JPS5850299B2 - Manufacturing method for precipitation-strengthened high-strength cold-rolled steel sheets - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a precipitation-strengthened high-strength cold-rolled steel sheet.

In recent years, for example, weight reduction has been avoided in the field of the automobile industry.
There is a strong demand for thinner steel plates, and the amount of high-strength steel plates inevitably tends to increase further.

Normally, methods such as solid solution strengthening, precipitation strengthening, strengthening by grain refinement, and transformation strengthening are known as methods for strengthening steel, but only the solid solution strengthening method and strengthening by grain refinement mentioned above are known. Therefore, it is difficult to produce a high-strength cold-rolled steel sheet with a tensile strength of 50 to 97 mm or more, and to utilize transformation strengthening, it is necessary to add a large amount of alloy.

Therefore, the precipitation strengthening method, which strengthens the steel by precipitating carbides and nitrides such as Nb, V, Ti, and B, is mainly used to manufacture high-strength cold-rolled steel sheets with a tensile strength of 50-7m4 or more. has been done.

Precipitation strengthening is a method in which steel is solution-treated at a high temperature and tempered at a low temperature to precipitate many fine precipitates, and the steel is strengthened by the stress field around the precipitates. In the manufacturing method of high-strength cold-rolled steel sheets by precipitation strengthening, precipitates are formed during annealing treatment (heated to a temperature higher than the recrystallization temperature of normal steel) to remove residual strain after cold rolling. Because it aggregates and becomes coarse,
It cannot be said that strengthening due to the miniaturization of precipitates has been sufficiently achieved, and furthermore, the precipitates that are once solidly dissolved during hot rolling are gradually removed during coiling at a temperature of 550 to 650°C after hot rolling. Because precipitation occurs due to cooling, the hot-rolled sheet becomes precipitation-strengthened, which poses problems such as requiring a large amount of energy in the next process of cold rolling. Ta.

This invention solves the above-mentioned problems in the conventional method for manufacturing high-strength cold-rolled steel sheets by precipitation strengthening, and provides a method for manufacturing high-strength cold-rolled steel sheets with higher strength. %, C: 0.005-0.15%, Si
: 1.5% or less, Mn2 0.01-3.0%, P: 0.
0.025% or less, S: 0.03% or less, solid solution AA:
Contains 0.01-0.30%, Nb: 0.01-0
．． 20%, Ti: 0.01-0.20%, and V: 0
．． 01 to 0.50%, with the remainder consisting of Fe and unavoidable impurities, and furthermore, the final finishing temperature in the hot rolling process of the steel with the above composition is determined. By setting the temperature to 750 to 950°C, and particularly by setting the winding temperature into the coil after hot rolling to a low temperature of 450°C or lower, precipitation of precipitates in the hot rolled sheet can be suppressed as much as possible and precipitation strengthening can be minimized. In addition to making it easier to perform hardening rolling in the next process, large precipitation strengthening is achieved in the subsequent annealing process, and then normal pickling, cold rolling, and annealing conditions, that is, cold rolling with a rolling reduction of 30% or more. The present invention is characterized by a method of manufacturing a high tensile strength cold rolled steel sheet by continuous annealing or box annealing at a temperature of 500 to 850°C.

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 annealing temperature are each limited as described above will be explained.

(a) Component composition ■ C If the content is less than 0.005%, the amount of carbide precipitated after annealing will be too small, making it impossible to secure the desired high strength; on the other hand, if the content exceeds 0.15% If it is included, a bainite structure will be formed during coil winding after hot rolling, which will significantly increase the strength of the hot rolled sheet and make cold rolling difficult. %.

■Si If the Si content exceeds 1.5%, scale generation during hot rolling becomes significant and causes roughness of the cold rolled steel sheet. Therefore, Si should not be contained in an amount exceeding 1.5%.

■ If the Mn content is less than 0.01%, it will not be possible to completely suppress the embrittlement phenomenon caused by the S component, while if the content exceeds 3.0%, this type of steel will have difficulty in melting. Therefore, the content was set at 0.01 to 3.0%.

■ P P is an unavoidable impurity that accompanies steel, but as its content increases, the steel becomes more brittle and there is a risk of cracking, so in order to reliably prevent cracking, The P content was set at 0.025% or less.

■ SS The S component is also contained as an unavoidable impurity like PI3, and if its content exceeds 0.03%, hot embrittlement is likely to occur, so it should not be contained in an amount exceeding 0.03%. Must not be 0 ■ Solid solution Al A7 has a strong deoxidizing effect, but its content is higher than that of solid solution A.
If the A7 content is less than 0.01%, it will not be possible to produce killed steel in a stable state, while if the solid solution A7 content exceeds 0.3%, it will only unnecessarily harden the steel and lead to high costs. Since it can be a cause of
%.

■ Nb, Ti, and V All of these components precipitate as carbides and nitrides and have the uniform effect of strengthening steel, but Nb:
less than 0.01%, Ti: less than 0.01%, and V: 0
．． If the content is less than 0.1%, the desired effect cannot be obtained in the above-mentioned action, but on the other hand, even if the content exceeds 0.20% of Nb, 0.20% of Ti, and 0.50% of V, precipitation hardening occurs. Since the performance is saturated and further improvement effect cannot be obtained, the respective contents are set to Nb: 0.01 to 0.2.
0%, Ti: 0.01-0.20%, V: 0.01
It was set at ~0.50%.

(b) Final finishing temperature When the temperature is less than 750°C, the strength of the hot rolled sheet increases due to work hardening, while when the temperature exceeds 950°C,
Since the hot-rolled sheet is rapidly cooled during coiling at a low temperature of 450°C or less in the next process, its hardness increases, and hot-rolled sheets with increased strength (hardness) become difficult to cold-roll. Therefore, in order to keep the strength of a hot-rolled sheet rolled at a low temperature of 450°C or lower, it is necessary to limit the temperature to a range of 750 to 950°C.

(c) Winding Temperature When a hot-rolled sheet is wound into a coil at a temperature exceeding 450°C, the hot-rolled sheet will be slowly cooled at this point, so that the formation of precipitates will occur, resulting in the coil In many cases, the strength of hot-rolled sheets coiled into steel sheets is increased due to precipitation strengthening, and a large amount of energy is required for the next step of cold rolling, and during annealing, precipitates coagulate and coarsen. This often occurs, making it impossible to maintain the desired high strength.

Therefore, in consideration of the strength stability of the hot-rolled sheet and cold-rolled product, the coiling temperature was determined to be 450° C. or lower.

(d) The rolling reduction ratio of cold rolling and the rolling reduction ratio of 30% or more at annealing temperature are the rolling reduction ratios in the normal cold rolling process,
Continuous or box annealing treatment at a temperature of 500 to 850°C is also normally applied to this type of steel, but if these conditions are not met, it is necessary to ensure the specified high strength and good workability. Of course, it cannot be done.

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

Example 1 Steel having the composition according to the present invention melted in a converter, that is, C: 0.05%, Si: 0.20%, Mn
: 1.23%, P: 0.020%, S: 0.007%
, Nb: 0.015%, V: 0.108%, Solid solution Al:
A steel containing 0.028% and 0.0038% N was hot rolled to a plate thickness of 2.8 mm at a final finishing temperature of 800 to 900°C, and then rolled to a thickness of 100 to 73 mm.
The hot-rolled sheets were wound into coils at various temperatures within the temperature range of 0° C., and the tensile strength of the hot-rolled sheets wound into coils was measured at each winding temperature.

The measurement results are shown in the drawing. These hot-rolled plates were then pickled and cold-rolled at a rolling reduction of 64% to a plate thickness of 1.0.
and then for each of these cold-rolled sheets,
Continuous annealing at 770°C for 1 minute and 660°C
and box annealing for 2 hours, respectively, and the tensile strength of the cold rolled sheets after annealing was measured.

The measurement results are shown in the drawing together with the tensile strength of the hot rolled sheet.

As is clear from the results shown in the drawings, if the coiling temperature after hot rolling exceeds 450°C, the hot rolled sheet will be slowly cooled during coil winding, so NbC, NbN,
Also, fine carbides and nitrides such as VN precipitate, and as a result, the strength of the hot-rolled sheet improves, making the next process of cold rolling difficult. This causes the ferrite to agglomerate and coarsen, which not only makes it impossible to fully utilize the required precipitation hardening, but also increases the grain size of the ferrite, resulting in a cold-rolled sheet with low strength.

On the other hand, the coiling temperature after hot rolling is within the range of the present invention.
When the temperature is 50°C or lower, the formation of precipitates in the hot-rolled sheet is suppressed, so the hot-rolled sheet is reliably maintained in a low strength state, making cold rolling easier, and furthermore, during annealing, the formation of precipitates is suppressed. Since these precipitates precipitate and exhibit high precipitation hardening ability, cold-rolled sheets with high strength can be obtained.

Example 2 Methods 1 to 13 of the present invention and comparative method a=f were carried out under the conditions shown in the attached table and below.

That is, six types of steel having the composition according to the present invention shown in the attached table were prepared, and hot rolled at the final finishing temperature and coiling temperature shown in the attached table to obtain a plate thickness of 3.
．． A hot-rolled plate between 2 and 3 was produced, and the tensile test results (according to JIS No. 5 tensile test piece) of the resulting hot-rolled plate were measured,
Then, each of these hot-rolled sheets was pickled and then
Cold rolling was performed at a rolling reduction of 3% to obtain a cold rolled sheet with a thickness of 1.2, and then each of the cold rolled sheets was
After holding at ℃ for 1 minute, continuous jet cooling annealing and 660℃
After holding at ℃ for 2 hours, cooling box annealing was performed at a cooling rate of about 40℃/h, followed by skin pass rolling at a rolling reduction of 1%, and the tensile strength of the resulting cold-rolled sheet was Test results (J
) was measured using a No. I85 tensile test piece.

The measurement results of the tensile test of the hot-rolled sheet and cold-rolled sheet are shown in the attached table.

From the results shown in the attached table, the final finishing temperature during hot rolling is within the range of the present invention, but the coiling temperature is 450, which is within the range of the present invention.
Comparative method exceeding ℃ When a = d, the strength of the hot-rolled sheet is high and the strength of the cold-rolled sheet is low, but the final finishing temperature and coiling temperature during hot rolling are In the case of methods 1 to 13 of the present invention, which are within the scope of the present invention, the strength of the hot-rolled sheets is low, while the strength of the cold-rolled sheets is extremely high, and the specified high strength is achieved. It is clear that there are

In addition, as seen in Comparative Methods e and f, although the coiling temperature is within the range of the present invention, if the final finishing temperature is outside the range of the present invention, the strength of the hot-rolled sheet becomes too high and Inter-rolling became extremely difficult.

As described above, according to the method of the present invention, the strength of the hot-rolled sheet can be kept low, so that the next step of cold rolling can be performed relatively easily. Because the fine precipitates are uniformly dispersed and precipitated, it has industrially useful effects such as extremely high strength.

[Brief explanation of drawings]

The drawing is a graph showing the relationship between the coiling temperature after hot rolling and the tensile strength of hot rolled sheets and cold rolled sheets.

Claims (1)

[Claims] l C: 0.005-0.15%, Si: 1.5% or less, Mn: 0.01-3.0%, P: 0.025%
Below, S: 0.03% or less, solid solution A#: 0.01-0.
Contains 30% and further Nb: 0.01-0.20
%, Ti: 0.01-0.20%, and V: 0.
01-0.50%, and the remainder is Fe and unavoidable impurities.
After hot rolling at a temperature of 450°C or lower, it is coiled to form a coil, followed by pickling and rolling reduction =
Cold-rolled by 30% or more, finally 500-850
A method for producing a precipitation-strengthened high-strength cold-rolled steel sheet, which is characterized by continuous annealing or box annealing at a temperature of °C.