JPH0244890B2 - TAIJIKOSEITOENSEINORYOKONA * REIENKOHANSEIZOHOHO - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing cold-rolled steel sheets with good aging resistance and ductility, and particularly aims to make it possible to advantageously achieve such performance by continuous annealing treatment. Generally speaking, in the annealing process of cold rolled steel sheets after cold rolling, continuous annealing treatment is replacing the conventional box annealing. The continuous annealing method is a revolutionary method that allows the production of soft cold-rolled steel sheets in a few minutes, which previously took several days using the conventional box annealing method. In order to complete annealing in a short time, continuous annealing involves rapid cooling and subsequent overaging treatment after recrystallization annealing, and the solid solution C in the steel is precipitated in a harmless form, namely Fe ₃ C (cementite). By doing so, the aim is to improve ductility due to softening and improve aging resistance due to solid solution C. An example of the heat cycle of the conventionally used continuous annealing method is shown in FIGS. 2 and 3. Figure 2 shows 350-400℃ after cooling at 10-30℃/sec after recrystallization annealing.
This is an example of performing overage processing for several minutes in
The figure shows water cooling to near room temperature (approximately 2000°C) after recrystallization annealing.
℃/sec), then reheated and over-aged for several minutes at 350 to 450℃. In the heat cycle shown in Figure 2, cooling before overaging is slow (10 to 30°C/Sec), so the degree of supersaturation of solid solution C is small, making it difficult for Fe ₃ C precipitation nuclei to form during the subsequent overaging. The rate of precipitation as Fe ₃ C is extremely slow. As a result, if sufficient overaging time (approximately 5 min or more) is allowed, the solid solution C will decrease, and the precipitated Fe ₃ C will exist in a form that is harmless to ductility, so it will have good aging resistance and ductility. However, in order to achieve this, when passing the steel plate through a continuous annealing furnace, it is necessary to slow down the passing speed and increase the residence time in the overaging treatment zone, or to increase the residence time in the overaging treatment zone. It is necessary to make the device itself longer, which increases the cost. On the other hand, if the overaging time is set to 5 minutes or less in the cycle shown in Figure 2, the solute C will not be sufficiently reduced even after the overaging treatment, and the increase in ductility and improvement in aging resistance due to softening will not occur. I can't wait. Next, before performing overaging treatment at 350 to 450°C as shown in Figure 3, water cooling to near room temperature (cooling rate approximately
When performing rapid cooling at a temperature of 2000°C/sec), sufficient supersaturation of solid solute C is obtained by ultra-rapid cooling (water cooling), and a large number of Fe 3 C precipitation nuclei are formed at the end of rapid cooling, which leads to the formation of a large number of Fe ₃ C precipitate nuclei during the next reheating. In the aging treatment process, solid solution C diffuses into these precipitation nuclei, and the precipitation of Fe ₃ C is completed in a short time. This treatment has the advantage that a large number of Fe ₃ C precipitation nuclei are formed, so that the solid solution C can diffuse over a short distance, and only a short time (for example, about 1 to 3 minutes) is required for the overaging treatment. However, it precipitated
Since Fe ₃ C is finely dispersed in large numbers within the crystal grains, precipitation strengthening occurs and significantly deteriorates ductility. As described above, the heat cycle of the conventional continuous annealing method shown in FIGS. 2 and 3 has the above-mentioned drawbacks. In response to this, various improvement methods have been devised, such as changing the heat cycle and adjusting the ingredients and hot rolling conditions, but it has not yet been possible to stably obtain sufficient performance. The inventors conducted various experiments to produce steel with good aging resistance and ductility through the continuous annealing process, and found that Fe ₃ C in the grains was removed at the end of rapid cooling during the continuous annealing process. We have found that by precipitating at appropriate distances, aging resistance can be improved without deterioration of ductility due to Fe ₃ C precipitation strengthening. In order to precipitate Fe ₃ C at appropriate intervals, the present invention preferably takes into consideration the crystal grain size, particularly restricts the rapid cooling rate, and
This is derived from a completely new idea that combines the overaging temperature and final cooling conditions. This invention contains C: 0.008 to 0.04% by weight (hereinafter simply expressed as %), Mn: 0.10 to 1.30%, and N: 0.008%.
A hot rolled steel strip having a composition containing at least 0.010% Al at a temperature of 650°C or above,
After winding at the winding temperature, it is pickled by a conventional method, cold rolled, and then continuously annealed by holding it at a temperature of 750 to 900°C for more than 10 seconds, preferably with a grain size number of 7.5. Heating to a temperature range of ~8.8℃ and slow cooling for 30 seconds or more until reaching the pre-overaging cooling start temperature in the range of 640~720℃, followed by rapid cooling and overaging cooling steps of 320~440℃. Rapid cooling rate to reach the temperature _TOR depending on the temperature of overaging treatment subjected to holding in the range of 60 to 210 seconds in the temperature range of °C
The combination of V _CR and final cooling rate V _L under the conditions shown in the shaded areas in Figure 1 a and b provides a means to solve the above problems, resulting in cold rolling with good aging resistance and ductility. This made it possible to manufacture steel plates. The reason why the amount of components in the steel is limited in this invention will be explained. If C exists in a solid solution state in steel, it not only causes deterioration of aging properties but also functions similarly to N in that it also worsens ductility, so the amount of C in solid solution should be reduced as much as possible. For this reason, in addition to degassing after steelmaking to reduce the C content to 0.005% or less, there are also methods to reduce C in solid solution by adding carbide-forming elements such as Ti, Nb, Zr, and V. ,
Furthermore, by performing rapid cooling and overaging treatment during continuous annealing, Fe ₃ C is precipitated in a short time,
There are three possible methods for reducing solid solution C. Of these, the first two have very little solid solution C, so
It hardly requires rapid cooling or overaging, but requires vacuum degassing or the use of expensive additive elements, which increases manufacturing costs. On the other hand, in the latter case of rapid cooling and over-aging treatment, the melting cost is the lowest, so if it is possible to shorten the over-aging time, it is possible to manufacture steel sheets with good aging resistance and ductility at low cost. becomes. This invention, of course, utilizes the latter, and therefore requires very strict limits on the C content. The lower limit of C was set at 0.008%, because if it is less than this value, the amount of supersaturated solid solution C obtained by rapid cooling will be very small because the C content itself is small, and as a result, the amount of C in solid solution obtained by rapid cooling will be very small. In the case of time aging, precipitation of Fe ₃ C is small and aging resistance and ductility cannot be improved. The reason for setting the upper limit of C to 0.04% is that an excessive increase in C increases carbide-based inclusions and suppresses grain growth, both of which are disadvantageous for ductility. Due to the refinement of crystal grains associated with an increase in C exceeding %, a sufficient degree of supersaturation cannot be obtained by rapid cooling before overaging, and long-term overaging of 5 minutes or more is required to reduce solid solution C, that is, to improve aging resistance. processing becomes necessary. As described above, in the present invention, it is necessary to keep the C content within the range of 0.008% to 0.04%. The lower limit of Mn is set at 0.1% to prevent cracking during hot rolling caused by S, while Mn exceeding 0.30%
Similar to increasing the amount of C, the addition of C causes grain refinement during continuous annealing, which is disadvantageous for ductility and aging resistance. Therefore, Mn is limited to 0.10 to 0.30%. Al is necessary to fix solid solution N, which is harmful to aging properties, as AlN by winding at a high temperature (650℃ or higher) during hot rolling, and is at least 0.01% to fix N.
is necessary. N degrades the aging resistance of steel sheets manufactured by continuous annealing, suppresses grain growth, and worsens ductility, so it is desirable to have as little as possible, and the upper limit is
It requires 0.0080%. Next, hot rolling conditions will be explained. In this invention, it is not necessary to specify the slab heating temperature or hot finishing temperature when hot rolling a slab into a hot rolled coil, but it is not limited to heating at 1200 to 1300°C, which is the usual slab heating condition. , 1000
By heating to ~1200°C, solid solution of N can be suppressed and further improvement in aging resistance can be expected. In addition, the hot rolling finish rolling temperature is not particularly specified as long as it is Ar ₃ points or higher (approximately 840°C or higher). However, regarding the winding temperature, by fixing solid solution N with Al and making it harmless, and coagulating C into a huge amount as Fe ₃ C during hot rolling,
Advantageous for grain growth and drawability during continuous annealing {111}
It is necessary to maintain the temperature at 650°C or higher in order to promote the development of texture. Next, the continuous annealing treatment conditions, which are the most important constituent factors of the present invention, will be explained together with the ingredients of the material. First, in the continuous annealing process, the material is rapidly heated to 750 to 900°C and then held at that temperature for 10 seconds or more. By using steel with the composition of this invention and heating it to 750 to 900°C by continuous annealing, the grain size can reach a grain size number of about 8.8, but when heating to a low temperature range of less than 750°C, , due to insufficient grain growth after completion of recrystallization, sufficient supersaturation of solid solution C could not be achieved by subsequent rapid cooling, and the precipitation of Fe ₃ C within the crystal grains due to overaging treatment was delayed, resulting in the end of overaging. It takes a long time to heat up, and the heating temperature exceeds 900℃.
When the particle size number is less than 7.5, roughness (orange peel) occurs during processing due to coarsening of crystal grains, which is not desirable as a product. The reason why this recrystallization temperature range is maintained for 10 seconds or more is because it takes at least 10 seconds for grain growth to be completed at that temperature. Next, from the recrystallization annealing temperature of 750 to 900 °C to the rapid cooling start temperature of 640 to 720 °C, slow cooling for 30 seconds or more is different from cooling for less than 30 seconds.
γ (austenite) created by annealing at high temperatures
The phase changes to fine pearlite by rapid cooling, which is disadvantageous for ductility, and the solid solution C concentration in the α phase decreases as the annealing temperature increases, even by rapid cooling before overaging treatment. This is because a sufficient degree of supersaturation cannot be obtained, and even after overaging treatment, solid solution C remains halfway, leading to deterioration of aging resistance. The reason why the slow cooling end temperature before rapid cooling, that is, the rapid cooling start temperature, is limited to 640 to 720°C is to render the γ phase formed by high-temperature annealing harmless by cooling it to below the Ar ₁ transformation point as described above. By setting the rapid cooling temperature to 640 to 720°C, the solid solute C in the α (ferrite) phase at the start of rapid cooling is set to the highest level (estimated solid solute C amount: 0.008 to 0.02
%) The purpose is to maximize the effectiveness of rapid cooling and subsequent overaging treatment. Next, the scope of rapid cooling and overaging treatment, which are the core of this invention, will be explained in detail. By performing rapid cooling, supersaturated solid solution C is left at the start of overaging, which causes
Nuclei of Fe ₃ C are formed within the crystal grains, and further Fe ₃ C grows. The precipitation state of Fe ₃ C and the amount of solute C, which greatly affect the material quality, are determined by the degree of supersaturation of solute C at the end of rapid cooling, which largely depends on the rapid cooling rate and crystal grain size. In addition, the overaging temperature is an important factor that determines how efficiently supersaturated solid solution C diffuses and precipitates toward the Fe ₃ C nuclei precipitated within the grains during a predetermined short-time overaging treatment. It is a factor. In addition, the final cooling to room temperature after overaging is
This is important in order to reduce the solid solution C remaining during over-aging to a level that does not pose a problem in terms of aging resistance. From the above viewpoint, steels with different compositions were melted in the laboratory, and the effects of various rapid cooling rates, overaging temperatures, and final cooling rates were investigated. (Experiment 1) The co-test materials were Steel A (C: 0.020%, Mn: 0.18%,
P: 0.013%, S: 0.010%, Al: 0.033%, N:
0.0041%) and Steel B (C: 0.053%, Mn: 0.27%,
P: 0.011%, S: 0.009%, Al: 0.035%, N:
0.0039%) were vacuum melted in the laboratory. After hot rolling, each test material was charged and held in a furnace at 700°C for 2 hours, cooled in the furnace, and subjected to a process equivalent to coil winding during hot rolling. After pickling and cold rolling, steel A was subjected to heat treatment equivalent to continuous annealing in the laboratory using heat cycles shown in FIGS. 4 and 5. Rapid cooling starts at 660°C and cools down to the overaging temperature at a rate of V _CR : 10 to 200°C/sec, or water cooling to room temperature (cooling rate approximately 2000°C/sec).
2 seconds), and in each case of reheating, the overaging temperature _TOR of 300° C. to 500° C. was held for 180 seconds, and then slowly cooled to room temperature at a rate of 3° C./second. In addition,
The grain size number of Steel A in this experiment was 8.3. Similarly, for Steel B, the overaging temperature was set at 350°C, and the cooling rate before this overaging treatment was set at 10~10°C.
A similar experiment was conducted with the temperature changed to 200°C/sec and water cooling. The test material was subjected to heat treatment equivalent to the continuous annealing described above.
The material was examined by subjecting it to 0.8% temper rolling. Regarding the material, A _I was examined as a measure of aging resistance, and total elongation was examined as a measure of ductility. The results are shown in Figures 6a and 6b. The results shown in parentheses are the results for Steel B, and the others are the results for Steel A. Using steel A according to this invention, and as rapid cooling conditions 30 ~
When the temperature is 200℃/sec and the rapid cooling rate and overaging temperature conditions are within the graphical region shown in the figure, A _I :
4.5Kg/mm2 or ^less and total elongation: 46% or more can be obtained, making it possible to manufacture steel sheets with good aging resistance and ductility. On the other hand, in Steel B of Comparative Example, good material quality was not obtained even if the rapid cooling rate was varied.
This is due to the high C content, which results in poor grain growth during continuous annealing (grain size number 9.2), and due to rapid cooling and over-aging treatment.
Precipitation of Fe ₃ C and reduction of solid solution C do not proceed sufficiently, and A _I ,
It is presumed that the total elongation was inferior. Steel A has an extremely fast cooling rate (water cooling: approx.
2000℃/sec), sufficient supersaturation of solid solution C is obtained by rapid cooling, and Fe ₃ C is finely precipitated, so A _I
is at a low level and is good, but Fe ₃ C precipitates too finely, so El deteriorates on the contrary. Furthermore, if the cooling rate is extremely slow (less than 30°C/sec), the degree of supersaturation of solid solution C at the end of rapid cooling is small, so precipitation of Fe ₃ C is unlikely to occur during overaging, and even if it occurs, Since it precipitates coarsely, a large amount of solid solute C remains, resulting in high A _I and poor El. On the other hand, the material quality (A _I , total elongation) is the best when the rapid cooling rate is 30 to 200° C./sec and the rapid cooling rate and overaging temperature are within the graphical ranges shown in FIGS. 6a and 6b. The grain size number of steel A is 7.9, and Ee ₃ C within the grain when both A _I and El are appropriate.
The average distance was 1.1-1.5μ. Note that when the rapid cooling rate is in the slow range of 30 to 70°C/sec, the degree of supersaturation of solid solute C at the end of rapid cooling is somewhat small, so the density of Fe ₃ C precipitation nuclei is coarse, resulting in a decrease in solid solute C. , for the accompanying growth of Fe ₃ C,
As shown in Figure 6 a and b, the overaging temperature is
A temperature around 400℃ is more desirable. On the other hand, when the rapid cooling rate is 70 to 200°C/sec, the degree of supersaturation of solid solution C at the end of rapid cooling is high and the density of Fe ₃ C precipitation nuclei is large, so at a low temperature of around 350°C, A decrease in solid solution C and an accompanying growth of Fe ₃ C occur. On the other hand, at the rapid cooling rate (70 to 200℃/sec), the overaging temperature
When the temperature is slightly higher, around 400°C, A _I becomes slightly higher, although the reason is not clear. Even in the rapid cooling rate range of this invention, if the overaging temperature is as high as 450°C or higher or as low as 300°C,
The material is inferior. The reason for this is that in the former, the overaging temperature is high, so the equilibrium solid solute C content is high at that temperature, and the solid solute C content remains high even if slowly cooled to room temperature, and in the latter, the overaging temperature It is presumed that the overaging was not completed in a short period of time because of the low In addition, in conjunction with this experiment, we also conducted an experiment in which the overaging end temperature was lower than the overaging starting temperature.If the difference between the overaging starting temperature and the overaging ending temperature is within 50℃, The intended purpose of the present invention can be achieved by setting the average value of the same end temperatures as the representative overaging temperature. The effect of overaging treatment time is small when it is less than 60 seconds, and when it exceeds 210 seconds, not only is the effect saturated, but the operation speed may be reduced or
It is necessary to lengthen the overaging treatment zone, which is disadvantageous in that it leads to a significant increase in costs. Next, the following experiment was conducted to investigate the relationship between overaging conditions and final cooling rate. (Experiment 2) Steel A from Experiment 1 was hot rolled and cold rolled under the same conditions as Experiment 1, and then heat treated in a cycle equivalent to continuous annealing. The continuous annealing cycle is the same as the cycle in Figure 4 until the start of rapid cooling, and then
According to this invention, rapid cooling from 660℃ to 30, 60,
The overaging temperature was divided into 100 and 200°C/sec, and the subsequent overaging treatment was carried out within the temperature range within the graphic area shown in Figure 7 (overaging time: 150 seconds), and the final cooling to room temperature was performed at 30°C. The heat treatment was performed at various speeds of less than /second. The material properties after temper rolling are summarized and plotted in Figure 7. Even if the rapid cooling rate and overaging temperature are set appropriately, unless the relationship between the overaging temperature and the rapid cooling rate falls within the figure shown in FIG. 7, a steel plate with good aging resistance and ductility cannot be manufactured. And the final cooling rate is 2℃/
If it is less than seconds, it is not preferable because it leads to a decrease in sheet threading speed or an increase in construction cost. Note that the ranges a and b in FIG. 1 in which the material quality is good as described above can only be achieved by limiting the grain size number regarding the steel composition and the annealing temperature during continuous annealing. As mentioned above, using Al-killed steel with adjusted material components, especially C and Mn, it is possible to
Heating to a temperature of 900℃, and thus the grain size number
If the subsequent rapid cooling rate, overaging temperature, and final cooling rate are selected within the shaded areas in Figure 1 a and b by adjusting the temperature to 7.5 to 8.8, a cold rolled steel sheet with good aging resistance and ductility can be obtained. It becomes possible to manufacture In order to achieve rapid cooling according to the present invention in a practical continuous annealing line, it is necessary to use a forced gas cooling method (cooling rate of 30 to 80°C/sec), which blows gas onto the sheet surface during coil threading, and a low-temperature roll. A method of cooling the plate surface by contacting it (cooling rate 30 to 200
℃/sec) and a method of cooling by spraying a mixture of gas and atomized liquid onto the plate surface (cooling rate 50-200℃/sec)
etc. may be used. Example 1 Four types of steel having different components shown in Table 1 were melted in a converter.

【table】

[Table] These steels were made into slabs with a thickness of 200 mm by continuous casting. These steels had a sufficiently low stop C value when tapped from the converter, so they were cast continuously after tapping without degassing. However, if the stop C value was high, degassing was performed to reduce the C content. Needless to say, it can be adjusted. After reheating these slabs, we hot rolled them to 2.8mm.
It was rolled and coiled at 680-720℃. Next, a cold rolled coil having a thickness of 0.8 mm was obtained by cold rolling after pickling, and the thus obtained cold rolled steel sheet was continuously annealed. Heat to 710-920℃ at a heating rate of about 15℃/second,
After holding for 30 seconds, it was gradually cooled to 650°C for 50 seconds. It was cooled from 650° C. at various cooling rates to various overaging temperatures, held at that temperature for 50 seconds to 210 seconds, and then cooled to room temperature at about 6° C./second.
For comparison, we also investigated the case of overaging by water cooling from 650°C to room temperature and reheating. after that
The material was examined by applying a 0.8% skin pass. If the cooling rate is less than 80°C/sec, use the forced gas jet cooling method in the actual line, or if the cooling rate is 80°C/sec or more and water cooling
°C/sec) was annealed on an experimental continuous annealing line. Table 2 shows the results.

[Table] As shown in Table 2, steels with compositions and annealing temperatures outside the limited range of the present invention have grain size numbers outside the range of the present invention. However, steel No. 3 has a grain size number of 7.6, which falls within the range of the present invention, but is outside the range because the C content is too low, at 0.007%. According to this, it is clear that if a steel plate having the composition of the present invention is treated under the continuous annealing conditions of the present invention, a cold rolled steel plate with excellent aging resistance and ductility can be produced. As described above, this invention limits the ingredients of the material and the annealing temperature during continuous annealing by limiting the grain size number to a range of 7.5 to 8.8, and furthermore, the rapid cooling rate during continuous annealing, overaging temperature,
By appropriately combining it with the final cooling rate, it is possible to produce a steel with good aging resistance and ductility, which is a completely new effect that has never existed before.

[Brief explanation of drawings]

1a and 1b are charts showing the limited ranges of overaging temperature, rapid cooling rate, and final cooling rate according to the present invention; FIGS. 2 and 3 are diagrams showing examples of heat cycles in conventional continuous annealing; Figures 4 and 5
Figure 6 is a diagram of the thermal cycle equivalent to continuous annealing used in Experiment 1, Figures 6a and b are diagrams showing the effects of rapid cooling rate and overaging temperature on A _I and total elongation, and Figure 7 The figure is a chart showing the results of Experiment 2 regarding the effects of A _I , overaging temperature on total elongation, and final cooling rate.

Claims (1)

[Claims] 1. A hot rolled steel strip having a composition containing 0.008 to 0.04% by weight of C, 0.10 to 0.30% by weight of Mn, and 0.008% by weight or less of N together with at least 0.010% by weight of Al, After the hot rolling, it is rolled up at a temperature of 650°C or higher, then pickled and cold rolled according to a conventional method, and then rapidly heated to a temperature within the range of 750 to 900°C for continuous annealing. After the annealing process, which is held for more than seconds, after that holding, slow cooling is performed for more than 30 seconds, followed by rapid cooling, to a temperature in the range of 320 to 440°C. The overaging treatment stage and the final cooling stage, which are held for 60 to 210 seconds at Regarding the rapid cooling rate VCR and the final cooling rate VL in the final cooling stage, the rapid cooling rate VCR is in the range of 30 to 200℃/sec, and when it is 30 to 100℃/sec, 2980≦7TOR＋6VCR≦3260, When the temperature exceeds 100℃/sec and reaches 200℃/sec, the following relationship is satisfied: 1800≦5TOR＋VCR≦2000, and the final cooling rate V _L is in the range of 2 to 20℃/sec, and 2 to 8
3TOR + 35V _L ≦1390 for temperatures up to ℃/sec, and 3TOR + 5V _L ≦1150 for temperatures exceeding 8℃/sec and up to 20℃/sec, resulting in good aging resistance and ductility. , Cold rolled steel plate manufacturing method. 2. The method for producing a cold rolled steel sheet having good aging resistance and ductility, as set forth in claim 1, wherein the annealing process is a step of adjusting the grain size of the cold rolled steel sheet to a grain size number of 7.5 to 8.8.