JPH03277741A - Dual-phase cold roller steel sheet excellent in workability, cold nonaging properties and baking hardenability and its manufacture - Google Patents

Abstract

PURPOSE:To obtain a dual-phase cold rolled steel sheet excellent in workability, cold nonaging properties and baking hardenability, in a dead soft steel sheet synergically mixed with Nb and B and having a structure of ferrite and acicular ferrite, by specifying its structure, the grain size and the total volume rate of the secondary phase. CONSTITUTION:Steel stock contg., by weight, <=0.01% C, <=0.1% Si, <=0.5% Al and <=0.02% N, contg. one or more kinds of 0.01 to 2.0% Mn and 0.005 to 5% Cr under the condition satisfying Mn+CrX2>=0.2, furthermore contg. one or more kinds of 0.005 to 0.1% Nb and 0.0005 to 0.01% B under the condition satisfying Nb+BX10>=0.02 and the balance Fe is used. This stock is hot-rolled, is then cold-rolled, is thereafter subjected to continuous annealing at a temp. higher than the Ac1 transformation point -50 deg.C and less than the Ac1 transformation point, is successively cooled at >=10 deg.C/sec cooling rate and is thereafter subjected to temper rolling at >=0.8% draft. Then, the dual-phase cold rolled steel sheet having a structure of ferrite and acicular ferrite with <=30mum grain size and <=5% total volume rate and having <=5% yield elongation can be obtd.

Description

[Detailed Description of the Invention] (Industrial Application Field) This invention is applicable to cold-rolled steel sheets that have excellent deep drawability, such as outer panels for automobiles, and which require high plastic deformation resistance for finished products. In particular, the present invention relates to a cold-rolled steel sheet that has excellent formability during processing, aging resistance when stored at room temperature, etc., and high baking hardenability (BH property), and a method for manufacturing the same.

(Prior Art) Press-formed products used for outer panels of automobiles and the like are frequently damaged by physical forces, and manufacturers of such products would like to use steel plates that are less susceptible to damage if possible. In such a skin plate, the resistance to dents and scratches due to collisions, for example, is called Punto resistance, and this property can generally be obtained by increasing the yield stress of the steel plate.

However, in press working, it is desired to reduce the yield stress of the steel plate from the viewpoint of the energy required during the working and the accuracy of the shape.

Steel plates that meet these conflicting demands are usually 1
BH (Bake
Hardening) steel plate. This steel plate usually has solid solute C or solid solute N, especially solid solute C, present in the steel, and paint baking is caused by the diffusion of solid solute C, etc., into mobile dislocations generated during processing due to high temperatures inside. It hardens by reducing the mobility of

The problem with the above-mentioned hardening mechanism is that some dislocations are already fixed by the solid solution components before processing, and therefore, during processing, wavy surface defects called stretcher strain occur due to elongation at yield point. At present, ultra-light reduction cold rolling for shape correction, called temper rolling after annealing, is used to separate mobile dislocations from solid solution components and create strain concentrated areas on the surface. Such inconveniences are prevented by trapping solid solution C and the like to prevent diffusion to mobile dislocations.

Although such measures are effective in the short term, they do not completely suppress the progression of the age hardening phenomenon itself, and this is especially noticeable in the case of high BH steel sheets with a BH amount of 3 kgf/mm or more at the lower yield point. However, even before processing, solid solution C easily diffuses into mobile dislocations due to long-term storage at room temperature or high-temperature treatment of up to 550°C in surface treatment lines, etc. Therefore, the usage conditions for these steel sheets are limited. The current situation is that it has become something that has been deprecated.

As a means to suppress aging at room temperature and obtain BH properties, rapid cooling treatment during annealing (Japanese Patent Publication No. 61-10014), partial fixation of C by carbonitride forming components (Japanese Patent Publication No. 61-10014),
1-9368, JP-A-61-281852), etc., but in both cases, it is difficult to obtain a high amount of BH because solid solution C is controlled in a delicate balance, and the component There has been a problem in that the compatibility between anti-aging properties and BH properties is easily lost due to changes in process conditions.

In contrast, applicants including the inventors have already
A completely new method of non-ageing and high BH in 2008 publication etc.
We are proposing shaped steel plates. That is, when an ultra-low carbon steel sheet to which Nb and B are jointly added is heated to a temperature range where α-phase and T-phase coexist above the transformation point and then rapidly cooled, it becomes a two-phase structure of acicular ferrite and ferrite. This structure contains solid solution C and has high BH properties, but since most of the solid solution C is trapped in the highly strained acicular ferrite, which is densely populated with dislocations, it has almost no yield point elongation even after annealing. It does not cause aging at room temperature.

However, this type of non-aging, high BH type steel sheet is inferior in workability, especially elongation value, compared to conventional non-aging, non-BH type soft steel sheets. Moreover, according to the inventors' investigation, there were cases in which non-BH type steel sheets could not be processed in actual deep drawing even if the r value was apparently higher than that. Furthermore, as the annealing temperature increases, the workability tends to deteriorate, making it difficult to achieve both the workability of the vest and the anti-aging properties of the vest.

(Problems to be Solved by the Invention) The purpose of the present invention is to improve the hardenability (BH property) of cold-rolled steel sheets used for automobile outer panels, etc. It has aging resistance against temperature rises below the recrystallization temperature (hereinafter referred to as room temperature non-aging in this specification), and is comparable in workability to non-aging, non-BH type soft deep drawing steel sheets. The purpose of this invention is to propose a cold-rolled steel sheet and a manufacturing method thereof.

(Means for Solving the Problems) This invention provides C: 0.01% or less (hereinafter simply expressed in %) or less, Si: 0.1% or less, Al: 0.01% or less.
5% or less and N: 0.02% or less, and Mn:
Contains one or two of 0.01 to 2.0% and Cr: 0.005 to 5% under conditions that satisfy Mn+CrX2≧0.2(%), and further Nb: 0.005
~0.1% and B: 0.0005~0.01% 1
Contains one or two species under conditions that satisfy Nb+BX10≧0.02 (%), and the remainder consists of Fe and unavoidable impurities, including ferrite and acicular particles with a particle size of 30 μm or less and a total volume ratio of 5% or less. It has a ferrite structure and has a yield point elongation of 0.5% or less,
Composite structure cold-rolled steel sheet (first invention) with excellent workability, non-aging property at room temperature, and baking hardenability, C: 0.01% or less, Si: 0.1% or less, Al
: 0.5% or less and N: 0.02% or less, and Mn: 0.01 to 2.0% and Cr: 0
．． 005 to 5% of one or two types, Mn+CrX2≧0
．． 2 (%) under conditions that satisfy the content, and further contains Nb: 0.005
~0.1% and B: 0.0005~0.01% 1
After hot-rolling and then cold-rolling a steel cable material containing one or two types of Nb+BX10≧0.02 (%) and the remainder consisting of Fe and unavoidable impurities, Ac, Transformation point higher than -50°C, Ac.

Continuous annealing at a temperature below the transformation point, followed by a cooling rate of 10
A method for producing a cold-rolled steel sheet with a composite structure having excellent workability, non-aging property at room temperature, and baking hardenability, which comprises cooling at a temperature of ℃/s or more and then skin-pass rolling with a reduction rate of 0.8% or more. (Second invention), and C: 0.01% or less, Si: 0.1% or less, Ti:
0.005 to 0.05%, Al: 0.5% or less,
N: 0.02% or less and S: 0.05% or less, C-(12/48Ti 12/32S-12/14N
)≧0.0005 (%), and Mn: 0.01 to 2.0% and Cr: 0.00
5 to 5% of one or two types of Mn + Cr
Contains 0.2 (%) under conditions satisfying Nb:
0.005-0.1% and B: 0.0005-0
．． 01% of one or two kinds of Nb+BX1.01%. O20,0
2 (%), and the remainder is composed of Fe and unavoidable impurities, resulting in a structure of ferrite and acicular ferrite with a grain size of 30 μm or less and a total volume fraction of 5% or less, and the elongation at yield point is characterized by being 0.5% or less,
Composite structure cold-rolled steel sheet (third invention) with excellent workability, non-aging property at room temperature, and baking hardenability, and c: o, oi% or less, Si: 0.1% or less, T
i: 0.005 to 0.05%, A1: 0.5% or less, N: 0.02% or less, and S: 0.05% or less, C-(12/48Ti-12/32S-12/14N )
≧0.0005 (%) and M
n: 0.01-2.0% and Cr: 0.005
~5% of one or two types, Mn + Cr X 2≧0
．． Nb:
0.005-0.1% and B: 0.0005-0.
01% of one or two types, Nb+BXIQ≧0.02
(%) with the remainder being Fe and unavoidable impurities. After hot rolling and then cold rolling, the material has Ac, a transformation point higher than -50°C, and Ac.

Continuous annealing at a temperature below the transformation point, followed by a cooling rate of 10
Manufacture of a cold-rolled steel sheet with a composite structure having excellent workability, non-aging property at room temperature, and baking hardenability, which is characterized by cooling at a temperature of ℃/s or more and then skin-pass rolling with a reduction rate of 0.8% or more. Method (fourth invention), and C: 0.01% or less, Si: 0.1% or less, P: 0.0
3-0.15%, Al 70.5% or less and N: 0
．． 0.02% or less, and Mn: 0.01 to 2.0
% and Cr: 0.005 to 5% of one or two types under conditions satisfying Mn+CrX2≧0.2(%), and further Nb: 0.00
5 to 0.1% and B: 0.0005 to 0.01%, containing one or two types under the condition that Nb + BX10≧0.02 (%) is satisfied, and the remainder is the composition of Fe and unavoidable impurities. It has a structure of ferrite and acicular ferrite with a grain size of 30 μm or less and a total volume fraction of 5% or less, and has an elongation at yield of 0.5% or less,
A cold-rolled steel sheet with a composite structure (fifth invention) having excellent workability, non-aging property at room temperature, and baking hardenability, and C: o, oi% or less, Si: 0.1% or less, P:
0.03-0.15%, Al: 0.5% or less and N
: Contains 0.02% or less, and Mn: 0.01
~2.0% and Cr: 0.0 (containing one or two types of 15 to 5% under conditions satisfying Mn + Cr Steel containing ~0.1% and B: 0.0005~0.01% of one or two types under the condition that Nb+BX10≧0.02 (%) is satisfied, and the remainder is Fe and inevitable impurities. After hot rolling and then cold rolling, the material has a transformation point higher than -50°C.

Continuous annealing at a temperature below the transformation point, followed by a cooling rate of 10
A method for producing a cold-rolled steel sheet with a composite structure having excellent workability, non-aging property at room temperature, and baking hardenability, which comprises cooling at a temperature of ℃/s or more and then skin-pass rolling with a reduction rate of 0.8% or more. (Sixth invention), and C: 0.01% or less, Si: 0.1% or less, Ti
: 0.005-0.05%, P: 0.03-0.15
%, Al: 0.5% or less, N: 0.02% or less, and S: 0.05% or less, C-(12/48Ti
12) 323-12 no 14N) ≧0.0005
(%), and Mn: 0.0
1 to 2.0% and Cr: 0.005 to 5%, one or both of which are contained under conditions that satisfy Mn + Cr 0
．． 1% and B: 0.0005 to 0.01% of one or two types are contained under the condition that Nb+BX10≧0.02 (%) is satisfied, and the remainder has a composition of Fe and inevitable impurities, and is ferrite. and an acicular ferrite structure with a grain size of 30 μm or less and a total volume fraction of 5% or less, and an elongation at yield point of 0.5% or less,
A cold-rolled steel sheet with a composite structure (seventh invention) that has excellent workability, non-aging properties at room temperature, and hardening properties, and C: 0.01% or less, Si: 0.1% or less, Ti:
0.005-0.05%, P: 0.03-0.15%
, Al: 0.5% or less, N: 0.02% or less, and S: 0.05% or less, C-(12/48Ti-12
/32S 12/14N)≧0.0005 (%), and Mn: 0.01 to 2.0
% and Cr: 0.005 to 5% of one or two types,
Contains under the condition that Mn + Cr
A steel material containing Nb-+-BXIO≧0.02 (%) with the balance consisting of Fe and unavoidable impurities is hot-rolled and then cold-rolled to reduce the Ac and transformation points. Higher than -50″C, Ac.

Continuous annealing at a temperature below the transformation point, followed by a cooling rate of 10
A method for producing a cold-rolled steel sheet with a composite structure having excellent workability, non-aging property at room temperature, and baking hardenability, which comprises cooling at a temperature of ℃/s or more and then skin-pass rolling with a reduction rate of 0.8% or more. (Eighth invention).

(Function) According to conventional wisdom, aging is suppressed by strain concentrated within the second phase grains and/or grain boundaries.
It did not work effectively unless it was about %. The same was true for copper plates having acicular ferrite as the second phase.

However, the inventors found that in a steel plate with acicular ferrite as the second phase, when the strain concentration is additionally reinforced from the outside to reduce the elongation at yield point to 0.5% or less, the second phase
It has been discovered that even if the phase fraction is lower than this, it has an aging suppressing effect.

The experiments that formed the basis of this invention will be described below.

C: 0.0030%, Si: 0.01%, Mn
: 0.61%, P: 0.015%, S: 0.00
7%, Al: 0.059%, N: 0.0025%,
A steel having a composition of Nb: 0.012% and B: 0.0013% was subjected to the steps of continuous casting, hot rolling, cold rolling, and continuous annealing while changing the annealing temperature, and the steel sheet was made to have a thickness of 0.8%.
! A cold-rolled In steel plate was used. Here, the slab heating temperature is 1
250"C, Ar, the transformation point was 850°C, the hot rolling end temperature was 880°C, the hot rolled plate thickness was 3.2 trm, and the hot rolling winding temperature was 620°C. Cold rolling was 0. Measurement of volumetric expansion coefficient after cold rolling (heating rate l
The transformation point was 910°C.The annealing temperature was varied from 810 to 950°C (soaking for 5 seconds), and the subsequent cooling rate was 25°C/s.

The BH amount of the steel plate thus obtained is 5.0 to 5.5 kg.
f/mm". Here, the BH amount is determined by the nominal stress at the nominal amount of 2% prestrain and the lower yield point stress (nominal) after aging treatment of 170°C 12O minutes after prestrain. I took the difference.

Each of the above cold rolled steel sheets was subjected to 1.2% temper rolling, and the second phase volume fraction, elongation value, room temperature non-aging property (yield point elongation ) is shown in Figure 1.

In the same figure, in order to obtain sufficient room-temperature non-aging properties without skin pass rolling, a volume fraction of the second phase of 10% or more is required.
When temper rolling (1.2%) is applied, the volume fraction of the second phase is 10%.
Even if it is less than 5%, it will not be aged at room temperature even with accelerated aging treatment (aging at 100°C for 110 hours; equivalent to carbon aging at 30°C for 16 months). In this range, the elongation value is also good and there is no aging deterioration.

The heat treatment (annealing) temperature at which the second phase volume fraction becomes 5% or less is lower than the Ac and transformation point measured by thermal expansion measurement. In this temperature range, acicular ferrite is only observed as minute grains at ferrite grain boundaries, especially at triple points. Therefore, it is considered that Mn was segregated at the grain boundaries and only this portion was transformed into the T phase at a temperature lower than the transformation temperature of the ferrite matrix during annealing.

Note that there is a condition in which the non-aging property deteriorates at room temperature just above the transformation point of the matrix, and this is thought to be due to the grain size rapidly coarsening as the transformation of the matrix begins, which prevents strain concentration due to skin pass rolling. .

Now, as mentioned above, it has been found that a steel sheet containing 5% or less of the total volume of acicular ferrite particles in the ferrite is also excellent in workability.

As shown in FIG. 1, the elongation values are better than those annealed above the ferrite phase transformation point. The same applies to the r value, and the actual processability with respect to the r value is also good.

FIG. 2 shows three types of cold-rolled steel sheets, namely the ferrite + fine-grain acicular ferrite type steel sheet (C:
0.0032-0.0036%, Si: 0.01%
, Mn: 0.40-0.42%, Cr: 0.08
%, P: 0.020%, S: 0.010%, Al
: 0.06%, N: 0.0026-0.002
7%, Nb: 0.015-0.025%, B:
0.0012%, slab heating temperature: 1200"C, A
r + transformation point: 830°C 1 hot rolling end temperature = 850°C 1 hot rolled sheet thickness: 3.5 mm, hot rolled sheet winding temperature = 600°C 1 cold rolled sheet thickness: 0.8 in, Ac, transformation point -890℃1
Annealing temperature: 870℃ 1 Soaking time: 1o seconds, cooling rate = 2
0°C/s, temper rolling reduction rate = 1.2%; non-aging, high BH
type), conventional ferrite + coarse grain acicular ferrite type control plate (C: 0.0030-0.0033%, Si:
0.01%, Mn: 0.40%, P: 0.018
%, S: 0.010%, Al: 0.05%,
N: 0.0024-0.0028%, Nb: 0
．． 015-0.025%, B: 0.0013%, slab heating temperature: 1200°C5Ar3 transformation point = 860°C
, Hot rolling end temperature: 890℃ 2 Hot rolled plate thickness: 3.5m+,
Hot-rolled coiling temperature: 2500℃, cold-rolled plate thickness: 0.8 am
, Ac, transformation point: 91O ℃ 2 annealing temperature: 940 ℃ 1 soaking time = 5 seconds, cooling rate = 20 ℃ / s, temper rolling reduction: 0.5%; non-aging / high BH type) and normal Ferritic single phase steel plate (C: 0.0022~0.0026%, S
i: 0.01~0.03%, Mn: 0.08~
0.12%, P: 0.008-0.012%, S:
0.005-0.015%, Al: 0.03-0.
05%, N: 0.0020-0.0028%, Nb
: 0.015-0.020%, slab heating temperature:
1200-1250°C-, Arz transformation point: 870°C 1. Hot rolling end temperature: 880-900°C 1. Hot-rolled sheet thickness: 3.5°C, hot-rolled sheet winding temperature: 580-660°C 1. Cold-rolled sheet Thickness 20, 8, Ac, transformation point: 930℃ 1 annealing temperature = 830 ~
88°℃ 1 soaking time: 0-20 seconds, cooling rate = 15-3
0″C/s; non-aging/non-BH type), r value, D, R.

(Limit drawing ratio) test (wrinkle presser crab 700 kgf
) shows the result of investigating the value. Here, T, S of each steel plate
.

(Tensile strength) level is 29-31kgf/lll1n
It is aligned to z.

In FIG. 2, the conventional ferrite + coarse grain acicular ferrite type steel sheet has the same r value as the ferrite single phase steel sheet, but when actually subjected to drawing processing, the drawing ratio is slightly lower. This is thought to be because when severe working is applied, the hard second phase restrains the deformation of the ferrite grains and reduces the anisotropy of plastic deformation due to crystal orientation. However, if the second phase is fine-grained and has a low volume fraction, it is processed without constraining the anisotropy of plastic deformation of the ferrite grains, so it is possible to obtain the same drawing ratio as a normal ferrite single-phase steel sheet. It is being

The inventors conducted further research based on the above findings, and
We have found the conditions necessary to obtain a high BH steel sheet with excellent workability and non-aging properties at room temperature.

First, the particle size of the second phase (acicular ferrite) is 30
It needs to be less than μm. If the particle size exceeds this, the processability will be adversely affected.

In addition, the volume fraction needs to be 5% or less, as already mentioned, in order to ensure workability and non-aging properties at room temperature. In the inventors' experiments, if even a small amount of the second phase was observed, the material had non-aging properties at room temperature, but the minimum value of the volume fraction of the second phase that could be confirmed at this time was 0.1%. .

In addition, in order to obtain the above particle size distribution, the presence of Mn is important for the formation mechanism of the fine second phase, but the inventors
It has been found that r also has a similar effect of promoting grain boundary segregation and γ phase formation. Specifically, the inclusion of Mn and/or Cr satisfying Mn+CrX2≧0.2% is a necessary condition for producing the fine γ phase during annealing heating. Furthermore, in order to convert this γ phase into acicular ferrite upon cooling, Nb and/or
Alternatively, it is necessary to contain B in a manner that satisfies Nb+BX10≧0.02% to delay the T→α transformation to the low temperature side.

In addition, even in steel sheets with the above-mentioned second phase distribution, if the initial yield point elongation exceeds 0.5%, a slight increase in yield point elongation due to accelerated aging is observed, so it is difficult to ensure room temperature non-aging properties. It is necessary to keep the elongation at yield point to 0.5% or less.

The reason for limiting the composition range of each component will be described below.

CTC is an important component in imparting BH properties, but if it exceeds 0.01%, it becomes difficult to maintain non-aging properties at room temperature even with the method of the present invention. Further, the lower the amount of C, the better the quality of the material, and if it exceeds 0.01%, good workability cannot be obtained. Therefore, the amount of C is set to 0.01% or less. Furthermore, in order to obtain high BH properties, the amount of C should be 0.000.
It is desirable that the content be 5% or more, and especially when adding Ti, which is a strong carbide forming component as described later, the solid solution C
It is essential that the amount is 0.0005% or more.

Si: A small amount of Si is not prohibited in order to increase the strength of the steel sheet, but if it is present in an amount exceeding 0.1%, it deteriorates the elongation and drawability of the copper sheet, so it is limited to 0.1% or less.

Al: Al is added to the mesh mainly for deoxidation during steel manufacturing. In addition, Al has the effect of fixing N as AlN and preventing a decrease in yield due to BN formation.
It is desirable to contain s% or more. However, addition of A1 exceeding 0.5% has a negative effect on the surface quality, so 0.5%
% or less.

Preferably it is 0.1% or less.

N: Not only does N deteriorate the deep drawability, but if it is not fixed with A1, it combines with B, greatly reducing the effect of adding B, so the larger the amount, the more Al is required, which is uneconomical.

Furthermore, since solid solution N has originally high aging properties at room temperature, baking is not used as a hardening component in this invention. Therefore, it is desirable to keep the amount of N as low as possible, but from the economical point of view of the process, the allowable amount is set to 0.02% or less. Preferably the amount of N is 0.0
It is best to keep it below 0.05%.

Mn, Cr: For the reasons already mentioned, Mn+Cr
It is necessary to add X2≧0.2%. Furthermore, both Mn and needles are components that can lower the transformation temperature of the entire steel sheet without deteriorating the deep drawability, and are also effective in increasing the strength of the steel sheet. However, the addition of less than 0.01% of Mn or C
Addition of less than 0.005% of r makes almost no contribution to lowering the transformation temperature or increasing the strength of the steel sheet. On the other hand, M
Addition of n exceeding 2.0% or excessive addition of Cr exceeding 5% deteriorates the balance between elongation and drawability of the steel sheet and strength. Furthermore, addition of more than 2.0% of Mn increases the endothermic reaction in the molten steel, and there is a risk that the temperature of the molten steel will drop, making vacuum degassing treatment impossible. Moreover, excessive addition of Cr exceeding 5.0% deteriorates the chemical conversion treatment property of the steel sheet surface. Therefore, Mn: 0.01~
2.0%, Cr: 0.005 to 5%, one or both of which must be contained, satisfying Mn+CrX2≧0.2%.

Nb, B: As described above, Nb and B, each alone or in combination, delay the T→α transformation during cooling to the low temperature side and promote the formation of an acicular ferrite+ferrite two-phase structure. It is also noticeable in the collective structure (111)
It has the effect of causing orientation accumulation and increasing workability (especially r value). Here, if Nb+BX10 is less than 0.02%, the two-phase structure and texture improvement will be insufficient, while if the Nb amount exceeds O81% or the B amount exceeds 0.01%, the addition effect will be insufficient. Not only does it become saturated, but it also causes a significant deterioration in elongation, lowering the strength-workability balance. Furthermore, if the amount of B added is less than 0.0005%, it will not contribute to the effect of joint addition, so there is no point in adding B at all.

The same applies to addition of less than 0.005% of Nb. Therefore, Nb: 0.005~0°1% and B
: 0.0005~0.01% of one or two types,
It is necessary to satisfy and contain Nb+BX10≧0.02%.

Ti, S: In this invention, there is no need to specifically specify S as a general component in steel, but it is desirable to reduce it within a range commensurate with the cost of the steel plate for processing, and it is preferably 0.05% or less. preferable. Furthermore, in the case of adding Ti, the amount of S added has an important meaning when considering the effective amount of Ti.

In this invention, Ti may be added to improve processability and yield. In order to produce the effect of Ti, it is necessary to add 0.005% or more, but 0.05%
Addition in excess of 20% is not only uneconomical in terms of the effect of addition, but also increases production costs due to an increase in the transformation point. In addition, in order to ensure BH properties, it is necessary to ensure the amount of solid solute C at 0.0005% or more, and effective Ti (TiN,
Regarding Ti) excluding TiS forming components, C-(12/4
8Ti 12/32 S -12/14 N)≧0.
0005(χ) must be satisfied. In addition, in the case of Ti addition, S is 0.0 to prevent a decrease in Ti yield.
5% or less. Therefore, the conditions for adding Ti are:
Ti: 0.005 to 0.05% and C (12/48Ti 12/32 S -12/14
N)≧0.0005(χ), and furthermore, s: o,
os% or less.

P: There is no need to specify P as a general component in t8, but 0.05% is required for applications that require softness.
The following is preferable. Further, as long as the steel sheet reinforcing component is added in an amount of 0.15% or less, the effects of the present invention will not be impaired. On the other hand, the effect of P on reinforcing the steel sheet is almost invisible unless it is added at least 0.03%, so when adding P for the purpose of strengthening the steel sheet, the amount of P should be 0.03 to 0.15%.
It needs to be %.

Next, the reasons for limiting the steel plate manufacturing conditions of this invention will be described below.

First, steel production may be carried out according to conventional methods, and the present invention does not particularly require limitations on these conditions, but from the viewpoint of cost and quality, it is desirable to use a continuous casting method.

For hot rolling, the usual method may be used, but fine 1.
In order to sufficiently concentrate Mn and/or Cr at grain boundaries to form grains, it is preferable that the coiling temperature after hot rolling is 550° C. or higher.

Cold rolling may also be carried out according to a conventional method, but a cold rolling reduction of 60% or more is desirable in order to obtain workability through recrystallization.

For annealing after cold rolling, a continuous annealing method is used since the box annealing method is insufficient to form a two-phase structure. Further, in order to obtain the fine second phase structure desired in the present invention, the annealing temperature is lower than the Ac transformation point of the ferrite phase, which is the first phase, as is clear from FIG.

The lower limit of the annealing temperature may be the temperature at which the T phase (austenite phase), which is the origin of the second phase, appears, but the temperature can be stabilized by setting the temperature to a temperature higher than -50°C below the Ac, transformation point of the ferrite phase. Two phases can be obtained. In addition, since the annealing is performed at a higher temperature than normal annealing, ferrite grains are likely to grow, and if soaked at the above temperature for more than 15 seconds, the room temperature non-aging property may deteriorate due to a decrease in the grain boundary area and a decrease in strain concentration. There is.

Therefore, the annealing soaking time is preferably 15 seconds or less.

In cooling subsequent to annealing, the cooling rate needs to be 10° C./s or more in order to convert at least a portion of the T phase into acicular ferrite during cooling. On the other hand, from the viewpoint of workability, the cooling rate is preferably 80° C./s or less.

In the fine acicular ferrite + ferrite dual-phase steel sheet thus obtained, it is necessary to strengthen the strain concentration inside the steel sheet as described above, and temper rolling is the most suitable method for this. In this case, the reduction ratio in skin pass rolling is required to be 0.8% or more, and preferably 1% or more, in order to strengthen strain concentration and obtain excellent anti-aging properties. Note that it is desirable to avoid a rolling reduction rate of more than 5%, as this will result in significant material deterioration.

The cold-rolled steel sheet of the present invention can also be applied to plated steel plates, and is particularly suitable as a base plate for hot-dip metal plated steel plates, which tend to undergo aging during the plating process.

(Example) Example 1 Steels having various component compositions shown in Table 1 were prepared.

These test steels were manufactured by continuous casting, and were rough rolled (reduction ratio 88%) and finish rolled (reduction ratio 88%) to a plate thickness of 3.
It was made into a hot coil of 5 VW, and then cold rolled to a thickness of 0.8 mm. Thereafter, it was pickled, continuously annealed, and temper rolled. Table 2 shows the main conditions of each process and the maximum grain size and volume fraction of the second phase (acicular ferrite phase) of the obtained steel plate according to a prior application investigation.

Table 3 shows the workability, BH properties, and aging characteristics of the cold rolled steel sheet thus obtained. The amount of Naori H was set at the lower yield point value shown in FIG.

As is clear from Table 3, all of the present invention examples have high BH
It can be seen that it not only shows excellent non-aging properties at room temperature, but also has excellent workability in elongation/strength balance (indicated by T, S, +E1 in Table 3) and r value.

On the other hand, Comparative Examples Nos. 10 to 13 whose components do not conform to this invention, and Comparative Examples Sou IB and I whose process conditions do not conform to this invention.
C, ID, IG, and IH all have poor BH properties or non-aging properties at room temperature compared to the examples of the present invention because a good second phase distribution or no second phase is obtained. , many have poor processability.

Furthermore, Comparative Examples IE and IF already had a high elongation at yield point before the accelerated aging treatment, and were out of the question.

Example 2 The present invention examples of Examples 1 and 3 were passed through a hot-dip galvanizing line. Soaking cycle of plating line is 550'C
It was 120 seconds.

The material of steel plate IA after plating is Y, S, : 16.5
kgf/mm”, T, S, :30.2 kgf/
mm2, elongation value 56.2%, r value 2.50, BH amount 5.
0 kgf/mm", and the elongation at yield point was 0.0%.

Also, steel plate 3 is Y, S,: 19.8 kgf/mmt
.

T, S,: 35.4 kgf/+am”, elongation value 5
2.0%, r value 2.41, BH amount 5. Okgf/■■2
, yield point elongation was 0.0%.

In all cases, the workability was almost the same as before plating, and the excellent high BH properties and non-aging properties at room temperature remained the same.

(Effects of the Invention) This invention has made it possible to stably industrially produce a steel plate that has excellent workability comparable to that of a steel plate for soft working, as well as high BH properties and non-aging properties at room temperature.

Such steel sheets are particularly suitable as steel sheets for processing external panels, and can be used under conditions where conventional BH steel sheets for processing cannot be used due to their aging properties, such as for long-term storage necessary to always maintain inventory, or for long-term storage. For export requiring a voyage of 500
The use of alloyed plating substrates that can withstand temperatures as high as ℃ opens the way for other applications.

[Brief explanation of drawings]

Figure 1 shows the effects of secondary effects on the workability and non-aging properties of steel sheets.
Graph showing the influence of phase axial ferrite amount and annealing temperature. Figure 2 is a graph showing the relationship with drawing ratio of the present invention steel and conventional steel. Figure 3 is baking hardening (BH property). FIG. How to measure r value and limit

Claims (1)

[Claims] 1. Contains C: 0.01 wt% or less, Si: 0.1 wt% or less, Al: 0.5 wt% or less, and N: 0.02 wt% or less, and Mn: 0.01 to 2. 0 wt% and Cr: 0.005 to 5 wt% under the conditions satisfying Mn+Cr×2≧0.2 (wt%), and further Nb: 0.005 to 0.1 wt%. and B: 0.0005 to 0.01 wt%, under the condition that Nb + B × 10 ≧ 0.02 (wt%), and the remainder is Fe and inevitable impurities, It has a structure of ferrite and acicular ferrite with a grain size of 30 μm or less and a total volume fraction of 5% or less, and has a yield point elongation of 0.5% or less, and has excellent workability, non-aging property at room temperature, and bake hardenability. Composite structure cold rolled steel sheet with excellent properties. 2. Contains C: 0.01 wt% or less, Si: 0.1 wt% or less, Al: 0.5 wt% or less, and N: 0.02 wt% or less, and Mn: 0.01 to 2.0 wt% and Cr: Contains one or two of 0.005 to 5 wt% under conditions that satisfy Mn+Cr×2≧0.2 (wt%), and further contains Nb: 0.005 to 0.1 wt% and B: 0.0005 ~0.01wt% of one or two types under conditions satisfying Nb+B×10≧0.02(wt%), with the remainder being Fe and unavoidable impurities, hot rolled and then After cold rolling, it is continuously annealed at a temperature higher than the Ac_1 transformation point -50°C and lower than the Ac_1 transformation point, and then cooled at a cooling rate of 10°C/s or more, followed by tempering with a rolling reduction of 0.8% or more. A method for producing a cold-rolled steel sheet with a composite structure that has excellent workability, non-aging properties at room temperature, and bake hardenability, and is characterized by subjecting it to rough rolling. 3. C: 0.01 wt% or less, Si: 0.1 wt% or less, Ti: 0.005 to 0.05 wt%, Al: 0.5 wt% or less, N: 0.02 wt% or less, and S: 0.05 wt%. % or less, C-(12/48Ti-12/32S-12/14N)
Mn + Cr (wt%) under conditions satisfying Nb+B×10≧0.02( wt%), and the remainder consists of Fe and unavoidable impurities, resulting in a structure of ferrite and acicular flights with a grain size of 30 μm or less and a total volume fraction of 5% or less, with an elongation at yield of 0. ．．
5% or less, a cold-rolled steel sheet with a composite structure having excellent workability, non-aging properties at room temperature, and bake hardenability. 4, C: 0.01wt% or less, Si: 0.1wt% or less, Ti: 0.005 to 0.05wt%, Al: 0.5wt% or less, N: 0.02wt% or less, and S: 0.05wt % or less, C-(12/48Ti-12/32S-12/14N)
Mn + Cr (wt%) under conditions satisfying Nb+B×10≧0.02( wt%), with the remainder consisting of Fe and unavoidable impurities. After hot rolling and then cold rolling, the steel material has an Ac_1 transformation point higher than -50°C and less than the Ac_1 transformation point. Processability, non-aging property at room temperature, and bake hardenability are characterized by continuous annealing at a temperature of A method for manufacturing cold-rolled steel sheet with a composite structure that has excellent properties. 5. Contains C: 0.01 wt% or less, Si: 0.1 wt% or less, P: 0.03 to 0.15 wt%, Al: 0.5 wt% or less, and N: 0.02 wt% or less, and Mn: Contains one or two of 0.01 to 2.0 wt% and Cr: 0.005 to 5 wt% under conditions that satisfy Mn+Cr×2≧0.2 (wt%), and further Nb: 0.005 ~0.1wt% and B:0.0005~0.01wt%, and one or two of them are contained under conditions that satisfy Nb+B×10≧0.02 (wt%), and the remainder is Fe and unavoidable impurities. It has a composition of ferrite and acicular ferrite with a grain size of 30 μm or less and a total volume fraction of 5% or less, and has a yield point elongation of 0.5% or less. A cold-rolled steel sheet with a composite structure that has excellent hardness and bake hardenability. 6. Contains C: 0.01 wt% or less, Si: 0.1 wt% or less, P: 0.03 to 0.15 wt%, Al: 0.5 wt% or less, and N: 0.02 wt% or less, and Mn: Contains one or two of 0.01 to 2.0 wt% and Cr: 0.005 to 5 wt% under conditions that satisfy Mn+Cr×2≧0.2 (wt%), and further Nb: 0.005 ~0.1wt% and B:0.0005~0.01wt%, and one or two of them are contained under conditions that satisfy Nb+B×10≧0.02 (wt%), and the remainder is Fe and unavoidable impurities. After hot rolling and then cold rolling, a steel material made of A method for producing a cold-rolled steel sheet with a composite structure having excellent workability, non-aging property at room temperature, and bake hardenability, the method comprising subsequently performing temper rolling at a rolling reduction of 0.8% or more. 7. C: 0.01 wt% or less, Si: 0.1 wt% or less, Ti: 0.005 to 0.05 wt%, P: 0.03 to 0.15 wt%, Al: 0.5 wt% or less, N: 0.02wt% or less and S: 0.05wt% or less, C-(12/48Ti-12/32S-12/14N)
Mn + Cr (wt%) under conditions satisfying Nb+B×10≧0.02( wt%), and the remainder consists of Fe and unavoidable impurities, resulting in a structure of ferrite and acicular ferrite with a grain size of 30 μm or less and a total volume fraction of 5% or less, and yield point elongation of 0. .5% or less, a cold-rolled steel sheet with a composite structure having excellent workability, non-aging properties at room temperature, and bake hardenability. 8, C: 0.01 wt% or less, Si: 0.1 wt% or less, Ti: 0.005 to 0.05 wt%, P: 0.03 to 0.15 wt%, Al: 0.5 wt% or less, N: 0.02wt% or less and S: 0.05wt% or less, C-(12/48Ti-12/32S-12/14N)
≧0.0005 (wt%), and one or two of Mn: 0.01 to 2.0 wt% and Cr: 0.005 to 5 wt%, according to the following formula Mn + Cr x 2 ≧ 0 .2 (wt%) under conditions that satisfy Nb: 0.005 to 0.1 wt% and B: 0.0005 to 0.01 wt%. After hot rolling and then cold rolling, a steel material containing 0.02 (wt%) under conditions that satisfies the Ac_1 transformation point -50°C, with the remainder consisting of Fe and unavoidable impurities, Workability, non-aging properties at room temperature, characterized by continuous annealing at a temperature below the Ac_1 transformation point, subsequent cooling at a cooling rate of 10°C/s or more, and then skin pass rolling with a rolling reduction of 0.8% or more. and a method for producing a cold-rolled steel sheet with a composite structure that has excellent bake hardenability.