JP2003003239A - High strength hot rolled steel sheet having excellent baking hardenability and production method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength hot rolled steel sheet which has excellent baking hardenability, and to which ductility and, particularly, the sufficient increase of strength by a baking finish after press forming are imparted for securing press formability and strength required for an automobile structural member, and to provide production method therefor. SOLUTION: A steel having a composition containing, by weight, 0.13 to 0.2% C, 0.01 to 2% Si and 0.5 to 2% Mn, and in which the content of S is controlled to <=0.015%, P to <=0.02%, and N to <=0.0035%, and, if required, containing one or more kinds selected from Ti, Nb, V, Mo, Ca and B, and the balance Fe with inevitable impurity elements is subjected to hot rolling, and, after that, its cooling rate is controlled to >=30 deg.C/s, and the steel sheet is coiled to produce the high strength hot rolled steel sheet having a structure containing ferrite and bainitic ferrite of >=20% in a volume ratio, and having excellent baking hardenability.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled steel sheet and a method for producing the same, and more particularly, to a high-strength hot-rolled steel sheet used for structural members for automobiles, etc. The present invention relates to a manufacturing method thereof.

[0002]

2. Description of the Related Art High-strength steel sheets have been widely applied to recent steel sheets for automobiles in order to reduce the weight and ensure energy absorption characteristics at the time of collision. However, in consideration of the formability of parts, there is an increasing demand for a steel plate having a so-called bake hardenability, which has a relatively low strength at the time of forming but the strength is increased by coating baking after processing.

Under such circumstances, hot-rolled steel sheets having bake hardenability for the purpose of being applied to structural members are disclosed in, for example, JP-A-10-183301 and JP-A-200183.
A technique for imparting room temperature aging resistance and high bake hardenability is disclosed by increasing the amount of N added as described in 0-297350 and by refining the structure by optimizing the cooling conditions after hot rolling for finishing. .

[0004]

However, although the refinement of the structure is an effective means for ensuring the strength, it is feared that the ductility may be lowered.

[0005]

[Means for Solving the Problems] In order to solve the above-mentioned problems, the present inventors have developed a low carbon steel (C: 0.05 to 0.25% -Mn: 0.8% -N:
0.0025%)) was used to investigate the effect of hot rolling conditions and microstructure on the bake hardenability in the laboratory, and the following was found.

The effects of the amount of C and the winding temperature on the bake hardenability (BH) are shown in FIGS. 1 and 2. The hot rolling conditions at this time were a heating temperature of 1200 ° C and a finishing temperature of 880 ° C, and a cooling rate after finishing rolling was 50 ° C / s. Fig. 1 Winding temperature: 55
This is the result when the temperature is 0 ° C. In addition, after applying a pre-strain of 2%, BH was heated at 170 ° C for 20 minutes and then re-stretched,
It was calculated from the difference between the maximum load before aging and the load at the upper yield point after aging. In particular, it was found that the amount of C has an optimum range for securing BH of 50 MPa or more. This is related to the precipitation of ε carbide, and so-called bainitic ferrite is formed in the high BH region. The bainitic ferrite referred to here is based on the definition of the bainite photograph collection I (published by the Iron and Steel Institute of Japan, 1992), and does not cause precipitation of carbides. In the same figure, as the cause in the low BH region, it is presumed that bainite is formed in the low C region and pearlite is formed in the high C region. . Regarding the influence of the coiling temperature, as a result of investigating steel containing 0.13% of C, it was found that there is an optimum range as shown in FIG. This is also considered to be related to the structure formation as described above, and is mainly due to the formation of bainite accompanied by the precipitation of cementite in the low winding temperature region, and the formation of pearlite in the high winding temperature region. It is presumed that. That is, it was newly discovered that in order to increase BH in the hot rolled steel sheet, it is necessary to control the microstructure so as to suppress the precipitation of carbides as much as possible in the hot rolled sheet stage. Therefore, when the influence of the volume ratio of bainitic ferrite was investigated,
It was found that the volume ratio of BH must be 20% or more to secure BH of 50 MPa or more.

The gist of the present invention is (1) by weight, C: more than 0.12 to 0.2%, Si: 0.01 to 2%, M
n: 0.5 to 2% included, S: 0.015% or less, P: 0.02% or less, N:
A high-strength hot-rolled steel sheet excellent in bake hardenability, which is 0.0045% or less, is composed of the balance Fe and unavoidable impurity elements, and has a structure that includes ferrite and 20% or more by volume of bainitic ferrite. . (2) A high-strength hot-rolled steel sheet having 0.007 to 0.1% of Ti, Nb and / or V in addition to the hot-rolled steel sheet according to (1) and having excellent bake hardenability. (3) 0.5% Mo in the hot rolled steel sheet according to (1) or (2)
A high-strength hot-rolled steel sheet containing the following, which has excellent bake hardenability. (4) 0.005% of Ca in the hot rolled steel sheet according to (1) to (3)
A high-strength hot-rolled steel sheet containing the following, which has excellent bake hardenability. (5) 0.003% of B in the hot rolled steel sheet according to (1) to (4)
A high-strength hot-rolled steel sheet containing the following, which has excellent bake hardenability. (6) A high-strength hot-rolled steel sheet excellent in bake hardenability, comprising a plating layer on the surface of the hot-rolled steel sheet according to (1) to (5). (7) After slab is made by continuous casting, rough rolling is performed immediately after reheating or casting, and finish rolling is finished in a temperature range of Ar ₃ transformation point or higher, and winding temperature is 500 to 650 ° C. The method for producing a high-strength hot-rolled steel sheet having excellent bake hardenability according to (1) to (5). (8) Finishing the rough rolling, winding the sheet bar once into a coil and subjecting it to finish rolling as it is, or finishing rolling after connecting to the preceding sheet bar, baking according to (7) A method for producing a high-strength hot-rolled steel sheet having excellent hardenability. (9) The method for producing a high-strength hot-rolled steel sheet having excellent bake hardenability according to (7) or (8), which comprises performing rough rolling immediately after casting into a slab of 100 mm or less.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION First, the reasons for limiting the component composition in the present invention will be described. If the content of C is less than 0.13%, the strength cannot be ensured sufficiently by ε carbide, so this is the lower limit. On the other hand, if it exceeds 0.2%, pearlite is likely to be formed and the weldability may be deteriorated. That is, BH
There is concern about the shortage of workability and the deterioration of weldability during the assembling of parts and the assembling of parts.

[0009] Si is one of the elements added to increase the strength of the steel sheet. However, excessive addition causes welding defects in the joints and parts assembly parts during steel plate manufacturing,
The upper limit is 2%.

[0010] Mn is also added when increasing the strength of steel, but excessive addition greatly deteriorates the ductility and the weldability in various welding methods, so the upper limit is 2%.
On the other hand, 0.5% to improve hot brittleness and secure strength with S
The above additions are necessary.

S is a type A inclusion (JIS G055
5) is formed and the ductility is deteriorated, so 0.015% is made the upper limit.

P is an element added mainly for the purpose of increasing the strength. However, if it is contained excessively, not only the ductility is lowered, but also the secondary workability is deteriorated.
02% or less.

Al is an element added for deoxidation.
If it is less than 0.005%, the intended effect will not be exhibited, while if it exceeds 0.1%, it remains in the steel as an oxide, which may cause a decrease in ductility.

The N content is preferably as small as possible, but an excessive decrease would significantly increase the cost in steelmaking, so the upper limit is 0.0045%.

Ti, Nb and V contribute to the increase in strength by forming carbides or nitrides. In order to develop the strengthening ability, 0.007% or more is required for each. On the other hand, if it is added in excess of 0.1%, the BH property deteriorates and it becomes impossible to obtain the characteristics of 50 MPa or more.

Mo is an element added to secure the strength, but is mainly added to improve the hardenability. In particular, the purpose is to ensure strength under the cooling conditions in the hot dip galvanizing process, but excessive addition causes deterioration of ductility, so the upper limit is 0.5%. In order to ensure hardenability, Mo is preferably 0.1% or more.

Ca is added for controlling the morphology of MnS formed in steel. In the present invention, since the amount of S is suppressed to a low level, excessive addition will rather cause inclusions to remain in the steel, so the content is made 0.005% or less. In order to keep the steelmaking cost low and to exhibit the above effects, 0.001% or more is preferably added.

B is added together with Mn for the purpose of stably forming bainitic ferrite necessary for securing strength at the stage of hot rolling. If added in excess of 0.003%, cracks will occur during the slab manufacturing stage, so this is the upper limit. Further, in order to effectively bring out the effect of B, 0.0001% or more is preferably added.

Note that trace amounts of Cu, N due to the use of scrap
The mixing of i, Sn and Cr does not impair the effect of the present invention.

Among the hot rolling conditions in the present invention, the finishing temperature and the coiling temperature are important factors for obtaining the desired steel sheet properties. That is, finish rolling needs to be carried out in a temperature range above the Ar ₃ transformation point. If it is carried out at a temperature lower than the transformation point, the structure becomes non-uniform, and there is a concern that ductility will deteriorate. The cooling rate after finish rolling is also an important factor in controlling the structure of the hot rolled sheet. In the case of the present invention, if the cooling rate becomes slower than 30 ° C./s, it becomes difficult to obtain sufficient bainitic ferrite and the BH property becomes low. Also, regarding the winding temperature, the microstructure formed in the steel sheet is set to 500 to 6 in order to form bainitic ferrite from the viewpoint of ensuring BH property.
It must be in the range of 50 ° C. If it is higher than 650 ℃, pearlite will be formed and high BH property cannot be obtained.
On the contrary, when it becomes lower, bainite is formed and the BH property is also impaired.

When carrying out the above-mentioned hot rolling, the effect of the present invention is not changed even if the preceding sheet bar is joined after the rough rolling by using laser welding or the like and the rolling is carried out.

Further, also in the case of producing a slab, it is not impairing the effect of the present invention to produce a thin slab having a so-called near net shape of 100 mm or less and to immediately produce it under the rolling conditions as described above.

Regarding the plating process, for example, when performing hot dip galvanizing, the surface of the steel sheet is heated to the same temperature as the temperature of the zinc bath and immersed in the zinc bath at that temperature. At that time, as a preferable condition, if the temperature becomes too high, the strength of the hot-rolled sheet is changed due to the change of the structure, so that the upper limit is 550 ° C.
On the other hand, if it is too low, poor plating and non-uniformity of the plating layer thickness are caused, so the lower limit is 420 ° C.

[0024]

EXAMPLES Example 1 C: 0.14%, Si: 0.012%, Mn: 0.85%, P: 0.018%, S: 0.006%, Al: 0.
Steel containing 027% and N: 0.0024% was tapped from the converter and continuously cast into a slab. After hot rolling at 1200 ° C., rough rolling was performed and then hot rolling was completed under the conditions shown in Table 1 to obtain a 2 mm hot rolled sheet. After that, 1% skin pass was applied to obtain a product.
The Ar ₃ transformation point here is 916-50 [C (%)] ＋ 27 [Si (%)] − 6
Approximately 855 ℃ when calculated with 4 [Mn]. Material evaluation is JIS Z
The No. 5 test piece described in 2201 was processed, and a tensile test was performed according to the test method described in JIS Z 2241. Regarding the amount of BH, after applying a tensile strain of 2%, heat treatment at 170 ° C. for 20 minutes, and then re-tensioning, the load at the time of applying 2% prestrain and the load at the upper yield point after aging Calculated from the difference. The results are shown in Table 2. In Nos. 1 to 5 according to the method of the present invention, 5
BH amount exceeding 0 MPa is obtained. On the other hand, the finishing temperature is Ar
_In No. 6 which is lower than the ₃ transformation point, not only the ductility is low due to the non-uniform structure of the hot-rolled sheet, but the bainitic ferrite formation is small and the BH content is also small. In addition, the volume ratio of bainitic ferrite is low due to the slow cooling rate after finish rolling, and No. 8 in which pearlite was formed due to the high winding temperature was also removed.
The quantity is low.

[0025]

[Table 1]

[0026]

[Table 2]

Example 2 Various steels shown in Table 3 were discharged from a converter and continuously cast into slabs. After hot rolling at 1150 to 1250 ° C., rough rolling and finish rolling were carried out to manufacture hot rolled sheets having plate thicknesses shown in Table 4. The finish rolling was completed in the temperature range above the Ar ₃ transformation point. Further, the amount of water in the cooling zone was adjusted so that the cooling rate after finish rolling was within the range of the present invention. After cooling, winding was performed at 600 ° C., and in the same manner as in Example 1, material evaluation by a tensile test and measurement of BH amount were performed. Also,
After applying a 7% prestrain by tension, 170 ° C as in the measurement of BH content
× Tensile strength (T
S) was measured. The results are shown in the table. According to the invention
A, B, C, D, E, G, H and I show high BH content and TS after 7% prestrain + BH is also higher than that of the base metal.
On the other hand, in the F steel where the amount of C deviates below the range of the present invention, BH
Although the amount is high, TS after applying 7% pre-strain + BH is not so high. In addition, J steel with a high C content deviates from the BH content because pearlite is formed in the hot-rolled sheet, and the TS after 7% prestrain + BH does not increase. K steel whose Mn amount exceeds the range of the present invention has low ductility, and there is a concern that workability may deteriorate. In addition, the L steel with a high S content deviates from the ductility because many A-based inclusions are formed.

[0028]

[Table 3]

[0029]

[Table 4]

Example 3 For the C and D steels according to the scope of the present invention of Example 2, a manufacturing process of immediately sending to a rough rolling process after casting by a thin slab continuous casting method, and a hot rolling process after the completion of rough rolling were preceded. It was manufactured by a so-called continuous hot rolling process in which the material is connected and rolled. Table 5 shows the manufacturing process. The finishing temperature, cooling conditions and winding temperature were the same as in Example 2. The obtained materials are shown in the same table. The obtained materials of a, a, u, d, o and mosquito have almost the same characteristics as those of the second embodiment.

[0031]

[Table 5]

[0032]

According to the present invention, a high-strength hot-rolled steel sheet having a high ductility and a high bake hardenability, which is particularly suitable for use as a structural member for automobiles, can be produced.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a C amount and a BH amount.

FIG. 2 is a diagram showing a relationship between CT (winding temperature) and BH amount.

FIG. 3 is a diagram showing the relationship between the volume ratio of bainitic ferrite and the amount of BH.

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Claims (9)

[Claims] 1. By weight ratio, C: 0.13 to 0.2%, Si: 0.01 to 2
%, Mn: 0.5 to 2%, S: 0.015% or less, P: 0.02% or less, N: 0.0045% or less, balance Fe and unavoidable impurity elements, and ferrite and volume ratio of 20% or more A high-strength hot-rolled steel sheet excellent in bake hardenability, characterized by exhibiting a structure containing bainitic ferrite. 2. A high-strength hot-rolled steel sheet, which comprises the hot-rolled steel sheet according to claim 1 and 0.007 to 0.1% of each of Ti, Nb, and V in an amount of 0.007 to 0.1%. 3. The hot-rolled steel sheet according to claim 1 or 2, wherein Mo
A high-strength hot-rolled steel sheet containing 0.5% or less of Bake Excellent in hardenability. 4. A high-strength hot-rolled steel sheet containing the hot-rolled steel sheet according to claim 1, which contains 0.005% or less of Ca and has excellent bake hardenability. 5. A high-strength hot-rolled steel sheet according to any one of claims 1 to 4, which contains B in an amount of 0.003% or less and which is excellent in bake hardenability. 6. A high-strength hot-rolled steel sheet having excellent bake hardenability, which has a plating layer on the surface of the hot-rolled steel sheet according to claim 1. 7. A slab is formed by continuous casting, and then rough rolling is performed immediately after reheating or casting, finishing rolling is completed in a temperature range of Ar ₃ transformation point or higher, and cooling rate is 30 ° C./s or higher. The method for producing a high-strength hot-rolled steel sheet having excellent bake hardenability according to any one of claims 1 to 5, which is cooled and wound in a temperature range of 500 to 650 ° C. 8. The method according to claim 7, wherein the rough rolling is completed, the sheet bar is once wound around a coil, and the sheet is directly subjected to finish rolling, or the sheet bar is connected to a preceding sheet bar and then finish rolling is performed. For producing a high-strength hot-rolled steel sheet having excellent bake hardenability. 9. The method for producing a high-strength hot-rolled steel sheet excellent in bake hardenability according to claim 7, wherein rough rolling is carried out immediately after casting into a slab of 100 mm or less.