JPS5841325B2 - Manufacturing method for high-strength cold-rolled steel sheets - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a cold rolled steel sheet having a high tensile strength of 50 kg/- or more at low cost.

In recent years, with the demand for weight reduction in the automobile industry,
Automotive parts are becoming thinner, and from this point of view, for example, the production of bumper reinforcing materials, door reinforcing beams, and even general auto parts requires 60 to 100 kg.
It has a tensile strength of g/m4 or more and a plate thickness of 0.8 to 1
．． High-strength cold-rolled steel sheets of grade 6 have come into wide use.

Conventionally, this type of high-strength cold-rolled steel sheet has been manufactured by manufacturing methods using methods such as (a) precipitation strengthening method, (b) transformation strengthening method, and (C) dislocation strengthening method.

Generally, when manufacturing cold rolled steel sheets, the temperature is 600℃ after cold rolling.
Recrystallization annealing is performed at the above temperature, but in the manufacturing method of high-strength cold-rolled steel sheets using the precipitation strengthening method described above, precipitates tend to aggregate and coarsen during this recrystallization annealing process. Therefore, it is virtually impossible to control the precipitation form of precipitates, and as a result, there is a problem that precipitation strengthening cannot be sufficiently achieved as in hot-rolled materials and thick plate materials.

In addition, the method using the above-mentioned transformation strengthening method is a recently developed method that has the advantage of producing cold-rolled steel sheets with good elongation due to the presence of martensite. Since it requires processing and the inclusion of a large amount of alloying elements, it is not necessarily an advantageous method from the viewpoint of manufacturing costs.
Taking box annealing as an example, this is a method conventionally used in the production of high-strength cold-rolled steel sheets as very annealing.In order to prevent complete recrystallization, the upper limit annealing temperature is set at a low temperature of about 600'C. On the other hand, in order to recover or partially recrystallize the cold-rolled structure and improve the elongation compared to the cold-rolled state, the lower limit temperature for annealing is generally set at about 300°C.

This method allows a cold rolled steel sheet with higher strength to be obtained even if the alloy components are smaller than those using the strengthening methods (a) and (b) above, and requires a lower annealing temperature.
Although it has advantages such as energy saving and low cost, the elongation of cold-rolled steel sheets is poorer than when using the other strengthening methods mentioned above, especially the elongation in the T direction (width direction). L
Because the elongation is poor compared to the elongation in the T direction (length direction), it is difficult to apply it to molding by roll forming, where elongation in the T direction is an important factor, and therefore its application is limited to special fields. The problem was that it was limited.

In addition to this, there is a method using solid solution strengthening method, but this method alone can produce up to 50 kg/mr? In order to produce a high-strength cold-rolled steel sheet with a tensile strength greater than , the content of alloying elements must be significantly increased, making it virtually impossible to produce.

From the above-mentioned viewpoint, the present inventors focused on a method for manufacturing cold-rolled steel sheets that utilizes the conventional dislocation strengthening method, which is the most advantageous in terms of manufacturing costs, and conducted research to improve the elongation in the T direction in particular. As a result, (a) In the conventional production method of cold-rolled steel sheets using dislocation strengthening, hot-rolled steel sheets are usually wound into coils at a temperature of 500 to 700°C after hot rolling. Layered carbides now exist in a coarse state, which is crushed during cold rolling to form voids. The elongation, especially the elongation in the T direction, is markedly reduced.

(b) Therefore, if the hot rolled sheet is wound into a coil at an extremely low temperature, the carbon can be in a solid solution state or a fine precipitate state in the hot rolled sheet, and if cold rolling is performed in this state, , Since the generation of voids is almost eliminated, the elongation in the T direction is significantly improved, and the carbides are finely dispersed in the strain relief annealing after cold rolling, so precipitation strengthening is sufficiently achieved.

The knowledge shown in sections (a) and (b) above has been obtained.

This invention has been made based on the above findings, and the target steel is C: 0.02 to 0.30% by weight,
Si: 1.2% or less, Mn: 0.10-2.50%, s
o#, A#: 0.01~0.20%, Fe and unavoidable impurities: The final finishing temperature in the hot rolling process of this steel is specified as 600~9.
By setting the temperature at 00°C and also at an extremely low temperature of 50 to 400°C for winding the coil after hot rolling, the carbon in the hot rolled sheet is brought into a solid solution state or a fine carbide state, which makes it difficult to prepare for the next process. 3. Suppressing the formation of voids during cold rolling, then pickling, cold rolling, and strain relief annealing under normal conditions, that is, cold rolling with a reduction ratio of 20% or more, and 3.
This method is characterized in that a high-strength dislocation-strengthened cold-rolled steel sheet with improved elongation in the T direction is produced by performing strain relief annealing at a temperature of 00° C. or higher.

Next, in the method of this invention, the component composition range, the final finishing temperature during hot rolling, the coil winding temperature after hot rolling,
The reason why the rolling reduction during cold rolling and the stress relief annealing temperature are limited to the above values will be explained.

(a) Ingredient composition ■ C If the content is less than 0.02%, the cost will increase,
However, if the content exceeds 0.3%, the weldability will deteriorate significantly, so the content was set at 0.02 to 0.30%. .

■5i The Si component has a solid solution strengthening effect and is therefore included as necessary. However, if its content exceeds 1.2%, scale generation during hot rolling becomes significant and 1.2 Because it causes roughening of the surface of the rolled steel plate and impairs its appearance.
The content must not exceed %.

■ Mn Like Si, the Mn component has a solid solution strengthening effect and an effect of preventing red heat embrittlement during hot rolling, but if its content is less than 0.10%, the desired effect is not achieved. On the other hand, if the content exceeds 2.50%, it becomes difficult to melt this type of steel and causes high costs.
Its content was determined to be 0.10 to 2.50%.

■ Sol, Al A7 has the effect of improving the cleanliness of steel through its strong deoxidizing effect, but if the 5ol-AA content is less than 0.01%, the desired effect cannot be obtained. Due to large variations in the strength of custom-made steel, if the content exceeds 0.20%, nozzle clogging is likely to occur during melting, especially during casting, so the content should be reduced to 0.01 to 0.20%.
% and determined pond (b) Saito answer A 4 Nijo Our shop crab [When the temperature is less than 600℃, the deformability of steel during hot rolling changes significantly, so hot rolled sheets with a constant thickness are During manufacturing, the temperature was limited to 600 to 900 degrees Celsius, as temperatures exceeding 900 degrees Celsius would make it impossible to wind at extremely low temperatures of 50 to 400 degrees Celsius in the next process. .

(C) Coiling temperature If the temperature is less than 50'C, the amount of solid solute carbon in the hot rolled sheet will be too large and dynamic strain aging will occur during cold rolling, which is undesirable. If the coiling temperature exceeds that temperature, layered carbides or pearlite will precipitate at the ferrite grain boundaries, resulting in cold rolling and
Since the elongation in the T direction after strain relief annealing significantly decreases, the temperature was set at 50 to 400°C.

(d) The rolling reduction ratio of cold rolling and the rolling reduction ratio of 20% or more in strain relief annealing are the conditions adopted in the normal cold rolling process, and the rolling reduction ratio of 20% or more in cold rolling and strain relief annealing is the condition adopted in the normal cold rolling process. Strain relief annealing is also normally applied to this type of steel, but if these conditions are not met,
Needless to say, it is not possible to ensure the predetermined high strength and high ductility.

Next, the method of the present invention will be specifically explained using examples.

Steel having the composition shown in Table 1 is melted in a converter, cast, and then hot-rolled to a plate thickness of 2.4 mm under the final finishing temperature conditions also shown in Table 1. or 2.8
Then, after winding at the winding temperatures shown in Table 1, and pickling, cold rolling and strain relief annealing were performed under the conditions shown in Table 1 to obtain a cold sheet with a thickness of 1.2 mm. Inventive methods 1 to 9 and comparative methods a to f, which consist of producing rolled steel sheets, were carried out, respectively.

Comparative methods a, C, and e indicate cases where the winding temperature is higher than the range of the present invention, and comparative methods, d and
, and f respectively indicate cases where the strain relief annealing temperature is out of the range of the present invention.

Next, cold rolled steel sheets 1 to 9 of the present invention obtained by the above-mentioned methods 1 to 9 of the present invention. And for each of the comparative cold rolled steel sheets a to f obtained by the above comparative methods a to f, JIS5
The tensile properties of the No. 1 test piece in the T direction and the L direction were measured.

The measurement results are shown in Table 2.

As is clear from the results shown in Tables 1 and 2, cold rolled steel sheets 1 to 9 of the present invention manufactured by applying the coiling temperature and strain relief annealing temperature within the range of the present invention were all Strength: Shows high strength of 50 kg/mvt or more, and T
The comparative cold-rolled steel sheets a, C, and e manufactured at coiling temperatures higher than the range of the present invention show almost the same high elongation in both the direction and the L direction. The strength is slightly inferior to that of cold-rolled steel sheets, and the elongation in the T direction is particularly significantly reduced.

Further, it has been shown that when the strain relief annealing temperature is outside the range of the present invention, the strength is improved but the ductility is not improved at all.

As mentioned above, according to the method of the present invention, the coil winding temperature after hot rolling is set to an extremely low temperature, thereby achieving high strength and almost the same high elongation value in both the T direction and the L direction. It is possible to produce high-strength cold-rolled steel sheets at low production costs.

Claims (1)

[Claims] IC: 0.02 to 0.30%, Si: 1.2% or less. Mn: 0.10-2.50%, 5ol-Al: 0
．． 01-0.20%, Fe and inevitable impurities: remainder (
After hot rolling a steel having a composition consisting of (more than 1% by weight) at a final finishing temperature of 600 to 900'C,
A high-temperature steel sheet that is characterized by being coiled at a temperature of 50 to 400°C, then pickled, cold-rolled at a rolling reduction of 20% or more, and then subjected to strain relief annealing at a temperature of 300°C or more. Manufacturing method for high-strength cold-rolled steel sheets.