JPS6024320A - Production of hot rolled steel sheet having excellent scale adhesion - Google Patents

Abstract

PURPOSE:To obtain easily a hot-rolled steel sheet for general working having excellent scale adhesion by charging a continuous casting billet having a specific compsn. into a heating furnace, then discharging the same from the furnace, hot rolling the billet and cooling the coiled sheet in a non-oxidative atmosphere. CONSTITUTION:A steel consisting of 0.03-0.25% C, <=0.05% Si, 0.20-1.00% Mn, <=0.020% P, <=0.020% S, <=0.05% Cu, <=0.05% Cr, <=0.08% Al, >=0.1% >=1 kind among Nb, Mo, V and Zr and the balance Fe with inevitable impurities is melted. The molten steel is continuously cast and the casting is immediately charged into a furnace or is charged therein after cooling. The heated billet is discharged therefrom at 950-1,200 deg.C and is hot-rolled. The hot-rolled sheet is coiled at 550-700 deg.C. The coil is cooled down to 350 deg.C in a non-oxidative atmosphere of N2, Ar or a gaseous mixture thereof.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Field of Application) The present invention relates to a method for producing hot rolled steel sheets for use in forming steel sheets, drums, shaped steels, and the like.

(Prior art) Steel plates manufactured by hot rolling continuously cast slabs have poor adhesion of secondary scale, which causes scale dust in subsequent processes, which poses an environmental problem. In some cases, the parts become unusable, or after pickling, a surface treatment such as bonding treatment is carried out, and then molding is performed to produce products such as drums and pipes, and then the surface is coated.

(Objective of the invention) The present invention does not generate dust in the post-process, can be used for full coil processing, does not require pickling or bonding treatment, and can be immediately molded and surface coated by the processing manufacturer. An object of the present invention is to provide a method for manufacturing a hot rolled steel sheet with excellent scale adhesion.

(Structure and operation of the invention) As mentioned above, hot-rolled steel sheets manufactured using continuously cast slabs have poor scale adhesion and require various surface treatments. As a result of research and development of a hot-rolled steel sheet with excellent scale adhesion that does not require a complex surface treatment process, the present inventors succeeded in developing a method that can achieve the objective by a much simpler means. .

The gist of the present invention is as follows.

(1) C0.03 to 0.25, SiO, 05 or less,
Mn 0.20-1.00%, Po, 020% or less, e
o, 020 or less, Cu 0.05% or less, Cr 080
After melting a steel containing 5% or less, At 0.08% or less, and the balance 9 consisting of Fe and unavoidable impurities, the hot billet obtained by continuous casting is immediately charged into a heating furnace or once made into a cold billet. The coil was then charged into a heating furnace, extracted at 950-1200°C, hot-rolled, and rolled at 550-700°C.
A method for producing a hot-rolled steel sheet with excellent scale adhesion, which comprises cooling until it reaches 350°C in a non-oxidizing atmosphere consisting of gas, Ar gas, or a mixture thereof.

(2) C0.03 to 0.25, SiO, 05 or less,
Mn 0.20-1.00%, Po 0.020% or less, 8
0.020 coefficient or less, CuO, 05% or less, CrO, 05
% or less, Ajo, 08% or less, in addition to Nb, Mo,
After melting steel containing one or more of V and Zr edges of 0.1 or less and the remainder consisting of Fe and unavoidable impurities, the hot copper piece obtained by continuous casting is immediately loaded into a heating furnace. or once it is made into cold pieces, it is charged into a heating furnace.9
Extracted at 50-1200℃ and hot rolled to 550-7
A hot-rolled steel sheet with excellent scale adhesion characterized by cooling a coil wound at 00°C in a non-oxidizing atmosphere consisting of N2 gas, Ar gas, or a mixture thereof until it reaches 350°C. manufacturing method.

This will be explained in more detail below.

The reason for limiting the components in the present invention is to provide a steel sheet that is suitable for processing purposes targeted by the present invention.

The reason for setting C0.03 to 0.25 is to give the hot rolled steel sheet according to the present invention properties suitable for use in pipes, drums, molded steel, automobile frames, cooking pots, etc. Mainly low carbon to medium carbon steels are suitable. In finished products, most of C becomes Fe5C (cementite), which worsens scale adhesion, so it is preferable to use A-grade C as much as possible, but the frame of your vehicle, building materials, pipes, etc. need strength, so to satisfy both The upper limit is 0.25%, and the upper limit of the C component suitable for drum scraping is 003, which is why the lower limit of the C component in the present invention is set at 0.03%.

Next, the reason for setting St to 0.05% or less is that if it exceeds this, the properties of the steel sheet that are the object of the present invention will be impaired.
Since i is used for deoxidation in At killed and A/nee Si killed steels, it is unavoidably contained.

In the present invention, the lower the St content, the better, and the range that does not impair the purpose is 0.05% or less.

Ushida Mn f: 0.20~1. The reason for setting it as OO% is that Mn is a heavy element that affects the adhesion of secondary scales, but if it is less than 0.201, there is a problem with the strength of the copper plate that is the object of the present invention, and if it is still more than 1.00, it is not economical. This is because they lose their sexuality.

Next, the reason for setting P and S to 0.020% or less is that P,
This is because both S concentrates at the interface between the scale and the base metal during heating and hot rolling, worsening the adhesion of the secondary scale, and 0.020
% is the limit. For this purpose, it is desirable for the content of both P and S to be less than the O, OIO ratio, but since both P and S removal increase processing costs, the content is set at 0.020% in consideration of economic tolerance. It should be determined as appropriate below.

The reason for setting At to 0.08% or less is that the inclusion of At is unavoidable when deoxidizing At to produce the target steel sheet, and as a result of investigating the limit of the effect on the purpose, it is 0.08%.
This is because good results were observed in % reduction. Then 0.08
An economic problem arises when the amount is exceeded.

Therefore, in the present invention, the content of A7 is set to 0.008% or less.

Similarly, the reason why Cu is kept below 0.05%91 and Cr is kept below 0.05% is that both Cu and Cr concentrate at the interface between the scale and the steel base during heating and hot rolling, worsening secondary scale adhesion. The limit for both Cu and Cr is 0.05%.

Further, like Cu and Cr, Sn, As, and Nl are also elements that deteriorate secondary scale adhesion, so it is not desirable to include them, but they are acceptable if they are included as unavoidable impurities.

Next, to improve strength and corrosion resistance, Nb, IVIo
, V.

Good results can be obtained by adding one or more of Zr to the above components in an amount of 0.1% or less.

However, if it exceeds 0.1%, it will concentrate on the surface of the base metal (the interface between the scale and the base metal) and deteriorate secondary scale adhesion, so it should be limited to less than 0.1%.

The steel having the above-mentioned components in the present invention is prepared by the well-known dephosphorization of molten iron,
After applying preliminary treatments such as S and Si removal as necessary, it is melted in a converter with top or bottom blowing or top and bottom blowing, and then it is continuously processed with or without preliminary treatment such as RH treatment. A casting device F'itc is used to form cast pieces, for example slabs or blooms. Then, the high-temperature slab obtained by the continuous casting machine VC is directly heated to ff.
The ZC material is charged into a heating furnace in the
temperature range. The reason why such low temperature heating of 950 to 1200°C is performed is that heating above 1200°C destroys Si and P.

Normal or high-temperature heating is not recommended because S, Cu, Cr, Sn, As, etc. will concentrate at the interface between the steel piece and scale or on the surface of the copper piece, and will also concentrate on the surface of the hot rolled steel sheet, deteriorating secondary scale adhesion. . Heating at a temperature below 950°C will worsen the biting and rolling properties during rough rolling.
This is also because it becomes difficult to secure the hot rolling finishing temperature.

Next, after hot rolling to the desired thickness, 550 to 70
Perform high temperature winding at 0″C.

The reason why this high temperature winding is limited to 550 to 700°C is as follows. In general, iron oxide is heated above 570℃, mainly Fe
Although it is an O phase, it transforms below 570°C and becomes F e s
It transforms into Oi phase and αFe. However, when rapidly cooled, the transformation does not proceed completely, resulting in iron oxide containing a FeO phase with slightly poor adhesion. When slowly cooled, Fe3O4 phase (αFe
550° just below the transformation temperature.
When a hot-rolled steel sheet is rolled at a temperature of C or above, it is gradually cooled by the heat retained in the coil and transformed into Fe3O4, yielding favorable results. This is the reason why the lower limit temperature is set at 550°C. i.e. 55
Below 0°C, the FeO phase increases in the iron oxide, which is not favorable. Next [If the winding temperature exceeds 700°C, the scale thickness will increase and the scale adhesion will deteriorate, so the winding temperature should be 7()0°C.
The upper limit is ℃. As a result, secondary scale is generated after rolling to finish f1, and the rolled bale also grows, and during this time, Si+P, S, Cu, Cr, etc. are concentrated at the interface between the scale and the base metal, worsening the adhesion of the secondary scale. So, the aforementioned 71V! Therefore, these elements and heating temperature were limited.

FIG. 1 shows an apparatus for cooling a coil in a non-oxidizing atmosphere. In Figure 1, coil 1 is sand 2 (seal sand)
The workpiece is placed on a work floor 3 covered with fl'fC, and then covered in an airtight manner with a sealing cover 4.

A gas supply pipe 5a provided on the side wall 4a of the airtight cover 4
, N2 gas from 5b, also gas supply pipe 6 a r6
Ar gas is supplied from b. 7a to 7d indicate blowing nozzles. 8a to 8d are on/off valves 1.9a to 9b'
is an exhaust pipe for N2 gas, and 10a to 10b are exhaust pipes for Ar gas.

The coil wound at 550-700 DEG C. is transported onto the work floor 3 by suitable transport means, and then the coil 1 is covered with a sealing cover 4. As shown in the figure, since the aforementioned seal sand 2 is provided on the work floor 3, the airtightness within the airtight cover 4 is sufficiently maintained. Water sealing may be used instead of sand sealing. Then, when N2 gas, for example, is blown from a supply source (not shown) through the blowing nozzles 7a and 7b, air is exhausted from the exhaust pipes 9a + 9b, and the inside of the airtight cover 4 is filled with N2 gas. Next, valves 8a and 8b,
Yohi 11 a.

Close 11b.

Depending on the need for cooling, the valves 8a and 8b may be kept open.
Open valves Ila and llb and exhaust pipe 9a.

Even if you make it so that it is always ejected from 9b, there is no problem. While blowing N2 gas, close the Ar gas valves 8c and 8(
1; As a matter of course, the exhaust valves llc and ld are closed.

When cooling the coil in an inert gas atmosphere like this, the surface of the coil, especially the top, is affected by air cooling. Hot rolling with excellent secondary scale adhesion can be achieved by oxidizing and forming secondary scales with a high content of Fe2o5, such as the tom and both edges, which have secondary scales with a high content of Fe3O4. It becomes 3 Oka boards.

Example 1 Medium carbon steel with the composition shown in Table 1 was melted and then continuously cast.
Charge the hot steel slab into the heating furnace and heat it to 950-1200℃.
After heating at hot rolling finishing temperature 820" C1 winding temperature 575 ~
After cooling at 650°C, it was charged into the apparatus shown in Fig. 1, and N2 gas was blown in at a flow rate of 3007/min for 30 minutes, and then a ryo was heated for 55 minutes.
0j! // minutes, and the air was blown until the coil surface temperature reached 350°C (this invention). As a comparison material, a material with the same composition heated to 1280°C was hot-rolled and rolled at the same winding temperature as above, and then charged into an N2 box (comparative profit A), or a material with a blower in the atmosphere. Cooled (comparative material B
). After that, a test material was taken from the center in the longitudinal direction of the coil.
Bending test pieces were taken from the middle end and the center, and the test results are shown in Table 2. Scale adhesion is bending radius 1.5t
After the 90° bending test, cellophane tape was pasted on the bent part, and after peeling off the tape, the state of scale peeling on the bent part of the test piece was visually observed, and it was good with no scale peeling; ○ mark, slight peeling r'[ 1° △ mark, Slightly peeling: x mark, Extremely peeling: bad: XX mark.

Table 1 (wtφ) Table 2 Example 2 Low alloy steel having the composition shown in Table 3 was melted, hot rolled and cooled in the same manner as in Example 1. Table 4 shows the results of a bending test to determine the adhesion of the secondary scale.

From the Examples shown in Table 4 and above, it can be seen that the hot-rolled steel sheet according to the present invention has extremely good scale adhesion, and the scale adhesion is particularly improved dramatically at the edges.

Example 3 Two-grade low alloy steel having the composition shown in Table 5 was hot-rolled and cooled in the same manner as in Example 1. Table 6 shows the results of a bending test to determine the adhesion of the secondary scale. Both component systems gave almost the same results.

Table 5 Table 4 Toro Table (Effects of the Invention) As mentioned above, the hot-rolled steel sheet with excellent scale adhesion produced by the method of the present invention does not generate dust due to scale in the post-process, and has a high temperature distribution over the entire width of the coil. It has good scale adhesion and does not require pickling or bonding treatment, so it can be used for manufacturing pipes, drums, etc., and can be painted as necessary after forming groove steel and angle steel. It is possible to do this and is extremely economical.

The present invention provides a means for economically producing such hot-rolled curled sheets, and is a method with high practical effects.

[Brief explanation of drawings]

FIG. 1 is a schematic illustration of an embodiment of an apparatus for carrying out the method of the present invention. DESCRIPTION OF SYMBOLS 1... Coil, 2... Seal sand, 3... Working floor, 4... Airtight cover, 5a-5b... Gas feed pipe (for N2), 6 a, 6 b-Gas feed Candy pipe (for Ar gas), 7 a + 7 b...Blowing nozzle, 8 a ~
8d...Opening/closing valve, 9a-9b...Exhaust pipe (N2
(for gas), 10a, 10b...exhaust pipe (for Ar gas)
, 11a to lld...opening/closing valves.

Claims (2)

[Claims] (1) C0.03~0.25chi, SiO,05 or less,
Mn 0.20-1.00%, Po 020% or less,
SO, 020 or less, CuO, 05% or less, Cr0.
After melting a steel with 0.05% or less AtO, 08% or less, and the remainder consisting of Fe and unavoidable impurities, the hot steel slab obtained by continuous casting is immediately charged into a heating furnace or is once made into a cold slab. The coil was charged into a heating furnace, extracted at 950-1200℃, hot-rolled, and wound at 550-700℃.
A method for producing a hot-rolled steel sheet with excellent scale adhesion, which comprises cooling until it reaches 350°C in a non-oxidizing atmosphere consisting of gas, Ar gas, or a mixture thereof. (2) CO, o3~0.25chi, SiO, 05chi or less,
Mn'0.20-1. OO%, Po, 02 (l or less,
So, 020% or less, Cu O, 05% or less, Cr0.
05% or less, At0008% or less, in addition to Nb, M
o+v, one or two or more of Zr to 0.
After melting steel containing less than 1% iron and the rest consisting of Fe and unavoidable impurities, the hot steel slab obtained by continuous casting is immediately charged into a heating furnace, or once it is turned into a cold slab, it is charged into a heating furnace. Extracted at 950-1200℃ and hot rolled to 550℃
A hot-rolled steel sheet with excellent scale adhesion characterized by cooling a coil wound at ~700°C in a non-oxidizing atmosphere consisting of N2 gas, Ar gas, or a mixture thereof until it reaches 350°C. Production method.