JPH07150331A - Production of high tensile steel plate for deep drawing - Google Patents

Abstract

PURPOSE:To produce the high tensile steel plate having improved balance of the strength and ductility and aging resistance of a carburized steel plate and an excellent deep drawability by specifying the chemical components of steel products and further, specifying the heat treatment and carburization quantity thereof. CONSTITUTION:This steel plate contains C, Al, S, N, Ti and Nb at specified contents and further contains the Ti and the Nb by satisfying relations (12/48) Ti*+(12/93)Nb>=C+0.003 by weight %, where Ti*=Ti-(48/32)S-48/14)N. The steel plate is subjected to finish cold rolling then to open recoil recrystallization annealing to hold the steel plate for >=30 minutes at 650 to 800 deg.C. The steel plate is carburized in a carburizing atmosphere kept at 500 to 750 deg.C and the carburization quantity thereof is specified to >=0.02% in an increase quantity of C in average of the plate thickness and specified to {(12/48)Ti*+(12/93)Nb- starting material C+0.003} or below. As a result, the high tensile steel plate having the excellent deep drawability is produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a deep-drawing high-strength steel sheet which is subjected to processing such as bending, press forming or drawing, mainly for automobile bodies, that is, cold rolling having high strength. The present invention relates to a method for manufacturing a steel plate such as a steel plate and a plated steel plate.

[0002]

2. Description of the Related Art In recent years, as environmental problems have been attracting attention, exhaust gas regulations for automobiles have been reexamined, and there has been an increasing demand for weight reduction of automobiles for improving fuel consumption. On the other hand, with the increasing tendency of personal injury due to car accidents, improving the safety of the car itself has become an important issue. In order to solve these problems, the tensile strength (TS) of the thin steel sheet for automobiles is 340 to 490 MPa.
However, there is an increasing demand for materials exhibiting excellent press formability (r value, elongation). However, in general, when the tensile strength is increased, the press formability is deteriorated, and the surface properties such as the chemical conversion processability and the plating property are also deteriorated. Therefore, in the development of the high tensile strength thin steel sheet, these properties are not deteriorated. Had to be considered.

By the way, there is a precipitation strengthening method as one of the methods for increasing the strength of a steel sheet. This precipitation strengthening method requires a smaller amount of alloying elements than the solid solution strengthening method and is advantageous for plating and chemical conversion treatment, but a large amount of precipitates are generated in the hot-rolled sheet, so cold-rolled sheet There was a problem that a good texture was not obtained by annealing and the workability was poor. Methods for dealing with these problems by producing precipitation-strengthened steel by a carburizing method have been proposed, for example, in JP-A-1-96330 and JP-A-63-38556.

However, with respect to these known techniques, for example, in the case of the technique disclosed in Japanese Patent Application Laid-Open No. 1-96330, there is a problem that sufficient workability cannot be obtained and the strength-ductility balance is poor, and the aging resistance It wasn't enough for sex. Further, the technique disclosed in Japanese Patent Laid-Open No. 63-38556 is intended to improve secondary work embrittlement resistance by carburizing during ultra-low carbon steel open coil annealing.
Before recrystallization, carbon penetrates into the steel sheet, causing deterioration in workability,
In addition, there is a problem that the grain size is reduced and the ductility is deteriorated particularly near the surface of the steel sheet due to the inhibition of grain growth by the infiltrated carbon or the precipitate formed by the infiltrated carbon. There was also the problem of lacking consideration for.

[0005]

SUMMARY OF THE INVENTION The present invention advantageously solves the above problems and overcomes the weaknesses in the strength-ductility balance, aging resistance and the like of steel sheets manufactured by the conventional carburizing method. The purpose is to propose a manufacturing method for

[0006]

Under the recognition of such a problem, the inventors have conducted researches to solve the problem, and in the production of such a high-strength steel sheet for deep drawing, an assembly that controls the formability. The present invention has been completed by discovering that the above problems can be solved at once by depositing a large amount of precipitates finely after the formation of the structure. That is, the present invention is a method of achieving the above novel findings by controlling the conditions of the composition of the starting material and the recrystallization annealing after cold rolling and the carburizing treatment, and its gist is as follows. Is. (1) C: 0.0050 mass% or less, Al: 0.010 to 0.10 mass%, S: 0.03 mass% or less, N: 0.005 mass% or less,
And at least one of Ti: 0.003 to 0.20 mass% and Nb: 0.003 to 0.20 mass% is selected, and at the same time, Ti and Nb are represented by the following formula; (12/48) Ti ^* ( mass%) + (12/93) Nb (mass%) ≧ C (mass%) + 0.0
03 However, Ti ^* (mass%) = Ti (mass%)-(48/32) S (mass%)-
(48/14) N (mass%) content is satisfied and the balance is Fe.
And a cold-rolled steel sheet made of steel consisting of unavoidable impurities as a starting material, then subjected to open coil recrystallization annealing at a temperature of 650 to 800 ° C. for 30 minutes or more, and then 50
Carburizing is performed in a carburizing atmosphere in the temperature range of 0 to 750 ° C, and the amount of carburizing is 0.002 ma as the amount of increase in C of the average plate thickness.
ss% or more and {(12/48) Ti ^* (mass%) + (12/93) Nb (mass%)
-A method for producing a high-tensile steel sheet for deep drawing, which is characterized by performing a treatment for leading C (mass%) + 0.003 mass%} or less of a starting material. (2) C: 0.0050 mass% or less, Al: 0.010 to 0.10 mass%, S: 0.03 mass% or less, N: 0.005 mass% or less,
And Si: 0.05-2.0 mass%, Mn: 0.05-3.0 mass%, P: 0.01-0.15 mass%, Mo: 0.05-2.0 mass%, Ni: 0.05-3.0 mass%, Cu: 0.05-3.0 mass% and Cr : Any one or two or more selected from 0.05 to 3.0 mass%, and any one or two selected from Ti: 0.003 to 0.20 mass% and Nb: 0.003 to 0.20 mass% At the same time as containing the seed, the Ti and Nb are represented by the following formula; (12/48) Ti ^* (mass%) + (12/93) Nb (mass%) ≧ C (mass%) + 0.0
03 However, Ti ^* (mass%) = Ti (mass%)-(48/32) S (mass%)-
(48/14) N (mass%) content is satisfied and the balance is Fe.
After making a cold-rolled steel sheet using steel consisting of unavoidable impurities as a starting material, open coil recrystallization annealing is performed at a temperature of 650 to 800 ° C. for 30 minutes or more, and then 50
Carburizing is performed in a carburizing atmosphere in the temperature range of 0 to 750 ° C, and the amount of carburizing is 0.002 ma as the amount of increase in C of the average plate thickness.
ss% or more and {(12/48) Ti ^* (mass%) + (12/93) Nb (mass%)
-C (mass%) of starting material + 0.003mass%} A method for producing a high-strength steel sheet for deep drawing, which is characterized in that the treatment is carried out to a value of not more than 0.003mass%. (3) In each of the manufacturing methods described in (1) or (2) above,
As a starting material, in addition to the above-mentioned steel composition, B: 0.0002 to 0.0040 mass% and Sb: 0.0O5 to 0.1 mass
%, A steel obtained by adding any one or two selected from the group. (4) The manufacturing method according to any one of (1) to (3) above, which further comprises plating after the carburization.

[0007]

First, the manufacturing method of the present invention was developed.
The experimental results will be described. The composition of the materials used in this experiment
The component composition is C: 0.002%, Mn: 0.9%, P: 0.04%, S: 0.0
07%, Al: 0.05%, N: 0.002%, Ti: 0.02% and Nb:
A cold-rolled steel sheet composed of 0.03% and having a thickness of 0.7 mm was used. This
Tensile strength (TS) of carburizing time
And the effect on the r value were investigated. In this survey
The test material used was the cold-rolled steel sheet soaked at 710 ° C. for 4 hours.
In the heat cycle of
Carburized, then pickled, and then 0.7% tempered
It was rolled and adjusted. Other atmospheres except during carburization
Is H₂3.0%, balance N₂Was used. Characteristics of these test materials
The values are also shown in Table 1. Here, the carburizing amount is the carburizing process.
Of the amount of C (thickness average C amount) by the through-thickness analysis before and after processing
Calculated by the difference, TS is taken from the C direction (direction perpendicular to the rolling direction).
It was determined by the JIS No. 5 test piece taken. Also, the r value is
L direction (rolling direction), C direction and D direction (in rolling direction)
JIS 45 test pieces from the direction of 45 °)
After sampling and applying a 15% pre-strain, each feature by the three-point method
Sex value r_L, R_CAnd r_DThe average value is r = (r_L
+ 2r_D+ R _C) / 4.

[0008]

[Table 1]

From Table 1, steels B and C which are carburized materials
Both are weakly carburized at 50ppm and strength is increased, but
It was found that the steel B carburized from the early stage of soaking has a smaller increase in TS than the steel C carburized only at the final stage of soaking, and has a small r value and poor workability. Although the details of the reason why such a result was obtained are not clear, steel B was carburized before the start of recrystallization, and the r value was deteriorated because solid solution C was present in the steel at the time of recrystallization. it is conceivable that. Also, steel B
The reason why the strength increase amount of is smaller than that of steel C is
It is considered that the precipitation was coarsened due to the long carburizing time of Steel B, and the contribution to the strength increase was small. Also,
For steel C, the C concentration profile in the steel sheet thickness direction was investigated from the analysis values of the samples of each layer in the thickness direction ground from the steel sheet surface. As a result, the internal C concentration is a constant value,
The C concentration was low in the surface layer portion. The reason why this phenomenon occurred in steel C is that C entered from the surface in the early stages of carburization and the C concentration on the surface temporarily increased, but it diffused during subsequent cooling and the concentration in the thickness direction became uniform. It seems that the steel sheet surface was decarburized.

From the above-mentioned experimental results at different carburizing times,
It was shown that the carburizing time must be performed after the recrystallization annealing in order to maintain a high TS and ensure an excellent r value.

Next, the composition of the components is C: 0.002%, Mn: 0.5
%, P: 0.07%, S: 0.007%, Al: 0.04%, N: 0.002
%, Ti: 0.05% and Nb: 0.01%, and the influence of carburizing temperature on the carburizing amount, strength increase and aging index was investigated using a cold rolled steel sheet having a thickness of 0.8 mm. The test material was obtained by subjecting the cold-rolled steel sheet to recrystallization annealing at 720 ° C. for 5 hours, followed by carburizing for 1 hour at various temperatures of 400 to 850 ° C., pickling, and 0.8% tempering. It was rolled and adjusted. The carburizing atmosphere was CO 0.8%, H ₂ 3.8%, and the balance N ₂ gas, and the atmosphere other than during the carburizing treatment was H ₂ 3% and the balance N ₂ . The aging index (AI) was determined from the amount of increase in the deformation stress before and after the aging treatment when the aging treatment was performed at 100 ° C. for 30 min after the tensile prestrain of 7.5% was applied.
If the AI value is 30 MPa or less, it can be said that there is a delayed aging effect. The test results are shown in FIG.

From FIG. 1 it can be seen that there is a preferred carburization temperature range for both TS and AI values. That is, in order to obtain a high-strength steel sheet excellent in deep drawability, 500 to 7
It is necessary to perform carburizing treatment in the temperature range of 50 ° C.

Based on the above experimental results, in the present invention, the following manufacturing method is adopted. Slab
It is manufactured according to the usual continuous casting method or ingot making method. Further, hot rolling and cold rolling may be carried out according to ordinary methods, and it is not particularly limited, but the cold rolling reduction is preferably 60% or more in order to obtain good workability. For the annealing after cold rolling, it is necessary to perform open coil annealing in order to carry out carburization at a relatively low temperature for a long time after the recrystallization annealing. The conditions for this recrystallization annealing must be maintained at 650 to 800 ° C. for 30 minutes or more. If the recrystallization annealing temperature is less than 650 ° C, good workability cannot be obtained, and if it exceeds 800 ° C, roughening of the surface due to coarsening of crystal grains and deterioration of ductility occur. This is because a crystal texture cannot be obtained.

After the above recrystallization annealing, in a carburizing atmosphere, 5
In the temperature range of 00 to 750 ° C, the amount of increase in C of the average plate thickness is made
Carburizing is required to obtain a carburizing amount of 0.002% or more and {(12/48) Ti ^* (%) + (12/93) Nb (%)-C% of starting material + 0.003%} or less. . If the carburizing temperature is lower than 500 ° C, a sufficient amount of carburizing cannot be obtained. On the other hand, if the temperature exceeds 750 ° C, a sufficient amount of carburizing can be obtained, but the precipitates become coarse and sufficient strength cannot be obtained. Since the TS-El balance also deteriorates, the carburizing process needs to be performed in the temperature range of 500 to 750 ° C.

Further, if the carburizing amount is less than 0.002 mass%, sufficient strength cannot be secured, while {(12/48) Ti ^* (mass%) + (1
2/93) Nb (mass%)-C (mass%) of starting material + 0.003 mass
%, The carburizing amount must be within the above range. The carburizing method is not particularly limited, but addition of a carburizing gas (CO, CH _4, etc.) to the atmosphere in the furnace, direct injection of a volatile C-containing liquid into the furnace, plasma carburization and the like can be applied.

The steel sheet thus obtained after the carburizing treatment by the method of the present invention can be used as a product as it is, but if necessary, for the purpose of straightening the plate shape, [plate thickness (mm) Even if temper rolling of +0.3]% or less is performed, the effect of the present invention is not impaired. Further, regarding the steel sheet obtained by the method of the present invention, even if the surface treatment such as Zn-based electroplating or hot dip galvanizing is performed, the effect is not impaired, and the alloy is compared with the high-strength thin steel sheet of the same strength. Since the amount of elements can be reduced, excellent plating properties can be obtained. Furthermore, in order to improve the chemical conversion treatability, weldability, press formability, corrosion resistance, etc., the surface of the thin steel sheet may be subjected to a special treatment.

Next, in the present invention, the reason why the steel composition of the starting material is limited to the above-mentioned essential constitution will be explained. C: 0.0050% or less; C is preferably as low as 0.0050% in order to obtain good workability, particularly good TS-El balance.
%, The TS-El balance decreases, so the content must be 0.0050% or less, preferably 0.0030% or less.

Al: 0.010 to 0.10%; Al is added for deoxidation and precipitation fixing of N in steel, but if it is less than 0.010%, inclusions increase and good workability cannot be obtained. If it is added in excess, the workability will be deteriorated and the surface properties will be deteriorated. Therefore, the content must be 0.010 to 0.10%.

S: 0.03% or less; S adversely affects the deep drawability, and when it exceeds 0.03%, the effect becomes particularly remarkable, so S is made 0.03% or less. 0.01% is particularly preferable
It is the following.

N: 0.005% or less N content is 0.00
If it exceeds 5%, the deep drawability deteriorates, so it is necessary to set it to 0.005% or less, preferably 0.002% or less.

Ti and / or Nb: 0.003 to 0.20 wt%; T
Both i and Nb fix the solid solution C, N, and S in the ferrite phase and effectively contribute to the improvement of workability. However, if the content of these is less than 0.003 wt%, the effect of addition is small, while if it exceeds 0.20 wt%, the workability deteriorates, so 0.003 to 0.20 wt% It is necessary to contain it within the range. In order to obtain an effective amount of strength increase, (12/48) Ti ^* (%) + (12/93) Nb (%) ≥ C (%) + 0.003 (However, Ti ^* (%) = Ti (% )-(48/32) S (%)-(48/14) N
(%)) Is used as a starting material to form an annealed steel sheet, which is then carburized to obtain good deep drawability and TS.
-El balance can be obtained.

Si: 0.05 to 2.0%, Mn: 0.05 to 3.0%,
P: 0.01 to 0.15%, Mo: 0.05 to 2.0%, Ni: 0.05 to 3.0
%, Cu: 0.05 to 3.0% and Cr: 0.05 to 3.0%, any one or more selected from two or more kinds; any of these elements is effective for strengthening the steel sheet, and combined with the above-mentioned precipitation strengthening. To get the required strength. However, each
Si: less than 0.05%, Mn: less than 0.05%, P: less than 0.01%, M
o: less than 0.05%, Ni: less than 0.05%, Cu: less than 0.05% and Cr: less than 0.05% have no effect, while
Si: 2.0%, Mn: 3.0%, P: 0.15%, Mo: 2.0%, N
When i is added in excess of 3.0%, Cu: 3.0% and Cr: 3.0%, workability, especially TS-El balance, is deteriorated, so it is necessary to limit the content to the above range.

B: 0.0002 to 0.0040%; B is an element effective for precipitating and fixing N in steel and securing secondary work embrittlement resistance, but if it is less than 0.0002%, that effect cannot be obtained.
If added in excess of 0.0040%, the recrystallization temperature rises and the workability deteriorates, so the addition amount must be in the range of 0.0002 to 0.0040%.

Sb: 0.0O5 to 0.1%; Sb is an element effective for preventing nitriding during open annealing, but if it is less than 0.05%, its effect cannot be obtained, and if it exceeds 0.1%. If added, it deteriorates workability, so the range of addition is 0.0O5.
Must be ~ 0.1%.

[0025]

Example A thin steel sheet obtained by hot-rolling and cold-rolling (steel reduction of 70 to 81%) a steel material having the composition shown in Table 2 was used as a starting material and was re-processed under the conditions shown in Table 3. Crystal annealing treatment and carburizing treatment were performed. Then for some things Electric Zn
-Ni plating (plating amount 20 g / m ² , Ni: 12%) or hot dip galvanizing (plating amount 45 g / m ² ) was applied. For these test materials, TS, El and AI are J taken from the C direction.
Measured with the IS5 test piece in the same manner as described above,
The r value was obtained as an average value of the characteristic values r _L , r _C, and r _D in the three directions of L, C, and D, as in the above method. The results obtained are shown in Table 3.

[0026]

[Table 2]

[0027]

[Table 3]

As shown in Table 3, it is understood that the thin steel sheet obtained by the method of the present invention has an excellent TS-EL balance represented by TS × El and a high r value. However, the comparative example is inferior in the characteristic value of at least one of these characteristics.

[0029]

As described above, according to the present invention, it is possible to provide a high-strength thin steel sheet having properties suitable for deep drawing, which greatly contributes to weight reduction and safety of automobiles.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between carburizing temperature and carburizing amount, tensile strength and aging index.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Reference number within the agency FI Technical indication C22C 38/14 38/50 (72) Inventor Makoto Imanaka 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba (72) Inventor Masahiko Morita, 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Prefecture Technical Research Division, Kawasaki Steel Co., Ltd. 1 chome (without street number) Kawasaki Steel Works Mizushima Steel Works

Claims (4)

[Claims] 1. C: 0.0050 mass% or less, Al: 0.010-0.10
mass%, S: 0.03 mass% or less, N: 0.005 mass% or less,
And at least one of Ti: 0.003 to 0.20 mass% and Nb: 0.003 to 0.20 mass% is selected, and at the same time, Ti and Nb are represented by the following formula; (12/48) Ti ^* ( mass%) + (12/93) Nb (mass%) ≧ C (mass%) + 0.0
03 However, Ti ^* (mass%) = Ti (mass%)-(48/32) S (mass%)-
(48/14) N (mass%) content is satisfied and the balance is Fe.
And a cold-rolled steel sheet made of steel consisting of unavoidable impurities as a starting material, then subjected to open coil recrystallization annealing at a temperature of 650 to 800 ° C. for 30 minutes or more, and then 50
Carburizing is performed in a carburizing atmosphere in the temperature range of 0 to 750 ° C, and the amount of carburizing is 0.002 ma as the amount of increase in C of the average plate thickness.
ss% or more and {(12/48) Ti ^* (mass%) + (12/93) Nb (mass%)
-A method for producing a high-tensile steel sheet for deep drawing, which is characterized by performing a treatment for leading C (mass%) + 0.003 mass%} or less of a starting material. 2. C: 0.0050 mass% or less, Al: 0.010-0.10
mass%, S: 0.03 mass% or less, N: 0.005 mass% or less,
And Si: 0.05-2.0 mass%, Mn: 0.05-3.0 mass%, P: 0.01-0.15 mass%, Mo: 0.05-2.0 mass%, Ni: 0.05-3.0 mass%, Cu: 0.05-3.0 mass% and Cr : Any one or two or more selected from 0.05 to 3.0 mass%, and any one or two selected from Ti: 0.003 to 0.20 mass% and Nb: 0.003 to 0.20 mass% At the same time as containing the seed, the Ti and Nb are represented by the following formula; (12/48) Ti ^* (mass%) + (12/93) Nb (mass%) ≧ C (mass%) + 0.0
03 However, Ti ^* (mass%) = Ti (mass%)-(48/32) S (mass%)-
(48/14) N (mass%) content is satisfied and the balance is Fe.
And a cold-rolled steel sheet made of steel consisting of unavoidable impurities as a starting material, then subjected to open coil recrystallization annealing at a temperature of 650 to 800 ° C. for 30 minutes or more, and then 50
Carburizing in a carburizing atmosphere in the temperature range of 0 to 750 ℃
ss% or more and {(12/48) Ti ^* (mass%) + (12/93) Nb (mass%)
-C (mass%) of starting material + 0.003mass%} A method for producing a high-strength steel sheet for deep drawing, which is characterized in that the treatment is carried out to a value of not more than 0.003mass%. 3. In each of the manufacturing methods according to claim 1 or 2, as a starting material, in addition to the composition of the above steel, B: 0.0002 to 0.0040 mass% and Sb: 0.0O5 to 0.1 mass.
%, A steel obtained by adding any one or two selected from the group. 4. The manufacturing method according to claim 1, further comprising a plating treatment after the carburizing treatment.