JPH09316619A - Production of high strength hot dip galvanized steel sheet excellent in deep drawability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a high strength hot dip galvanized steel sheet having high strength, excellent in press formability and deep drawability and used for an automotive inner sheet or the like. SOLUTION: A steel slab having a compsn. contg., by weight, 0.001 to 0.01% C, 0.05 to 0.20% P, 1.0 to 4.0% Mn, 0.005 to 0.5% Mo, 0.5 to 2.5% Cu, 0.005 to 0.1% Al and 0.0005 to 0.003% B, contg. 2.0% Si, 1.3% Ni, 0.02% S and <=0.007% N and furthermore contg. one or more kinds among Ti: [(48/12)×%C+(48/14)×%N+(48/32)×%S] to 0.10%, 0.01 to 0.1% Nb and 0.01 to 0.1% V is subjected to hot rolling so as to regulate the hot rolling finishing temp. to the Ar3 transformation point or above, the average cooling rate after the finish rolling to >=20 deg.C/sec and the hot rolling coiling temp. to 450 to 650 deg.C. After cold rolling, it is hot dip galvanized in continuous galvanizing equipment so as to regulate the annealing temp. to 800 to 920 deg.C, the average rate from the annealing temp. to the precipitation treating temp. to 5 to 120 deg.C/sec, the recrystallization annealing temp. to 500 to 650 deg.C, its time to 0.5 to 5min and the inlet side temp,. to 500 to 550 deg.C.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-strength hot-dip galvanized steel sheet which has high strength and is excellent in press formability and deep drawability and is used as an inner plate for automobiles.

[0002]

2. Description of the Related Art As steel plates for automobiles, high-strength steel plates are widely used because of their safety, reduction of vehicle body weight, reduction of material usage, and the like. Since this type of automobile steel sheet undergoes severe forming, it is required to have a high Rankford value, which is an index of good press workability, particularly deep drawability. Many proposals have been made for high-strength cold-rolled steel sheets. For example, in JP-A-62-205231, 490 N / mm ^{2 is} obtained by adding Si, Mn and P to low carbon steel and optimizing the conditions of hot rolling.
It has a high level of strength above the grade. However, the Rankford value was only about 1.0, and the steel material was inferior in deep drawability. As a means for improving the deep drawability, Japanese Examined Patent Publication No. 62-3
According to Japanese Patent No. 4804, a small amount of Mn is added on the basis of an ultra low carbon steel to which Ti is added, and cold rolling and annealing conditions are optimized. When using this method, about 2.0
Rank Ford value is obtained, but the strength is 390 N / m
It is as low as m ² or less. Further, since a steel sheet for automobiles is required to have corrosion resistance, a hot-dip galvanized steel sheet is also proposed in JP-A-62-260046. However, the hot-dip galvanized steel sheet manufactured by the conventional method has a low strength of 390 N / mm ² or less, and does not sufficiently satisfy the properties required as a steel sheet for automobiles.

[0003]

When the strength of a steel sheet is increased by the conventional method, the Rankford value becomes low and the formability of the steel sheet deteriorates. On the contrary, when the Rankford value is increased to improve the moldability, the strength tends to be insufficient. A method for improving both the strength and the Rankford value, which show such contradictory tendencies, has not been put to practical use so far. Therefore, it is unavoidable to select a steel material with emphasis on either strength or Rankford value. The present invention has been devised to solve such a problem, and by controlling the morphology of Cu precipitates and the metallographic structure of hot-rolled sheet, which greatly influence the strength and the Rankford value, Required as steel plate for etc. 49
It is an object of the present invention to obtain a high-strength hot-dip galvanized steel sheet having a high strength of 0 N / mm ² or more and a Rank Ford value of 1.4 or more and excellent in deep drawability.

[0004]

In order to achieve the object of the method for producing a high strength cold rolled steel sheet according to the present invention, C: 0.001
.About.0.01 wt%, Si: 2.0 wt% or less, P: 0.
05 to 0.20% by weight, Mn: 1.0 to 4.0% by weight,
Mo: 0.005-0.5 wt%, Cu: 0.5-2.
5% by weight, Ni: 1.3% by weight or less, S: 0.02% by weight or less, Al: 0.005-0.1% by weight, N: 0.0
07% by weight or less, B: 0.0005 to 0.003% by weight
In addition, Ti: [(48/12) ×% C + (48 /
14) x% N + (48/32) x% S] to 0.10 wt%, Nb: 0.01 to 0.1 wt% and V: 0.01 to
A steel slab containing 0.1% by weight of one or two or more kinds, a hot rolling finish temperature of Ar ₃ transformation point or more, an average cooling rate after finish rolling of 20 ° C./sec or more, and a hot rolling coiling temperature of 450. ~
After performing hot rolling at 650 ° C. and cold rolling, the annealing temperature is 800 to 920 ° C. in a continuous hot dip galvanizing facility, and the average cooling rate from the annealing temperature to the precipitation treatment temperature is 5 to 120 ° C.
/ Sec recrystallization annealing and processing temperature 500-650 ° C,
It is characterized in that after a Cu precipitation heat treatment for a treatment time of 0.5 to 5 minutes, hot dip galvanizing is performed at an inlet temperature of 500 to 550 ° C. Similarly, steel slabs having the same composition are similarly recrystallized by continuous annealing equipment, temper-rolled, and Cu-treated by continuous hot dip galvanizing equipment at a treatment temperature of 500 to 650 ° C. and a treatment time of 0.5 to 5 minutes. It is also possible to perform a precipitation heat treatment and then perform hot dip galvanizing at an inlet temperature of 500 to 550 ° C.

[0005]

The present inventors investigated the influence of the morphology of Cu precipitates and the metal structure on the strength and Rankford value of an ultra-low carbon steel containing Cu which is a precipitation strengthening element. As a result, the ultra low carbon steel containing Cu as a precipitation strengthening element and Mn and Mo as elements for improving hardenability are subjected to hot rolling and annealing under appropriate conditions to obtain 490
It has been found that a steel sheet having a high strength of N / mm ² or more and a Rankford value of 1.4 or more can be obtained. That is, the elements Mn and Mo that improve the hardenability are contained in an ultra-low carbon steel containing Cu, and hot rolling is performed under the conditions where the hot rolling finishing temperature, the average cooling rate after finish rolling, and the hot rolling coiling temperature are specified. Then, a strong hot rolled texture is formed. This strong hot-rolled texture promotes the development of cold-rolled texture and further enhances the degree of accumulation of the recrystallized texture of {554} <225> orientation formed during annealing.

Further, since the hot rolling coiling temperature is set in a temperature range in which the precipitation of Cu precipitates is promoted, coarse Cu precipitates having a grain size of about 0.1 μm are deposited during hot rolling coiling. . The coarsened Cu precipitate has a function of further improving the degree of integration in the {554} <225> orientation, which is a recrystallization texture during annealing. The coarsened Cu precipitate during hot rolling is subjected to high temperature annealing after cold rolling and is once solid-dissolved in the matrix. By controlling the cooling rate after soaking, the precipitation treatment temperature, and the precipitation treatment time, the solid-soluted Cu is finely precipitated in an extremely short time to enhance the strength of the steel sheet. Also,
When the annealing temperature is set to the Ac ₁ transformation point or higher, the microstructure of the cold-rolled annealed sheet becomes a composite structure of polygonal ferrite and pseudo bainite or a single phase structure of pseudo bainite, and the strength is further enhanced. Furthermore, by specifying the inlet temperature immediately before the hot dip galvanizing treatment, surface texture defects on the steel sheet surface are eliminated.

The alloy components, content, manufacturing conditions, etc. contained in the steel material used in the present invention will be described below. C: 0.001 to 0.01% by weight A lower C content is preferable for improving deep drawability and ductility, and carbonitrides such as TiC and NbC and Ti ₄ C ₂ S _{2 are used.}
It becomes fixed as carbosulfide. Further, when the C content is lowered, the addition amount of Ti, Nb or the like necessary for fixing C can be reduced. When the C content exceeds 0.01% by weight, the amount of Ti, Nb, etc. necessary for fixing C is remarkably increased, which not only increases the cost but also adversely affects the Rankford value. However, in order to make the C content lower than 0.001% by weight, the manufacturing cost in the manufacturing process increases. Si: 2.0 wt% or less It is an alloying element effective in increasing the strength of the steel sheet. However, if the Si content exceeds 2.0% by weight, the ductility and Rankford value tend to decrease. When the Si content is 0.5% by weight or more, the hot-dip galvanizing property is deteriorated and defects such as non-plating are likely to occur. The deterioration of the hot-dip galvanizing property that causes the generation of defects of this type is eliminated by performing electroplating after cold rolling.

P: 0.05 to 0.20% by weight It has the effects of improving the strength of the steel sheet and improving the corrosion resistance of the steel sheet. In order to express these effects,
A P content of 0.05% by weight or more is required. But,
When a large amount of P exceeding 0.20% by weight is contained, secondary work cracking is significantly promoted. Mn: 1.0 to 4.0% by weight It is an alloying element that improves quench hardenability. Also, Mn
Even if the cooling rate after finish rolling is relatively small due to the inclusion, the microstructure of the hot-rolled sheet exhibits a pseudo-bainite structure, and a strong hot-rolled texture is formed with the formation of the pseudo-bainite structure. This strong hot-rolled texture promotes the development of cold-rolled texture and further enhances the degree of integration of the recrystallized texture having the {554} <225> orientation formed during annealing. In order to obtain this effect, it is necessary to contain 1.0% by weight or more of Mn, but if a large amount of Mn is contained in excess of 4.0% by weight, the ductility is greatly reduced.

Mo: 0.005 to 0.5% by weight Like Mn, it has the effect of improving the quench hardenability.
Further, even if the cooling rate after finish rolling is relatively small due to the inclusion of Mo, the microstructure of the hot-rolled sheet exhibits a pseudo-bainite structure and a strong hot-rolled texture is formed due to the formation of the pseudo-bainite structure. This strong hot-rolled texture promotes the development of cold-rolled texture and is formed during annealing {554} <
The degree of accumulation of recrystallized texture having a 225> orientation is increased.
To obtain this effect, it is necessary to contain 0.005% by weight or more of Mo, but if a large amount of Mo is contained in excess of 0.5% by weight, the ductility is greatly reduced. Cu: 0.5 to 2.5% by weight An alloying element effective in increasing the strength of the steel sheet, and 0.5
The effect becomes remarkable when the content is more than wt%. But,
If a large amount of Cu exceeding 2.5% by weight is contained, the ductility is greatly reduced. The preferable range of Cu content is 1.0
~ 2.0% by weight.

Ni: 1.3 wt% or less In Cu-added steel, it is an alloy component added as necessary to prevent hot red hot brittleness. Generally, Cu
It is preferable to add 1/2 or more of the addition amount of Ni.
However, since it is an extremely expensive element, the upper limit is set to 1.3% by weight in order to suppress an increase in steel material cost. S: 0.02 wt% or less A harmful element that binds to Mn to form a non-metallic inclusion and causes defects such as work cracks during press working. Therefore, in the present invention, the upper limit of the S content is restricted to 0.02% by weight. Al: 0.005 to 0.1% by weight It is an alloying element added as a deoxidizer, and needs to be 0.005% by weight or more. However, a large amount of Al content exceeding 0.1% by weight is a cause of increasing inclusions such as Al ₂ O ₃ and deteriorates workability and surface quality.

N: 0.007% by weight or less If left as solid solution N, it is a harmful element that deteriorates deep drawability. Therefore, it is required to precipitate as TiN and reduce the solid solution N. However, as the N content increases, the TiN precipitation amount increases and suppresses the development of {554} recrystallization texture. Therefore, in the present invention, the upper limit of the N content is specified to be 0.007% by weight. B: 0.0005 to 0.003 wt% Segregates at crystal grain boundaries preferentially over P, and exhibits an effect of suppressing grain boundary embrittlement due to P. This effect is exhibited when the content of B is 0.0005% by weight or more. However, when a large amount of B exceeds 0.003% by weight, grain growth is hindered, and the Rankford value and the ductility of the steel sheet deteriorate.

Ti: [(48/12) ×% C + (48 /
14) ×% N + (48/32) ×% S] to 0.10% by weight It has an effect of fixing S, N and C, which are detrimental to the development of recrystallized texture of {554} <225> orientation. The lower limit of the Ti content is [(48 /
12) x% C + (48/14) x% N + (48/32)
X% S]. However, the effect of addition of Ti is saturated at 0.10% by weight, and even if it is added more than that, the effect commensurate with the increase of Ti is not seen. Nb, V: 0.01 to 0.1% by weight It acts to fix S, N and C, and promotes the development of a recrystallized texture having a {554} orientation. The action of Nb and V appears at a content of 0.01% by weight or more,
It saturates at 0% by weight, and even if it is added more than that, the effect commensurate with the increase in amount is not seen. The steel targeted by the present invention is manufactured into a slab by continuous casting after degassing the molten steel adjusted to have a predetermined composition in a converter, an electric furnace or the like in an RH facility or the like.
This slab is directly sent as it is, or is once cooled to be a cold piece and then reheated, and then sent to the hot rolling step.

Hot rolling: In hot rolling, the hot rolling finish temperature is higher than the Ar ₃ transformation point and the average cooling rate after finish rolling is 2
The hot rolling coiling temperature is set to 450 to 650 ° C at 0 ° C / sec or more. By controlling the hot rolling conditions in this way, a strong hot rolled texture is formed. The strong hot-rolled texture promotes the development of the cold-rolled texture, and also has the effect of increasing the degree of accumulation of the recrystallized texture having the {554} <225> orientation formed during annealing. Further, since the hot rolling coiling temperature is set in a temperature range in which the precipitation of Cu is promoted, a coarse Cu precipitate having a particle size of about 0.1 μm is deposited during the hot rolling coiling. The coarsened Cu precipitates further increase the degree of integration in the {554} <225> orientation, which is a recrystallization texture during annealing.

Hot-dip galvanizing: The hot-rolled steel sheet is pickled and cold-rolled under normal conditions and sent to an in-line annealing type continuous hot-dip galvanizing line. 0.5 wt% Si
The cold-rolled steel sheet containing the above is subjected to electroplating in order to improve the hot-dip galvanizing property, followed by recrystallization annealing and Cu precipitation treatment, and then hot-dip galvanizing. The electroplating prevents plating defects such as non-plating due to the oxide of the easily oxidizable element Si generated during recrystallization annealing and Cu precipitation. Specifically, a pre-plated layer of Ni, Fe, Fe-B, Fe-P or the like is formed by electroplating. In the annealing step, the recrystallization texture effective for the Rankford value is generated, and the coarse Cu precipitate is re-dissolved, so that the annealing temperature is 8
The temperature is set to 00 ° C or higher. In particular, when the annealing temperature is set to the A _c1 transformation point or higher, the microstructure of the cold-rolled annealed sheet exhibits a composite structure of polygonal ferrite and pseudo bainite or a single phase structure of pseudo bainite, so that further strengthening can be achieved. . However, if the annealing temperature exceeds 920 ° C, production using ordinary equipment becomes difficult. The cooling rate after soaking is
To prevent the formation of coarse Cu precipitates during the cooling process, 5
℃ / sec or more is set. However, if the cooling rate is higher than 120 ° C./sec, production using ordinary equipment becomes difficult.

The annealed steel sheet has a temperature of 500 to 650 ° C. and a time of 0.5 to 5 minutes in order to prevent solid solution and coarse precipitation of Cu and precipitate fine Cu precipitates in a short time. Cu precipitation treatment is performed. Thereby, the strength of the steel material is increased. Further, by setting the precipitation treatment temperature to 500 ° C. or higher, the inlet temperature can be kept high in the next hot-dip galvanizing treatment, and the hot-dip galvanizing property is also improved. The temperature on the inlet side to the hot dip plating bath is maintained in the range of 500 to 550 ° C. When the inlet side temperature is lower than 500 ° C., the molten zinc wettability of the steel sheet decreases, and defects such as pinholes and non-plating tend to occur on the steel sheet surface. But 55
At an inlet temperature exceeding 0 ° C, the powdering resistance of the alloyed steel sheet deteriorates. Even if the recrystallization annealing described above is performed in the continuous annealing equipment and the Cu precipitation treatment and the hot dip galvanizing are performed in the continuous hot dip galvanizing equipment, the steel sheet having the required properties can be manufactured.

[0016]

【Example】

Example 1: Steels of steel type numbers 1 to 16 having the compositions shown in Table 1 were melted, and the slab heating temperature was 1250 ° C. and the finishing temperature was 920.
C., hot rolling was performed under the conditions of an average cooling rate from finishing temperature to hot rolling coiling temperature of 20.degree. C./sec and hot rolling coiling temperature of 550.degree. C. to produce a hot rolled sheet having a plate thickness of 4 mm.

[0017]

After pickling the obtained hot rolled sheet, the sheet thickness is 1 mm.
Cold rolling was performed at a rolling rate of 75%. In addition, about steel type number 4,6,10,12 which contains 0.5 weight% or more of Si, Fe-B electroplating was given in order to improve hot dip galvanizing property. Then, recrystallization annealing was performed in a continuous hot dip galvanizing facility under the conditions of an annealing temperature of 850 ° C., a soaking time of 60 seconds, and an average cooling rate of 40 ° C./second from the annealing temperature to the precipitation treatment temperature, and subsequently the precipitation treatment temperature. A Cu precipitation treatment was performed at 550 ° C. for a precipitation treatment time of 1 minute. Then, temper rolling with an elongation of about 1% was performed, and a tensile test was performed on JIS No. 5 test pieces. As can be seen in Table 2 showing the test results, the steels of steel numbers 1 to 12 according to the present invention are 490 N / mm.
Exhibits a strength of ² or more and has an excellent balance of strength and ductility.
Had a high Rank Ford value of 4 or more. On the other hand, in steels of steel type numbers 13 to 16, Mn, Mo, Cu,
Since the contents of C and Ti were out of the range specified in the present invention, the Rankford value was greatly reduced.

[0019]

Example 2: Steel type numbers 2, 7, 9, 1 in Table 1
Hot-rolled sheets were produced from 3,15 steel under the same conditions as in Example 1. The obtained hot-rolled sheet was pickled and cold-rolled to a sheet thickness of 1 mm at a rolling rate of 75%. Then, after performing a treatment at an annealing temperature of 850 ° C. and a soaking time of 60 seconds in a continuous annealing facility,
Cooling was performed from the annealing temperature to 200 ° C or lower at an average cooling rate of 40 ° C / sec. After that, temper rolling with an elongation of 1% is performed, Cu is subjected to a precipitation treatment temperature of 550 ° C. and a precipitation treatment time of 1 minute in a continuous hot dip galvanizing facility, and an inlet side temperature of 520 ° C.
Hot-dip galvanized. Further, temper rolling with an elongation rate of about 0.5% was performed, and a tensile test was performed on JIS No. 5 test pieces.
As can be seen in Table 3 showing the test results, the steel grade numbers 2, 7, 9 according to the present invention show a strength of 490 N / mm ² or more, an excellent balance of strength and ductility, and a high rank of 1.4 or more. Had a Ford value. On the other hand, steel type number 1
In Comparative Examples 3 and 15, the Mn, Mo, and Cu contents were out of the ranges specified in the present invention, so that the Rankford value was significantly reduced.

[0021]

Example 3: Using steel No. 5 shown in Table 1, hot rolling was performed under the conditions shown in Table 4, pickling, cold rolling at a rolling ratio of 75%, and then Table 4 Hot-dip galvanizing was performed under the conditions of. Then, temper rolling with an elongation of about 1%
A tensile test was performed on the IS5 test piece. As shown in Table 5 showing the test results, the steel sheets of Group A manufactured according to the present invention show a strength of 490 N / mm ² or more,
It had a high Rank Ford value of 1.4 or higher. On the other hand, in the steel sheets (B1, B2) of Group B, the hot rolling conditions or the hot dip galvanizing conditions were out of the ranges specified in the present invention, so that the Rankford value was greatly reduced. In the steel sheets B3 and B4, the hot rolling conditions or the annealing conditions were out of the range regulated by the present invention, so that the strength was greatly reduced. In the steel sheet of B5, the inlet temperature was out of the range specified in the present invention, so that non-plating occurred on the steel sheet surface.

[0023]

[0024]

[0025]

As described above, in the present invention, the hardenability improving elements Mn and Mo and the precipitation strengthening element Cu are used.
Is added, and hot rolling conditions, annealing conditions, and hot dip galvanizing conditions are set to form {554} during annealing.
Increase the degree of recrystallization texture with <225> orientation,
We manufacture hot-dip galvanized steel sheets with high strength and high Rankford value. The high-strength hot-dip galvanized steel sheet thus obtained is used as a structural material, a member, etc. in a wide range of fields including steel sheets for automobiles by utilizing excellent workability such as deep drawability.

Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI Technical display area C23C 2/02 C23C 2/02

Claims (2)

[Claims] 1. C: 0.001-0.01% by weight, S
i: 2.0 wt% or less, P: 0.05 to 0.20 wt%, Mn: 1.0 to 4.0 wt%, Mo: 0.005 to
0.5% by weight, Cu: 0.5 to 2.5% by weight, Ni:
1.3% by weight or less, S: 0.02% by weight or less, Al:
0.005 to 0.1% by weight, N: 0.007% by weight or less, B: 0.0005 to 0.003% by weight, and further Ti: [(48/12) ×% C + (48/14) ×% N
+ (48/32) ×% S] to 0.10% by weight, Nb:
0.01 to 0.1% by weight and V: 0.01 to 0.1% by weight of steel slab containing one or more kinds, hot rolling finish temperature of Ar ₃ transformation point or higher, average after finish rolling. Hot rolling is performed at a cooling rate of 20 ° C / sec or more and a hot rolling coiling temperature of 450 to 650 ° C, and after cold rolling, precipitation is performed at an annealing temperature of 800 to 920 ° C and an annealing temperature in a continuous hot dip galvanizing facility. After performing recrystallization annealing at an average cooling rate up to the processing temperature of 5 to 120 ° C./second and Cu precipitation heat treatment at a processing temperature of 500 to 650 ° C. for a processing time of 0.5 to 5 minutes, the steel sheet inlet temperature 500 A method for producing a high-strength hot-dip galvanized steel sheet having excellent deep drawability, which is hot-dip galvanized at 550C. 2. A steel slab having the composition according to claim 1, wherein the hot rolling finish temperature is Ar ₃ transformation point or higher, the average cooling rate after finish rolling is 20 ° C./sec or higher, and the hot rolling coiling temperature is 450-6.
Hot rolling is performed at 50 ° C, and after cold rolling, the annealing temperature is 800 to 920 ° C from the annealing temperature to 200 ° C in the continuous annealing equipment.
Recrystallization annealing was performed at an average cooling rate up to the following of 5 to 120 ° C./second, and after temper rolling, treatment temperature was 500 to 650 ° C. and treatment time was 0.5 to 5 minutes in a continuous hot dip galvanizing facility. Precipitation heat treatment is applied, and then the inlet temperature is 500 to 550.
A method for producing a high-strength hot-dip galvanized steel sheet having excellent deep drawability, which is hot dip galvanized at ℃.