JPH09209039A - Production of high strength cold rolled steel sheet excellent in deep drawability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a high strength cold rolled steel sheet having high strength, furthermore excellent in press formability and deep drawability and used as an automobile inner sheet or the like. SOLUTION: A steel slab having a compsn. contg., by weight, 0.001 to 0.01% C, <=2.0% Si, 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 to 1.0% Ni, <=0.02% S 0.005 to 0.1% Al, <=0.007% N and 0.0005 to 0.003% B 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 in such a manner that the hot rolling finishing temp. is regulated to the Ar3 transformation point or above, the average cooling rate after the finish rolling is regulated to >=20 deg.C/sec and the hot rolling coiling temp. is regulated to 450 to 650 deg.C, is subjected to cold rolling, is subjected to recrystallization annealing in such a manner that the average cooling rate to a precipitation treating temp. after annealing at 800 to 920 deg.C is regulated to 5 to 120 deg.C/sec and is successively subjected to precipitation heat treatment at 400 to 650 deg.C for 0.5 to 10min.

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 cold-rolled steel sheet which has high strength and is excellent in press formability and deep drawability and which 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. In this regard, Japanese Patent Publication No. 62-34804 discloses an ultra-low carbon steel with Ti added as a base and a small amount of M.
The deep drawability is improved by adding n and optimizing the cold rolling and annealing conditions. When using this method,
A Rankford value of about 2.0 is obtained, but strength is 39
It is as low as 0 N / mm ² or less. In addition, Japanese Unexamined Patent Application Publication No.
Japanese Patent No. 173242 discloses a cold-rolled steel sheet for working in which Mn and P are added to an ultra-low carbon steel in which Ti and Nb are added together. However, the strength of the steel sheet manufactured by this method is also as low as 440 N / mm ² or less.

[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
An object is to obtain a high strength cold rolled 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: 0 to 1.0% by weight, S: 0.02% by weight or less, Al: 0.005 to 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. ~
Hot rolling is performed at 650 ° C., cold rolling is carried into the continuous annealing equipment, and an annealing temperature of 800 to 920 ° C. and an average cooling rate from the annealing temperature to the precipitation treatment temperature of 5 to 120 ° C./sec are used again. Crystal annealing is applied, followed by a processing temperature of 400 to 65
It is characterized in that precipitation heat treatment is performed at 0 ° C. for a treatment time of 0.5 to 10 minutes. Further, steel slabs having the same composition may be similarly subjected to recrystallization annealing and then subjected to precipitation heat treatment at a treatment temperature of 300 to 650 ° C. and a treatment time of 0.5 minutes to 5 hours.
Between the recrystallization annealing and the precipitation heat treatment, it is also possible to subject the steel sheet to temper rolling or product processing.

[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. The steel composition targeted by the present invention includes Mn and Mo for improving hardenability in an extremely low carbon steel containing Cu. When controlling the hot rolling finish temperature, the average cooling rate after finish rolling, and the hot rolling coiling temperature of this steel, a strong hot rolling texture is formed. Hot rolled texture promotes the development of cold rolled texture,
Further, the degree of integration of the recrystallized texture of {554} <225> orientation formed during annealing is increased.

Further, since the hot rolling coiling temperature is set in the temperature range in which the precipitation of Cu precipitates is promoted, coarse Cu precipitates are deposited during the hot rolling coiling. The coarsened Cu precipitate has a recrystallization texture during annealing {554} <22.
5> It has the effect of further improving the degree of integration in the direction. The coarsened Cu precipitate during hot rolling is subjected to high temperature annealing after cold rolling and is once solid-dissolved in the matrix. Cu in solid solution
Controls the cooling rate after soaking, the precipitation treatment temperature, and the precipitation treatment time to precipitate as a fine compound on the cold-rolled annealed sheet, thereby increasing the strength of the steel sheet. Also, the annealing temperature is set to Ac
When the temperature is set at 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.

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.

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 with 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: 0 to 1.0 wt% In a 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 1.0% by weight in order to suppress an increase in steel material cost even when Ni is added. 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 also 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. When Ti is added, the lower limit of the content is [(48/12) ×% C + (48/14) ×% in order to fix S, N and C.
N + (48/32) ×% S]. However, Ti
The effect of addition is saturated at 0.10% by weight, and even if added more than that, the effect commensurate with the Ti increase is not seen. Nb, V: 0.01 to 0.1% by weight It acts to fix S, N and C, and promotes the development element of the recrystallized texture having {554} orientation. The action of Nb and V is exhibited at a content of 0.01% by weight or more, but
It is saturated at 10% by weight, and even if it is added more than that, the effect commensurate with the increase in amount is not observed. Steel targeted by the present invention is a converter,
Molten steel adjusted to a predetermined composition in an electric furnace or the like is degassed by an RH facility or the like, and then manufactured into a slab by continuous casting. 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 the temperature range where the precipitation of Cu is promoted, coarse Cu precipitates are deposited during the hot rolling coiling. Coarse C
The u precipitate is a recrystallization texture during annealing {554}
Further increase the degree of integration in the <225> direction.

Recrystallization annealing: The steel sheet after hot rolling is pickled and cold rolled under normal conditions and sent to a continuous annealing step. The annealing temperature is set to 800 ° C. or higher in order to generate a recrystallized texture effective for the Rankford value and to re-dissolve the coarse Cu precipitate in the matrix.
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, and the strength is further enhanced. However, at a high annealing temperature exceeding 920 ° C., it becomes difficult to produce using a normal continuous annealing equipment. The steel sheet soaked to the annealing temperature is cooled at a rate of 5 ° C./sec or more in order to prevent the formation of coarse Cu precipitates in the cooling process. However, the cooling rate is 120 ° C /
If it exceeds the second, production using a normal continuous annealing equipment becomes difficult.

Precipitation heat treatment: For Cu precipitation treatment, a step of performing recrystallization annealing in a continuous annealing equipment, followed by cooling to the precipitation treatment temperature and performing the precipitation treatment, recrystallization annealing in the continuous annealing theory section, and up to 200 ° C. or less After cooling, a step of performing precipitation treatment with continuous annealing / batch annealing equipment, and recrystallization annealing with continuous annealing equipment, cooling to 200 ° C or less, temper rolling or forming processing of the steel sheet, and then continuous annealing -It is performed in the process of performing precipitation treatment in batch annealing equipment. The precipitation treatment condition is that the precipitation treatment temperature is 4 when the continuous annealing type equipment is used.
0 to 650 ° C, precipitation treatment time 0.5 to 10 minutes, precipitation treatment temperature 30 when using batch annealing type equipment
The temperature is set to 0 to 650 ° C. and the precipitation treatment temperature is set to 0.5 minutes to 5 hours. The conditions for the precipitation treatment are set in such a range in order to prevent solid solution of Cu in steel and precipitation of coarse Cu precipitates.

[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%. Next, recrystallization annealing is performed under 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, a precipitation treatment temperature of 550 ° C. and a precipitation treatment time of 1 Heat treatment for minutes was performed. 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 from Table 2 showing the test results, the steels of steel types Nos. 1 to 12 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 the steels Nos. 3 to 16, the Mn, Mo, Cu, C, and Ti contents were out of the ranges specified in the present invention, so that 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, the annealing temperature 850
℃, soaking time 60 seconds, after applying the annealing temperature 2
It was cooled to below 00 ° C at an average cooling rate of 40 ° C / sec. Then, precipitation heat treatment was performed at 500 ° C. for 3 hours. 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 3 which shows the test results, the steel grade numbers 2, 7, 9 according to the invention are 490
It exhibited a strength of N / mm ² or more, an excellent balance of strength and ductility, and a high Rank Ford value of 1.4 or more.
On the other hand, in the comparative examples of steel type numbers 13 and 15, Mn,
Since the contents of Mo and Cu were out of the range specified in the present invention, the Rankford value was greatly 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 The recrystallization annealing and the precipitation heat treatment were performed under the above conditions. Next, 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 5 which shows the test results, the steel sheets of Group A produced according to the present invention have 490 N /
It exhibited a strength of mm ² or more and a high Rank Ford value of 1.4 or more. On the other hand, in the steel sheets (B1 and B2) of the B group, the hot rolling conditions or the annealing conditions were out of the ranges specified in the present invention, and thus the Rankford value was greatly reduced. In the steel sheets B3 and B4, the precipitation heat treatment conditions were out of the range regulated by the present invention, so that the strength was greatly reduced.

[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 the hot rolling conditions and annealing conditions are set to increase the degree of accumulation of the recrystallized texture having the {554} <225> orientation formed during annealing, and a high strength and high Rank Ford value steel sheet. Are manufactured. The high-strength steel sheet obtained in this manner makes use of excellent workability such as deep drawability, and is used as a structural material in a wide range of fields including steel sheets for automobiles.
It is used as a member.

Claims (3)

[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: 0
.About.1.0 wt%, S: 0.02 wt% 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 T
i: [(48/12) ×% C + (48/14) ×% N +
(48/32) ×% S] to 0.10% by weight, Nb: 0.
01-0.1% by weight and V: 0.01-0.1% by weight of steel slab containing one or more kinds, hot rolling finish temperature is Ar ₃ transformation point or higher, average cooling rate after finish rolling. 20
C./sec or more, hot rolling at a hot rolling coiling temperature of 450 to 650.degree. C. is carried out, and after cold rolling, carried into a continuous annealing facility,
Recrystallization annealing is performed at an annealing temperature of 800 to 920 ° C, an average cooling rate from the annealing temperature to the precipitation treatment temperature of 5 to 120 ° C / sec, and subsequently, a treatment temperature of 400 to 650 ° C and a treatment time of 0.5 to 10 minutes. The method for producing a high-strength cold-rolled steel sheet excellent in deep drawability, which is subjected to the precipitation heat treatment of. 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.
After performing recrystallization annealing at an average cooling rate up to the following of 5 to 120 ° C./sec, a precipitation heat treatment at a processing temperature of 300 to 650 ° C. and a processing time of 0.5 minutes to 5 hours is performed. Manufacturing method of high strength cold rolled steel sheet. 3. A steel slab having the composition of claim 1 has a hot rolling finish temperature of Ar ₃ transformation point or higher, an average cooling rate after finish rolling of 20 ° C./sec or higher, and a hot rolling coiling temperature of 450 to 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.
After performing recrystallization annealing at an average cooling rate up to the following of 5 to 120 ° C./second, and subjecting the obtained steel sheet to temper rolling or product processing, processing temperature 300 to 650 ° C., processing time 0.5
A method for producing a high-strength cold-rolled steel sheet which is excellent in deep drawability and which is subjected to precipitation heat treatment for 5 to 5 hours.