JP2001107144A - Method for producing hot rolled steel sheet excellent in formability and hardenability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a hot rolled steel sheet, in which sufficient formability applicable to parts for car body structure, or the like, is obtained and the strength can easily be made high by quenching after forming. SOLUTION: In the method for producing a hot rolled steel sheet, a steel slab containing, by mass, 0.05 to 0.2% C, <=0.1% Si, 0.8 to 2% Mn, <=0.02% P, <=0.02% S, <=0.005% N, 0.0003 to 0.004% B, sol.Al of 0.01 to 0.1% and also >=9.6×N(%) and Ti: <=3.4×N(%), and the balance iron with inevitable impurities is hot-rolled and is coiled at >=480 deg.C to form into a hot rolled coil, and after that, the coil is annealed at >=500 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a hot-rolled steel sheet having excellent formability and hardenability, which is used for a material such as an automobile body structural member.

[0002]

2. Description of the Related Art Data on the destruction of a vehicle body in the event of an automobile collision is disclosed, and the data governs the sales of the vehicle. For example, the need for ensuring the safety of passengers is increasing year by year, and legal regulations are becoming stricter. There is a tendency. The car body has a so-called crushable body structure,
In the event of a collision, some parts deform moderately to absorb the impact,
The cabin has a high-strength structure and is designed to protect passengers inside the vehicle. In other words, higher strength is required for a part of the vehicle body. For this reason, use thick molded products in the necessary parts,
Countermeasures such as attaching a reinforcing material or forming using a high-strength steel plate have been taken.

However, increasing the plate thickness leads to an increase in weight, which conflicts with the demand for improved fuel efficiency or energy saving. For reinforcement, spot welding of rib material and parts for shock absorption are incorporated, but this also increases the weight of the car body.
In practice, it has been widely adopted to incorporate hardened and reinforced reinforcing members made of steel pipes inside the doors of vehicles. Also,
High-strength steel sheets are inferior in press formability, so that complicated press working is difficult. In addition, the residual stress due to processing increases as the strength increases, and the risk of delayed fracture increases.
There is a limit to increasing the strength of a steel sheet.

In order to reinforce the vehicle body structure, the desired portion is locally heated by high frequency without increasing the thickness of a steel plate molded part applied to a specific portion of the vehicle body that needs to be reinforced and using a high-strength steel plate. Japanese Patent Application Laid-Open No. Hei 6-116630 discloses a method of improving the strength by performing a quenching process. If quenching and strengthening are performed after press forming, it is possible to eliminate the difficulty of forming a complicated shape and the problem of large residual stress after forming as in the case of forming a high-strength steel sheet. However, although the above-mentioned publication discloses an induction hardening method, there is no description about the type of steel plate to be applied.
Some steel sheets do not show an increase in strength due to quenching. Particularly, steel sheets having better press formability cannot be expected to have quenched reinforcement.

Further, Japanese Patent Application Laid-Open No. 10-17933 discloses an invention of a method of changing the hardness corresponding to the strength distribution by local induction hardening in order to obtain a strength distribution required for a press-formed part of a vehicle body. It has been disclosed. As an example of the application, a pillar, also called a center pillar, arranged between a front seat and a rear seat on the side of the vehicle body is taken up. The expected load distribution of the strut at the time of collision is greatest at the center in the length direction, and in order to respond to the collision load, the conventional method increases the thickness of the entire strut or welds by overlapping the reinforcement at the center I needed to. On the other hand, in the above method, the central part in the longitudinal direction is hardened by induction hardening so that the thickness is not increased to an unnecessary part, and a reinforcing material is not required, and the weight is reduced. is there. In this case, a steel sheet having a C content of 0.05 to 0.25% can be used as the material steel sheet for supporting column formation, and is JIS standard SAPH370 to SAPH440 or SPFC3.
Steel plates such as 70 to SPFC440 are considered to be good.

[0006] However, in the steel sheets of these standards, the mechanical properties of the steel sheet as a material such as strength and elongation are guaranteed, but the hardenability and the strength after quenching are completely unknown.

Although not a vehicle body structural member, a part obtained by processing a steel pipe having a required strength by quenching is widely used as a reinforcing material incorporated in a door for protecting the inside of a vehicle cabin against a side collision. Many of these are manufactured by processing the end of an electric resistance welded steel pipe made of a steel plate into a state suitable for installation inside a door, and induction hardening.

For example, JP-A-4-52249 discloses that C: 0.10 to 0.20%, Mn: 0.2 to 1.5.
0%, Si: 0.05 to 0.50%, Al: 0.01 to
0.10%, Ti: 0.01 to 0.10% and B:
An ERW steel pipe made of a steel sheet containing 0.0005 to 0.010% is induction hardened and has a tensile strength of 1275 MPa (1
The invention of an automobile reinforcing material of 30 kgf / mm ² or more is disclosed. If a steel plate made of such steel is used for a vehicle body structural member, it is thought that local strength can be improved by induction hardening as described above, but even if simple bending such as ERW steel pipe is possible, It is considered difficult to apply to complicated press forming such as a vehicle body structure.

As a steel sheet to be used after being subjected to quenching and tempering, there is a polished strip steel of JIS-G3311. In this case, even the steel with the lowest carbon content excluding the alloy steel has a C: 0.2
7 to 0.33%, Si: 0.15 to 0.35%, Mn:
0.60 to 0.90%, and the content of each of C, Si and Mn is high. Even if sufficient strength can be obtained by quenching, workability as an annealed material steel sheet is poor. The required press workability cannot be obtained.

[0010]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing a hot-rolled steel sheet having sufficient formability applicable to parts for vehicle body structure and the like, and which can be easily strengthened by quenching after forming. To provide.

[0011]

SUMMARY OF THE INVENTION The present inventors have sought to enhance the structure of an automobile body by performing quenching after forming, without significantly changing the shape of members and the strength of applied materials. Various experiments and examinations were conducted to develop a thin steel plate that can be used. As a result, the following findings were obtained.

A) The strength required for strengthening the vehicle body structure is 785 to 1765 M in consideration of the performance and economy of steel.
If the composition is designed to be about Pa (80 to 180 kgf / mm ² ) and sufficient quenching can be achieved with low C steel with a carbon content of 0.05 to 0.2%, it is easy to secure workability. Become.

B) In order to improve hardenability, it is not preferable to use hardenability improving elements such as Cr and Si. This is because a thin steel sheet for an automobile must have good plating property and chemical conversion treatment property of a paint base for corrosion protection, but these are hindered. In addition, since cost reduction is strictly required, expensive additive elements should be avoided.

C) In general, the content of the hardenability improving element should be as low as possible, since the hardenability improving element hardens the steel and reduces the workability when the added amount thereof is increased.

D) It is best to effectively use Mn and B as hardenability improving elements. Mn has an effect of greatly improving hardenability, is inexpensive, and has little effect on plating properties and chemical conversion treatment properties. However, when the content is large, the workability deteriorates and the corrosion resistance as steel tends to deteriorate, so that the amount of addition is limited. In order to compensate for this, it is preferable to use B in combination.

E) Since the effect of B is exerted with a small amount of addition, the hardenability of the steel can be greatly improved without substantially affecting other general properties of the steel.

F) However, in order to improve the quenchability by adding B, it is necessary to keep a small amount of added B in a solid solution state. Conventionally, B is easily oxidized. In this case, N, which easily binds to B, was fixed by adding Ti. However, steel containing B and Ti in combination has poor workability and is not suitable for use in automobile parts. The cause lies in Ti added to fix N.

G) That is, in order to fix N sufficiently,
Conventionally, the steel contains Ti in such a manner that all N in the steel becomes TiN, but this excess Ti becomes TiC and precipitates in the steel, which significantly deteriorates elongation.

H) In order to prevent the precipitation of TiC and secure the workability, the Ti content may be set to 3.4 times or less of the N content, but N not fixed by Ti is replaced by BN. And the amount of solid solution B decreases, so that sufficient strength cannot often be obtained after quenching if hot rolling is performed.

I) However, the quenching property can be remarkably improved by annealing the hot-rolled sheet. It is considered that the reason is that BN precipitated on the hot-rolled sheet is decomposed during annealing, N is re-precipitated as more stable AlN after cooling, and B remains in a solid solution state.

J) However, in order to precipitate AlN, it is necessary to contain a sufficient amount of Al with respect to N, and the amount of sol. Al needs to be 9.6 times or more of the N content. is there.

The present invention has been made based on the above findings, and the gist thereof is as follows.

In mass%, C: 0.05-0.2%, S
i: 0.1% or less, Mn: 0.8 to 2%, P: 0.02
%, S: 0.02% or less, N: 0.005% or less,
B: 0.0003-0.004%, sol. Al: 0.0
1 to 0.1% and 9.6 × N (%) or more, and T
i: A steel slab containing 3.4 × N (%) or less, the balance being iron and unavoidable impurities is hot-rolled and wound at a temperature of 480 ° C. or more to form a hot-rolled coil, and then a temperature of 500 ° C. or more. A method for producing a hot-rolled steel sheet having excellent formability and hardenability, which is annealed at a temperature.

[0024]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for limiting the chemical composition and production conditions of steel according to the present invention will be described below. In addition,% display of a chemical composition shows all mass%.

1) Chemical composition C: C is an important element for determining the strength of steel after quenching. If the C content is less than 0.05%, the strength required for strengthening the vehicle body cannot be obtained even if sufficient quenching is performed. On the other hand, if the content exceeds 0.2%, press formability becomes insufficient. Therefore, the C content is 0.05 to
0.2%. More preferably, 0.05% or more.
Less than 1%.

Si: Si is set to 0.1% or less. Si is generally used for deoxidizing steel, and has an effect of increasing strength and improving hardenability. However, when B is added, deoxidation by Si is not sufficient, and in the present invention, Al having a stronger deoxidizing effect is used, so that it is not particularly required. In the case of a thin steel sheet used for an automobile body, plating and painting are performed, but since the presence of Si deteriorates the plating property of the steel surface and the chemical conversion treatment property of the paint base, the content is preferably as small as possible. Therefore, it is set to 0.1% or less as a range having almost no influence, but it is preferable to set the range to 0.0% or less.
It is at most 4%, more preferably at most 0.01%.

Mn: The content of Mn is set to 0.8 to 2%. In the present invention, Mn is an important element for securing the hardenability of steel. If it is less than 0.8%, hardenability may be insufficient, and sufficient strength may not be obtained. On the other hand, when the content exceeds 2%, the workability of the steel such as a decrease in elongation is deteriorated, and further, the steel itself tends to rust and the corrosion resistance is deteriorated. Preferred is 0.8-1.5%.

P: P is one of the unavoidable impurities of steel, and deteriorates the toughness of the steel after quenching. Therefore, the smaller the content of P, the better. The range that does not exert a significant adverse effect is limited to 0.02% or less, but is preferably 0.005% or less.

S: S is also one of the inevitable impurities and degrades the workability and toughness of steel, so the smaller the content, the better. Like P, the range of not having a significant adverse effect was limited to 0.02% or less, but is preferably 0.005% or less.

N: N is set to 0.005% or less. N binds to B and inhibits B's hardenability improving effect. In order to suppress the adverse effect of N, it is made harmless by combining with Ti or Al. However, the resulting fine precipitates such as TiN and AlN impair the workability of steel, so that if the N content is small, The smaller the better, the better. The content of N is set to 0.005% or less so as not to show a noticeable effect.

B: The content of B is 0.0003 to 0.0
04%. B remarkably improves the hardenability, and the effect is particularly large as the content of C is reduced as in the present invention. The effect of improving the hardenability is almost constant irrespective of the content, but if it is less than 0.0003%, the effect is not sufficiently exhibited. Further, if the content increases, the steel becomes brittle, so that the content is limited to at most 0.004%.

Sol. Al: The content of sol. Al (acid-soluble Al) is 0.01 to 0.1%. Al is mainly added for the purpose of deoxidizing molten steel during steelmaking. If the sol.Al remaining in the steel due to the deoxidation treatment is less than 0.01%, the deoxidation is insufficient, and solid solution B may not remain due to oxidation.
In the case of the present invention, at least 0.01% or more of sol.Al must be contained in order to sufficiently fix N. However, even if a large amount is contained, the effect is saturated and the cost is increased only by meaningless addition.
%.

Sol.Al (%) ≧ 9.6 × N (%) · Al is sufficiently contained in order to prevent the effect of adding B from being inhibited by N. Therefore, it is necessary to contain sol.Al satisfying the following expression (1). This equation is the Al required to make all N in steel AlN.
Means that sol.Al is contained in an amount at least 5 times the chemical equivalent of

Sol.Al (%) ≧ 9.6 × N (%) (1) By including a sufficient amount of Al with respect to N, N can be rendered harmless.

Ti: Ti is an element to be contained if necessary, and when contained, the content is such that Ti (%) ≦ 3.4 × N (%) (2). Ti is extremely effective for fixing N, and when B is added, it is generally added for the purpose of fixing N. In the case of the present invention, even if Ti is not added, N is fixed as long as the content of Al satisfies the above formula (1). However, it is preferable to add a small amount of Ti in order to securely fix N and exert the effect of B more. However, if a large amount is added, the workability deteriorates, and it is considered that the workability deterioration is caused by the formation of TiC.
Was kept within a range where TiC was not formed, that is, within a range satisfying the above-mentioned formula (2). In addition, in terms of workability deterioration, the effect will be affected if the amount of TiN increases, so that even if the content of Ti is within the range of the expression (2), the content of.
It is preferably at most 01%. When N is contained for the purpose of fixing, it is desirable to contain at least 0.002% or more.

2) Manufacturing method The slab obtained by melting steel having the above chemical composition and then hot-rolling the slab and the conditions thereof may be the same as usual, and there is no particular restriction. However, the winding temperature after hot rolling needs to be 480 ° C. or higher. This is because winding at a temperature lower than 480 ° C. may be quenched by cooling to the winding temperature, and the steel sheet is significantly hardened. Note that if the winding temperature is increased, uneven cooling is likely to occur, and it is difficult to ensure uniformity of the characteristics over the entire length of the coil.
It is desirable to wind the film at a temperature lower than 0 ° C.

The reason why the hot-rolled steel sheet is annealed is to improve the hardenability. It is considered that the quenching property is improved by annealing because the amount of solid solution B effective for improving quenching property is insufficient when hot-rolled, but increases due to annealing of hot-rolled steel sheet.

N in steel that could not be fixed by Ti
When the temperature decreases, if Al or B is present,
Precipitates such as N and BN are formed. However, when these two elements coexist, B has a diffusion rate in steel of Al
BN is formed mainly in the cooling process after rolling, and the amount of solute B in the hot-rolled sheet decreases.
However, since AlN is more stable than BN, if sufficient Al is present, annealing of the hot-rolled sheet will cause BN to decompose and become all AlN, and B will be dissolved. It is speculated that as a result, hardenability is improved.

If the annealing temperature is lower than 500 ° C., it takes too much time for BN to decompose and form AlN, and productivity is impaired. Desirably, it is 700 ° C. or higher. The upper limit is preferably 850 ° C.

[0040]

EXAMPLE A steel having the composition shown in Table 1 was produced by using an experimental vacuum melting furnace. As the steel composition, C: 0.12%, M
n: 1.5%, sol. Al: 0.045%, N: 0.
003%, Ti: 0.005%, and B: 0.0006% were set as target reference compositions. To determine the effects of the contents of Mn, Ti, B, Al, and N, the respective contents were changed. The components were kept as constant as possible.

[0041]

[Table 1]

These ingots were forged into slabs for hot rolling having a thickness of 20 mm, heated to 1200 ° C., and then hot-rolled in a temperature range from 1150 ° C. to 900 ° C. to finish them to a thickness of 4 mm. Coil after cooling to 550 ° C or 580 ° C under cooling conditions assuming a hot strip mill by forced air cooling or water spray, immediately put the obtained steel sheet into a furnace maintained at 550 ° C or 580 ° C, and winding the coil. It cooled on the conditions assumed.

From the obtained hot rolled steel sheet, a thickness of 1.2 mm
A test piece having a width of 90 mm and a length of 240 mm was sampled and, as it was, or after being subjected to a hot-rolled sheet annealing at 700 ° C. for 20 hours, a heating temperature of 950 ° C. for 2 seconds by an induction quenching apparatus having an output of 200 kW and a frequency of 150 kHz. After the holding, a quenching treatment was performed under water cooling conditions. After that, JIS5
The test piece was molded into a tensile test piece.

Table 2 shows the test results of the steel sheet as hot rolled and after the hot rolled sheet annealing.

[0045]

[Table 2]

As is clear from Table 2, as shown in Test Nos. 1 to 4, as the Mn content increases, the elongation decreases in the state of the material before quenching and the workability deteriorates.
Strength after quenching increases. It can be seen that the elongation before processing is sufficiently large and the strength after quenching is large within the range specified in the present invention.

Regarding the effect of the Ti content on the elongation of the steel sheet before quenching, as can be seen from the results of Test Nos. 5 to 9, as the Ti content increases, the elongation decreases and the workability deteriorates.

FIG. 1 shows the relationship between Ti (%) /
It is a figure which shows the relationship between N (%) and elongation before quenching of the hot-rolled steel plate which performed the hot-rolled sheet annealing.

FIG. 1 shows that Ti (%) / N (%) is 3.4%.
That is, it can be understood that elongation is deteriorated when an excessive amount of Ti is contained in an amount that makes all of Ti become TiN.

The effect of B can be seen in Test Nos. 10 to 12, but if it is less than 0.0003%, sufficient strength cannot be obtained.
Hardenability becomes insufficient.

FIG. 2 is a graph showing Al (%) /
FIG. 4 is a graph showing the relationship between N (%) and the tensile strength after quenching;
Nos. 13 to 20 with different amounts and N amounts, and Mn, Ti
FIG. 9 is a diagram in which the results of specimens having substantially the same amount of B and the like are combined.

From FIG. 2, the atomic weight ratio of Al and N is 1.93.
Where 5 times the atomic weight ratio, ie, Al (%) /
When N (%) is less than 9.6, it is understood that the tensile strength is low and the hardenability is insufficient.

As is clear from Table 2, the test number 25
Although the chemical composition of the steels No. to No. 32 is within the range of the present invention, the strength after quenching is insufficient because the hot-rolled sheet annealing has not been performed.

[0054]

According to the present invention, it is possible to obtain a hot-rolled steel sheet which has a sufficient formability applicable to processing such as press forming and which can be easily strengthened by quenching after forming. By utilizing this hot-rolled steel sheet for, for example, the body structure of an automobile, it is possible to make the cabin structure stronger and lighter. This satisfies social demands such as enhancing the safety of passengers in the event of an automobile collision and saving energy by reducing the weight of the automobile.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the ratio between the Ti content and the N content of steel and the elongation of the steel sheet.

FIG. 2 is a diagram showing the relationship between the ratio of the Al content to the N content of steel and the tensile strength of the steel sheet after induction hardening.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4K037 EA01 EA02 EA05 EA06 EA15 EA18 EA23 EA25 EA27 EA31 EB05 FE01 FE02 FF01 FF02 FF03 FM04 JA06

Claims (1)

[Claims] (1) In terms of mass%, C: 0.05 to 0.2%, S
i: 0.1% or less, Mn: 0.8 to 2%, P: 0.02
%, S: 0.02% or less, N: 0.005% or less,
B: 0.0003-0.004%, sol. Al: 0.0
1 to 0.1% and 9.6 × N (%) or more, and T
i: A steel slab containing 3.4 × N (%) or less, the balance being iron and unavoidable impurities is hot-rolled and wound at a temperature of 480 ° C. or more to form a hot-rolled coil, and then a temperature of 500 ° C. or more. A method for producing a hot-rolled steel sheet having excellent formability and hardenability, characterized by annealing at a temperature.