JPH0647706B2 - Cold-rolled steel sheet for deep drawing with excellent secondary work cracking resistance and method for producing the same - Google Patents

Description

TECHNICAL FIELD The present invention relates to a cold-rolled steel sheet for deep drawing which is excellent in secondary work crack resistance and a method for producing the same.

[Conventional technology]

For example, parts such as automobile quarter panels, oil pans, and odd-shaped air cleaner cases are formed by extremely severe pressing. The cold rolled steel sheet used for manufacturing such molded products is required to have high ductility, ultra-deep drawability and non-aging property. In response to this requirement, carbon and nitride forming elements Ti, Nb and Cr are added individually or in combination to fix C and N in the steel to improve ductility and ensure non-aging, and It has already been proposed to use a non-aged cold-rolled steel sheet for deep drawing in which a recrystallization texture of {111} plane orientation effective for improving the deep drawability is developed by the action of Nb carbonitride. There is.

For example, JP-B-44-18066, JP-A-59-67322 and JP-A-59-89727 describe Ti-added steel as disclosed in JP-B-54-54.
-1245, Japanese Patent Publication No. 59-34778, and Japanese Patent Laid-Open No. 58-81
952 discloses Nb-added steel, JP-B-50-30572 and JP-A-58-18572 disclose Ti-Cr-added steel, and JP-A-59-
67319 discloses a Ti-Nb-added steel, JP-A-59-123720 discloses a Ti-Nb-Cr-added steel, and JP-A-59-140333 discloses a T-added steel.
iB-added steel, and Japanese Patent Laid-Open No. 59-193221 discloses Ti-Nb-
Each of the B-added steels is disclosed.

[Problems to be solved by the invention]

Among the cold-rolled steel sheets for super deep drawing in which C and N in the steel are sufficiently fixed by the carbonitride forming element described in the above-mentioned publication, those to which B is not added are extremely severe presses. There is a problem that cracking occurs due to brittle fracture during secondary processing after forming. This brittle fracture is due to the fact that there is no solid solution C in the steel, so there is no segregation of C at the ferrite grain boundaries, which promotes segregation of P at the grain boundaries, resulting in embrittlement of the grain boundaries.

In the Ti-B-added steel and the Ti-Nb-B-added steel disclosed in JP-A-59-193221 and JP-A-59-140333,
It is known that the added B segregates at the grain boundaries and prevents the segregation of P at the grain boundaries, so that secondary work cracks are less likely to occur during the secondary work after severe press forming. However, B
If the amount of addition is too large, the recrystallization temperature rises and the presence of a large amount of solid solution B causes deterioration of ductility and deep drawability. Further, a steel sheet containing a large amount of B tends to cause non-uniform plating that causes color unevenness when subjected to surface treatment, for example, unichrome plating, and when subjected to galvanizing treatment. ， There is a drawback that uneven plating occurs. On the contrary, if the amount of B added is too small, the sufficient effect of improving the resistance to secondary work cracking cannot be obtained.

Therefore, in order to effectively improve the secondary work cracking resistance without deteriorating the ductility and deep drawability by adding B, it is necessary to properly control the B addition amount. It is not always easy in terms of steelmaking technology.

[Means for solving problems]

INDUSTRIAL APPLICABILITY The present invention improves the secondary work cracking resistance of a steel sheet in which C and N in the steel are fixed by adding Ti alone or at least one of Nb and Cr in addition to Ti without using the means of adding B. The purpose is to

According to the present invention, the secondary work cracking resistance of the cold rolled steel sheet in which Ti alone or Ti and at least one of Nb and Cr are added together to fix C and N in the steel is It was found that the improvement can be achieved by forming a very fine-grained carburized layer having an appropriate thickness and an appropriate C concentration.

Thus, the present invention provides Ti alone or Ti plus Nb and Cr.
Is a cold-rolled steel sheet in which C and N in the steel are fixed by adding at least one of the above, and has a carburized layer on both surface layers, and the ratio of the total carburized layer thickness to the steel sheet thickness is 1/4 ~ 1/12, the C concentration average of the carburized layer is 0.02 to 0.10% by weight, the ferrite grain size No. of the carburized layer is 9.5 or more, and the ferrite grain size in the center of the plate thickness
Provided is a cold-rolled steel sheet for deep drawing which is excellent in secondary work cracking resistance and has a No. of 6.0 or more.

The present invention also provides an extremely low carbon steel containing Ti alone or at least one compound in addition to Ti, which is sufficient to fix C and N in the steel melted by vacuum degassing, into a slab according to a conventional method. , If necessary, perform slab care, hot rolling, scale removal by pickling, cold rolling with a total rolling ratio of 60% or more, and open coil annealing at a dew point of -70 to 0 ° C. In the atmosphere gas condition, the annealing heating temperature is above the recrystallization temperature and below 830 ° C, and the annealing time is within 5 hours, the ratio of total carburized layer thickness to sheet thickness is 1/4 to 1/12 on both surface layers of the steel sheet. Provided is a method for producing a cold-rolled steel sheet for deep drawing, which is excellent in secondary work cracking resistance, characterized by forming a carburized layer.

For the purpose of the present invention of improving the secondary work cracking resistance, the ratio of the total thickness of both carburized layers to the plate thickness of the steel sheet is 1/12 or more, and the average C concentration of the carburized layer is 0.02 weight. % Or more, the ferrite grain size No. of the carburized layer is 9.5 or more, and the ferrite grain size No. of the central portion of the plate thickness is 6.0 or more. In other words, unless these conditions are satisfied, substantial improvement in secondary work crack resistance cannot be obtained. On the other hand, although the upper limit of the ratio of the total thickness of both carburized layers to the plate thickness of the steel plate and the upper limit of the average C concentration of the carburized layer are not so critical, the ratio of the total thickness of both carburized layers to the plate thickness of the steel plate is not so critical. Substantially exceeds 1/4 or the average C concentration in the carburized layer is 0.1
Since the ductility and deep drawability of the product tend to deteriorate when the content exceeds 0% by weight, the ratio of the total thickness of both carburized layers to the steel plate thickness is 1/12 to 1/4 and the carburized layer is C concentration average of 0.02 to 0.10% by weight.

According to the present invention, by forming an appropriate carburized layer on the surface layer portion of the cold rolled steel sheet, the accurate working mechanism that the secondary work crack resistance can be improved without deteriorating the ductility and deep drawability is still a Although it has not been clarified, firstly, the carburization suppresses the growth of recrystallized grains in the surface layer and the surface layer becomes very fine, and secondly, it does not precipitate as TiC in the carburized carbon. It is considered that segregation of carbon of about 1 to 2 ppm at the grain boundaries to prevent segregation of P at the grain boundaries improves the secondary work crack resistance. Since the cold-rolled steel sheet of the present invention is non-aging after carburizing and annealing,
It can be said that the amount of C segregated at the grain boundaries is very small.

Next, a method for manufacturing a cold-rolled steel sheet for deep drawing having excellent secondary work cracking resistance according to the present invention will be described.

The cold-rolled steel sheet for deep drawing having excellent secondary work cracking resistance according to the present invention is a total steel made of steel in which C and N in the steel are fixed by adding Ti alone or at least one of Nb and Cr in addition to Ti. The cold rolled steel sheet with a rolling reduction of 60% or more can be conveniently manufactured by open coil annealing under the condition that a predetermined carburized layer is formed.

Regarding the chemical composition of steel, there is no special limitation except that C and N in the steel are fixed by addition of Ti alone or by addition of at least one of Nb and Cr in addition to Ti. For the purpose of obtaining a cold-rolled steel sheet having both deep drawability, the chemical composition of the cold-rolled steel sheet before open coil annealing is preferably in the following range.

The lower the content of C, the higher the ductility of the cold rolled steel sheet. In particular, when the C content is substantially higher than 0.01%, a large amount of carbonitride forming elements are required, and in addition, an increase in the carbonitride precipitation amount deteriorates the press formability. On the other hand, making the C content substantially lower than 0.001% is impractical in a practical steelmaking furnace. Therefore, the preferable C content is 0.001 to 0.01%.

Generally, Si is added for the purpose of deoxidizing molten steel, and Mn is mainly added for the purpose of preventing hot embrittlement, but if Si and Mn are added in too large amounts, ductility deteriorates. Therefore, it is preferable that Si and Mn are the amounts in the range normally contained in a normal cold-rolled steel sheet, Si is 0.1% or less, and Mn is 0.5% or less.

Al is generally added for the purpose of deoxidizing molten steel, but if the amount is substantially less than 0.01% as sol.Al in the steel, the purpose of deoxidation cannot be sufficiently achieved. On the other hand, as the sol.Al content exceeds 0.10%, the deoxidizing effect becomes saturated and non-metallic inclusions increase, causing surface defects. Therefore, sol.Al is preferably 0.01 to 0.10%.

Cr, when added in combination with Ti or Ti and Nb, exerts the effect of improving deep drawability and bulging property. For this purpose, Cr of 0.06% or more is required. However, the addition of Cr in an amount exceeding 0.20% saturates this effect and only increases the manufacturing cost.
Therefore, it is preferable to add Cr in the range of 0.06 to 0.20%, if necessary.

If too much P is added, the yield strength and the tensile strength will be increased, and the amount of segregation at the grain boundaries will increase, which will promote secondary work cracking. Therefore, the upper limit of the P content is preferably 0.03%.

N, S and O are strongly bonded to Ti to form nitrides,
Since sulfides and oxides are formed, if their contents are too large, not only the Ti addition amount necessary for fixing C in steel increases, but also the press formability and surface properties deteriorate. There is a disadvantage. Therefore, it is preferable that N is 0.007% or less, S is 0.015% or less, and O is 0.01% or less.

Ti secures the non-aging property of the cold-rolled steel sheet by fixing C and N, and the generated TiC develops a recrystallized grain texture of {111} plane orientation effective for improving deep drawability. Therefore, for the purposes of the present invention, nitrides,
It is necessary that the effective Ti amount obtained by subtracting the Ti amount as sulfides and oxides from the total Ti amount is 4 × C% or more. However, when the Ti content substantially exceeds 0.40%, not only the effect is saturated, but also the intragranular strength is increased, which promotes deterioration of secondary work crack resistance. Therefore, the upper limit of the Ti content is preferably 0.40%.

When Nb is contained in an amount of 0.03% or more, it has an effect of improving the in-plane anisotropy of r value of the cold rolled steel sheet, but excessive Nb causes deterioration of ductility. Therefore, it is preferable to add Nb in the range of 0.03 to 0.10% if necessary.

In the production of the cold-rolled steel sheet of the present invention, the steel is melted in a steelmaking furnace according to a conventional method, but it is preferable to carry out vacuum degassing treatment before ingot casting or continuous casting. In the vacuum degassing process, Al can be added for the purpose of deoxidizing.

After vacuum degassing, a slab is manufactured by ingot-making and slab-rolling or by continuous casting, and slab care is performed if necessary, followed by hot rolling. Alternatively, after continuous casting, cold slabs may be once formed and then reheated for hot rolling, or hot slabs may be directly charged into a heating furnace for hot rolling.

When performing hot rolling, from the perspective of improving deep drawability,
It is preferable to set the hot rolling finishing temperature to ₃ Ar or higher. Also,
The hot rolling temperature is preferably in the range of 600 to 750 ° C.

The hot-rolled steel sheet is subjected to cold rolling after pickling. This cold rolling develops a texture of {111} plane orientation, which is advantageous for deep drawability, to achieve a total cold rolling rate of 60% or more. It is necessary to do in.

A predetermined carburizing layer is formed on both surface layers of the cold-rolled steel sheet thus obtained by open coil annealing. At that time, various conditions, particularly annealing temperature, soaking time and annealing atmosphere are set to a predetermined carburizing rate. Control is required so that the layers are formed, as described below.

The annealing temperature is in the range of not lower than the recrystallization temperature (preferably about 730 ° C or higher) and 830 ° C or lower. When annealing is performed at a temperature substantially higher than 830 ° C, ferrite grains become coarse and a product with good resistance to secondary work cracking cannot be obtained. The annealing time is
In most cases, it should be within 5 hours. If the annealing is performed for an unreasonably long time, the carburization becomes excessive, and as a result, the thickness of the carburized layer and / or the C concentration of the carburized layer becomes excessive, which tends to deteriorate the ductility and deep drawability of the obtained product.

Atmospheric gas, HNX gas _{(H 2: 3~10%, N} 2: 97
˜90%, CO ₂ : 0.2% or less, CO: 0.2% or less), and then high-purity AX gas (H ₂ : 75%, N ₂ : 25%)
It is good to say The carburization of the surface layer of the steel sheet is promoted by about 500 to 1500 ppm of CO contained in the atmosphere. As the CO source, CO of about 2000 ppm existing in HNX gas remains after the AX gas is switched after barging, and when dust or oil inevitably present in the annealing furnace serves as the CO source. is there. However, HN as a purge gas
The use of X gas is for economic reasons. Even if N ₂ gas or AX gas is used as a purge gas, CO of about 500 to 1200 ppm exists in the AX gas atmosphere during soaking. Therefore, there is no problem in promoting carburization.

The dew point of the atmosphere needs to be controlled within the range of -70 to 0 ° C. If the dew point substantially exceeds 0 ° C, CO
Even if 500 to 1500 ppm is present, the carburizing power of the atmosphere is weak, so the prescribed carburized layer is often not formed.
On the other hand, the lower the dew point, the stronger the carburizing power of the atmosphere. However, on a commercial scale, the limit is -70 ° C, and controlling the dew point below -70 ° C is not economically feasible.

1 is a schematic sectional view of a cold-rolled steel sheet according to the present invention. As shown in FIG. 1, the cold-rolled steel sheet of the present invention has a carburized layer having a thickness of t ₁ on one surface side and a carburized layer having a thickness of t ₂ on the other surface side. Assuming that the cold-rolled steel sheet thickness is t ₀ , the ratio of the total carburized layer thickness to the sheet thickness t _C = (t ₁ + t ₂ ) /
t ₀ is in the range of 1/12 to 1/4.

FIG. 2 is a photograph of a metal structure showing a cross section in the plate thickness direction on one surface layer side of the cold-rolled steel sheet of the present invention according to Test No. 2 described later (magnification:
However, in this example, the thickness t ₁ of the carburized layer on the surface side shown in the photograph obtained from the metallographic photograph was 100μ.

FIG. 3 is a diagram showing a line analysis result of C concentration by EPMA of a section in the plate thickness direction on one surface layer side of the cold-rolled steel sheet of the present invention according to Test No. 2 described later. The thickness of the carburized layer obtained from the figure agrees with the value obtained from the metallographic photograph of Fig. 2. In the case of test No. 2, since t ₁ = t ₂ = 100 μ and t ₀ = 1.2 mm, t _C = (t ₁ + t ₂ ) / t ₀ = 1/6 was calculated.

The average C concentration in the carburized layer is t _C thus obtained,
Mean = [x% of C concentration of total thickness of the carburized average C concentration (x%) and the total thickness average C concentration before carburization (y%) from the following equation carburized layer - (1-t _C) y %] / T _C, which can be conveniently calculated, but this value is in the range of 0.02 to 0.10%.

Also, the ferrite grain size No. of the carburized layer is 9.5 or more,
The ferrite grain size No. at the center of the plate thickness is 6.0 or more.

Example A 180 ton converter and degassing equipment were used to adjust the composition of the steel with the chemical composition values shown in Table 1, and each molten steel was continuously cast to form a slab. The heating temperature from each slab was 1250-12.
70 ℃, Hot rolling finishing temperature 900〜930 ℃, Hot rolling coiling temperature 700〜730
Hot-rolled coils were obtained under the hot-rolling conditions of ℃, pickled, and then cold-rolled at the cold rolling rate shown in Table 2 to a plate thickness of 1.2 mm. For each coil, as shown in Table 2, annealing temperature 750 to 850 ° C, annealing time 1 to
Carburized layer consisting of fine-grained ferrite on both surface layers of the steel sheet after replacement with HNX gas under the condition of dew point -50 to + 50 ° C during soaking for 6 hours Was formed.

Table 2 shows the mechanical properties and the secondary work cracking resistance limit temperature of the cold rolled steel sheet obtained in each test.

In the secondary work cracking resistance test, after blanking the test material to 90 mmφ, the cup was made with a three-stage drawing (total drawing ratio = 2.7) of the primary drawing 50φ, the secondary drawing 40φ, and the tertiary drawing 33φ. After molding and trimming the obtained cup to a height of 40 mm, the cup was placed in the refrigerant adjusted to each test temperature, and the apex angle was 60 °.
The conical cone-shaped punch was pushed in, and the lower limit temperature at which brittle fracture, which is called vertical cracking, did not occur was measured. If the critical temperature for secondary work cracking is substantially higher than -50 ° C, the resistance to secondary work cracking must be considered to be poor.

As an evaluation of the deep drawability, it can be said that the deep drawability is good when the total elongation (T.El) is 45% or more and the value is 1.8 or more. If any of these conditions are not met, deep drawability must be considered poor.

As is clear from Table 2, the test steel A of the Ti-added steel, which is the test No. 2 of the present invention, has not only a high total elongation of 48.3%, but also a high value of 2.17 and a high resistance to corrosion. The sub-working cracking limit temperature is as low as -70 ° C, and the ductility, deep drawability and resistance to secondary working cracking are extremely good. On the other hand, in the test No. 1 (comparative example) in which the ratio of the total carburized layer thickness to the plate thickness is 1/2, which is outside the scope of the present invention, the secondary work crack resistance is good, but the total elongation is 3
6.3%, which is very low for Ti-added steel. Further, the deep drawability indicated by the value is also unsatisfactory. Test Nos. 3 and 4 (comparative examples) with high atmospheric dew points of + 20 ° C and + 50 ° C, respectively, do not have a carburized layer and are inferior in secondary work cracking resistance.

Test Steel B of Ti-Nb-Cr-added steel, Test Nos. 6 of the present invention example
Nos. 7, 8 and 13 had a high total elongation of 49.3 to 50.3% and a good value of 2.05 to 2.18. Furthermore, the secondary work cracking resistance temperature was low at -50 to -70 ° C and Excellent secondary work cracking resistance. Test Nos. 10 and 11 (comparative example)
Since the carburized layer consisting of fine-grained ferrite is not formed, the secondary working crack resistance temperature is relatively high at -30 to -40 ° C, and the secondary working crack resistance is poor. Also, test No. 1
In the case of No. 2 (Comparative Example), the annealing temperature was too high, so the ferrite grain size No. at the center of the plate thickness was 4.0, which is outside the range of the present invention, the total elongation was relatively low, and the secondary work cracking resistance temperature was Is -1
The resistance to secondary work cracking is very poor at 0 ° C. Test No. 14
In No. 1 to No. 16, the effects of cold rolling rate were investigated by changing the thickness of hot rolled sheet.
In 4 (comparative example), the value is as low as 1.53 and the target deep drawability cannot be obtained. On the other hand, Test No. 1 which is an example of the present invention
In 5 and 16, the target characteristics are obtained.

〔The invention's effect〕

In accordance with the invention, Ti alone or in addition to Ti, Nb and
At the time of open coil annealing of a cold-rolled steel sheet in which at least one of Cr is added and C and N in the steel are fixed,
If the ferrite grain size in the central part of the steel sheet is properly controlled and appropriate carburized layers are formed on both surface layers of the steel sheet, it is possible to obtain a cold-rolled steel sheet for deep drawing with excellent secondary work cracking resistance. .

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view of a cold-rolled steel sheet according to the present invention, and FIG. 2 is a micrograph showing a metal structure of a cross-section in the plate thickness direction on one surface layer side of the cold-rolled steel sheet according to the present invention according to Test No. 2 (magnification: 100
2) and FIG. 3 are graphs showing the results of line analysis of the C concentration by EPMA of the section in the plate thickness direction on the one surface layer side of the cold-rolled steel sheet of the present invention according to Test No. 2.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification number Internal reference number FI Technical display location C23C 8/22 7516-4K (72) Inventor Kazuu Takagi 5 Ishizu Nishimachi, Sakai City, Osaka Prefecture Nisshin (72) Inventor Satoshi Usuki, No. 5 Ishizu Nishimachi, Sakai City, Osaka Prefecture Nisshin Steel Co., Ltd. Hanshin Works

Claims (2)

[Claims] 1. A cold-rolled steel sheet in which C and N in steel are fixed by adding Ti alone or a combination of Ti and at least one of Nb and Cr, and having a carburized layer on both surface layers, The ratio of layer thickness to plate thickness is 1/4 to 1/12, and the C concentration average of the carburized layer is 0.0
2 to 0.10% by weight, the ferrite grain size No. of the carburized layer is 9.5 or more, and the ferrite grain size No. of the central part of the plate thickness is 6.0 or more, for deep drawing with excellent secondary work cracking resistance. Cold rolled steel sheet. 2. An ultra-low carbon steel containing Ti alone or a combination of Ti and at least one of Nb and Cr sufficient to fix C and N in the steel melted by vacuum degassing according to a conventional method. , Slab, hot rolled, after scale removal,
When performing open coil annealing with cold rolling at a total rolling rate of 60% or more, the annealing heating temperature is from recrystallization temperature to 830 ° C and the annealing time is 5 at the atmosphere gas condition with dew point of -70 to 0 ° C. Cold drawing for deep drawing with excellent secondary work crack resistance characterized by forming a carburized layer with a ratio of total carburized layer thickness to plate thickness of 1/4 to 1/12 on both surface layers of the steel sheet within a time period Manufacturing method of rolled steel sheet.