JPS6233291B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing cold rolled steel sheets with good drawability, ductility, and aging resistance by continuous annealing. Cold-rolled steel sheets with good drawability and ductility are conventionally manufactured by box annealing. However, the box annealing method not only takes several days to process, but because the coil is heat treated, the heating and cooling rates differ in the radial direction of the coil, making it difficult to obtain a homogeneous material over the entire coil. Ta. Using the continuous annealing method,
It is possible to eliminate these drawbacks of the box annealing method. However, continuous annealing involves rapid heating and cooling, which results in poor crystal grain growth.
Furthermore, since the precipitation of C dissolved in solid solution in the steel does not proceed, the steel is hard and has poor drawability and aging resistance. In order to eliminate these drawbacks of continuous annealing, we promote grain growth in an orientation that is advantageous for drawability by coiling at a high temperature during hot rolling, and after rapid cooling during continuous annealing, we
A method has been proposed in which overaging treatment is carried out at ~500°C for several seconds to several minutes to promote the precipitation of unprecipitated solid solution C and improve aging properties, but high temperature coiling during hot rolling is difficult. The steel sheet manufactured by this method is still inferior to the box-annealed material in terms of drawability, ductility, and aging resistance, which is accompanied by a decrease in pickling properties. On the other hand, since the main cause of deterioration of the aging resistance of continuously annealed materials is solid solution C, we have improved the aging resistance by using ultra-low carbon steel material with a reduced C content of 0.0050% or less. Methods to improve this have been proposed. In general, in order to produce steel sheets for drawing, it is essential to finish hot rolling at the Ar ₃ transformation point or higher, but this reduction in C content is advantageous in terms of aging resistance. However, since it raises the Ar ₃ transformation point, in order to finish rolling in the γ (austenite) region during hot rolling, it is necessary to increase the slab heating temperature and change the hot rolling reduction schedule, which leads to problems in terms of energy saving. This is a big minus. Another cause of deterioration of the aging resistance of continuously annealed materials is N dissolved in steel. In order to reduce solid solution N, we have developed a method of coiling Al-killed steel pieces at high temperatures during hot rolling and fixing them as AlN. A known method is to add 20 to 50 ppm of B to steel and fix N as BN. Improvement of aging property and drawability by solid solution C and N,
For the purpose of improving ductility, reduce the C content of high-temperature rolled material of Al-killed steel and B-added Al-killed steel to an extremely low C range of 0.01% or less, and appropriately combine the slab heating temperatures of these steels. Although many methods have been developed, they have not been sufficient to produce steel with good aging resistance, drawability, and ductility. Furthermore, a common feature of the conventional continuous annealing method for manufacturing cold rolled steel sheets is that the finishing temperature of hot rolling is set to Ar ₃ or higher in order to improve drawability. The present inventors have discovered that even if the hot-rolling finishing temperature is set to Ar ₃ points or lower, drawability, ductility, and aging resistance are comparable to or higher than conventional hot-rolled steel sheets manufactured at Ar ₃ points or higher. As a result of repeated research with the aim of manufacturing steel sheets as materials, we used a B-added ultra-low carbon Al-killed steel material with a specified range of C, B, and N components, and combined the slab heating temperature and hot rolling conditions. They discovered that a steel sheet with good drawability, ductility, and aging resistance can be obtained even with low-temperature finishing at an Ar point of ₃ or less during hot rolling. The present invention was created based on these findings, and its gist is that C: 0.0040% or less, Mn: 0.40% or less, sol.Al: 0.008 to 0.090%,
Steel containing N: 0.0010 to 0.0050%, B: 0.0005 to 0.0050%, and B/N = 0.2 to 1.0, with the balance being Fe and unavoidable impurities, or with respect to the composition of this steel, further Ti, Nb, One or two of V and Zr
A steel slab made by adding 0.002 to 0.02% of
After heating to a temperature below 1180℃, the finishing temperature is 680℃.
A cold rolled steel sheet with good drawability, ductility and aging resistance, characterized by hot rolling at ~800°C and a coiling temperature of 580°C or less, followed by cold rolling and continuous annealing. It is in the manufacturing method. The present invention will be explained in detail below. As described in the following experiment, the present inventors produced small steel ingots with different amounts of C, Al, N, and B in the laboratory, and made them into slabs with a thickness of 30 mm by blooming rolling. A hot-rolled sheet of 2.8 mm was obtained by changing the heating temperature and hot-rolling finishing temperature, immediately charged into a furnace at 530°C, held for 2 hours, and then slowly cooled in the furnace to simulate winding of the hot-rolled sheet. . Next, after pickling, the test piece was cold rolled to 0.8 mm, and then inserted into a fluidized bed heat treatment furnace at 800°C until the plate temperature reached 800°C.
After reaching the temperature, continuous annealing is performed by holding the temperature for 15 seconds and then rapidly cooling it (heating rate of approximately 50℃/sec, cooling rate of approximately 20℃).
°C/sec) and 0.6% of the material after temper rolling. Experiment Mn: 0.26%, P: 0.015%, S: 0.011%, Al:
0.035%, N: 0.0028%, B: 0.0013% (B/N=
In addition to 0.464), small steel ingots with compositions varying in C content from 0.0005 to 0.0080% were used, slab heating temperature: 1250℃, 1100℃, hot rolling finishing temperature: 890℃, 760℃.
After processing at a coiling equivalent temperature of 530°C, cold rolling and continuous annealing were performed to examine the material properties. Value, total elongation, as a measure of drawability, ductility, and aging resistance.
AI (statute of limitations index) was used. AI is pre-strained to 7.5% and then accelerated aging at 100℃ for 30 minutes.
The amount of increase in yield stress was defined as AI. AI≦3
Kg/mm ² Preferably, if AI≦2.5Kg/mm ² , the aging resistance is equivalent to box annealed material, and aging at room temperature can be almost ignored. Figure 1 shows the results.
When C: 0.0040% or less, preferably C: 0.0030
% or less, it shows excellent values, total elongation, and AI, but it is an even better material especially when the slab heating temperature is 1100℃ and the hot rolling finishing temperature is 760℃, which is below Ar ₃ points (marked with ○ in the figure). has. Experiment C: 0.0020%, Mn: 0.24%, P: 0.013%,
Based on S: 0.010%, Al: 0.039%, N:
Steel with B: 0.0010 to 0.0080% and B: 0.0003 to 0.0080% was manufactured in the laboratory, and the slab heating temperature was
1100℃, hot rolling finishing temperature 760℃, coiling equivalent temperature 530℃
Hot rolling was completed under these conditions, followed by cold rolling and continuous annealing to examine the material properties. The results are shown in FIGS. 2, 3, and 4. The numbers at the points in these charts are the respective measurements. Value ≧1.6, total elongation ≧49 as material equivalent to non-aging steel plate for deep drawing obtained by conventional box annealing.
%, AI≦3Kg/mm ² as a standard, and the areas that satisfy all of these are shown in Figures 2, 3, and 4. This area is N: 0.0010-0.0050%, B:
This is a region that satisfies 0.0005 to 0.0050% and B/N: 0.2 to 1.0. These component conditions are essential requirements for the slab material in the present invention. Therefore, the reason why the lower limit of N is set to 0.0010% is because it is the lower limit that can be implemented using current steel manufacturing technology.
The upper limit of N was set at 0.0050% because adding more than this significantly suppresses the growth of crystal grains during annealing and reduces drawability and ductility. In order to achieve this, it is preferable that N: 0.0040% or less. The amount of B should be kept at a minimum in order to achieve the effect of B addition.
Requires addition of 0.0005%. The effect of B addition is
The reason for this is not clear, but it is thought to be due to the fact that the suppressing power of grain growth during annealing of BN in which nitrides are formed with N is extremely small. Furthermore, there is a range of BN content that is effective in promoting grain growth during annealing, and B exceeds 0.0050% or B/N
If the value exceeds 1.0, BN or B dissolved in solid solution increases and becomes harmful to grain growth during annealing, making it impossible to obtain a good material. Moreover, when B/N is lower than 0.2, the amount of effective precipitates is extremely small and there is no effect. N remaining without being combined with B during slab heating is not harmful to aging resistance since it combines with Al and precipitates as AlN during slab heating and hot rolling. A minimum of 0.008% of Al is required to fix N that was not fixed by B. However, if excessive Al is added, solid solution strengthening of Al will occur, so the upper limit is set at 0.080%. Especially Al: 0.015~
0.060% is most preferred. Excessive addition of Mn significantly deteriorates the material quality of the annealed plate, so the upper limit is set at 0.40%. The contents of Si, P, and S are not particularly stipulated, but are within the range that unavoidably remain in the normal steelmaking process, Si: 0.10% or less, P: 0.030
% or less and S: 0.020% or less does not have any adverse effect on the present invention. Regarding the effective amounts of N and B in the ultra-low C steel slab of the present invention, both the slab heating temperature and the hot rolling finishing temperature are within a specific temperature range, as is clear from the large effect of precipitates in the hot rolling stage. It is only when it becomes effective. The heating temperature of the slab is preferably 1180°C or lower, which is lower than the usual slab heating temperature of 1250 to 1300°C. This temperature range is
It is considered that BN does not dissolve and the entire amount of B is fixed as BN, which is favorable for grain growth during annealing. This is also the temperature range necessary to fix the remaining N with Al. The lower limit is set at 950°C from the viewpoint of rolling properties. The most preferable range in terms of material is 1000 to 1160°C. Next, the hot rolling finishing temperature, which is the most important in the present invention, will be explained. Conventional steel sheet manufacturing technologies for drawing include box annealing,
In both continuous annealing methods, it is desirable to finish the hot rolling at Ar ₃ points or higher, that is, in the entire austenite region. If hot rolling is completed in the two-phase region of α (ferrite) + γ (austenite) or in the α region when the Ar is ₃ points or less during hot rolling, the (110) (100) orientation, which is disadvantageous for drawability, develops parallel to the sheet surface. ,
It is said that this inhibits the development of (111), which is advantageous for drawability, in parallel to the sheet surface during annealing. In contrast, the present inventors have discovered that there is a hot rolling finishing temperature that is effective for grain growth during annealing, particularly for developing (111) planes parallel to the sheet surface, in steel sheet materials with the above-mentioned specific composition. It is. This will be explained below through experiments. Experiment C: 0.0023%, Mn: 0.25%, P: 0.019%,
S: 0.018%, Al: 0.023%, N: 0.0038%, B:
Using a small steel ingot with a composition of 0.0035% (B/N = 0.92), the finishing temperature was set after heating the slab at a temperature of 1120℃.
A 2.8mm hot-rolled plate is heated to 650-930℃, and immediately heated to 530℃.
The hot-rolled sheet was charged into a furnace at a temperature of 0.degree. In addition, some samples were subjected to a treatment equivalent to high-temperature coiling of hot-rolled sheets, in which the samples were placed in a furnace at 680°C after finishing hot rolling, held for 2 hours, and then cooled in the furnace. The steps after pickling were the same as in the experiment. Figure 5 shows the results. The circle mark in the figure is the result of the material treated at 530°C and equivalent to low CT winding. According to Figure 5, when hot rolling is finished at a finishing temperature of 680°C to 830°C, both the total elongation and the value are good, and especially when the hot rolling finishing temperature is 700°C to 800°C, the material quality is extremely good. I know it will happen. The reason for this is not clear, but due to low-temperature finishing at Ar ₃ points or less during hot rolling finishing, part of the processing strain introduced during finishing is retained even after reaching room temperature, and this and BN precipitates are It is thought that this interaction causes the development of (111) planes parallel to the sheet surface during annealing, resulting in better drawability and better overall elongation due to grain growth caused by low-temperature finishing. By setting the upper limit to 800°C, the temperature difference between the center and the ends in the strip width direction is reduced during cooling, making the material more uniform, and low-temperature heating saves energy and reduces oxidation scale on the surface of the steel sheet. This reduces the amount of adhesion, which helps reduce the cost of pickling in the post-process and improves the yield of the steel sheet itself. In this way, finishing is done at low temperature,
The material when wound at a high temperature of 680°C is indicated by the x mark in Figure 5, but the material when wound at a high temperature is the same as that when wound at a low temperature. There is no improvement in the quality of the material due to removal. Moreover, high-temperature web material is accompanied by an increase in descaling costs during pickling. In contrast, in the present invention, by performing hot-rolling at a low temperature of Ar ₃ or less, it is possible to obtain a material equivalent to that of a high-temperature rolled material even if it is rolled at a low temperature, so it is possible to reduce pickling costs. becomes. The upper limit of the coiling temperature during hot rolling in the present invention is 580°C in order to prevent cost increases during pickling. Experiment Using the experimental steel as the basic composition, as shown in Table 1, Ti, Nb, V,
Steel with a small amount of Zr added was cast in a laboratory using vacuum melting, and after heating at a slab heating temperature of 1100℃, the finishing temperature was
A hot-rolled sheet of 2.8 mm was prepared at 780°C, immediately charged into a furnace at 530°C, held for 2 hours, and then cooled in the furnace to perform a treatment equivalent to low-temperature coiling of the hot-rolled sheet. The steps after pickling were the same as in the experiment. The results are shown in Table 2. According to this, Ti = 0.002 (Ti/C = 0.21
By adding a small amount of carbide-forming elements such as (atomic ratio), the value and total elongation do not change, and only the aging index decreases. I know what I can get.
In Table 1, no decrease in AI is observed when Ti is contained at 0.001%, but this may be because Ti does not combine with C and does not effectively reduce AI due to its combination with unavoidable impurities. It is thought that this was due to a dry spell. Therefore, the appropriate lower limit for the total amount of one or more of Ti, Nb, V, and Zr is 0.002%. next
The reason why the upper limit of the total amount of one or more of Ti, Nb, V, and Zr is set to 0.02% is that even if more than this is added, the effect will be saturated and the cost will increase. Therefore, one or two of Ti, Nb, V, and Zr
The upper limit for the total of species and above is set at 0.02%.

【table】

【table】

[Table] Based on the above facts, the present invention is based on the above-mentioned method, in which a steel slab having the above composition is heated at a low temperature, hot-rolled at a low-temperature finish, and coiled at a low temperature to produce a hot-rolled sheet, which is then pickled and then cold-rolled into a cold-rolled steel sheet, which is then continuously annealed. It is. Continuous annealing conditions do not need to be particularly specified, and as long as the maximum temperature of the steel plate is equal to or higher than the recrystallization temperature, there are no particular restrictions on the heating rate, cooling rate, or presence or absence of overaging. Examples of the present invention will be described below along with comparative examples. Example Five types of steel with different compositions were tapped by performing RH degassing for 20 minutes after tapping from a converter, and were made into slabs with a thickness of 200 mm by continuous casting. After heating these slabs in a heating furnace to 1050-1150℃ and keeping it for 60 minutes,
Finish rolling with hot rolling at 720~800℃,
It was wound at 530°C to form a 3.2 mm hot rolled coil. Table 3 shows the steel composition, slab heating temperature, and hot rolling finishing temperature. For comparison, steel No. 4 was also subjected to slab heating and hot rolling finishing temperatures outside the conditions of the present invention. The coil was pickled, cold-rolled to 0.8 mm, and then continuously annealed. The conditions for continuous annealing are a heating rate of approximately 20°C/sec, soaking at 800°C for 20 seconds,
The cooling rate is approximately 50°C/sec. After that, it was subjected to 0.6% temper rolling. Table 4 shows the materials of the steel plates manufactured in this manner. As is clear from this table, the cold-rolled steel sheet manufactured by the present invention has a drawability (value) of
Both ductility (total elongation) and aging resistance (AI) are excellent. As described above in detail, the present invention uses a steel slab with a specific composition containing B and N, and in particular, heats the slab before hot rolling to 1180°C, and the finish rolling temperature ranges from 680 to 800°C.
This is a method for producing cold rolled steel sheets, which consists of hot rolling at a coiling temperature of 580°C or less, followed by cold rolling, followed by continuous annealing of rapid heating and rapid cooling. This makes it possible to produce cold-rolled steel sheets with excellent toughness, ductility, and aging resistance.

【table】

【table】

【table】 [Brief explanation of the drawing]

Figure 1 is a chart showing the relationship between the value, total elongation, and AI of a cold rolled steel sheet and the amount of material C, slab heating temperature, and hot rolling finishing temperature.
Figure 3 is a diagram showing the relationship between total elongation and B and N quantities, Figure 4 is a diagram showing the relationship between AI and B and N quantities, and Figure 5 is a diagram showing the relationship between AI and B and N quantities. is a chart showing the relationship between the value and total elongation of a cold rolled steel sheet, the hot rolling finishing temperature, and the hot rolling winding temperature.

Claims (1)

[Claims] 1% by weight, C≦0.0040%, Mn≦0.40%, sol.
Al: 0.008~0.090%, N: 0.0010~0.0050%,
B: Contains 0.0005 to 0.0050%, and B/N = 0.2 to
1.0, with the balance consisting of Fe and unavoidable impurities, is heated to a temperature of 950°C to 1180°C, then the finishing temperature is 680°C to 800°C, and the coiling temperature is 580°C.
A method for producing a cold-rolled steel sheet with good drawability, ductility, and aging resistance by continuous annealing, which comprises hot rolling as described below, followed by cold rolling, and then continuous annealing at a temperature higher than the recrystallization temperature. . 2 In weight%, C≦0.0040%, Mn≦0.40%, sol.
Al: 0.008~0.090%, N: 0.0010~0.0050%,
B: Contains 0.0005 to 0.0050%, and further contains 0.002 to 0.002 or more of Ti, Nb, V, and Zr.
A steel slab containing 0.02%, B/N = 0.2 to 1.0, and the balance consisting of Fe and unavoidable impurities,
After heating to a temperature of 950℃~1180℃, finish temperature
Drawability, ductility and A method for producing cold-rolled steel sheets with good aging resistance.