JPS625214B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for producing hot rolled steel sheets for pipes, drums, shaped steel and other general processing. (Prior art) Steel plates manufactured by hot rolling continuously cast slabs are
Because of the poor adhesion of secondary scale, a method has been adopted in which surface treatment such as bonding treatment is carried out after pickling, followed by molding to produce products such as drums and pipes, followed by surface coating. (Objective of the invention) The present invention provides a method for producing hot rolled steel sheets with excellent scale adhesion, which does not require pickling or bonding treatment and can be immediately formed and surface coated at a processing manufacturer. be. (Structure and operation of the invention) As mentioned above, hot-rolled steel sheets manufactured from continuously cast slabs have poor scale adhesion and require various surface treatments, but these are disadvantageous economically. As a result of conducting research and development to develop a hot rolled steel sheet with excellent scale adhesion that does not require the conventional complicated surface treatment process, the present inventors succeeded in developing a method that achieves the objective by simpler means. The gist of the present invention is C0.03-0.25%, Si0.03% or less, Mn0.20-1.00%, P0.020% or less, S0.020% or less, Al0.05% or less, Cu0.03% or less, After melting a low carbon steel with a Cr content of 0.03% or less and the remainder consisting of Fe and unavoidable impurities, the continuously cast slab is directly hot rolled or an induction heating device is used.
After lightly heating the edges and other areas where the temperature tends to drop using a gas heating device, hot rolling is performed to 550 to 700℃.
A method for producing a hot-rolled steel sheet with excellent scale adhesion, characterized by cooling a coil wound with _N2 gas, argon gas, or a mixture thereof until it reaches 350°C, the method comprising: This will be explained in more detail. The reason for limiting the components in the present invention is to provide a steel sheet that is suitable for the processing applications targeted by the present invention, and the reason for setting C0.03 to 0.25% is to provide a steel sheet that is suitable for the processing applications targeted by the present invention. This is to give properties suitable for pipes, drums, molded steel, automobile frames, cooking pots, etc., and low to medium carbon steels are mainly suitable for these. C is mostly Fe ₃ C in finished products
(cementite) and worsens scale adhesion, so it is desirable to have it as low as possible, but automobile frames, building materials, pipes, etc. require strength, and to satisfy both, 0.25% is the upper limit.
In addition, the limit of the C component suitable for drum materials, etc. is 0.03%, which is the lower limit of the C component in the present invention of 0.03%.
This is the reason why it is set as %. Next, the reason why Si is set to 0.03% 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. Si concentrates on the surface of the steel base during heating or hot rolling and after rolling up, impairing the adhesion of the scale to the base steel. In particular, heating usually deteriorates the quality, so in the present invention it is better to directly roll the continuous slab and have a lower Si content, preferably 0.03% or less. Also, the reason why Mn is set at 0.20 to 1.00% is that Mn is a desirable element for adhesion of secondary scales, but
If it is less than 0.20%, there will be a problem in terms of the strength of the steel sheet, which is the object of the present invention, and if it exceeds 1.00%, it will be uneconomical. 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 secondary scale and the steel base, impairing scale adhesion, and the limit is 0.020%. For this purpose, it is desirable for the content of both P and S to be 0.010% or less, but since processing costs for both P and S removal increase, the content is determined as appropriate at 0.030% or less, taking into account the economic tolerance range. Should. In addition, the reason why Al is set at 0.05% or less is that when deoxidizing Al for the purpose of manufacturing steel sheets, the inclusion of Al is unavoidable, and after investigating the limit of the effect on the purpose, good results were found up to 0.05%. This is because it was recognized. However, if it exceeds 0.05%, an economic problem will occur.
Therefore, in the present invention, the Al content is set to 0.05% or less. Similarly, the reason for setting Cu to 0.03% or less and Cr to 0.03% or less 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. In the invention, the limit is 0.03%, the lower limit for Cu is 0.003%, and for Cr is 0.005%.
It is. Since the present invention uses a direct rolling method instead of reheating, there is no need for high temperatures or long heating times during reheating, which reduces the concentration of impurity elements on the surface of the steel base, and reduces the concentration of impurity elements in the steel components. Even if the content of Si, P, S, Cu, Cr, etc. is slightly higher than that of the reheated material, there is an advantage in that the scale adhesion of the hot rolled steel sheet is improved. In the present invention, the steel having the above-mentioned components is prepared by carrying out the well-known preliminary treatment of hot metal, such as deP, S, and Si, as necessary, and then using a top-, bottom-blowing, or top-bottom blowing converter. It is melted and then cast into a billet, e.g. slab or bloom, in a continuous casting machine with or without pre-treatment such as RH treatment. Next, the high-temperature slab obtained by the continuous casting device is directly hot-rolled as it is, or it is lightly heated, that is, by heating the parts where the temperature tends to drop, such as the ends, using an induction heating device, a gas heating device, etc. Further, the sheet is hot-rolled to a desired thickness using rough and finish rolling equipment, and then coiled at a high temperature of 550 to 700°C. In the present invention, since direct rolling is performed, elements such as Si, P, S,
Cu, Cr, etc. are reduced, improving secondary scale adhesion. The reason why this high temperature winding is limited to 550 to 700°C is as follows. In general, iron oxide is mainly in the FeO phase at temperatures above 570°C, but at temperatures below 570°C it transforms into the Fe ₃ O ₄ phase and αFe. However, if it is rapidly cooled, the transformation does not proceed completely and it becomes iron oxide containing a FeO phase with slightly poor adhesion. Slow cooling completely transforms into Fe ₃ O ₄ phase (including αFe) with good adhesion, so when a hot rolled steel sheet is rolled up at 550°C or higher, just below the transformation temperature, Fe ₃ O ₄ is slowly cooled by the heat retained in the coil. This results in favorable results. This is the reason why the lower limit temperature is set at 550°C. i.e. 550℃
In the following, FeO phase increases in iron oxide, which is not preferable. Next, the upper limit of the winding temperature is 700°C because the scale becomes thicker and the scale adhesion deteriorates if it is rolled at a temperature higher than 700°C. Next, the high-temperature wound coil is placed in the embodiment shown in FIG. 1 and cooled until the average coil temperature reaches 350°C. The reason why we limited the cooling end temperature in a non-oxidizing atmosphere to 350℃ is that iron oxide FeO on the surface of the coil wound at 550 to 700℃ transforms to Fe ₃ O ₄ below the transformation temperature (approximately 570℃). After that, if exposed to air at high temperature, scale adhesion will be poor.
Since it becomes Fe ₂ O ₃ , the temperature is as low as possible, i.e.
When cooled to about 350℃ in a non-oxidizing atmosphere,
The hot-rolled steel sheet has secondary scale with a high content of Fe ₃ O ₄ and has excellent secondary scale adhesion. In FIG. 1, the coil 1 is placed on a support 4 provided on a work floor 2 with a water sealing device 3 and then placed in an airtight state by means of a sealing cover 8. Instead of the water seal, a sand seal or a combination of both may be used. The airtight cover 8 has a blowing nozzle 6 connected to the gas supply pipes 5a to 5f for N ₂ gas, Ar gas, etc. on the side wall.
a to 6f are fixed. 7a to 7f are on-off valves, 9a is an exhaust pipe for _N2 , and 9b is an exhaust pipe for Ar. Now, the coil that has been hot-rolled and wound at 550 to 700 DEG C. is placed on the support base 4 using an appropriate conveyance means such as a table roll or a crane, and covered with a sealing cover 8. The edge 8a of the sealing cover is immersed in water to maintain airtightness. The blowing nozzle 6 is then supplied from a supply source (not shown).
If, for example, N ₂ gas is blown through a to 6f, air will be exhausted from the exhaust pipe 9a, and the inside of the airtight cover 8 will be
After being filled with N ₂ gas, valve 10a is closed. Depending on the need for cooling, the valve 10a may be kept open to constantly exhaust air from the exhaust pipe 9a. Note that 11a to 11e are expandable tubes that form part of the gas supply tube, and are expanded and contracted to facilitate movement when the airtight cover 8 is lifted or suspended by a crane (not shown). Used for the purpose of enhancing sex. Instead of the telescopic tubes 11a to 11e, detachable valve joints for gas pipes with good workability may be used. Now, when the coil is cooled in an inert atmosphere as described above, the parts of the coil surface that are easily oxidized and generate secondary scale containing a large amount of Fe ₂ O ₃ , especially the top, bottom and both edges, are susceptible to oxidation during normal air cooling.
The hot-rolled steel sheet has secondary scale with a high content of Fe ₃ O ₄ and has excellent secondary scale adhesion. Scale adhesion in the present invention was determined by attaching cellophane tape to the outer surface of the central part of a bending test piece, bending it by 90 degrees with the same radius of curvature as the thickness of the test piece, and then peeling off the cello tape. Those with good scale adhesion were marked with ◯, partially peeled off, those with slightly poor scale adhesion were marked with △, and most of them were peeled off, and those with poor scale were marked with ×. Based on these measurement results, in the present invention, 650℃
In the case of , it can be seen that the scale adhesion is extremely good not only at both ends but also over the entire length and width. FIG. 2 is a schematic explanatory diagram of different apparatuses for carrying out the method of the present invention, in which a support stand 12 is provided surrounding a pit 17 drilled in the work floor 2.
A sealing cover 80 is removably arranged on the upper surface via a flexible seal stand 13, and the coil 1 is mounted on a perforated support plate 18 which is constructed on the support stand 12 by a transportation device such as a crane (not shown). However, before that, the sealing cover 80 is lifted up by a lifting crane (not shown), and after the coil 1 has been placed, it is hung down. 6g and 6h are blowing nozzles provided in the airtight cover 80 and the pit 17 of the work floor 2, including porous blowing ports 14a and 14b, and manifolds 14c and 14d.
and is connected to inert gas supply pipes 16a and 16b. 15 indicates an on-off valve. 90a and 90b are exhaust pipes, which are not shown through blow nozzles 6g and 6h from above and below.
When a predetermined amount of gas is supplied from an inert gas supply device such as N ₂ or Ar, a perforated support plate 18 is formed on the lower side of the coil.
through a through hole (not shown) provided in the
On the upper side, gas is blown directly onto the end face of the coil 1. Then, the inside of the airtight cover 80 is quickly filled with inert gas, and a highly adhesive scale is formed on the entire surface of the coil 1, including both end faces and edges. Instead of supporting the coil 1 by the perforated support plate 18 in the above device example, if an appropriate number of cross beams are arranged on the support base 12 to support the coil 1, gas will pass through the space between the cross beams and reach the lower end surface of the coil 1. Similarly, edge oxidation prevention can be effectively achieved. In this way, it is permissible to change the design of the device as long as it does not deviate from the intended scope. Example 1 After melting low carbon steel having the composition shown in Table 1, it was continuously cast and directly rolled at a hot rolling finishing temperature of 840°C and a coiling temperature of 550°C and 650°C. _N2 gas was blown into the coil at a flow rate of 800/min for 3 minutes, then the flow rate was increased to 100/min, and the coil surface temperature was
Blowing was continued until the temperature reached 350°C. For comparison, steel with the same composition was rolled up at 575°C and 650°C and then air cooled (forced cooling using a blower). As a result, the scale adhesion at the widthwise ends of the air-cooled material was poor in the taping bending test, whereas the coil produced by the method of the present invention was good over the entire length and width. A comparison is shown in FIG. In Figure 3, the width direction of the central part of the coil is plotted on the horizontal axis, the scale adhesion is plotted on the vertical axis, and the winding temperature is plotted.
This is a comparison between the case of the present invention sealed with _N2 at 650℃ and 550℃, and the case of a normal air-cooled coil.
It can be seen that in the present invention, scale adhesion, especially at the edges, is dramatically improved. The ends in the width direction of the coil, which were air-cooled after being wound at 650°C, had a scale containing a lot of Fe ₂ O ₃ and had particularly poor adhesion.

[Table] Example 2 After melting steel with the composition shown in Table 2, it was continuously cast and directly rolled to a hot-rolled finish temperature of 820°C and a coiling temperature of 600°C.
After winding up the coil, it is loaded into the device shown in Figure 1, and N ₂ gas is blown into the left and right supply pipes at a flow rate of 1 m ^{3 /} min for 2 minutes. Blowing was continued until the surface temperature reached 350℃. The total amount of N ₂ gas blown was about 35 m ³ . As a comparative material, hot steel slabs obtained by continuous casting were used as 1270
A coil obtained by heating at ℃ was cooled in N ₂ gas to a temperature of 350℃ or less, and the scale adhesion was compared. All of the coils manufactured by the method of the present invention had good scale adhesion, whereas the comparative materials were inferior. The comparison is shown in FIG.

[Table] Figure 4 similarly shows the width direction dimension of the central part of the coil, scale adhesion on the vertical axis, and the winding temperature.
This is a comparison between the case of the present invention sealed with N ₂ at 600°C and the conventional reheated coil, and the scale adhesion is improved on average in the present invention. On the other hand, the scale adhesion of reheated coils, especially at the ends, is somewhat poor, and the concentration of C and Si at the interface between the steel base and scale is higher than that of directly rolled materials. Example 3 After melting steel with the composition shown in Table 3, it was continuously cast and directly rolled to a hot rolling finish temperature of 840°C and a coiling temperature of 650°C.
After winding the coil, it is loaded into the device shown in Figure 1, and N ₂ gas is blown into the left and right supply pipes at a flow rate of 0.8 m ^{3 /} min for 2 minutes. Blowing was continued until the surface temperature of the part reached 350℃. The total amount of N ₂ gas blown was approximately 33 m ³ . As shown in Figure 5, the coil manufactured with the components of the present invention has good scale adhesion, and the comparison material with higher components has less Si, P, S, Cr, Cu, etc. on the base metal/scale than the present invention material. It is high at the interface and has poor scale adhesion.

[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 require pickling or bonding treatment, and can be used for manufacturing pipes, drums, etc. After manufacturing channel steel, angle steel, etc. by forming processing, it is possible to paint as necessary, making it extremely economical. The present invention provides a means for economically producing such hot rolled steel sheets, and is a method with high practical effects.

[Brief explanation of the drawing]

FIG. 1 is a schematic explanatory diagram of an embodiment apparatus for implementing the method of the present invention, FIG. 2 is a schematic explanatory diagram of different apparatuses for implementing the method of the present invention, and FIG.
FIGS. 4 and 5 are graphs comparing scale adhesion between the coil according to the present invention and a comparison material coil. 1...Coil, 2...Working floor, 3...Water seal device,
4...Support stand, 5a-5f...Gas feed pipe, 6a-6
f...Blowing nozzle, 7a-7f...Opening/closing valve, 8...
Airtight cover, 9...exhaust pipe, 10...valve, 11a
~11e... Telescopic tube, 12... Support stand, 13... Flexible seal stand, 14a, 14b... Inlet,
14c, 14d...manifold, 15...open/close valve, 16a, 16b...inert gas supply pipe, 17...
Pit, 18... Perforated support plate, 90a, 90b... Exhaust pipe.

Claims (1)

[Claims] 1 C0.03~0.25%, Si0.03% or less, Mn0.20~
1.00%, P0.020% or less, S0.020% or less, Al0.05%
Below, Cu is 0.03% or less, Cr is 0.03% or less,
After melting low-carbon steel, the remainder of which is Fe and unavoidable impurities, the continuously cast slab is directly hot-rolled, or parts where the temperature tends to drop, such as the edges, are heated using an induction heating device, gas heating device, etc. After being lightly heated, the coil is hot-rolled at 550 to 700℃ and then cooled in a non-oxidizing atmosphere consisting of _N2 gas, Ar gas, or a mixture thereof until it reaches 350℃. A method for producing hot-rolled steel sheets with excellent scale adhesion.