JPS6043416B2 - Method for producing cold-rolled steel sheets with excellent workability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for winding a hot-rolled strip, and an object of the present invention is to appropriately prevent the generation of hot-rolled coarse grains.

It is a well-known fact that when producing a steel plate for deep drawing using a continuous annealing method, high-speed winding is carried out in hot rolling in order to soften the steel sheet and improve the strength.

On the other hand, it is well known that the higher the winding temperature is set, the softer the steel becomes, and in order to obtain a soft steel sheet, winding is performed at the highest temperature possible. Items that regulate the winding temperature include: 1. Regulation due to poor scale peeling, 2. Coil deformation during winding and extraction, and 3. Quality deterioration due to generation of hot rolled coarse grains.

Of these, 1 can be almost eliminated by skin bathing and roll pending before pickling. Regarding problem 2, the temperature at which coil deformation becomes a problem is quite high, and it is possible to sufficiently soften the coil at a temperature lower than that.

On the other hand, regarding problem 3, various factors are mixed,
It is in a very unstable state. Conventionally, in order to stabilize this, the temperature level has been lowered to a considerably safe side, and this has been a bottleneck in the production of highly formable steel sheets. For this reason, various studies have been made in the past in order to produce highly formable steel sheets using continuous annealing.

As a material for this high workability, aluminum killed steel is mainly considered as a steel type that exhibits less aging deterioration and has excellent ductility (particularly biaxial ductility) because it has few inclusions. On the other hand, in terms of components, actions such as lowering [Mn], [C], and [N] have been taken to reduce strength, improve values, and improve grain growth, and consideration has been given to increasing crystal grain size. Ta. )
Against this background, the present invention was proposed as a result of further research, and it provides a method for stably manufacturing high-formability steel sheets by continuous annealing, and in particular, The purpose is to prevent the generation of coarse grains by using materials with high quality and by winding at a sufficient winding temperature.

The present invention will be explained in detail below. In general, the main cause of the generation of coarse grains during high-temperature coiling is thought to be due to grain growth that occurs when the residual strain generated during hot rolling and coiling recovers at high temperatures. is unlikely to occur. However, preventing these residual strains from remaining is a very difficult problem under typical manufacturing conditions, and it is necessary to prevent them under other conditions.
Therefore, the inventors of the present invention investigated various items and discovered that there is a close relationship between the occurrence of coarse grains and the components in aluminum killed steel.

In other words, the generation of coarse grains during high-temperature coiling of aluminum-killed steel is affected by the amount of [N] and [SOlAl].
The effect of h is small in areas where precipitates exist. FIG. 1 shows the effects of [N] and [SOlAl] on the generation of coarse grains.

As shown in the figure, when [N] decreases below a certain level, coarse particles tend to occur, and even if the amount of [SOLAl] increases relative to the amount of [N], coarse particles also tend to occur. I understand. Also, the winding temperature and descending point (YP),
The total elongation (El) and the average Rankford value (〒) have a relationship as shown in Figure 2 (the figure shows the relationship with YP as a representative example), and unless coarse grains occur, these values The higher the value, the better. Therefore, in order to prevent the generation of coarse grains, the coiling temperature after hot rolling should be adjusted to the amount of [N] and [SOlA].
l) I learned that the following formula should be satisfied in relation to the quantity. In the present invention, a highly formable steel plate is obtained by continuous annealing. In order to achieve this, the copper component range is SOl, Al: 0.
020-0.100%, [C]: 0.06% or less, [
Mn]: 0.30% or less, [P]: 0.020% or less, [S]: 0.025% or less, [N]: 0.00
It is recommended to reduce the amount to 70% or less.

That is, when [C] exceeds 0.06%, the strength increases;
This is because the value decreases, and [Mn] is also regulated for the same reason as [C] and for uniformity in the longitudinal direction of the product.
[P] is limited to 0.020% or less because it increases strength and reduces elongation, and [S] does not leave solid solution [S], so the upper limit is limited to 0.025%. The reason why the lower limit of SOlAl was set to 0.020% is that even if it is less than this, [N] will not be fixed as Al-N during high-temperature coiling of hot rolling and in subsequent steps, but will be in a solid solution state, resulting in poor aging properties. This is because it adversely affects workability, and the reason why the upper limit was set at 0.100% is because if it exceeds this, the strength will increase due to an increase in the amount of SOlAl, the surface quality will deteriorate, and the cost will also increase. It is. This is because if [N] exceeds 0.0070%, the number of Al--N precipitates increases, grain growth is inhibited, and the precipitates cause a decrease in strength and elongation. Further, in the present invention, the amount of [N] is 0.
．． If 0070% or less, [N]/[SOlAl]≦0
．． When it reaches 067, coarse grains tend to occur.

For this reason, coarse particles are particularly likely to occur [N]: 0.
For those below 0040%, [N]/SOIAI
By setting the ratio to 0.067 or more, it is possible to prevent the generation of coarse particles. The reason why such an effect can be obtained is considered as follows. That is, A during high temperature winding
The size and number of I-N precipitates affect grain growth, and when [N] is large, the number of precipitates is large and the overall coarse length is inhibited, making it difficult to form coarse grains. on the other hand,
When the number of precipitates is small and large (SO
Corresponding to the case where lAl is low), grain growth is good and grains grow uniformly, so on the contrary, coarse grains are less likely to occur. In the region between these regions, there are a mixture of areas that inhibit grain growth and areas where grain growth is easy, so it is thought that the areas that are easy to grow become extremely large, resulting in the generation of coarse grains. The reason why the upper limit of the winding temperature is limited to 780° C. in the above formula is that if it exceeds this value, there is a risk of so-called coil breakage.

[Example] As an example of the present invention, as shown in Table 1, when a combination of various components was finished at a temperature of Ar'3 or higher, and then the winding temperature was varied in the range of 700 ° C to 750 ° C by water cooling. This shows the occurrence of coarse grains.

Table 2 also shows the results of these materials after removing scale.
Cold rolling is performed at a rolling reduction of 60% or more, and 690% is applied in a continuous annealing furnace.
〜720℃×15m11n heating, 350℃ after rapid cooling
Perform over-edging at 500℃ x 1 min or more 1
．． Mechanical properties are shown after applying an elongation rate of around 0%.
As is clear from the above examples, according to the present invention,
In particular, by coiling killed steel at a predetermined coiling temperature after hot rolling, it is possible to prevent the generation of coarse grains and to stably produce cold rolled steel sheets with excellent workability. This is a great invention.

[Brief explanation of the drawing]

Figure 1 is a graph to explain the influence of [N] and [SOlAl] on the generation of coarse grains, and Figure 2 is a graph for explaining the influence of [N] and [SOlAl] on the generation of coarse grains. Temperature and Y
This is a club showing the relationship between P and the area where coarse grains are generated.

Claims (1)

[Claims] 1. In Al-killed steel that is hot-rolled and coiled at a high temperature, the hot-rolled strip is coiled at a coiling temperature that satisfies the following formula,
A method for producing a cold-rolled steel sheet with excellent workability, which is then subjected to normal cold rolling and continuous annealing. T≦15×10^3([N]-0.067[solAl
] +720℃<780℃ However, T: Coiling temperature (℃) [N]: N concentration (%) [solAl]: solAl concentration (%)