JPH0438825B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention is a continuous high-strength cold-rolled steel sheet that has excellent stretchability and deep drawability, as well as good bake hardenability, room temperature aging resistance, and dent resistance, and has a TS level of 35 to 45 kg/mm ² . This invention relates to a manufacturing method by annealing. Steel plates used for the outer panels of automobile bodies must be thin and have high strength in order to reduce the weight of the vehicle.Such high-strength steel plates are used for automobile body doors, roofs, hoods, fenders, etc. When applied to the outer panel of a steel plate, the following conditions must be met. (1) It must have excellent press formability and shape freezing properties so that it can be pressed into a predetermined shape. (2) A beautiful surface can be obtained after pressing. (3) It must have excellent dent resistance, meaning it will not dent even when pressed with fingers or hit by small stones. (4) Good corrosion resistance and spot weldability after painting. Of the above conditions, (4) is mostly determined by the composition of the steel, but in order to satisfy the conditions (1) to (3) above, the steel plate must have the following mechanical properties. Must have. That is, YP<27Kg/mm ² , TS:
35-45Kg/ ^mm2 , El>37%, n>0.200,>1.4,
Bake hardening amount>4Kg/ ^mm2 , room temperature aging property: YPEl<1.0% after 16 days at 38°C. In this way, it maintains a strength level of TS: 35 to 45 kg/mm ² , which is suitable for the outer panel of an automobile body, and at the same time has excellent stretchability, deep drawability, and shape fixability, as well as good bake hardenability and room temperature resistance. There has been a need to manufacture steel sheets that meet all the requirements, such as having good aging properties. Table 1 below shows the mechanical properties of steel sheets obtained by conventional manufacturing methods, separately for batch annealing and continuous annealing.

[Table] As is clear from Table 1 above, batch annealed materials generally have excellent press formability such as stretchability and deep drawability, but they do not have bake hardenability. When steel plates are applied to the outer panels of automobile bodies, there is a problem in that the dent resistance of the panels deteriorates as the gauge of the plate thickness decreases. In order to solve these problems, a proposal for imparting bake hardenability to batch annealed materials was proposed in JP-A-54-
No. 107415, Japanese Patent Application No. 143813 (1984)
73824) etc. However, in any case, batch annealing is a manufacturing method that requires a long processing time and a large amount of energy, and if possible, a continuous annealing method is desired. As shown in Table 1, conventional continuously annealed materials have somewhat poor press formability, but have excellent bake hardenability. Therefore, when a continuously annealed material is applied to the outer panel of an automobile body, an increase in panel strength can be expected in the paint baking process, and there is an advantage that dent resistance properties are improved. However, continuous annealing materials have inferior press formability, particularly deep drawability, compared to batch annealing materials, and therefore have the disadvantage that the parts to which they can be applied are limited. As described above, a method for producing high-strength cold-rolled steel sheets that has all the desirable characteristics as a material for the outer panels of automobile bodies, especially a method for producing them by continuous annealing, has not yet been proposed, and such a proposal is strongly desired. There is. The present invention was developed in view of the above-mentioned current situation, and its purpose is to combine excellent stretchability, deep drawability, bake hardenability, room temperature aging resistance, and dent resistance, and to achieve TS :35~45Kg/ ^mm2
An object of the present invention is to provide a method for manufacturing a high-strength cold-rolled steel sheet having a strength level of 1000 by continuous annealing. By the way, bake hardenability refers to solid solution C and N in steel sheets.
Although it is a kind of age hardening property caused by the aging property, the use of solid solution N is not preferable when considering the room temperature aging property. For this reason, it is preferable to use Al-killed steel in which solute N precipitates and fixes as AlN, leaving a significant amount of solid solute C in place. From this point of view, in the present invention, the steel type is a low-carbon Al-killed steel, and in order to satisfy all the above-mentioned required mechanical properties, a specific composition system and heat treatment method are adopted. be. That is, in the present invention, the component system is particularly C: 0.005
~0.02%, Si≦0.1%, Mn: 0.10-0.70%, P:
0.03%~0.15%, S≦0.020%, _N2 : 0.0020~
0.0080%, Sol.Al: 0.030~0.070%, balance Fe and unavoidable impurities as low carbon Al killed steel, finishing temperature 850~950℃, rolling temperature
After hot rolling at 500-750℃ and pickling, 60-95
A coil that has been cold rolled at a cold rolling rate of 50% is continuously annealed at a temperature above the recrystallization temperature and below 900°C, and then temper rolled. The reasons for limiting the components of the steel of the present invention will be explained below. When the C content is in the range of 0.005 to 0.020%, the Al-killed steel has bake hardenability, has relatively good room temperature aging properties, and can ensure good deep drawability. That is, in continuous annealing, stable bake hardenability can be obtained when C is 0.005% or more. Also the first
The figure shows the amount of C and the amount of recovery of yield point elongation due to aging at room temperature (38° C. x 16 days), and it can be seen that if the amount of C is within the range of the present invention, good aging resistance at room temperature is exhibited. The reason why such low carbon Al-killed steel exhibits bake hardenability and room temperature aging resistance is due to solid solution.
This is not because of N, which will be described later. In the case of continuous annealing, C
At >0.020%, bake hardenability can be obtained, but deep drawability deteriorates. Moreover, when C<0.005%, the absolute amount of solid solution C becomes too small, and as mentioned above, stable bake hardenability cannot be obtained. Also, C<
If it is 0.005%, in order to have excellent deep drawability and stable strength, it is necessary to add P, Mn, and Si in amounts exceeding the upper limit shown below. Although Si is an effective element for strengthening steel, it is desirable to keep it at 0.1% or less in cases where beautiful appearance and excellent press formability are required, such as in the outer panels of automobile bodies. Mn is required at 0.10% or more to prevent hot embrittlement of steel, but adding a large amount of Mn causes deterioration of press formability, especially deep drawability, so the upper limit should be 0.70%. . P has the effect of improving the formability, or improving the strength of the steel sheet while maintaining this, and further imparting bake hardenability, and is an important additive element in the present invention. For this reason, it is necessary to add P at least 0.03%. However, the addition of a large amount of P has a negative effect on weldability and brittleness, so the upper limit is set at 0.15%. S is an element that forms sulfides in steel and impairs formability, and should be kept at 0.020% or less. Sol.Al is necessary to fix N in the steel, prevent excessive room temperature aging due to N, and obtain bake hardenability due to solid solution C alone. Furthermore, in order to obtain excellent press formability as an Al-killed steel, Sol.
It is necessary to control the amounts of Al and N appropriately, Sol.Al is 0.030 to 0.070% and N is 0.0020 to 0.0080.
%. In order to avoid material deterioration due to low-temperature finishing, steel with the above composition system is used during the hot rolling stage.
Performs high-temperature finishing of 850℃ or higher. However, high-temperature finish rolling at a temperature exceeding 950°C coarsens the ferrite grains of the hot-rolled steel sheet and deteriorates the deep drawability after cold-rolling and annealing. Therefore, the upper limit of finishing temperature is set at 950°C. The winding temperature can be any temperature in the range of 500 to 700°C. That is, the winding temperature can be a normal winding temperature of about 500 to 650°C, but it can also be rolled at an even higher temperature, and in this case, the following actions and effects can be expected. That is, when the winding temperature is increased, the carbide becomes coarser, and as a result, the precipitated nuclei of intrinsic C are reduced, and the amount of solid solute C is increased, which has the effect of improving bake hardenability. Furthermore, high-temperature winding has the advantage that good press formability can be obtained due to the synergistic effect with continuous annealing, which is rapid heating and short-time annealing. In cold rolling subsequent to pickling after hot rolling, the cold rolling rate is 60 to 95%. The deep drawability of a cold-rolled steel sheet shows the highest value when the rolling reduction is around 80%, and decreases if the rolling reduction is too low or too high. If the rolling ratio is less than 60% or more than 95%, good deep drawability cannot be obtained. Therefore, the cold rolling rate is preferably 70% or more. The subsequent continuous annealing may be carried out by any conventional method as long as the temperature is higher than the recrystallization temperature. However, the upper limit of the annealing temperature should be 900°C; if the temperature exceeds this upper limit, good surface texture and shape cannot be obtained. Next, Table 2 shows the mechanical properties of the steel sheets obtained by the method of the present invention in comparison with comparative materials. Note that the amount of C was adjusted by vacuum degassing treatment.

【table】

[Table] As is clear from the same table, while the comparative materials have not reached the point where they satisfy all of the mechanical properties, the steel sheets obtained by the method of the present invention all have Mechanical properties applicable to the outer panels of automobile bodies as described above, that is, YP<27Kg/mm ² ,
TS: 35~45Kg/ ^mm2 , El＞37%, n＞0.200, r〓＞
1.4, bake hardening amount>4Kg/ ^mm2 , YPEl<1.0% after aging at 38°C for 16 days, and it is clear that it also has excellent dent resistance after painting. Furthermore, since the material of the present invention does not contain a large amount of alloying elements, it has good spot weldability and paint corrosion resistance. As is clear from the above explanation, according to the method of the present invention, it has excellent elongation properties, good bake hardenability, room temperature aging resistance, and dent resistance, and has a strength level of TS: 35 to 45 kg/mm ² . There is an excellent effect that a high-strength cold-rolled steel sheet having the following properties can be obtained by continuous annealing.

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between C content and room temperature aging property.

Claims (1)

[Claims] 1 C: 0.005~0.02%, Si≦0.1%, Mn: 0.10~
0.70%, P: 0.03% to 0.15%, S≦0.020%, _N2 :
0.0020~0.0080%, Sol.Al: 0.030~0.070%, remainder
Finishing temperature of steel consisting of Fe and unavoidable impurities is 850
Hot rolling at ~950℃, winding temperature 500~750℃,
It is characterized by being cold rolled at a cold rolling rate of 60 to 95% and then continuously annealed at a temperature above the recrystallization temperature and below 900°C.
A manufacturing method using continuous annealing of high-strength cold-rolled steel sheets with a TS level of 35 to 45 Kg/ ^mm2 .