JPS60228617A - Manufacture of nonaging cold rolled steel plate by continuous casting and continuous annealing method - Google Patents

Abstract

PURPOSE:To manufacture a cold rolled steel plate having nonaging characteristic by continuous annealing by hot rolling and cold rolling a continuously cast slab of low carbon steel contg. specified quantities of Mn, P to a plate material, then heat treating said material under a specified condition. CONSTITUTION:The slab or steel contg., by weight 0.01-0.10% C, 0.05-0.3% Mn, 0.005-0.015% S, <0.1% P, 0.005-0.10% Al, <0.0030% N and if necessary, <0.0030% B is manufactured by continuous casting method. During said casting, the slab is cooled in 1,100-1,300 deg.C temp. range by 5-40 deg.C/min cooling rate, then reheated to <=1,150 deg.C, hot rolled, cold rolled, then recrystallization annealed continuously by the heat cycle indicated in the figure 1. Thereafter, said plate is cooled rapidly by >=30 deg.C/sec cooling rate, then over age treated at 230-400 deg.C for 2-10min. The cold rolled steel plate having nonaging characteristic is manufactured by continuous annealing.

Description

Detailed Description of the Invention (Technical Field of the Invention) The present invention relates to decarburization by vacuum degassing in steelmaking, Ti,
JIS () 31 without adding elements such as Nb
418 PCs! The present invention relates to a method of manufacturing a cold rolled steel sheet having properties of D or higher, particularly non-aging properties, by continuous annealing.

(Prior Art) Due to its good workability, soft cold-rolled steel sheets are used as final products after undergoing severe forming processes, mainly for automobiles. However, this workability may deteriorate over time, and this deterioration over time is called aging. Among soft cold-rolled steel sheets, those used for applications that undergo particularly severe forming should not have this aging property.

This aging property is caused by the interstitial solid solution of O + and N in the steel fixing the mobile dislocations introduced during the final temper rolling process, which increases the yield point and decreases the elongation at break. ,
This is because deterioration such as yield point elongation occurs.

Of the 0 and N that cause this aging property, N is a trace amount, so by using aluminum kill 1:' steel, it can be fixed in the form of aluminum nitride, or by adding I3, nitride can be fixed as boron. Therefore, the statute of limitations due to N can be avoided.

On the other hand, solid solution O has an extremely small solid solubility limit in cementite at low temperatures, so if it is cooled over time as in box annealing,
It barely lingers. However, in continuous annealing, solid solution C remains due to cooling in a short time, resulting in large C aging. In order to reduce this solid solution C, generally the degree of supercooling is increased by cooling under continuous annealing conditions, and then a cementite precipitation treatment called overaging is performed.

In the present invention, fine manganese sulfide (MnS) is produced during casting.
By generating this Mn5f site, heterogeneous nucleation is caused during rapid cooling-overaging of full continuous annealing,
This method aims at reducing solid solution carbon by promoting nucleation and growth, that is, cementite precipitation.

On the other hand, it has been often reported that if the cooling rate after annealing is extremely high, fine cementite is generated within the crystal grains.

For example, "Tetsu to Hagane" No. 62 (1976) No. 624
p +1 (1t o
.

11c) reports that fine carbides are observed in a steel plate that has been water-cooled from 700'C at 2000°C/S and then subjected to over-aging treatment. If the density of carbides is thick, the diffusion distance required for their growth will be shortened, and the reduction of solute carbon will proceed quickly. However, due to precipitation hardening and dispersion hardening caused by these fine carbides, the steel itself will become hard and have low ductility. becomes.

Therefore, it is necessary to control the intragranular carbide density within a certain appropriate range, but the above report does not take this into account. In addition, rapid cooling at 2000'C/s has the disadvantage that the shape of the steel sheet collapses due to quenching distortion.Furthermore, such rapid cooling requires water cooling, so after cooling at water temperature t, the steel plate is allowed to swell to the overaging temperature. However, there remains the problem of thermal energy loss and surface oxidation due to water cooling.

As a prior art that utilizes MnS to improve the properties of cold rolled steel sheets by continuous annealing as in the present invention,
There is an invention described in Japanese Patent Publication No. 56-8891. However, in this invention, the effect of MnS is only suggested (column 7, line 25 and below) of the same publication, and the effect of the sliver is mainly an improvement in the Lankford value (7 values).
There is no consideration given to carbon aging suppression as in the present invention. In this prior art, low-temperature heating hot rolling is an important component and is similar to the case of reheating hot rolling (direct hot rolling: direct rolling) of the present invention, but as described above, its purpose is Therefore, continuous casting conditions and continuous annealing conditions are important constituent elements in the present invention, whereas these cannot be found in this prior art.

Similarly, as a prior art technology that aims to improve the material quality of continuously annealed cold-rolled steel sheets through low-temperature heating and hot rolling, JP-A-51-9
There are inventions described in Publication No. 016 and Japanese Patent Publication No. 56-11732. However, the former does not mention aging at all, and the latter presents conditions for effective use of BN (boron nitride), and the purpose of the present invention is that aging is only hinted at at the end. It is as if there is no mention of non-prescription claims.

Aging property is often indicated by aging index (AI) or yield point elongation (Yp-Ell) after accelerated aging at 100°C for 60 minutes, but in order to be considered as non-aging property, at least A
3 K9f/m, rn' or less in I, and YpE7
The ratio should be 0.4 or less, preferably 2KLif/rn, ie, 2 or less, for AI and 0 for Yp-El.

However, when using so-called ordinary steel, it is necessary to make it non-aging by continuous annealing, that is, to reduce the solute carbon (in order to reach the above level, the solute carbon needs to be reduced to 1 to 2 ppm+t or less). It is extremely difficult to produce continuous annealed, truly soft, non-aging cold rolled steel sheets, so-called IF steel (Interstitial F steel).
There is only one called ree steel. IF steel is manufactured by reducing the carbon content to below 50 ppm through vacuum degassing during steel manufacturing, and adding strong carbonitride-forming elements such as Ti and Nb to more than the stoichiometric amount of C and H. This steel is completely unaged. [2 However, this I F
The production of steel requires special steelmaking equipment and operations as mentioned above, and has the economic disadvantage of high production costs due to the use of expensive alloys.

(Objective of the Invention) The present invention solves the above-mentioned drawbacks and is suitable for special steel manufacturing equipment and controls.
After obtaining the S distribution and completing hot rolling while maintaining this distribution, heterogeneous nucleation of cementite is caused on this MnS under specific continuous annealing conditions, and cementite precipitation and growth.
In other words, it aims to promote the reduction of solid solution carbon and achieve non-aging.

(Structure of the Invention) The gist of the present invention is that CO, 01 to 0.10w
t%, Mn0.05-0.3wt%, So, 005
-0.015 wt%, PO,1,wt% or less, A-IO,005-0,100wt%, N
O,0030wt% or less, as necessary B O,00
Steel containing 30 wt % or less and the balance Fe and unavoidable impurities is continuously cast into a slab in the temperature range of 1100 to 1300°C to a temperature of 5 to 100°C, and then directly cast into cold pieces or hot pieces. After heating to 1150'C or less, hot rolling, then cold rolling, followed by short recrystallization annealing at 30°C/S.
This is a method for producing a non-aging cold rolled steel sheet by continuous casting and continuous annealing, which comprises cooling at the above cooling rate and overaging at 230 to 400°C for 2 to 10 minutes.

Hereinafter, the constituent elements of the present invention will be explained and the numerical limitation conditions thereof will be described.

O is in the normal low carbon range of 0.01 to 0. ]Owt
%. In the present invention, carbon precipitation is promoted by quench aging in continuous annealing, but the intragrain carbides generated in this way gradually deteriorate workability compared to the case of only grain boundary carbides. Therefore, to compensate for this, C
It is preferable to make it less than 0°03 wt%. C is 0
．． If it is less than 0.01 wt %, the degree of O supersaturation will not be high during continuous annealing, which is unsuitable.

Mn and S are each 0.05-0.3 wt%.

It is necessary to set it to 0.005-0.015 wt%. In the present invention, since MnS is utilized, S is 0.005. wt
% or more is required. When S exceeds 0.015 w+, large MnS in the form of inclusions increases.1. make the characteristics worse. When Mn is less than the lower limit, FcS is generated and hot embrittlement occurs.

Moreover, when Mn exceeds 3.3 wl %, workability is completely lost.

The present invention can be applied to a phosphorus-added high-strength cold-rolled steel sheet, and in that case, the upper limit of P is 50.1 wt%. If this value is exceeded, the steel becomes brittle and its workability deteriorates.

0.0.2 wl % when applied to normal mild steel plate
The following is fine. In particular, when good workability is required, 0.
01. Preferably, the concentration is low, less than wt%.

0.005 w % of kl is required for deoxidation and fixation as N i AlN, but 0.1 wt
Addition exceeding % will increase inclusions and worsen processability. In this sense, N is a harmful element, and the upper limit is set at 0.0030 wt%. In order to more thoroughly fix N, it is preferable to add B, which has a stronger affinity for N than Al, in an amount within 0.0030wi% that does not cause any harm such as embrittlement.

Next, continuous casting conditions important to the present invention will be described.

00.02 wt%, Mn 0.1 wt%, S
O.01wt%.

PO,005wt%, AlO,'04wt%
A large number of steels with a target composition of 1.5wt N Ooo were melted and cooled to various cooling rates depending on the combination of thickness, cooling from the surface, and retention to form slabs. The cooling rate is that of the center layer, and is shown as an average value from 1300℃ to 1100℃, but heat transfer is based on the measured surface temperature! I determined it by calculating N. After that, 1. ．． After heating to 080°C, hot rolling was carried out so that the final hot water temperature reached 890°C, and winding treatment was performed at 700°C. Furthermore, this hot-rolled sheet was cold-rolled by 80% to obtain a 0.8
After making it thick, it was annealed using a heat cycle shown in FIG. However, the soaking conditions were 820°C for 1 minute and the cooling rate V was 100'C/s.

Table 1 shows a comparison of the above cooling rate, the density of cementite grains in the final annealed plate, and the amount of solid solute carbon determined by the AI and internal friction methods.

1 2 Note that AI is determined by aging at 100° C. for 60 minutes after prestraining 10 chips, and is expressed as the increase in yield point before and after aging. In addition, the cementite density was measured by taking a photograph at 3000x magnification with a scanning electron microscope after performing cementite exposure etching.
3π8 or more (actual size of 0.3 μm or more) The number of mentites was measured.As can be seen from Table 1,
When the cooling rate in continuous casting is within a certain range, the number of cementite in the annealed plate increases, solute carbon decreases, and AI
will also become smaller. In other words, it approaches non-aging.

In order to investigate why the cooling rate of continuous casting has such an effect, we conducted a transmission electron microscopy observation of a sample of an annealed plate that had been subjected to cementite dissolution heat treatment and then rapidly cooled. When the number of MnS with a size of 0.01 μm or more was measured using this method, the cooling rate of continuous casting was 2.1 and 20.2.
, 100.1℃/5 corresponding to Ill#l, respectively.
．． 3 x 105.1.5 x 10', 8.3
It was 106 pieces/2 units. From this, MnS of a certain size or more becomes a cementite nucleation site.
If the cooling rate of continuous casting is too fast, the MnS will be too fine and will not become sites, and if the cooling rate of continuous casting is too slow, the MnS will be large and the absolute number of resites will be insufficient even if the number is small. I can explain.

Thus, in the present invention, the cooling rate of continuous casting is extremely important, and as shown in Table 1, the cooling rate must be within the range of 5 to b°C. At temperatures above 1300°C, there is little precipitation of MnS, and the cooling rate in this area has no meaning. Same <11
Below 00℃, the diffusion of solute elements is insufficient, resulting in burnt JM
Precipitation of nS is less likely to occur. Note that the cooling rate of continuous casting in the present invention is that of the central layer excluding the surface layer 5 of the continuous casting slab. This is because the surface layer shows complicated temperature changes due to water cooling, roll contact, etc., and must be excluded.

It is necessary to prevent the MnS produced in this way from becoming coarse. Therefore, it must not be reheated above 1150°C. Normally, this is the heating temperature for hot rolling, so it is necessary to keep the hot rolling heating temperature below 1150°C (preferably above 1000'C) or to perform direct rolling without heating. .

The hot-rolling/cold-rolling conditions may be normal, and the finishing hot-rolling temperature is A r
3 transformation points or more, the cold rolling rate may be 40 to 90%.

High-temperature winding treatment at 650 to 750° C. and high cold rolling reduction at 70 to 90%, which are often used in continuous annealing, are preferable conditions because they improve the 7 value and promote grain growth.

In this way, the dispersed state of MnS is not effective unless conditions are adopted to cause appropriate heterogeneous nucleation on MnS during the cooling-overaging process of continuous annealing, which is a process to reduce solid solution carbon. Fast.Quick cooling at 30℃/s or more (preferably 50 to 200℃/s) after annealing and 230 to 4
It is retained at 00°C. This is because nucleation does not occur unless there is heterogeneous nucleation into MnS and a considerable degree of carbon supersaturation. The conditions to achieve this are first 30℃/
Rapid cooling of S or higher is required. 230-400℃, where carbon is relatively easy to diffuse if a certain degree of supersaturation is maintained.
Sufficient nucleation occurs by retaining the material at a certain temperature, and then during the remainder of the retention time sufficient nucleation occurs, or in other words, a reduction in solid solution carbon is achieved. If it is below 230°C or above 400°C, nucleation is not sufficient, and the temperature is too low. :, If it is less than 2 minutes, the growth is not sufficient,
The effect is saturated in 10 minutes.

In order to carry out these processes more thoroughly, the first
As shown in the figure, the cooling rate V is set to 70 °C/s or more,
300-400 after holding at 230-350℃ for 0.5-2 minutes
After heating at 5O-100C, leave the remaining time at 230C.
It is preferable to take a cycle of cooling to ~300°C.

Other recrystallization conditions that are usually adopted are +7)
Generally, the temperature is 730-820℃ for 5POD class, and 760-850℃ for 5POE class, and the time is 0.5℃.
It takes about 3 minutes.

Finally, temper rolling is performed, but the elongation rate is 0.6 to 2.0.
%.

The cooling rate for continuous annealing is set at an upper limit of 500° C./S, as various problems will occur if the thickness is too high as described above.

(Example) Steel having the composition shown in Table 2 was melted in a converter, and continuously cast and hot rolled under the conditions also shown in Table 2.

These hot rolled copper strips were pickled and then cold rolled by 80% to obtain cold rolled steel strips with a thickness of 0.8 mm, and subsequently subjected to continuous annealing. The conditions and 1
, 2% mechanical properties after temper rolling are shown in Table 3. In addition,
In Table 2, steel with code E has a tensile strength of 35 Kq.
This is an example of a f/rrun' grade high-strength cold-rolled steel sheet with thinning added.

o 4 -112- I Tomio Q 刹 φ = Knee OO 1:lO:O As is clear from Table 3, the steel according to the invention has a yield point strength of 19 Kqf/ in soft material after artificial aging. Fin 2 or less 2 or more 46% or more, high strength steel plate 25 KL each
i, 7"/rn- or less, exhibits good workability of 40 modulus or more, and has aging resistance of 0.2% or less in Yp-El, and A
I is 3 Kqf/mx'' or less, which is extremely good.

On the other hand, when the conditions of the present invention are not met, the mechanical test values of both soft materials and high-strength materials are significantly inferior.

〔Effect of the invention〕

Continuous manufacturing processes are the trend of the times. In particular, the fact that continuous annealing can be performed using continuous casting on cold-rolled steel sheets is
It has an extremely large effect. However, conventionally, continuous annealing has only been able to produce high-grade cold-rolled steel sheets using expensive IP steel, and for this reason, switching to continuous annealing for all cold-rolled steel sheets has been considered nothing more than a dream.

With the present invention, this has become a reality, and furthermore, warm piece charging and direct rolling do not pose any problem, and in fact, these advancing technologies can be incorporated as preferred forms, and the economic benefits of the present invention. It can be said that the technical significance is extremely large.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a temperature-hour relationship (heat cycle) under preferable continuous annealing conditions of the present invention. Agent Patent attorney Masamitsu Akizawa and 2 others June 1980 tt1 Director of the Japan Patent Office 1. Indication of the case Patent Application No. 83505 of 1983 2. Name of the invention: Continuous casting - Non-aging property due to continuous annealing method Manufacturing method for cold-rolled steel sheets 3, relationship with the amendment case Applicant Address (residence) 2-6-3 Otemachi, Chiyoda-ku, Tokyo Name (665) Nippon Steel Corporation 4; Agent
Residence: Taiyo Building, 12-1 Nihonbashi Kabutocho, Chuo-ku, Tokyo”
”:i#P=:;;II Island date Showa year month day (shipment) Contents of amendment 1. Delete [(direct hot rolling: direct rolling)” written in lines 11 to 12 on page 5 of the specification. . 1-

Claims (1)

[Claims] (1100,01-0.10 wt%. Mn 0.05-0.3 wt%. So, 005-0.015 wt%. P 0.1 wt Upper and lower AlO, 005-0 .100 wt%. N O, 0030 wt % or less. If necessary, Bo, 0030 wt % or less, balance Fe
and unavoidable impurities during continuous casting.
A slab is prepared in the temperature range of 0 to 1,300°C for 5 to 50 minutes, and then directly or after heating a cold or hot piece to 1,150°C or less, hot-rolled, then cold-rolled, and then recrystallized for a short time after annealing for 30 minutes. A method for producing a non-aging cold rolled steel sheet by a continuous casting and one continuous annealing method, which comprises cooling at a cooling rate of at least C/S and overaging at 230 to 400 C for 2 to IO minutes.