JPS6132375B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method of manufacturing a steel plate for ultra-deep drawing. Two types of steel plates for ultra-deep drawing are known: Ti killed steel plates (Japanese Patent Publication No. 18066/1982) and Nb killed steel plates (Japanese Patent Publication No. 1245/1983). For example, the technology of Nb killed steel sheets has significantly improved compared to when the patent application was filed, and the technology disclosed in the patent publication is not economical and therefore impractical. C<
Since it has become possible to easily reduce C to a range of 500 ppm, there have recently been reports in academic literature that it is possible to produce with lower amounts of carbon and Nb than the ranges in the above-mentioned patent publications. These steel sheets are designed to have extremely low carbon, and are made by adding Ti or Nb, which has a strong carbonitride-forming ability, to create steel sheets with almost no interstitial elements C and N. It also has the advantage of being able to produce products on the same level as box annealing. However, when these steel plates are actually produced by continuous annealing, they have the following drawbacks. Ti-killed steel sheets are susceptible to secondary processing cracks. In particular, if a material with good quality is targeted and the amount of Tj added is in a range equivalent to or higher than the amount of C and N, secondary processing cracks are likely to occur as the amount of P increases. Another drawback is that the r value deteriorates considerably when P is added. Furthermore, when manufacturing alloyed galvanized steel sheets on a Sendzimer-type continuous hot-dip galvanizing line, which is a type of continuous annealing, powdering (alloying progresses excessively and the plating layer is coated during press forming) is produced. It also has the disadvantage of being prone to the phenomenon of peeling off. However, for this steel type, the hot rolling winding temperature is
It has the advantage of being able to produce stable materials by continuous annealing even at the usual 600-650℃. On the other hand, Nb killed steel requires hot rolling and high temperature coiling (coiling temperature 700°C). At normal coiling temperatures, the complete recrystallization temperature becomes extremely high, and unrecrystallized areas may remain in the continuous annealing furnace's possible temperature range (approximately 850°C or less), and depending on the amount of Nb. There are large variations in material. It has been variously reported that when high-temperature coiling is performed, a steel plate with a high r value can be obtained at an annealing temperature of about 800 to 850°C, except for the ends of the hot-rolled coil. . However, high-temperature winding not only makes the scale thicker and drastically reduces pickling efficiency, but also makes it impossible to obtain sufficient material at the ends of the coil due to the fast cooling rate, resulting in a decrease in yield.
Some Nb-killed steels are particularly large. As a result of a detailed study of the advantages and disadvantages of Ti-killed steel and Nb-killed steel, the present inventors first considered the above-mentioned difference in behavior as follows. Ti is an extremely strong nitride-forming element, and although the carbide in which TiN has already been formed in the hot-rolling furnace has a lower precipitation temperature than the nitride,
It is thought that a considerable amount of nitrides will precipitate even during winding at 600 to 650°C using nitrides as precipitation sites. Therefore, even at a low coiling temperature, a considerable amount of precipitates are precipitated as large precipitates in the hot coil, and only a small amount of precipitates are precipitated during continuous annealing after cold rolling. Therefore, the recrystallization temperature does not need to become extremely high, and the material is considered to be fairly homogeneous. However, most of the C and N precipitate as carbonitrides, and the grain boundaries become clean, and impurity elements such as P, which cause grain boundary embrittlement, segregate there, causing secondary work cracking. Conceivable. Furthermore, when manufacturing alloyed galvanized steel sheets with Ti-killed steel, the Ti in the steel promotes the alloying reaction between the base iron and molten zinc, which tends to cause overalloying and powdering. it is conceivable that. On the other hand, Nb has a considerably inferior nitride formation ability compared to Ti. In fact, in Nb-added steel, Nb forms carbides, but nitrogen precipitates as AlN. AlN is difficult to form during low-temperature coiling, and is not formed in hot-rolled sheets unless the coiling temperature is 700°C or higher.AlN is finely precipitated during continuous annealing after cold rolling, and is used to increase yield strength and reduce elongation. This causes material deterioration such as deterioration. Therefore, even if the front and rear ends of the hot-rolled coil are coiled at a high temperature, the material remains close to that used for low-temperature coiling because the cooling rate is fast. In addition, due to slight variations in hot-rolling winding temperature and differences in the degree of AlN formation at the center and ends of the coil,
It is thought that deterioration of the material at the front and rear ends of the hot-rolled coil and variations in the material inside the coil occur. However, since Nb has inferior carbonitride formation ability compared to Ti, several ppm of carbon is segregated at grain boundaries, and this increases the binding energy of grain boundaries, even if the P content is quite high. It is considered that secondary processing cracking is not a cause for concern. In addition, during the production of alloyed galvanized steel sheets, Nb does not tend to accelerate the alloying reaction between base iron and molten zinc as much as Ti.
Powdering is less likely to occur in Nb-killed steel than in Ti-killed steel. Based on this idea, the following is the concept of manufacturing ultra-deep drawable steel sheets that have good homogeneity of the material inside the coil and no worries about secondary processing cracks, and that powdering is difficult to occur when manufacturing alloyed galvanized steel sheets. It became something like That is, N is not AlN but Ti
The basic concept of the present invention is a steel sheet in which TiN is precipitated as a composite carbide of [Ti, Nb]C before finish hot rolling. As will be described later, the steel of the present invention is superior to Ti-killed steel in that the r value does not deteriorate even when P is added to increase the strength, and secondary processing cracks are extremely unlikely to occur. Furthermore, when producing an alloyed galvanized steel sheet, it has the advantage that powdering is less likely to occur. Next, compared to Nb killed steel,
Because N is fixed as TiN instead of AlN, almost the same level of material can be obtained even with low-temperature winding without high-temperature winding, and the material quality in the coil length direction and coil width direction is extremely high. It is superior in that it is homogeneous. These advantages clearly indicate that the steel of the present invention has various superior material properties that are not possessed by the conventional Ti-killed steel and Nb-killed steel, and the steel of the present invention is extremely advantageous. be. Furthermore, the steel of the present invention with multiple additions of Ti and Nb has unique properties that cannot be predicted from the properties of conventional Ti-killed steel and Nb-killed steel, because the anisotropy of the r value is extremely small. It is. in general,
The r value of Ti-killed steel and Nb-killed grooves is poorest in the rolling direction (L direction) or 45° direction, and best in the direction perpendicular to rolling (C direction). However, the steel of the present invention has an r value that is almost equal in L, C, and 45° directions, or is somewhat larger in the 45° direction, regardless of the cold rolling rate. A high r-value steel sheet with such anisotropy has not been known at all so far, and it is academically interesting, but it also has great practical advantages. In particular, it is easy to imagine that extremely excellent formability is exhibited in the case of a rectangular cylindrical drawing (corners angled at 45°).
However, even in the case of cylindrical deep drawing, the fracture limit is often determined by the minimum r value. In addition, low anisotropy is a great advantage for applications where uniformity of plate thickness after drawing is important, such as outer cylinders for dry cell batteries, and for applications where it is desired to minimize the occurrence of selvage. Therefore, it is thought that the appearance of new steel sheets such as the steel of the present invention will increase public interest in the anisotropy of the r value, rather than simply looking at the average value in three directions. As described above, the steel of the present invention is a completely new steel plate that is superior in all respects to steels with only Ti or Nb added, and has completely unexpected properties, making it extremely advantageous. Next, we will discuss the component range. As mentioned above,
The amount of Ti added is determined by the relationship with N, and should be added at least in an amount sufficient to fix N. That is, 48/14 [N (%) - 0.002% (20 ppm)] <Ti
It is. If Ti is added in an amount exceeding the equivalent amount to N (i.e. 48/14N (%)), it will form sulfides or become carbides, deteriorating secondary workability, and worsening the quality of the material when P is added. At most, the amount of Ti added should be equal to or less than the equivalent amount of C and N. That is, Ti (%) < [48/
12C (%) + 48/14N (%)]. On the other hand, Nb is determined by the relationship with C, and is 0.3 times or more the amount of C in terms of atomic ratio, that is, Nb (%) > 0.3 × 93/12C (%) = 2.33 (%),
And it is desirable to add it in an amount of 0.003% or more and less than 0.025%. When Nb (%) / C (%) < 2.33, there is a problem that a composite carbide of [Ti, Nb] C is not formed and solid solution C remains, making it impossible to obtain a non-aging steel sheet.
At 0.025% Nb, the material property values show behavior similar to conventional Nb killed steel, the recrystallization temperature increases, and material deterioration at the front and rear ends of the coil increases, which deviates from the principle of the present invention. FIGS. 1 and 2 show the scope of the present invention from the relationship between the amounts of Ti and Nb. Figure 1 shows the amount of Nb fixed at a constant amount (0.022%).
These are material property values when changing the amount of Ti.
C; 0.005, Si; 0.01, Mn; 0.25, P; 0.02,
A sample of S; 0.01, Sol.Al; 0.06, N; 0.005 (each %) was hot-rolled at 720°C, and a;
Center part in the coil length direction, b; front and rear ends of the coil are shown. When the amount of Ti is insufficient to fix N, i.e. 48/14 [N (%) - 0.002%] = 0.010%>
In the case of Ti, the material deterioration at the front and rear ends of the coil is large, and the anisotropy of the r value shows the characteristics of the r value of ultra-low carbon steel with a trace amount of Nb added.
The effect of compound addition is small. On the other hand, when Ti is added in an amount equivalent to or more than C and N, the material deterioration at the front and rear ends of the coil is very small, but the anisotropy of the r value is
It is almost the same as that of Ti Killed.
That is, only when an appropriate amount of Ti is added, the effect of improving the texture due to the combined addition of Ti and Nb appears, and the material exhibits excellent r-value characteristics with extremely low anisotropy. Such excellent r-value isotropy is due to Ti, Nb
This is due to the texture that cannot be obtained with steel with only Ti and Nb added. Furthermore, as shown in Figure 1, the amount of Ti added is 0.010% because the lower limit is Ti > 48/14 [N (%) - 0.002], and the upper limit is Ti < [4.00C (%) + 3.43N]. (%)〕
0.037%. If Ti is less than 0.010%, the ratio of Ti fixing N as TiN at the front and rear ends of the coil will be small, and conversely, the ratio of N precipitated as AlN will increase, and the size of AlN precipitates will be extremely small, so the coil This is undesirable because it deteriorates the material of the front and rear ends. When the amount of Ti is 0.037 or more, the total amount of C and N in the steel is
Fixed by Ti. In this case, the material properties are the same as those of steel with only Ti added, and primary workability is deteriorated and powdering is more likely to occur, which is undesirable. Figure 2 shows the material properties when a constant amount (0.02%) sufficient to fix the amount of Ti to N is added and the amount of Nb added is varied. The chemical composition of the sample is at almost the same level as in Figure 1.
If the Nb content is too low relative to the C content, it will exhibit characteristics similar to those of simple ultra-low carbon steel as shown in Figure 1, and 45
The r value in the ° direction is extremely small, and the anisotropy is extremely large. Furthermore, the material deterioration at the front and rear ends of the coil is large, and non-aging properties cannot be obtained, as in the case of FIG. 1. When the amount of Nb exceeds 0.025%, the anisotropy of the r value follows the same trend, but there is a drawback that the value of r _c becomes low, and the material deterioration at the front and rear ends of the coil length is extremely large, which is unique to Nb killed steel. Similar to characteristics. As can be seen from FIG. 2, only by adding a suitable combination of Ti and Nb can the anisotropy of the r value be extremely small and the excellent properties of uniform material in the longitudinal direction of the coil can be obtained. Such properties cannot be obtained with steels to which only Ti and Nb are added, indicating that the combined addition of Ti and Nb is non-essential. The following are the reasons why, compared to the addition of Nb alone, a highly stable material is produced that does not depend on the coiling temperature, and compared to the addition of Ti alone, a high quality material can be obtained even with a small amount of alloying element added. by. When the addition amount of Ti and Nb is well-balanced as in the range of the present invention, a composite precipitate of [Ti, Nb]C is formed, but this has a higher precipitation initiation temperature than TiC and NbC, and a large Since it precipitates as a precipitate, it seems to exhibit good recrystallization behavior even at a low coiling temperature. It is thought that this results in an isotropic r value. On the other hand, in Nb-added steel or when 0.025% Nb is added, NbC is produced, and the precipitation state changes greatly depending on the coiling temperature, and in the case of low-temperature coiling, fine NbC is continuous. The material quality deteriorates significantly due to the increased recrystallization temperature during annealing. In addition, when Ti is added alone, Ti and C
If the atomic ratio of +N is not greater than 1, the material will deteriorate significantly. This is because unless the amount of Ti is increased, TiC will not precipitate sufficiently in the hot rolled sheet, and will precipitate finely during continuous annealing. It is thought that it becomes hard and has poor ductility. As detailed above, Nb<0.025% and Nb
When Ti is added in combination, a steel plate with excellent ductility and deep drawability can be obtained. As stated above, the composition range of the steel of the present invention is Ti
The amount added depends on the amount of N in the steel, Ti (%) > 48/14
[N (%) - 0.002%) and Ti (%) < [48/12C
(%) + 48/14N (%)], and the amount of Nb added is determined by the amount of C in the steel, Nb (%) > 0.3 × 93/12
C (%) = 2.33C (%) and 0.003%≦Nb (%)<
The content is 0.025%. The range of components other than Ti and Nb is C: 0.007% or less,
It consists of Si: 0.8% or less, Mn: 1.0% or less, P: 0.1% or less, Al: 0.01 to 0.1%, N: 80ppm or less, and other unavoidable impurities. When the amount of C is large, Nb is inevitably added to fix C.
A correspondingly larger amount is required, and the amount of [Ti, Nb]C produced increases, which inhibits the growth of crystal grains, leading to a decrease in r value, increase in yield strength, and decrease in elongation. Therefore, from the perspective of producing ultra-deep drawable steel sheets, C
shall be 0.007% or less. When producing hot-dip galvanized steel sheets, Si tends to reduce the adhesion of the plating layer film.
Should be 0.8% or less. Particularly when alloying treatment is not performed, a content of 0.3% or less is desirable. If Mn is added in large amounts, the r value will deteriorate significantly.
The upper limit is 1.0%, but a lower value is preferable from the viewpoint of high r value. The steel of the present invention is difficult to cause secondary processing cracks due to the combined addition of Ti and Nb, but if a large amount of P is included,
The upper limit of P is set to 0.1% because the amount of grain boundary segregation increases, embrittles the grain boundaries, and promotes secondary work cracking. Al is added as a deoxidizer for molten steel before adding Ti and Nb, but if it is too small, deoxidation by Al will not be sufficient and Ti and Nb will act as deoxidizers, resulting in a decrease in the yield of Ti and Nb. becomes significant. On the other hand, if it is added in a large amount, Al ₂ O ₃ inclusions will increase, which is not preferable. For the above reasons, Al is set to 0.01 to 0.1%. N is fixed to Ti as TiN, but if the N content is large, the required amount of Ti increases, which is not preferable. Therefore, N should be 80 ppm or less. Examples will be described below. Example 1 Table 1 shows the chemical composition of the steel of the present invention and the test steel used for comparison.

[Table] The above test steel was hot-rolled to a thickness of 4.0 mm at two levels: a hot finishing temperature of 910°C, a coiling temperature of 720°C, and 620°C, and then cold-rolled to 0.8 mm. Annealing was performed on a continuous annealing line using the annealing cycle shown in the figure. That is, the temperature was maintained at 800 to 850°C for 30 seconds, and the cooling rate to about 400°C was 5 to 100°C/sec. Table 2 shows the material test results of the cold-rolled steel sheets obtained in this way. In Table 2, (1) is an example of a winding temperature of 720°C, and Table 2 (2) is an example of a winding temperature of 620°C.

【table】

[Table] Table 4 shows an outline of the distribution of mechanical properties in the coil length direction of the test steel. A in the figure; Ti,
Nb-added steel (sample steel 2) B; Ti-killed steel (6) C; Nb
Killed steel (4) is shown, a: coiling temperature is 720°C, b: coiling temperature is 620°C. It is clear from Table 2 and FIG. 2 that the Ti and Nb composite steel of the present invention has extremely superior material properties compared to conventional Ti-killed steel and Nb-killed steel. Nb killed steel has a very high recrystallization temperature at the normal coiling temperature of 620°C, resulting in high yield strength and low elongation. Even in high-temperature web material with a winding temperature of 720°C, the cooling rate is high at the ends of the coil, so the material is close to that of normal web material, resulting in extremely low yields. On the contrary,
Ti-killed steel has enough Ti to precipitate C and N.
When (6) is added, the material has excellent uniformity in the length direction of the coil. However, the amount of Ti added is
If the amount is insufficient than the amount to precipitate Ti/
When C+N (atomic ratio)<1 (7), the material quality deteriorates significantly. On the other hand, Ti and Nb composite added steel
It showed excellent material uniformity, almost the same as Ti-killed steel with a sufficient amount of Ti added. Next, we conducted a secondary work cracking test (drawing ratio 3.0) and found that the crack initiation temperature in Ti-killed steel was approximately 30°C compared to Nb-killed steel, Ti, and Nb-added steel, as shown in Figure 5. It has become clear that it has the disadvantage of being expensive. On the other hand, steel with Ti and Nb additions has a good level equivalent to that of Nb killed steel. However, when the cooling rate after annealing is slow, such as in box-annealed materials, grain boundary segregation of P occurs during cooling, raising the temperature at which brittleness occurs, so the steel of the present invention must be manufactured by continuous annealing. It is. A further noteworthy point is the anisotropy of the r value. As shown in Figure 4, Δr is relatively small for all steel types in normal temperature rolled material, but in high temperature rolled material, Δr is relatively small.
Δr is very large in Ti-killed steel and Nb-killed steel. Figure 6 shows typical values _of the in-plane anisotropy of the r value for each steel type. It is highly likely that On the other hand
Unlike Nb-killed steel, Ti and Nb composite additive steel does not have an extremely low value for low-temperature web material and has extremely small anisotropy, and compared to r _L and r _C , r ₄₅ ° is almost equal. Alternatively, it exhibits a slightly larger value, and exhibits particularly excellent formability in the case of rectangular tube drawing. FIG. 7 shows the behavior of the r value when the cold rolling rate is changed. Figure a: Winding temperature 720
°C, b: This is an example of a winding temperature of 620 °C. As already mentioned, the anisotropy of the r value of Ti and Nb composite steel is significantly lower than that of Ti-killed steel and Nb-killed steel, but this property is observed regardless of the cold rolling rate. It is. Furthermore, compared to Ti-killed steel and Nb-killed steel, Ti and Nb-added steel have a relatively high r value even at a low cold rolling rate, and can be said to be an excellent steel type from the standpoint of actual operation. In addition, as shown in Table 2, Ti and Nb composite added steel has an excellent work hardening coefficient n value, Ti killed steel,
Like Nb killed steel, it exhibits non-aging properties. Example 2 Table 3 shows the compositions of the steel of the present invention and the test steel used for comparison.

[Table] The above test steels are made by adding alloying elements (mainly P) to the Ti and Nb composite addition steel of the present invention, conventional Ti killed steel, and Nb killed steel to increase their strength. These steels were hot rolled to a thickness of 4.0 mm at a hot finishing temperature of 910°C and a coiling temperature of 720°C, then cold rolled to 0.8 mm, and then transferred to a continuous annealing line using the annealing cycle shown in Figure 3. and annealed. Table 4 shows the material test results of the cold-rolled steel sheets obtained in this way.

【table】

[Table] Figure 8 shows the distribution of material property values in the coil length direction for each sample steel. In the figure, A: Ti, Nb composite addition steel (sample steel 8, 9), B: Ti killed steel (11),
C: Shown for Nb killed steel (10). The following is clear from Table 4 and Figure 8. When P is added to Ti-killed steel, Ti, Nb
r at the center of the coil compared to additive steel and Nb-killed steel.
The disadvantage is that the value is about 0.2 lower. Furthermore, when P was added to Ti-killed steel, the secondary processing crack initiation temperature shown in FIG. 5 showed a tendency to further increase. With Nb killed steel, there is significant material deterioration at the end of the coil. Compared to these conventional steels, the Ti and Nbb composite addition steel has an r value as high as that of the Nb killed steel at the center in the longitudinal direction of the coil, and the material distribution in the longitudinal direction of the coil is similar to that of the Ti killed steel. It is extremely uniform. Furthermore, the anisotropy of r value is extremely small.
It has excellent properties not found in Ti-killed steel and Nb-killed steel. The superiority of the steel of the present invention became clear even when the strength was increased by adding alloying elements in this manner. Example 3 Among the test steels shown in Tables 1 and 3, 2, 3,
Examples 5, 6, 8, 10, and 11 were subjected to cold rolling under the same conditions as in Example 2, and then hot-dip galvanized steel sheets were manufactured using the cycle shown in FIG. In the figure, 800 to 850℃ x 30sec, (a), approx. 450℃
℃ at a cooling rate of 3 to 100℃/sec (b), treated in a zinc bath at 450 to 500℃ (c), and (a) and (b) to about 500 to 560℃
shows the alloying treatment (d). Cycle (a) corresponds to the case where alloying treatment is not performed, and cycle (b) corresponds to the case where alloying treatment is performed to produce an alloyed galvanized steel sheet. Although the mechanical properties were hardly affected by the presence or absence of alloying treatment, Table 5 shows the material property values when alloying treatment (b) was performed.

[Table] Material property values of each test steel are shown in Example 1 and Example 2.
This indicates that the steel of the present invention is extremely superior as a hot-dip galvanized steel sheet. In the case of alloyed galvanized steel sheets, if alloying progresses excessively, there is a risk that a brittle alloy layer will grow and cause powdering during press forming. Table 6 shows the results of a powdering test performed on samples cut out from 10 locations (10 locations in total) for 10 coils manufactured for each type of steel.

[Table] Ti-killed steel has a very high powdering rate because Ti promotes alloying of base iron and molten zinc and promotes overalloying. The Ti and Nb composite additive steel of the present invention has excellent powdering resistance, which is almost at the same level as Nb killed steel, and from this point of view as well, it is optimal as a good alloyed galvanized steel sheet material. As described above, by adding Ti and Nb in combination, various excellent properties that cannot be obtained with a material in which Ti and Nb are added individually are obtained, demonstrating the novelty of the present invention.

[Brief explanation of the drawing]

Figure 1 is an explanatory diagram showing the influence of the amount of Ti on the material properties of steel with Ti and Nb composite additions. Figure 2 is an explanatory diagram showing the influence of the amount of Nb on the material properties of steel with Ti and Nb composite additions. Figure 4 is an explanatory diagram showing the annealing cycle, Figure 4 is an explanatory diagram showing the distribution of material test values in the longitudinal direction of the coil, Figure 5 is an explanatory diagram showing the temperature at which secondary processing cracks occur, and Figure 6 is an explanatory diagram showing the r value and r Figure 7 is an explanatory diagram showing the anisotropy of values, Figure 7 is an explanatory diagram showing the dependence of r value on cold rolling rate, Figure 8 is an explanatory diagram showing the distribution of material test values in the coil length direction, and Figure 9 is an explanatory diagram showing the distribution of material test values in the coil length direction. It is an explanatory view showing an annealing cycle.

Claims (1)

[Claims] 1 C: 0.007% or less, Si: 0.8% or less, Mn: 1.0
% or less, P: 0.1% or less, Al: 0.01-0.1%, N:
Consisting of less than 80ppm and other unavoidable impurities,
And Ti and Nb are added, Ti is 48/14 [N (%) -
0.002%〕＜Ti, and Ti(%)＜[4.00C(%)＋
3.43N (%)], Ti is contained within the range of 0.010% to 0.037%, and Nb is Nb (%) > 2.33C (%).
The steel containing the added ingredients in an amount of 0.003% or more and less than 0.025% is heated to 700℃ after hot rolling and cold rolling.
A method for producing a steel plate for ultra-deep drawing, characterized by continuous annealing at a temperature below the Ac ₃ transformation point.