JPS624820A - Manufacture of hot rolled steel sheet for intense working having superior adhesion to scale - Google Patents

Abstract

PURPOSE:To obtain a hot rolled steel sheet suitable for hard working and having superior adhesion to scale by subjecting a steel slab contg. specified amounts of C, Si, Mn, P, S, Al and Cr to heating, hot rolling at a specified temp., rapid cooling and coiling. CONSTITUTION:A steel consisting of, by weight, 0.03-0.25% C, >0.04-0.5% Si, 0.2-2.0% Mn, <=0.025% P, <=0.025% S, <=0.08% Al, 0.1-0.5% Cr and the balance Fe with inevitable elements is manufactured by refining. The steel is formed into a slab by continuous casting or through an ingot, and the slab is put in a heating furnace or cooled once and reheated. The slab is then hot rolled at the Ar3 point + 50 deg.C or below, the pouring of water is started within 5sec after the hot rolling to rapidly cool the resulting steel sheet at >=40 deg.C/sec cooling rate and the cooled steel sheet is coiled at 360-600 deg.C. The steel sheet may be coiled at <360 deg.C after the rapid cooling and cooled at 0.5 deg.C/min cooling rate. Thus, a hot rolled steel sheet for intense working having high adhesion to scale over the whole width is surely obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial 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 working of square steel pipes and the like.

[Conventional technology]

The secondary scale of steel plates manufactured by hot rolling steel slabs obtained by continuous casting or ingot forming cannot be avoided in part due to the forming process, so conventionally, after pickling, phosphate treatment, etc. After surface treatment, molding and processing are performed to produce drums,
The method used was to make products such as pipes and then coat the surface. 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 uses 0.02 to 0.1 Cr, N
Add 0.02 to 0.1% of 1 and set the winding temperature to 450 to 7.
The temperature was 50°C.

However, the Japanese Patent Publication No. 54-31734 does not provide any countermeasures against the warping tar crack and the nugget fracture of the spot weld. Furthermore, N1 is expensive and economically disadvantageous.

Also, patent application No. 58-170855 and patent application No. 60-315
The steel sheet obtained by method 00 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 not suitable for difficult processing applications such as square pipes. If used, the adhesion of the scale will be insufficient and the user will not be satisfied.

[Problem to be solved by the invention]

The object of the present invention is to provide a manufacturing method that can reliably obtain a hot rolled steel plate with excellent scale adhesion suitable for difficult processing at a low cost, without penetrator cracking during flash butt welding or nugget fracture in spot welds. That is.

[Means for solving problems]

Means for solving the above problems in the present invention are as follows.

That is, (1) C0.03-0.25 wt%, 0.04(
Si≦0.5wt%, Mn 0.2-2. 0wt%,
P≦0.025wt%, S≦0.025wt, Al≦
0. O'8 wt%, 0.1% Cr≦0.5 wt%
After melting a steel containing Fe and other unavoidable elements, the steel slab obtained by continuous casting method or ingot making method is fed into a heating furnace or once cooled and then reheated. death,(
Scale adhesion characterized in that after hot rolling at a temperature of Ar3+50℃ or lower, water injection cooling is started within 5 seconds and the rolling is performed at a temperature of 600℃ or lower and 360℃ or higher with rapid cooling at a cooling rate of 40℃/second or higher. A method for producing hot-rolled steel sheets for heavy working with excellent properties.

(z) co, o3~0.25 wt chi, 0.04 (Si
≦0.5wt%, Mn 0.2-2. Owt, %, P≦
0.025 wt%, S≦0.025 wt%, Al≦
A steel billet obtained by a continuous casting method or an ingot method after melting a steel containing 0.08 vt%, 0.1%>Cr≦0.5 wt%, with the balance consisting of Fe and unavoidable elements. (Ar
After hot rolling at a temperature of 3+50℃ or lower, water injection cooling is started within 5 seconds and rapid cooling is performed at a cooling rate of 40℃/second or higher.
Roll it up at a temperature below 0℃, then reduce the cooling rate to 0.5℃
1. A method for producing a hot-rolled steel sheet for heavy working which has excellent scale adhesion.

It is.

[Effect]

The effects brought about by limiting the components and hot rolling conditions in the present invention will be described. By setting the CQ to 03 to 0.25 inches, the strength and secondary scale adhesion required for the application of the hot rolled steel sheet according to the present invention, ie, square steel pipes, etc., can be obtained. That is, if it is less than 0.03%, there is a problem in the strength of the target steel plate, and if it exceeds 0.25%, there is a lot of Fe5C (cementite) near the scale/substrate interface, which deteriorates the adhesion of 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 for setting Si to 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 C-)-Mn/20+Si/3O-)-2P-)-
The smaller the value of 48, the less likely the nugget fracture will occur in the spot weld. Also, the strength is Ceq = (! 0Mn
/ 6 + Si /4, and in order to obtain the desired strength, especially when manufacturing high-strength steel sheets, it is preferable to increase Si compared to C1Mn for spot welding, and the amount of addition is Mn. /Si should be taken into account when deciding. From these facts, the amount of 81 added is from more than 0.09 to 0.
, 5% or less, the cyscale adhesion is not deteriorated (Mn/5i=4 to 23 is reliably obtained).

By setting P to 0.025% or less and S to 0.025% or less, it is possible to prevent pSs from concentrating at the scale/substrate interface during heating and hot rolling, and from worsening secondary scale adhesion. There is.

For the purpose of the present invention, it is preferable that both p and s be small. However, since processing costs for both P removal and S removal increase, the content may be appropriately determined to be below this value, taking into consideration the economical tolerance range.

Next, when M is deoxidized during the molten steel treatment process for producing steel sheets, which is the object of the present invention, the inclusion of M compounds is unavoidable.As a result of investigating the influence of M on scale adhesion, it was found that scale adhesion can be maintained well. The allowable range of M is 0.08
% or less.

In addition, by controlling Cr from more than 0.1% to 0.5% or less, fine ROM carbide is formed, the amount of cementite is reduced, and the scale of Cr-Si-0 noncompounds and base iron are reduced. Since the scale is formed in an anchor ring shape in between, the scale adhesion is much improved compared to a hot-rolled steel sheet manufactured under the hot-rolling conditions described below with Cr added in an amount of 0.1% or less. The effect of this addition of Cr on improving adhesion is up to Q5, and beyond this point economical efficiency is lost.

Furthermore, it is preferable to add elements such as Nb, Mo, V, and Zr within an economically acceptable range in order to satisfy the required material properties, particularly the desire to improve strength.

Next, the hot rolling conditions will be described.

C: 0.09-0.15 wt%, sl: 0.07
~0.20wt%, Mn: 0.40~0.75w
t%, P: 0.014-0.016wt%, S:
0.0 0 7~0.0 1 5vt, %, fiJ
, : 0. Ol 4-0. 'O25wt% 0.1
(We investigated the scale adhesion, scale thickness, scale composition, and composition at the edge in the width direction of steel plates manufactured by changing various hot rolling conditions for slabs with Cr≦0.5wt%, and organized the relationships among them. Shown in Figure 1.2.Scale thickness ≦10μ, Fe2
If the conditions of 03≦25%, Fe, and 04275% are satisfied, the scale adhesion rating Cr is 0 to 1 or less and passes.

[Cr: The outer surface of a curved steel plate bent at 90 degrees by pressing a punch with a radius R = 1.5 x plate thickness is taped, and the scale peeling status is divided into 10 categories based on area and ranked as a score].

In the present invention, it is particularly preferable to set the heating temperature to 1,100°C or less, and the reason for this is that elements (Sl, p, s
The aim is to reduce the enrichment at the scale/substrate interface during the heating stage of (e.g.)
It is desirable to appropriately determine the temperature at 0°C or lower.

The reason why the finishing rolling temperature is limited to (Ar3+5°C) or lower in the present invention is to keep the temperature low within a range that does not leave a mixed grain structure or a rolled structure in order 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 10 μm or less.

The reason why the winding temperature was set to 600°C or less in the above cases ① and ② is as shown in Figure 6, which shows the relationship between scale adhesion, finishing temperature, and winding temperature.If the winding temperature exceeds 600°C, the scale adhesion passes the test. I don't have anything. Further, as shown in Fig. 4.5, if the winding temperature is 600° C. or less, it is possible to make the amount of Fe2O3 produced less than 3 inches and the scale thickness less than 10 μm.

Generally, the scale production amount (y) is y = K... (
t: 'time), K = ni-e-'R'I' (K: constant determined by atmospheric conditions, etc., Q: 33000 cak7'm
o l, R: gas constant, T: temperature). Therefore, it is preferable that the residence time at high temperature be short.
According to the findings obtained from the inventor's experiments, after finish rolling,
The time required to start cooling must be 5 seconds or less.

The reason why the cooling rate during cooling after finish rolling was set to 40° C./S or higher was to reduce the scale thickness while ensuring scale adhesion.

Observation of the scale layer under a microscope showed that the cooling rate was 4.
If it is slow, less than 0℃/sec, FeO→Fe3O
α-Fe due to 4+ Fe transformation (occurs below 570°C)
is present on the scale surface side, transformation starts during cooling and progresses from the scale surface side, and if the cooling rate is fast, α-Fe is observed on the entire scale layer or on the substrate interface side, From these facts, it is considered that the transformation was progressing from the steel base side as well, and that the produced Fe3O4 and the steel base had good consistency, and the scale adhesion was improved. This phenomenon also depends on the winding temperature; when it exceeds 60°C, transformation proceeds from the scale surface regardless of the cooling rate, and scale adhesion is poor. However, by adding Cr, cementite transforms into fine Cr carbide, reducing the amount of cementite under the interface, and 0r-8i-0 uncompounds are generated at the interface between the scale and the steel, showing an anchoring effect. Even when the winding temperature is higher than 500°C and lower than 600°C, the scale adhesion is greatly improved compared to the one without Cr addition.

Cooling at a cooling rate of 40℃/s or more, winding temperature 600℃
Hereinafter, even if the cooling rate is slow (less than 0.5°C/min) after winding at 360°C or higher, the transformation proceeds from both the scale surface and the steel base interface, and the scale becomes Fe containing α-Fe on the entire surface.
Compared to 3O4, scale adhesion is good. Moreover, C
By adding more than 0.1 inch to less than 0.5 inch of r, cementite is converted to fine Cr carbide, reducing the amount of cementite under the interface, and Cr-8i-0 uncompounds are added to the scale matrix interface. The scale adhesion is further improved due to the anchoring effect. However, when the winding temperature is lower than 360℃ and the subsequent cooling rate is lower than 0.5℃/min, α-Fe advances only from the base steel side, but α-Fe exists in islands. , scale adhesion is poor and undesirable.

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 Fe3O4 can be suppressed and the FeO bio Adhesion is good if the scale is .

This is because FeO is soft, and FeO//・α-Fe (
The azimuth relationship of (subway) is (100) [011] Fo. ////
(100) (α■1) It is considered that ct-Fe has good consistency and good adhesion. 0.5 after winding
To cool at a cooling rate of ℃/min or higher, stop stacking in a well-ventilated place in the yard and use natural ventilation or forced ventilation with a fan, N2, A in a box, etc.
Cooling means such as forced cooling by a fan using r-gas, water cooling, immersion cooling, etc. or a combination thereof may be used.

〔Example〕

The composition of the steel used in the examples and comparative examples of the present invention.

Shown in Table 1. Steel has a composition system based on Al, Si, and K, and steels ■ to ■ have a composition system in which Cr is added to the composition of steel ■, and the addition of Cr increases in the order of ■, ◎, and [F].

Table 2 shows the hot-rolling conditions for the inventive example and comparative example using the above steel, and the lightweight section steel (C
(channel) and scale adhesion after forming to a heavily worked square steel pipe.

Table 1 Comparative example of composition
This is a coil wound at 50°C. There was scale peeling even when forming lightweight section steel for light processing, which is not at a satisfactory level. Comparative Example (2) is a coil made of steel (4) at a rolling temperature of 870°C and rolled at 650°C. Although scale adhesion was improved compared to Comparative Example ■, scale peeling was still large even when forming lightweight sections and square steel pipes.
Not at a satisfactory level. Comparative Example (2) is a coil made of steel (4) at a rolling temperature of 870°C and rolled at 550°C. Compared to Comparative Example 2, the scale adhesion has improved, and although some scale peeling is seen when forming lightweight section steel, it is at a satisfactory level. It is not at a satisfactory level.

Comparative example ■ is a steel rolling temperature of 830°C and 45°C.
This is a coil wound at 0°C. Although the scale adhesion was slightly improved compared to Comparative Example ■, it was not at a satisfactory level for forming square steel pipes. Comparative Example (2) is a coil made of steel (4) at a rolling temperature of 830°C and a rolling temperature of 450'Cf. Compared to Comparative Example ■, the scale adhesion is slightly improved, and although it is at a satisfactory level for lightweight section steel, it is not at a satisfactory level for forming square steel pipes. In ◎, the rolling temperature was 870℃, and the rolling temperature was 550℃.
This is a coil wound at ℃. The scale adhesion was considerably improved compared to Comparative Example (3), and although there was slight peeling of scale when forming lightweight sections and square steel pipes, it was at a satisfactory level. Inventive example (2) is a coil made of steel (◎) at a rolling temperature of 870°C and wound at 550°C. In addition, in the invention example (■), the rolling temperature was 870°C in steel [F],
This is a coil wound at 550°C. In both cases, the scale adhesion was improved compared to Comparative Example ■ and Invention Example ■, and the scale did not peel off at all when forming lightweight section steel.
Although some scale peeling is observed in the square steel pipe, it is at a satisfactory level.

Example (■) of the present invention uses steel ◎ at a rolling temperature of 830°C, and
This is a coil wound at 00℃. The scale adhesion was almost the same as that of Examples ① and ② of the present invention, and was at a satisfactory level.

Inventive Example 0 Steel O was rolled at a rolling temperature of 830°C.
This is a coil wound at 50°C. Inventive Example (2) is a coil made of steel O, rolled at a rolling temperature of 830°C and wound at 450°C. All of these products have extremely good scale adhesion, and even when forming square steel pipes that undergo heavy processing, most of the scale peels off, which is a completely satisfactory level.

In the present invention example (2), the rolling temperature was 830°C in steel O, and 3
This is a coil wound at 50°C. It shows almost the same scale adhesion as Examples ① and ② of the present invention, and is at a satisfactory level.

Example (2) of the present invention is a coil made of steel (2) heated at 1090°C, rolled at a rolling temperature of 830°C, and rolled at 550°C.

Inventive Examples ⑅ and ⑅ have improved scale adhesion and are at a completely satisfactory level, with no scale peeling off even during the forming of square steel pipes that undergo heavy processing.

Example (2) of the present invention is a coil made of steel (2) heated at 1080°C, rolled at a rolling temperature of 830°C, and rolled at 350°C.

The scale adhesion was almost the same as that of the invention examples (2), (2), (2), and (2), and was at a satisfactory level.

〔Effect of the invention〕

The present invention has two features according to the above configuration.

It is possible to reliably obtain a hot-rolled steel sheet at a low cost that is free from venerator cracking during pot welding and nugget fracture at the pot welded portion, and has excellent scale adhesion and is suitable for difficult-to-process applications.

As a result, there is less dust generated by scale in the post-process, and the scale adhesion is good over the entire length and width of the coil, further increasing yield when manufacturing difficult-to-process items such as square steel pipes.
It is extremely economical as it can reduce costs and can be painted if necessary.

[Brief explanation of drawings]

FIG. 1 shows the relationship between the scale adhesion score, the scale thickness, and the scale composition Fe2O3%. FIG. 2 shows the relationship between the scale adhesion score, scale thickness, and scale composition Fe3O4%. Figure 3 shows the relationship between rolling finishing temperature and scale thickness. Figure 4 shows the relationship between the winding temperature and the scale thickness. FIG. 5 shows the relationship between the winding temperature and the scale composition Fe2O3%. FIG. 6 shows the relationship between the scale adhesion score, rolling finishing temperature, and winding temperature.

Claims (3)

[Claims] (1) C0.03-0.25wt%, 0.04<Si≦
0.5wt%, Mn0.2-2.0wt%, P≦0.0
25wt%, S≦0.025wt%, Al≦0.08w
After melting steel containing t%, 0.1%<Cr≦0.5wt%, with the remainder consisting of Fe and unavoidable elements, the steel slab obtained by continuous casting method or ingot-forming method is placed in a heating furnace. Either by operation or by cooling it once and then reheating it (Ar_3+50℃).
After hot rolling at the following temperature, water injection cooling is started within 5 seconds and rapid cooling is performed at a cooling rate of 40°C/second or more to 600°C or less.36
A method for producing a hot-rolled steel sheet for heavy working with excellent scale adhesion, characterized by rolling at a temperature of 0° C. or higher. (2) C0.03-0.25wt%, 0.04≦Si≦
0.5wt%, Mn0.2-2.0wt%, P≦0.0
25wt%, S≦0.25wt%, Al≦0.08wt
%, 0.1%>Cr≦0.5wt%, the balance being F
After melting steel consisting of e and inevitable elements, the steel slab obtained by continuous casting method or ingot-forming method is put into a heating furnace or once cooled and then reheated to below (Ar_3+50℃). After hot rolling at a temperature of A method for producing a hot-rolled steel sheet for heavy working that has excellent scale adhesion. (3) The method for producing a hot-rolled steel sheet for heavy working with excellent scale adhesion according to claim 1 or 2, wherein the reheating temperature is 1100° C. or lower.