JP3436857B2 - Thin steel sheet excellent in press formability with few defects and method for producing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin steel sheet having few defects and excellent press formability, and a method for producing the same. More specifically, the present invention relates to a steel sheet having few surface defects and defects such as cracks at the time of pressing and excellent in formability, and a manufacturing method thereof, which is suitable as a steel sheet to be used for press forming such as automobile and home electric appliance parts. And a method for manufacturing the same.

[0002]

2. Description of the Related Art Steel sheets generally used for automobiles, home electric appliances and the like are made of Al-killed steel in which undeoxidized molten steel produced in a converter is deoxidized with Al. In such Al deoxidized steel, Al added during deoxidation reacts with oxygen in the molten steel, or Al remaining in the steel after deoxidation is oxidized by oxygen such as slag or air to generate alumina. Since this alumina is hard, it is not crushed by rolling or processing and remains in the steel sheet in the form of lumps, and if it exists on the surface of the steel sheet, it becomes a surface flaw and impairs the surface quality.If it exists inside the steel sheet, it cracks during press forming. It may cause defects such as scratches and flaws. Therefore, with respect to these aluminas, the control of oxygen in the slag and the atmosphere prevents the formation of alumina due to the oxidation of Al in the molten steel, and the reduction of the alumina in the molten steel by promoting the levitation of the molten steel by blowing gas or flux into the molten steel. , Ca in molten steel
Addition of Al has been detoxified by controlling the morphology of alumina into calcium aluminate, which is easily crushed during rolling and processing.

However, as long as deoxidation is carried out with Al, the simple substance of alumina cannot be generated and the removal thereof is insufficient. In addition, the method of adding Ca also increases the cost of the alloy because Ca is expensive and the yield is extremely poor. Also, since alumina is contained in the inclusions, solid alumina partially crystallizes in the inclusions during cooling, and rolling occurs. Even if it is not crushed, it will not be crushed and a residual defect will occur. Further, the calcium aluminate formed by adding Ca is likely to be enlarged, and if such inclusions cannot be floated and remain, it becomes a big defect. To solve these problems A
Deoxidation with elements other than 1 is conceivable.
As shown in Japanese Patent Publication No. 29005, there is a method of deoxidizing only Ti without adding Al or Si at all.
Oxygen in the molten steel before addition of Ti has a very high value due to deoxidation only by n, and when Ti is added to such molten steel, a large amount of titanium oxide having a large grain size is generated and remains in the molten steel. This is a defect because it is hard to be crushed similarly to alumina. Therefore, as seen in Japanese Examined Patent Publication No. 2-9646, Al is added before Ti is added for preliminary deoxidation,
There is a method of adding Ti after reducing molten steel oxygen.

[0004]

In the above method, since Al is added in a state where oxygen in molten steel is high, it is necessary to add a large amount of Al, so a large amount of alumina is produced and remains as it is. Further, since Al has a stronger oxidizing power than Ti, when Ti is added, alumina is not reduced and becomes inclusions having a high alumina concentration, so that it remains without being crushed even by rolling, etc. It is easy to occur. Further, since Al has a strong deoxidizing power, the control of oxygen is unstable. On the other hand, most of the produced titanium oxide has a large particle size and is present in the molten steel as titanium oxide which is not easily crushed, and a part of the titanium oxide is not fully floated and remains, which easily causes defects. As described above, in the ordinary manufacturing method, it is general that the steel contains inclusions having a size of about 100 μm, and since these inclusions become the starting points of cracks during press forming, such inclusions are If it is contained in a large amount, it has a drawback that defects such as press cracks frequently occur.

Further, steel containing a large amount of acid-soluble Al has a high recrystallization temperature, and particularly Ti and Nb are added to extremely low carbon steel.
Since the fine carbonitrides exist in the IF steel to which carbonitride-forming elements such as the above have been added, it has been necessary to perform annealing at a higher temperature than general low carbon Al-killed steel. As a method for solving this, JP-A-62-308
Although deoxidation is performed with Al as in Japanese Patent Laid-Open No. 22-22, there is a technique for limiting excess Al other than that used for deoxidation, that is, acid soluble Al to 0.010 or less. However, since it is deoxidized with Al, it is inevitable that the oxide of Al remains in the steel, and the occurrence of defects due to the oxide of Al cannot be suppressed even in this steel. Furthermore, when deoxidizing so that the acid-soluble Al does not remain, deoxidizing becomes insufficient, bubbles often remain in the steel, and the surface properties are impaired due to defects called blowholes, which enables stable practical application. Was not.

The present invention has been made in order to advantageously solve the above-mentioned problems, and it is a composition in which inclusions in steel are fine and there is no partially hard crystallization phase, and the entire inclusions are easily deformed / crushed. Since the inclusion defects in the steel can be controlled at a low cost and the content of acid-soluble Al in the steel can be extremely reduced, the recrystallization temperature is low and when annealing is performed at the same temperature as conventional ones. It is an object of the present invention to provide a steel sheet and a method for producing the steel sheet that can obtain high press formability.

[0007]

The features of the present invention are as follows.
Roha (1) In wt%, C: 0.0001 to 0.0070%, Si: 0.001-2.0%, Mn: 0.05 to 3.0%, P: 0.001 to 0.150%, S: 0.001 to 0.050%, N: 0.0005 to 0.0080%, Acid-soluble Al: 0.005% or less, Ti: 0.004 to 0.040% Steel containing iron and the balance iron and inevitable impuritiesDuring ~
To, JTan oxide, manganese oxide, silicon oxide,
AluminaIs compounded and30% or less of aluminaIs
Oxide systemcompositeDeficiency characterized by containing inclusions
Thin steel plate with few pits and excellent press formability. And after refining
Deoxidize the amount of oxygen in molten steel to 250 ppm or less, then
TiAfter adding AlAfter adding C: 0.0001-
0.0070%, Si: 0.001-2.0%, Mn:
0.05-3.0%, P: 0.001-0.150%,
S: 0.001 to 0.050%, N: 0.00055
0.0080%, acid-soluble Al: 0.005% or less, T
i: 0.004 to 0.040%, the balance iron and
Consisting of inevitable impuritiesMeltingsteelDuring ~To, JTan oxide, man
Gun oxide, silicon oxide, aluminaIs it composite
One30% or less of aluminaIsOxide systemcompositeIncluding inclusions
Thank youMeltingAfter continuous casting-hot rolling of steel, 600 ° C ~
It is wound at 800 ° C and then descaled.
Then, cold rolling is performed at a rolling ratio of 50% to 95%, and 650
To 900 ° C, which is characterized by annealing in a temperature range
It is a method for manufacturing thin steel sheets with few defects and excellent press formability.
It

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors artificially synthesized inclusions of various compositions, embedded them in steel, and conducted rolling experiments in a laboratory. As a result, a small amount of alumina is contained in the inclusions, and titanium oxide (Ti0x, X = 1.5 to 2.0), manganese oxide (MnO), silicon oxide (Si).
0 ₂ ) and alumina (Al ₂ O ₃ ) as the main ingredients, the melting point is relatively low, the formation of a hard crystallization phase with a high melting point during cooling can be suppressed, and it can be finely divided by rolling or the like. It was found to be crushed. Steel in which such inclusions having different alumina contents and compositions are dispersed are melted and cast in a laboratory, hot-rolled, pickled, cold-rolled by a conventional method,
A thin steel plate was annealed and temper-rolled and press-formed, but defects such as cracks occurred in some of the steel plates. As a result of investigating the vicinity of this defect, it was found that extended inclusions were detected in the defect portion, and the size thereof was measured and converted to the size of the cast piece, which were all larger than 50 μm.

From the above, it was speculated that inclusions of the above composition having a size of 50 μm or less would not cause defects in the cast slab. Therefore, 30% by weight or less of alumina having a size of 50 μm or less was used, and titanium oxide and manganese oxide were contained. , Silicon oxide,
Steel in which only inclusions having a composition containing alumina as the main component are dispersed is melted and cast in a laboratory, and hot-rolled by a usual method.
When pickled, cold-rolled, annealed, temper-rolled to give a steel sheet and press-formed, the press-formability is good,
It was confirmed that the occurrence of defects was extremely small. As a result of conducting an experiment while changing the Ti concentration to be added, it was found that an inclusion having a composition containing titanium oxide, manganese oxide, silicon oxide, and alumina as the main components (alumina content of 30% by weight or less) It is necessary to set the Ti concentration to 0.040% or less. This is because if Ti is too high, the deoxidizing power of Ti is higher than that of Mn and Si, so it does not combine with these oxides, and inclusions with a high melting point and high titanium oxide content similar to alumina are generated. by. On the other hand, the lower limit of Ti is set to 0.004% in order to prevent generation of bubbles due to insufficient deoxidation during continuous casting. The amount of Ti is preferably an amount obtained by adding 0.002% necessary for deoxidation to the amount required for the precipitation fixation of C and N, since C and N are deposited and fixed using the residue used for deoxidation. That is all. The amount of Ti required for depositing and fixing C and N differs depending on whether Nb is added or not, and details will be described later. By adding Al after adding Ti, the oxygen concentration at the time of adding Al decreases.
The addition amount of Al is small, the content of alumina in the generated inclusions is small, and even if the inclusions contain alumina, there are almost no defects during pressing. Also, Ti
The inclusions composed mainly of titanium oxide, manganese oxide, silicon oxide, and alumina formed at the time of addition are A
Without being reduced by l, it becomes an inclusion containing titanium oxide, manganese oxide, silicon oxide, and alumina as the main components. If the total amount of Al is 0.001% or more, oxygen can be reduced to 20 ppm or less, CO gas is not generated, casting is possible, and generation of pores due to CO gas in the steel material can be suppressed. it can.

Next, the manufacturing method of the present invention will be described in detail. First, molten steel containing 0.02 to 0.10% C is melted in a converter. At this time, when C in the molten steel is higher than the desired C concentration, decarburization treatment is performed by a vacuum degassing device or the like after tapping to reduce the C concentration to a predetermined value. When the concentration is lower than the desired C concentration, C (carburizing agent) may be added after tapping to obtain a predetermined C concentration. In addition, Fe-Mn may be added as necessary when tapping molten steel. Next, one or two types of Mn and Si are added to the molten steel that has been tapped, or preliminary deoxidation by vacuum degassing treatment is performed to reduce oxygen in the molten steel to 250 ppm or less. It is necessary not to add Al before adding Ti in order not to generate simple substance alumina, and when deoxidizing with Mn and / or Si by adding Fe-Mn or Fe-Si. , M
The amount of n or Si added may be adjusted in consideration of the amount that increases from the amount of Si or Mn contained in the Ti alloy added during deoxidation. If the oxygen in the molten steel is higher than 250 ppm, it becomes necessary to add a large amount of Ti alloy, the degree of supersaturation at the time of deoxidation becomes large, as described later, and when Ti is added, a high melting point titanium oxide similar to alumina. May be generated in large numbers, and complex inclusions may not be stably formed, and these may aggregate to form large inclusions. In this way, in the molten steel in which the oxygen in the molten steel is adjusted to 250 ppm or less, a chemical composition of Ti: 10 to 70% by weight and the balance of Fe, Mn, and Si are 1 to 3 types and inevitable impurities. By adding an alloy consisting of
0.002 to 0.030% is contained as a molten steel component. By setting the Ti concentration in the molten steel to 0.030% or less in this way, it becomes possible to form a composite inclusion having a composition containing titanium oxide, manganese oxide, silica, and alumina as main components. If the Ti concentration in the molten steel is too high, the deoxidizing power of Ti is higher than that of Mn or Si, so it does not combine with these oxides, and titanium oxide with a high melting point similar to alumina becomes the main component inclusion. .

Further, if the degree of supersaturation at the time of deoxidation is reduced, the nucleation rate becomes slower, and the number of inclusions formed and the diameter of inclusions become smaller. Since the degree of supersaturation is determined by the product of Ti and oxygen, Ti in the deoxidized alloy can be used as a method for reducing the degree of supersaturation.
It is effective to reduce the content and oxygen in molten steel during deoxidation. If the Ti content in the deoxidized alloy is high, a portion with a high Ti concentration is generated around the deoxidized alloy added to the molten steel, and the degree of supersaturation increases, so use a deoxidized alloy with a low Ti content. Preferably. Therefore, an experiment in which the oxygen concentration in the molten steel and the Ti content in the alloy were changed
As a result of examination, as the oxygen concentration and the Ti content in the alloy decrease, the inclusion diameter decreases, and
It was found that inclusions of 50 μm or less at the maximum can be obtained by deoxidizing the alloy with the content of 50 ppm or less and the Ti content of 70% or less. Increasing the Ti content increases the diameter of the inclusions, and at the time of deoxidation, inclusions having a high content of titanium oxide are produced, which remain in the molten steel and are mixed. If the Ti content is too low, the amount of alloy to be added becomes too large and the temperature of the molten steel lowers, making it difficult to solidify or cast the molten steel, and it takes a long time to add and impairs the productivity. Further, when the Ti content is high, it is effective to add it little by little because the portion having a high degree of supersaturation is partially reduced. Further, by making Ti an alloy with Fe, Si, or Mn, the activity of Ti is reduced and the region where the concentration is high is partially reduced, so that the degree of supersaturation is further reduced, and titanium oxide and manganese oxide are oxidized. It promotes the formation of composite inclusions mainly consisting of materials, silica, and alumina. Ti may be added once, or may be added in two or more divided portions.
If Ti has a high oxygen content in the molten steel after the addition and CO gas may be generated during casting to cause boiling of the molten steel in the mold, Al may be added in an amount of 0.001% or more after the addition of Ti. Good. This reduces oxygen to 20 ppm or less, and enables casting without boiling due to CO gas generation. Since oxygen is reduced by deoxidation by adding Ti, the amount of Al added is smaller than that when Al is added before Ti addition, the amount of inclusions produced is small, and coarse inclusions are also produced. do not do. 0.005% Al
By the following, the inclusions are titanium oxides, silicon oxides, manganese oxides, and inclusions that exist in a liquid state in the molten steel containing 30% or less of alumina as the main component. The molten steel thus melted can be cast by a continuous casting machine through a tundish in the same manner as usual.

[0012] Finally, if the Mn content contained in the steel is reduced to less than 0.05%, the refining time will be long and the economy will be greatly impaired.
If it exceeds 0%, the workability of the steel sheet is significantly deteriorated, and high workability cannot be expected. The upper limit is 3.0%. If the amount of Si is reduced to less than 0.001%, sufficient pretreatment is required and a large cost burden is applied to refining and the economic efficiency is impaired. Since the property deteriorates significantly, the upper limit is 2.0%. P is
If the content is reduced to less than 0.001%, it takes time and cost to perform hot metal pretreatment, and economical efficiency is greatly impaired.
The lower limit is 1%, and if it exceeds 0.050%, the workability deteriorates, so 0.050% is the upper limit. S is 0.001
If the content is reduced to less than%, it takes time and cost for hot metal pretreatment, and economical efficiency is greatly impaired. Therefore, the lower limit is 0.001%, and if it exceeds 0.030%, workability and corrosion resistance deteriorate, so 0.030%. Is the upper limit. N is 0.0005
If it is less than 0.00%, the cost will be greatly increased in the refining stage and the economy will be greatly impaired. Therefore, 0.0005% is set as the lower limit, and if 0.0080% is exceeded, N for solid solution N is eliminated. Since a large amount of addition is required and the morphology control of inclusions, which is the object of the present application, becomes impossible, the upper limit is 0.0080%. In order to reduce the amount of solid solution N, N is changed to TiN
As stated above, to fix as at least Ti
[%]> 3.43 N [%] is required. further,
In order to improve the workability, it is preferable to add Ti in an amount equal to or more than C. In this case, the Ti addition amount is

## EQU1 ## Ti [%]> 4C [%] + 3.43N [%]. The upper limit of Ti is 0.040%. This is because if the amount exceeds this amount, a large amount of Ti must be added during deoxidation, and the composition of the inclusions, which is the feature of the present invention, cannot be obtained. Nb is added mainly for precipitating and fixing C in order to improve workability. The addition amount is preferably Nb [%] when Ti is not added.
> 6.64 N [%], when N is deposited and fixed by adding Ti, Nb> 0.8 × 7.75 C [%] is added.
As the lower limit of the addition amount, if it is less than 0.004%, the effect of improving the workability is lost, so 0.004% is the lower limit, and if it exceeds 0.050%, the workability is rather improved by the presence of solute Nb. As it will deteriorate, 0.05
The upper limit is 0%. B is added to prevent secondary processing brittleness. B is an element that is effective in preventing embrittlement called secondary work embrittlement, which is often seen when solid solution C existing at grain boundaries disappears, and the steel sheet of the present invention can be applied to parts to which severe drawing is applied. Add when applied. If the addition amount is less than 0.0004%, the effect of preventing secondary work embrittlement is lost, so 0.0004% is the lower limit, and if it exceeds 0.0070%, adverse effects such as an increase in the recrystallization temperature occur. Then, it becomes difficult to manufacture in a normal steel plate manufacturing process, so 0.0070% is made the upper limit.

The molten steel thus melted is passed through a tundish in the same manner as usual and cast by a continuous casting machine. Further, this slab is hot-rolled by the same method as usual and then wound in a temperature range of 600 ° C to 800 ° C. Even if the amount of Ti is simply added in an amount equal to or more than N, the total amount of N is Ti
Since it does not precipitate as N, the winding temperature is 600 ° C.
In the range of ˜800 ° C., N is precipitated as TiN as much as possible, and the amount of solute N is 2 ppm or less. Solid N content 2ppm
If it exceeds 5, the susceptibility to cracking will increase, cracks will occur even in inclusions with a size of about 50 μm, and the aging property will deteriorate, causing defects called stress trains. It should be 2 ppm or less. When the coiling temperature is lower than 600 ° C, precipitation and fixation of N by Ti is insufficient, and solid solution N exceeds 2 ppm,
Since the susceptibility to cracking due to inclusions deteriorates and the aging property of the product deteriorates, the lower limit is 600 ° C and 800 ° C
If it exceeds, the crystal grains become coarse and a high r value cannot be obtained after cold rolling annealing. Therefore, the upper limit is 800 ° C. Then,
Perform descaling. Generally, pickling is performed, but mechanical scale removal may be performed. Then, cold rolling is performed and continuous annealing is performed. The temperature of continuous annealing is 650 ° C to 90
The temperature is 0900 ° C. If the temperature is lower than 650 ° C, recrystallization does not occur and the workability deteriorates, so the lower limit is 650 ° C, and if it exceeds 900 ° C, the high-temperature strength of the steel sheet weakens, causing a phenomenon called drawing in the continuous annealing furnace and causing problems such as fracture. Therefore, the upper limit is 900 ° C. Then, skin pass rolling is performed to obtain a steel plate. Further, thereafter, plating, resin coating or the like may be applied for corrosion resistance and design. The continuous annealing may be performed in a hot dip galvanizing line, or immediately after the annealing, hot-dip galvanized steel sheet such as hot-dip galvanized steel sheet, alloyed hot-dip galvanized steel sheet, hot-dip aluminized steel sheet or the like can be obtained. .

[0014]

EXAMPLE Steels having the components shown in Table 1 were melted in a 270 ton converter. Further, this molten steel was subjected to vacuum degassing treatment and the deoxidizer shown in Table 2 was added to adjust the oxygen concentration and the composition of inclusions. Then, it was continuously cast into a steel ingot. The composition of the obtained steel and the composition and size of the inclusions in the ingot are shown in Table 3 and Table 4 together. Then, hot rolling was performed. Finish rolling and winding at that time were performed at the temperatures shown in Table 5.
Then, after pickling and cold rolling, annealing was performed at the temperatures shown in Table 5. Table 5 also shows the recrystallization temperature measured before the annealing. After annealing, skin pass rolling was performed at a reduction rate of 1% to obtain a cold rolled steel sheet. Using the obtained cold-rolled steel sheet, about 10,000 pieces were processed at a drawing ratio of 2.2. The number of cracks caused by inclusions generated at this time was investigated, and the defect occurrence rate during pressing was determined. The results are shown in Table 5. From Table 5, according to the method of the present invention, the recrystallization temperature is lower than that of the comparative method, and mechanical property values equivalent to those of the comparative material are obtained even at a low annealing temperature. Further, it can be seen that the method of the present invention has an extremely low defect occurrence rate during pressing as compared with the comparative method.

[0015]

EFFECTS OF THE INVENTION According to the present invention, inclusions in steel can be made finer to significantly reduce defects due to inclusions during manufacturing, and the recrystallization temperature can be lowered. It is possible to obtain an excellent effect such that it can be annealed at a temperature and a press forming steel sheet having excellent quality and economical efficiency and having few defects can be obtained.

[0016]

[Table 1]

[0017]

[Table 2]

[0018]

[Table 3]

[0019]

[Table 4]

[0020]

[Table 5]

Claims (9)

(57) [Claims] 1. In weight percent, C: 0.0001 to 0.0070%, Si: 0.001-2.0%, Mn: 0.05 to 3.0%, P: 0.001 to 0.150%, S: 0.001 to 0.050%, N: 0.0005 to 0.0080%, Acid-soluble Al: 0.005% or less, Ti: 0.004 to 0.040% Steel containing iron and the balance iron and inevitable impuritiesDuring ~
To, JTan oxide, manganese oxide, silicon oxide,
AluminaIs compounded and30% or less of aluminaIs
Oxide systemcompositeDeficiency characterized by containing inclusions
Thin steel plate with few pits and excellent press formability. 2. A thin steel sheet having few defects and excellent in press formability according to claim 1, wherein Nb: 0.004 to 0.050% is contained. 3. A thin steel sheet having few defects and excellent press formability according to claim 1, wherein B: 0.0004 to 0.0050% is contained. 4. The amount of oxygen in the molten steel after refining is set to 250 pp.
deoxidized to m or less, then TiAfter adding AlAddition
Then C: 0.0001 to 0.0070%, Si: 0.0
01-2.0%, Mn: 0.05-3.0%, P: 0.
001 to 0.150%, S: 0.001 to 0.050
%, N: 0.0005 to 0.0080%, acid-soluble Al:
0.005% or less, Ti: 0.004 to 0.040%
Contains, balance iron and unavoidable impuritiesMeltingsteelDuring ~
To, JTan oxide, manganese oxide, silicon oxide,
AluminaIs compounded and30% or less of aluminaIs
Oxide systemcompositeIncluded inclusionsMeltingContinuous casting of steel-heat
After hot rolling, wind at 600 ℃ ~ 800 ℃, and then remove
Scaled and then rolled at 50% to 95%
Cold rolling and annealing in the temperature range of 650-900 ° C
Excellent press formability with few defects characterized by being applied
Method for manufacturing thin steel sheet. 5. Nb: 0.004 to 0.050% and / or
Alternatively, B: 0.0004 to 0.0050% is contained, and the method for producing a thin steel sheet having few defects and excellent in press formability according to claim 4. 6. The press formability according to claim 4, wherein the molten steel after refining is subjected to vacuum degassing treatment to deoxidize the oxygen content in the steel to 250 ppm or less, and then Ti is added. Excellent manufacturing method for thin steel sheet. 7. Addition of one or two of Si and Mn,
The method for producing a thin steel sheet having few defects and excellent press formability according to claim 4, wherein Ti is added after deoxidizing the oxygen content in the steel to 250 ppm or less. 8. The amount of oxygen in the molten steel after refining is 250 ppm.
After deoxidation to the following, an alloy having a chemical composition of Ti: 10 to 75%, the remaining one to three kinds of Fe, Mn and Si, and inevitable impurities is added. 5. A method for producing a thin steel sheet having few defects described above and excellent in press formability. 9. A chemical composition of Al: 1 when Al is added.
The defect according to claim 4 or claim 5 or claim 6, characterized in that an alloy comprising 0 to 80% by weight, the remainder of 1 to 3 of Fe, Mn and Si and unavoidable impurities is added. A method for manufacturing a thin steel sheet with less press formability and excellent press formability.