JPS61130423A - Production of cold rolled steel sheet having excellent deep drawability - Google Patents

Abstract

PURPOSE:To produce a cold-rolled steel sheet having excellent deep drawability by subjecting an extra-low C billet incorporated therein with a specific compsn. ratio of Ti or Nb to temp. control at an adequate temp. then to rolling at a high finishing temp. in the stage of heating said billet and subjecting the same to continuous hot rolling. CONSTITUTION:The billet which contains 0.01wt% C and <=0.4% Mn, contains further at least one kind of 0.01-0.15% Ti and 0.01-0.15% Nb, has >=4 Ti/C or >=8 Nb/C when either Ti or Nb is incorporated or >=8 (Ti+Nb)/C when both Ti and Nb are incorporated and consists of the balance Fe and unavoidable impurities is subjected to the temp. control in a temp. range of 800-1,000 deg.C in the stage of heating the billet to >=1,000 deg.C and subjecting the same to continuous annealing. The billet is then rolled at the finishing temp. of >=700 deg.C by which the billet is rolled to a hot-rolled steel strip having an intermediate thickness. The above-mentioned steel strip is thereafter subjected to pickling, cold rolling and annealing and the cold-rolled steel sheet having a high (r) value and excellent deep drawability is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a cold-rolled steel sheet with excellent deep drawability. The present invention relates to a method for stably manufacturing cold-rolled steel sheets with excellent deep drawability by controlling the temperature.

In general, cold-rolled steel sheets for processing used for automotive exterior panels, gasoline tanks, and the like are required to have excellent stretch formability, deep drawability, aging resistance, and the like. As is well known, the stretch formability of a steel plate is better as the yield strength is lower and the elongation and work hardening index are higher. Deep drawability is determined by the Lankford value (r
The higher the r value, the higher the deep drawing limit. In addition, when C and N remain in solid solution in a steel sheet, a disorder called stretcher strain occurs due to room temperature aging during press forming, so steel sheets for press forming are required to have aging resistance. be done.

By adding carbonitride-forming elements such as Ti and Nb to ultra-low C steel with a C content of 0.01% or less, and performing box annealing or continuous annealing after hot rolling and cold rolling, excellent cold rolled steel sheets can be obtained. It is already known that press workability can be imparted. Furthermore, the deep drawability (r value) of these steel sheets can be further improved by lowering the heating temperature of the steel prior to hot rolling from the conventional temperature of 1200°C or higher to about 1100°C or lower. is also known. However, such low-temperature heating of steel slabs can be easily adopted in a batch furnace heating method for small-volume production, but it is difficult to continuously load large quantities of steel slabs, weighing several hundred tons, into a heating furnace. The continuous heating method used often causes inconvenience in actual production if the low-temperature heating material is small.

As a result of intensive research to solve the above-mentioned problems in the production of cold-rolled steel sheets with excellent deep drawability, the present inventors found that
In addition to regulating the chemical composition of the steel used, the temperature is controlled at a predetermined temperature during continuous hot rolling of this steel billet, and
It has been discovered that by specifying the finishing temperature, it is possible to produce cold-rolled steel sheets with excellent r-values, and furthermore, that the r-values of the resulting cold-rolled steel sheets can be further improved by low-temperature heating of the steel slabs. , which led to the present invention.

The method for producing a cold rolled steel sheet with excellent deep drawability according to the present invention is as follows:
Contains 0.01% or less of C, 0.4% or less of Mn, and further contains at least one member selected from the group consisting of 0.01 to 0.15% of Ti and 0.01 to 0.15% of Nb. and when it contains either Ti or Nb, T i / C
a2 or Nb/C≧8, and when both Ti and Nb are included, (Ti+Nb)/C≧8, and after heating a steel billet consisting of the balance iron and unavoidable impurities to a temperature of 100θ℃ or higher, continuous When hot rolling, 8
Temperature controlled in the range of 00 to 1000℃, 700℃
It is characterized in that it is rolled at the above finishing temperature to form a hot-rolled steel strip with an intermediate thickness, and then pickled, cold rolled, and annealed.

In the method of the present invention, temperature control refers to holding the rolled material at a predetermined temperature for a predetermined time during continuous hot rolling of steel, and then hot rolling is continued. In the present invention, the controlled temperature is in the range of 800 to 1000°C,
The required holding time at this temperature varies depending on the chemical composition of the steel, but in order to obtain a cold rolled steel sheet with a high r value, it is usually
It is more than 1 minute. In this way, after heating steel having a predetermined chemical composition, the temperature of the rolled material is adjusted during continuous hot rolling according to the present invention, and then hot rolling is continued to produce hot rolled material of intermediate thickness. When a cold rolled steel plate is obtained by obtaining a steel strip and then pickling it by a conventional method, cold rolling it, and annealing it, the tensile strength and yield strength of this cold rolled steel plate are determined depending on the temperature control temperature. At about 1000°C, it is almost the same as when no temperature control is applied, and there is a tendency to decrease as the temperature control temperature decreases.On the other hand, the r value and elongation are observed when the temperature control temperature is 900°C.
It is maximum when it is near. That is, when IML conditioning is applied during hot rolling in this manner, a cold rolled steel sheet with excellent elongation and r value can be obtained compared to a case where temperature control is not applied. A particularly preferable temperature control temperature is in the range of 850 to 950°C. ! The adjustment time is 2 to 10 minutes. The effect of such temperature control is similarly observed when a steel billet is heated at a so-called low temperature.

As mentioned above, it is not necessarily clear why the elongation and r value of ultra-low C-Ti steel and ultra-low C-Nb steel are improved by temperature control during hot rolling, but It appears that the precipitation of TiC and NbC is closely related to the above effect.

Next, the reason for limiting the chemical composition of the steel slab in the present invention will be explained.

When C is added in a large amount exceeding 0.01%, a large amount of TiC or NbC precipitates, hardens the steel and deteriorates deep drawability.
The upper limit of the amount added is 0.01%.

Particularly preferred is a range of 0.006% or less.

When Mn is added in a large amount, it increases the recrystallization temperature during annealing and deteriorates deep drawability, so the upper limit of the amount added is set to 0.4%.

In the method of the present invention, Ti improves the deep drawability of the cold rolled steel sheet,
In particular, it is an important element for increasing the r value, and when the amount added is less than 0.01%, there is no such effect.On the other hand, when added in excess of 0.15%, the r value is Although there is no particular problem from the viewpoint of improvement, since the manufacturing cost becomes high, the upper limit of the amount added is set to 0.15% in practical terms. However, in the present invention, the amount of Ti is regulated by the amount of C, and when the Ti/C weight ratio is smaller than 4, the r value of the obtained cold rolled steel sheet tends to decrease. is within the above range, and the T i /C weight ratio is required to be 4 or more.

In the method of the present invention, Nb is also an important element for increasing the r value like Ti, and when the amount added is less than 0.01%, it has no effect on improving the r value; ，
If it is added in excess of 15%, not only will the effect of improving the r value become saturated, but it will also increase manufacturing costs, so the upper limit of the amount added is set at 0.15%. However, the amount of Nb is also regulated by the amount of C. That is, when adding Nb alone, Nb
The b/C weight ratio is required to be 8 or more.

This is because when the Nb/C weight ratio is smaller than 8, the r value of the obtained cold rolled steel sheet tends to decrease.

Ti and Nb may be added individually, or may be added in combination. However, when using this composite addition, (Nb
+Ti)/C≧8.

In the method of the present invention, the steel billet is hot rolled after being treated as necessary. In the present invention, the heating temperature of the steel slab prior to hot rolling is 1000°C or higher. If the steel heating temperature is lower than 1000℃, 70
It is difficult to ensure a finishing temperature of 0°C or higher. When the finishing temperature is lower than 700° C., a part of the processed structure remains in the U weave of the hot rolled steel sheet, reducing the r value of the cold rolled steel sheet. The upper limit of the steel heating temperature is preferably 1250° C. in consideration of energy saving, durability of refractory bricks, etc. In addition, in the method of the present invention, a hot steel billet obtained by continuous casting or blooming may be directly hot rolled.

In the method of the present invention, it is preferable to apply a rolling reduction of 50% or more before the temperature of the rolled material is adjusted, from the viewpoint of crimping defects in the steel billet. In actual operation, it is preferable to perform temperature control during the rough rolling process or after the rough rolling is completed.

After performing hot rolling with temperature control as described above to obtain a hot rolled steel strip with an intermediate thickness, it is cooled according to a conventional method, and is usually rolled for 7
Wind it into a coil at a temperature below 00℃. When cold rolling this hot rolled steel strip after pickling the oxide scale according to a conventional method, the rolling reduction is not particularly limited, but in order to ensure sufficient deep drawability, the rolling reduction should be between 60 and 95%. It is preferable that

Annealing after cold rolling may be either box annealing or continuous annealing, and the annealing temperature is preferably in the range of 700 to 850°C.

As described above, according to the method of the present invention, when a steel billet having a predetermined chemical composition is heated and then subjected to continuous hot rolling, cooling with good deep drawability is achieved by controlling the temperature at a predetermined temperature. A rolled steel plate can be obtained. It can also be applied to a low temperature heating method for steel slabs.

The present invention will be explained in detail below.

Examples Steels ■, ■, and ■ having the chemical compositions shown in Table 1 are melted into slabs in a small melting furnace, heated under the conditions listed in Table 2, and subjected to various treatments during hot rolling. The temperature was controlled to obtain a hot-rolled steel strip with a thickness of 3.2 mm. This hot-rolled steel strip was pickled, cold-rolled to a thickness of 0.8 μ, continuously annealed at a temperature of 850° C., and then subjected to 0.5% temper rolling.

The mechanical properties of the cold rolled steel sheet thus obtained are shown in Table 2. The results for steels ■ and ■ are shown in Figures 1 and 2, respectively.

As described above, it is clear that all cold rolled steel sheets manufactured according to the method of the present invention have high r values and elongations, and have excellent deep drawability. As shown in Figure 1, for ultra-low C-Ti steel I, @adjustment temperature 800-1020
In the temperature range of 900°C, the tensile strength and yield strength of cold-rolled steel sheets decrease as the temperature control temperature decreases, while the r value and elongation are maximum near the temperature control temperature of 900°C. Temperature control temperature 10
At 20° C., the mechanical properties of the cold-rolled steel sheet are almost the same as those without temperature control. Therefore, the temperature control temperature should be set to 800~
By controlling the temperature to 100°C, preferably 850 to 950°C, a cold rolled steel sheet with excellent r value and elongation can be obtained. Furthermore, it has been observed that the r-value is further improved by heating the steel at low temperatures prior to hot rolling.

As shown in Figure 2, regarding ultra-low C-Nb steel ■,
The same tendency as the above Ti-based steel is observed.

[Brief explanation of drawings]

Figure 1 is a graph showing the relationship between the temperature control temperature in the hot rolling process of ultra-low C-Ti steel specified in the present invention and the mechanical properties of the obtained cold-rolled steel sheet. C-Nb
A similar graph for steels. Fig. 1 Temperature 1 degree (°C) Fig. 2 Temperature C)

Claims (1)

[Claims] (1) Contains 0.01% or less of C, 0.4% or less of Mn, and further at least one member selected from the group consisting of 0.01 to 0.15% of Ti and 0.01 to 0.15% of Nb. and contains either Ti or Nb, Ti/C≧4 or Nb/C≧8, and when both Ti and Nb are contained, (Ti+Nb)/C≧8, the balance being iron. When continuously hot rolling a steel billet containing unavoidable impurities to a temperature of 1000°C or higher,
Temperature controlled in the range of 00 to 1000℃, 700℃
A cold-rolled steel sheet with excellent deep drawability characterized by being rolled at a finishing temperature above to form a hot-rolled steel strip with an intermediate thickness, followed by pickling, cold rolling, and annealing. manufacturing method.