JPS63219528A - Manufacture of cold-rolled steel sheet excellent in workability - Google Patents

Abstract

PURPOSE:To improve workability by using a steel of low-cost type, by subjecting a steel slab in which respective amounts of C, Mn, S, Sol.Al, and N are specified to hot rolling and winding under prescribed conditions and then by applying cold rolling and annealing to the hot-rolled plate. CONSTITUTION:A steel containing, by weight, 0.01-0.06% C, 0.03-0.2% Mn, <=0.01% S, 0.01-0.08% Sol.Al, and N in an amount satisfying N(%)<=[0.32-Mn(%)]/57 is refined. A slab of this steel is heated to <=1,100 deg.C, hot rolled at a temp. of the Ar3 transformation point or above, cooled rapidly, and wound up at <=500 deg.C. Subsequently, the resulting wound-up steel plate is cold-rolled and annealed.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention provides a cold rolled steel sheet with excellent workability (hereinafter referred to as "cold rolled steel sheet"), particularly a cold rolled steel sheet that is low cost and has a deep surface that is less rough during press working. The cold-rolled steel sheets to which the present invention is directed are used in large quantities for high-grade applications such as automobile exterior panels, but are particularly required to be low in cost. The present invention relates to a method for manufacturing cold-rolled steel sheets that satisfies both of these requirements.

(Prior Art) Conventionally, AQ killed steel for deep drawing, among products equivalent to JIS G3141 class 3 cold rolled steel plate for deep drawing (symbol 5PCE), has been mainly used for automobile outer panels.

This steel type has AQ and N of O, OS% and 0, respectively.
Use low C-M killed steel adjusted to about 0.005%. Then, in order to make AQN a complete solid solution, it is heated to a high temperature of 1200°C or more, and after hot rolling, it is rapidly cooled to 600°C or less and coiled. During the crystal annealing process, 8QN is precipitated to form a texture favorable for deep drawability. When even better deep drawability is required, a method is generally adopted in which the amount of solid solute C is reduced by further decarburizing annealing or vacuum degassing treatment. In this way, the physical properties required for deep drawability, low yield strength, high ductility (high elongation) and high Lankford value (average value in each direction) r (
(hereinafter simply referred to as r value). however,
The above-mentioned decarburization or degassing treatment is not preferable from the viewpoint of realizing cost reduction because it adds one step.

On the other hand, since it is desirable to reduce the heating temperature of the slab to 1200°C or less in terms of reducing thermal energy costs, a method for producing cold rolled steel sheets with excellent press workability by reducing the heating temperature of the slab to 1200°C or less has been published in Japanese Patent Application. Showa 49-1296
This is proposed in Publication No. 22, but this is because press workability is not comprehensively judged based on many physical properties, but only based on the reduction rate converted from the conical cup value (JIS Z 2249). , it is unclear whether it is necessarily effective as a practical steel, and the steel type is simply specified as low carbon steel, but according to the examples, all C = 0.0
0.5% pole (1 is carbon steel, so cost increase is inevitable.

Furthermore, it is found in JP-A No. 61-23721 that cold-rolled steel sheets for deep drawing can be manufactured by setting the cumulative reduction rate at 1000°C or less to 87% or more, but this is difficult from the viewpoint of steel composition. N: Very low N of 0.00025% or less, limiting the amount of Mn and C. Also, although this publication does not mention the winding temperature, looking at the examples, all
℃ or higher, and high-temperature winding is performed. moreover,
There is no mention of so-called direct rolling, in which the slab is hot-rolled by temperature control without cooling after casting.

(Problems to be Solved by the Invention) An object of the present invention is to provide a method for manufacturing at low cost a cold-rolled steel sheet for deep drawing with less surface roughness during press working.

Another object of the present invention is to provide a method for producing an inexpensive cold-rolled steel sheet with a CIo of 01 to 0.06% and excellent deep drawability.

(Means for Solving the Problems) First, in order to achieve the above object, the present inventors focused on reducing material costs and thermal energy costs in order to realize low costs. In particular, regarding thermal energy costs, since the products to be manufactured are mass-produced, it is necessary to pay attention to the impact on the costs of other products that use the same line. Furthermore, simplifying the heat treatment as much as possible will also reduce management costs.

Thus, the present inventors conducted research and development targeting AQ killed steel, and as a result, they decided to control the ferrite crystal grains to equiaxed grains (isotropic) as a countermeasure against roughening of the surface during press working. It was discovered that the above objectives could be achieved by selecting the ingredients, heat treatment, and processing conditions, and as a result of further studies, the present invention was completed.

That is, the gist of the present invention is that in weight %, C: 0. OL~0.06%, Mn: 0.03~
0.20%, S: 0.01% or less, sol, AQ:
Contains 0.01 to 0.08%, and N: N%≦
(%)≦[0.32-Mne This is a method for producing a cold-rolled steel sheet with excellent workability, which comprises rolling the steel sheet, followed by rapid cooling and coiling at 500°C or less, then cold-rolling the thus obtained steel sheet, and annealing it. .

As described above, according to the present invention, a steel type having the following chemical composition was selected as a relatively low-cost slab material with excellent workability for cold-rolled steel sheets for deep drawing.

C: Q, 01-0.06%, MnsO, 05
~0.20%, S: 0.01% or less, sol, 8Q + 0.01~0.08%, and NUN (
g)≦(%)≦[0.32-Mn(g)]157, and the remainder is re and unavoidable impurities.

Note that since Mn is 0.03 to 0.20%, the maximum range is N: 0.0051% or less.

Steel processing and heat treatment are carried out in the following order:
Do it with conditions.

The slab is heated to a temperature of 1100° C. or lower, hot rolled at 3 Ar points or higher, then rapidly cooled and rolled at 500° C. or lower.

The steel plate thus obtained is cold rolled and annealed.

In conventional AQ killed steel, elongated grains were obtained by controlling the aggregated Al1 texture using AQN finely precipitated during recrystallization annealing, but the feature of the present invention is to improve the texture without using this control. 7. Therefore, fine MnS and AQN become grain growth inhibitors during recrystallization.
A process is in place to suppress the precipitation of That is, in the present invention, heating the slab to 1100° C. or lower is considered to have the effect of not allowing many of these precipitates to form a solid solution in the case of non-direct delivery, and causing them to coarsen. Even in the case of direct shipping, although the mechanism is not well understood, low Mn (<0.2%
), it is considered that AQN is precipitated and coarsened during heat retention at 1100° C. or lower, with the precipitated MnS as the nucleus. Therefore, even in the case of the direct feeding method, an equiaxed grain m weave can be obtained without being affected by AQN during recrystallization annealing. In addition, the purpose of rapidly cooling and coiling to below 500°C after hot rolling is to prevent the slight amount of solid solute N from precipitating finely after hot rolling. This is different from the purpose of keeping it in a supersaturated state.

As is clear from the above, in the present invention, from the viewpoint of steel composition, ultra-low carbon steel (C<0.01%), ultra-low nitrogen steel (for example, N<0.0005%), or special We aimed to reduce material costs by not using expensive materials such as steel. Additionally, from the perspective of the manufacturing process, thermal energy costs were reduced by lowering the slab heating temperature and winding temperature. In that case, a decrease in the winding temperature means that the cooling heat can be used as thermal energy of the cooling water by the difference. Furthermore, it has been found that the thermal history before slab heating does not affect the deep drawability of the steel material with the above composition, so the thermal energy for slab heating can be reduced by direct rolling as shown in Table 2 of Example 2 below. This allowed for significant savings. Therefore, in the present invention, the steel slab includes not only steel slabs produced by the blooming rolling method, but also those produced by the ordinary continuous casting method, and even those produced by direct rolling. In the case of such direct rolling, the steel slab is not cooled to a temperature lower than the Ar2 point during the cooling process.
It is kept at a temperature of 1100 degrees Celsius or less.

(Function) Next, the reason why the chemical components are limited as described above in the present invention will be explained below. In addition, in this specification, "%
” unless otherwise specified, is 1% by weight.

C: If C is contained in a large amount, it inhibits the growth of ferrite grains and deteriorates workability, so the upper limit was set at 0606%. Further, in order to reduce C to less than 0.01%, a special decarburization process is required, which goes against the goal of low cost, so the lower limit is set to 0.01%.
It was set as 01%.

Gate n: FIG. 1 is a graph showing the relationship between Mnl and F value obtained by collecting some data of Examples described later.

When Mn exceeds 0.2% in low-temperature heating at 1100° C. or lower, the 7 value decreases. The Mn content is preferably around 0.1%; if the Mn content is too low, there is a risk of brittleness during hot rolling. Therefore, in the present invention, 0.03 to 0.2
The range was 0%.

Preferably 0.05 to 0.20%, more preferably 0
．． 07% to 0.14%.

S: In order to prevent brittleness observed during hot rolling and to suppress the precipitation of fine MnS immediately after hot rolling, the smaller the amount of S, the better, and the upper limit was set at 0.01%.

sol, 8Q: Since it is a feature of the present invention that the AQN set & II weaving control force is not utilized, the amount of AQ that fixes N is sufficient. Since adding more than necessary will lead to higher material costs, it is set in the range of 0.01 to 0.08%.

N: The upper limit of the content of N varies depending on the amount of Mn. The relationship is shown in FIG. 2. FIG. 2, like the case of FIG. 1, summarizes some data of the examples described later.
It is a graph showing the F value in relation to the amount of N. N%≦(%)
≦[0.32-Mn (%)) 157, good workability can be obtained as shown in the figure.

Steel having a composition that satisfies the above conditions is melted and then made into a slab. This slab was hot-rolled, cold-rolled, and annealed under the conditions described above to form equiaxed grains with high ductility (
A cold-rolled steel sheet with high elongation (high elongation) and high 7 value can be obtained.

The reason why the slab heating temperature performed prior to hot rolling was set to 1100° C. or lower is that the present invention does not require high-temperature heating unlike the conventional method. The hot rolling is completed at an Ar point of 2 or more, and the reduction amount at that time is not particularly limited, but is preferably 60% or more. The reason why the coiling temperature is 500°C or less is to control the fine precipitation of AQN. Therefore, after hot rolling, it is cooled to the coiling temperature. Inter-rolling and annealing are not particularly limited,
It is sufficient to apply conventional methods.

One of the features of the steel of the present invention is that its workability does not change depending on the thermal history before heating the slab, so it can be hot rolled after the slab is cast and the temperature is adjusted by keeping it warm without cooling it too much. In other words, high-quality cold-rolled steel sheets can be obtained even by direct hot rolling.

Next, the present invention will be explained in more detail with reference to Examples.

(Example) The component steel shown in Table 1 was melted, hot rolled under the conditions shown in Table 2, finished at 870°C above the Ars point, and after quenching, the number of turns was applied. cold rolling, 680
After annealing for 8 hours at ℃, 1% skin pass rolling was performed.

The temperature at the end of hot rolling was 870°C. those si+
Characteristic values of Ii (yield strength, tensile strength, elongation and 7 values)
are summarized in Table 2.

Among the test steels in Table 1, steel symbols A-G conform to the component amounts of the present invention, but steel symbols H and later are comparative current steels, among which steel symbols H-J have N components, and M for steel symbol I
Even in the n component, the Mn component in the steel symbol exceeds the content of the present invention.

As is clear from the results shown in Table 2, the steel sheets manufactured by the method of the present invention first exhibit good deep drawability, but in particular, the steel code B has excellent properties in general, especially in r value. ing. For steel symbol B, high temperature slab heating (122
0° C.) or high-temperature winding (700° C.), the manufacturing method of the present invention shows better characteristic values, especially a value of 7.

FIG. 3 is a graph comparing the r values of steel type B, which has components within the range of the present invention, and steel type G, which has the current composition, by changing the slab heating temperature and coiling temperature. A steel having the following composition shows an excellent r value in the slab heating temperature range of the present invention (shaded area in the drawing).

Table 1 (Note) Umbrella; outside the scope of the present invention Table 2 Table 2 (Continued) (Note) Umbrella After varnish slab casting, low-temperature heating (1020° C.) was performed without cooling.

(Effects of the Invention) The present invention has been described in detail above, and it can be seen that according to the present invention, the following practically excellent effects can be obtained.

(11 ultra-low carbon steel (C<0.01%), ultra-low nitrogen 14 (
For example, N < 0.0005%) or a relatively low cost steel grade can be used without special steel.

(2) Keep the heating temperature of the slab below 1100℃, hot rolling and
The coiling temperature after quenching is selected to be 500° C. or less, and then cold rolling and annealing are performed to produce a cold rolled steel sheet with excellent deep drawability. This process saves thermal energy costs compared to conventional methods.

(3) Furthermore, a steel sheet with similar deep drawability can be obtained by direct rolling, in which hot rolling is performed without cooling after slab casting, with temperature control performed, so that thermal energy and energy costs can be further reduced.

(4) According to the manufacturing method of the present invention, a cold-rolled steel sheet with an F value, which characterizes deep drawability, of 2 or more and superior workability compared to conventional methods can be obtained.

[Brief explanation of drawings]

Figure 1 is a graph showing the relationship between Mnl and the 7 values by varying the C content and slab heating temperature in two stages, and Figure 2 shows the relationship between the 7 values of the Mn and N component amounts. The graph and FIG. 3 are graphs comparing the T values of the current component m (Steel type G in Table 1) and the steel targeted for the present invention (Steel type B in Table 1).

Claims (1)

[Claims] In weight%, C: 0.01 to 0.06%, Mn: 0.03 to 0.20
%, S: 0.01% or less, sol. Al: 0.01~0
．． 08%, and N:N(%)≦[0.32-M
A steel slab that satisfies n(%)]/57 and has a composition with the balance consisting of Fe and unavoidable impurities is heated to a temperature of 1100°C or lower, hot rolled at a temperature of Ar_3 transformation point or higher, and then rapidly cooled. A method for producing a cold-rolled steel sheet with excellent workability, which comprises rolling the steel sheet at 500° C. or lower, then cold rolling and annealing the steel sheet thus obtained.