JPH07103423B2 - Highly efficient manufacturing method of cold rolled steel sheet with extremely excellent workability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to an ultra-low carbon cold material having extremely excellent deep drawing workability, which is particularly suitable for applications such as automobile outer panels where ultra deep drawing performance is required. The present invention relates to a method for efficiently manufacturing a rolled steel sheet.

(Prior Art) An ultra-low carbon Ti-added cold-rolled steel sheet was invented as a cold-rolled steel sheet for ultra-deep drawing exceeding JIS G 3141 SPCE class, which is the standard for cold-rolled steel sheet, and Japanese Patent Publication No. 44-18066). The applications of steel sheets have expanded dramatically. Along with that, the improvement and improvement of this steel has been greatly promoted since then. At present, the workability by adding Ti and Nb in combination, particularly the deep drawability, is further improved, and the secondary workability is improved by adding B. As prior art to these, for example, JP-A-59-140333 and JP-A-61-11.
3724, JP-A-61-113725 and the like.

(Problems to be solved by the invention) The properties of this ultra-low carbon Ti-added steel after cold rolling / annealing are significantly influenced by the hot rolling conditions as well as the components mainly for high purification. However, this is a fatal drawback for mass-consumable materials such as steel sheets for automobiles, which causes a decrease in yield and impairs economic efficiency. This is an extremely serious problem in the case of the steel, which is costly because it requires vacuum degassing and the like in the first place.

These ultra-low carbon cold-rolled steel sheets have excellent workability in the first place, but in the present invention, the deep drawability represented by the Rankford value (hereinafter referred to as the value) of them is further improved to give the ultra-deep drawability. In addition to the above, the present invention provides the following manufacturing method for increasing the industrial stability.

(Means for Solving the Problem) The present invention solves such a problem by taking a specific hot rolling of a high-purity steel having a specific component, particularly finishing rolling conditions to winding, which are unprecedented. The main point is mass%, C: 0.0040% or less, N: 0.0040%
Below, Mn: 0.05-0.4%, S: 0.015% or less, acid-soluble Al (hereinafter sol.Al): 0.005-0.100%, Ti: 0.04-0.085%, B: 0.0
When steel containing 001 to 0.0010% and the balance unavoidable impurity elements is heated to 1200 ° C or lower and hot-rolled, the rough finish thickness is set to 45 mm or more, and the effective strain represented by the following formula is ε.
After finishing rolling at a temperature of 880 ° C or higher with an _eff of 45% or higher, cooling is started within 1 s, cooled to 830 ° C or lower at an average cooling rate of 20 ° C / s or higher, and then 680-800. It is wound at a temperature of ℃, and then cold rolled at a cold rolling rate of 75 to 90%.
This is a highly efficient method for producing a cold-rolled steel sheet having extremely excellent workability, which is characterized by performing continuous annealing at a temperature of 0 to 870 ° C.

Here, ε _eff = final pass rolling reduction (%) + 1/2 final 1st-stage preceding pass rolling reduction (%) + 1/4 final 2nd-stage preceding pass rolling reduction (%).

That is, Nb which is usually used as a component is not used, and Ti is added in a much larger amount than the stoichiometric equivalents of carbon and nitrogen. This is to secure a certain amount of solid solution Ti. Furthermore, in hot rolling, by heating at a relatively low temperature and increasing the rough finish thickness, the overall finish reduction rate is increased, and a specific post-stage reduction in which the rolling effect is exerted closer to the final finish stage After performing hot rolling, cooling is started immediately to freeze the effect as much as possible, and it is wound at a high temperature.

Although the mechanism of this effect is not yet clear, it is thought that the requirement expected for hot-rolled sheet as a pretreatment for such high-purity steel is to provide a fine-grained matrix with extremely few impurities.

The two are in the first place conflicting with each other, and their control is extremely delicate. However, by increasing the total finishing reduction rate, strain-induced precipitation of precipitates is promoted and the precipitation treatment in γ becomes complete. By setting the finishing final stage region under high pressure, fine recrystallized γ grains can be stably obtained.

Then, the hot-rolled plate crystal grains formed through the growth of the γ grains, the γ-α transformation, and the growth of the α grains are rapidly cooled immediately after rolling, so that the as-rolled state is frozen. Finally, the product is wound in a temperature range where the α-grains are almost no longer grown, and the precipitation and coarsening of the precipitates are thoroughly made by utilizing the low solubility in α.

Coarsening of the precipitate is thus performed both in γ and in α, and hot rolling grain refinement is performed in a region where the final final stage reduction is increased and saturated, so that the state in the hot rolled sheet is stable and preferable. Put in a state.

(Operation) Next, the operation of each requirement and the reason for limiting the numerical value will be described.

C, N: C, N is an interstitial solid solution element and is harmful to the texture formation. Therefore, it is necessary to further reduce it as much as possible compared with ordinary ultra low carbon steel. Therefore, each is set to 0.0040% or less.
C: 0.0025% and N: 0.0020% are preferable.

Mn: Substitution type solid solution element. If it is too much, it hardens the steel and impairs ductility. However, it also has the role of forming S and MnS in steel and avoiding hot embrittlement due to S, and therefore 0.05-0.4%
And Low Mn improves both ductility and value, so 0.15% or less is preferable.

Since it becomes S: MnS and becomes a harmful inclusion, it is better to reduce it as much as possible. Therefore, 0.015% or less. Preferably Mn
0.15% or less and S is 0.008% or less.

sol.Al:Al is necessary for deoxidation, so 0.005-0.10% in steel
To remain. If it is less than the lower limit, sufficient deoxidation cannot be achieved, and if it exceeds the upper limit, inclusions increase and the ductility of the steel is impaired.

Ti: Ti needs to be 0.04 to 0.085%. First, Ti fixes N in the form of TiN to avoid the adverse effect of solid solution N. Also, most of C
Also fixed in the form of TiC. Further, solid solution Ti has an effect of affecting recrystallization of the hot rolled sheet and re-granulating it. Therefore, in the present invention, a large amount of Ti is added to secure the solid solution Ti. Addition of less than 0.04% does not have these effects. However, if added in excess of 0.085%, the adverse effect of impurities will be more than the above effect. Therefore, the workability deteriorates.

B: B is added to improve secondary workability. In a high-purity ultra-low carbon steel such as this steel, there is no solid solution carbon that is a grain boundary strengthening element, and therefore the grain boundary strength is low. This occurs in the form of vertical cracks when secondary processing such as mouth widening is performed after undergoing ultra-deep drawing which is the main processing type of the present invention. B is added to prevent this secondary processing brittleness. If it is less than 1 ppm, it has no effect, and if it exceeds 10 ppm, the solid solution B has an adverse effect and deteriorates the value.

Hot rolling heating temperature: 1200 ° C or less. If heated above this temperature, the γ-grains before rolling become too large, and the hot-rolled sheet grains become mixed grains even after hot-rolling according to this method, which does not have a predetermined structure,
Furthermore, by dissolving various precipitates unnecessarily, it becomes difficult to carry out the precipitation / coarsening treatment in the subsequent hot rolling step.
From this point of view, the hot rolling heating temperature is preferably 1100 ° C. or lower.

Rough finish thickness: 45mm or more. As a result, the rolling ratio in the relatively low temperature range called finish rolling is increased, and the strain-induced precipitation of various precipitates is promoted and coarsened. Normal 40
At about mm, this effect is not perfect, which eventually leads to material deterioration and variation. It should preferably be 55 mm or more.

ε _eff : In the present steel, due to the above effect, the grain boundary and the inside of the grain are cleaned with few impurities. Therefore, ordinary rolling cannot obtain fine crystal grains. It is a specific finishing rolling condition that overcomes this, especially 3 in the latter stage of finishing.
It was found that the rolling rate of the pass is very important, and as a result of various studies as an index showing the effect of this post-stage reduction rate industrially, ε _eff shown in the following equation was derived. That is, ε _eff = final pass rolling reduction (%) +1/2 final one-stage preceding pass rolling reduction (%) +1/4 final two-stage preceding pass rolling reduction (%).

FIG. 1 is a diagram in which the values after cold rolling and annealing are plotted in the relationship between the finish rolling finish temperature and ε _eff .

Ingredients are C: 15-23ppm, N: 12-20ppm, Mn: 0.08-0.14
%, S: 0.003 to 0.007%, sol.Al: 0.02 to 0.035%, Ti: 0.0
43 ~ 0.083%, B: 0.0003 ~ 0.0006%, hot rolling heating temperature: 11
00 ~ 1130 ℃, rough finish thickness: 55 ~ 60 mm, 0.3 ~ 0.4 s after finishing
After that, it is rapidly cooled to about 800-780 ℃ at an average of 30-40 ℃ / s and 730-760.
It was wound up at ℃. Subsequently, after 80% cold rolling, continuous annealing was performed at 850 ° C., a 0.3% pressure was applied, and the test was performed.

As can be seen from the figure, when ε _eff is 45% or more, the value is stable over a wide range of finish rolling end temperature and shows an extremely high value of about 2.2 or more. Ε _eff is 6 for more stable and higher price
0% or more is preferable.

Finish rolling finish temperature: 950 to 880 ° C. As shown in FIG. 1, if the temperature exceeds this value, stable high processing characteristics cannot be obtained no matter how high ε _eff is increased. Further, if the temperature is lower than 880 ° C., it may be partially rolled in the α region, and not only the material does not come out, but also defects such as rough skin occur.

Cooling conditions after rolling: The hot-rolled structure obtained as described above is cooled within 1 s so as not to coarsen the grain size, and is cooled to 830 ° C or less at an average of 20 ° C / s or more. . If this condition is removed, the crystal becomes coarse and a constant material cannot be obtained. In this case, the time to start cooling is especially important.
It should be within 1 s, preferably within 0.5 s. If the cooling rate is less than 20 ° C / s, coarsening of grains occurs during cooling. 830 ° C
Since the grain growth is less likely to occur below, the end point of this cooling is 830 ° C.

Winding temperature: The effect of heat retention after winding promotes precipitation and coarsens the precipitate. Below 680 ℃, this effect is less than 800
If the temperature exceeds ℃, the rolling structure may not be sufficiently frozen and the crystal grains may become coarse, so the coiling temperature was set to 680 to 800 ℃. The winding temperature is preferably 720 ° C. or higher in order to achieve sufficient precipitation coarsening.

Cold rolling rate: The cold rolling rate must be 75% or more to achieve a high value. It is preferably 78% or more. On the other hand, in this steel, although the rolling reduction is about to improve even if it is increased to about 93%, it is an industrially difficult region, so the upper limit was set to 90%.

Annealing temperature: The annealing temperature for continuous annealing is 780 ° C because it is sufficiently soft and has a high value texture. On the other hand, the upper limit is 870 ℃
And In annealing at a high temperature exceeding this, in the steel such as the present steel in which grain growth is likely to occur, the crystal grains as a product become too large, and roughening occurs after press forming. As the annealing temperature, high temperature annealing of 830 ° C. or higher which is relatively high in continuous annealing is preferable.

Although the constituent features and functions of the present invention have been described above, the melting of the steel of the present invention is usually carried out in a converter to obtain ultra-low carbon by vacuum degassing such as RH. Then, it is usually made into a billet by continuous casting. Hot rolling is finish rolling by a tandem rolling mill having five or more rolling mills.
In order to increase the total finishing reduction ratio according to the present invention and to protect the finishing finish temperature according to the present invention, a delay may be performed to reach a predetermined temperature before finish rolling.

After hot rolling, high-temperature winding is performed. Since both ends of the hot-rolled coil are rapidly cooled, it is preferable to take a U-shaped winding temperature pattern in which the ends become even hotter in order to compensate for this. The hot rolled coil is pickled, cold rolled, and then continuously annealed. The soaking temperature of continuous annealing is as described above, but the holding time may be 40s to 180s which is usually taken.

The cooling conditions after annealing are also not specified, and the overaging zone is usually provided, but the temperature conditions are not specified either. The pressure adjustment after annealing should be limited to the unavoidable range for shape correction. From the material, it is preferable not to regulate the pressure, but considering the shape correction, 0.2-0.8%, preferably 0.2-0.5%
% Is the proper pressure regulation rate.

(Examples) Steels having the components shown in Table 1 were melted in a converter and continuously cast into slabs. At this time, RH vacuum degassing was used.

Subsequently, treatment was performed under the hot rolling, cold rolling and continuous annealing conditions shown in Table 2, and a representative portion of each coil was subjected to a material test.

The tensile test was performed according to the method described in Z 2241 using JIS Z 2201,5 test piece.

In addition, the secondary work resistance is as follows: first, a 50 mm flat bottom punch is used to perform deep drawing with a drawing ratio of 2.2, and subsequently, the process of expanding this mouth is performed at various temperatures, and the transition temperature of brittle-ductile fracture is measured. I asked.

Table 3 shows the material test results of the central portion of the coil longitudinal direction.

In this table, n is a work hardening index, which was calculated from the stresses of 10% and 20% strain according to the nth power law. In rave showy plane average temperature Lankford value, r ₄₅ is the Lankford value of 45 ° direction to the rolling direction.

The steels of treatment Nos. 1, 7, 9, 13, 14 and 15 according to the invention are
It can be seen that the elongation is 55% or more and the n value is 0.27 or more, which is extremely high ductility, and the extremely high deep drawability of ≧ 2.45 and r ₄₅ ≧ 2.15. Moreover, the transition temperature is sufficiently low for secondary workability.

On the other hand, the steels of other treatment Nos. In which the composition, the hot rolling conditions or the cold rolling / continuous annealing conditions are different from the conditions of the present invention, do not have the desired high properties of the present steel.

Further, FIG. 2 shows the material distribution in the coil longitudinal direction of Table 2, Treatment No. 14 (condition of the present invention) and Treatment No. 16 of the same table (comparison condition). In the former case, the finishing temperature is 895 to 940.
The coiling temperature was 770 to 780 ℃ at the coil end and 740 to 755 ℃ at other parts. In the latter case, the temperatures were 885 to 930 ℃, 760 to 785 ℃ and 745 to 760 ℃, respectively.

As is clear from the figure, in the No. 14 coil according to the present invention, stable and high elongation can be obtained over the entire length, whereas in the comparative coil, the material variation is large although it is at a considerably high level.

(Effects of the Invention) The steel of the present invention may be used as a cold-rolled steel sheet, or may be used as a plated steel sheet that has been subjected to electroplating or electrical composite plating after continuous annealing. Further, the present steel component does not particularly impair the hot dip galvanizing property or the alloying property of the subsequent plated layer. Therefore, as long as the continuous annealing conditions are satisfied, a continuous annealed hot dip galvanized steel sheet or an alloyed hot dip galvanized steel sheet may be used.

[Brief description of drawings]

FIG. 1 is a table showing the values after cold rolling and continuous annealing in relation to the finish rolling finish temperature and the finish rolling effective strain ε _eff, and FIG. 2 is the material distribution in the longitudinal direction of the coil used in the examples. FIG.

Claims (1)

[Claims] [Claim 1] C: 0.0040% or less, N: 0.0040% or less, Mn: 0.05 to 0.4%, S: 0.015% or less, acid soluble Al: 0.005 to 0.100%, Ti: 0.04 to 0.085% , B: 0.0001 to 0.0010%, when heating steel consisting of the balance unavoidable impurity elements after heating to 1200 ° C or lower and hot rolling, the rough finish thickness is set to 45 mm or more, and the effective strain ε _eff shown by the following formula is 45% or more. 950 ℃ ~
After finishing rolling at a temperature of 880 ° C or higher, start cooling within 1 s, cool to 830 ° C or lower at an average cooling rate of 20 ° C / s or higher, and then wind at a temperature of 680 to 800 ° C, and continue. 75
A highly efficient method for producing a cold rolled steel sheet with extremely excellent workability, which comprises performing cold rolling at a cold rolling rate of up to 90% and performing continuous annealing at a temperature of 780 to 870 ° C. However, ε _eff = Final pass reduction rate (%) + 1/2 Final 1st stage preceding pass reduction rate (%) +1/4 Final 2nd stage previous pass reduction rate (%)