JPH02225644A - Non-age baked hardenability cold rolled steel sheet having excellent local ductility manufactured by continuous annealing - Google Patents

Abstract

PURPOSE:To obtain the low-carbon Al killed cold rolled steel sheet of SPCE class by continuous annealing which shows prescribed value of transgranular carbide density and has specified local ductility, baking hardenability of coating, good formability and BH properties by subjecting a low-carbon Al killed steel to hot rolling, thereafter cold rolling, continuous annealing and skinpass rolling. CONSTITUTION:A steel contg., by weight, 0.008 to 0.025% C, 0.05 to 0.20% Mn, 0.004 to 0.015% S, 0.03 to 0.15% acid soluble Al and regulatively contg. <=0.012% P and <=0.003% N as impurities is treated as follows. Namely, the steel is subjected to normal hot rolling and is successively to cold rolling, continuous annealing and skinpass rolling. In this way, the amt. of solid soln. C is regulated to 2 to 5ppm and the transgranular carbide density (delta-number/mm<2>) is regulated to the range in the inequalities by the relationship with the aging index (Al-kgf/mm<2>). Then, the non-age baked hardenability cold rolled steel sheet having <=19kgf/mm<2> yield strength (YP) after subjected to aging treatment (100 deg.CX1h), >=30.9mm LDH0, the index of local ductility and 2 to 5kgf/mm<2> baking hardenability of coating can be obtd.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention is directed to IF steel (Interstjtlal Atom
s FreeSteel (hereinafter referred to as IF steel) (
Steel is made by lowering carbon to an extremely low level of several pI), and then adding T1 and Nb, which have a strong affinity for carbon, to completely eliminate solid solution carbon and nitrogen in the steel. , JIS-G3141 grade by continuous annealing, using a low-order 1-killed steel with a normal carbon level, which has both non-aging properties and local ductility comparable to box-annealed materials, and also has bake hardenability (hereinafter referred to as BH properties). 3F! Related to cold-rolled steel sheet for deep drawing "symbol 5PCEJ".

(Prior art) The production of 5PCE class steels by continuous annealing has been done mostly using IF steel. This IFj% is based on the invention described in Japanese Patent Publication No. 44111086, and many improvements and improvement inventions have been made recently. However, IF steel requires a long time to undergo vacuum degassing treatment during steelmaking, and is economically unsuitable for box annealing, which is the conventional method.
The situation is that it is inferior to fl guild steel, and major restrictions are required on steelmaking equipment. Furthermore, it becomes extremely difficult to impart BH properties. Attempts have also been made to manufacture continuously annealed 5PCE class steel using normal Hemega guild steel instead of IF steel, but there are two basic complaints about it: poor strain aging resistance and poor local ductility, so it is no longer used. It hasn't arrived yet.

(Problem to be solved by the invention) In recent years, many inventions have been made with the aim of reducing solute carbon and suppressing aging by optimizing the cooling and overaging treatment of continuous annealing. being done. In order to prevent stretcher strain that impairs the appearance quality of press-formed products, the aging index (hereinafter referred to as AI) should be set to 3)c.
It is generally well known that gf/- or less is required.

On the other hand, in order to extract enough solid solution carbon, which causes strain aging, through a short-time overaging treatment such as continuous annealing, it is advantageous to distribute more carbides, which serve as precipitation sites, within the grains. However, Takahashi et al.
5ociety or^IME, 1982. p5
1), Hosoya et al.
5occulty of AIME.

1984, p. 81) and Japanese Patent Publication No. 59-59832 reveal that an increase in intragranular carbide density increases yield strength (YP) and decreases ductility. Therefore, a cold-rolled steel sheet that is not aged and has excellent workability actually needs to have an appropriate amount of solid solute carbon and intragranular carbide density.

Solute carbon, which causes strain aging, has the advantage of hardening the material through baking coating after press forming and improving the Punt resistance of the panel. It is most desirable that no Letcher strain occurs and high BH properties are ensured. However, conventional IF steel or box-annealed AN guild steel has almost no residual solid solute carbon, so although it is completely non-aging, it is extremely difficult to impart BH properties to it.

Conventionally, there is no clear evaluation index other than local ductility, and it cannot be evaluated by ductility in a normal tensile test.
Furthermore, the material factors that influence local ductility were not clear.

However, based on the results of the research conducted by the present inventors, A and K are methods for evaluating local ductility, which have been unclear until now.

LCHo (Met, Eng) proposed by G71OSH
, Quart.

15 (1975) 53) was found to be effective as described below (the test method is approximately ^, K, Cll
This is in line with the proposal of 08II, but see Example 1 for details. ).

It has also become clear that optimizing the balance between the intragranular carbide density and the amount of solid solute carbon is the most fundamental technology in order to achieve both non-aging properties and workability, and to ensure BH properties. . Therefore, the present invention provides a deep drawing material that has both non-aging properties and BH properties by controlling the intragranular carbide density and the amount of solid solute carbon, and also overcomes the disadvantage of poor local ductility of conventional AD-killed steel sheets produced by continuous annealing. The company provides cold-rolled steel sheets.

(Means for Solving the Problems) The gist of the present invention is that C: 0.008 to 0.025 in weight%
%, Mn: 0.05-0.20%, S: 0.004-
0.015%, acid-soluble Al: 0.03 to 0.15%, P as impurities: 0.012% or less, N: 0.0%
030% or less, with the balance consisting of Fe and unavoidable impurities, the steel is subjected to normal hot rolling, followed by cold rolling and continuous annealing.
A steel plate (strip) obtained by temper rolling, with a solid solution carbon content of 5 pp1 or less and 2 ppm or more, and an intragranular carbide density (ρ).

ko/-) as the aging index (AI, kg f /sj)
The range of the following formula, which is a function of , is o ≦ (-0, 1941
x105. Ka ρ≦(-0, B4L5X A l
+4.80411) X 105 After aging treatment (100℃
If the yield strength (YP) of ×1 h) is 19) cg f /- or less, LD is an index of local ductility! -1° is 30.9mm
The above is a cold rolled steel sheet having a bake hardening ability of 2 to 5 kgf/mm2.

(Function) First, the reason for numerical limitations on each component of C, Mn, P, S, Al, and N in the present invention will be described.

The lower limit of C is limited from the viewpoint of statute of limitations. That is, in overaging precipitation, it is necessary to increase the degree of supersaturation of C in order to increase the driving force for precipitation, but if the C content is low, the degree of supersaturation of C will not increase no matter how much quenching is performed after annealing. In this sense, C needs to be 0.01)II% or more. Also C
The upper limit is limited by workability, and if it is 0.025% or more, the desired workability cannot be obtained. The lower limit of Mn and the upper limit of S are limited from the viewpoint of preventing FeS generation. That is, when the Mn/S ratio is small, S becomes FeS and precipitates, which causes hot embrittlement. Therefore, Mn needs to be 0.05% or more, and S needs to be 0.015% or less. On the other hand, the lower limit of S is limited in order to secure an appropriate amount of MnS.

In other words, MnS is a precipitation site for cementite that precipitates inside grains, and by dispersing MnS appropriately, the nucleation density of cementite is increased appropriately, the diffusion distance of carbon is shortened, and the time required for diffusion is shortened, resulting in non-aging. promote the development of Therefore, S is set to 0.004% or more. Further, the upper limit of Mn needs to be 0.15% or less from the viewpoint of workability. Al needs to be 0.03% or more to promote AIN precipitation, but if it is too much, it will harden the steel and deteriorate the r value, so it needs to be 0.15% or less.

P and N are each 0.012% from the viewpoint of workability.

It needs to be 0.0020% or less.

Next, in order to examine the relationship between the amount of solid solute carbon and the BH property, the BH properties of materials having various amounts of solid solute carbon were investigated. The results are shown in FIG. A from the point of view of stretcher strain
It is necessary to satisfy I≦3 kgf/-, and for this purpose, it is necessary to reduce the amount of residual solid solution carbon to 5 ppm or less. On the other hand, the lower limit is limited by BH, 2) BH above cgf/-
In order to provide this, the amount of residual solid solute carbon must be 2 ppm or more.

Up to 5 kg f/m while satisfying the above conditions
It can be seen from FIG. 1 that it is possible to provide a BH of m2.

The problem of local ductility in continuous annealing materials of low-carbon Al-killed steel, which had been unclear until now, was investigated in detail from various perspectives, and it was discovered that this is plane strain fracture that occurs at actual press sites, and that this accounts for the majority of fractures. There was found. Therefore, local ductility means ductility in a plane strain state, where diffusion constriction and local constriction occur almost simultaneously, and this is the most severe deformation condition for the material. but,
Ductility under plane strain deformation cannot be evaluated by ordinary tensile tests because plane strain conditions cannot be obtained.

As a method for evaluating ductility under such plane strain deformation, A.
K, GHO8HI7) We focused on the proposed LDHo and found it to be the simplest and most effective. Therefore, in the present invention, LDH was adopted as an evaluation index of local ductility. Next, we investigated the governing factors of LDHo. The results are shown in Figures 2 and 3. Figure 2 shows AI and solute carbon content and LDH
The relationship between o was investigated before and after aging treatment (100° C. x 1 h).

FIG. 3 shows an investigation of the relationship between LDHo and carbide density for materials in which the solid solution carbon I was set to Oppm and the carbide density was continuously changed. The amount of solid solute carbon was determined by the internal friction method, and the carbide density was determined by an extracted replica electron micrograph taken at a magnification of 10,000 times. From these results, LD)i
It was found that o deteriorated significantly due to strain aging and intragranular carbide density.

On the other hand, in order to extract a sufficient amount of solid solute carbon through a short-time overaging treatment such as continuous annealing, it is advantageous to distribute more carbides, which serve as precipitation sites, within the grains. However, when the intragranular carbide density is excessively present, the yield point (YP) increases and LDHo decreases. Furthermore, if precipitation of solid solution carbon is insufficient, YP increases and LDHo decreases due to strain aging. Therefore, it is necessary to appropriately control the amount of solid solute carbon and the intragranular carbide density.

Therefore, in order to clarify this condition, materials having various solid solution carbon contents and intragranular carbide densities were prepared by heat treatment as shown in FIG. In the heat treatment (A) in FIG. 4, the intragranular carbide density increases as the quenching end point temperature (TE) decreases, and the amount of solid solute carbon decreases as the overaging time (tOA) increases.

Further, in FIG. 4(B), the intragranular carbide density increases as the TE decreases, and the amount of solid solute carbon decreases as the tOA increases. For the sample thus obtained, LDH was applied immediately after temper rolling and after aging treatment (100°C
o test and tensile test were conducted, and the results shown in FIG. 5 were obtained. Figure 5 shows the LDHo and y after aging treatment (100°C
p. As a result, if the following conditions are satisfied, YP≦19kgE/sJ (upper limit of A, 9 guild steel by box annealing) and LDHo≧30.9■嘗 (average of 1 guild steel by box annealing) even after aging treatment. It turned out to be satisfying.

Aging Index (AI) from the perspective of stretcher strain
3 kg f/d or less, shape freezing property (Y P
, kg f /mJ) and local ductility (L D HO, I
The intragranular carbide density (ρ, co/−) is calculated from the viewpoint of ρ≦(
-0,1941XA I +3.8831) Xl05
(1) ρ≦(−0,8415XA I +4.8041
1) Control to Xl05(2).

In Aj7 killed steel produced by conventional continuous annealing, the balance between the amount of solid solute carbon and the density of intragranular carbides has not been optimized, so the above range of ρ has not been used. . Therefore, the present invention is a cold-rolled steel sheet for AJL guild deep drawing by continuous annealing, which was conventionally considered impossible to manufacture, with an optimized balance between the amount of solid solute carbon and the density of carbides in the grains, and furthermore, it is a cold-rolled steel sheet for AJL guild deep drawing that is made by continuous annealing, which was previously considered impossible to manufacture. This cold-rolled steel sheet has BH properties that are difficult to impart to A1 guild steel.

Next, how useful the present invention is compared to box-annealed A1 guild steel and IF steel will be explained using examples.

(Example-1) After tapping steel having the chemical composition shown in Table 1 and manufacturing a slab by continuous casting method, this slab was
Heating to 1050℃, finishing temperature is 860-880℃
The sheet was hot rolled to a thickness of 4.0 mm, then cooled on a runout table at an average cooling rate of 15° C./S, and then wound into a coil at 690° C.

This hot-rolled steel strip was pickled and then cold-rolled to 0.8 strands to obtain a cold-rolled steel strip, and subsequently, this cold-rolled steel strip was subjected to continuous annealing. The conditions for continuous annealing are: annealing temperature 2820℃, soaking time: 1m
In, first slow cooling: cooling at 5.5°C/S to 690°C,
Subsequent rapid cooling: cooling at 120°C/S until 250°C, proper cooling condition = 2S hold at 250°C, reheating condition: heating at 30°C/S until -340°C, overaging condition: end temperature at 270°C
The overaging time was set to 200 s. Thereafter, skin bath rolling was carried out at an elongation rate of about 1.0%, and the sample was subjected to a test.

As tests, tensile and LDHo tests were conducted immediately after skin bath rolling and after artificial aging treatment at 100° C. for 1 hour. The tensile test was conducted using JI322201, No. 5 test piece.
This was carried out according to the method described in No. 2241. In addition, AI, BH properties, solid solution carbon content, and intragranular carbide density were also determined. AI is 10
? 6 After pre-straining, artificially accelerated aging at 100°C x 6 (1 mIn) is performed and the increase in yield strength is shown as the increase in yield strength before and after this aging. It is shown by the increase in yield strength.The amount of solid solute carbon was determined by the internal friction method, and the intragranular carbide density was determined by an extracted replica electron micrograph at 10,000 times magnification.LDHo
The test was conducted using a 1001-φ ball head punch and an IHmmφ die at a punch speed of 40! I11/1111s
Confirm that there is no material inflow and apply wrinkle holding pressure (BHF)
This was carried out using anti-rust oil lubrication in consideration of actual press conditions of 30 tons.

The test results are shown in Table 2. As is clear from this table, steels A, B, and C according to the present invention exhibit formability comparable to that of box-annealed materials, and have B of 2 to 5) cg f / aJ.
It has H. On the other hand, ff4D has A I −4,5
k [f /mm2, which is unsuitable from the point of view of stretcher strain, and furthermore, YP increases significantly after aging treatment due to strain aging, and LDHo also decreases significantly. Steel E has a high intragranular carbide density, so immediately after the skin bath, YP is already high < LDHo is low. In addition, scrap F has high AI and intragranular carbide density, and is unsuitable in all respects.

(Example-2) Using steel B in Table 1, continuous casting-hot rolling, pickling, and cold rolling were performed under the same manufacturing conditions as in Example-1, and continuous annealing was performed under the conditions shown in Table 3. went. After that, about 1. (Skin bath rolling was performed at an elongation rate of 1% and the test was performed. The test method was the same as in Example-1.

The test results are shown in Table 4. The heat treatment codes in Table 4 correspond to those in Table 3. f in Table 4! j4a.

Since the material b is Al-4.5 kgf/mm2, it is unsuitable from the viewpoint of stretcher strain, and furthermore, YP increases significantly after aging treatment due to strain aging, and LD) io also decreases significantly. Steels c and d have a low Al-1, 0.5 kg f/■1, and do not have problems such as deterioration of formability due to strain aging or generation of stretcher strain, but they have high intragranular carbide densities, making it difficult for skin to form. Immediately after, YP is already high.
LDH is low. Furthermore, it has almost no bake hardenability. In contrast to these steels, the steel e+'+g of the present invention exhibits formability comparable to that of box annealed materials, and has a B of 2 to 5 kg f/mJ.
Has H property.

/ (Effects of the invention) Cold-rolled steel sheets are consumed in large quantities for automobiles, electrical products, and building materials.
It is also a material for surface-treated steel sheets. The present invention relates to a cold-rolled steel sheet used for such applications, and the present invention relates to a cold-rolled steel sheet using continuous annealing to achieve unprecedentedly good formability and BH properties.
It is a cB class low carbon Al guild cold rolled steel sheet. Therefore, an extremely large industrial effect is expected.

[Brief explanation of the drawing]

The drawings show the technical contents of the present invention, and FIG.
A chart showing the relationship between I, BH, and YP-El. Figure 2 is L
A chart showing the influence of the amount of solid solute carbon on DHo. FIG. 3 is a chart showing the influence of intragranular carbide density on LDHo. FIGS. 4(A) and 4(B) are diagrams showing the heat treatment cycle of the sample material. Figure 5 shows AI. Intragranular carbide density and LD after aging treatment (100°C x 1h)
It is a chart showing the relationship between Ho, YP and BH. Fig. 2 Influence of rotational speed on 1DH σ Pes Ij゛S AI (ky//, vIKsu3 S 8 /3 60 Persistent each member t (pp Wei) Fig. 3 LDHar'JA°t'h-R4 compound NHK I ward AI and BH rotten system 1 Toyu Higashi T (trn) Figure 4 manual retail amendment (voluntary) September 2, 1999

Claims (1)

[Claims] In weight%, C: 0.008-0.025%, Mn: 0.
Contains 0.05 to 0.20%, S: 0.004 to 0.015%, acid-soluble Al: 0.03 to 0.15%, and P as impurities of 0.012% or less and N of 0.0030%. % or less, and the remaining amount of Fe and unavoidable impurities is a steel plate obtained by normal hot rolling, followed by cold rolling, continuous annealing, and temper rolling, the steel plate having a solid solute carbon content of is less than 5 ppm2
ppm or more, the intragranular carbide density (ρ, co/mm^2) is within the range of the following formula, which is a function of the aging index (AI, kgf/mm^2), and ρ≦(-0.1941×AI+3.883
1)×10^5, and ρ≦(−0.8415×AI+4
．． 8048)×10^5 After aging treatment (100℃×1h)
yield strength (YP) of 19 kgf/mm^2 or less, LDH_0, an index of local ductility, of 30.9 mm or more,
A non-ageing bake hardenable cold rolled steel sheet with excellent local ductility produced by continuous annealing and characterized by having a bake hardenability of ~5 kgf/mm^2.