JPH0359125B2 - - 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 having high strength and excellent scale adhesion for light processing and used for automobile suspension parts, pipes, drums, etc. [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 0.1% Ni.
0.02 to 0.1% was added and the winding temperature was 450 to 750°C. [Problems to be Solved by the Invention] The method disclosed in Japanese Patent Publication No. 54-31734 has the following problems. There are welding problems in the applications to which it is applied, and there is no mention of countermeasures against penetrator cracking during flash butt welding and nugget fracture in spot welds, and Ni is expensive and economically disadvantageous. The present invention solves the above-mentioned problem of weldability and provides a method for inexpensively producing a hot-rolled steel sheet with high strength and excellent scale adhesion suitable for light processing applications. [Means for Solving the Problems] The inventors of the present invention have conducted various studies to solve the above-mentioned problems, and have found that: a. Decrease the thickness of the scale...Regardless of the composition of the scale, the deformability at room temperature is , it is inferior to the matrix of local railways. Therefore, it is necessary to make it as thin as possible. b Scale composition: Fe ₂ O ₃ , Fe ₃ O ₄ , and FeO exist, but Fe ₂ O ₃ , which has high hardness at room temperature and poor deformability, should be avoided. Fe ₃ O ₄ is produced by the transformation of FeO, which is stable at high temperatures (above 570°C), but the amount and transformation process are important and affect scale adhesion. When FeO has good compatibility with the matrix, which is the base iron, scale adhesion improves, partly because it is soft. c Scale/substrate interface or just below the interface...P,
This is influenced by the concentration phenomenon of S, Cu, etc., the amount of other iron cementite directly under the interface, and the presence of oxides and nitrides. obtained new knowledge. The present invention has been made based on this new knowledge, and the features of the means are as follows. C0.03~0.25wt%, 0.04<Si≦0.5wt%,
Mn0.2~2.0wt%, P≦0.025wt%, S≦
After melting steel containing 0.015wt%, Al≦0.08wt%, and Cr≦0.1wt%, with the balance consisting of Fe and unavoidable elements, the steel slab obtained by continuous casting method or ingot forming method is heated in a heating furnace. After being charged into cold pieces or reheated and hot-rolled 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. below 600℃ and 360℃
A method for producing a hot rolled steel sheet with excellent scale adhesion, characterized by rolling at a temperature above. C0.03~0.25wt%, 0.04<Si≦0.5wt%,
Mn0.2~2.0wt%, P≦0.025wt%, S≦
After melting steel containing 0.015wt%, Al≦0.08wt%, and Cr≦0.1wt%, with the balance consisting of Fe and unavoidable elements, the steel slab obtained by continuous casting method or ingot forming method is heated in a heating furnace. After being charged into cold pieces or reheated and hot-rolled 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. A method for producing a hot-rolled steel sheet with excellent scale adhesion, characterized by rolling at a temperature of less than 360°C, and then cooling at a rate of 0.5°C/min or more. [Effects] The effects brought about by limiting the components and hot rolling conditions in the present invention will be described. By setting the C content to 0.03 to 0.25%, the hot rolled steel sheet according to the present invention can be used in applications such as the strength required for the brake and accelerator pedal arms, frames, etc. of automobile suspensions.
Adhesion of the next scale is 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 less than 0.25%.
In the case of super steel, Fe ₃ C (cementite) also increases near the scale/substrate interface, worsening 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 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 have more Si than Mn, and the amount of addition needs to be determined with consideration to Mn/Si. For these reasons, by setting the Si addition amount to 0.05 to 0.5%, 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 0.015% 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 the processing cost increases for both P removal and S removal, the content may be appropriately determined to be less than this value in consideration of economical tolerance. Next, when Al is deoxidized in the molten steel processing process for manufacturing 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 the present invention, by setting Cr to 0.1% or less, fine chromium carbide is formed and the amount of cementite is reduced, so the scale is even greater than that of a hot rolled steel sheet manufactured under the hot rolling conditions described below without adding Cr. This is because adhesion is improved. Figures 8 and 10 show the scale adhesion of hot-rolled steel sheets with Cr added.
As shown in the figure. The effect of this addition of Cr on improving adhesion is limited to 0.1%, and beyond this, economical efficiency is lost. 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%,
Scale adhesion, scale thickness,
The scale composition was investigated and the relationships are summarized and shown in Figures 1 and 2. Scale thickness ≦8μ, Fe ₂ O ₃ ≦3
%, Fe ₃ O ₄ ≧85%, and the scale adhesion rating Gr is 5 or less, passing the test. [Gr: The peeling status of the taping-type scale on the outer surface of a bent steel plate piece bent at 90° by pressing a punch with radius R = 1.5 x plate thickness is classified into 9 areas and ranked as a score]. However, even if all of the above conditions are not satisfied, if one condition is significantly large or small, the scale adhesion is passed. In the present invention, the rolling finishing temperature is set to (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, and to reduce the scale thickness. FIG. 3 shows the relationship between scale thickness and rolling finishing temperature. The upper solid line represents the upper limit, and the lower solid line represents the lower limit, so data exists between the two solid lines. As shown in FIG. 3, by setting the temperature to 840°C or less, the scale thickness can be reduced to 8μ or less. The reason why the winding temperature is set to 500℃ or less in the present invention is as shown in Figure 6, which shows the scale adhesion and finishing temperature.
The relationship between the winding temperature is shown below, and there is no sample that passes the scale adhesion test when the winding temperature exceeds 500°C. Furthermore, as shown in Figures 4 and 5, which have the same surface shape as Figure 3, if the winding temperature is 500℃ or less, it is possible to reduce the amount of Fe ₂ O ₃ produced to 3% or less and the scale thickness to 8μ or less. It is. Generally, the amount of scale produced (y) is y=K・√t (t: time), K=k・e ^-Q/RT (k: constant determined by atmospheric conditions, etc., Q: 33000 cal/mol, R: Gas constant, T:
temperature). 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. In addition, the reason why the winding temperature was set to 600°C or less in the above is that due to the effect of scale adhesion due to the addition of Cr, the scale thickness and the amount of Fe ₂ O ₃ produced that were specified in the case of no Cr addition were relaxed, and the allowable The upper limit of the winding temperature is 600℃. 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 proceeding from the steel substrate side and that the formed Fe ₃ O ₄ is compatible with the steel substrate. This is thought to be due to the improved scale adhesion. This phenomenon also depends on the winding temperature; above 500°C, transformation proceeds from the scale surface regardless of the cooling rate, resulting in poor scale adhesion. If the cooling rate is 40℃/s or more, the winding temperature is 500℃ or less. Even if the cooling rate is faster than 0.5℃/min after winding, or the winding temperature is 500℃ or less.
Even if it is cooled at a slow cooling rate of less than 0.5°C/min after winding at temperatures below 500°C and above 360°C, transformation proceeds from both the scale surface and the substrate interface, and the scale remains α throughout the entire surface.
It becomes Fe ₃ O ₄ containing -Fe, and has good scale adhesion. However, if the winding temperature is less than 360℃ and the subsequent cooling rate is less than 0.5℃/min, α-Fe will proceed only from the base steel side, but
Fe exists in the form of islands, and scale adhesion is poor, which is not preferable. 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 the main scale is used. This is because FeO is soft and FeO//α−
The orientation relationship of Fe (substrate) is (100) [011] _FeO //
(100) [001] = _-Fe is considered to have good consistency and good adhesion. To cool at a cooling rate of 0.5°C/min or higher after winding, place the product in a well-ventilated place in the yard, stop stacking, use natural ventilation or forced air cooling with a fan, use N ₂ in a box, Cooling means such as forced cooling using a fan using Ar gas, water spray cooling, immersion cooling, etc. or a combination thereof can be used. [Example] The composition of the steel used in the comparative example of the example of the present invention
Mn/Si is shown in Table 1. Steel A has a composition system based on Al-Si-K, and steel B has a composition system in which Cr is added to the composition of steel A. Table 2 shows the hot rolling conditions of Examples and Comparative Examples using the above steel. In Comparative Example 1, steel A was rolled at a rolling temperature of 870°C and the coil was wound at 550°C, and the scale adhesion in the longitudinal direction is shown in FIG. There was significant scale peeling on the inner and outer windings and ends of the coil, and the scale adhesion criterion (Gr≦5, hereinafter referred to as Gr≦5) was not satisfied. Comparative example 2 uses steel B with a rolling temperature of 870°C.
Figure 8 shows the scale adhesion in the longitudinal and width directions of a coil wound at 650°C. There is large peeling of scale on the inner and outer parts and parts of the ends of the coil.
Gr≦5 is not satisfied. In Example 1, steel B was rolled at a rolling temperature of 870°C, and the scale adhesion in the longitudinal and width directions of a coil wound at 550°C is shown in FIG. Gr≦5 was satisfied in the entire length and width directions of the coil, and the scale adhesion was improved compared to Comparative Examples 1 and 2, and there was an improvement effect by adding Cr and lowering the winding temperature. Comparative Example 3 uses steel A at a rolling temperature of
The coil was heated at 830℃ and wound at 450℃.
Figure 10 shows the scale adhesion in the width direction. Gr≦5 is satisfied in the entire length and width directions of the coil, and the scale adhesion is greatly improved compared to Comparative Example 1, and there is an improvement effect by low temperature rolling and low temperature winding.
Example 2 uses steel B with a rolling temperature of 830°C.
Figure 11 shows the scale adhesion in the longitudinal and width directions of a coil wound at 450°C. Gr≦5 is satisfied in the entire length and width directions of the coil, and the scale adhesion is further improved than in Example 1 and Comparative Example 3, and there is an improvement effect due to the addition of Cr, low-temperature rolling, and low-temperature winding. Table 3 below shows the composition and hot rolling conditions of the steel used in other Examples and Comparative Examples of the present invention, as well as the scale adhesion evaluation results. Comparative examples C-2, D-2, E-2, F-2 are
Satisfying Gr≦5, Comparative Examples C-1, D-1, E-1,
Scale adhesion is greatly improved compared to F-1, and there are improvement effects due to low-temperature rolling, water cooling started quickly after rolling, improved cooling rate, and low-temperature winding. Examples C-3, C-4, C-5, and D-4 are hot-rolled steel sheets manufactured based on Claim 1, and satisfy Gr≦5, and Comparative Examples C-1 and D-1. The scale adhesion was greatly improved compared to Comparative Example C-
2, D-2, E-2, and F-3, the scale adhesion was further improved, and the rolling temperature, cooling temperature, winding temperature, and Cr addition had the effect of improving. Comparative example D-3 satisfies Gr≦5, Comparative example D-
Scale adhesion is improved compared to Comparative Example D-
The scale adhesion was further improved than in Example 2, and in addition to the rolling temperature, cooling rate, and degree of winding, forced cooling after winding had an improvement effect. Example D-5 is a hot-rolled steel sheet manufactured based on claim 2, which satisfies Gr≦5, has greatly improved scale adhesion compared to Comparative Example D-1, and has a significantly improved scale adhesion compared to Comparative Example D-1. 2. The scale adhesion was further improved than D-3, and there were improvements in the rolling temperature, cooling rate, winding temperature, addition of Cr, and forced cooling after winding.

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[Table] * Score ≦5…Pass
>5... Reject [Effect of the invention] The present invention is a method for producing a hot rolled steel sheet with excellent scale adhesion without pickling after hot rolling, and the above-described structure prevents penetrator cracks and spots during flash butt welding. It is possible to reliably obtain a hot-rolled steel sheet at a low cost that does not cause internal nugget fracture during welding, has high strength suitable for soft processing, and has excellent scale adhesion. As a result, there is less dust generated by scale in subsequent processes, and scale adhesion is good over the entire length and width of the coil, further increasing yields and reducing costs when manufacturing automobile suspension parts and frames. This makes it possible to paint as needed, making it extremely economical.

[Brief explanation of drawings]

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. FIG. 7 shows the scale adhesion of Comparative Example 1 in the longitudinal and width directions of the coil. FIG. 8 shows the scale adhesion of Comparative Example 2 in the longitudinal and width directions of the coil. FIG. 9 shows the scale adhesion of Example 1 in the longitudinal and width directions of the coil. FIG. 10 shows the scale adhesion of Comparative Example 3 in the longitudinal and width directions of the coil. FIG. 11 shows the scale adhesion of Example 2 in the longitudinal and width directions of the coil.

Claims (1)

[Claims] 1 C0.03-0.25wt%, 0.04＜Si≦0.5wt%,
Mn0.2~2.0wt%, P≦0.025wt%, S≦0.015wt
%, Al≦0.08wt%, Cr≦0.1wt%, and the balance is Fe and unavoidable elements. After melting the steel, the steel slab obtained by continuous casting method or ingot forming method is loaded into a heating furnace. or reheat it after cooling it,
After hot rolling at a temperature below (Ar ₃ +50℃), 5
A method for producing a hot-rolled steel sheet with excellent scale adhesion, characterized by starting water injection cooling within seconds, rapidly cooling at a cooling rate of 40°C/second or more, and rolling at a temperature of 600°C or lower and 360°C or higher. 2 C0.03~0.25wt%, 0.04<Si≦0.5wt%,
Mn0.2~2.0wt%, P≦0.025wt%, S≦0.015wt
%, Al≦0.08wt%, Cr≦0.1wt%, and the balance is Fe and unavoidable elements. After melting the steel, the steel slab obtained by continuous casting method or ingot forming method is loaded into a heating furnace. or reheat it after cooling it,
After hot rolling at a temperature below (Ar ₃ +50℃), 5
Heat with excellent scale adhesion, characterized by starting water injection cooling within seconds, rapidly cooling at a cooling rate of 40°C/second or more, and rolling it up at a temperature below 360°C, followed by a cooling rate of 0.5°C/minute or more. Method of manufacturing rolled steel plate.