JP3333414B2 - High-strength hot-rolled steel sheet for heat curing with excellent stretch flangeability and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention has good moldability,
A high-strength hot-rolled steel sheet for heat-hardening that can be further improved in strength by applying a heat-curing heat treatment after molding, and this steel sheet is required to have strength and rigidity, toughness, fatigue properties, etc., mechanical structural parts, automobile parts, It is suitably used as a general steel sheet product.

[0002]

2. Description of the Related Art In recent years, steel sheets for automobiles have been widely reduced in strength and thickness from the viewpoints of improving safety and fuel efficiency of automobiles. Since these steel sheets are formed by cold working such as pressing, excellent formability is also required. In the case of a hot-rolled steel sheet, particularly, stretch flangeability is often regarded as important. On the other hand, a structural member is required to have high strength and excellent fatigue properties, but as the strength increases, workability generally deteriorates.

[0003] Therefore, various steel plates have been developed which have excellent workability during cold working, and which are then subjected to a predetermined hardening heat treatment to increase the strength. For example, Japanese Patent Application Laid-Open Publication No.
Some are already industrialized and put to practical use, as described in JP-A-79717 and JP-A-2-19747. However, this type of steel sheet requires a relatively large amount (about 1%) of expensive Cu to be added in order to secure a sufficient increase in strength. In addition, in order to prevent cracking caused by slab heating caused by Cu, After all expensive N
It is necessary to add i at the same time, which leads to an increase in manufacturing cost. Cu that cannot be removed during scrap refining also becomes an obstacle from the viewpoint of recycling.

As disclosed in JP-A-2-15145 and JP-A-3-277717, the T
Hot rolled steel sheets for heat hardening utilizing precipitation strengthening elements such as i and Nb have also been developed. All of these are intended to improve the strength by dissolving Ti or the like in a hot-rolled steel sheet, performing forming and then performing a heat-curing heat treatment to precipitate as carbides.

[0005]

However, these hot-rolled hot-rolled steel sheets are mainly developed with a view to improving strength by heat treatment. Therefore, although the material before heat treatment has low strength, The workability is not necessarily superior to other high workability steel sheets. In particular, stretch flangeability is an important work property, but satisfactory properties are not obtained.

[0006] In high-strength steel sheets for heavy working,
Steel sheets excellent in stretch flangeability have already been provided. For example, Japanese Patent Application Laid-Open No. 57-101649 discloses Nb, T
It is described that a high-strength hot-rolled steel sheet having excellent stretch flangeability can be obtained by controlling the volume ratio of ferrite and bainite for steel containing i. Japanese Patent Application Laid-Open No. 6-172920 discloses an excellent TS-
A method is disclosed in which a steel to which Nb and Ti are added in combination to obtain El balance and stretch flangeability is hot-rolled, and then cooled to form a ferrite + bainite structure.

However, these high-strength steel sheets are made of Nb, T
Although i was added, no consideration was given to heat-curability, so that a sufficient strength increase could not be obtained stably.

The present invention has been made in view of such a problem, and has a strength of 690 N / mm ² or more as hot rolled and an excellent stretch flangeability, and has a higher strength by heat curing heat treatment after molding. An object of the present invention is to provide a high-strength hot-rolled steel sheet for heat hardening, which can be used for heat treatment.

[0009]

The present inventors have repeatedly studied a hot-rolled steel sheet having both heat-curability and stretch flangeability, and found that not only the heat-hardness and stretch flangeability but also the microstructure of the hot-rolled steel sheet are not limited to the components. Greatly affected, T
In the hot-rolled steel sheet for heat hardening utilizing the precipitation of i, the microstructure is changed to ferrite and bainite controlled to a predetermined volume ratio, and further, by controlling the size of cementite present in these structures, excellent elongation is achieved. The inventors have found that not only can the flange property be secured, but also a large amount of heat curing can be obtained, and the present invention has been completed.

That is, the high-strength hot-rolled steel sheet for heat hardening of the present invention has a C content of 0.05 to 0.20% and a Si content of 0.01 to 100% by weight.
0.8%, Mn: 0.5-2.5%, P: 0.005-0.1%, S
: 0.01% or less, Al: 0.01 to 0.08%, Ti: 0.08 to 0.
20%, the balance consisting of Fe and unavoidable impurities,
It has a structure composed of ferrite and 30 to 90% by volume of bainite, and the average particle size of cementite contained in the structure is 3 μm or less.

[0011] As the steel component, Nb: 0.01
To 0.1%, and / or one or more components selected from at least one of the following first to third groups. Group 1: V: 0.01 to 0.5% Group 2: Cr: 0.05 to 0.8%, Mo: 0.05 to 1.0%,
B: 0.0005 to 0.01% Group 3; Ca: 0.005% or less, rare earth element: 0.05% or less

Hereinafter, the present invention will be described in detail. First, the reasons for limiting the components will be described. All units are wt%
(Mass%). C: 0.05 to 0.20% C is effective for strengthening steel. It is necessary for bonding with Ti or the like at the time of heat treatment after cold working to increase the strength and also for forming bainite which is preferable for stretch flangeability. For this purpose, 0.05% or more is required. It needs to be added. However, if added excessively, the ductility is significantly deteriorated and the weldability is also reduced.
20%. A desirable range is 0.07 to 0.15%.

Si: 0.01 to 0.8% Si is very effective as a solid solution strengthening element in improving the tensile strength, but excessive addition deteriorates the surface properties and the chemical conversion property, so the upper limit is 0.8%. And .

Mn: 0.5 to 2.5% Mn is also a solid solution strengthening element and is effective in improving tensile strength, and is also an element necessary for suppressing the formation of coarse pearlite and forming bainite. To exert this effect effectively, it is necessary to add 0.5% or more. However, if added excessively, it not only reduces ductility but also impairs weldability, so the upper limit is made 2.5%.

P: 0.005 to 0.1% P has a function of improving the strength of steel, but excessive addition deteriorates workability and toughness.
The upper limit is set to 0.1%.

S: 0.01% or less S is preferably as small as possible because it deteriorates ductility. In the present invention, it is set to 0.01% or less for improving stretch flangeability.

Al: 0.01 to 0.08% Al is added for deoxidation. If it is less than 0.01%, the content is too small. On the other hand, if it exceeds 0.08%, the amount of alumina-based inclusions increases and the workability deteriorates.
8%.

Ti: 0.08 to 0.20% Ti is an important element in the steel sheet of the present invention. That is, not only is it effective for obtaining the desired ferrite + bainite structure together with Mn and Cr added as necessary, but also Ti dissolved in the hot rolling stage becomes fine TiC by heat treatment. And has the effect of significantly increasing the strength. 890 N / mm ² after heat treatment
To secure the above TS, it is necessary to add 0.08% or more. However, if added excessively, the workability deteriorates. Therefore, the upper limit is made 0.20%.

The hot-rolled steel sheet of the present invention contains the above components as essential components, and the balance substantially consists of Fe.
0.01 to 0.1%, and / or one or more components selected from at least one of the following first to third groups.

Nb: 0.01-0.1% Nb coexists with Mn and affects the transformed structure after hot rolling.
Like Ti, it has a function of making it easier to obtain a bainite structure. Further, since it is also a precipitation strengthening element, it suppresses precipitation during hot rolling and winding, as in Ti, and exhibits heat treatment curability. In order to exhibit these effects, 0.01% or more must be added. However, excessive addition causes an increase in the yield ratio and a decrease in ductility. Therefore, the upper limit was set to 0.1%.

First group: V: 0.01 to 0.5% V is a precipitation strengthening element, and exhibits heat treatment curability by suppressing precipitation during hot rolling and winding like Ti.

Second group: Cr: 0.05-0.8%, Mo: 0.05
1.0%, B: 0.0005 to 0.01% Cr, Mo, and B are elements that improve hardenability and advantageously provide a desired structure. In order to exert this effect effectively, the lower limit of each element was set, and the amount at which the effect was saturated was set as the upper limit from an economic viewpoint.

Group 3: Ca: 0.005% or less, rare earth element: 0.05% or less Ca and rare earth element are ductile,
In particular, it has the effect of improving stretch flangeability. In order to exert this effect effectively, the lower limit of each element was set, and the amount at which the effect was saturated was set as the upper limit from an economic viewpoint.

Next, the microstructure of the hot rolled steel sheet of the present invention will be described. Microstructure is 30 ~ by volume ratio with ferrite
Consists of 90% bainite. Ferrite is necessary for ensuring elongation, while the hard phase composed of bainite is inferior in stretch flangeability to pearlite as compared to bainite, and the increase in strength due to subsequent hardening heat treatment is small. In addition, martensite significantly deteriorates the stretch flangeability of a hot-rolled steel sheet, and also has a low heat-curing property due to softening of the martensite during hardening heat treatment. The reason why the volume ratio of bainite is 30 to 90% is that if it is less than 30%, the strength is reduced and the stretch flangeability is not sufficient, while if it exceeds 90%, the decrease in elongation is large.

Further, the size of the cementite present in the ferrite and bainite structures must be 3 μm or less on average. This is for the following reason. In order to form TiC by hardening heat treatment and to increase the strength, solid solution Ti and solid solution C are required. However, the amount of solid solution C is small at the stage of hot-rolled steel sheet, and most of it is cementite (Fe).
₃ C). Therefore, it is necessary to dissolve cementite at the time of heat treatment for curing, but coarse cementite is difficult to dissolve, so that in the present invention, fine cementite of 3 μm or less is used. As a result, cementite is rapidly dissolved, and TiC can be sufficiently formed. Also,
The refinement of cementite is also effective for improving the workability, especially the stretch flangeability, and the particle size of cementite is preferably 2 μm or less.

Next, a description will be given of a preferred industrial production method of the hot-rolled steel sheet of the present invention. However, the hot-rolled steel sheet of the present invention is not limited to those manufactured by such a method.

The steel slab of the steel component is heated to 1150 ° C. or higher, preferably 1200 ° C. or higher, and hot-rolled for solutionizing precipitation strengthening elements such as Ti and Nb. It is preferable that the finishing temperature of the hot rolling be ₃ points or more and less than 900 ° C. A
If r is less than ₃ points, a processed structure remains, the workability is deteriorated, and the heat curability is also lowered. On the other hand, when the finishing temperature is 9
When the temperature is lower than 00 ° C., an appropriate amount of ferrite can be stably obtained, and thus elongation can be secured. When the finishing temperature is 900 ° C. or higher, the amount of ferrite becomes extremely small, and the deterioration of elongation becomes large.

The winding temperature is preferably set to 300 ° C. or higher and lower than 450 ° C. If the temperature is lower than 300 ° C., martensite is generated, so that the stretch flangeability is reduced. In addition, martensite is softened by a subsequent heat treatment, which is not preferable for heat curability. 450 ° C
In the above, even if a predetermined amount of bainite is obtained, cementite in the structure is coarsened and the amount of TiC deposited is also increased, so that sufficient heat curability cannot be obtained, and the stretch flangeability also decreases. Become like

The hardening heat treatment after the molding is performed by solid solution T
i. Heating may be performed so as to precipitate solid solution Nb. Usually, heating may be performed at a temperature of 500 to 750 ° C. for 1 minute or more.

[0030]

EXAMPLE A slab having a thickness of 5 mm to 30 mm was produced from steel having the chemical composition shown in Table 1 in order to change the rolling reduction.
After heating these at 1250 ° C. for 60 minutes, hot rolling was performed. Rolling is in the non-recrystallization temperature range of 830 to 950 ° C
At the finishing temperature shown in Table 2, rolling to 3.0 mm, cooling to a temperature equivalent to the winding temperature, and
After holding for 0 minutes, the furnace was cooled.

A JIS No. 5 tensile test piece was sampled from the obtained hot-rolled steel sheet and subjected to a tensile test and a hole expansion test for evaluating stretch flangeability. In the hole expansion test, the initial hole diameter di = 1
A punched hole of 0 mmφ was made, and a conical punch having a vertex angle of 60 ° was pushed in to obtain a hole diameter db when a crack penetrated the plate thickness, and a hole expansion ratio λ was calculated by the following equation. λ = ((db−di) / di) × 100%

Further, in order to evaluate the heat-curing property, the test piece was subjected to a heat-curing heat treatment at 600 ° C. × 10 minutes, and was subjected to JIS5.
A tensile test was performed using the No. test piece. Table 2 shows the obtained results.

[0033]

[Table 1]

[0034]

[Table 2]

Table 2 shows that the steel sheet of the present invention (Sample Nos. 1 to 7)
Have excellent workability of TS × λ ≧ 50,000 before heat treatment, and have a tensile strength of 150 N after heat treatment.
/ Mm ² or more.

On the other hand, in Sample No. 8 (Comparative Example), although the workability was good, a remarkable scale pattern occurred because the Si content exceeded the range of the invention. In addition, it can be seen that the other comparative samples have insufficient workability as hot rolled and insufficient heat curing amount.

[0037]

The high-strength hot-rolled steel sheet for heat hardening of the present invention is
Contains 0.08 to 0.20% Ti and has an average particle size of 3 μm
Since it has a two-phase structure consisting of ferrite containing cementite and a predetermined amount of bainite, it has excellent workability, especially stretch flangeability, and also has a large amount of heat hardening, and has both excellent workability and heat hardening. It is suitable as. Further, the production method of the present invention is excellent in productivity and suitable as an industrial production method of the high-strength hot-rolled steel sheet for heat curing.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-9-170048 (JP, A) JP-A-8-325644 (JP, A) JP-A-7-286243 (JP, A) 240356 (JP, A) JP-A-5-105963 (JP, A) JP-A-61-130454 (JP, A) JP-A-58-42725 (JP, A) JP-A-8-144393 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (4)

(57) [Claims] C .: 0.05 to 0.20% by weight, Si:
0.01 to 0.8%, Mn: 0.5 to 2.5%, P: 0.005 to 0.1
%, S: 0.01% or less, Al: 0.01 to 0.08%, Ti: 0.
It has a structure composed of ferrite and 30 to 90% by volume of bainite, and has an average grain size of cementite of 3 μm or less. A high-strength hot-rolled steel sheet for heat-hardening with excellent stretch flangeability. 2. The steel component further comprises Nb: 0.01 to
The high-strength hot-rolled steel sheet for heat hardening according to claim 1, which contains 0.1%. 3. As a steel component, a first group; V: 0.01 to 0.5%; a second group; Cr: 0.05 to 0.8%; Mo: 0.05 to 1.0%;
B: 0.0005 to 0.01% Third group; Ca: 0.005% or less, rare earth element: 0.05% or less The high strength for heat curing according to claim 1 or 2 containing at least one component selected from the group consisting of: Hot rolled steel sheet. 4. The steel having the composition described in claim 1, 2 or 3 is heated to 1150 ° C. or more, hot rolling is completed at a finishing temperature of ₃ points or more and less than 900 ° C., and 300 ° C. or more and less than 450 ° C. A method for producing a high-strength hot-rolled steel sheet for heat-hardening, which is excellent in stretch flangeability, characterized by being wound at a temperature of 1.