JPS6052527A - Production of non-aging cold rolled steel sheet by continuous annealing - Google Patents

Abstract

PURPOSE:To produce economically a mild non-aging cold rolled steel sheet without using special equipment by cold rolling an Al killed and hot rolled steel sheet contg. specified amt. of C and MnS having a specific grain size and annealing continuously the steel sheet under specific conditions. CONSTITUTION:The MnS having 0.05-0.2mu diameter of the MnS in an Al killed and hot rolled steel sheet contg. 0.01-0.05wt% C and contg, if necessary, 0.0005-0.0040% B is made 20-200ppm. After such steel sheet is cold rolled to about 50-90% cold draft, the steel sheet is heated at 1-100 deg.C/sec heating rate and is held for about 20-180sec at about 700-850 deg.C, by which the steel sheet is subjected to recrystallization annealing. The steel sheet is then cooled at an average cooling rate of 50-400 deg.C/sec from 650 deg.C to <=400 deg.C and is further heat- treated for 3-10min at 370-250 deg.Cso that the overaging precipitation of solid- soln. carbon is effected. The mild non-aging cold rolled steel sheet is thus obtd. without requiring special equipment and operation and without using costly Ti, Nb, etc.

Description

[Detailed description of the invention] Industrial application field (technical field to which the invention pertains)
This invention relates to a method for manufacturing non-aging cold rolled steel sheets by continuous annealing without decarburizing by vacuum degassing in steel manufacturing or using elements such as Tf and Nb.

BACKGROUND OF THE INVENTION Due to its good workability, soft cold-rolled steel sheets are used as steel sheets that are made into final products after undergoing severe forming processes, mainly for automobiles. However, this processability may deteriorate over time, and this deterioration over time is called aging property.

Among soft cold-rolled steel sheets, those used for applications that undergo particularly severe forming must not have this aging property. This is because the movable dislocations introduced during rolling are fixed, resulting in deterioration such as an increase in the yield point, a decrease in the elongation at break, and the occurrence of elongation at the yield point.

Of the O and N that are responsible for this aging property, N can be fixed in the form of aluminum nitride by making aluminum killed steel because it is in very small amounts, or can be fixed in the form of boron nitride by adding B. The statute of limitations due to N can be the same.

On the other hand, solid solution 0 has an extremely small cementite solid g limit at low temperatures, so if it is cooled over time like box annealing, the tl
Hardly any residue remains. However, in continuous annealing, since the steel is cooled in a short time, solid solution 0 remains, resulting in large C aging. In order to reduce this solid solution 0, generally the degree of supercooling is increased by continuous annealing cooling, and then a precipitation treatment called overaging is performed. This is thought to reduce the solid solution C to 5-101) pm. Various cooling patterns have been proposed to improve the efficiency of precipitation. For example, in order to maximize the degree of supercooling of 0, a method has been proposed in which after annealing, water cooling is performed at a rate of 1000 μm or more, and then tempering is performed. According to this method, solid solution 0 becomes fine cementite and is reduced to the equivalent of precipitation, but it has the disadvantage that it is already hard before aging due to precipitation strengthening due to this fine cementite. The only continuous annealed steel sheet that can be called a truly soft, non-aging cold rolled steel sheet is so-called IP steel (Interstl tial B'ee steel). IP steel is made by reducing T0 to about 50 ppm or less through vacuum degassing during steel manufacturing, and
Strong carbonitride-forming elements such as i and Nb are added to 0. Produced with more than a stoichiometric amount of N, this steel is completely non-aging. However, the production of this IP steel requires special steelmaking equipment and operations as mentioned above. Since expensive alloys are used in this process, there is an economic disadvantage in that the manufacturing cost is high.

OBJECTS OF THE INVENTION The present invention solves the above-mentioned drawbacks, does not require special steel-making equipment or operations, and can be produced using expensive Tt, Nb, or zr.

■By using manganese sulfide (hereinafter referred to as Mn8), which is universally contained in steel, without using elements such as The purpose is to provide a manufacturing method using continuous annealing.

Structure of the Invention The gist of the present invention is that 0.0% of manganese sulfide is contained in an aluminum killed hot-rolled steel sheet containing 0.01 to 0.05 wt% of carbon and, if necessary, 0.0005 to 0.0040% of B. 20 to 0.05 to 0.2ttm diameter
? OOppm, followed by cold rolling, and continuous annealing. After recrystallization annealing, heat treatment is performed from 650°C to 400°C for 50 to 400°C, and then hardened by heat treatment at 370 to 250°C for 3 to 10 degrees. This is a method for producing a non-aging cold rolled steel sheet by continuous annealing, which is characterized by performing over-aging precipitation of molten carbon.

In other words, the present invention involves adding an appropriate amount of MnS, which is universally contained in steel, to a hot-rolled sheet with an appropriate size that hardly changes to #1 during cold rolling or short-time recrystallization annealing. ensure that
The aim is to utilize this as a site for heterogeneous nucleation during 00 precipitation during cooling after continuous annealing.

The configuration of the present invention will be explained below.

First, the basic component is aluminum killed steel containing 0.01 to 0.05 wt% of 0. This amount of C is the amount normally used for soft cold-rolled steel sheets'), and if it is up to o-oiwt%, degassing treatment is unnecessary or only a light treatment is required.The upper limit of 0.05WtX is due to the softness Determined from the limit. If you are aiming for more stable and soft, 0 is 0.01~
The content is preferably 0.03 wt%.

Aluminum killed steel is a common type of soft, highly worked cold-rolled steel sheet.

The main purpose of the present invention is to provide a method for suppressing N aging and C aging, which are related to aging properties.

However, in order to obtain a soft, non-aging cold-rolled steel sheet, it is necessary to fix N, and typical methods for this purpose include fixing it as aluminum nitride or as polymer nitride. The former requires hot rolling and winding at a high temperature of 680°C or higher. In the case of ridges, it is necessary to add 5 to 40 pI)m of B. N to 151) to enable these actions.
I) It is preferable to reduce it to below m.

Next, 20.4QQppm of MJI8 having a diameter of 0.05 to 0.2μ is required. Bore diameter is O-05
If it is less than ttm, it becomes a heterogeneous nucleation site for cementite, and if it exceeds 0.2 nss, the number of sites will be insufficient, or a large amount of MJI 8 will be required, making solution treatment difficult or reducing ductility due to deterioration of cleanliness. This causes a problem. If the amount is less than 201) pm, the number of sites will be insufficient, and if it exceeds 40 ppm, solution treatment will be difficult and size adjustment will be difficult. In addition, cleanliness deterioration occurs at 0M/n.
j3 is spherical, but being slightly flattened or angular does not impair its effectiveness as a sight.

According to the research conducted by the present inventors, such Mn8 distribution was first determined by changing the steel composition to Mn 0.03 to 0.20wt%, 8o, o
o i o ~o, o i owt%, and Mn%X
$1.0X10~L, SX10, hot rolling heating temperature 1050~1200℃, hot rolling winding temperature t-680
It is obtained by setting the temperature to 750°C. If these conditions are removed, Mn8 will become too fine, or conversely, it will become coarse, and K will precipitate aluminum nitride using the Mn8-iron interface as a precipitation site.

It has been confirmed that such composites are unlikely to become sites for cementite precipitation.

Subsequently, cold rolling is performed, and the cold rolling rate is set to 50 to 9 ON, as is usually done. 8 for a high rank Ford value
It is preferably 0 to 85%.

Continuous annealing may also be carried out under the usual conditions, with a heating rate of 1
~b 20B~180a.

Next, the cooling process is important, and even when using MnS as a cementite precipitation site, a certain degree of supercooling and storage of 0 is necessary. Therefore, it is necessary to increase the temperature to an average cooling rate of 50° C./1 or more. However, if this cooling rate is too high as atb, fine precipitation will occur due to uniform nucleation of cementite and the steel will harden, so the upper limit is set at 400'p/f. Cementite precipitation between these regions is thought to be mainly at ferrite grain boundaries, so in order to suppress this, it is preferable to increase the crystal grain size. Therefore, for K, high temperature annealing of 800°C or higher is desirable. Moreover, the degree of supersaturation increases as the amount of 00 dissolved during annealing increases, so it is preferable to perform slow cooling from annealing 5J1 degrees to the A1 transformation point.

Furthermore, in order to secure this supersaturation degree and move on to the next overaging,
It is preferable that the end point of this cooling is set to 300° C. or lower, and then reheating is performed to proceed to the next overaging treatment.

Overaging treatment is performed at a relatively low temperature of 370-250α and 3-10-
conduct. In this process, cementite nucleates and grows at MnS, and the solid solution 0 rapidly decreases. 370℃
If the temperature exceeds 100, the nuclei become coarser, the product number decreases, and subsequent precipitation is delayed. Below 250°C, the diffusion rate is low;
The solid solution 0 remains without reaching the nucleus. When i is less than 3-, the diffusion time is insufficient and the effect is saturated at 10-. The overaging treatment may be an isothermal treatment, but the temperature is increased at first and then lowered.
A stepped or sloped pattern is preferred.

Note that, as the middle stage of rapid cooling to 650 to 400° C. or lower, any means such as water cooling, hot water cooling, air/water cooling, metal contact cooling, gas jet cooling, salt bath immersion, etc. may be used.

Further, as an impurity in the steel, P interacts with the solid solution 0 and reduces the diffusion of the solid solution 0, so it is preferable to reduce it to 0,0 s% or less.

Example 1 00.022%, Mlo, 11%, Po, 004%, S
O,0043% (Mnxs 4.73X10), ht
023jX, NO, 0012%.

The steel was melted in a converter and made into a slab by continuous casting.

At this time, 0.0007N of B was added to some of the molten steel, and B steel was also produced. This slab was rolled at various heating temperatures per winding temperature to obtain hot rolled coils. In addition, some steels are heat treated to change the form of MnS. The hot rolling finishing temperature was 890°C, and the thickness was 4.0 haze. In this way, hot rolled steel plates with symbols A to E as shown in Table 1 were obtained.

In addition, in order to measure the Mn8 morphology of this steel,
After three cycles of cementite solution treatment, the mixture was quenched to remove 9 parts of cementite from the precipitate. Then, the size and number of MnS were measured by observing the thin film with a transmission electron microscope.

In addition, the conversion to weight was calculated by assuming that 80 molar volume of Mn was three times that of iron. The number of measurements is 1000 per test piece.
There were ~5000 pieces.

The results are shown in Table 1.

Table 1 The hot-rolled coil thus produced was cold-rolled to a pickling thickness of 0.8 mm in a normal process. This coil was subsequently continuously annealed. The condition is heating speed! 10℃/li *Annealing is 8
00°C, 1-, then cooled at a cooling rate of 5°C/41 to 6500, then cooled at various cooling rates, and then
Overaging was performed in various patterns shown in the figure. Figure 1 (a
) is a sloped JR pattern (b) is a 2R pattern, and (C) is an isothermal holding pattern. Finally, 1.2X temper rolling was performed.

The results are shown in Table 2.

For comparison, the mechanical test values of nine-box annealed aluminum killed steel manufactured in a normal process are also shown.

The tensile test was conducted according to JI8, and the test piece was 1155.
Also, the aging index is I00℃ after IOX pre-strain,
Aging was performed at 60°C, and the difference in yield point strength before and after aging was shown.

Mechanical test values that are considered to be substantially non-aging are: aging index within 2 f/-, mechanical test value after 60-year artificial aging at 100℃, yield point elongation within 0.2%, and yield point strength. 191w/
/- or less, the noise is 45% or more.

As is clear from Table 2, the cup 2゜a, 10 according to the present invention
．． 11.1? , 16; satisfies soft non-aging properties, but A4.5 in which the form of MnS is outside the scope of the present invention
．． GT '7 steel plate is hard after aging and has a high aging index. Even if the Mn8 morphology is sufficient, the A1.8.9°IL 14.15m steel sheet with a different cooling pattern after annealing also has high aging resistance and is insufficient as a non-aging soft cold rolled steel sheet. be.

Effects of the Invention According to the present invention, as is clear from the above embodiments, it is possible to economically produce a soft non-aging cold rolled steel sheet without imposing any burden on steel manufacturing.

As a result, high-grade non-aging steel sheets were conventionally box-annealed, and low-grade steels were produced by continuous annealing, and in order to produce high-grade steel by continuous annealing, it was necessary to use the recommended IF'fi. can now be manufactured by continuous annealing without using expensive IP steel. As a result, the advantages of continuous annealing, such as high productivity, uniform quality, energy and labor savings, short delivery times, and ease of manufacturing high-strength steel sheets, can be enjoyed, together with the fact that IF steel is not used. , the economic effects are extremely large.

[Brief explanation of drawings]

FIG. 1 shows the overaging heat treatment pattern used in the example. Agent Patent Attorney Masaaki Akizawa 2 Mitsugai 4 (α) (b) (C) 1. Indication of the case Patent Application No. 159579-1982 2. Name of the invention Manufacture of non-aging cold rolled steel sheet by continuous annealing Method 3, Relationship with the case of the person making the amendment Applicant Address (Residence) 2-6-3 Otemachi, Chiyoda-ku, Tokyo Name (665) Nippon Steel Corporation 4, Agent Residence Taiyo Building, 12-1, Nihonbashi Kabuto-cho, Chuo-ku, Tokyo Detailed explanation) 8. Contents of the amendment As per the attached sheet, amend ``From the contents of the amendment'' to ``By addition of B.'' 2. ``Fee Steel'' in line 16 on page 3 of the same 'A-3
ee steel”.

Claims (1)

[Claims] (1) 7A containing 0.01 to 0.05 wt% of carbon and 0.0005 to 0.004 ON of B as necessary
-0% of the manganese sulfide in the mini-ram-killed hot-rolled steel sheet.
20-200p for those with a diameter of 0.05-0.2μ
K at 9, followed by cold rolling and continuous annealing.
650t after recrystallization annealing: 50~400C or less
Cooled at an average cooling rate of 4ooc/s, and further
A method for producing a non-aging cold-rolled steel sheet by continuous annealing, characterized in that over-aging precipitation of solid solution carbon is carried out by heat treatment at 250° C. and 3 to 1°C.