JPS5943531B2 - Manufacturing method for high-strength cold-rolled steel sheets with excellent workability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to the production of a high-strength cold-rolled steel sheet with a tensile strength of 40 kg/- or more and excellent workability, and which has a tensile strength of 40 kg/- or more and a temperature between A1 transformation point and A3 transformation point after hot rolling. Si, Cr, Ti, and V can be removed by rolling it over a temperature range.

The present invention provides a high-strength cold-rolled steel sheet with excellent workability without using large amounts of expensive elements such as Mo.

Recently, the necessity of high-strength cold-rolled steel sheets has been recognized from the standpoint of resource saving, energy saving, and environmental protection.

For example, efforts are being made to reduce the weight of car bodies in order to meet both exhaust gas component regulations and fuel ratio reductions, but it is necessary to reduce the thickness of the material to reduce the weight while still ensuring sufficient car body strength. High-strength steel plates are absolutely necessary.

However, as the tensile strength of a steel plate increases, its elongation decreases, so its uses are limited.

Therefore, it is desirable to produce a steel plate that has both strength and workability.

Such steel plates use large amounts of alloying elements such as Si, Cr, and Ti to improve both strength and ductility.
Alternatively, by taking advantage of the fast cooling rate of continuous annealing, annealing is performed in the temperature range between A1 transformation point and A3 transformation point during continuous annealing to form a ferrite phase and a martensite phase, a bainite phase, a sorbite phase, or a troostite phase. Some aim to improve strength and ductility by creating a composite structure.

The former is not only expensive because it uses a large amount of special alloying elements, but also has problems such as poor descaling properties and a large load being applied during rolling.

The latter uses composite structure steel, but various conditions are required to create such a structure, and in order to make composite structure steel through normal hot rolling and continuous annealing processes, Si, Si, A1 during continuous annealing by adding a large amount of elements such as V and Mo
It is necessary to maintain the temperature between the transformation point and the A3 transformation point for several minutes or more.

Alternatively, if these elements are not added, it is necessary to achieve a cooling rate that is impossible in a normal continuous annealing process, and in order to achieve such a fast cooling rate, water cooling equipment must be installed in the continuous annealing process. However, processes that use water cooling are not common.

The present invention was researched and developed in order to overcome the various drawbacks mentioned above, and is characterized by C: 0.03 to 0.
．． 30%, Mn: 0.7-3.0%, Si: 0.5
% or less, 5olAl: 0.1% or less, the balance consists of iron and unavoidable impurities. Steel is melted and rolled at a temperature between A1 transformation point and A3 transformation point after hot rolling, and the annealing process is performed at A1 transformation point. To produce a steel plate with high strength, excellent elongation, and excellent workability with a low yield ratio by holding the steel plate between the A3 transformation point for 15 seconds to 30 minutes and then cooling at a rate of 1°C/5 ecll. .

The reason why steel sheets with high strength and excellent workability can be obtained by the above method is that the secondary phase such as martensite phase, bainite phase, sorbite phase or troostite phase
It is presumed that an increase in strength is achieved through phase dispersion, and that the concentration of C in this second phase reduces the amount of C in the ferrite phase, thereby improving workability.

That is, when rolling at a temperature in the range of A1 transformation point to A3 transformation point after hot rolling, the austenite phase in which Mn and C are sufficiently enriched is already appropriately dispersed, so that the subsequent continuous During heat retention during annealing, there is no need to diffuse and concentrate Mn and C into the austenite phase. Therefore, not only can the annealing time be shortened, but also the martensitic phase and It can be a bainite phase, a sorbite phase, or a troostite phase.

Second, during slow cooling after hot rolling, the austenite phase contains a large amount of Mn and C, so when it transforms into a cementite + ferrite phase, the cementite phase is prevented from coarsening and becomes a dense pearlite structure. However, the introduction of strain into the pearlite phase during cold rolling increases, and the reverse transformation from pearlite phase to austenite phase during annealing is promoted.
It is estimated that it has the effect of completing the transformation in a short time annealing.

The significance of winding the winding temperature between the A1 transformation point and the A3 transformation point in this way is significant; it has excellent strength and ductility, which was previously impossible with normal continuous annealing furnace type annealing, and has a large yield with work hardening ability. A steel plate with a low ratio can be easily produced.

The steel sheet manufactured in this manner has the characteristic that it can be rapidly hardened by skin pass processing and can be made into a steel sheet with a high yield ratio.

The present invention will be explained in detail below.

The present invention is C2 0.03-0.30%, Mn: 0.7-3
．． 0%, Si0.5% or less, Sol, At: 0.1
% or less, the balance iron and unavoidable impurity elements are melted in a converter, open hearth, or electric furnace, and made into steel ingots or slabs by continuous casting.

The reason why the lower limit of the amount of C was set at 0.03% is that it is impossible to obtain the required strength with less than this, so the upper limit was set at 0.03%.
．． The reason why it is set at 30% is that if it exceeds this value, it will not be possible to form a steel plate with excellent workability.

The lower limit of the Mn content was set at 0.7% because if it is less than this, a steel plate with excellent strength and ductility will not flow at a normal cooling rate after being maintained at a temperature between A1 transformation point and A3 transformation point during continuous annealing. The reason why the upper limit was set at 3.0% is because if the Mn content exceeds 3.0%, the increase in strength will be small even if the amount of Mn is increased, and descaling properties will be poor and oxidation of the steel plate surface will become a problem. This is because

Although Si does not necessarily need to be contained, Si-killed or Si-semi-killed steel may be used.

In this case, even if it is present in an amount exceeding 0.5%, the effect will be small and the price of the steel sheet will only increase, and descaling properties and oxidation on the surface of the steel sheet will also become a problem. There is no need to include it.

A/, the content is also the same as Si and is not necessarily At.
Although it is not necessary to contain At-killed steel, it may be used.

In the case of A4 killed steel, even if the content exceeds 0.1%, it will not affect the strength and ductility, but it will cause problems in terms of descaling and oxidation of the steel plate surface, so the upper limit was set at 0.1%.

After hot rolling, the slab of the above components is rolled up at a temperature between the A1 transformation point and the A3 transformation point.

The reason why the winding temperature is set to A1 transformation point to A3 transformation point is that by winding at this temperature, M
It becomes possible to segregate n and C and stabilize the composite structure in the subsequent annealing process.

In addition, the rolling load during cold rolling can be reduced and cold rolling can be stably performed.

The hot-rolled sheet rolled up between the A1 transformation point and the A3 transformation point is subjected to cold rolling after descaling.

Next, rapid heating and short-time rapid cooling annealing is performed.The reason why the annealing is held at a temperature in the range between A1 transformation point and A3 transformation point for 15 seconds or more is to obtain an austenite phase in the ferrite phase. If it is less than that, the iron carbide phase will not completely transform into the austenite phase.

The upper limit of the annealing time is set at 30 minutes because it is sufficient to obtain an iron carbide phase in the austenite phase, and any longer time is meaningless, and industrially, it is necessary to take a longer time. This is because it is difficult.

The austenite phase generated in this way is 1°C/S
At a cooling rate of ec or higher, it changes to a martensite phase, a bainite phase, a sorbite phase, or a troostite phase, forming a composite structure together with a ferrite phase.

The steel produced in this manner may be treated as it is or may be subjected to light temper rolling if necessary.

Furthermore, the steel produced in this way contains Zn, Sn, and Cr.
Surface treatment such as plating may be applied, and Zn during continuous annealing.
It may be plated.

Examples will be described below.

Example A C: 0.11%, Si: 0.05%, melted in a converter
Mn: 1.52%, P: 0.015%, S: 0.0
16%, 5o4At: 0.034% slab
．． After hot rolling to 3 mm, it was rolled up at 750°C and 600°C, and after pickling, it was rolled to 0.7 m using a tandem cold rolling mill.
It was rolled to m.

This copper strip was heated in a continuous annealing furnace at 800°C for 1 minute or at 70°C.
Annealing was performed at 0°C for 1 minute, and cooling was performed at an average cooling rate of 10°C/sec.

Note that the cooling rate in Comparative Example 2 was 0.5° C./sec.

The mechanical properties of the samples thus produced are shown in Table 1.

As is clear from Table 1, the winding temperature after hot rolling is A1
The annealing temperature during continuous annealing is set to 750°C above the transformation point and A1.
By holding the steel plate at 800° C. for 15 seconds or more, which is between the transformation point and the A3 transformation point, a steel plate with high strength, good elongation, low yield ratio, and excellent workability can be obtained.

It can be seen that when the cooling rate after annealing was set to 0.5° C./sec, good strength and ductility could not be achieved, and a steel sheet with a low yield ratio and high work hardenability could not be obtained.

Example B Steels with different Mn contents shown in Table 2 were hot-rolled at 750°C, pickled, and then cold-rolled to a thickness of 0.7 mm using a tandem cold rolling mill. .

This copper strip was held at 800° C. for 1 minute in a continuous annealing furnace, and then cooled at an average cooling rate of 10° C./SeC.

The mechanical property values of this sample are shown in Table 2.

As is clear from Table 2, when the Mn content is 0.7% or more, the strength increases, the yield ratio decreases, and the strength-ductility balance improves, making it possible to manufacture steel sheets with high strength and excellent workability. That is clear.

As explained above, the present invention is a steel plate that can be produced more cheaply and easily using conventional equipment than conventional high-strength steel plates, has high strength, excellent ductility, and a low yield ratio, and has industrial value. is extremely large.

Claims (1)

[Claims] IC: 0.03~0.30%, Mn: 0.7~
3.0%, Si: 0.5% or less, 5olAl 20, 1
% or less, the balance iron and unavoidable impurity elements are rolled after hot rolling at a temperature range between A1 transformation point and A3 transformation point, cold rolled, and then rolled at a temperature between A1 transformation point and A3 transformation point. After annealing for seconds to 30 minutes, 1℃/Sec
A method for producing a high-strength cold-rolled steel sheet with excellent workability, characterized by cooling at a temperature above.