JPH0140895B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention has outstanding stretchability, excellent deep drawability, and ultra-low C with excellent secondary workability after pressing.
This invention relates to a method for producing aluminum killed cold rolled steel sheets. There are three conventional manufacturing methods for cold-rolled steel sheets with excellent stretchability and deep drawability. (a) Aluminum killed steel decarburized and annealed using an open coil. (b) Manufactured by box annealing or continuous annealing of ultra-low C aluminum killed steel. (c) Ultra-low C aluminum killed steel with Ti or Nb added. First, in (a), the precipitation of AlN is effectively utilized during heating during recrystallization annealing to impart excellent deep drawability, and furthermore, the C content in the steel is reduced through decarburization treatment to improve stretchability. be. Therefore, the amount of N is usually
40 ppm or more, the Al content is 0.02 to 0.08%, and the slab heating temperature is normally 1200°C or higher to completely solutionize the AlN, and the heating rate for recrystallization annealing is also slow heating ( 10 to 40℃/hr), and decarburization also takes a long time. Therefore, productivity is poor and it is not preferable from the point of view of energy saving. (b) does not require C-removal annealing, but when manufactured by box annealing, it has the same drawbacks as the box annealing method because it is manufactured using the same method as in (a) above. Also
When manufacturing by continuous annealing in (b), the desired properties cannot be obtained unless high-temperature winding is performed. Therefore, there are problems such as a decrease in the pickling properties of the hot-rolled sheet and deterioration of the material of the coil top and bottom parts due to high-temperature winding. Also, (c) can be manufactured by either box annealing or continuous annealing, but the addition of special elements increases the cost and the overhang properties are inferior to (a) and (b). Furthermore, conventional methods (a), (b), and (c) have the following common drawbacks. In other words, cold-rolled steel sheets for overhang deep drawing are subjected to severe press working, so secondary workability (referring to the toughness of the processed product after press working) is not a problem when the degree of work is low. However, ultra-low C materials in particular generally have poor secondary workability, and all of the above-mentioned steel sheets had problems with this secondary workability. The present inventors completed the present invention as a result of various studies to solve the above-mentioned drawbacks. That is, the gist of the present invention is that C: 0.005% by weight.
Below, Mn: 0.5% or less, Al: 0.005-0.05%, P:
0.008% or less, N: 0.0025% or less, and P
4C, P+5N: Steel with a composition of 0.0175% or less, the balance consisting of iron and unavoidable impurities, is hot rolled at 850°C or higher, cold rolled at a cold rolling rate of 50% or higher, and then at a recrystallization temperature or higher, A A method for manufacturing cold rolled steel sheets characterized by continuous annealing or box annealing at a temperature of ₃ points or less, and further including Ti: 0.10% or less in the above steel.
A method for manufacturing a cold-rolled steel sheet containing one or more of Nb: 0.10% or less and B: 0.003% or less. The present invention will be explained in detail below. First, the steel components constituting the present invention will be explained. The most important component of the present invention is to specify CP and N as a close and inseparable relationship. First, in the present invention, the amounts of C, P and N are each C: 0.005.
% or less, P: 0.008% or less, N: 0.0025% or less, and the conditional expressions of P4C and P+5N0.0175% must be satisfied. These conditions are intended to improve stretchability, deep drawability, and secondary workability at the same time. This will be explained in more detail below. First, an upper limit of 0.005% for the amount of C was set in order to obtain excellent stretchability and deep drawability. It has long been known that in order to improve workability, it is better to have as little C as possible, and ultra-low C steel with 0.01% or less is manufactured, but if C exceeds 0.005%, C does not precipitate as Fe ₃ C, but forms a supersaturated solid solution in the steel, increases aging resistance, and loses the characteristics of the present invention.
Moreover, if the amount of C is simply reduced, secondary processing cracks are likely to occur after press working. For example, C
It is known that if the content is 0.005% or less, secondary processing cracks will occur even if the degree of press working is not too great. In the present invention, in order to prevent secondary processing cracks even when performing severe press working with a drawing ratio of about 3.5, the P content is limited to 0.008% or less, and at the same time, as the C content decreases, the P content becomes P4C.
Numerous experimental results have shown that it is effective to also reduce the amount. Note that reducing the amount of P along with the reduction of the amount of C means reducing the amount of P as described below.
It also greatly contributes to improving deep drawability and stretchability by reducing the amount of steel. Therefore, in the present invention, since the reduction in the amount of C also leads to the reduction in the amount of P, the effect is significantly greater than in conventional steels. In order to maximize the characteristics of the present invention, the amount of C should be 0.004% or less, and the amount of P should be 0.004% or less.
3c is preferable. Limiting the amounts of P and N also has extremely important significance in the present invention. Figure 1 shows an example of C: 0.003-0.004%,
Figure 2 shows the relationship between the P content, N content and secondary workability of steel with a composition of Mn: 0.20~0.25% and Al: 0.01~0.04%.
It shows the relationship between N content, N content, value, and elongation, all of which are expressed as contour lines of the average value of a large number of experiments. In addition, the upper limit of P amount in the figure is C≒0.0035.
It is expressed as a percentage of P: 0.14% (P=4C). Other manufacturing conditions are as follows. Hot-rolled slab heating temperature 1050-1200℃ Hot-rolling finishing temperature 890℃ or higher Hot-rolling temperature 550-650℃ Cold rolling rate 80-85% Annealing conditions 750℃ x 1 minute (continuous annealing method) Temper rolling rate 1.0 % In the secondary processing test shown in Figure 1, cups with various drawing ratios were drawn, and the cups were expanded using an inverted conical punch at 0℃, and it was investigated whether brittle cracks occurred in the cups. The secondary workability is evaluated at the maximum drawing ratio at which no brittle cracks occur, and the larger this value is, the better the secondary workability is. The numbers in FIG. 1 indicate the maximum drawing ratios at which secondary processing cracks do not occur, and the larger this number is, the better the secondary processing properties are. In FIG. 2, solid lines indicate elongation, dashed lines indicate values, and numbers indicate elongation and values, respectively. As can be seen from Figures 1 and 2, P not only affects secondary workability, but also affects elongation, which has a strong correlation with stretchability, and values that have a strong correlation with deep drawability. At 0.008% P, the effect of improving elongation becomes extremely large, and at the same time, as the amount of P decreases, the r value also improves. Regarding N, a new fact was also discovered that reducing the amount of N improves secondary workability.
Below 25ppm, the value increases rapidly and the elongation also improves. In the present invention, in order to balance stretchability, deep drawability and secondary workability, and to maximize performance, P0.008%, 20.0025% and P+5N are used.
Limited to 0.0175%. This resulted in an elongation of over 52%,
It is possible to ensure an r value of 1.6 or more and a critical drawing ratio for secondary processing cracking of 3.5 or more. Of course, by further reducing P and N, these characteristics can be further improved, and P
By setting N to 0.010% and N to 0.0020%, a cold rolled steel sheet having the highest quality stretchability, deep drawability and secondary workability can be manufactured. Mn is required at about 0.05% to prevent hot embrittlement caused by S during hot rolling, but Mn/S
In order to stably satisfy the conditions of 15, the lower limit is set to 0.10.
% is preferable. On the other hand, if it exceeds 0.5%, the steel plate becomes hard and the characteristics of the present invention are impaired. The amount of Mn that maximizes the characteristics of the present invention is 0.40% or less. Al is uniquely different from conventional methods in that AlN is not used during recrystallization annealing in the present invention. The amount of Al is required to be at least 0.005% in order to obtain killed steel.
On the other hand, if the Al content exceeds 0.05%, the steel plate becomes slightly hard and costs increase. The preferred range is
It is 0.010-0.040%. Although S is not specified, Mn/S is commonly used to prevent hot embrittlement.
It is preferable to set it to 15. The basic components of the present invention have been described above, but in addition to the basic components, Ti, Nb, and B can be added as appropriate. Ti, Nb, and B bond with N or C, respectively, so in the present invention, which aims at low C and low N, the features of the present invention are further improved by adding these elements. When adding these elements, Ti0.10%,
One or more of Nb 0.10% and B 0.0030% may be used, but if these upper limits are exceeded, the effect will be saturated and the steel will become expensive. The steel billets (slabs) of the present invention are melted in a normal melting furnace such as a converter or an electric furnace, and the molten steel melted by vacuum degassing is then ingot-formed, bloomed, or continuously cast. In the present invention, the hot rolling conditions are sufficient as long as the finishing temperature is 850°C or higher, and the characteristics of the present invention are not affected by the slab heating temperature and winding temperature. Therefore, the heating temperature is free, but from the point of view of energy saving
It is desirable to heat at 1200℃ or less. Alternatively, hot pieces that have been continuously cast or bloomed may be directly hot rolled. Alternatively, hot charging may be performed in which continuously cast and bloomed hot pieces are charged into a heating furnace. In order to make the most of the features of the present invention, the finishing inlet temperature during finishing hot rolling should be 1000°C or less, and the reduction rate should be as large as possible in the low temperature range (more preferably, the total rolling reduction in the final two passes should be lowered). 40% or more), and it is desirable to perform forced cooling at 30°C/sec or more immediately after hot rolling. As described in the examples, it can be seen that when the hot rolling finishing inlet temperature is set to 1000°C or less, the value, which is an index of deep drawability, improves. In order to make the most of this effect, the slab heating temperature should be set to 1100℃.
It is preferable to do the following. The reason for this is not clear, but when the ultra-low C steel of the present invention is subjected to normal hot rolling, the crystal grains after hot rolling tend to become coarse. On the other hand, if hot rolling is performed at a low temperature and forced cooling is started immediately, the grains after hot rolling will become finer.
It is thought that the value improves due to the effect of making the grains of the hot-rolled sheet finer. Although the features of the present invention are not affected by the winding temperature, it is preferably 550 to 650°C from the viewpoint of improving pickling properties. make things worse). The hot rolled coil is subsequently descaled and subjected to cold rolling. Cold rolling is carried out at a cold rolling rate of 50% or more, as is commonly practiced. However, in the steel of the present invention, the workability is improved when the cold rolling rate is higher than that of ordinary steel, so the cold rolling rate is preferably 73% or more. Annealing may be continuous annealing or box annealing. Continuous annealing is performed above the recrystallization temperature A ₃ as is normally done.
Recrystallization treatment is performed below the temperature, followed by cooling and, if necessary, overaging treatment. Typical annealing conditions are 700
After performing recrystallization treatment at ~800°C for 3 minutes or less, it is cooled. When performing overaging treatment, the treatment is performed at 200 to 450°C for 5 minutes or less. In the case of box annealing, recrystallization treatment is performed above the recrystallization temperature and below the _A3 point, but in the present invention, the heating rate is different from the conventional method and there is no particular need for gradual heating. Typical annealing conditions include recrystallization at 650-750°C for 1-5 hours. The annealed steel plate is subjected to skin pass rolling as required and is then used as a finished product. Further, since the steel sheet manufactured by the method of the present invention does not lose any of the features of the present invention even if it is subjected to surface treatment, it can also be applied to tinplate, Zn-plated, and turn-plated steel sheets. Example The steel shown in Table 1 is melted in a converter, the amount of C is reduced to a predetermined amount by vacuum degassing treatment, the slab is made by continuous casting, and after heating at 1050 to 1200 ° C. , hot rolled under the conditions shown in Table 1, descaled, cold rolled to 0.8 mm, annealed, 1.5%
Temper rolling was carried out. The material properties of the obtained cold rolled steel sheet are also shown in Table 1. The tensile test piece used was a JIS No. 5 test piece, and the secondary workability was measured by expanding cylinders that had been deep drawn at various drawing ratios using an inverted conical die at 0°C, and using the maximum drawing ratio that did not cause brittle cracking. . Steel sheets manufactured within the scope of the present invention not only have extremely excellent elongation, which has a strong correlation with stretchability, and values that have a strong correlation with deep drawability, but also have extremely excellent secondary workability, and are the best. It can be said that it has superior press formability. Coil Nos. 3 and 4, 13 and 14 have significantly different values even though they were manufactured under exactly the same conditions except for the hot rolling finishing inlet temperature. That is, it can be seen that the r value is greatly improved by reducing the hot rolling finishing inlet temperature to 100°C or less. It can be seen that even if any one condition deviates from the scope of the present invention, any one of the properties of stretchability, deep drawability, and secondary workability is inferior. The present invention has the following outstanding features compared to the prior art. (a) Not only is the secondary workability after press working excellent, but the stretchability is significantly improved compared to conventional products. Also, the deep drawability is equal to or better than that of conventional products. (b) There is almost no dependence on the slab heating temperature, so it is possible to heat the slab at a low temperature. (c) Manufacture is possible regardless of the annealing method, and in the case of box annealing or open coil annealing, there is no need for gradual heating as in the past, improving productivity. Furthermore, the continuous annealing method does not require high-temperature winding, and low-temperature winding of 650°C or less is possible. Therefore, there is no variation in material quality and no deterioration in pickling properties.

【table】

[Table] * Maximum drawing ratio without secondary processing cracks

[Brief explanation of drawings]

FIG. 1 is a diagram showing the relationship between secondary workability and the amount of P and the amount of N, and FIG. 2 is a diagram showing the relationship between the elongation value and the amount of P and N.

Claims (1)

[Claims] 1. C: 0.005% or less, Mn: 0.5% as weight%
Below, Al: 0.005 to 0.05%, N: 0.0025% or less,
P: 0.008% or less, and P4C, P+5N:
Steel with a composition of 0.0175% or less and the balance consisting of iron and unavoidable impurities is hot rolled at 850℃ or higher to produce 50%
A method for producing a cold-rolled steel sheet, which is characterized in that cold rolling is performed at a cold-rolling rate of the above, and then continuous annealing or box annealing is performed at a temperature above the recrystallization temperature and below A ₃ points. 2 C: 0.005% or less, Mn: 0.50% as weight%
Below, Al: 0.005 to 0.05%, N: 0.0025% or less,
P: 0.008% or less, and P4C, P+5N:
0.0175% or less, Ti: 0.10% or less, Nb:
0.10% or less, B: 0.003% or less, and the balance is iron and unavoidable impurities, hot rolled at 850℃ or higher,
% or more, and furthermore, the recrystallization temperature or more,
A A method for manufacturing cold rolled steel sheets characterized by continuous annealing or box annealing at temperatures below ₃ points.