JPH05195146A - Cold rolled steel sheet for deep drawing excellent in quality stability and its production - Google Patents

Abstract

PURPOSE:To provide a cold rolled steel sheet nearly free from a variation in quality in the longitudinal direction of a coil and having stable quality distribution. CONSTITUTION:This cold rolled steel sheet consists of 0.015-0.025% C, <=0.1% Si, 0.05-0.25% Mn, <=0.1% P, <=0.1% S, 0.02-0.06% sol. Al, <=0.0040% N and the balance Fe, satisfies inequalities MnXC<=4X10<-3> and 48N/14<=Ti<=3.2C+4.8 N/14-0.04 and has <0.8% DELTAEl and <1.0kgf/mm<2> DELTAYP in the longitudinal direction of a coil. This steel sheet is regulated to a prescribed thickness by cold rolling after hot rolling and it is annealed in a continuous annealing equipment with an overaging zone.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a continuously annealed cold rolled steel sheet using a low carbon Al killed steel containing a small amount of Ti and a method for producing the same, which is a cold rolled steel sheet for deep drawing with stable material. Is to be provided and manufactured at an extremely low cost.

[0002]

2. Description of the Related Art Since the continuous annealing process was developed as an annealing technology for cold-rolled steel sheets, it has been manufactured by a box annealing method until then, from cold-rolled steel sheets for general processing (CQ) to cold rolling for non-aging ultra-deep drawing. A number of techniques have been developed for producing products such as steel plates (EDDQ) by continuous annealing, and today, this continuous annealing has become established as an annealing technique that covers all standards of cold-rolled steel sheets.

Since the continuous annealing method heats and cools in an extremely short time as compared with the box annealing method, a low carbon Al-killed cold-rolled steel sheet in which solid solution / precipitation of carbon (C) in steel occurs during the annealing process. In this case, it is important to control the recrystallization texture in the temperature rising process and to accelerate the precipitation of solute carbon in the cooling process. Generally, for the former, the hot rolling temperature is increased to increase the nitrogen content in the steel (N ) As AlN and at the same time precipitate C in the steel as coarse cementite, and the latter is dealt with by over-aging treatment in the temperature range of 300 to 450 ° C. after quenching after annealing and soaking. To be done. But,
Even with the above technology, it is difficult to manufacture a cold-rolled steel sheet for non-aging high deep drawing using low carbon Al killed steel as a raw material. For such applications, the C content in the steel can be reduced to 50 ppm or less. Cold-rolled steel sheet made of so-called IF (Interstitial Free) steel, in which carbon, nitride forming elements such as Ti, Nb, and Zr are added to the steel, is used.

From the above viewpoint, general-purpose products (CQ to DQ)
Regarding the above, low carbon Al killed steel is used from the viewpoint of material cost, and IF steel is often used for high grade products (DQ, DDQ to EDDQ). In particular, the IF does not require over-aging treatment as well as high-temperature winding during hot rolling.
Steel is also preferable from the viewpoint of the stability of the material, and its use ratio has been increasing recently.

However, this one inherently has the following problems. 1. High material cost, 2. It is unavoidable to clean the slab surface. Securing of hot rolling finish temperature is stricter than low carbon steel, 4. 4. High temperature annealing is inevitable because of high recrystallization temperature. Secondary processing embrittlement easily occurs, 6. 6. When boron (B) is added in combination as a countermeasure for secondary processing embrittlement, not only the recrystallization temperature rises but the deep drawability deteriorates. When used as a base steel sheet for a galvannealed steel sheet, an abnormal alloying reaction is likely to occur.

Especially when the degassing capacity during steelmaking is insufficient, it is extremely important to establish a stable manufacturing technique using low carbon Al killed steel as a raw material, not limited to general-purpose products. That is, the cause of the material variation when the low carbon Al killed steel is used is mainly the high temperature winding process during hot rolling. That is, N
In order to completely precipitate and fix it as AlN, it is essential to wind it at a high temperature close to 700 ° C, but at the end in the longitudinal direction of the coil, the cooling rate after winding is fast, so on the runout table of that part. Even if the cooling of the
Cannot be sufficiently precipitated and coarsened. Therefore, the material in the longitudinal direction changes due to the non-uniform deposition state of AlN. In addition, excessively high-temperature winding not only deteriorates the descaling property, but also causes coarsening of hot-rolled plate crystal grains.

Therefore, the following techniques have been disclosed as means for solving such a problem. That is, for the purpose of lowering the hot rolling coiling temperature, a technique of adding B and precipitating and fixing N as BN (JP-A-55-77910, JP-A-56).
-166330). This technique makes it possible to improve the productivity and stabilize the material by adding B, which can effectively precipitate and fix N even at a low temperature of 680 ° C. or lower.

As means for lowering the hot rolling coiling temperature without adding B, C≤0.02%, Mn≤0.15%, Al≥0.1.
There is a technology (“Iron and Steel”, vol.76 (1990) .p.1536) of heating the slab at a temperature of 1150 ° C. or less and then hot rolling using the steel whose composition has been adjusted to 05%. .. With this technique, a high r-value can be obtained even by winding at about 650 ° C. by adjusting the above components.

On the other hand, Japanese Unexamined Patent Publication (Kokai) No. 57-169022 discloses a technique for precipitating and fixing N in steel with Ti.

[0010]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
77910 and those disclosed in JP-A-56-166330 are B
The addition range of B is narrow and the addition of B is not always an effective means for the r value, and a high r value cannot be obtained at an annealing temperature of 800 ° C. or lower.

In the technology published in the "Iron and Steel" magazine, N is
The point that it is fixed as AlN is the same as the conventional technical idea, and not only has the same disadvantages, but the content of Al is 0.05% or more.
Addition becomes unavoidable. Further, heating the slab at a temperature of 1150 ° C. or lower causes variations in the material due to nonuniform heating.

Further, in Japanese Patent Laid-Open No. 57-169022,
In order to obtain a relatively high r value, the C content in steel is higher than the Mn content, and high temperature annealing at around 800 ° C. is inevitable.

[0013]

Means for Solving the Problems The present invention has been studied to solve the technical problems in the conventional ones as described above, and has a specific component composition and is added with an appropriate amount of Ti. Succeeding in obtaining a cold-rolled steel sheet for deep drawing which is excellent in material stability by reducing material variation caused by precipitation fixing N in steel as AlN or BN, and is as follows.

(1) C: 0.015 to 0.025 in wt%
%, Si: 0.1% or less, Mn: 0.05 to 25%, P: 0.03
% Or less, S: 0.01% or less, sol.Al: 0.02 to 0.06
%, N: 0.0040% or less and Mn × C ≦ 4 × 10
^-3 , and Ti containing 48N / 14≤Ti≤3.2C + 48N / 14-0.04, with the balance being Fe and unavoidable impurities, and the material variation in the coil longitudinal direction is | ΔYP
| Is less than 1.0 kgf / mm ² , and | ΔEl | is less than 0.8%, a cold-rolled steel sheet for deep drawing with excellent material stability. (2) Material variation in the coil longitudinal direction is | Δr _m |
The cold-rolled steel sheet for deep drawing having excellent material stability as described in the above item (1), characterized in that it is 0.1 or less. (3) wt%, C: 0.015 to 0.025%, Si: 0.1
% Or less, Mn: 0.05 to 0.25%, P: 0.03% or less,
S: 0.01% or less, sol.Al: 0.02 to 0.06%, N: 0.0.
0040% or less, Mn × C ≦ 4 × 10 ⁻³ ,
In addition, steel containing Ti satisfying 48N / 14 ≦ Ti ≦ 3.2C + 48N / 14-0.04 and the balance Fe and unavoidable impurities was hot-rolled and then cold-rolled to a predetermined thickness. A method for producing a cold-rolled steel sheet for deep drawing, which is excellent in material stability, characterized by annealing a steel strip in a continuous annealing equipment having an overaging zone. (4) A hot-rolled steel strip obtained by finishing rolling the steel having the composition according to the above item (3) in a range not lower than the Ar ₃ point after continuous casting and winding the steel strip in a range of 640 ° C. to 700 ° C. When performing continuous annealing after descaling and cold rolling to a predetermined plate thickness, after soaking at a temperature of recrystallization temperature or higher, the average cooling rate at 600 ° C or lower is 40 ° C /
Cool down to 400 ℃ or less under the condition of more than seconds, 300 ℃ ~
A method of manufacturing a cold-rolled steel sheet for deep drawing having excellent material stability, which comprises performing overaging treatment for 30 seconds or more in a temperature range of 400 ° C.

[0018]

The operation of the present invention will be described in detail. In the present invention, Ti is 48N / 14≤Ti≤3.2C + 48N.
It is a continuous annealed cold rolled steel sheet using low carbon Al killed steel added in the range of /14-0.04 (wt%) and its manufacturing technology. The technical idea is to change N in steel before hot rolling.
By precipitating and fixing as TiN, it is to avoid precipitation of N in the hot rolling and winding stage. Further, in the present invention,
Considering that Ti precipitates and fixes C in steel as TiC,
The lower limit of the addition amount is limited to the range in which only TiN is precipitated and fixed. That is, in order to secure the delayed aging property after continuous annealing in low carbon steel, overaging treatment is indispensable. For that purpose, it is essential that about 100 ppm of supersaturated solid solution C be present at the start of overaging treatment, and as a steel component
It is desirable to contain about 0.02% of C.

From the above-mentioned relationship, in the present invention, even if a portion of C in the steel as TiC is precipitated and fixed, a supersaturated solid solution state of C in the steel is realized at the start of the overaging treatment. In order to secure an effective carbon amount of 0.015% or more, the upper limit of the Ti amount is defined as a function including the C amount in steel. Particularly, in the present invention, when Ti becomes excessive, a part of S in the steel is precipitated and fixed as TiS together with Mn, and a part of C in the steel is taken in as Ti ₄ C ₂ S ₂ in the hot rolling and winding stage. Form a complex precipitate in the form. This reduces the amount of effective carbon for obtaining the supersaturated solid solution state of C during overaging treatment. Therefore, in the present invention, S in steel is basically precipitated as MnS.

FIG. 1 shows the yield strength (YP) between the central portion in the longitudinal direction of the coil and the position of the end portion 20 m of the continuously annealed cold rolled steel sheet manufactured by using the hot rolled steel sheet having a hot rolling coiling temperature of 640 ° C. ) Difference (ΔYP) between Ti and N (a)
And the aging index (Al) after continuous annealing (Ti-4
The relationship (b) between 8N / 14) and C is shown. ΔTP ≦ 1kgf
/ Mm ² and AI <4 kgf / mm ² are satisfied in the range of Ti 48 N / 14 ≦ Ti ≦ 3.2 C + 48 N / 14-0.
When using low carbon Al killed steel specified in the range of 04 (wt%), excellent material stability and delayed aging are obtained compared to the continuous annealed cold rolled steel sheet using conventional low carbon Al killed steel. can get.

FIG. 2 shows steels 9 and 11 in Table 1 which will be described later.
No. 13 shows the material variation of the total coil length after continuous annealing, which was manufactured using the material hot-rolled at 700 ° C., but Ti in the range of 48 N / 14 ≦ Ti should be added. As a result, the variation in the material of the total coil length becomes
It is understood that the effect becomes more remarkable by satisfying the condition of Mn × C ≦ 3 × 10 ⁻³ .

The reasons for limiting the components of the present invention are as follows. C: 0.015 to 0.025%. In the present invention, the solid solution C is precipitated by the overaging treatment, and for that purpose, it is necessary to increase the supersaturation degree of the solid solution C which is a driving force for the precipitation of the solid solution C in the overaging treatment. 1
In order to stably realize supersaturated solid solution C of about 00 ppm, an initial carbon content of 0.015% is essential, and below that,
It is difficult to precipitate solid solution C sufficiently by overaging treatment. On the other hand, when the addition amount is large, the coarsening of the carbide (cementite) becomes insufficient under the hot rolling and winding conditions at 700 ° C. or less, and a high r value cannot be obtained. In addition, the increase in grain boundary carbides decreases the elongation, so the upper limit is 0.02.
5%.

Si: 0.1% or less. Si is not a particularly important element in the present invention in terms of characteristics, but its upper limit is set for the purpose of maintaining excellent chemical conversion treatability.
It is set to 0.1%.

Mn: 0.05 to 0.25%. Mn is obtained by precipitating and fixing S in steel as MnS,
It has an effect of preventing hot cracking due to FeS formation. Therefore, the lower limit is 0.05% from the viewpoint of preventing hot cracking.
And On the other hand, with respect to the upper limit, if a large amount is added, the coarsening of cementite during hot rolling and winding will be insufficient and the r value will deteriorate, so the upper limit of Mn is limited to 0.25%.

Mn × C ≦ 4 × 10 ⁻³ . In the present invention, in addition to the above-described limitation of C and Mn components, Mn
The value of × C is restricted to 4 × 10 ⁻³ or less. As shown in FIG. 3, when r values are summarized by Mn × C for materials annealed at 750 ° C. to 760 ° C. in an actual machine, even if annealed at 750 ° C. in the range of Mn × C ≦ 4 × 10 ⁻³ , 1.7. This is because it has been clarified that r-values exceeding 0 can be stably obtained.

P: 0.03% or less. In low carbon Al killed steel, P is an element that deteriorates the r value. Therefore, in the present invention, the upper limit is set to 0.03% as a range in which the influence on the r value is relatively small.

S: 0.01% or less. From the viewpoint of hot cracking, S is preferably low. In particular,
When the amount of Mn is regulated from the viewpoint of coarsening of cementite during hot rolling, the upper limit is set to 0.01% so that MnS can be sufficiently fixed. On the other hand, if the amount of S is reduced too much, scale peeling during hot rolling may be insufficient and scale surface defects may occur. In practice, 0.003% or more is preferable, but it is necessary to regulate the lower limit in terms of material. Absent.

Sol.Al: 0.02 to 0.06%. In the conventional low carbon Al killed steel, N in the steel is precipitated and fixed as AlN, but in the present invention, it is fixed as TiN, so that it is not necessary to add N for fixing. But,
Not only the yield of Ti is improved by deoxidizing with Al during steel making, but also nozzle clogging during casting can be prevented and a slab with good surface properties can be obtained, so the lower limit of Al is 0.02%.
Limited to. On the other hand, with respect to the upper limit, when a large amount is added, alumina inclusions increase and no merit in terms of material is recognized, so the upper limit is set to 0.06%.

N: 0.0040% or less. In the present invention, since N is precipitated and fixed as TiN, a lower N amount contributes to the reduction of the Ti addition amount. Therefore, in the present invention, the upper limit that can be stably managed as general-purpose steel is 0.0
Restricted to 040%.

| ΔYP | in the longitudinal direction of the coil is 1.0
By setting less than kgf / mm ² and | ΔEl | less than 0.8%, substantially uniform deep drawability can be obtained at any position in the coil longitudinal direction, and stable forming workability can be obtained. │Δ
More preferable range of YP | is less than 0.7 kgf / mm ² , | Δ
The more preferable range of El | is less than 0.5%.

| Δr _m | in the longitudinal direction of the coil is 0.1
By the following, stable and uniform deep drawing can be uniformly obtained over the entire length of the coil. A more preferable range of | Δr _m | in the entire length in the longitudinal direction of the coil is 0.08 or less, and more preferably 0.06 or less.

When the steel of the present invention having the above-mentioned component system is annealed by a general continuous annealing process, the material stability is remarkably excellent as compared with the conventional low carbon Al killed steel. In particular, even if it is annealed in a continuous hot-dip galvanizing line where sufficient overaging treatment is difficult, similar material stability can be obtained.

However, in order to obtain a particularly excellent material using the steel disclosed in the present invention, the hot rolling coiling temperature: 640
℃ ~ 700 ℃, 600 ℃ in the primary cooling process after annealing
The following average cooling rate ≧ 40 ° C./sec, overaging temperature: 300
C.-400 degreeC conditions are preferable.

FIG. 4 shows the effect of hot rolling temperature on the properties of steel 4 (comparative steel) and steel 5 (inventive steel) in Table 1 of Examples described later, after continuous annealing. However, it can be seen that the invention steel has a high r _m value in a wide winding temperature range and a small material variation in the coil longitudinal direction. That is, with regard to continuous annealing, the present invention does not set conditions that are significantly different from the conventional process,
The above range was defined for the purpose of securing the amount of supersaturated solid solution C at the start of the overaging treatment and precipitating the solid solution C by the overaging treatment for a short time.

[0036]

EXAMPLES Specific examples of the present invention will be described. The present invention steels and comparative steels specifically adopted by the present inventors are as shown in Table 1 below.

[0037]

[Table 1]

The production example of the above steel will be described below. (Production Example 1) A continuously cast slab of the composition shown in Table 1 above was heated to 1200 ° C, and then rough rolling and continuous hot rolling were performed to finish a hot rolled sheet of 2.8 mm thickness at a final finishing temperature of 890 ° C. After the run-out cooling, it was wound up at 680 ° C. The steel strip was pickled, cold-rolled into a cold rolled steel strip having a thickness of 0.7 mm, and continuously annealed under the following conditions. Annealing temperature: 750
℃, 600 ℃ ~ 400 ℃ primary cooling conditions: roll cooling (cooling rate ≈ 100 ℃ / sec), overaging conditions: 350 ℃
× 2.5 minutes. The materials obtained after 1.0% temper rolling are shown in Table 2 below.
Shown in.

[0039]

[Table 2]

That is, in the case of the present invention, | ΔYP | is 1 kg.
f / mm ² or less, especially 0.7 kgf / mm ² or less, | ΔE
l | is also low at -0.8% or less, particularly at -0.5% or less. Furthermore |
Δr _m | is -0.1 or less and AI is 4 kg.
It was confirmed that the steel sheet has a characteristic of excellent quality stability in the longitudinal direction of the coil of f / mm ² or less and is a preferable steel sheet for deep drawing.

(Production Example 2) Steel No.-1 in Table 1 above,
After heating 2, 4, 8 continuous cast slabs to 1080 ℃, the final finishing temperature is 910 ℃ by rough rolling and continuous hot rolling.
Finished to 2.8mm thick hot rolled sheet and 650 after runout cooling
It was wound up at ℃. After pickling the steel strip, it was cold rolled to 0.
After forming a cold rolled steel strip having a thickness of 7 mm, continuous annealing was performed under the following conditions. Annealing temperature: 770 ° C, primary cooling condition from 600 ° C to 400 ° C: Roll cooling (cooling rate ≈ 100 ° C /
Second), overaging condition: 350 ° C. × 2.5 minutes. After annealing 1.0
The following Table 3 shows the materials evaluated by performing% temper rolling.

[0042]

[Table 3]

(Production Example 3) Steel No.-1 shown in Table 1 above,
After heating 4, 5 continuous cast slabs to 1200 ° C, final finishing temperature is 2.8 at 910 ° C by rough rolling and continuous hot rolling.
Finished to a hot rolled sheet with a thickness of mm and cooled to 650 ° C for 6 after runout cooling.
It was wound up at 70 ° C. After pickling the steel strip by cold rolling
After forming a cold-rolled steel strip having a thickness of 0.7 mm, continuous annealing was performed under the following conditions. Annealing temperature: 800 ° C, 750 ° C, cooling conditions: gas jet cooling, overaging treatment conditions: 400 ° C x 3 minutes. The materials evaluated by performing 1.0% temper rolling after annealing are as shown in Table 4 below.

[0044]

[Table 4]

[0045]

As described above, according to the present invention, a cold-rolled steel sheet for deep drawing having a stable material distribution with little material change in the longitudinal direction of the coil is provided at low cost, and a preferable manufacturing method thereof is obtained. It is an invention that has a large effect industrially.

[Brief description of drawings]

FIG. 1 shows the relationship between Ti and N on the yield strength difference between the central portion and the end portion in the longitudinal direction of a coil of a continuously annealed cold rolled steel sheet having a coiling temperature of 640 ° C., and an aging index (A) after continuous annealing.
It is a chart showing the relationship between (Ti-48N / 14) and C affecting I).

FIG. 2 is a chart showing material variation of the total coil length after continuous annealing of a material hot rolled at 700 ° C.

FIG. 3 is a chart showing the relationship between Mn × C and r _m value.

FIG. 4 is a table showing the effect of hot rolling coiling temperature on the properties after continuous firing for representative examples of the present invention steel and comparative steel.

Claims (4)

[Claims] 1. C .: 0.015-0.025%, S: wt%
i: 0.1% or less, Mn: 0.05 to 0.25%, P: 0.03% or less,
S: 0.01% or less, sol.Al: 0.02 to 0.06%, N: 0.0040% or less, Mn × C ≦ 4 × 10 ⁻³ , and 48 N / 14 ≦ Ti Includes Ti satisfying ≦ 3.2C + 48N / 14-0.04, the balance is Fe and unavoidable impurities, and material variation in the coil longitudinal direction is | ΔYP
| Is less than 1.0 kgf / mm ² , and | ΔEl | is less than 0.8%, a cold-rolled steel sheet for deep drawing with excellent material stability. 2. The material variation in the coil longitudinal direction is | Δ
The cold rolled steel sheet for deep drawing having excellent material stability according to claim 1, wherein r _m | is 0.1 or less. 3. W: wt%, C: 0.015 to 0.025%, S
i: 0.1% or less, Mn: 0.05 to 0.25%, P: 0.03% or less,
S: 0.01% or less, sol.Al: 0.2 to 0.06%, N: 0.0040% or less, Mn × C ≦ 4 × 10 ⁻³ , and 48 N / 14 ≦ Ti A steel strip containing Ti satisfying ≤3.2C + 48N / 14-0.04, hot rolling a steel consisting of balance Fe and unavoidable impurities, and then cold rolling to a predetermined thickness, is overaged. A method for producing a cold-rolled steel sheet for deep drawing, which is excellent in material stability, characterized by annealing in a continuous annealing equipment having a strip. 4. A hot-rolled steel strip obtained by finishing rolling of a steel having the composition according to claim 3 after continuous casting in a range not lower than the Ar ₃ point and winding in a range of 640 ° C. to 700 ° C. When performing continuous annealing after descaling and cold rolling to a predetermined plate thickness, after soaking at a temperature of recrystallization temperature or higher, the average cooling rate at 600 ° C or lower is 40 ° C /
Cool down to 400 ℃ or less under the condition of more than seconds, 300 ℃ ~
A method of manufacturing a cold-rolled steel sheet for deep drawing having excellent material stability, which comprises performing overaging treatment for 30 seconds or more in a temperature range of 400 ° C.