although it is not clear why the coiling temperature at which the average particle size of the carbide containing V is 30 nm or less depends on the C and V contents, the precipitation rate of the carbide containing V depends on the C and V contents, and The inventors think that the temperature range in which the growth rate is maximized is different.
When the coiling temperature exceeds [5500 / {6.7 + log ([% V] × [% C])}]-400 (° C.), the precipitated carbide becomes coarse, and the hot rolled sheet has a mean particle size of 30 nm or less. In some cases, a hot-rolled sheet having a structure in which precipitation occurs is not obtained, and sufficient strength increase due to precipitation may not be obtained during tempering after quenching. For this reason, the coiling temperature was defined as [5500 / {6.7 + log ([% V] × [% C])}] − 400 (° C.) or less.

The lower limit of the coiling temperature is not strictly limited in terms of the material, but if it falls below 450 ° C, the fraction of low-temperature transformation phase martensite phase and bainite phase increases, and the volume fraction of ferrite phase becomes less than 60%. In some cases, the moldability may be significantly deteriorated. Accordingly, the hot rolling coiling temperature is set to 450 ° C. or higher. Further, when higher formability is required, in the cooling between finish rolling and winding, in order to generate more ferrite phases advantageous for improving formability as a thin steel plate, It is preferable to provide a slow cooling region.
The cooling rate from finish rolling to winding is not particularly limited as long as the cooling rate is equal to or higher than that of standing cooling. However, from the viewpoint of refinement of carbide containing V, the cooling rate should be 10 ° C / s or more. Is preferred. More preferably, it is 20 ° C./s or more.

In the hot rolling in the production of a hot rolled steel sheet suitable as a thin steel sheet for quenching and tempering according to the present invention, part or all of the finish rolling may be lubricated rolling in order to reduce the rolling load during hot rolling. Performing lubrication rolling is also effective from the viewpoint of uniform steel plate shape and uniform material. The wear coefficient during this lubrication rolling is preferably in the range of 0.25 to 0.10. Moreover, it is preferable to set it as the continuous rolling process which joins the sheet | seat bars which precede and follow, and finish-rolls continuously. The application of the continuous rolling process is also desirable from the viewpoint of the operational stability of hot rolling.
Further, after hot rolling, temper rolling of 10% or less may be performed for shape correction, adjustment of surface roughness and the like.
Moreover, the hot-rolled steel sheet suitable as the thin steel sheet for quenching and tempering according to the present invention can be subjected to surface treatment before forming. Examples of the surface treatment include galvanization (including alloy system), tin plating, enamel and the like. Furthermore, after annealing or galvanization, special treatment may be applied to improve chemical conversion properties, weldability, press formability, corrosion resistance, and the like.

As mentioned above, although the case where the thin steel plate for hardening and tempering was manufactured as a hot-rolled steel plate was demonstrated, when manufacturing as a cold-rolled steel plate, the following processes are further required.
After hot rolling, prior to cold rolling, pickling is performed according to a conventional method. However, in the case of a very thin scale, the pickling can be omitted and directly subjected to cold rolling.
Cold rolling is not particularly limited as long as it is possible to obtain a cold-rolled sheet having a desired size and shape. However, the rolling reduction should be 20% or more from the viewpoint of surface flatness and structure uniformity. Is preferred.

Next, the cold-rolled sheet is annealed to obtain a cold-rolled sheet. This annealing is preferably performed in either a continuous annealing line or a continuous hot dip galvanizing line.
The annealing conditions are defined as follows.
Annealing temperature: Ac ₁ transformation point or more, (Ac ₃ transformation point +200) ° C. or less Ac ₁ transformation point or more, (Ac ₃ transformation point +200) ° C. The retention time of the following: 10～300S
If the annealing temperature, which is the highest temperature during annealing, is less than the Ac ₁ transformation point, the residual stress of cold rolling cannot be removed sufficiently, and there is a large risk of deformation during processing at room temperature, making stable molding difficult. There is a possibility. On the other hand, if it exceeds (Ac ₃ transformation point +200) ° C., the austenite grain size becomes coarse, and the formability of the cold-rolled thin steel sheet and the strength-ductility balance of the automotive member are lowered.
Further, the temperature range from the Ac ₁ transformation point to (Ac ₃ transformation point +200) ° C. is the temperature range where carbides containing V precipitate, and holding for a long time leads to coarsening of the carbides containing V. The time needs to be 300 s or less. More preferably, it is 120 s or less. On the other hand, if the holding time is less than 10 seconds, there is a problem that it is difficult to obtain a sufficiently uniform structure.
Here, the holding time means a residence time in the above temperature range. The Ac ₁ transformation point and Ac ₃ transformation point can be obtained by measuring the thermal expansion coefficient.

Cooling after annealing: After holding in the above temperature range, average cooling rate up to 500 ° C: 10-50 ° C / s
For the cooling after annealing, it is necessary to cool at an average cooling rate of 10 to 50 ° C./s to at least 500 ° C. after holding in the temperature range of the above Ac ₁ transformation point to (Ac ₃ transformation point +200) ° C. When the average cooling rate is less than 10 ° C./s, not only the productivity is reduced, but also carbides containing V are precipitated and coarsened during cooling, and the cold-rolled annealed plate contains carbides containing V having an average particle size of 30 nm or less. In some cases, it does not have a precipitated structure, and a sufficient strength increase due to precipitation may not be obtained after molding, quenching, and tempering. In addition, the strength-ductility balance deteriorates. For this reason, the average cooling rate up to 500 ° C. after holding in the above temperature range, that is, the average cooling rate from the Ac ₁ transformation point temperature, which is the lower limit temperature of the above temperature range, to 500 ° C. is 10 ° C./s or more. More preferably, it is 15 ° C./s or more. On the other hand, with respect to the upper limit of the average cooling rate, if it is too fast, it becomes difficult to make the volume fraction of the ferrite phase 60% or more, and the formability near room temperature tends to decrease. To 50 ° C./s.

Although there is no particular limitation after the above cooling, the temperature range from 500 ° C to 350 ° C greatly affects the strength of the low-temperature transformation phase (martensite and bainite). In order to improve the workability, it is preferable that the residence time between 500 ° C. and 350 ° C. is set to 30 s or longer for cooling. If the residence time is less than 30 s, the low-temperature transformation phase tends to harden and processing at room temperature may be difficult. On the other hand, the upper limit of the residence time is preferably about 600 s from the viewpoint of productivity.
In addition, you may perform temper rolling of 10% or less after said annealing for adjustment of shape correction, surface roughness, etc.
Thus, a cold sheet steel for quenching and tempering can be obtained.

The hot steel sheet and the cold steel sheet for quenching and tempering obtained as described above are formed into a predetermined shape at room temperature and then subjected to quenching and tempering to obtain an ultra-high strength automotive member.
Hereinafter, the quenching and tempering process will be described.

Heating temperature: Ac ₃ transformation point or higher, (Ac ₃ transformation point +200) ° C. or lower In order to obtain a structure mainly composed of martensite by cooling (quenching) after heating, it is heated to a temperature higher than the Ac ₃ transformation point. There is a need. Moreover, from the viewpoint of dissolving carbide containing V precipitated on the quenched and tempered thin steel sheet, it is desirable that the temperature be as high as possible. However, heating to a temperature exceeding (Ac ₃ transformation point +200) ° C. increases the cost and during heating. Since oxidation of the steel sheet surface becomes a problem and the austenite grain size becomes coarse, it is necessary to set (Ac ₃ transformation point +200) ° C. More preferably, it is (Ac ₃ transformation point +150) ° C. or lower. In addition, as a heating method of the steel plate here, high-frequency heating is generally used, but resistance heating, heating with a gas burner, or the like may be used. Further, the holding time in such a heating temperature region is preferably 1 to 600 s.

Cooling rate: 20 ° C./s or more The high-strength automotive member of the present invention has a structure mainly composed of martensite even at a relatively slow cooling rate by controlling the addition amount of C, Mn, and V and adding B. be able to. The cooling rate required to make the molded member uniform in properties after quenching varies somewhat depending on the plate thickness and composition, but the average cooling rate up to 300 ° C. may be 20 ° C./s or more. Moreover, in order to manufacture more stably, it is preferable to set it as 30 degrees C / s or more.
As for the cooling method after heating, any of water cooling, mist cooling, gas cooling, and air cooling may be used as long as the above cooling rate can be satisfied.

Tempering conditions: Conditions that satisfy the following formula (1)
11000−3000 [% V] ≦ Tb (20 + logt) ≦ 15000−1000 [% V] (1)
However, Tb ≦ Ac ₁ transformation point where [% V] is the V content (mass%)
Tb is the tempering temperature (℃)
t is the holding time at the tempering temperature (s)
The inventors consider that the heat treatment conditions for precipitating carbide containing V with the desired size and distribution depend on the V content, which is considered to affect the precipitation rate and growth rate of the carbide. The effects of V content, heat treatment temperature, and time on the size and distribution of the steel were examined in detail.
As a result, in order to obtain a high-strength automotive member having an excellent balance between strength and ductility and less softening of the heat affected zone, the tempering temperature Tb is lower than the Ac ₁ transformation point after quenching. It has been found that the tempering treatment needs to be performed under the condition that the holding time t (s) satisfies 11000−3000 [% V] ≦ Tb (20 + logt) ≦ 15000−1000 [% V].

That is, when Tb (20 + logt) is less than 11000-3000 [% V], as described above, 1000 particles / μm ³ or more of carbides containing V having a particle size of 20 nm or less are precipitated, and the particle size is 20 nm or less. When the average particle size of carbides containing V is not 10 nm or less, and the Tb (20 + logt) exceeds 15000-1000 [% V], the average particle size of carbides containing V becomes coarser than 10 nm. In any case, it is impossible to obtain a high-strength automotive member excellent in the intended strength-ductility balance and welding heat-affected zone softening characteristics.
Further, when the tempering temperature Tb exceeds Ac ₁ transformation point, austenite completely be another organization's product, the strength is greatly reduced, it is necessary to not more than Ac ₁ transformation point.
The cooling method after tempering is not particularly defined, and can be performed by methods such as water cooling, mist cooling, gas cooling, and air cooling.

Next, the process after finish rolling of the hot forming thin steel sheet will be described.
After finish rolling, the steel sheet is cooled to a temperature Ta (° C.) or less expressed by the following formula (2) and wound up.
Ta (° C.) = [5500 / {6.7 + log ([% V] × [% C])}] − 400 (2)
Here, [% C] and [% V] are the contents of each element (% by mass).
Also in the case of a thin steel sheet for hot forming, the control of the coiling temperature after the finish rolling is extremely important in controlling the average particle size of carbides containing V targeted in the present invention to 30 nm or less.
The inventors consider that the particle size of the carbide containing V depends on the contents of C and V, which are considered to affect the precipitation rate and growth rate of the carbide, and the inclusion of C and V on the carbide particle size. The effect of quantity and winding temperature was investigated.
As a result, [5500 / {6.7 + log ([% V] × [% C])}]-400 (° C.) or less is cooled and wound to control the average particle size of carbides containing V to 30 nm or less. It became clear that we could do it.
The reason why the coiling temperature at which the average particle size of the carbide containing V is 30 nm or less depends on the content of C and V is as described above, and the precipitation rate of the carbide containing V depends on the content of C and V, as described above. This is probably because the temperature range where the growth rate is maximized is different.

Here, when the coiling temperature exceeds [5500 / {6.7 + log ([% V] × [% C])}] − 400 (° C.), the precipitated carbide becomes coarse, and the hot rolled sheet has an average particle size of 30 nm. In some cases, a hot rolled sheet having a structure in which the following carbides are precipitated is not obtained, and sufficient strength increase due to precipitation may not be obtained during tempering after hot forming. For this reason, the coiling temperature was limited to [5500 / {6.7 + log ([% V] × [% C])}] − 400 (° C.) or less. The lower limit of the coiling temperature is not strictly limited in terms of material, but if it is below 200 ° C., the shape of the steel sheet is significantly disturbed, and the risk of causing problems in actual use of the steel sheet increases. Therefore, the hot rolling coiling temperature is preferably 200 ° C. or higher. Furthermore, when high material uniformity is required, the temperature is preferably 300 ° C. or higher.
The cooling rate from finish rolling to winding is not particularly limited as long as the cooling rate is equal to or higher than that of standing cooling. However, from the viewpoint of refinement of carbide containing V, the cooling rate should be 10 ° C / s or more. Is preferred. More preferably, it is 20 ° C./s or more.

In addition, in hot rolling in the production of a hot rolled steel sheet suitable as a thin steel sheet for hot forming according to the present invention, part or all of finish rolling may be lubricated rolling in order to reduce rolling load during hot rolling. Further, after hot rolling, temper rolling of 10% or less may be performed for the purpose of shape correction, surface roughness adjustment, etc., as in the case of the quenching and tempering thin steel sheet.
In addition, the hot-rolled steel sheet suitable as the thin steel sheet for hot forming according to the present invention can be subjected to surface treatment before hot forming. Examples of the surface treatment include galvanization (including alloy system), tin plating, enamel and the like. In addition, the hot-rolled steel sheet suitable as the thin steel sheet for hot forming according to the present invention is subjected to special treatment after annealing or galvanization to improve chemical conversion processability, weldability, hot press formability, corrosion resistance, etc. May be performed.

As mentioned above, although the case where the thin steel plate for hot forming was made as a hot-rolled steel plate was demonstrated, when manufacturing as a cold-rolled steel plate, the following processes are required.
After hot rolling, prior to cold rolling, pickling is performed according to a conventional method. However, in the case of a very thin scale, the pickling can be omitted and directly subjected to cold rolling.
Cold rolling is not particularly limited as long as it is possible to obtain a cold-rolled sheet having a desired size and shape. However, the rolling reduction should be 20% or more from the viewpoint of surface flatness and structure uniformity. Is preferred.

Next, the cold-rolled sheet is annealed to obtain a cold-rolled sheet. This annealing is preferably performed in either a continuous annealing line or a continuous hot dip galvanizing line.
The annealing conditions are defined as follows.
Annealing temperature: Ac ₁ transformation point or more, (Ac ₃ transformation point +200) ° C. or less Ac ₁ transformation point or more, (Ac ₃ transformation point +200) ° C. The retention time of the following: 10～300S
If the annealing temperature is less than the Ac ₁ transformation point, the residual stress of cold rolling cannot be sufficiently removed, and there is a large possibility of deformation during heating during hot forming, which may make stable forming difficult. on the other hand,
When (Ac ₃ transformation point +200) ° C. is exceeded, the austenite grain size becomes coarse and the strength-ductility balance is lowered.
Note that this temperature range is a temperature range where carbides containing V are precipitated, and holding for a long time leads to coarsening of carbides containing V, so the holding time needs to be 300 s or less. More preferably, it is 120 s or less. On the other hand, when the holding time is less than 10 seconds, there is a problem that it is difficult to obtain a sufficiently uniform structure.

Cooling after annealing: After holding in the above temperature range, average cooling rate up to 500 ° C: 10 ° C / s or more Cooling after annealing is from the Ac ₁ transformation point which is the above annealing temperature to (Ac ₃ transformation point +200) After holding in the temperature range of ° C., it is necessary to cool at an average cooling rate of at least 10 ° C./s to at least 500 ° C. When the average cooling rate is less than 10 ° C./s, not only the productivity is reduced, but also carbides containing V are precipitated and coarsened during cooling, and the cold-rolled annealed plate contains carbides containing V having an average particle size of 30 nm or less. In some cases, the structure does not have a precipitated structure, and a sufficient increase in strength due to precipitation may not be obtained during tempering after hot forming. For this reason, the average cooling rate from the annealing temperature to 500 ° C. is 10 ° C./s or more. More preferably, it is 15 ° C./s or more.
Thus, a cold thin steel sheet for hot forming can be obtained.

The hot thin steel sheet and cold thin steel sheet steel sheet for hot forming obtained as described above are subjected to tempering treatment after hot forming to obtain an ultra-high strength automotive member.
Hereinafter, the hot forming process will be described.
Heating temperature: Ac ₃ transformation point or higher, (Ac ₃ transformation point +200) ° C. or lower This heating temperature is the same reason as explained for the quenching and tempering thin steel sheet, and the Ac ₃ transformation point to (Ac ₃ transformation point +200) ° C. It was limited to the temperature range. Further, the holding time in such a heating temperature region is preferably 1 to 600 s.

Hot forming temperature: Ac ₃ transformation point or higher Hot forming should be carried out at the above-mentioned heating temperature, preferably after holding for 1 to 600 s, at the Ac ₃ transformation point or higher. This is because when the hot forming temperature is less than the Ac ₃ transformation point, ferrite is generated, and the desired structure cannot be obtained after cooling, and the deformation resistance becomes large and the forming becomes difficult. Therefore, the hot forming temperature is set to the Ac ₃ transformation point or higher. From the viewpoint of facilitating the structure control during cooling, a more preferable hot forming temperature is (Ac ₃ transformation point + 50) ° C. or higher.

Cooling rate: 20 ° C./s or more This cooling rate was also set to 20 ° C./s or more for the same reason as described for the thin steel plate for quenching and tempering. More preferably, it is 30 ° C./s or more.

Tempering conditions: Conditions that satisfy the following formula (1)
11000−3000 [% V] ≦ Tb (20 + logt) ≦ 15000−1000 [% V] (1)
However, Tb ≦ Ac ₁ transformation point where [% V] is the V content (mass%)
Tb is the tempering temperature (℃)
t is the holding time at the tempering temperature (s)
This tempering condition was also set to a range satisfying the above formula (1) for the same reason as described for the quenched and tempered thin steel sheet.

Example 1
This example is a case where an automobile member is manufactured using a thin steel plate for quenching and tempering.
Molten steel having the composition shown in Table 1 was melted in a converter and made into a steel slab by a continuous casting method. The Ac ₁ transformation point and Ac ₃ transformation point were determined by measuring thermal expansion.
Subsequently, these steel slabs were made into hot-rolled sheets with a thickness of 4.0 mm under the conditions shown in Table 2, and subjected to temper rolling with an elongation of 1.0% after pickling.
A test piece was collected from the hot-rolled sheet thus obtained, and the structure was observed. In addition, a tensile test was conducted to examine tensile properties. The obtained results are also shown in Table 2.
Further, from the obtained steel plate, a specimen of 300 mm × 220 mm was taken with the longitudinal direction as the rolling direction, and processed into the shape shown in FIG. 4 to create a test member. JIS No. 5 test specimens were collected to investigate the tensile properties after quenching and tempering, and the flange portion was subjected to the following CO ₂ laser welding to investigate the softening properties of the heat-affected zone of the welded heat affected zone.
The quenching and tempering conditions are shown in Table 3, and the obtained results are shown in Table 4.

The details of the test method are as follows.
(1) Microstructure observation Specimens are collected from the obtained steel plate and test member, and the cross section (C cross section) perpendicular to the rolling direction is imaged using an optical microscope or a scanning electron microscope, and image analysis is performed. The type of structure such as a ferrite phase and a tempered martensite phase was identified using an apparatus, and the structure fraction (area ratio) was obtained and used as the volume ratio.
The average particle size of carbides containing V was observed with a transmission electron microscope at a magnification of 200,000 times and over 10 fields, and V and C were detected by elemental analysis using EDX (energy dispersive X-ray spectroscopy). The area of each precipitate was determined using an image analysis device, and the average particle size was determined by the above-mentioned method after converting to an equivalent circle diameter. The density of carbides containing V is determined by selecting carbides containing V having a particle size of 20 nm or less by the same method as described above, counting the number, and dividing by the total volume of the observation field (area × thin film thickness). And the density of the precipitates. The thickness of the thin film used in the transmission electron microscope was measured by EELS (Electron Energy Loss Spectroscopy).

(2) Tensile test A JIS No. 5 tensile test piece (thickness: 1.0 mm) with the long axis perpendicular to the rolling direction was taken from the obtained steel sheet and subjected to a tensile test in accordance with the provisions of JIS Z 2241. The tensile properties (yield stress (YS), tensile strength (TS), elongation (EI), yield ratio (YR)) were determined.

(3) anti-weld heat affected zone softening properties resistant HAZ softening properties, by CO ₂ laser welding, laser power: 3 kW, welding speed: 4m / min, the laser focal position: sheet steel surface, shielding gas: the Ar Welding under conditions, base material not affected by welding, and Vickers hardness at quarter thickness position of the thickness section from weld melt to heat affected zone, measured at 0.1mm intervals under the condition of load: 200g Then, the difference ΔHv between the average Vickers hardness of the base material and the maximum Vickers hardness of the heat affected zone was evaluated.

As shown in Table 2, all of the hot-rolled thin steel sheets for quenching and tempering obtained according to the present invention had a ferrite phase volume ratio of 60% or more and an average particle size of carbides containing V of 30 nm or less.
Further, as apparent from Table 4, when this hot-rolled thin steel sheet for quenching and tempering is actually subjected to quenching and tempering treatment after forming, it has excellent strength-ductility such that TS × El is 12000 MPa ·% or more. Along with the balance, an excellent welding heat-affected zone softening property of ΔHv of 50 or less is obtained.
Note that steel plate No. 10 with a ferrite phase volume fraction of less than 60% had good member properties after quenching and tempering, but the TS × El as a hot-rolled steel plate was low, and it was formed before quenching and tempering. There was a problem with sex.
On the other hand, in the comparative example outside the proper range of the present invention, the strength-ductility balance (TS × EI) was less than 12000 MPa ·%, or ΔHv exceeded 50.

Example 2
The hot-rolled sheet obtained in Example 1 was pickled and cold-rolled at a rolling reduction of 75% to obtain a cold-rolled sheet having a thickness of 1.0 mm. Next, the cold-rolled sheet was annealed under the conditions shown in Table 5 in a continuous annealing line, and further subjected to temper rolling with an elongation of 1.0%.
A specimen was collected from the cold-rolled annealed plate thus obtained, and the structure was observed. In addition, a tensile test was conducted to examine tensile properties. The obtained results are also shown in Table 5.
Further, test members were prepared in the same manner as in Example 1, and the tensile properties and the weld heat-affected zone softening properties after quenching and tempering under the conditions shown in Table 6 were also investigated. The results obtained are shown in Table 7.

As shown in Table 5, all of the cold-rolled thin steel sheets for quenching and tempering obtained according to the present invention,
The volume fraction of the ferrite phase was 60% or more, and the average particle size of the carbide containing V was 30 nm or less.
Further, as is apparent from Table 7, when this cold-rolled thin steel sheet for quenching and tempering was actually subjected to quenching and tempering treatment after forming, TS × El was excellent in strength-ductility of 12000 MPa ·% or more. Along with the balance, an excellent welding heat-affected zone softening property of ΔHv of 50 or less could be obtained. Steel plate No. 12 with a ferrite phase volume fraction of less than 60% had good member properties after quenching and tempering, but TS × El as a cold-rolled steel plate was low, and formability at room temperature was low. It was inferior.
On the other hand, in the comparative example outside the proper range of the present invention, the strength-ductility balance (TS × EI) was less than 12000 MPa ·%, or ΔHv exceeded 50.

Example 3
This example is a case where an automobile member is manufactured using a thin steel sheet for hot forming.
Molten steel having the composition shown in Table 1 was melted in a converter and made into a steel slab by a continuous casting method. Subsequently, these steel slabs were formed into hot rolled sheets having a thickness of 4.0 mm under the conditions shown in Table 8, and after pickling, temper rolling was performed at an elongation of 1.0%.
A test piece was collected from the hot-rolled sheet thus obtained, and the structure was observed. In addition, a tensile test was conducted to examine tensile properties. The obtained results are also shown in Table 8.
Further, from the obtained steel sheet, a specimen of 300 mm × 220 mm was taken with the rolling direction as the longitudinal direction, and after hot forming into the shape of FIG. . JIS No. 5 test piece with the rolling direction as the tensile direction was taken from the vertical wall of the prepared test member, and the tensile properties after hot forming were investigated, and the flange portion was the same as in Examples 1 and 2 above. Then, CO ₂ laser welding was conducted and the welding heat-affected zone softening characteristics were investigated.
Table 9 shows the hot forming conditions, and Table 10 shows the results obtained.

As shown in Table 8, the hot-rolled thin steel sheet for hot forming obtained according to the present invention had an average particle size of carbides containing V of 30 nm or less.
Further, as apparent from Table 10, when this hot-rolled thin steel sheet for hot forming was actually subjected to tempering after hot forming, TS × El was excellent strength of 12000 MPa ·% or more− Excellent weld heat-affected zone softening properties with ductility balance and ΔHv of 50 or less could be obtained.
On the other hand, in the comparative example outside the proper range of the present invention, the strength-ductility balance (TS × EI) was less than 12000 MPa ·%.

Example 4
The hot-rolled sheet obtained in Example 3 was pickled and cold-rolled at a rolling reduction of 75% to obtain a cold-rolled sheet having a thickness of 1.0 mm. Next, the cold-rolled sheet was annealed under the conditions shown in Table 11 in a continuous annealing line, and further subjected to temper rolling with an elongation of 1.0%.
A specimen was collected from the cold-rolled annealed plate thus obtained, and the structure was observed. In addition, a tensile test was conducted to examine tensile properties. The obtained results are also shown in Table 11.
Further, the tensile properties and the weld heat-affected zone softening properties after hot working and tempering were also investigated as in Example 3 under the conditions shown in Table 12. The results obtained are shown in Table 13.

As shown in Table 11, all of the cold-rolled thin steel sheets for hot forming obtained according to the present invention had an average particle size of carbides containing V of 30 nm or less.
Further, as apparent from Table 13, when this cold-rolled thin steel sheet for hot forming was actually subjected to tempering after hot forming, TS × El was excellent strength of 12000 MPa ·% or more− Excellent weld heat-affected zone softening properties with ductility balance and ΔHv of 50 or less could be obtained.
On the other hand, in the comparative example outside the proper range of the present invention, the strength-ductility balance (TS × EI) was less than 12000 MPa ·%.

  In the production of automobile parts, the present invention optimizes the component composition, hot-rolling conditions and cold-rolling annealing conditions, as well as quenching and tempering conditions or hot-forming conditions, thereby reducing the microstructure in the final product stage. By controlling and properly producing carbide containing V with the tempered martensite phase as the main phase, it is possible to obtain excellent weld-heat affected zone softening characteristics as well as excellent strength-ductility balance.

It is the figure which showed the influence of the amount of C, Mn, and V which has on TSxEl by the relationship of (10Mn + V) / C. It is the figure which showed the influence of the amount of C, Mn, and V on the welding heat affected zone softening property (ΔHv) in the relationship of (10Mn + V) / C. It is the figure which showed the influence of the amount of Cr and Mo which has on TSxEl by the relationship of (2Cr + Mo) / 2V. It is the figure which showed the shape and dimension of the test member after a shaping | molding process.

Claims (15)

In mass% C: 0.10 to 0.25%,
Si: 1.5% or less,
Mn: 1.0-3.0%
P: 0.10% or less,
S: 0.005% or less,
Al: 0.01 to 0.5%,
N: 0.010% or less and V: 0.10 to 1.0%
Include, and satisfied (10Mn + V) / C ≧ 50, the balance being Fe and unavoidable impurities, the volume ratio of the tempered martensite phase is 80% or more, a particle size: 20 nm carbide containing the following V An automobile member characterized in that 1000 / μm ³ or more is precipitated and the average particle size of the carbide containing V of 20 nm or less is 10 nm or less. The member according to claim 1, further comprising mass%.
Nb: 0.1% or less,
Ti: 0.1% or less and B: 0.0050% selected from among the following one or automotive parts, characterized in that a composition containing two or more. The member according to claim 1 or 2, further comprising mass%.
Cr: 0.005 to 1.0% and
Mo: 0.005 to 0.5%
An automotive member comprising a composition containing one or two selected from the above in a range satisfying (2Cr + Mo) /2V≦2.0. In any one of Claims 1-3, a member is further in the mass%.
Cu: 0.5-5.0%
An automotive member comprising a composition containing The member according to claim 4, further comprising:
Ni: 0.1-2.0%
An automotive member comprising a composition containing In mass% C: 0.10 to 0.25%,
Si: 1.5% or less,
Mn: 1.0-3.0%
P: 0.10% or less,
S: 0.005% or less,
Al: 0.01 to 0.5%,
N: 0.010% or less and V: 0.10 to 1.0%
Includes and satisfies (10Mn + V) / C ≧ 50, the balance being Fe and unavoidable impurities, the volume ratio of the ferrite phase is 60% or more, the particle size was determined for the following deposit 80 nm V After forming a steel plate having an average particle size of carbide containing 30 nm or less into a predetermined shape at room temperature, it is heated to a temperature not lower than the Ac ₃ transformation point and not higher than (Ac ₃ transformation point +200) ° C., and then 300 ° C. Cooling at an average cooling rate of 20 ° C./s or more to the following, and further tempering under conditions satisfying the following expression (1):
Record
11000−3000 [% V] ≦ Tb (20 + logt) ≦ 15000−1000 [% V] (1)
However, Tb ≦ Ac ₁ transformation point where [% V] is the V content (mass%)
Tb is the tempering temperature (℃)
t is the holding time at the tempering temperature (s) In mass% C: 0.10 to 0.25%,
Si: 1.5% or less,
Mn: 1.0-3.0%
P: 0.10% or less,
S: 0.005% or less,
Al: 0.01 to 0.5%,
N: 0.010% or less and V: 0.10 to 1.0%
Hints, and satisfied (10Mn + V) / C ≧ 50, the balance being Fe and unavoidable impurities, grain size with an average particle diameter of carbide containing V calculated for the following precipitates 80nm is 30nm or less A steel sheet is heated to a temperature not lower than the Ac ₃ transformation point and not higher than (Ac ₃ transformation point +200) ° C., formed into a predetermined shape, and then cooled to an average cooling rate of 300 ° C. or lower at a rate of 20 ° C./s or higher. A method for producing a member for an automobile, which is cooled and further subjected to tempering under conditions satisfying the following expression (1).
Record
11000−3000 [% V] ≦ Tb (20 + logt) ≦ 15000−1000 [% V] (1)
However, Tb ≦ Ac ₁ transformation point where [% V] is the V content (mass%)
Tb is the tempering temperature (℃)
t is the holding time at the tempering temperature (s) In mass% C: 0.10 to 0.25%,
Si: 1.5% or less,
Mn: 1.0-3.0%
P: 0.10% or less,
S: 0.005% or less,
Al: 0.01 to 0.5%,
N: 0.010% or less and V: 0.10 to 1.0%
It includes and satisfies (10Mn + V) / C ≧ 50, the balance being a steel slab having the composition of Fe and unavoidable impurities, was heated to above 1000 ° C., and a sheet bar by rough rolling and then finish rolling delivery temperature : After finish rolling under conditions of 800 ° C or higher, cool to 450 ° C or higher and to temperature Ta (° C) or lower expressed by the following formula (2), wind up, and then obtain the hot-rolled sheet obtained After forming into a specified shape at room temperature, heat it to a temperature of Ac ₃ transformation point or higher and (Ac ₃ transformation point +200) ° C. or lower, and then cool down to 300 ° C. or lower at an average cooling rate of 20 ° C./s or higher. And a method for producing a member for an automobile, further comprising tempering under conditions satisfying the following expression (1):
Record
11000−3000 [% V] ≦ Tb (20 + logt) ≦ 15000−1000 [% V] (1)
Ta (° C.) = [5500 / {6.7 + log ([% V] × [% C])}] − 400 (2)
However, Tb ≦ Ac ₁ transformation point Here, [% C] and [% V] are the contents of each element (mass%), respectively.
Tb is the tempering temperature (℃)
t is the holding time at the tempering temperature (s) In mass% C: 0.10 to 0.25%,
Si: 1.5% or less,
Mn: 1.0-3.0%
P: 0.10% or less,
S: 0.005% or less,
Al: 0.01 to 0.5%,
N: 0.010% or less and V: 0.10 to 1.0%
It includes and satisfies (10Mn + V) / C ≧ 50, the balance being a steel slab having the composition of Fe and unavoidable impurities, was heated to above 1000 ° C., and a sheet bar by rough rolling and then finish rolling delivery temperature : After finish rolling under conditions of 800 ° C or higher, the steel sheet is cooled to a temperature Ta (° C) or lower expressed by the following formula (2), wound up, and then rewound. After cold-rolled sheet by hot rolling, the cold-rolled sheet is heated to a temperature range not lower than Ac ₁ transformation point and not higher than (Ac ₃ transformation point +200) ° C., and maintained at this temperature range for 10 to 300 seconds, at least 500 ° C. The average cooling rate is 10 to 50 ° C./s until the cold-rolled annealed sheet is formed into a predetermined shape at room temperature, and then the Ac ₃ transformation point or higher (Ac ₃ transformation point +200). ) Heated to a temperature of ℃ or less, then cooled to 300 ℃ or less, average cooling rate: 20 ℃ / s or more, further down (1) A method of manufacturing automobile members and characterized by applying tempering treatment under conditions satisfying the equation.
Record
11000−3000 [% V] ≦ Tb (20 + logt) ≦ 15000−1000 [% V] (1)
Ta (° C.) = [5500 / {6.7 + log ([% V] × [% C])}] − 400 (2)
However, Tb ≦ Ac ₁ transformation point Here, [% C] and [% V] are the contents of each element (mass%), respectively.
Tb is the tempering temperature (℃)
t is the holding time at the tempering temperature (s) In mass% C: 0.10 to 0.25%,
Si: 1.5% or less,
Mn: 1.0-3.0%
P: 0.10% or less,
S: 0.005% or less,
Al: 0.01 to 0.5%,
N: 0.010% or less and V: 0.10 to 1.0%
It includes and satisfies (10Mn + V) / C ≧ 50, the balance being a steel slab having the composition of Fe and unavoidable impurities, was heated to above 1000 ° C., and a sheet bar by rough rolling and then finish rolling delivery temperature : After finish rolling under conditions of 800 ° C or higher, the steel sheet is cooled to a temperature Ta (° C) or lower expressed by the following formula (2), wound, and then rewound, and then the hot rolled sheet is moved to the Ac ₃ transformation point or higher. , and heated to (Ac ₃ transformation point +200) ° C. below the temperature, after formed into a predetermined shape, the average cooling rate to 300 ° C. or less: 20 ° C. / s cooled at speeds above, further the following equation (1) The manufacturing method of the member for motor vehicles characterized by performing a tempering process on the conditions which satisfy | fill.
Record
11000−3000 [% V] ≦ Tb (20 + logt) ≦ 15000−1000 [% V] (1)
Ta (° C.) = [5500 / {6.7 + log ([% V] × [% C])}] − 400 (2)
However, Tb ≦ Ac ₁ transformation point Here, [% C] and [% V] are the contents of each element (mass%), respectively.
Tb is the tempering temperature (℃)
t is the holding time at the tempering temperature (s) In mass% C: 0.10 to 0.25%,
Si: 1.5% or less,
Mn: 1.0-3.0%
P: 0.10% or less,
S: 0.005% or less,
Al: 0.01 to 0.5%,
N: 0.010% or less and V: 0.10 to 1.0%
It includes and satisfies (10Mn + V) / C ≧ 50, the balance being a steel slab having the composition of Fe and unavoidable impurities, was heated to above 1000 ° C., and a sheet bar by rough rolling and then finish rolling delivery temperature : After finish rolling under conditions of 800 ° C or higher, the steel sheet is cooled to a temperature Ta (° C) or lower expressed by the following formula (2), wound up, and then rewound. After cold-rolled sheet by hot rolling, the cold-rolled sheet is heated to a temperature range not lower than Ac ₁ transformation point and not higher than (Ac ₃ transformation point +200) ° C. Is cooled at an average cooling rate of 10 ° C / s or higher, and the obtained cold-rolled annealed sheet is then heated to a temperature not lower than Ac ₃ transformation point and not higher than (Ac ₃ transformation point +200) ° C. After cooling to 300 ° C or less, the product is cooled at an average cooling rate of 20 ° C / s or more to 300 ° C or less, and the following equation (1) is satisfied. Method for producing automotive parts, characterized in that performing the tempering treatment under the conditions of.
Record
11000−3000 [% V] ≦ Tb (20 + logt) ≦ 15000−1000 [% V] (1)
Ta (° C.) = [5500 / {6.7 + log ([% V] × [% C])}] − 400 (2)
However, Tb ≦ Ac ₁ transformation point Here, [% C] and [% V] are the contents of each element (mass%), respectively.
Tb is the tempering temperature (℃)
t is the holding time at the tempering temperature (s) The steel slab according to any one of claims 6 to 11, further comprising mass%.
Nb: 0.1% or less,
Ti: 0.1% or less and B: 0.0050% 1 kind selected from among the following or manufacturing process of automotive parts, characterized in that a composition containing two or more. The steel slab according to any one of claims 6 to 12, further in mass%.
Cr: 0.005 to 1.0% and
Mo: 0.005 to 0.5%
A method for producing a member for automobile, comprising a composition containing one or two selected from the above in a range satisfying (2Cr + Mo) /2V≦2.0. The steel slab according to any one of claims 6 to 13, further in mass%.
Cu: 0.5-5.0%
The manufacturing method of the member for motor vehicles characterized by comprising the composition containing this. 15. The steel slab according to claim 14, further comprising mass%.
Ni: 0.1-2.0%
The manufacturing method of the member for motor vehicles characterized by comprising the composition containing this.

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