JPH0137455B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention is directed to a non-aging hot-dip galvanized steel sheet with extremely excellent formability, especially a hot-dip galvanized steel sheet that is non-aging and has high deep drawability and ductility even after hot-dip galvanizing treatment. The present invention relates to a method for producing a steel plate. In order to extend the service life of thin steel sheets used for automobile exterior panels, there is an increasing demand for steel sheets that have been surface-treated. Although various methods have been developed as such surface treatment methods, the continuous hot-dip galvanizing method is considered to be the most suitable in terms of manufacturing cost and characteristics. By the way, since the outer and inner panels of automobiles are subjected to advanced press forming, they are also required to have excellent formability, and therefore non-aging hot-dip galvanized steel sheets with high value and large elongation are required. becomes. (Prior Art) As a method for manufacturing such a hot-dip galvanized steel sheet, (1) after plating a low-carbon steel cold-rolled steel sheet in a normal process, that is, a continuous annealing-continuous plating process,
A method of overaging to make it non-aging (Japanese Patent Publication No. 1972-1972, etc.); (2) Adding a small amount of Ti to the steel sheet material so as not to impede the plating properties, as well as carbide-forming elements such as Nb. There is a method in which C is added to fix C in the steel and make it non-aging (Japanese Patent Application Laid-open No. 35616/1983). (Problems to be Solved by the Invention) However, none of the above methods has been able to provide sufficient drawability, that is, elongation and value. The present invention advantageously solves the above problems,
An advantageous method for producing hot-dip galvanized steel sheets that not only exhibit extremely high values and high elongation values even after plating treatment, and have excellent deep drawability with virtually no aging, but also have excellent surface quality. The purpose is to propose. (Means for solving the problem) The inventors have determined the material composition, hot rolling conditions, and continuous annealing conditions after cold rolling that affect the aging properties and deep drawability of ultra-low carbon steel containing Nb. After a detailed study, we obtained the following new findings. 1 N to 4 times or more of AlN equivalent and 0.01%
For steels containing Al, hot-rolled sheets are treated at a hot rolling reduction rate of 90% or more, a rolling speed of 70 m/min or more, and a coiling temperature of 600 to 800°C, then cold rolled, and then continuously annealed. If you do,
If the amount of Nb added to C is about 1/3 or more of the NbC equivalent, non-aging property is exhibited. 2 In the case of the above hot rolling and annealing conditions, NbC
When the excess Nb is 0.02% or less, the elongation reaches a maximum value of 48% or more, and the value is as high as when the excess Nb is 0.02% or more.
Indicates 1.9 or higher. 3. The steel sheet of the above composition has excellent hot-dip galvanizing properties, and even after such galvanizing treatment, the deterioration of the various properties described above is extremely small. This invention is based on the above knowledge. That is, this invention includes: C: 0.008% or less, Si: 0.20% or less, Mn: 0.04 to 0.30%, P: 0.03% or less, S: 0.02% or less, and N: 0.01% or less, and further contains 0.01 to 0.10%. and at least 4 times N% Al, 0.01 to 0.07% and (C%
×3) to (C%×8+0.02%) along with Nb that satisfies the range, and the balance is essentially Fe, rolling reduction: 90% or more, rolling speed: 70
m/min or higher and finishing temperature: 830°C or higher, then coiled at a temperature range of 600 to 800°C, and then cold rolled at a rolling reduction of 70 to 90%. It is characterized in that it is coated, then annealed at a temperature range of 700 to 900°C for 10 seconds to 5 minutes, then cooled to a temperature not lower than the bath temperature of the hot-dip galvanizing bath, and then hot-dip galvanizing treatment is continuously applied. This is a method for producing non-aging hot-dip galvanized steel sheets with extremely excellent formability. The experimental data that led to the present invention will be explained in detail below. Steel slabs produced by varying the C and Nb contents shown in Table 1 were rolled at a rolling reduction rate of 90% and a minimum rolling speed.
After hot rolling at 70 m/min, finishing temperature 870°C, and coiling temperature 680°C, a cold-rolled sheet with the final thickness obtained by cold rolling at a reduction rate of 80% was subjected to continuous annealing. Parameter α≡ on characteristic values (AI value, El value, value) when annealing is performed at 830℃ for 40 seconds
Nb (%)/C (%) and parameter β≡Nb (%)
-8C (%) relationships are shown in Figures 1 and 2.

[Table] From Figure 1, when parameter α is 3 or more,
The AI value, that is, the aging index, is less than 1 Kg/mm ² and the value is more than 1.9, so that a substantially non-aging steel plate with a high value can be obtained, and from Figure 2, the El value (elongation) follows the parameter β. and β changes as
It can be seen that a sufficiently high value can be obtained when the content is 0.02% or less. By repeating this experiment, it was found that Nb is required to be more than three times as much as C (%), but β≡Nb (%) - 8
It has been found that ×C (%), that is, Nb (%) unbonded with C, needs to be 0.02% or less. Note that within the above range, the Nb content is 0.03
Within the range of ~0.06% and 6 x C (%) ~ 8 x
Generally speaking, it is preferable to be within the range of C (%) + 0.010% from the viewpoint of the balance of characteristic values. If C exceeds 0.008%, the value and elongation will drop significantly, so it needs to be lower than that, and C 0.006% is best. Al needs to be added at least 0.01% and at least 4 times the amount of N (%) in order to fix N as AlN. Otherwise, since N in the steel combines with Nb in the steel, a large amount of C that is not fixed by Nb remains in the steel, resulting in an inability to reduce the AI value sufficiently.
However, addition of 0.1% or more of AI should be avoided as it increases inclusions caused by alumina clusters in the steel and causes surface flaws. If the N content is high, it becomes necessary to increase the Al content, and in particular, if the N content exceeds 0.01%, surface defects will increase due to an increase in inclusions caused by alumina clusters, so the N content should be 0.01%. It is necessary to do the following. Regarding Mn, it is sufficient to set it to the level contained in ordinary cold-rolled steel sheets, and it may be set to 0.04 to 0.030%. As for Si, if it is contained in a large amount, it will significantly deteriorate the fitting properties, so it needs to be kept at 0.20% or less. It is preferable to reduce other impurities such as P, S, and O as much as possible, but the level of each impurity contained in ordinary cold rolled steel sheets is 0.030%,
It is acceptable if it is less than 0.020% or 0.008%. Next, the manufacturing method of the present invention will be specifically explained step by step. For melting the steel sheet material, any commonly used methods can be used alone or in combination. However, it is necessary to decarburize C in advance at the stage of molten steel, and RH
It is advantageous to perform vacuum decarburization treatment by the method, DH method, etc. In addition, pure oxygen bottom-blowing converter method (Q-BOP)
It is advantageous to directly produce ultra-low carbon using a method (method). Furthermore, either the conventional ingot forming method or the continuous casting method can be used. A slab obtained by continuous casting or a slab obtained by blooming a steel ingot produced by a conventional ingot-forming method is subjected to continuous hot rolling. In this case, the heating temperature of the slab should be 1,150°C or higher, which is necessary to form a solid solution of NbC in the steel, and a general temperature range of 1,150 to 1,300°C is sufficient. Now, in a slab heated to a temperature in the above range,
Subsequently, continuous hot rolling is performed, and in the present invention, the reduction ratio and rolling speed in this continuous hot rolling are particularly important. In other words, the rolling reduction ratio is such that the total rolling reduction from the rough rolling of the slab to when it exits the finishing stand group is 90% or more, and the rolling speed is such that the rolling speed of the finishing stand group is at least 70 m/min, preferably 80 m/min.
It has been revealed that non-aging and high El values can be obtained by hot rolling under conditions of m/min or higher. The reason for this has not yet been clearly elucidated, but when hot rolling is carried out under the conditions of high reduction and high rolling speed as described above, fine Nb (for example, less than 1000A) is produced during this hot rolling stage. C, N),
Precipitation of AlN, MnS, etc. is promoted, these serve as precipitation sites and promote the precipitation of FexCy, and furthermore, as a result of the subsequent thermal history, composite precipitates are formed, resulting in the formation of complex precipitates in the steel. It is believed that the solid solute C is sufficiently reduced, thus resulting in substantially no aging, a high value, and improved elongation. And even after the hot-rolled sheet thus obtained is subjected to subsequent cold rolling, annealing, and plating treatment, the deterioration in value and El value is extremely small. In this regard, if the total rolling reduction in hot rolling is less than 90% or the rolling speed is slower than 70 m/min, the desired effect will not be obtained no matter how you adjust the subsequent processing conditions. Nakatsuta. According to the present invention, the hot rolling finishing temperature needs to be 830°C or higher. This is because if a finishing temperature lower than this temperature is used, the value, elongation, and aging properties will deteriorate. Also, the winding temperature must be 600°C or higher.
This is because if you wind it at a temperature lower than this temperature,
This is because the fixation of C by Nb and the fixation of N by Al are insufficient, making it difficult to obtain a non-aging steel plate. From the point of view of AI value, value, and El value, a high winding temperature, that is, a range of 680 to 750°C is effective. Furthermore, if the temperature is 800℃ or higher, C and N in the steel
This is not desirable because the fixation of the particles reaches saturation while the formation of scale increases. In order to maintain the coiling temperature within this temperature range, measures such as weakening the water cooling after finish rolling or omitting the water cooling altogether may be taken. The hot rolled coil thus obtained is then subjected to oxide scale pickling according to a conventional method and then cold rolled, or after cold rolling, scale is removed by pickling or grinding. If the rolling reduction during cold rolling is less than 70%, the desired high value will not be obtained, while if it exceeds 90%, the value will be high but the anisotropy will increase, so the cold rolling reduction is 70%. ~90% more preferably 75
It should be ~85%. Recrystallization annealing and hot-dip galvanizing are then performed continuously, and the recrystallization annealing temperature and time during plating performed in the continuous hot-dip galvanizing line for cold-rolled sheets are respectively 700 to 900°C. ,
It is necessary to carry out the heating for 10 seconds to 5 minutes, and it is important to select appropriate conditions within this range according to the target material. If the temperature is lower than 700℃, recrystallization becomes difficult and the yield point becomes extremely high.
If the temperature exceeds 900°C, solid solution of NbC will occur, making it difficult to ensure non-aging properties. Note that at a high temperature between 700 and 900°C, the strength is lower, but the value and elongation are higher, and it is particularly preferable to soak at 780 to 880°C for 30 to 90 seconds. After that, it is sufficient to follow the conventional method, that is, to cool to a temperature of 500℃ or just above the temperature of the fastening bath, followed by hot-dip galvanizing, and alloying treatment as necessary.The cooling rate at this time is particularly regulated. It will not be done. Also, for shape correction, 2% or less, preferably 1%
There is no problem in applying the following skin pass rolling. When galvanized, the material may deteriorate somewhat compared to when it is not plated, but even if it does deteriorate, the magnitude will be 0.1 in value.
~0.2, which is only about 1 to 3% in terms of El value. (Example) Next, an example of the present invention will be described. (1) Steel production/ingot making Steel is tapped in a pure oxygen top-blowing converter (LD converter), and steel is tapped in a pure oxygen bottom-blowing converter (Q-BOP).
Molten steel having the composition shown in Table 2 was obtained through a degassing step.

[Table] Degassing time is 125 minutes for steel, 23 minutes for steel,
It was hot in 35 minutes. Nb and Al were added just before the end of the degassing process. The steel was made into a 220mm thick slab by ingot rolling. The steel was made into slabs of the same thickness by continuous longevity manufacturing. (2) Hot rolling After surface treatment of the above slab, steel is rolled in a heating furnace.
: 1280°C for 35 minutes, steel: 1300°C for 30 minutes (temperatures are measured values on the slab surface). Subsequently, continuous rolling was performed using a 4-row rough rolling mill and a 7-stand finishing mill to obtain a 3.2 mm thick hot rolled steel strip. At this time, the rolling reduction rate when converting the sheet bar into a steel strip in finish rolling is
It was 92% for steel and 93% for steel. In addition, the rolling degree of each stand of the finishing mill (approximately corresponds to the threading speed on the exit side of each roll) is steel: 1st stand 98
m/min, 7th stand 660m/min, steel: 1st
The speed was set at 103 m/min at the stand and 745 m/min at the 7th stand. In either case, the temperature on the entry side of finish rolling is 1030~
The finishing temperature was controlled at 1050°C and 860-920°C. The winding temperature is steel: 770℃, steel: 660℃, steel:
The temperature was 710℃. (3) Cold rolling, annealing Hot rolled steel strip is pickled and cold rolled to a thickness of 0.7mm (reduction ratio: 78%) or 0.8mm thickness (reduction ratio: 75%).
%) cold-rolled coil. Recrystallization annealing was performed on a continuous hot-dip galvanizing line under the following conditions. Steel: 830-860°C, soaked for 40 seconds Steel: 780-820°C, soaked for 25 seconds Steel: 860-880°C, soaked for 40 seconds After cooling all steel types to 480°C, take a plating bath led inside. The cooling rate to the plating bath of approximately 460℃ is 3
~5℃/sec, 580℃ after steel plating,
Alloying treatment was performed for 10 seconds. The plated coil was made into a product by skin pass 0.6 to 0.7%. The mechanical properties of the product are shown in Table 3.
Furthermore, the plating properties are shown in Table 4.

【table】

[Table] Note) Low carbon that is said to have good plating properties
When compared with plain rimmed steel, it was rated ◎ if there were no problems as determined visually.
The above steels have excellent formability,
It can be seen that a non-aging hot-dip galvanized steel sheet with good stickability can be obtained. (Effects of the Invention) According to this invention, the Nb content can be reduced compared to the conventional method, which is not only advantageous in terms of cost, but also allows for non-aging even under such low Nb conditions and even after plating treatment. In addition to being large and expensive,
A hot-dip galvanized steel sheet with excellent elongation properties can be obtained.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the relationship between the steel plate parameter α and the AI value, and FIG. 2 is a diagram showing the relationship between the steel plate parameter β and El (%).

Claims (1)

[Claims] 1 C: 0.008% or less, Si: 0.20% or less, Mn: 0.04 to 0.30%, P: 0.03% or less, S: 0.02% or less, and N: 0.01% or less, further comprising 0.01 to 0.01%. 0.10% and at least N
%4 times Al, 0.01 to 0.07% and (C%×
3) Satisfy the range of ~(C%×8+0.02%)
A steel billet containing Nb and the remainder having a composition of substantially Fe is rolled at a rolling rate of 90% or more and a rolling speed of 70 m/min.
min or higher and finishing temperature: 830℃ or higher, then coiled at a temperature range of 600 to 800℃, cold rolled at a rolling reduction of 70 to 90%, and then 10 seconds to 5 in the temperature range of 700 to 900℃
Non-aging hot-dip galvanizing with extremely excellent formability, characterized by being annealed for 1 minute, then cooling to a temperature not lower than the bath temperature of the hot-dip galvanizing bath, followed by continuous hot-dip galvanizing treatment. Method of manufacturing steel plates.