JP2003013178A - High strength steel sheet superior in age hardening resistance and bake hardening property - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength steel sheet superior in aging resistance and bake hardening property. SOLUTION: This high strength steel sheet includes, by mass %, 0.06% to 0.3% C, 0.8% to 2% Si, 0.5% to 3% Mn, 0.01% or less (including 0%) sol.Al, 0.001%+(7/13)[sol.Al]<N<0.02% (where, [sol.Al] indicates the content (mass%) of sol.Al), 0.15% or less (excluding 0%) P, and 0.02% or less (including 0%) S, and includes, by space factor, 70% or more ferrite and 3-30% retained austenite. The steel sheet may further include one or more selected from the group consisting of bainite, pearlite, and martensite in the metal structure.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a high-strength steel sheet having excellent aging resistance and bake hardenability, and in particular, has excellent aging resistance without deterioration of properties such as workability with time, In addition, the present invention relates to a high-strength steel sheet which exhibits excellent bake hardenability (hereinafter sometimes referred to as BH [Bake Harding] property) capable of ensuring high strength by coating baking.

The present invention is widely used in the industrial fields of automobiles, electric machines, machines, etc., but in the following, as a typical application example, the case where it is used in the body of an automobile will be mainly described.

[0003]

2. Description of the Related Art In recent years, steel sheets for automobiles have been required to be thinned from the viewpoint of improving fuel efficiency, and to be further strengthened in order to improve safety in the event of a collision. Further, it is required that the molding process can exhibit excellent molding processability as represented by press working. However, since the improvement of strength is likely to cause deterioration of the formability, in the case of an automobile steel sheet that requires complicated processing,
As a characteristic of the steel sheet, it is required that the steel sheet is relatively soft at the time of forming and easy to be formed, and that a large amount of baking and hardening can secure high strength in coating baking after the forming.

Under these circumstances, a steel sheet for improving the above-mentioned bake hardenability has been proposed in the past, and a quasi-IF (Inte
In rstitial free type BH steel, dislocations are fixed by solid solution carbon of about 30 ppm to secure bake hardenability. However, since the quasi-IF type BH steel originally has a small amount of solid solution C, the strength obtained by bake hardening is as low as 440 MPa at most.

In DP steel (Dual Phase Steel),
The dislocations are introduced into the matrix ferrite by the martensitic transformation, and the YP value is low as it is. However, there is a case where the dislocations introduced by the above-mentioned dislocations and working are fixed by the baking of the paint and the YP is increased by hardening. .

[0006] Further, so-called TR, which aims at toughening by plastic working by allowing residual austenite (hereinafter sometimes referred to as residual γ) to be present in the metal structure in an amount of several% to several tens%, is called TR.
Some IP steels also have improved bake hardenability. For example, Japanese Patent Application Laid-Open No. 2001-11565 discloses a technique in which the amount of bake-hardening is increased in order to increase the collision absorption energy, with the objective of simultaneously achieving the collision safety of passenger cars and the weight reduction of the vehicle body.

Incidentally, as a mechanism capable of enhancing the bake hardenability in such TRIP steel, it is generally considered that C originally in the ferrite is fixed to the work dislocation like the DP steel. There is. However, this idea cannot explain how to secure a high bake hardening amount of 50 MPa or more. Residual γ is transformed to martensite by plastic working before bake hardening, C in the martensite is released, and during coating baking, the C is fixed to dislocations in the ferrite introduced by working and hardening occurs. It is considered to be a thing.

Among the TRIP steels having an excellent balance of strength and workability, it has been proposed that the bake hardenability at the time of coating baking is improved as described above.
Deterioration of characteristics over time, such as an increase in YP and a decrease in elongation,
That is, the occurrence of aging becomes a problem. As its generation mechanism,
Dislocations are introduced by skin pass during production or martensitic transformation that occurs during production. On the other hand, TRIP steel has residual γ containing a large amount of solute carbon. It is considered that C constituting γ diffuses and moves and is fixed to dislocations, resulting in deterioration of characteristics such as increase in YP and decrease in elongation. Therefore, even if it exhibits excellent formability immediately after the steel sheet is manufactured, the aging as described above proceeds thereafter,
There is a problem that the characteristics are deteriorated when the user handles the steel sheet.

[0009]

The present invention has been made in view of the above circumstances, and an object thereof is to provide a high-strength steel sheet having both excellent aging resistance and bake hardenability.

[0010]

The high-strength steel sheet excellent in aging resistance and bake hardenability of the present invention which can solve the above-mentioned problems means a mass% of C: 0.06% or more and 0.3% or more. Below, S
i: 0.8% or more and 2% or less, Mn: 0.5% or more and 3% or less, sol. Al: 0.01% or less (including 0%),
0.001% + (7/13) [sol.Al] <N <0.02%
([Sol.Al] indicates the content (mass%) of sol.Al), P: 0.15% or less (not including 0%), S:
0.02% or less (including 0%) is satisfied, and in the space factor, ferrite: 70% or more, retained austenite: 3
The present invention has the gist of containing at least 30%, and may further contain at least one selected from the group consisting of pearlite, bainite, and martensite.

Further, in the present invention, Cr in mass% is
And / or Mo containing 1% or less (not including 0%) in total; Ni: 0.5% or less (not including 0%) and / or Cu: 0.5% or less (0%) Not containing); Ti: 0.1% or less (0
%), Nb: 0.1% or less (0% is not included), and V: 0.1% or less (0% is not included), at least one selected from the group consisting of;
B: containing 0.003% or less (not including 0%); Ca: 0.003% or less (not including 0%),
And / or REM: those containing 0.003% or less (not including 0%) are both preferable embodiments of the present invention.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION Under the above-mentioned circumstances, the present inventors can maintain good workability without causing aging, and perform coating baking to secure high strength. With the aim of realizing a high-strength steel sheet with excellent bake hardenability, we have conducted intensive research. As a result, the presence of the solute N in the residual γ can effectively suppress the decomposition of the residual γ secured to obtain a good strength-workability balance, and the dislocation introduced during the production can be improved. It was found that the solid solution C of, that is, aging can be effectively suppressed. As a result of further pursuit of the quantitative action effect of the solid solution N, the present invention has been conceived.

First, in the steel sheet of the present invention, it will be explained that solid solution N was secured to stabilize the residual γ and the quantitative action and effect were examined. In addition, all the units of the following chemical components are mass%.

Sol. Al (acid-soluble Al): 0.01
% Or less (including 0%) 0.001% + (7/13) [sol.Al] <N <0.02% (the above [sol.Al] is the content (mass%) of sol.Al)
(Shown) The residual γ introduced to enhance the workability in the high-strength region is decomposed for some reason to constitute the residual γ.
To prevent dislocations from sticking to dislocations and causing aging,
It is necessary to suppress the decomposition of the residual γ. Solid solution N is an austenite stabilizing element and is known to be effective in stabilizing residual γ. However, the solid solution N
Is easy to form a metal component and nitride in steel,
In order to secure the solid solution N that effectively acts on the stabilization of the residual γ, it is necessary to limit the amount of coexisting metal elements. In the present invention, since the solid solution N easily forms a nitride with Al, the sol. Limiting the amount of Al (acid-soluble Al), solid solution N
It was decided to secure.

In order to secure a sufficient amount of solute N and to prevent the AlN from deteriorating the characteristics such as elongation and stretch flangeability even when AlN is formed, the above sol is used. ． It was found that the amount of Al needs to be 0.01% or less, preferably 0.005% or less.
In addition, the sol. As a method for reducing the amount of Al, it is possible to perform deoxidation in the steelmaking stage by using Si instead of Al.

On the other hand, the lower limit of the N content was also specified from the viewpoint of securing the amount of solid solution N. That is, the space factor described later is 3 to
It has been found that 0.001% or more of solid solution N is required to stabilize 30% of retained austenite and suppress generation of aging due to residual γ decomposition. Therefore, the total amount of N is the same as the sol. Considering the Al content, at least [0.001% + (7/13) [sol.Al]]
It turned out to be necessary.

On the other hand, if the N content is too high, bubbles will be generated in the ingot during the production, causing cracks and fractures in the hot rolling process. Therefore, the N content is 0.02% or less. Need to be kept to. It is preferably 0.01% or less.

Next, the reasons for defining other chemical components will be described below.

C: 0.06% or more and 0.3% or less C is 0.06% because it is an essential element for securing high strength and securing a predetermined amount of residual γ at room temperature.
Above, it is necessary to contain 0.1% or more.
However, if it is contained excessively, weldability and workability are deteriorated, so the content is controlled to 0.3% or less, preferably 0.2% or less.

Si: 0.8% or more and 2% or less Si is an element that effectively suppresses the decomposition of residual γ to form carbides, and is also useful as a solid solution strengthening element. In order to effectively exhibit such an effect, it is necessary to add Si by 0.8% or more, preferably 1.0% or more. However, the above effect is saturated even if the above elements are added in excess, and hot brittleness occurs, so the upper limit is made 2%. It is preferably 1.8% or less.

Mn: 0.5% to 3% Mn is an element necessary for stabilizing the austenite structure and obtaining a desired residual γ. In order to exert such an effect effectively, it is necessary to add 0.5% or more, preferably 1.0% or more. However, if added in excess of 3%, adverse effects such as slab cracking are observed. It is preferably 2% or less.

P: 0.15% or less (not including 0%) P is an element effective for ensuring a desired residual γ.
In order to effectively exhibit such an effect, it is recommended to add 0.03% or more, and more preferably 0.05% or more. However, if over 0.15% is added, the secondary workability deteriorates. It is more preferably 0.1% or less.

S: 0.02% or less (including 0%) S is an element that forms sulfide-based inclusions such as MnS and acts as a starting point of cracking and deteriorates workability.
% Or less, preferably 0.01% or less.

The basic chemical composition of the steel sheet of the present invention is as described above, but it is also effective to add the following elements.

Cr and / or Mo in total less than 1%
(0% is not included) Cr and Mo are elements that are effective in improving hardenability and increasing the strength of steel. Therefore, Cr and / or Mo should be added in a total amount of 0.1% or more. Is recommended.
However, if added excessively, ferrite transformation and bainite transformation will be suppressed too much and the formation of residual γ will be hindered. Therefore, it is preferable to keep Cr and / or Mo to 1% or less in total, and more preferably 0% in total. .8%
It is the following.

Ni: 0.5% or less (not including 0%)
And / or Cu: 0.5% or less (not including 0%) These elements are elements that are useful as a strengthening element for steel, and are effective for stabilizing residual γ and securing a predetermined amount. In order to effectively exhibit such an effect, Ni: 0.
It is recommended to add 1% or more, and / or Cu: 0.1% or more. However, even if these elements are added excessively, the above effect is saturated, so N
It is preferable to suppress i: 0.5% or less, and / or Cu: 0.5% or less.

At least one of Ti: 0.1% or less (not including 0%), Nb: 0.1% or less (not including 0%), V: 0.1% or less (not including 0%) These elements have effects of precipitation strengthening and microstructure refinement, and are useful elements for increasing strength. In order to effectively exhibit such an effect, Ti: 0.01% or more (more preferably 0.02% or more), Nb: 0.01% or more (more preferably 0.02% or more), V: It is recommended to add 0.01% or more (more preferably 0.02% or more), respectively. However, if any element is added in excess of 0.1%, the above effect is saturated and it is economically useless. More preferably, Ti: 0.08% or less, Nb: 0.
08% or less and V: 0.08% or less.

B: 0.003% or less (not including 0%) B is an element having the effects of improving hardenability and increasing the strength of steel, so 0.0005% or more is recommended to be added. It However, excessive addition causes embrittlement of the grain boundaries and causes cracking during casting or rolling, so
The upper limit is 0.003%, preferably 0.002%.

Ca: 0.003% or less (not including 0%
And / or REM: 0.003% or less (not including 0%) Ca and REM (rare earth elements) are elements effective for controlling the form of sulfide in steel and improving workability. Here, examples of rare earth elements used in the present invention include Sc, Y, and lanthanoids. In order to effectively exhibit the above-mentioned effects, each of them is 0.0003% or more (more preferably 0.0003% or more).
05% or more) is recommended. However, 0.0
Even if added over 03%, the above effect will be saturated,
It is economically wasteful. More preferably, it is 0.0025% or less.

The preferred elements contained in the present invention are as described above, and the balance component is substantially Fe. Needless to say, the inclusion of a trace amount of unavoidable impurities in the steel sheet is allowed.
It is also possible to use a steel plate positively containing other elements within a range that does not adversely affect the operation of the present invention.

Further, in the steel sheet of the present invention, its metallographic structure is also defined as follows in order to secure excellent workability in the high strength region.

Ferrite: 70% or more in space factor In order to secure excellent elongation characteristics and workability at high strength, ferrite in the metal structure is 70% in space factor.
Above, it is necessary to secure at least 75%. However, if the space factor of ferrite is too high, it becomes difficult to secure the required strength, and the conventional composite structure or TRI
Similar to P steel, many voids are generated at the interface between the ferrite and the second phase and the stretch flangeability deteriorates. Therefore, it is recommended to set the upper limit to 95%.

Retained austenite (retained γ): in space factor
The residual γ of 3 to 30% is useful for improving the total elongation, and it is necessary to secure the residual γ in the space factor of 3% or more, preferably 5% or more in order to effectively exert such an effect. is there. On the other hand, if the space factor of the residual γ becomes too large, the stretch flangeability deteriorates, so the upper limit was set to 30%. It is more preferably 25% or less.

Others: Perlite, bainite, and
One or more selected from the group consisting of martensite
The steel sheet of the present invention ( including 0% in terms of product ratio) may be composed only of the above-described structure (that is, a mixed structure of ferrite and residual γ), but other types of different types are possible as long as the action of the present invention is not impaired. The structure may contain one or more selected from the group consisting of pearlite, bainite, and martensite. Although these structures can inevitably remain in the production process of the present invention, the smaller the amount, the more preferable.

The method for producing the steel sheet of the present invention is not particularly limited, but in order to obtain the structure of the present invention, it is recommended to produce it by the following method.

That is, in the hot rolling step, it is recommended that the hot rolling finishing temperature (FDT) is a temperature of A _r3 point or higher as the condition. This avoids rolling in the two-phase region,
This is because it is effective in obtaining the desired tissue.

Further, in the cooling after the finish rolling, a desired structure can be obtained by controlling the cooling rate (CR). Particularly, in the case of producing a hot rolled steel sheet, after the finish rolling, 30 ℃ up to 700 ± 100 ℃
Cooling at an average cooling rate of / s or more, and air cooling in the temperature range is 1
It is preferable to carry out a two-stage cooling in which the heating is carried out at 350 to 450 ° C. after being carried out for about 30 seconds, because the residual γ can be secured.

In the case of cold-rolling after hot-rolling, it is not necessary to control the cooling rate, but in order to reduce the load during cold rolling, hardening of the hot-rolled steel sheet is suppressed and ductility deterioration is suppressed. From the viewpoint of suppressing the precipitation of AlN, which is the cause, it is desirable that the winding is performed at 600 ° C. or higher.

When the continuous annealing treatment or the plating treatment is performed, the heat treatment method is as follows: heating and holding at a temperature of A ₁ point or more and A ₃ point or less for 10 to 600 seconds, and then at an average cooling rate of 3 ° C./s or more. It is recommended to cool to a temperature of 300 ° C. or higher and 480 ° C. or lower while avoiding the pearlite transformation and maintain the temperature range for 1 second or longer to generate a desired mixed structure and residual γ.

In order to secure a better elongation in addition to the bake hardenability, it is recommended that the cooling after the annealing be performed as follows. That is, by cooling to a temperature of (A ₁ point to 600 ° C.) at an average cooling rate of 15 ° C./s or less, ferrite is generated and C in the ferrite is concentrated to γ, and then 300 This is a method of cooling to a temperature of ℃ to 480 ° C at an average cooling rate of 20 ° C / s or more to suppress the transformation of γ into pearlite and obtain a residual γ structure.

When the plating treatment is carried out, the plating layer may be alloyed at 400 to 650 ° C.

[0042]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to the following examples, and may be appropriately applied within a range compatible with the gist of the preceding and the following. Modifications can be made and implemented, and all of them are included in the technical scope of the present invention.

Steel materials having the chemical composition shown in Table 1 were vacuum-melted to obtain experimental slabs, which were then hot-rolled. The experiment No. in Table 2 described later. In 1-17,
The heating performed during hot rolling was performed at 900 ° C., and the winding was performed at 600 ° C. to obtain a hot-rolled steel sheet having a sheet thickness of 2.4 to 3.2 mm. Further, the obtained hot-rolled steel sheet was pickled and then cold-rolled to obtain a steel sheet having a plate thickness of 1.0 to 1.6 mm. After that, the experiment No. Nos. 1 to 16 are continuous plating lines (CGL), and Experiment No. No. 17 was heat-treated in a continuous annealing line (CAL) and then wound with a skin pass having a surface reduction rate of 0.5 to 2%. In addition, the experiment No. In No. 18, heating was performed at 900 ° C. during hot rolling, and winding after finish rolling was performed at 380 ° C. to obtain a as-hot-rolled steel plate having the above plate thickness.

The metal structure of the steel sheet thus obtained is
After repeller corrosion, scanning electron microscope (SE
M) was observed, and the area ratio of other structures such as ferrite and bainite was determined from the structure photograph. Residual γ
The X-ray measurement was performed to determine the space factor.

The tensile strength (YS), elongation [total elongation (El)], bake hardening amount (BH amount), and natural aging amount of the obtained steel sheet were measured in the following manner.

First, a JIS No. 5 test piece was used for the tensile test, and tensile strength (YS) and elongation [total elongation (El)] were used.
Was measured. The bake hardening amount (BH amount) is a value obtained by measuring the 2% pre-strain after manufacturing the steel sheet and after performing the heat treatment on the steel sheet at 170 ° C. for 20 minutes to obtain the stress difference. is there.

The aging resistance was not evaluated by a commonly used accelerated test (AI value), but was evaluated by determining the natural aging amount by allowing it to stand at room temperature. Specifically, a tensile test was performed on a steel sheet immediately after production and a steel sheet left at room temperature for 3 months after production, and the difference in YP average value was taken as the natural aging amount. The tensile test was performed using JIS No. 5 test piece in the same manner as above, and the YP
The average value is the average of the values measured three times immediately after the steel sheet was manufactured and three months after the steel sheet was manufactured. The results are shown in Table 2.

[0048]

[Table 1]

[0049]

[Table 2]

From these results, the following can be considered. In addition, the following No. Are all Experiment Nos. In Table 2. Is shown.

First, No. 2, 4, 6 and 9-18 are
Both satisfy the requirements specified in the present invention,
It can be seen that a steel sheet with good characteristics is obtained.

On the other hand, in the following examples which do not satisfy any of the requirements defined by the present invention, the following problems have occurred. That is, No. Since No. 1 had a small amount of C, sufficient strength could not be secured, resulting in poor strength-ductility balance. Moreover, it has not been possible to secure a sufficient bake hardening amount. No. 3, 5, and 7 are sol. A
Since the amount of l was too large and all of the contained N was used for the formation of AlN, and solid solution N could not be secured, natural aging occurred. In addition, No. In No. 8, since the Si content was too small to secure a sufficient residual γ, elongation was unfavorable and natural aging occurred.

[0053]

The present invention is constituted as described above, and by ensuring the solid solution N as specified in the present invention, excellent aging resistance capable of maintaining a good strength-workability balance. It was possible to obtain a steel sheet having the above-mentioned properties and exhibiting excellent bake hardenability during coating baking. The realization of such a steel sheet has made it possible to provide a stable steel sheet without deterioration of the excellent strength-workability balance over time.

Continued front page    (72) Inventor Koichi Makii             1-5-5 Takatsukadai, Nishi-ku, Kobe City Stock Association             Company Kobe Steel Works, Kobe Research Institute (72) Inventor Shigenobu Namba             1-5-5 Takatsukadai, Nishi-ku, Kobe City Stock Association             Company Kobe Steel Works, Kobe Research Institute (72) Inventor Shunichi Hashimoto             1 Kanazawa Town, Kakogawa City, Hyogo Prefecture             To Steel Works, Kakogawa Works

Claims (6)

[Claims] 1. In mass%, C: 0.06% or more and 0.3% or less, Si: 0.8% or more and 2% or less, Mn: 0.5% or more and 3% or less, sol. Al: 0.01% or less (including 0%), 0.001% + (7/13) [sol.Al] <N <0.02%
([Sol.Al] indicates the content (mass%) of sol.Al) P: 0.15% or less (not including 0%), S: 0.02% or less (including 0%) ,and,
In terms of space factor, ferrite: 70% or more, retained austenite: 3 to 30%, and further selected from the group consisting of pearlite, bainite, and martensite 1
A high-strength steel sheet excellent in aging resistance and bake hardenability, characterized in that it may contain one or more kinds. 2. The high strength excellent in aging resistance and bake hardenability according to claim 1, further comprising Cr and / or Mo in a total amount of 1% or less (not including 0%) in mass%. steel sheet. 3. The method according to claim 1, further comprising, in mass%, Ni: 0.5% or less (0% is not included), and / or Cu: 0.5% or less (0% is not included). A high-strength steel sheet excellent in aging resistance and bake hardenability according to 2. 4. In mass%, Ti: 0.1% or less (0% is not included), Nb: 0.1% or less (0% is not included), and V:
The high-strength steel sheet excellent in aging resistance and bake hardenability according to any one of claims 1 to 3, containing at least one selected from the group consisting of 0.1% or less (not including 0%). 5. Further, in mass%, B: 0.003% or less (not including 0%) is contained, and the high aging resistance and bake hardenability are excellent. Strength steel plate. 6. Further, in mass%, Ca: 0.003% or less (not including 0%), and / or REM: 0.003% or less (not including 0%).
A high-strength steel sheet having excellent aging resistance and bake hardenability according to any one of claims 1 to 5.