JPS61272321A - Manufacture of ultra high-strength cold rolled steel sheet - Google Patents

Abstract

PURPOSE:To manufacture the titled steel sheet superior in ductility and workability, by applying hot rolling, pickling, cold rolling and continuous annealing to steel slab, then cooling and holding the sheet at a specified temp. range, then cooling it up to room temp. CONSTITUTION:Continuously cast slab of molten steel contg. by weight, 0.1-0.3% C, 0.25-2.0% Si, 1.5-2.5% Mn, <0.010% P, <0.003% S, 0.02-0.10% sol. Al, or further 0.0002-0.0030% Ca is hot rolled conventionally, the scale is removed by pickling, and the plate is cold rolled to steel strip having 1.4mm thickness. This is annealed continuously at >=750 deg.C, then cooled to 350-500 deg.C range by 3-70 deg.C/sec rate. The sheet is held at the temp. range for time range satisfying a formula (1), then cooled to room temp. The titled steel sheet superior in ductility and workability, having >80kgf/mm<2> tensile strength, >16% elongation, min. bending radius r(mm) of >=two times the sheet thickness t(mm) can be manufactured.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention provides a tensile strength sokgf/■ with excellent ductility and workability.
The present invention relates to a cracking method by continuous annealing of two or more ultra-high strength cold rolled steel sheets.

(Conventional technology) The application of high-strength steel plates in the automobile industry for the purpose of reducing the weight of car bodies and ensuring passenger safety by improving collision strength is steadily progressing, and the characteristics of high-strength steel plates suitable for reeds and car body parts are being developed. It has been clearly K. In particular, the application of ultra-high strength copper plates with a tensile strength of 80'qf/vm" or higher to parts whose main purpose is to deform and absorb energy during a collision, such as bumpers, side members, or doors. This is effective in terms of both weight reduction and safety, and has recently been actively studied in conjunction with the fact that in the United States it is now legal to install a doper on the door.

Such ultra-high strength steel plates have conventionally been manufactured by the following method. Namely, (1) Recovery annealing method: A method in which the annealing temperature is lower than the recrystallization temperature of the steel sheet when the steel sheet is completely annealed, and only the elongation is recovered with almost no reduction in the as-rolled strength.

(2) Precipitation strengthening method: A method of adding elements that have a strong tendency to form carbonitrides, such as Ti + Nb, and obtaining strength by dispersing fine carbonitrides.

(3) Transformation strengthening method: A steel plate is annealed in the α+γ or γ region and then rapidly cooled to form a hard quenched structure (martensis).
1- A method of obtaining high strength.

etc.

(Problems with Prior Art) However, each of these methods has the following drawbacks.

In the recovery annealing method (1), as described in Japanese Patent Publication No. 57-9409, by increasing the strength of the original plate,
Although it is possible to manufacture ultra-high strength steel plates with a tensile strength of 100 kg7ws", the ductility of the resulting steel plates is very low, at most 6 to 8 inches, and therefore the workability is very poor. In addition to this, recovery If the ductility is insufficient, the ductility will further decrease, and if it is excessive, the material will soften rapidly. Therefore, in order to obtain the desired properties with less variation, it is necessary to control the annealing temperature within a very narrow range, making it stable. It is virtually difficult to manufacture products of such quality.

As an ultra-high strength steel plate using the precipitation strengthening method, the
There is one described in No. 22407.

As is clear from the description in this publication, obtaining ultra-high strength through precipitation strengthening requires large amounts of expensive alloying elements such as TI and Nb, making it extremely expensive. If it exceeds this range, there are disadvantages such as rapid deterioration in elongation, resulting in lower workability, and increased strength in the hot rolling stage, resulting in lower cold workability.

As an example of using a quenched structure, there is
There is a method using water quenching as shown in Publication No. 2.

With such rapid cooling, martensite can be easily obtained even with a small amount of alloy, but at 100'Kg/lax''
When trying to obtain a certain tensile strength.

Since a large amount of martensite is required, the coarsening of martensite and the formation of a band structure are unavoidable.

Ductility decreases. The steel described in Japanese Patent Publication No. 59-42052 also has a tensile strength of 100 kg/a" and an elongation of 12 inches, which is only a slight improvement in the ductility of the precipitation-strengthened copper described in Japanese Patent Publication No. 54-22407. In addition, the slow cooling method requires an extremely large amount of alloying elements, which increases the cost, and at the same time has the same structural defects as the water quenching method described above.It is necessary to avoid such harm due to martensite reinforcement. Therefore, Japanese Patent Application Laid-Open No. 53 (1983) aimed at producing ultra-high strength steel sheets by combining precipitation strengthening and recovery strengthening.
There is a technique described in Publication No.-5018. However, this method also results in a tensile strength of 1 due to the above-mentioned three drawbacks.
001 w/w'' and an elongation of only 12 to 13 inches at most has been obtained.

As described above, the current ultra-high-strength cold-rolled steel sheets have low workability, and therefore can only be applied to parts that are mainly bent, such as roll forming with a simple shape and a large bending radius.

(Problems to be Solved by the Invention) The present invention overcomes the drawbacks of the conventional method by paying special consideration to the composition of the starting steel material and the continuous annealing conditions, and achieves excellent workability that allows press forming. Has a tensile strength of 80kl? f
The object of the present invention is to provide an ultra-high strength cold-rolled steel sheet with a strength of /- or more.

(Means for Solving the Problems) The present inventors have conducted extensive research to overcome the drawbacks of the conventional methods described above, and have found that by adjusting the chemical composition of the starting steel material, the initiation of martensitic transformation is brought to the lower temperature side. By combining the cooling rate after annealing, the cooling end temperature, and the retention time at that temperature, multiple martensite is not generated and the strength-ductility balance is maintained by other low-temperature transformation formation phases. We have found that it is possible to significantly improve The present invention has been made based on this knowledge, and its gist is as follows.

(1) Heavy! i% C: 0.1-0.3%, Si:
0.25-2.0%, Mn: 1.5-2.5%,
P: 0.010% or less.

S: 0.00396 or less, 5o At: 0.0
2S Steel containing 0.10% T' gold and the balance Fe is melted and 1 is hot rolled, pickled, and cold rolled in the usual manner, then continuously annealed at a temperature of 750°C or higher, and then heated to 350°C. Cool to a temperature above 500°C at a cooling rate of 3°C/s to 70°C 78°C, maintain the temperature within a time range that satisfies the following formula (1), and then cool to room temperature. A manufacturing method for ultra-high strength cold-rolled steel sheets.

6000≧Tθ(6)×tθ(min)≧2000 ・
... (1) However, % (℃): Cooling end point temperature tθ (minutes): Holding time at cooling end point temperature (2) Weight [C in %; 0.1 to 0.3%, Si: 0.25 to 2
．． 0%, Mn: 1.5-2.5%, P: 0.
010% or less.

S: 0.003% or less, troL,)d, :
0.02~0.10% +Ca: 0.0002-
Copper containing 0.0030% and the balance consisting of Fe is melted, hot rolled and pickled using the usual method.

After cold rolling, it is continuously annealed at a temperature of 750°C or higher, and then heated at a temperature of 350°C or higher and lower than 500°C for 3°C/s or more7.
Production of an ultra-high strength cold-rolled steel sheet characterized by cooling at a cooling rate of 0°C/s or less, holding the temperature within a time range that satisfies the following formula (1), and then cooling to room temperature. Method.

6000≧Tθ(6)×tθ(min)≧2000...
・ (1) However, % (°C): Cooling end point temperature tBC minutes): Retention time at cooling end point temperature or less Ingredients of the present invention, &! The reasons for limiting the construction conditions will be explained in detail.

First, we will discuss the reasons for limiting the ingredients. C is an essential element for strengthening steel, but if the amount is less than 0.1 inch, the desired strength cannot be obtained and the temperature at which martensitic transformation begins becomes high. If it exceeds 0.391, the strength after hot rolling increases and cold workability decreases, and the appearance of a band structure of coarse martensite is unavoidable, and although strength is obtained, ductility deteriorates significantly. In addition, in order to obtain a product with a stable strength-ductility balance, the CI should be set to 0.
The range is preferably 16% or more and 0.25% or less.

St has the effect of strengthening steel and at the same time greatly improving ductility. In order to exhibit such an effect, it is necessary to add 0.251 or more, but if the addition amount exceeds 2.0%, the occurrence of St scale will be significant during the hot rolling stage, and the This not only reduces workability, but also significantly deteriorates workability, weldability, and corrosion resistance. In order to maximize the effect of improving the strength-ductility balance while suppressing the damage caused by scale by adding St, it is preferable that the addition i- is 0.8% or more and 1.7% or less.

Mn has the function of strengthening the steel, improving the hardenability of the steel, and reducing the cooling rate sensitivity of low-temperature transformation. That is, low-temperature transformation can be obtained even at low cooling rates.

If Mn is less than 1.596, this effect is not sufficient, and if it exceeds 2.5%, the formation of a band structure due to segregation becomes significant, resulting in a marked deterioration in workability.

Since op and s deteriorate the workability and weldability of steel,
Ultra-high strength steel plates such as the steel of the present invention require particularly strict control, with the upper limit of P being 0.010% and the upper limit of S being 0.01%.
003%. In addition, if further improvement in workability and weldability is desired, P' should be 0.0070% or less.

It is preferable that 8 is 0.0O10 or less.

Further, it is known that Ca combines with S and changes the form of sulfide into a form that is harmless to workability, and in the case of the present invention, Ca is added when control of the form of sulfide is necessary. In this case, if the addition 1 is less than 2% O,0O0, there is no effect on morphology control, and if it exceeds 0.003096, the effect will be saturated, and oxides harmful to processability will be generated, so 0.00
0.02% or more and 0.0030% or less.

At has a strong deoxidizing effect and is an effective element for improving the cleanliness of steel, but its content is 0.02' for sot and At.
If it is less than s, the desired effect cannot be obtained, and if it is less than 0, 1096i
If the content exceeds 1i, it will reduce cold workability.
f sot and Atte were set at 0.02 to 0.10%.

Steel with the above components is melted in a converter or electric furnace, made into a slab by continuous casting or blooming, then hot rolled and pickled under normal conditions.

Cold rolled.

The conditions for hot rolling are normal conditions and are not particularly regulated, but in order to suppress the formation of 81 scale as much as possible and ensure performance such as workability, corrosion resistance, and weldability, the heating temperature should be set to 100°C. More than 11501? : Hereinafter, it is desirable that the finishing temperature is 830°C or more and 880'C or less, and the winding temperature is 600°C in order to prevent the formation of a hardened structure during the hot rolling stage and to suppress the formation of a knurled structure. More than 700
``It is desirable that the range be below C.

The cold rolling method may be a normal rolling method or an asymmetric rolling method. From the viewpoint of making the ferrite grains fine after recrystallization and improving ductility, it is desirable that the rolling reduction ratio be 40% or more in terms of area reduction ratio.

After forming the cold rolled steel strip in this way, it is annealed by a continuous annealing method, and the annealing temperature at that time must be 750° C. or higher. If the annealing temperature is less than 750'C, hardly any austenite phase will be formed during annealing, and as a result, the number of low-temperature transformation formed phases will decrease, making it impossible to obtain the desired strength. Further, the upper limit is considered to be 900° C., considering the equipment aspect of the current continuous annealing equipment for thin plates. The temperature range in which the steel having the components of the present invention is annealed in the two-phase region is the heating rate,
The ingredients vary slightly depending on the eyelid, but approximately 750-830
℃ range, and annealing at the low temperature side of this temperature range,
As the distribution of alloying elements into austenite increases compared to that at high temperatures, the structure after annealing becomes a two-phase structure of ferrite + coarse martensite, and at the same time becomes a band structure, which tends to reduce workability. Therefore, the setting of the end point temperature of cooling and the holding time at that temperature is limited to a narrow range. On the other hand, annealing at the high temperature side of this temperature range or higher temperatures increases the austenite (t) during annealing, so
On the contrary, the amount of alloy is reduced, making it easier to obtain a uniform structure. Such a temperature range is 820 to 880°C, and it is desirable to perform annealing within this range. Cooling rate after annealing is 3℃/s
It must be above 70°C/s. If the cooling rate is less than 3° C./s, the desired low-temperature transformation product phase cannot be obtained, and it becomes extremely uneconomical to realize such a low cooling rate in terms of equipment. Furthermore, even if the lower limit of the range of components of the present invention is taken, a cooling rate of 70°C/B is sufficient to obtain a low-temperature transformation phase, and if it exceeds 70'C/me, workability decreases due to quenching distortion. It's remarkable.

In addition, the shape of the steel plate also deteriorates. When more preferable components, hot rolling, cold rolling and annealing temperature ranges are taken,
The cooling rate is desirably 5° C./m or more and 30° C./s or less. The end temperature of cooling and the holding time at that temperature are extremely important factors that determine the properties of the low-temperature transformation phase, t', and the cooling end temperature Tθ (6) is 350°C or more and 500°C.
The holding time tθ (minutes) at this end point temperature must be in the range determined by the following equation (FIG. 1).

6000≧TB It is known that the 7-phase light + martensitic structure has high workability and is called dual phase steel. However, the present inventors have found that in the case of the present invention, when increasing the martensite volume fraction in order to increase the strength, the formability deteriorates on the contrary. Therefore, the cooling end point temperature was set at 350° C. or higher based on the knowledge that generation and contamination of a large amount of martensite should be avoided. Moreover, when the temperature exceeds 500° C., the low-temperature transformation phase itself does not occur.

The holding time at the end point temperature is determined by the i point source temperature; if the end point temperature is high, the transformation rate is slow, so the holding time may be short, but if the transformation temperature is low, a long time is required.

In order to determine the range of Tθ and tθ, the present inventors used a cold-rolled steel sheet having components within the range of the present invention and varied Tθ and tθ within the range of the annealing temperature and cooling rate of the continuous annealing conditions of the present invention. The resulting mechanical properties were investigated. The result is Tθ,
Value of tθ ki'? FIG. 2 shows the parameters expressed as parameters. This figure shows that the elongation is better than the tensile strength. In other words, in order to obtain a material with a large tensile strength x elongation value and excellent bendability, Tθ x tθ must be in the range of 2000 or more and 6000 or less, further improving strength, elasticity balance, and bendability. To do this, Tθ×tθ is 32
It is preferable to set it to 00 or more and 5200 or less.

After this holding, it is cooled to room temperature, but the cooling rate at that time is not particularly restricted. In addition, after cooling to room temperature, the product is normally subjected to skin-pass rolling, but according to the method of the present invention, the shape of the steel sheet is excellent and no skin-pass rolling is required, but the shape accuracy is extremely strict. , the elongation rate is 0.5
The effects of the present invention are not impaired even if temper rolling is performed below H.

(Example 1) Steel A-L having the chemical composition shown in Table 1? It was melted in a converter and made into a slab by continuous casting. This slab was heated to a temperature of 1080°C and a finishing temperature of 850°C.

Hot-rolled at a winding temperature of 650°C to obtain a 3.5-〇 hot-rolled steel strip with a thickness of 3.5-〇, followed by ordinary pickling, and then cold-rolled into a cold-rolled steel strip with a thickness of 1.4 ttm. And so. Then the second
Annealing was carried out using the continuous annealing cycle shown in Table A7, but the annealing time was 3 minutes, and the cooling to room temperature after being held at the cooling end temperature was air-cooled. A test piece was taken from the center of the entire length of this annealed coil, and its mechanical properties (2) bendability were investigated. The mechanical properties were measured by a tensile test using a JIS No. s test piece, and the bending property was measured using a 45°V bending test using an as-sheared test piece @ 50 ram and a length of 150 m. ,,) was evaluated.

The results are shown in Table 3. In A, where C is low outside the range of the present invention, the strength is poor, and in B, where C is high, the ductility deteriorates and the bendability is poor. C whose si is low and outside the range of the present invention lacks ductility. In addition, in D where Mn is higher than the range of the present invention, elongation 1 bendability deteriorates,
Also, if the E is set to a low value, the strength will not be achieved. Furthermore, F and G, in which P and S are outside the range of the present invention, have inferior bendability compared to the present invention steel. On the other hand, the steel of the present invention H-L has good elongation compared to its strength and is also very good in bendability.

Table 3 (Example 2) Among the cold rolled steel strips prepared in Example 1 and having the chemical compositions shown in Table 1, the cold rolled steel strips having the composition of the steel code, which is the steel of the present invention, were used to prepare the second Annealing was performed under the continuous annealing conditions shown in the table. The annealing time at that time was 3 minutes, and air cooling was used for cooling to room temperature after holding at the cooling end temperature. Samples for mechanical testing were taken as in Example 1 and weighed in the same manner.

The results are shown in Table 4. J161, in which the annealing temperature was completely outside the range of the present invention, and J161, in which the holding temperature after cooling was completely outside the range of the present invention, did not exhibit strength. Further, in A3, in which the holding temperature after cooling was outside the range of the present invention, the strength-ductility balance was poor and the bendability was also deteriorated. In Mums 4 and 5, in which the holding time at the cooling end point temperature was outside the range of the present invention, both ductility and bendability were deteriorated compared to the steel of the present invention. On the other hand, according to the present invention, A6 which is made of Ming steel
- &9 are excellent in both strength-ductility balance and bendability.

Table 4 (Effects of the Invention) The steel sheet manufactured by the present invention can be formed by conventional bending-based processes such as rolls of 7 ohm, and can be easily formed by pressing into items with uneven cross sections such as door guard bars. can. Furthermore, during bending, as is clear from this embodiment, the bending radius can be made extremely small, thereby reducing springback and the like.

As described above, according to the present invention, a steel plate having high strength and excellent workability can be obtained, so that it contributes not only to the weight reduction of automobile parts but also to a wide range of industries.

[Brief explanation of drawings]

Figure 1 shows the relationship between the cooling end point temperature Tθ (6) and the holding time tθ (minutes) at the cooling end point temperature, and the area surrounded by the broken line and solid line in Figure 1 is the scope of the present invention. . Figure 2 shows that the steel of the present invention having the components of the first surface treatment is used, and Tθ is within the range of the annealing temperature and cooling rate of the continuous annealing conditions of the present invention.
Mechanical properties obtained by changing and ta? FIG. 3 is a diagram showing the value of Tθ×tθ as a parameter. Figure 2/It) t) 261) l) J6t) θatm sa
aθ67) 121 ml) 70xtθ

Claims (3)

[Claims] (1) C: 0.1-0.3%, Si: 0.25 in weight%
~2.0%, Mn: 1.5-2.5%, P: 0.010
% or less, S: 0.003% or less, sol. Al: 0.0
A steel containing 2 to 0.10% Fe with the remainder being Fe is melted, hot rolled, pickled, and cold rolled in the usual manner, and then continuously annealed at a temperature of 750°C or higher. ℃ or more 50
Cool to a temperature below 0°C at a cooling rate of 3°C/s or more and 70°C/s or less, maintain the temperature within a time range that satisfies the following formula (1), and then cool to room temperature. A manufacturing method for ultra-high strength cold-rolled steel sheets. 6000≧T_θ (°C)×t_θ (minutes)≧2000...
・(1) However, T_θ (℃): Cooling end point temperature t_θ (minutes): Holding time at cooling end point temperature (2) C: 0.1-0.3%, Si: 0.25 in weight%
~2.0%, Mn: 1.5-2.5%, P: 0.010
% or less, S: 0.003% or less, sol. Al: 0.0
2-0.10%, Ca: 0.0002-0.0030%
A steel containing Fe with the balance being Fe is melted, hot rolled, pickled, and cold rolled in the usual manner, and then continuously annealed at 750°C.
Annealed at a temperature above, cooled to a temperature of 350°C or more and less than 500°C at a cooling rate of 3°C/s or more and 70°C/s or less, within a time range that satisfies the following formula (1) within that temperature range. A method for producing ultra-high strength cold-rolled steel sheets, which is characterized by cooling to room temperature after inspection. 6000≧T_θ (°C)×t_θ (minutes)≧2000...
・(1) However, T_θ (℃): Cooling end point temperature t_θ (minutes): Holding time at cooling end point temperature (3) Tensile strength 80kgf/mm^2 or more, elongation 16%
As described above, the ultra-high strength cooling according to claim 1 or 2, characterized in that the minimum bending radius r (mm) is not more than twice the plate thickness t (mm) (that is, r≦2t). Method of manufacturing rolled steel plate.