JPH0730408B2 - Method for producing hot-rolled thin steel sheet having bake hardenability by normal temperature aging - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a high-strength hot-rolled thin steel sheet, which is suitable for use in an automobile body or the like that requires high strength after forming and processing. This is intended to improve the so-called bake hardenability, in which the yield point greatly increases when subjected to the baking treatment in the paint baking process after the molding process, and the delayed aging at room temperature.

(Prior Art) Conventionally, high-strength thin steel sheets have been widely used for automobile body exterior plates and the like in order to reduce the weight of the vehicle body. Since such a high-strength thin steel sheet for automobiles is subjected to forming processing typified by press working, it is not only necessary that it is relatively soft and has good formability, but it also has sufficient strength. Therefore, in recent years, a steel sheet that is relatively soft at the time of forming but has the property that the strength increases by age hardening in the paint baking process after forming, that is, a steel plate with a large bake hardenability is used. Has become.

The bake hardening amount, which is an index of bake hardenability, is generally measured as follows. That is, first, a pre-strain of about 2%, which is equivalent to press molding, is applied, and then a heat treatment is performed at 170 ° C. for 20 minutes, which is equivalent to baking. Then, the difference between the deformation stress at the time of 2% pre-strain and the yield stress after the heat treatment is calculated, and the value is taken as the bake hardening amount.

By the way, there are various conventional steel sheets having bake hardenability, but recently, as a high strength and high ductility steel sheet having a particularly high bake hardenability, for example, as described in JP-B-55-48575, nitrogen is used. (N) High N with increased amount
Quenched and tempered steel, and "Japan Institute of Metals" 19
(1980) P439 or "Journal of the Japan Institute of Metals" 19 (1980) P1
0, “Iron and Steel” Vol 68 (1982) No. 9, P1348, dual phase steel sheet (Dual hase steel sheet) and the like are known.

(Problems to be Solved by the Invention) However, the steel sheet described in Japanese Patent Publication No. 55-48575 described above has a problem that the variation of the material is large because the N content is difficult to adjust.

On the other hand, the above-mentioned two-phase steel sheet has an average bake hardening amount of 5 kgf.
/ mm ² about the only, yet if you further increase the bake hardenability, could not avoid the problem of material deterioration due to room temperature aging. Further, the strength level of the two-phase structure steel sheet becomes too high, and it was difficult to manufacture a steel sheet having good workability of TS of about 40 to 60 kgf / mm ² .

This invention was developed in view of the above circumstances, high bake hardenability, specifically showing a high bake hardenability of 5 kgf / mm ² or more, and almost no aging deterioration at room temperature,
Moreover, it is an object of the present invention to provide a high-strength hot-rolled thin steel sheet having good workability of TS: 40 to 60 kgf / mm ² .

(Means for Solving the Problems) As a result of various studies to achieve the above-mentioned object, the inventors of the present invention have achieved the desired result by appropriately controlling rolling conditions and cooling conditions including the chemical composition. The inventors have found that the object can be realized advantageously, and have completed the present invention.

That is, the present invention contains C: 0.02-0.10 wt% (hereinafter simply referred to as%), Mn: 0.5-1.2%, P: 0.045-0.10%, Al: 0.02-0.05% and N: 0.005-0.020%. It consists of hot rolling the composition steel at a temperature of Ar ₃ -20 ° C or higher, then cooling it at an average cooling rate of 30 ° C / s or higher, and then winding it in the temperature range of 150 to 450 ° C. A method for producing a hot-rolled thin steel sheet which is always late-aged and has bake hardenability.

The present invention will be specifically described below.

First, the reason why the component composition of the starting material in the present invention is limited to the above range will be described.

C: 0.02 to 0.10% If C is less than 0.02%, the transformation temperature is too high and the lower limit of the finish rolling temperature becomes high, which is disadvantageous in operation, while C is 0.
If it exceeds 10%, it is difficult to perform the welding in the spot welding process for forming and assembling thin plates, and the bake hardenability tends to decrease, so the C content is 0.02 to 0.
Limited to 10% range.

Mn: 0.5 to 1.2% Mn is important as a strengthening element, but in the present invention, not only as a strengthening element, but also the effect of suppressing aging deterioration at room temperature by interaction with C and N is utilized. This effect is small at less than 0.5%, so the lower limit was made 0.50%.

On the other hand, if a large amount of more than 1.2% is added, strength can be expected to increase, but ductility deteriorates, and problems such as cost increase and weldability deterioration occur. Therefore, the upper limit was made 1.2%.

P: 0.045 to 0.10% P is a useful element that strengthens steel while suppressing aging deterioration at room temperature due to the interaction with C and N, like Mn described above. However, if the content is less than 0.045%, the effect of addition is poor. On the other hand, if it exceeds 0.10%, not only the effect of addition almost reaches saturation but also problems such as secondary work embrittlement are not preferable, so P is 0.045%. It was supposed to be added in the range of 0.10%.

Here, the secondary work brittleness is a phenomenon in which a material that has once been subjected to strong cold working is significantly embrittled.

Al: 0.02 to 0.05% Al is useful as a deoxidizing element and as an element for suppressing room temperature aging by fixing solid solution N, but if the content is less than 0.02%, its addition effect is poor, while 0.05% is added. If it is added in excess, it will remain undissolved as AlN when the slab is reheated at a low temperature, and the solid solution N will decrease in the hot-rolled sheet state, and not only great bake hardenability cannot be expected, but also Al ₂ O ₃ clusters, etc. The deterioration of the surface properties due to Therefore Al is 0.
It was decided to add in the range of 02 to 0.050%.

N: 0.005 to 0.020% If N is less than 0.0050%, large bake hardening due to solid solution N cannot be expected, so the lower limit was made 0.005%. On the other hand, the amount of N
Even if the content exceeds 0.0200%, the amount of bake hardening hardly increases, and rather, it causes disadvantages such as difficulty in melting and deterioration of hot rolling property. Therefore, the upper limit was set to 0.0200%.

Although the basic components have been described above, Si can be further added as a strength improving element in the present invention.

Si: 1% or less Si is an element that is advantageous for improving the strength without a large decrease in ductility, but if it exceeds 1%, the steel hardens and the rolling property deteriorates, so Si should be 1% or less. It is preferable to contain it.

Next, the reasons for limiting the hot rolling conditions will be described.

First, the finish rolling temperature is preferably relatively high in order to enhance the bake hardenability. This temperature corresponds to the so-called Ar ₃ transformation point, but considering this point, Ar ₃
-20 ° C is the lower limit temperature, and bake hardenability deteriorates when finishing at lower temperatures.

However, simply applying a high-temperature finish does not improve all the materials, and it is not preferable in terms of workability because the crystal grains become coarse. 0.08C-0.10Si-1.00Mn-
Using 0.0070N-0.07P-0.040Al steel as a material, after heating it to 1200 ° C, the finish rolling temperature was variously changed and the change of the bake hardening (BH) amount was investigated.

The obtained results are shown in FIG. At this time, the cooling rate and the winding temperature after the completion of rolling were constant at 40 ° C / s and 200 ° C, respectively. The Ar ₃ point is about 815 ° C.

As is clear from the figure, by finishing hot rolling at a temperature of Ar ₃ −20 ° C. or higher, excellent BH of 5 kgf / mm ² or higher can be obtained.
Sex has been obtained.

Then, after cooling, it is wound up.However, in order to prevent precipitation of AlN during cooling and to obtain good bake hardenability by forming a fine grain ferrite structure, it is important to control the cooling rate. Therefore, specifically, it is necessary to set the cooling rate after finish rolling until winding to an average rate of 30 ° C./s or more.

Regarding the coiling temperature, there is an appropriate temperature range in order to suppress precipitation of AlN, promote appropriate precipitation of C, and obtain an appropriate fine grain ferrite structure.
When the coiling temperature is lower than 150 ° C, the amount of solid solution C becomes too large and the aging deterioration becomes remarkable at room temperature, while when it exceeds 450 ° C, the solid solution C and N are largely reduced, resulting in deterioration of bake hardenability. Therefore, the winding temperature is limited to the range of 150 to 450 ° C.

(Operation) In the present invention, in particular, N addition and P addition have characteristic features, and by combining this with controlled rolling and controlled cooling, N and C are left in a solid solution state to bake hardenability. In addition, the aging at room temperature is suppressed by the interaction between P and these solid solution elements.

(Example) A steel slab having the chemical composition shown in Table 1 was hot-rolled under the conditions shown in Table 2 and the results of examining the tensile properties, BH properties and weldability of the hot rolled steel sheet obtained are shown in Table 2. Write together.

Here, ΔY.El is the increase amount of Y.El in% when aged for 6 months at 40 ° C after 1% skin pass, and
BH had a yield stress of 2% prestrain and 170% aging at 170 ° C for 20 minutes, and 2% deformation stress before aging (flow str
ss) and the difference in stress.

In the present invention, ΔY.El is required to be approximately 0.5% or less, and BH is required to be 5 kgf / mm ² or more.

As is clear from the table, all of the hot-rolled sheets obtained according to the present invention have a good relationship between TS and El, and Δ
Y.El was 0, and BH exhibited a good bake hardenability of 5 kgf / mm ² or more.

(Effects of the Invention) Thus, according to the present invention, it is possible to easily obtain a hot-rolled thin steel sheet excellent in room-temperature slow aging property and bake hardenability, and conventionally a high-strength cold-rolled steel sheet is mainly applied to a vehicle body. It is also advantageous because it can be used for applications such as exterior plates.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between finish rolling temperature and BH amount.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shinobu Okano 1 Kawasaki-cho, Chiba-shi, Chiba Kawasaki Steel Co., Ltd. Technical Research Headquarters (56) References JP-A-62-180021 (JP, A)

Claims (1)

[Claims] 1. A composition containing C: 0.02-0.10 wt%, Mn: 0.5-0.2 wt%, P: 0.045-0.10 wt%, Al: 0.02-0.05 wt% and N: 0.005-0.020 wt%. Steel is hot-rolled at a temperature of Ar ₃ −20 ° C. or higher, then cooled at an average cooling rate of 30 ° C./s or higher, and then wound in a temperature range of 150 to 450 ° C., A method for producing a hot-rolled thin steel sheet which is always aged and has bake hardenability.