JPH0665685A - Cold rolled sheet of ultrahigh tensile strength steel and its production - Google Patents

Abstract

PURPOSE:To produce a cold rolled sheet of ultrahigh tensile strength steel excellent in bendability while taking advantage of short delivery date and low cost by subjecting a slab of steel with specific composition to hot rolling, to cold rolling, and to continuous annealing under specific conditions and then performing overaging treatment. CONSTITUTION:A slab of a steel which has a composition containing, by weight, 0.080-0.20% C, <0.50% Si, 1.50-3.00% Mn, <0.030% P, <0.008% S, 0.01-0.10% Al, and >0.50-1.50% Cr or further containing 0.20-1.00% Mo so that (Cr+Mo) becomes 0.70 to 1.50% is hot-rolled, finish-rolled at a temp. not lower than the Ar3 transformation point, and coiled at 600-700 deg.C. The resulting hot rolled steel plate is pickled to undergo the removal of surface scale and cold-rolled to final sheet thickness. The resulting sheet is continuously annealed at 800-900 deg.C for 20-200sec and cooled. Then the sheet is held at <=430 deg.C for 60-600sec to undergo overaging treatment. By this method, the cold rolled sheet of ultrahigh tensile strength having a structure composed essentially of fine and uniform bainite, having >=980N/mm<2> tensile strength, and excellent in bendability can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to an ultrahigh-strength cold-rolled steel sheet having a tensile strength of 980 N / mm ² or more, which is suitable for use in, for example, automobile door guard bars and bumper reinforcements, and the production thereof. Regarding the method.

[0002]

2. Description of the Related Art Conventionally, as the number of traffic accidents has increased, there has been an increasing demand for improving the safety of automobiles. In particular, in order to secure the collision strength in the case of a side collision, a plate-shaped or Pipe-shaped door guard bars have been fixed. On the other hand, in order to improve the fuel efficiency and power performance of automobiles, the weight reduction of the vehicle body is also progressing in parallel, and for example, thinning and strengthening of steel plates used for vehicle body constituting members such as bumper reinforcement are being promoted.

In particular, for members such as a door guard bar and a bumper reinforcement that require high strength, it has hitherto been a problem.
High-tensile cold-rolled steel sheets of 90 to 780 N / mm ² class were used, but with the increasing emphasis on safety and weight reduction in recent years, ultra high strength of 980 to 1470 N / mm ² class has recently been achieved. Tension cold-rolled steel sheets have come into use.

By the way, in general, for ultra-high-strength cold-rolled steel sheets with TS ≧ 980 N / mm ² , the forming method applied to mild steel sheets such as draw forming and stretch forming cannot be applied. Be the subject. Therefore, it is important for the ultra-high-strength cold-rolled steel sheet to have good bendability.

As an ultra-high-strength steel sheet having good workability, a dual-phase steel (dual-phase steel) is known in which hard martensite is dispersed in a soft ferrite material to improve strength and workability at the same time. Widely used. However, although this dual-phase steel has good ductility, it lacks bendability and cannot be applied to parts manufactured by severe bending. So, from the past,
Various techniques for improving the bendability of ultra-high-strength cold-rolled steel sheets have been proposed.

For example, Japanese Patent Laid-Open No. 63-14817 discloses that
C: 0.03 to 0.20% (hereinafter, "%" means "% by weight" unless otherwise specified), Si: 0.3 to 1.5%, Mn: 0.5 to 2.6% In addition, if necessary, Ti: 0.01 to 0.25% and Nb:
In addition to 0.01 to 0.3% of 1 type or 2 types, B: 0.0003 to 0.
Steel containing 01% and the balance Fe and unavoidable impurities is subjected to hot rolling, heat treatment, cold rolling, annealing, quenching and over-aging treatment under specific conditions to obtain 785 to 147
A technique for producing a high-strength cold-rolled steel sheet having a tensile strength of 0 N / mm ² and excellent bending properties is disclosed in JP-A-63-38526, in which C: more than 0.1 to 0.2% and Si: 0.7%. Below, Mn: 1.5 ~
2.5%, Ti: 0.05 to 0.15%, S: 0.008% or less, Al: 0.
Steel containing 1% or less, balance Fe and unavoidable impurities,
Directly sent or heated to over 1200 ℃ and hot rolled to 850
After finishing hot rolling in the temperature range of ~ 950 ° C, winding in the temperature range of 500 ~ 650 ° C, followed by cold rolling, annealing for 30 ~ 300 seconds at the recrystallization temperature or higher and 930 ° C or lower. Then, the sample is cooled at an average cooling rate of 3 to 200 ° C / sec, and then the following formula according to the annealing temperature 1100-ST ≤ T ≤ 1200-ST ... where ST: annealing temperature (° C Each of the technologies for producing an ultra-high tensile cold-rolled steel sheet excellent in bending formability is proposed by holding at a temperature T (° C.) within a range satisfying the above condition (2) for 2 to 10 minutes.

Both of these techniques use C, Si and Mn.
In the continuous annealing process after cold rolling, using a steel containing as a main component, the overaged zone that is part of the continuous annealing equipment is used to specify the cold-rolled steel sheet in the overaged zone after annealing and cooling. It is characterized by improving the bendability of the ultra-high-strength cold-rolled steel sheet as a structure mainly composed of fine and uniform bainite by retaining only the temperature range and time.

[0008]

According to these conventional techniques, it is possible to manufacture an ultrahigh-strength cold-rolled steel sheet having good bendability by using the most common C, Si and Mn-containing steel. Therefore, there is an advantage that the cost increase can be suppressed as much as possible. On the other hand, however, these techniques have the problems listed below.

Since the retaining conditions (overaging conditions) have a great influence on the strength and bendability of the cold rolled steel sheet, it is necessary to strictly control the retaining conditions. Therefore, in the actual production on the continuous annealing line, it is necessary to connect dummy coils for temperature adjustment called connecting materials before and after the target coil. By the way, the overageing zone is often long and its heat capacity is large.
A large weight dummy coil, such as around 0 tons, is required, which leads to an increase in manufacturing cost due to manufacturing restrictions and a reduction in productivity (an increase in loss due to processing of dummy material).

It is preferable to anneal in the austenite region as much as possible in order to obtain a bainite-based uniform structure, but for that purpose it is necessary to carry out continuous annealing at an annealing temperature close to 900 ° C., which causes an increase in energy cost.

An object of the present invention is to provide an ultra-high tensile cold-rolled steel sheet excellent in bendability which can be manufactured without killing the features of the continuous annealing method of short delivery time and low cost, and to retain this high-tensile cold-rolled steel sheet. It is to provide a method for manufacturing while suppressing the influence of time and annealing temperature.

[0012]

Means for Solving the Problems The inventors of the present invention have made various studies to solve the above-mentioned problems, obtained important findings of the contents listed below, and further earnestly studied to complete the present invention. did. (i) C, Si, and Mn-containing steels to which Cr and, if necessary, Mo are added in appropriate amounts each have improved bendability as compared with steels containing no C, Si and Mn.

(Ii) C, Si and Mn-containing steels to which Cr and, if necessary, Mo are respectively added in appropriate amounts are annealed at a temperature of 800 ° C. or higher, and the target minimum bending radius of the ultrahigh strength cold-rolled steel sheet is 1.5.
mm or less can be obtained. On the other hand, non-added C, Si and
Annealing at 850 ° C or higher is required for Mn-containing steel.

(Iii) C, Si, and Mn-containing steels to which Cr and, if necessary, Mo are added in appropriate amounts, respectively, have little influence of overaging treatment temperature on tensile strength and bendability, and have good properties over a wide range. Shows tensile strength and bending performance.

The gist of the present invention is as follows.
C: 0.080 to 0.200%, Si: 0.50% or less, Mn: 1.50 to
3.00%, P: 0.030% or less, S: 0.008% or less, Al: 0.
980 N / mm ² or more, characterized by having a steel composition consisting of 01 to 0.10%, Cr: more than 0.50 to 1.50%, balance Fe and unavoidable impurities, and having a fine and uniform bainite-based structure. It is a super-high-strength cold-rolled steel sheet having excellent tensile strength and excellent bendability.

In another aspect, the present invention provides C: 0.080-0.
200%, Si: 0.50% or less, Mn: 1.50 to 3.00%, P: 0.
030% or less, S: 0.008% or less, Al: 0.01 to 0.10%, C
r: over 0.50% to 1.50%, Mo: 0.20 to 1.00 as required
% And Cr + Mo: Steel having a steel composition containing 0.70 to 1.50% is hot-rolled at a finishing temperature of Ar ₃ transformation point or higher, wound in a temperature range of 600 to 700 ° C, then pickled and cooled. Hot-rolling, continuous annealing in the temperature range of 800-900 ° C for 20-200 seconds, cooling, and then overaging treatment in the temperature range of 430 ° C or less for 60-600 seconds. Is a method for producing an ultra-high-strength cold-rolled steel sheet having a tensile strength of 980 N / mm ² or more and excellent in bendability.

[0017]

The operation of the present invention will be described in detail below. First, the reason for limiting the composition of the ultrahigh strength cold rolled steel sheet according to the present invention will be described.

C: C is an element effective for improving strength, and when the C content is less than 0.080%, the target TS ≧ 98.
It becomes difficult to form martensite and bainite necessary for obtaining 0 N / mm ² , while when the C content exceeds 0.200%, the strength of the spot welded portion used when manufacturing the door guard bar is reduced. Therefore, the C content is limited to 0.080% or more and 0.200% or less.

Si: Si is an element that improves the hardenability of the material, and is an effective element in the production of ultra-high-strength cold-rolled steel sheet. However, on the other hand, the production of ferrite is also promoted,
If the Si content exceeds 0.50%, the amount of ferrite generated increases, the nonuniformity of the structure is promoted, and the bendability deteriorates. Therefore, the Si content is limited to 0.50% or less.

Mn: Mn is an element that improves the hardenability of the material like Si, but if it is less than 1.50%, its effect is reduced and the desired strength cannot be obtained. On the other hand, when the Mn content exceeds 3.00%, the bainite structure that is the object of the present invention becomes difficult to be generated, and the amount of metal Mn used in the steelmaking stage increases, which causes an increase in cost. Therefore, the Mn content is limited to 1.50% or more and 3.00% or less.

P: P is an element that causes deterioration of toughness in an ultra-high-strength cold-rolled steel sheet, so that brittle fracture may occur not only in the final product stage but also in the cold rolling stage. Therefore, it is desired to reduce the P content as much as possible, but if it is extremely decreased, the cost will increase, and the content of about 0.030% is allowed. Therefore, the P content is limited to 0.030% or less.

S: When the S content exceeds 0.008%, expanded MnS is produced and the bendability deteriorates. Therefore, the S content is limited to 0.008% or less.

Al: Al is an element effective for deoxidation for improving the cleanliness of steel, and it is necessary to add 0.01% or more for that, but the effect is saturated even if added over 0.10%. The cost will only increase. Therefore, the Al content is 0.01% or more.
It is limited to 0.10% or less.

Cr: Cr is an extremely important element in the present invention, and is an element that promotes bainite transformation to bring about a fine and uniform structure and contributes to the improvement of bendability. The effect of improving the bendability may be 0.50% or more.
In the case of adding together with Cr, Mo + should be 0.70% or more. On the other hand, if the added amount is more than 1.50%, the effect is saturated and only the cost is increased. Therefore, the Cr content is limited to more than 0.50% and 1.50% or less.

Mo, Cr + Mo: Mo is an optional additive element added according to need in the present invention, and promotes bainite transformation similarly to Cr, but it is effective to add Cr in combination. . When adding Mo, it is effective to add 0.20% or more. On the other hand, Mo is expensive, so 1.00%
If added too much, the cost will increase. Therefore, when Mo is added, its content is preferably limited to 0.20% or more and 1.00% or less.

By adding Cr and Mo in combination, a fine and uniform bainite-based structure is formed, which is effective in improving bendability, and the effect of the overaging treatment conditions in continuous annealing on the characteristics. It is possible to manufacture an ultra-high-strength cold-rolled steel sheet that is small and has good bendability under a relatively wide range of overaging conditions. Considering only the improvement of bendability, the effect is recognized even with a small amount of Cr + Mo, but in order to reduce the influence of overaging treatment conditions to eliminate the use of the dummy coil mentioned above, or to minimize the amount used 0.
Addition of 70% or more is effective. On the other hand, if it exceeds 1.50%, the effect will be saturated and only increase the cost. Therefore, when adding Mo, the amount of Cr + Mo is 0.70% or more 1.
It is desirable to limit it to 50% or less. In the usual case, compositions other than the above are Fe and inevitable impurities.

The ultra-high-strength cold-rolled steel sheet according to the present invention has a fine and uniform structure mainly composed of bainite. Bainite has an area ratio of 50% or more. The term "fine" means the degree of fineness obtained by the present invention, and specifically, it is about 10 to 12 in terms of grain size. Further, the term “uniform” means the degree of uniformity obtained by the present invention, and means that the difference in structure between the plate thickness and the plate width direction is small.

Structures other than bainite are martensite and ferrite, and their area ratios are about 0 to 30% and 0 to 20%, respectively. The ultrahigh-strength cold-rolled steel sheet according to the present invention having the above composition and structure is 980 N /
It has a tensile strength of mm ² or more and excellent bendability.

In the present invention, the steel having the above composition is provided with Ar.
Hot rolling is performed at a finishing temperature of ₃ transformation points or higher. The steel for ultra-high-strength cold-rolled steel sheet has a large deformation resistance even during hot rolling, and especially when the finishing temperature is less than Ar ₃ point, the deformation resistance rapidly increases, making it difficult to perform hot rolling. Therefore, the finishing temperature is limited to the Ar ₃ transformation point or higher. Desirably, it is 870-950 ° C.

After hot rolling, it is wound in a temperature range of 600 ° C. or higher and 700 ° C. or lower. If the coiling temperature is lower than 600 ° C, bainite and martensite are generated, and the material becomes hard and cold rolling becomes difficult. On the other hand, when the coiling temperature exceeds 700 ° C, the band structure is excessively developed due to segregation of Mn and the like, and even after cold rolling-continuous annealing, a nonuniform structure is formed in the sheet thickness direction, and bendability deteriorates. Therefore, the winding temperature is limited to 600 ° C or higher and 700 ° C or lower. After winding, follow the normal process,
Perform pickling and cold rolling. The steps of pickling and cold rolling may be ordinary steps without any limitation.

After the cold rolling is completed, continuous annealing is performed under the condition of holding in the temperature range of 800 to 900 ° C. for 20 to 200 seconds and cooling. In order to obtain an ultra-high-strength cold-rolled steel sheet with good bendability, it is desirable to perform continuous annealing in the austenite single-phase region as much as possible. For that purpose, it is necessary to perform continuous annealing at 800 ° C or higher for 20 seconds or longer. is there. On the other hand, if the annealing temperature and annealing time exceed 900 ° C and 200 seconds, respectively,
A part of the austenite crystal grains during the continuous annealing becomes coarse and non-uniform, conversely the bendability is deteriorated, and annealing is performed at high temperature for a long time, which causes an increase in energy cost. Therefore, the annealing temperature is limited to 800 ° C to 900 ° C, and the annealing time is limited to 20 seconds to 200 seconds. After annealing, overaging is continued, but the cooling rate during that period is 40 to 150 ° C.
/ s is preferable.

The overaging treatment is carried out at a temperature range of 430 ° C. or lower at 60 to
Perform under the condition of holding for 600 seconds. The ultra-high-strength cold-rolled steel sheet according to the present invention has little influence of overaging treatment conditions after annealing,
It is characterized by not being restricted by the annealing schedule of continuous annealing, but when the overaging treatment temperature exceeds 430 ° C, the structure may become coarse and the strength may decrease. In addition, the holding time must be 60 seconds or more to obtain a fine structure mainly composed of bainite. On the other hand, if it exceeds 600 seconds, it is necessary to reduce the strip speed of continuous annealing, which lowers the productivity. .

Thus, according to the present invention, 980 N / mm
It becomes possible to manufacture an ultra-high-strength cold-rolled steel sheet having a tensile strength of ² or more and excellent in bendability. Further, the present invention will be described in detail with reference to examples, but this is an example of the present invention and the present invention is not limited thereto.

[0034]

Example 1 Steels A to D shown in Table 1 were melted by using an existing converter, the components were adjusted by a vacuum degassing furnace (RH degassing furnace), and then cast by a continuous casting machine to form a slab. Got

[0035]

[Table 1]

Heating temperature of the obtained slab: 1200 to 1250
C., finishing temperature: 870 to 900.degree. C., coiling temperature: 630.degree. After pickling, cold rolling was performed to 1.2 mm to form a cold rolled coil, and a sample was cut out and the influence of continuous annealing conditions was investigated by a continuous annealing simulator.

The tensile test was conducted by producing JIS No. 5 test pieces, and the bending test was conducted by bending at 90 degrees with a V block.
The minimum bending radius at which cracking did not occur was determined. The bending test was carried out with the shear cut on one end surface. The effect of annealing temperature was investigated by the heat cycle shown in FIG. 1 (continuous annealing condition: 700 to 880 ° C. × 40 seconds, overaging treatment condition: 300 ° C. × 120 seconds).
The results obtained are shown graphically in FIG.

From FIG. 2, A with Cr or Cr + Mo added
Steels and B steels have better bendability than C steels that do not add Cr and Mo and D steels that add Si, and the target minimum bending radius of ultra-high-strength steel sheets by annealing at 800 ℃ or more. 1.5 mm or less is obtained. In contrast, C steel and D steel are 850 ℃
The above annealing is required.

Next, the heat cycle shown in FIG. 3 (continuous annealing condition: 820 ° C. × 40 seconds, overaging treatment condition: 250 to 500 ° C.)
The effect of overaging treatment temperature was investigated for 120 seconds). The results obtained are shown graphically in FIG. From FIG. 4, it can be seen that the steels A and B have little influence of the overaging treatment temperature on the tensile strength and the bending performance, and show good tensile strength and bending performance over a wide range.

[0040]

Example 2 Steels 1 to 13 having the steel compositions shown in Table 2 were
In the following steps, melting, hot rolling, winding, pickling, cold rolling, continuous annealing, cooling (after annealing 5 to 10 ℃ / 700 ℃
Slow cooling in seconds, then cooling to overaging temperature at 80 to 120 ° C./second), overaging treatment and temper rolling were carried out to produce an ultra high tensile cold rolled steel sheet.

[0041]

[Table 2]

(1) Steelmaking: Converter → Vacuum degassing (RH): Composition adjustment → Continuous casting: Casting → Slab 240t (mm) (2) Hot rolling: Heating: 1200-1220 ° C, Finishing temperature: 870
~ 900 ℃, winding: 630 ~ 660 ℃ Plate thickness: 2.4 mm → pickling →
Cold rolling: 2.4 mm → 1.2 mm (3) Continuous annealing: See Table 2 for continuous annealing conditions and overaging treatment conditions.
Reference (4) Temper rolling: Reduction ratio 0.3 to 0.5% Sample N, which is the ultra-high-strength cold-rolled steel sheet produced in this way
JIS No. 5 tensile test pieces were cut out from o.1 to sample No. 13 and subjected to a tensile test to measure YP, TS and El, and at the same time, a minimum bending radius was measured by a bending test in which 90 ° bending was performed by a V block. . The results are summarized in Table 3.

[0043]

[Table 3]

As is apparent from Table 3, according to the present invention,
It has become possible to manufacture ultra-high-strength cold-rolled steel sheets with a tensile strength of 980 N / mm ² or more and excellent bendability. Therefore, it is a material suitable for use in, for example, door guard bars of automobiles, bumper reinforcements, and the like.

[0045]

As described in detail above, according to the present invention, TS
It is possible to manufacture an ultra-high-strength cold-rolled steel sheet having a good bending performance of ≧ 980 N / mm ² while suppressing the effects of annealing in continuous annealing and overaging treatment conditions to be small. Therefore, it is possible to take advantage of the short delivery time and low cost that are features of continuous annealing.

[Brief description of drawings]

1 is a heat cycle showing the annealing conditions used in Example 1. FIG.

2 is a graph showing the results of Example 1. FIG.

3 is a heat cycle showing overaging treatment conditions used in Example 1. FIG.

FIG. 4 is a graph showing the results of Example 1.

Claims (3)

[Claims] 1. By weight%, C: 0.080 to 0.200%, Si: 0.50% or less, Mn: 1.50 to
3.00%, P: 0.030% or less, S: 0.008% or less, Al: 0.01 to 0.10
%, Cr: more than 0.50% to 1.50%, having a steel composition consisting of the balance Fe and unavoidable impurities, and having a fine and uniform structure mainly composed of bainite, tensile of 980 N / mm ² or more Ultra-high-strength cold-rolled steel sheet with strength and excellent bendability. 2. By weight%, C: 0.080 to 0.200%, Si: 0.50% or less, Mn: 1.50 to
3.00%, P: 0.030% or less, S: 0.008% or less, Al: 0.01 to 0.10
%, Cr: over 0.50% to 1.50%, steel having a steel composition containing 60% is subjected to hot rolling at a finishing temperature of at least the Ar ₃ transformation point, and 60
After winding in the temperature range of 0 to 700 ° C, pickling and cold rolling are performed, and continuous annealing is performed in the temperature range of 800 to 900 ° C for 20 to 200 seconds to cool and then 430 ° C. A method for producing an ultra-high-strength cold-rolled steel sheet having a tensile strength of 980 N / mm ² or more and excellent in bendability, which is characterized by performing an overaging treatment in which the temperature is maintained for 60 to 600 seconds in the following temperature range. 3. The tensile strength of 980 N / mm ² or more according to claim 2, wherein the steel further contains Mo: 0.20 to 1.00% and Cr + Mo: 0.70 to 1.50% by weight. Of a super-high-strength cold-rolled steel sheet having high bendability and excellent bendability.