JPS58217638A - Preparation of cold rolled steel plate good in ageing resistance and ductility - Google Patents

Abstract

PURPOSE:To prepare a cold rolled steel plate good in ageing resistance and ductility, by applying heat treatment after annealing to steel strip having a specific composition containing C, Mn, N, Al or the like and obtained through hot rolling, pickling and cold rolling under a specific condition. CONSTITUTION:A hot rolled steel strip containing, on the basis of a wt%, 0.008-0.04% C, 0.10-0.30% Mn, 0.008% or less N and 0.010% or more Al and wound up at 650 deg.C or more is subjected to pickling and cold rolling according to a usual method. In this next step, the steel plate is rapidly heating to 750- 950 deg.C and, after held at said temp. for 10sec or more, the heated steel plate is continuously annealed to adjust the crystal particle size of the cold rolled steel plate to 7.5-8.8 in G.S. No. Subsequently, the heated steel plate is gradually cooled to 640-720 deg.C over 30sec or more and quenching (a cooling speed; VCR), overageing treatment for holding a temp. TCR fo 320-440 deg.C over 60-210sec and final cooling (a cooling speed; VL) are advanced under a condition shown by the oblique line regions of the figures A, B to obtain a cold rolled steel plate good in ageing resistance and ductility.

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. be.

Now, in general, 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, the continuous annealing method involves rapid cooling after recrystallization annealing, followed by overaging treatment, and the O dissolved in solid solution in the steel is precipitated in a harmless form, namely Fe50 (cementite). This aims to improve ductility by softening the steel and improve aging resistance by eliminating solid solution.

An example of the heat cycle of the conventionally used continuous annealing method is shown in Fig. 181 and Fig. 8. Figure 3 shows an example in which over-aging treatment is carried out for several minutes at 10-400°C after recrystallization annealing, and Figure 8 shows an example in which after recrystallization annealing, water-cooling (approximately 200°C) is performed to near room temperature.
0℃/seo) L, reheat to 850-450℃
In the heat cycle shown in Figure 8, where over-aging treatment is performed for several minutes at
Because it is slow like SEO, the degree of supersaturation of solid solution 0 is small.
Precipitation nuclei of Fe50 are difficult to form, and the rate at which Fe30 is precipitated during the subsequent overaging treatment is extremely slow. As a result, if sufficient overaging time (approximately 5 min or more) is allowed,
Since solid solution O is reduced and the precipitated Fe30 exists in a form that is harmless to ductility, it is possible to manufacture steel sheets with good aging resistance and ductility. When passing a steel plate through a continuous annealing furnace, it is necessary to either slow down the passing speed and lengthen the residence time in the overaging zone, or increase the length of the overaging furnace itself, which increases costs. On the other hand, if the overaging time is set to 6 minutes or less in the cycle shown in Figure 2, the solid solution 0 will not be sufficiently reduced even if the overaging treatment is performed, and the increase in ductility due to softening and the improvement in aging resistance will not occur. Unexpected Zero Next, as shown in Figure 8, when rapid cooling is performed by water cooling to near room temperature (cooling rate approximately 000°C/sec) before overaging treatment at 850 to 450°C, ultra-rapid cooling called water cooling is performed. As a result, a sufficient degree of supersaturation of solid solution 0 is obtained, and a large number of Fe30 precipitate nuclei are formed at the end of rapid cooling, and in the next overaging treatment step after reheating, solid solution 0 diffuses into these precipitated nuclei, and Fe30 precipitates quickly. The precipitation of Fe50 is completed. In this process, F
Since a large number of e50 precipitation nuclei are formed, it has the advantage that the solid solution C can diffuse over a short distance and only requires a short overaging treatment (for example, 1 to 8 minutes). However, since the precipitated Fe50 is finely dispersed in large numbers within the crystal grains, precipitation strengthening occurs and the ductility is significantly deteriorated.

Like this! The heat cycle of the conventional continuous annealing method shown in FIGS. 1 and 8 has the above-mentioned drawbacks.

In response to this, various methods have been devised to improve the hammer, such as changing the heat cycle and adjusting the ingredients and hot rolling conditions, but it has not yet been possible to stably obtain excellent performance.

The inventors have found that the continuous annealing treatment improves both aging resistance and ductility.
As a result of repeated experiments in order to produce good steel,
At the end of rapid cooling in the continuous annealing process, Fe in the grains
By precipitating 50 at appropriate distance intervals, Fe1
It has been found that the aging resistance can be improved without deterioration of ductility due to precipitation strengthening of No. 30. This invention is Fe50
In order to precipitate at appropriate intervals, it is more desirable to take into account the grain size, especially to limit the rapid cooling rate, and to combine this with the overaging temperature and final cooling conditions, an entirely new idea that has never existed before. It originates from.

In this invention, O + 0.008 to 0.04 weight (hereinafter simply referred to as excellent), Mn + 0.10 to 0.801, and N
j At least 0.01 below 0,00894
When a hot-rolled steel strip having a composition containing 0 n of A is coiled at a coiling temperature of 650°C or higher, it is pickled by a conventional method, cold rolled, and then continuously annealed.
Hold the temperature within the range of 0 to 900°C for 10 seconds or more, then heat to a temperature range that preferably adjusts the grain size number to 7.6 to 8.8, and then heat to a temperature range of 640 to 710°C before overaging. An over-aging treatment in which slow cooling is performed for 80 seconds or more until the cooling start temperature is reached, and subsequent rapid cooling and over-aging cooling steps are maintained within a temperature range of 820 to 440°C for 60 to 810 seconds. Figure 1 (a) shows the rapid cooling rate (VOR') and final cooling rate (VL) up to the temperature TOH according to the
) and (b), the combination of proceeding under the conditions shown in the shaded area provides a solution to the above-mentioned line layer and makes it possible to manufacture a cold-rolled steel sheet with good aging resistance and ductility.

The reason why the amount of components in the steel is limited in this invention will be explained.

If O 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 these solid solutions should be reduced as much as possible. Therefore, in addition to degassing after melting during steel manufacturing to reduce the 0 content to 0.00154 or less, Ti
* A method of adding equalizer forming elements such as Nb + Zr IV to reduce 0 in the solid solution state, and by performing rapid cooling and over-aging treatment during continuous annealing, it precipitates as Fe50 in a short time and reduces the 0 in solid solution. There are eight possible ways to do this. Of these, the former two have very little solid solution 0 to begin with, so rapid cooling and over-aging treatment are rarely necessary.
It is necessary to use vacuum degassing or expensive additive elements, causing an increase in 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. This invention naturally utilizes the latter, and therefore very strict restrictions are required on the 0 content.

The lower limit of 0 was set to 0.008.If the value is less than this value, the amount of supersaturated solid solution obtained by rapid cooling is
Since the zero content is small, it becomes very small, and as a result, in short-time overaging with a tolerance of 1, the precipitation of FeλO is small and the aging resistance ductility cannot be improved. The reason for setting the upper limit of 0 to 0.041 is that an excessive increase in 0 increases carbide-based inclusions and suppresses grain growth, both of which are disadvantageous for ductility. Due to the refinement of grains associated with an increase in O content exceeding 0.04%, a sufficient degree of supersaturation cannot be obtained by rapid cooling before overaging, and long-term aging of 6 minutes or more is necessary to reduce the solid solution O content, that is, to improve aging resistance. Requires aging treatment. As described above, in this invention, the range of O is
, ooas to 0.04%.

The lower limit of In is set at 0.1 in order to prevent cracking during hot rolling caused by S. On the other hand, addition of Mn exceeding 0.130 causes grain refinement during continuous annealing as well as increasing the amount of In. This is disadvantageous for ductility and aging resistance. Therefore, In is limited to 0.1O to O, aO*.

A is necessary to fix hard fiN, which is harmful to aging properties, as AjN by winding at a high temperature (650° C. or higher) during hot rolling, and a minimum of 0.01 is required to fix N.

Since N deteriorates the aging resistance of steel sheets manufactured by continuous annealing, suppresses grain growth, and impairs ductility, it is desirable to reduce the amount as much as possible, and the upper limit should be o, ooso.

Next, hot rolling conditions will be explained.

In this invention, the slab heating temperature and hot finishing temperature when hot-rolling a slab to form a hot-rolled coil are not particularly specified, but the normal slab heating conditions are 1200
Not only N12100℃ heating but also 1000~1ji-
By heating to 00°C, solid solution of N can be suppressed and further improvement in aging resistance can be expected. Further, the finish rolling temperature of hot rolling is not particularly specified as long as it is 3 points or more of unevenness (approximately 840° C. or more). However, regarding the coiling temperature, by fixing the solid solution Nt-A to make it harmless and agglomerating O into a huge amount as Fe30 during hot rolling, it is advantageous for grain growth and drawability during continuous annealing (111). 650℃ for the purpose of promoting the development of texture
It is necessary to do the above.

The table shows the ingredients of the material as well as the continuous annealing treatment conditions, which are the most important component of the present invention. .

By using the steel having the composition of this invention and heating it to 760 to 900°C by continuous annealing, the crystal grain size reaches a grain size number of about 8.8, but it is heated to a low temperature range of less than 750°C. In this case, since the grain growth after the completion of recrystallization is not optical, the subsequent rapid cooling does not allow optical supersaturation of solid solution 0, and the precipitation of Fe50 in the crystal grains due to overaging treatment is delayed. It takes a long time to finish overaging, and if the heating temperature exceeds 900°C and the particle size number is less than 7.5, roughness (orange beer) occurs during processing due to coarsening of crystal grains, resulting in poor quality of the product. undesirable as such. The reason why this recrystallization temperature range is maintained for more than 10 seconds is because it takes at least 10 seconds for grain growth to complete at that temperature.Next, rapid cooling is started from a recrystallization annealing temperature of 160 to 900°C. Slow cooling for 80 seconds or more to a temperature of 640 to 720°C is because if the cooling time is less than 80 seconds, the r (austenite) phase created by high-temperature annealing will turn into fine pearlite due to rapid cooling. In addition, the solid solution 0 concentration in the α phase decreases as the annealing temperature increases, and sufficient supersaturation cannot be obtained even with rapid cooling before overaging, resulting in a disadvantageous effect on ductility. This is because the solid solution 0 remains in the middle, resulting in 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 l-phase produced by high-temperature annealing harmless by cooling it to below the Ar1 g state point. and the rapid cooling temperature to 64
By heating the temperature to 0 to 720°C, α(
The purpose is to set 0 solid solution in the ferrite phase as the highest level and to maximize the effect 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 O is left at the start of overaging, and as a result, Fe30 nuclei are formed within the crystal grains during overaging, and Fe50 grows roughly.

Precipitation state of Fe30 and solid solution O have a large effect on material quality
The amount is determined by the degree of supersaturation of the solid solution 0 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 0 diffuses and precipitates toward the Fe30 nuclei precipitated within the grains during a predetermined short-time overaging treatment. be.

Furthermore, final cooling to room temperature after overaging is important for reducing solid solution O remaining during overaging to an extent that does not pose a problem in terms of aging resistance.

From the above point of view, steels with different compositions were melted in the laboratory, and the rapid cooling rate, overaging temperature, and final cooling rate were varied.
We investigated its effects.

(Experiment 1) The test material was steel laminate (Q + 0.01101, Mn +
0.189G, P: 0.018 Excellent, S + 0.0
Section 10, Az: o, oaa i N +0.0041
Excellent) and Steel B (,0: 0.0581!,,Mn;
o, g'y・excellent, P: 0.011 excellent, S + 0
．． 009 Excellent, Alj 0.085 Excellent, N I 0.00
891) were vacuum melted in the laboratory. After hot rolling, each test material was placed 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 the heat cycles shown in Figures 4 and 6.
Rapid cooling starts from 660℃ and VO up to the overaging temperature.
R: 10~b When cooling, water cooling to room temperature (cooling rate approximately 000℃)
/ seconds) and then reheated, 8
The overaging temperature (TOR) of 00°C to 500°C was maintained for 180 seconds, and then slowly cooled to room temperature at a rate of δ°C/second. Note that the grain size number of Steel A in this experiment was 8.8.

Similarly, for steel B, the overaging temperature was set to 850.
℃ and the cooling rate before over-aging treatment was 10-b.
The material was examined by temper rolling.

Regarding the material, artificiality was examined as a measure of aging resistance, and total elongation was examined as a measure of ductility. The results are shown in FIG. 6 (~lω). The results shown within 0 are the results for steel B, and the others are the results for steel. When steel A according to the present invention is used and the rapid cooling condition is 80 to b degrees within the graphic region shown in the figure, AI=4.
5ψ- or less, total elongation: 46 inches or more, aging resistance,
It becomes possible to manufacture steel sheets with good 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 because the crystal grain growth during continuous annealing is poor due to the high J (grain size number 9.2) Fe50 due to rapid cooling and overaging treatment.
It is presumed that the precipitation of 0 and the reduction of solid solution 0 did not progress sufficiently, resulting in poor AH1 total elongation.

! Rapid cooling speed is extremely fast with ilA (water cooling technology goo
o℃/sec), the degree of supersaturation of solid solution 0 is sufficiently obtained by rapid cooling, and Fe50 is finely precipitated, so AI is at a low level and good, but since Fe50 is too finely precipitated, Ej is on the contrary. to degrade. Also, the cooling rate is extremely slow (8
(less than 0°C/sec), the supersaturation degree of solid solution 0 at the end of rapid cooling is small, so precipitation of Fe50 occurs during overaging.
, even if it occurs, it precipitates coarsely, resulting in a large amount of solid solution 0 remaining, resulting in high Ai and poor Ej.

On the other hand, when the rapid cooling rate is 80~b, the rapid cooling rate and overaging temperature are as shown in Figure 6 a + b.
The material quality (AI, total elongation) is the best when it is within the figure range. In addition, the grain size number of the steel sheet is 7.9, and A
When both I and Ej were appropriate, the average distance of Ee3Q within the crystal grain was 1.1 to 1.6 μ.

When the rapid cooling rate is in the 80~b range, the degree of supersaturation of solid solution 0 at the end of rapid cooling is somewhat small, so the density of Fe50 precipitation nuclei is coarse, resulting in a decrease in solid solution 0 and the accompanying growth of ye3o. For this reason, it is more desirable that the overaging temperature be around 400°C, as shown in FIG. 6, a and b. On the other hand, the rapid cooling rate is 70~
In the case of WOO℃/sec, the degree of supersaturation of solid solution 0 at the end of rapid cooling is high, and the degree of precipitation nuclei of Fe30 is large, so a50
At low temperatures around 'c, a decrease in solid solution O and accompanying Fe5
0 growth occurs. On the other hand, when the rapid cooling rate (70-b) is set to a slightly higher aging temperature of around 400°C, the AI becomes slightly higher, although the reason is not clear.

Even in the rapid cooling rate range of this invention, the overaging temperature is 450
If the temperature is as high as 800°C or higher, or as low as 800°C, the quality of the material is poor. The reason for this is that in the former case, the overaging temperature is high, so the equilibrium solid solution 0 amount at that temperature is high and the solid solute 0 amount remains high even if slowly cooled to room temperature, and in the latter case, the overaging temperature It is presumed that the overaging was not completed in a short time because of the low temperature. If the difference between the temperature and the end temperature is within 50°C, the intended purpose of the present invention can be met by setting the average value of the overaging start temperature and the overaging end temperature as the representative overaging temperature.

The effect of overaging treatment time is small if it is less than 60 seconds, and if it exceeds 210 seconds, not only is the effect saturated, but it is also necessary to reduce the operating speed or lengthen the overaging treatment zone. This has the disadvantage of significantly increasing 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 shown in Fig. 4 until the start of rapid cooling, and then the rapid cooling from 660°C is repeated at 80°, 60°, and 100° according to the present invention. and 800°C/sec, and as a subsequent overaging treatment, the overaging temperature was carried out within the temperature range within the graphic area of Figure 7 (overaging time 150 seconds), and the final cooling to room temperature was performed for 800°C/sec.
Heat treatment was performed at various rates below 0°C/sec. The material properties after temper rolling are summarized and plotted in FIG.

Even if the rapid cooling rate and overaging temperature are set appropriately, unless the relationship between the overaging temperature and the final cooling rate falls within the diagram of FIG. 7, a steel plate with good aging resistance and ductility cannot be manufactured. If the final cooling rate is less than 2° C./sec, it is not preferable because it leads to a decrease in the sheet passing rate or an increase in construction costs.

In addition, the range where the material quality is good in this way is (a) in Figure 1.
(b) can only be achieved by limiting the composition of the steel, the annealing temperature during continuous annealing, and the grain size number.

As mentioned above, using the material composition, especially O%MnttB) 14-node munokilled steel, 750 to 90
Heating to a temperature of 0°C, the grain size number is 7.6~
8.8, the subsequent rapid cooling rate, overaging temperature, and final cooling rate are as shown in Figure 1 (11).
, (b), it becomes possible to manufacture a cold-rolled steel sheet with good aging resistance and ductility.

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 variable 80 to 80°C/sec), which blows gas onto the sheet surface during coil threading, and a low temperature method. A method of cooling the plate surface by bringing it into contact with a roll (cooling rate 80~b) A method of cooling the plate surface by spraying it (cooling rate 50~b) Example 1 4Al steels with different components shown in Table 1 were heated in a converter. Melted.

These steels were made into slabs with a thickness of 2 oosm by continuous casting. These steels were continuously cast after tapping without degassing because the zero value of the blowstop at the time of tapping from the converter was extremely low. However, if the zero value of the blowstop was high, degassing was performed to reduce the O content. It goes without saying that you can join the two.

After reheating these slabs, they were hot rolled to 2.811 s and wound up at 680 to 720°C. Next, a cold-rolled coil with a thickness of 0.8118 was obtained by cold rolling after pickling, and the cold-rolled steel sheet thus obtained was continuously annealed at a heating rate of about 15°C/
Heat to 710-920℃ in seconds, hold for 80 seconds, and then heat to 660℃.
It was slowly cooled to ℃ in 50 seconds.

It was cooled from 660° C. at various cooling rates to various overage temperatures, held at that temperature for 60 seconds to 810 seconds, and then cooled to room temperature at about 1° C./second. For comparison, a case of overaging by cooling with water from 650° C. to room temperature and then reheating was also investigated.

After that, a skin pass of 0.8% was applied and the material was examined.If the cooling rate is less than 80℃/sec, we will use the forced gas jet cooling method on the actual line.
/sec) was annealed on an experimental continuous annealing line. Table 8
The results are shown in

Table 3 Perfume-tuning steels whose components and annealing temperatures are outside the scope of the present invention have grain size numbers outside the scope of the present invention, as shown in Table 8.

However, the steel of Perfume 8 has a grain size number of 7.6, which falls within the range of the present invention, but 0 is too low, 0.0071, so it is outside the range. 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, in this invention, the grain size number is set to 7.5 in terms of limiting the ingredients of the material and the annealing temperature during continuous annealing.
After limiting the temperature to a range of ~8.8, a steel with good aging resistance and ductility can be produced by appropriately combining a rapid cooling rate of 1 degree during continuous annealing, an overaging temperature, and a final cooling rate. It is possible to bring about completely new effects that have never existed before.

[Brief explanation of the drawing]

Figures 1 (a) and (b) are charts showing the limited ranges of overaging temperature, rapid cooling rate, and final cooling rate according to the present invention, and Figures LiA and 8 are diagrams showing the heat cycle in conventional continuous annealing. Striped diagrams showing examples; Figures 4 and 5 are diagrams of thermal cycles equivalent to continuous annealing used in Experiment 1; Figures 6 (a) and (b) are rapid cooling effects on AI and total elongation. 2 is a diagram showing the effect of rate and overaging temperature;
Figure 7 is a chart showing the results of Experiment 2 regarding the effects of AI, overaging temperature on total elongation, and final cooling rate. At the shrine) Figure 1 (b)

Claims (1)

[Claims] L 010.008 to 0.04 weight, Mn + 0.
10 to 0.80% by weight of N; 0.008% by weight or less together with Aj of at least 0.010% by weight, and heated to 660°C or more after hot rolling. After that, the material is wound up at a temperature of and, after holding, a pretreatment step of slow cooling for 80 seconds or more to a temperature in the range of 640 to 7110°C, followed by rapid cooling, and holding for 60 to 810 seconds at a temperature in the range of 880 to 440°C. The rapid cooling rate (TOR) of the subsequent quenching process in the pretreatment stage ( VOR) and the cooling rate (VL) in the final cooling stage. A method for producing a cold rolled steel sheet. 2. The annealing process changes the grain size of the cold rolled steel sheet to 7.
5 to 8.8, the method according to I.