JPH0128817B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing a steel plate for ultra-deep drawing that has extremely excellent secondary workability. Conventionally, ultra-low carbon steels with added carbonitride-forming elements have been developed as deep-drawable steel sheets for continuous annealing, but such steel sheets become brittle and fracture when subjected to secondary processing after severe deep drawing. In particular, when producing high-strength steel sheets by adding P, Si, Mn, etc., the above-mentioned secondary work embrittlement is extremely difficult because P and Si have a strong tendency to embrittle steel sheets. It is more likely to occur. B, like C, is said to have the effect of strengthening grain boundaries, but as a result of actual research and study by the present inventors, we found that the effect of B addition on the steel sheet material varies depending on the steel type and manufacturing conditions. The present invention has been completed based on the following new findings that the invention is different. When B is added to Ti-added ultra-low carbon steel,
Although the secondary workability tends to improve, the improvement effect is relatively small, and there are cases where the deterioration of deep drawability (r value) and ductility (El) is extremely large compared to materials without B additives. . In Ti-added steel, Ti is O in the steel,
Since the tendency to form precipitates with N, S, and C is extremely strong, the grain boundaries are extremely clean and the grain boundary strength is extremely weak. Even when B is added, the brittle grain boundary properties remain, so the effect of improving secondary workability is relatively small. When B is added to Nb-added ultra-low carbon steel,
When the amount of B added is small, the effect of improving secondary workability is small, and on the contrary, when the amount of B added is increased to the extent that the effect of improving secondary workability appears, the above-mentioned Ti-only added steel Similarly, the deterioration of r value and El is extremely large. The cause of this phenomenon is that in the case of Nb-added steel,
Since B has a greater tendency to form nitrides than Nb and Al, the added B forms BN and very little exists in the form of solid solution B, which has the effect of improving secondary workability. Therefore, when a trace amount of B is added, the effect is small. In order to secure B existing as solid solution B, N
It is necessary to add an amount of B greater than the amount required to form BN. However, since BN has a strong tendency to deteriorate the r value and El, the material deteriorates significantly and is not preferred as a steel sheet for deep drawing. Furthermore, since the amount of B that becomes BN is determined by the amount of N in the steel width, the amount of B added needs to be increased in consideration of safety, taking into account the fluctuations in the N content during actual manufacturing. Even when B exists as a solid solution, B tends to deteriorate the material, so Nb-added steel, which requires a large amount of B, has the disadvantage of extremely large material deterioration and material fluctuation. In addition to the above-mentioned problems caused by the addition of B, the present inventors have obtained new knowledge that conventional ultra-low carbon steel sheets for deep drawing have the following drawbacks. In steel with only Ti added, if the amount of Ti added is less than the equivalent amount of C and N, carbide (TiC) will precipitate finely, resulting in significant deterioration of material properties such as ductility, yield strength, deep drawability, and aging resistance. There is a tendency to Therefore, in order to obtain a material with excellent deep drawability, Ti
It is necessary to make the addition amount equal to or higher than the equivalent amount of C and N. In this case, there is almost no solid solution C in the steel sheet, resulting in extremely poor secondary workability, and the addition amount of Ti increases. , the deterioration of the phosphate treatment applied as a base treatment for painting is significant. For Nb-added steel, there are restrictions on hot rolling coiling temperature, annealing temperature, and cooling rate after annealing. Nb-added steel requires hot rolling and high temperature coiling (coiling temperature ≧700℃). At normal coiling temperatures, the complete recrystallization temperature becomes very high, and the possible temperature range of continuous annealing furnaces (usually about
(below 850℃), unrecrystallized areas remain, and the material quality varies greatly depending on the amount of Nb. This is related to the formation of AlN and NbC, and is thought to be because these precipitates do not have sufficient size in the hot-rolled sheet to suppress recrystallization. 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 longitudinal ends of the hot-rolled coil. It is. This is related to the formation of AlN and NbC, and is because these precipitates form in the hot-rolled sheet as large-sized precipitates during high-temperature coiling. However, high-temperature winding not only makes the scale thicker and drastically reduces pickling efficiency, but also because the cooling rate at the end of the coil is fast, the material remains at the same temperature as the normal winding temperature. Since the material cannot be obtained, the decrease in yield is particularly large in Nb-added steel. The second problem is the annealing temperature after cold rolling and the cooling rate after annealing. As stated in JP-A-55-141526 and JP-A-55-141555, annealing at high temperatures (approximately 900°C or higher)
Since AlN and NbC are redissolved, solid solution C and N are formed, and slow aging cannot be achieved unless slow cooling is performed after annealing. Therefore, it becomes a problem from the viewpoint of operability and economy. The present invention also relates to the production of steel that eliminates the drawbacks of these conventional Ti-added steels and Nb-added steels. In other words, the objective was to develop a steel sheet that has excellent deep drawability and is resistant to secondary processing cracking even when subjected to severe deep drawing processing, and is insensitive to hot rolling winding conditions. The gist of it is C:
0.005% or less, Si: 0.8% or less, Mn: 1.0% or less,
Consisting of P: 0.1% or less, Al: 0.01 to 0.1%, N: 0.005% or less and other unavoidable impurities, and B,
The essential condition is to add Ti and Nb in combination, and B
is added within the range of 2ppm or more and less than 10ppm, and Ti is added within the range of 2ppm or more and less than 10ppm.
48/14 [N (%) - 0.003%] ≦Ti (%) ≦48/12C
(%) + 48/14N (%), and Nb is 7C (%) > Nb (%) > 2C (%) and
Steel with a content within the range of 0.003%≦Nb<0.035% and a composition satisfying 0.017%<Ti+Nb (%)<0.06% is hot rolled at a heating temperature of 1300℃ or less, and then descaled. , After rolling, continuous annealing is performed at a temperature above the recrystallization temperature and below AC ₃ points,
This is a method for producing a cold-rolled steel sheet for ultra-deep drawing, which has extremely excellent secondary workability, and is characterized by cooling between 650°C and 450°C at an average cooling rate of over 50°C/sec. The basic principle of the steel of the present invention will be described below. The basic idea of the present invention is to significantly increase grain boundary strength through grain boundary concentration of solid solution B and solid solution C present in the steel sheet, and to provide extremely excellent secondary workability. Furthermore, Ti is added in combination in order to exhibit the above effects by turning the B added into the steel into a solid solution B.
The effect of the composite addition of Ti is to precipitate and fix N in the steel as TiN, thereby preventing the added B from forming BN and forming solid solution B. Therefore, the amount of B added is effective even in a small amount, and the deterioration of ductility (El) and deep drawability (r value) due to B addition can be suppressed. Furthermore, the effect of composite addition of Nb is that some of the C in the steel is precipitated and fixed as NbC, and solid solution C is
The objective is to reduce the amount so that it is substantially non-aging. The reason why the steel of the present invention has superior deep drawability and secondary workability compared to conventional steels with only Ti added and steels with only Nb added is because of the combined addition of trace amounts of B, Ti, and Nb. This is due to a number of things. That is, due to the small amount of Ti added in a composite manner, N in the steel is already precipitated and fixed as TiN in the hot rolling furnace. Since TiN is extremely stable as a nitride, it does not change in any way during the hot rolling, cold rolling, and recrystallization annealing processes. The precipitation form remains unchanged. Since B added to steel has a smaller tendency to form nitrides than Ti, it exists as solid solution B, and has the effect of increasing grain boundary strength with a small amount of addition. On the other hand, when B is added to steel with only Nb added, B has a greater tendency to form nitrides than Nb and Al, so the added B forms BN. Therefore, when the amount of B added is small, there is no solid solution B that has the effect of improving secondary processability (B is
(exists as BN), so it has no effect on improving secondary workability. To ensure that B exists as a solid solution, N and
It is necessary to add B in an amount greater than the amount required to form BN, but since BN and solid solution B have a strong tendency to deteriorate r value and El, increasing the amount of B added will lead to material deterioration, resulting in deep drawing. It is not preferred as a steel plate for industrial use. In addition, when B is added to steel with only Ti added,
As is well known, the deterioration of the r value and El is extremely large compared to the material to which B is not added. Furthermore, Ti is O, N,
Because it has an extremely strong tendency to form precipitates with S and C,
The grain boundaries are extremely clean and the grain boundary strength is very weak. Therefore, in order to improve the properties of brittle grain boundaries by adding B, it is necessary to increase the amount of B added, which is not preferable from the viewpoint of material quality. On the other hand, the addition of Ti in the steel of the present invention
This serves to precipitate and fix N as TiN, and does not cause the drawbacks seen in the steel with only Ti added above. The results of repeatedly comparing the materials of the steel of the present invention and the above-mentioned steel with only Ti and Nb added by fixing the amount of added B to a constant value at various additive amounts also show that the steel of the present invention has the best ductility and deep drawability. It also showed clear superiority in terms of secondary processability. The steel of the present invention has a composite addition of Nb along with B and Ti, but due to the coexistence of Ti and Nb, (Ti, Nb)
Since composite precipitates such as C are formed and begin to precipitate before finishing during hot rolling (ie, in the austenite temperature range), a fairly good material can be obtained even at a low winding temperature. The reason why the steel of the present invention has superior material properties compared to conventional steels with only Nb added is that by precipitating and fixing N as TiN, the addition of a small amount of B can stably improve secondary workability.N of
TiN has already been precipitated in the hot rolling heating furnace.
The factor of variation in the winding temperature due to the material has been eliminated. By the combined addition of Ti and Nb,
The advantage is that composite precipitates such as C can be formed before finishing and a fairly good quality material can be obtained even at a low winding temperature. Next, we will discuss the component range. First, the amount of B added must be within the range of 2 ppm or more and less than 10 ppm. The addition of B in the steel of the present invention has the effect of improving secondary workability,
This effect is due to B existing in solid solution. In the steel of the present invention, N is replaced with TiN by the composite addition of Ti.
Since B is precipitated as B, the added B becomes solid solution B, and even a small amount is sufficiently effective. As the amount of B added increases, the r value and El tend to deteriorate slightly. Considering the characteristics of the steel of the present invention, which is a steel plate for ultra-deep drawing, the upper limit is set to 2 ppm or more and less than 10 ppm. Ti is added to fix N and eliminate its harmful effects, and needs to be added in an amount of 48/14 [N (%) - 0.003%] or more. In other words, the lower limit of the amount of Ti added is
Calculations show that the amount of N that cannot be precipitated and fixed by Ti is 30 ppm or less. In normal aluminium-killed steel, 30 ppm of N has an adverse effect, but when Ti is added in combination,
AlN was precipitated using TiN as a precipitation nucleus (Ti, Al)
It was found that a composite precipitate of N is formed and becomes a stable precipitate even at extremely high temperatures, so that it has the same effect as precipitating substantially the entire amount of N as TiN. In order to make the above effect sufficiently noticeable, 0.002%
It is desirable to add Ti in the above amount. Furthermore, if more than the equivalent of the sum of C and N is added, the properties similar to Ti-added steel will become stronger and secondary processing cracks will occur more easily, so the upper limit should be set at 48/12C (%) + 48/14N (%). less than From ductility, yield strength and economic considerations
The amount of Ti added is 48/14N (%) or less, which does not generate TiC, and is most preferably 0.015% or less. The amount of Nb added requires 2C (%) or more to form a composite precipitate, and 0.003%
The effect is small when the amount of Nb added is less than 7C.
(%) or more, the precipitates will have a composition close to that of NbC, and the drawbacks of steel with only Nb added will clearly appear, which is undesirable. Most preferably Nb<
The amount added is 0.025%. Moreover, the feature of the present invention is that solid solution C remains, and the chemical equivalent of Nb with C, that is, 93/12C (%)
If you add more than [=7.75C (%)], there will be no solid solution C,
The features of the present invention cannot be brought out. For this reason, Nb must be kept below 7.75C (%), but in order to obtain excellent secondary workability, it must be further reduced.
It is necessary to reduce the amount of Nb and secure the minimum amount of solid solution C, and therefore the upper limit of Nb is set at 7.0C (%). Furthermore, cold-rolled steel sheets are subjected to phosphate treatment (bonding treatment) as a base treatment for painting, but they also need to have excellent bonding properties. but,
Ultra-low carbon steel has the property that the addition of Nb or Ti significantly deteriorates bonding properties. In order to ensure good chemical conversion properties, especially for areas where welded parts are cleaned with a grinder and new surfaces are exposed, the amount of Ti and Nb added must be Ti (%) + Nb (%).
It is necessary to limit it to <0.06%. The most desirable range is Ti (%) + Nb (%) < 0.05%. In order to form a composite precipitate such as (Ti, Nb)C, the amounts of Ti and Nb should be adjusted as shown in the example.
(%) + Nb (%) > 0.017% is required. Next, the range of elements other than B, Ti, and Nb will be described. When the amount of C is large, it is necessary to fix C.
As the amount of Nb increases, manufacturing costs increase, and the amount of composite precipitates produced increases, precipitation strengthening factors become large, leading to deterioration of material quality. For this reason 0.005
% or less. Si is sometimes added when making high-strength steel sheets, but it is an element that promotes brittleness and also inhibits chemical conversion treatment properties, so it should be kept at 0.8% or less. Mn can also be used to increase the strength. However, since it has the property of deteriorating the r value and the cost of ferroalloy is high, it is limited to 1.0% or less. P is an element with the greatest strengthening ability and is added to increase strength, but if it is included in a large amount, the amount of grain boundary segregation increases and causes embrittlement, that is, secondary work cracking, so the upper limit is 0.1 %. Substantially the total amount of N is fixed as (Ti, Al)N, but if the N content is large, a large amount of Ti is required to be added, so it is set to 0.005% or less. C, N
By limiting the amount to an extremely low amount of 50ppm or less,
The amount of precipitates is reduced, ductility is good, yield strength is low, and the negative effects of increasing the amount of Ti and Nb added tend to be alleviated. Next, the manufacturing conditions will be described. In the steel of the present invention, N is rendered harmless by precipitating and fixing it with Ti, and (Ti, Nb)C composite precipitates are precipitated at high temperatures by the combined addition of Ti and Nb. By setting the temperature to 1300° C. or lower, these precipitates or precipitation nuclei are sufficiently present in the heating furnace. As a result, it has become possible to precipitate virtually the entire amount of N as (Ti, Al)N by adding a small amount of Ti, and (Ti, Nb)C composite precipitates can be precipitated before finishing. Precipitation begins in the high temperature range. Therefore, we have obtained the new knowledge that even at a low coiling temperature, the precipitates are considerably agglomerated in the hot rolled sheet state, and the material behavior is insensitive to the coiling temperature. By limiting the heating temperature to 1300° C. or less, the degree of aggregation of precipitates is improved and the adverse effects thereof are reduced, so the upper limits of the amounts of Ti and Nb added are also slightly relaxed. In addition, since it improves the quality of the material, especially the r value, it has a positive effect on the secondary workability, and the effect of B addition is noticeable and is sufficiently effective when added in an amount of 2 ppm or more. Promoting coarse agglomeration of precipitates also has a positive effect on chemical conversion treatment properties, relaxing the upper limit on the total amount of Ti and Nb added. That is, (Ti, Nb)C, (Ti,
Precipitates such as Al)N are less soluble in acids than Fe ₃ C, making it difficult for phosphate crystals to precipitate and having a negative impact on chemical conversion properties. The material density is reduced and chemical conversion treatment properties are improved. In the steel of the present invention, there is no need to specify other hot rolling conditions. However, as the hot rolling finishing temperature decreases, the r value and El tend to decrease.
A finishing temperature of 850°C or higher is preferred. There is no need to particularly specify the winding temperature for the above reasons. There is no need to particularly specify the cold rolling conditions. As the cold rolling rate increases, deep drawability tends to improve, and secondary work brittle cracking is less likely to occur as the r value of the steel sheet increases. Therefore, in order to further improve the characteristics of the steel of the present invention, A cold rolling rate of 50% or more is most preferred. Since the steel of the present invention requires only a small amount of Ti and Nb to be added,
Although the recrystallization temperature is low, increasing the cold rolling rate
Furthermore, it is effective in lowering the recrystallization temperature and lowering the annealing temperature. Regarding the annealing conditions, continuous annealing is performed at a temperature above the recrystallization temperature and below AC ₃ points. Since the cooling rate of box-shaped annealing is extremely slow, diffusion of P to grain boundaries occurs during cooling, which is undesirable. When manufacturing cold-rolled steel sheets, it is necessary to limit the cooling rate after annealing. Although the steel of the present invention is a material with extremely excellent secondary workability, if the cooling rate is too slow, secondary workability tends to occur easily due to grain boundary segregation of P and the like. Considering the diffusion of P into the grain boundaries, the temperature ranges from 650℃ to 450℃.
The problem is the cooling rate between
It should be over sec. Examples will be described below. Example 1 Steel slabs having the components shown in Table 1 were melted and hot rolled under the hot rolling conditions shown in Table 1. The finishing temperature is 890-910°C in all cases. The hot-rolled sheet thickness was 3.8 mm, and after pickling, it was cold rolled to 0.8 mm and then annealed in a continuous annealing furnace. The annealing cycle is approx.
After heating to 780-820℃ at 10℃/sec and holding in the temperature range for 40 seconds, average cooling rate 50-820℃ to room temperature.
It was cooled at 100°C/sec. FIG. 1 shows an annealing cycle. After applying a skin pass of 0.8%, it was subjected to a material test.
The results are shown in Table 2 along with the results of chemical conversion treatment and secondary processing cracking tests. For some materials with high coiling temperatures, the longitudinal center of the hot rolled coil (upper stage),
The material of the position corresponding to the longitudinal end (lower stage) is shown.

【table】

【table】

[Table] All of the products of the present invention (test steel Nos. 1 to 4) showed good results. Test steel No. 5 has a large amount of Ti and Nb added (Ti (%) +
Nb (%)>0.06%), chemical conversion treatment properties are poor. No.
No. 6 has a high hot rolling heating temperature, so the composite addition effect of Ti is small, and compared to No. 1, the material quality and secondary workability are inferior. No. 7 does not contain B, so secondary processing cracks tend to occur, while No. 8, on the other hand, has too much B added.
YP, El, r values are not good. Since No. 9 has a large amount of Ti added, its properties are similar to those of Ti-added steel, and its secondary workability and chemical conversion treatment properties are poor.
No. 10 has a small amount of Nb, so there is a lot of solid solution C, and the aging property is high and the material quality is also inferior. No. 11 has too much Nb, making the material similar to Nb-added steel, and a good material cannot be obtained at a coiling temperature of 700°C or lower. Nos. 12 to 14 are materials to which Ti is not added, and in this case, B forms BN with N, so B has no effect of improving secondary workability (No. 12). In addition, material deterioration is large when the winding temperature is low (No. 13). As shown in No. 14, increasing the amount of B added improves secondary workability, but the material quality is inferior. Nos. 15 and 16 are cases in which only B is added without adding Nb to Ti-added steel, but in this case, the addition of B causes significant material deterioration, the effect of improving secondary workability itself is small, and furthermore, Processability is poor. Comparative Example 1 Next, a case where the cooling rate is low will be described. Using steel slabs having compositions No. 2 and 4 shown in Table 1, they were wound into a 3.8 mm thick coil at a heating temperature of 1200°C, a finishing temperature of 900°C, and a winding temperature of 700°C and 650°C, respectively. After pickling, it was cold rolled into a 0.8 mm coil, and then subjected to continuous annealing and a 0.8% skin pass using cycles I to E in Figures 2 and 3. Table 3 shows the cooling conditions after continuous annealing, and Table 4 shows the material properties of cold rolled steel sheets obtained under these conditions.

【table】

[Table] shows the maximum drawing ratio without cracking. The tensile test value is in the longitudinal direction of the hot coil.
The position corresponding to the heart is shown.
The average cooling rate between 650℃ and 450℃ is 30℃/
sec, 10° C./sec, 3° C./sec, and 2° C./sec, the secondary workability is at a high level but tends to be lower compared to Example 1. In order to maximize the properties of the steel of the present invention, which has extremely excellent secondary workability, the cooling rate must be set to 50℃/
Should be over sec. Example 2 Using a steel slab with the composition shown in Table 5, the heating temperature was
It was hot rolled at 1180°C, finishing temperature 890°C, and coiling temperature 680°C to form a 3.8 mm coil. After pickling and cold rolling to form a 0.8 mm coil, it was continuously annealed in the cycle shown in Figure 1, subjected to a skin pass of 0.8%, and then subjected to material testing. The results are shown in Table 6.

【table】

[Table] Conventionally, high-strength steel sheets with high r-values have TS=40
Kgf/mm class ² was the limit. In order to provide further strength, it is necessary to add reinforcing elements such as P and Si, but these elements significantly promote embrittlement and are likely to cause secondary processing cracks, which is an inhibiting factor. It was hot. At the same time, if an attempt was made to improve the secondary workability by adding B, the material quality would deteriorate significantly. As shown in Table 6, when B is added to the conventional steel with only Ti and Nb added, the material quality deteriorates significantly, and the effect of improving secondary workability is very small with a small amount of B. With the addition of a small amount of B, the steel of the present invention has extremely excellent secondary workability, and from the viewpoint of material quality, B
There is no adverse effect of addition of P, Si, or Mn. Therefore, the steel of the present invention is a high-strength steel plate with high strength,
It is also extremely advantageous for the production of thick steel plates that are susceptible to secondary workability.

[Brief explanation of drawings]

FIGS. 1, 2, and 3 are explanatory diagrams showing a heat treatment cycle in an embodiment of the present invention.

Claims (1)

[Claims] 1. C: 0.005% or less, Si: 0.8% or less, Mn: 1.0% or less, P: 0.1% or less, Al: 0.01 to 0.1%, N: 0.005% or less, and unavoidable impurities. and B, Ti, Nb
The essential condition is that B is added in combination, and B is 2ppm.
Added within the range of 10 ppm or more, and Ti is 48/14
[N (%) - 0.003%] ≦Ti (%) ≦48/12C (%) +
Contains within the range that satisfies the condition of 48/14N (%),
Nb is 7C (%) > Nb > 2C (%) and 0.003%≦
Steel with a content that satisfies Nb (%) < 0.035% and a composition that satisfies 0.017% < Ti (%) + Nb (%) < 0.06% is hot-rolled at a hot rolling heating temperature of 1300℃ or less. After rolling, descaling treatment, cold rolling, continuous annealing at a temperature above the recrystallization temperature and below AC ₃ points, and then cooling between 650℃ and 450℃ at an average cooling rate of over 50℃/sec. A method for producing a cold-rolled steel sheet for ultra-deep drawing, which has extremely excellent secondary workability.