JPH0368927B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for producing an ultra-high strength cold-rolled steel sheet having excellent workability and a tensile strength of 100 Kgf/mm ² or more. (Conventional technology) Strengthening of automotive steel sheets is progressing more and more from the viewpoint of energy and safety of automobiles, and tensile strength of 100 kg has been achieved.
It has been put into practical use to the extent that it exceeds f/mm class ² .
Typical examples of the use of ultra-high-strength cold-rolled steel sheets are door reinforcements and bumper reinforcements based on U.S. federal safety standards. However, as the strength of the material increases, its ductility generally deteriorates, making it difficult to form. When the tensile strength exceeds 100 Kgf/mm ² class, press molding is extremely difficult, and roll forming is usually used. Roll forming gradually obtains a predetermined cross-sectional shape using multiple forming rolls, and while it is suitable for processing difficult-to-form materials, it has drawbacks such as low productivity and difficulty in producing complex shapes. The current state of the manufacturing method for ultra-high strength cold-rolled steel sheets used in this manner is detailed in the materials listed in Tetsu-to-Hagane, No. 68 (1982), No. 9, pages 1348-1354. Typical manufacturing methods include those described in Japanese Patent Publication No. 59-42052 and Japanese Patent Publication No. 61-8125. The former achieves ultra-high strength by rapidly cooling in jet water to increase the martensite volume fraction, and the latter achieves ultra-high strength through a structure in which martensite is dispersed in an unrecrystallized ferrite matrix. Although imparting a certain degree of workability is part of the purpose of all of them, they do not have complicated press moldability. Despite this current situation, there was a strong movement among automobile companies to improve productivity and pursue new designs.
There has been an increasing demand for ultra-high-strength steel plates to be processed by press forming, which is highly productive and easy to form into complex shapes. On the other hand, the factors governing the press formability of such ultra-high strength steel plates are different from those of mild steel plates, and experience with mild steel plates does not apply. For example, there is no simple positive correlation between elongation at break and formability in a simple tensile test. Although it is not a conventional technology, a composition similar to the steel composition of the present invention is described in Japanese Patent Publication No. 53-47331, but this technology does not relate to ultra-high strength cold-rolled steel sheets, and The extension conditions are also significantly different. (Purpose of the invention) In view of the current situation, a tensile strength of 100 kg, which can withstand press molding based on the actual press molding scale, has been developed.
It is an object of the present invention to provide a method for producing an ultra-high strength cold rolled steel sheet having f/mm ² or more. (Structure of the invention) The gist of the present invention is that C: more than 0.10
0.20%, Si: 0.6% or less, Mn: 2.2-3.0%, S:
Average cooling rate after hot rolling of steel consisting of 0.008% or less, Al: 0.01 to 0.1%, and the remainder unavoidable impurity elements: 30
Cool at ~100℃/sec, wind at 500~630℃,
Subsequently, after cold rolling, [A _c3 transformation point -20℃] or higher,
Processability, which consists of heat treatment at a temperature of 930℃ or less and for a period of time shown by the following formula, followed by cooling at an average cooling rate of 5 to 30℃/sec, and holding at 250 to 450℃ for 1 to 10 minutes. The method lies in the production of superior ultra-high strength cold-rolled steel sheets. logt≧(1070−T)/130 Where, t: Time (seconds) T: Temperature (°C) In other words, it is a steel that does not contain Ti or Nb, and has a higher Mn content than conventional technology. limited C
-Using Mn aluminum killed steel, after carrying out a specific hot rolling process, it is subjected to a first stage heat treatment with a specified lower limit of temperature and time, followed by a specific continuous cooling,
It is characterized by stopping continuous cooling midway through and performing a specific second-stage heat treatment. The reason for limiting the chemical composition of the target steel of the present invention will be explained. In the present invention, the tensile strength is stably 100Kg.
C exceeds 0.10% in order to achieve f/mm ² or more. On the other hand, if C exceeds 0.20%, it deteriorates formability and causes poor weldability. In order to obtain good weld strength in spot welding, C is 0.16.
% or less. Si has the effect of solid solution strengthening and has _little ductility deterioration, so it is often used in high-strength steel sheets.
Since it promotes separation, the amount added should be within 0.6%.
It should preferably be within 0.3%. Next, Mn is an important element in the present invention. In other words, Mn has the effect of lowering the Ac ₃ transformation point of the steel, making it easier to obtain a uniform γ phase in the first heat treatment, increasing the hardenability of the steel, and preventing the formation of structures such as coarse pearlite at an appropriate primary cooling rate. There is. . Mn needs to be at least 2.2% to produce such an effect. The upper limit shall be 3.0%. This is because the effect is saturated around this value, and increasing it any further will only make the steel unnecessarily expensive. S is a harmful substance that forms sulfide inclusions and inhibits the formability of steel. Therefore, the upper limit is 0.008%
It was determined that 0.004 by making full use of the latest ultra-low sulfide technology
% or less is preferable. Furthermore, it is more preferable to add 0.0005 to 0.0050% of Ca to change the sulfide composition as needed. Al is necessary as a deoxidizing agent for steel. If it is less than 0.01%, sufficient deoxidation will not be achieved, and if it exceeds 0.1%, inclusions in the steel will increase. Steel is usually cast into a slab by continuous casting and then hot rolled either directly or after heating. When heating, the heating temperature is usually 1000 to 1300°C, but direct hot rolling or low temperature heating hot rolling at 1100°C or less is preferred in order to prevent coarsening of crystal grains before hot rolling. Hot rolling finish temperature is 800
The temperature may be up to 950°C, but if it is too low, a band-like structure tends to occur and the formability of the steel deteriorates, so it is preferable to finish at 850°C or higher. After hot rolling, the strip is cooled at an average cooling rate of 30 to 100°C/sec to a temperature of 500 to 630°C.
It is necessary to wind it at ℃. If the cooling rate is less than 30℃/sec, a band-like structure is likely to be formed;
If the steel is rolled with ultra-high strength, coarse pearlite will be produced and a good structure will not be obtained after cold rolling and annealing. Currently, the upper limit of the cooling rate for hot strip mills is about 100℃/sec, and there is no need to increase it any higher.
℃/sec or less. If the coiling temperature is too low, hard structures such as martensite will increase, making cold rolling difficult, so the lower limit was set at 500°C. The obtained hot-rolled steel sheet is cold-rolled after removing scale, and the cooling rate may be 40 to 80%, which is the same as usual. The conditions of the subsequent heat treatment are extremely important for the present invention. The cold-rolled strip is first processed at [Ac ₃ transformation point -
It must be heated to a temperature of 20°C] or higher and 930°C or lower. This heating plays the role of recrystallization annealing of the steel, sufficient solutionization of carbides, homogenization of the structure, and adjustment of _austenite grain size. difficult,
It is preferable to heat at a temperature of Ac ₃ or higher. Furthermore, if the temperature exceeds 930°C, the structure becomes too coarse and sufficient properties cannot be obtained. Here, the Ac ₃ transformation point is a temperature defined by the following equation. Ac ₃ (°C) = 879-346 [C (%)] + 65 [Si (%)] -18 [Mn (%)] + 544 [Al (%)] After this heat treatment, average cooling at 5 to 30 °C/sec Cool at speed to 250-450°C and hold at this temperature for 1-10 minutes. If the average cooling rate is less than 5° C./sec, pearlite transformation occurs during cooling, which not only reduces strength but also impairs formability. Furthermore, if the average cooling rate exceeds 30° C./sec, the quenching strain increases and voids occur in the steel, resulting in poor formability. In the present invention, the main structure is a uniform bainite structure, but if the holding temperature is 250°C for less than 1 minute, the amount of martensite formed increases, and if the holding temperature exceeds 450°C, the amount of pearlite formed increases, both of which result in poor formability. is not good. In addition, the retention time was set at 10 minutes as the effect was saturated in about 10 minutes. Note that this holding is not necessarily limited to the case where the temperature is maintained at a constant temperature, but also includes cases where the temperature is kept at a constant temperature, and where the temperature is kept at a constant temperature. Furthermore, in the present invention, the first stage heat treatment time is important. In order to clarify this, a molding test similar to the actual press molding as shown in FIG. 1 was conducted. The width and length of the specimen are 200 mm, and the width of the punch and die are 50 mm and 54 mm, respectively.
The punch and die shoulder radii are each 5 mm. The weight of the wrinkle presser was 60 tons. FIG. 2 shows the moldable height at that time in relation to the heat treatment temperature and time. The material composition and history are as follows.
All are within the scope of the present invention. Ingredients: 0.15%C-0.21%Si-2.60%Mn-0.0010%
S-0.07%Al hot rolling Finish temperature: 860℃, average cooling rate: 45℃/sec,
Coiling temperature: 550℃, cold rolling rate: 66% (3.5mm → 1.2mm) This material was heat treated at 750 to 950℃ for varying times from 10 to 1000 seconds, and immediately cooled at an average cooling rate of 15℃/sec. The mixture was cooled to 360°C and held at this temperature for 6 minutes. It was subjected to a forming test after 0.8% temper rolling. The Ac ₃ transformation point of this material is 808°C. In the figure, the numbers indicate the molding height (mm). The mold is 5mm
Available in pitch up to 35mm. This indicates that the rupture occurred at a type that was one step higher than the numerical value. ≧ is expressed like this because we have not conducted a 40mm test. As is clear from Figure 2, [Ac ₃ transformation point −20
In the range of 930°C to 930°C, formability has a strong correlation with retention time, and the lower the temperature, the longer it takes. These conditions are located at higher temperatures and for longer periods of time than those normally adopted for continuous annealing of cold-rolled steel sheets. Furthermore, when the temperature is outside the above range, moldability is poor regardless of the time. Although the reason for the above is not clear, it is presumed that such conditions arise from the above-mentioned homogenization of the structure, solutionization of carbides, adjustment of austenite crystal grain size, etc. From the figure, the following formula was obtained as an experimental formula to stably obtain a molding height of 30 mm or more. logt≧(1070−T)/130 where t: time (seconds) T: temperature (° C.) This is the basis for limiting the numerical values. Although any equipment may be used for this heat treatment as long as it satisfies the above-mentioned conditions, a continuous annealing equipment for cold-rolled steel sheets that allows mass production, prevents surface oxidation, and has an overaging furnace is preferred. Also, there is no particular question about the temperature increase rate for the first stage heat treatment or the cooling rate after the second stage heat treatment;
Cooling rates of ˜20° C./s are usually employed in the latter case water cooling. (Example) Steel having the components shown in Table 1 was melted and made into a slab by continuous casting. Steels a, b and g are according to the invention, while steels c to e differ from the invention in one of their components. The Ac ₃ transformation points of each steel are also shown in Table 1.

[Table] After heating this slab to 1050 to 1100°C, it was hot rolled and heat treated under the conditions shown in Table 2. Note that conditions not listed in the table are shown below. Hot rolling end temperature: 850-880℃, hot rolling thickness: 3.5mm,
Cold rolling rate: 66%, product thickness: 1.2mm, temper rolling rate:
0.6-0.8% In Table 2, codes 4, 5, 7, 14 and 16
is a manufacturing method according to the present invention, but steels with other symbols differ from the present invention in the conditions enclosed by broken lines. Table 3 shows the results of mechanical tests and forming tests for each steel. The tensile test was carried out using a JIS Z 2201 No. 5 test piece (longitudinal direction is the rolling direction) according to the method described in JIS Z 2241. The bending test was conducted using a JIS 2204 No. 3 test piece (longitudinal direction perpendicular to rolling direction, end surface machined) according to the V-block method described in JIS 2248. The inner radius is 0.5mm and the bending angle is 90 degrees. OK means no cracks. Further, U-forming was performed according to the method described.

【table】

【table】

[Table] As is clear from Table 3, the steel produced according to the method of the present invention has a tensile strength of 100 Kgf/mm ² or more and a high yield strength, and no cracks were observed in the bending test at r = 0.5 mm. Moreover, the U-forming height is also sufficiently large. (Effects of the Invention) Press molding is a central production method in the automobile industry, which requires mass production and high productivity. On the other hand, the adoption of ultra-high strength cold-rolled steel sheets is also socially essential from the standpoint of safety and energy conservation. According to the present invention, it is possible to provide an ultra-high strength cold-rolled steel sheet having excellent workability, so it can be said that the present invention is an industrially useful invention that can fully meet the above two needs. Furthermore, the present invention is applicable not only to automobiles but also to electric,
Of course, it is also useful for increasing the strength of press-molded materials such as building materials, and the present invention has great significance from its wide range of applications.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing a molding test method corresponding to press molding, and FIG. 2 is a chart showing the molding height when the temperature and time are varied in the first stage heat treatment.

Claims (1)

[Claims] 1 C: more than 0.10 to 0.20%, Si: 0.6% or less, Mn:
2.2-3.0%, S: 0.008% or less, Al: 0.01-0.1%,
After hot rolling, the steel consisting of the remaining unavoidable impurity elements is cooled at an average cooling rate of 30 to 100℃/sec, and
After winding at ℃, followed by cold rolling, heat treatment at a temperature of [Ac ₃ transformation point - 20℃] or higher and 930℃ or lower for a period of time expressed by the following formula, followed by 5~30℃.
A method for producing an ultra-high strength cold rolled steel sheet with excellent workability, which comprises cooling at an average cooling rate of °C/sec and holding at 250 to 450 °C for 1 to 10 minutes. logt≧(1070−T)/130 where, t: time (seconds) T: temperature (°C)