JPH0571650B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a method for manufacturing a hot rolled steel sheet with high strength and excellent scale adhesion, which is used for heavy processing of square steel pipes and the like. [Prior art] Because the secondary scale of steel sheets manufactured by hot rolling steel slabs obtained by continuous casting or ingot-forming cannot be avoided in part due to forming, pickling has traditionally been used. After that, the material was subjected to surface treatments such as phosphate treatment, then molded and processed into products such as drums and pipes, and then the surface was coated.
However, since it is economically disadvantageous, recently Japanese Patent Publication No. 31734/1983 has been proposed as a method for producing hot rolled steel sheets with excellent scale adhesion that does not require a surface treatment process. This method contains 0.02-0.1% Cr and Ni
0.02 to 0.1% was added, and the winding temperature was 450 to 750°C. However, the hot-rolled steel sheet obtained by the method disclosed in Japanese Patent Publication No. 54-31734 has welding problems in the applications to which it is applied. I'm not used to it. Furthermore, Ni is expensive and economically disadvantageous. Also, JP-A-60-63319 and JP-A-61-
The steel sheet obtained by the method of Publication No. 194112 is a hot-rolled steel sheet with high strength and excellent scale adhesion that is suitable for light processing applications such as automobile suspension parts, pipes, and drums, but it is suitable for difficult-to-process processing such as square pipes. When used for other purposes, the adhesion of the scale is insufficient and users are not satisfied. [Problems to be Solved by the Invention] The present invention is capable of producing hot-rolled steel sheets at low cost that are free from penetrator cracking during flash butt welding and nugget fractures at spot welds, and have excellent scale adhesion and are suitable for difficult machining. The purpose is to provide a manufacturing method that can be used to obtain the desired results. [Means for solving the problems] The means for solving the above problems in the present invention are as follows. That is, (1) C: 0.03 to 0.25wt%, 0.04<Si≦0.5wt%,
Mn: 0.2-2.0wt%, P≦0.025wt%, S≦
0.025wt%, Al≦0.08wt%, 0.1<Cr≦0.5wt%
After melting the steel containing Fe and the rest consisting of Fe and unavoidable elements, the steel slab obtained by continuous casting method or ingot-forming method is charged into a heating furnace or once cooled and then reheated. After hot rolling at a temperature below (Ar ₃ + 50℃), water injection cooling is started within 5 seconds and rapid cooling is performed at a cooling rate of 40℃/second or higher to 600℃ or below.
A method for producing a hot-rolled steel sheet for heavy working with excellent scale adhesion, which is characterized by rolling at a temperature of 360°C or higher. (2) C: 0.03-0.25wt%, 0.04<Si≦0.5wt%,
Mn: 0.2-2.0wt%, P≦0.025wt%, S≦
0.025wt%, Al≦0.08wt%, 0.1<Cr≦0.5wt%
After melting the steel containing Fe and the rest consisting of Fe and unavoidable elements, the steel slab obtained by continuous casting method or ingot-forming method is charged into a heating furnace or once cooled and then reheated. After hot rolling at a temperature below (Ar ₃ + 50℃), water injection cooling was started within 5 seconds, and rapid cooling was performed at a cooling rate of 40℃/second or higher, and rolling was performed at a temperature below 360℃. 0.5℃/
A method for producing a hot-rolled steel sheet for heavy working which has excellent scale adhesion. It is. [Effects] The effects brought about by limiting the components and hot rolling conditions in the present invention will be described. By setting C to 0.03 to 0.25%, the strength and secondary scale adhesion required for the use of the hot rolled steel sheet according to the present invention, ie, square steel pipes, etc., can be obtained. In other words, if it is less than 0.03%, there will be problems with the strength of the target steel plate, and if it is more than 0.25%, Fe ₃ C (cementite) will be formed near the scale/substrate interface.
, which deteriorates the adhesion of the secondary scale. By controlling Mn to 0.2 to 2.0%, the desired strength of the steel plate can be ensured without deteriorating scale adhesion. That is, if it is less than 0.2%, the desired strength of the steel plate cannot be obtained, and if it exceeds 2.0%, economic efficiency is lost and scale adhesion deteriorates. The reason why Si is set at 0.04 to 0.5% is that Si is added as an element that plays a very important role in terms of weldability. In other words, if Mn/Si is in the range of 4 to 23, penetrator cracking will not occur during flash butt welding. Also, the smaller the value of C+Mn/20+Si/30+2P+4S, the less likely the nugget fracture will occur in the spot weld. Also, the strength is Ceq=C+Mn/6
It is proportional to +Si/4, and in order to obtain the desired strength, especially when manufacturing high-strength steel plates, C,
It is preferable for spot welding to use more Si than Mn, and it is necessary to determine the amount by considering the amount of Mn/Si added. For these reasons, the amount of Si added is from over 0.09 to 0.5%.
By setting the following, Mn/Si=4 to 23 can be reliably obtained without deteriorating scale adhesion. By setting the P content to 0.025% or less and the S content to 0.025% or less, P and S will not scale/scale during heating and hot rolling.
This prevents the secondary scale from condensing at the interface of the substrate and impairing the adhesion of the secondary scale. For the purpose of the present invention, it is preferable that both P and S are small. However, since processing costs increase for both P removal and S removal, the content may be appropriately determined to be below this value in consideration of economical tolerance. Next, when Al is deoxidized during the molten steel processing process to manufacture steel sheets, which is the objective of the present invention, the inclusion of Al is unavoidable, so as a result of investigating the influence of Al on scale adhesion, it was found that scale adhesion can be maintained well. It was recognized that the acceptable range of Al that could be produced was 0.08% or less. In addition, by reducing the Cr content from more than 0.1% to 0.5% or less, fine chromium carbide is formed, reducing the amount of cementite, and creating an anchor between the scale of the Cr-Si-O compound and the base metal. Since the scale is formed into a shape, scale adhesion is further improved compared to a hot-rolled steel sheet manufactured under the hot-rolling conditions described below without adding 0.1% or less of Cr. The effect of this Cr addition on improving adhesion is up to 0.5%, and beyond this it loses economic efficiency. Furthermore, it is preferable to add the elements Nb, Mo, V, and Zr within an economically acceptable range in order to satisfy the required material properties, especially the desire to improve strength. Next, the hot rolling conditions will be described. C: 0.09-0.15wt%, Si: 0.07-0.20wt%,
Mn: 0.40-0.75wt%, P: 0.014-0.016wt%,
S: 0.007~0.015wt%, Al: 0.014~0.025wt%,
The scale adhesion, scale thickness, and scale composition at the edges in the width direction of the steel plates manufactured by changing various hot rolling conditions for slabs with 0.1<Cr0.5wt% were investigated, and the relationships among them were adjusted. Shown in Figure 2. If the following conditions are satisfied: scale thickness ≦10μ, Fe ₂ O ₃ ≦25%, and Fe ₃ O ₄ ≧75%, then the scale adhesion rating Cr is 0 to 1 or less and passes. [Cr: The outer surface of a steel plate bent at 90° by pressing a punch with radius R = 1.5 x plate thickness is taped, and the degree of scale peeling is classified into 10 areas and ranked as a score]. In the present invention, it is particularly preferable to set the heating temperature to 1100°C or less, because the heating temperature of elements (Si, P, S, etc.) that are enriched at the scale/substrate interface and deteriorate the adhesion of secondary scale. In order to reduce the enrichment at the scale/substrate interface, it is desirable to appropriately determine the heating temperature at 1100°C or less within the range that allows finish rolling. In the present invention, the rolling finishing temperature (Ar ₃ +50℃)
The reason for the following limitations is to keep the temperature low within a range that does not leave a mixed grain structure or a rolled structure.
The goal is to reduce the scale thickness. As shown in FIG. 3, by setting the temperature to 280°C or less, the scale thickness can be reduced to 10μ or less. The reason why the winding temperature was set at 600°C or lower in the above is that Figure 6 shows the relationship between scale adhesion, finishing temperature, and winding temperature.
At temperatures exceeding ℃, there is no product that passes the scale adhesion test.
Further, as shown in FIGS. 4 and 5, if the winding temperature is set to 600° C. or lower, it is possible to reduce the amount of Fe ₂ O ₃ produced to 3% or less and the scale thickness to 10 μm or less. Generally, the scale generation amount (y) is y=k・√(t: time), K
=K・e ^-Q/RT (K: constant determined by atmospheric conditions, etc.
Q: 33000cal/mol, R: gas constant, T: temperature)
It can be expressed as Therefore, it is preferable that the residence time at high temperature be short, and according to the findings obtained from experiments by the present inventors, the time from finish rolling to the start of cooling needs to be 5 seconds or less. The cooling rate during cooling after finish rolling was set to 40° C./s or more in order to reduce the scale thickness while ensuring scale adhesion. Observation of the scale layer under a microscope shows that if the cooling rate is slow (less than 40℃/sec), FeO→
Fe ₃ O ₄ + α− due to Fe transformation (occurs below 570℃)
Fe exists on the scale surface side, and transformation starts during cooling and progresses from the scale surface side.
When the cooling rate is fast, α-Fe is observed on the entire scale layer or on the steel substrate interface side, indicating that transformation is also progressing from the steel substrate side, and the generated Fe ₃ O ₄
This is thought to be due to the good compatibility between the scale and the base steel, resulting in improved scale adhesion. This phenomenon also depends on the winding temperature; above 600°C, transformation proceeds from the scale surface regardless of the cooling rate, resulting in poor scale adhesion. However, by adding Cr, cementite transforms into fine Cr carbide, reducing the amount of cementite under the interface, and Cr-Si-O compounds are generated at the interface between the scale and the base metal, exhibiting an anchoring effect. Winding temperature is over 500℃~600℃
Even in the following cases, the scale adhesion is greatly improved compared to those without Cr addition. Even if cooling is performed at a cooling rate of 40°C/s or higher, the winding temperature is 600°C or lower, and cooling is performed at a slow cooling rate of less than 0.5°C/min after winding at 3,600°C or higher, transformation occurs from both the scale surface and the substrate interface. As the scale progresses, the entire surface becomes Fe ₃ O ₄ containing α-Fe, and the scale adhesion is good. Moreover, Cr
By adding more than 0.1% to less than 0.5% of
The scale adhesion is further improved due to the anchoring effect of the system compound generated at the scale base iron interface. However, if the winding temperature is less than 360℃ and the subsequent cooling rate is less than 0.5℃/min, α
-Fe advances only from the sub-rail side, but α-Fe advances from the sub-rail side, but α-Fe exists in islands,
Scale adhesion is poor and unfavorable. However, even in the case of winding at a cooling rate of 40°C/s or more and less than 360°C, if the cooling rate is 0.5°C/min or more after winding, the transformation of FeO to Fe ₃ O ₄ can be suppressed and FeO Adhesion is good if it is a biological scale. This is because FeO is soft and FeOα−
The orientation relationship of Fe (substrate) is (100) [011] _FeO (100)
[001] = _-Fe is considered to have good consistency and good adhesion. To cool the product at a cooling rate of 0.5℃/min or higher after winding, place it in a well-ventilated place in the yard, stop stacking, and use natural ventilation or forced draft cooling with a fan, or cool it in a box.
Forced cooling using a fan using _N2 , Ar gas, etc.
Cooling means such as water spray cooling, immersion cooling, etc. or a combination thereof may be used. [Example] Table 1 shows the components of the steel used in the Examples and Comparative Examples of the present invention. Steel is a composition system based on Al ₁ -Si, -K, and steel ~ is a composition system in which Cr is added to the steel composition, and the addition of Cr is greater in the order of <<<. Table 2 shows the hot rolling conditions of the inventive example and comparative example using the above steel, and the scale adhesion after forming into lightly worked lightweight section steel (C channel) and heavily worked square steel pipe using Edge hoop. shows.

【table】

【table】

〔Effect of the invention〕

With the above-described structure, the present invention can inexpensively and reliably produce a hot-rolled steel sheet that is free from venerator cracking during flash butt welding and nugget fracture at spot welds, and has excellent scale adhesion and is suitable for difficult-to-process applications. It is something that can be obtained. As a result, there is less dust generated by scale in post-processing, and scale adhesion is good over the entire length and width of the coil, further increasing yields and reducing costs when manufacturing difficult-to-process items such as square steel pipes. Then,
It can be painted if necessary and is extremely economical.

[Brief explanation of the drawing]

Figure 1 shows the scale adhesion rating and scale thickness.
The relationship with scale composition Fe ₂ O ₃ % is shown. FIG. 2 shows the relationship between the scale adhesion score, scale thickness, and scale composition Fe ₃ O ₄ %. Figure 3 shows the relationship between rolling finishing temperature and scale thickness. FIG. 4 shows the relationship between winding temperature and scale thickness. FIG. 5 shows the relationship between the winding temperature and the scale composition Fe ₂ O ₃ %. FIG. 6 shows the relationship between the scale adhesion score, rolling finishing temperature, and winding temperature.

Claims (1)

[Claims] 1 C: 0.03 to 0.25wt%, 0.04<Si≦0.5wt%,
Mn: 0.2-2.0wt%, P≦0.025wt%, S≦
Steel slabs obtained by continuous casting or ingot forming after melting steel containing 0.025wt%, Al≦0.08wt%, 0.1<Cr≦0.5wt%, with the balance consisting of Fe and inevitable elements. After charging into a heating furnace or turning it into cold pieces and then reheating and hot rolling at a temperature below (Ar ₃ +50℃), water injection cooling is started within 5 seconds to increase the cooling rate.
A method for producing a hot-rolled steel sheet for heavy working with excellent scale adhesion, characterized by quenching at a rate of 40°C/second or more and rolling at a temperature of 600°C or lower and 360°C or higher. 2. The method for producing a hot-rolled steel sheet for heavy working with excellent scale adhesion according to claim 1, wherein the reheating temperature is 1100°C or less. 3 C: 0.03-0.25wt%, 0.04<Si≦0.5wt%,
Mn: 0.2-2.0wt%, P≦0.025wt%, S≦
Steel containing 0.025wt%, Al≦0.08wt%, 0.1<Cr≦0.5wt%, with the balance consisting of Fe and inevitable elements, is melted and then obtained by continuous casting method or ingot forming method. After charging into a heating furnace or turning it into cold pieces and then reheating and hot rolling at a temperature below (Ar ₃ +50℃), water injection cooling is started within 5 seconds to increase the cooling rate.
A method for producing a hot-rolled steel sheet for heavy working with excellent scale adhesion, characterized by rapid cooling at a rate of 40°C/sec or more and rolling at a temperature below 360°C, followed by a cooling rate of 0.5°C/min or more. 4. The method for producing a hot-rolled steel sheet for heavy working with excellent scale adhesion according to claim 3, wherein the reheating temperature is 1100°C or less.