JPH0673504A - Thick steel plate for structural use having excellent scale adhesion and high young's modulus and its production - Google Patents

Abstract

PURPOSE:To provide a thick steel plate for structural use having excellent scale adhesion and high Young's modulus. CONSTITUTION:In the process for subjecting a cast slab of structural steel, where rolling is started at >=Ac3 deg.C, to rolling in two phase region at a temp. lower than the Ar3 point at >=50% draft, rolling is done while applying cooling at a cooling rate of >=2 deg.C/sec surface cooling rate under the condition where the relationship between cooling time and total rolling time satisfies (cooling time)/(total rolling time)<0.2 at 1200-Ar1 deg.C average steel plate temp. By this method, the thick steel plate for structural use, which has scale having <=10mum thickness and >=70% compositional ratio of Fe3O4 on the surface of steel plate and further has a structure having >=10 aspect ratio of texture colony and where the Young's modulus in the direction perpendicular to rolling direction is regulated to >=23000kgf/mm<2>, can stably be produced with superior productivity. This steel plate has excellent scale adhesion.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a structural thick steel plate having good scale adhesion and a high Young's modulus, and a method for producing the steel plate with high productivity.

[0002]

2. Description of the Related Art In recent years, in the field of industrial machines and the like, with the increase in size of machines, there is an increasing demand for steel sheets that are thin but have high rigidity from the viewpoint of weight reduction by thinning the steel materials used. Regarding the scale properties of steel sheets, the demand for tightness of the scale of the steel sheets used is strict because of problems such as uneven coating when directly painting with bent parts and prevention of indentation flaws due to scale peeled off during press forming. In addition, from the viewpoint of paintability and aesthetics of steel sheets, especially in the field of industrial machinery where there are many forming operations, it is extremely easy to peel from the aspect of the working environment,
There is also an increasing demand for steel sheets with less red scale, which tends to become powdery.

By the way, generally, the rigidity of a steel sheet is proportional to Young's modulus if the shape is constant. In conventional steels, the Young's modulus was considered to be constant at approximately 21,000 kgf / mm ² except for special cases such as single crystals and electromagnetic steel sheets, and therefore, it was not seen as a material property to be particularly noted. But,
In recent years, improvement in rigidity in a specific direction in use has been demanded, and application of Young's modulus in a direction perpendicular to the rolling direction (hereinafter, referred to as C direction) has been studied. According to this method, it is possible to increase the rigidity of the structure without increasing the plate thickness or changing the shape.

On the other hand, there are various proposals for high Young's modulus steels, all of which are intended to develop a rolling texture by rolling in a two-phase region or a ferrite region and improve the Young's modulus in a specific direction of the steel sheet. For example, Japanese Patent Publication Sho 58-
Japanese Patent Publication No. 14849 discloses a method for producing a high Young's modulus steel material. The manufacturing method disclosed here is a method of rolling a steel having a defined chemical composition in a two-phase region, controlling the cooling rate up to 300 ° C. after rolling finishing, and then tempering it at a temperature of 700 ° C. or less, thereby It is said that the Young's modulus can be increased by about 10%.

Further, Japanese Patent Publication No. 62-4448 discloses that
Steel with C less than 0.03% by weight, Ar ₃ or less 600
Specifies the rolling reduction in the temperature range of ℃ or more, 450 ℃ or more 7
A method for increasing Young's modulus in the C direction up to 24300 kgf / mm ² by winding at 20 ° C. or less is described. However, these methods have the following problems in practical use, and improvement is awaited. In the method described in JP-B-58-14849, α- which significantly accelerates the formation of a texture in order to improve Young's modulus.
Although the γ2 phase region large reduction rolling method is adopted, a significant waiting time occurs in the temperature decrease up to the 2 phase region, resulting in a marked decrease in productivity. Further, the method described in Japanese Patent Publication No. 62-4448 requires the compositional limitation of C ≦ 0.03% and is a manufacturing method for a steel sheet having a tensile strength of 30 kgf / mm ² or less. Tensile strength of target structural steel 40
It does not satisfy the strength of kgf / mm ² or more.

Further, as a method for improving the adhesion of the scale, in the field of hot-rolled wire rods, there is proposed a method for reducing the scale thickness as described in, for example, Iron and Steel, 65 (1979), S390. Has been done. As an example of applying a method for making a tight scale by reducing the scale thickness to a hot-rolled steel sheet, for example, JP-A-58-15
As described in Japanese Patent No. 7517, a method of covering the finish rolling mill and the water cooling device with a laminar water cooling device to shut off from the atmosphere, Japanese Patent Laid-Open No. Sho 6-62.
No. 0-24320, JP-A No. 60-77922, a method of cooling a low carbon aluminum killed steel after completion of rolling to a low temperature in a non-acidic atmosphere, JP-A No. 61-12.
A method of cooling to a low temperature in an inert gas or reducing gas atmosphere immediately after finish rolling as described in JP-A-3403, and Cr as disclosed in JP-A-61-195702.
It has been proposed to cool low-carbon aluminum killed steel to which is added immediately after rolling.

However, these methods are, in practice,
In order to realize it, equipment for shutting off the steel sheet passing at high speed from the atmosphere, or equipment for rapidly cooling the steel sheet passing at high speed to a low temperature immediately after rolling, etc. are required.
It has the drawback of enormous increase in equipment cost. As a method of improving these methods and making the scale thin and controlling the composition of the scale, the components of the main elements are defined as described in JP-A-64-83615, and only the cold speed after rolling is defined. A method is proposed, but the tensile strength is 30
The present invention relates to a manufacturing method for a steel plate of about kgf / mm ² , and the tensile strength of the structural steel of the present invention is 40 kg.
It does not satisfy the strength of f / mm ² or more. Further, the hot-rolled steel sheet produced in the continuous rolling step described above is in a very advantageous situation in reducing the scale because the high temperature residence time is significantly shorter than that of the thick steel sheet. There are few useful methods for scale control during production with relatively long rolling times in reverse rolling as used in rolling.

[0008]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a high-strength structural steel material having good scale adhesion and a high Young's modulus, and a method for producing the steel material with high productivity and economically. It is what

[0009]

The gist of the present invention is as follows. The steel sheet has a thickness of 10 μm or less, a Fe ₃ O ₄ composition ratio of 70% or more on a scale, and a texture colony aspect ratio of 10 or more. Young's modulus of 23000kgf / mm ²
The structural thick steel plate having good scale adhesion and high Young's modulus characterized by the above.

In the process of rolling the slab of structural steel that started rolling from Ac ₃ ° C. or higher at a rolling reduction of 50% or more at less than Ar ₃ points, the average temperature of the steel plate is 1200 ° C. to Ar.
The relationship between the cooling time and the total rolling time between ₁ ° C is the cooling time /
Total rolling time> 0.2, surface cooling rate is 2 ° C /
A method for producing a structural thick steel sheet having good scale adhesion and high Young's modulus, which is characterized by rolling while cooling at a cooling rate of at least 2 seconds. After completion of the rolling, the cooling rate of 5 ° C./second or higher is 65
A method for producing a structural thick steel plate having good scale adhesion and high Young's modulus, which is characterized by accelerated cooling to a temperature of 0 ° C. or less. A method for producing a structural thick steel sheet having good scale adhesion and high Young's modulus, which is characterized by performing accelerated cooling to a temperature of 650 ° C. or less at a cooling rate of 5 ° C./second or more within 10 seconds after the completion of rolling. . The method for producing a structural thick steel sheet having good scale adhesion and high Young's modulus, which is characterized by performing quenching and tempering treatment after completion of the rolling.

[0011]

The structural steel targeted by the present invention is described in, for example, Japanese Patent Publication No. 58-14849, and as described below,
The types and amounts of additive elements conventionally used in the art to obtain the required material in ordinary welded structural steel are also used. Therefore, the steel containing these is the subject steel of the present invention.

The reason for adding each element and the amount thereof are shown below. C is added as an effective component for improving the strength of the steel, but if it exceeds 0.20% in an excessive amount, island martensite is precipitated in HAZ (Heat Affected Zone) and the HAZ toughness is significantly deteriorated. Therefore, the content is regulated to 0.20% or less.

Si is necessary as a deoxidizing element for molten steel,
Although it is added as a strength-increasing element, if it is less than 0.01%, the deoxidizing effect is insufficient, and if it exceeds 1.0%, the workability of the steel decreases and the toughness of the HAZ decreases.
The addition amount is regulated to 0.01 to 1.0%.

Mn is also required as a deoxidizing component element,
If it is less than 0.3%, the cleanliness of the steel decreases and the workability is impaired. In addition, it is necessary to add 0.3% or more as a component for improving the strength of the wire. However, Mn significantly deteriorates the weldability due to excessive addition, so the upper limit is 2.0%.

Al and N are necessary because the grain size of the steel can be refined by Al nitride. However, if the addition amount is small, the effect is not obtained, and if it is excessive, the toughness of the steel is deteriorated. Therefore, the addition amount of Al is 0.001 to 0.20%.
And N inevitably contained is limited to 0.020% or less.

The basic components of steel targeted by the present invention are as described above. On the basis of this, when alloying elements are added according to the required properties for the purpose of increasing the strength of the base metal or improving the toughness of the joint, it becomes difficult to secure weldability if they are added too much. Therefore, as the alloy, Ni, Cr, M
o, Cu, W, P, Co, V, Nb, Ti, Zr, T
a, Hf, rare earth element, Y, Ca, Mg, Te, Se,
One or more kinds of B may be added, but the total amount added is 4.5
Regulate within%.

The lower limit temperature of the temperature range for cooling is also related to the rolling end temperature, and if the cooling is carried out at the Ar ₁ point or less, the rolling end temperature may not be able to secure a predetermined temperature, so it was set. When the surface of the steel sheet, which has the lowest temperature, falls below the Ar ₁ point temperature, the Young's modulus is improved, but the ferrite transformed from austenite is excessively processed and the toughness of the surface layer portion is extremely deteriorated. It is desirable that the temperature is not higher than the Ar ₁ point temperature.

In carrying out the present invention, when the above-mentioned temperature history of Ac ₃ points or more is applied, it is set to 1350 ° C. or lower in view of the balance of the rolling electric power unit and the heating fuel unit and the handling condition of the slab. desirable. Further, after the completion of rolling, when accelerated cooling is continued to a temperature of 650 ° C. or lower at a cooling rate of 5 ° C./sec or more and the time until the start of cooling after rolling is controlled, it is effective for suppressing the growth of scale. .

In order to solve the problems of the above-mentioned prior art, the inventors of the present invention investigated various experiments from the viewpoint of improving Young's modulus and scale adhesion by using a general structural steel having the following chemical components. Was repeated.

C: 0.05 to 0.15% S
i: 0.15 to 0.25% Mn: 0.8 to 1.6% Al: 0.01
~ 0.05% N: 0.0020-0.0050%

FIG. 1 shows the relationship between the Young's modulus by angle from the rolling direction (L direction) and the two-phase rolling reduction. As a result of this investigation, it was found that the Young's modulus in the C direction was improved by 10% or more by setting the rolling reduction of the two-phase region rolling to 50% or more.

Further, since the Young's modulus is due to the texture, a detailed investigation of the microstructure of the steel sheet having the improved Young's modulus revealed that it corresponds to the colony of the texture. The result is shown in FIG. still,
The texture colony is DE FERRI METALLO
GRAPHIA, I, CNRN. (1966) p. 97
It was measured by the heating coloring method described in. This method is a method in which the color of the oxide film when heated is different due to the different crystal orientation.

Next, for the steel sheet obtained in the experiment, the relationship between the thickness of the scale, the ratio of Fe ₃ O ₄ in the scale composition and the adhesion of the scale was investigated, and FIG. 3 was obtained. Adhesion tends to be improved when the thickness is 10 μm or less, but it is clear that even if the scale thickness is 10 μm or less, the adhesion is not improved unless the ratio of Fe ₃ O ₄ in the scale composition is 70% or more. became.

Further, the scale thickness and composition of the above steel sheet are summarized from the process conditions (cooling rate and cooling time / total rolling time when cooled between 1200 ° C. and Ar ₃ point during rolling), and FIG. 4 is obtained. It was From Figure 4, the cooling rate of the surface is 2 ℃ /
When the cooling time / total rolling time is 0.2 seconds or more and the thickness is 10 μm or less, Fe _{3 in the} composition is less than 10 μm.
It was learned that the ratio of O ₄ is 70% or more.

From the above, the cause of good adhesion of the scale is related to the thickness of the scale and the scale composition. In the present invention, both the scale thickness and the composition are controlled. The adhesion of the scale is related to the difference between the scale composition and the bonding force of the base steel.
It is known that e ₃ O ₄ has good adhesion to the base metal and the color of the scale is black.

Regarding the reason for forming the above scale, the following two points can be inferred from the viewpoint of thinning the scale and controlling the composition. Thinning scale It is speculated that the reduction of the high temperature residence time by cooling promotes the reduction of the absolute value of the thickness of the base metal that is oxidized, and the scale is thinned. Control of scale composition In the present invention, since the surface is extremely rapidly cooled during rolling,
FeO formed at high temperature is the eutectoid transformation point of FeO (4FeO
It is assumed that the time of exposure to 570 ° C. or lower, which is (Fe ₃ O ₄ + Fe), becomes long as a result, and a large amount of Fe ₃ O ₄ is produced.

[0027]

【Example】

Steel to be tested The steel components of the present invention may be any combination as long as they are elements and addition amounts of the above-mentioned general structural steel. The chemical components used in the examples are shown in Table 1. It is also a typical chemical composition with different strength levels in the field of structural steel.

[0028]

[Table 1]

Production conditions and material results Tables 2 and 3 show the production conditions and the obtained materials, scale thickness, composition and adhesion. The ratio of Fe ₂ O ₃ and the color of the scale are also shown. The relationship between scale color and composition is
It is known that Fe ₃ O ₄ has a black color, and the more the ratio of Fe ₂ O ₃ , the more it exhibits a red color.

[0030]

[Table 2]

[0031]

[Table 3]

The weld bead adhesion was evaluated by the following method. Welding method: CO ₂ arc welding, horizontal fillet welding, wire 1.6 mmφ Welding conditions: current 400 A, voltage 37 V, target angle 55
°, torch angle of 90 °, speed of 50 cpm were determined as shown in FIG.

The test steels shown in Table 1 are steel Nos. 1 and 40 of 40 kg class steel, steel Nos. 3 to 6 of 50 kg class steel, and steel No. 7 of 60 kg class steel. If necessary, V, Nb, N
Alloying elements such as i, Ti, Cu, Cr and Mo are added.

No. In each of the examples of the present invention of A1 to A11, structural steel sheets having excellent properties of material, Young's modulus, and scale were obtained. On the other hand, Comparative Example No. which did not perform sufficient cooling during rolling. At least one of Young's modulus, scale thickness, and composition of B1 to B9 did not satisfy the predetermined characteristics. In addition, the cutting property of the steel sheet, which has been recently required, by laser, and the weld bead adhesive property were also evaluated, and it was found that the steel of the present invention has extremely excellent properties.

[0035]

INDUSTRIAL APPLICABILITY The present invention makes it possible to smoothly and stably manufacture structural steel sheets having a small red scale under extremely high productivity, and to bring about economical advantages to the industrial field mainly in the field concerned. The effect is extremely large.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a rolling reduction and a Young's modulus in a two-phase region.

FIG. 2 is a diagram showing the relationship between Young's modulus and aspect ratio of a textured colony.

FIG. 3 is a diagram showing the relationship between the scale thickness, the ratio of Fe ₃ O ₄ in the scale composition, and the adhesion of the scale.

FIG. 4 is a diagram showing a relationship between cooling time / total rolling time for each cooling rate, scale thickness, and composition ratio.

5 (a) and 5 (b) are schematic diagrams of evaluation of weld bead adhesiveness.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Hiroshi Yoshikawa 1st Nishinosu, Oita-shi, Oita-shi Nippon Steel Co., Ltd. Oita Works (72) Yoshiki Kawashima 1st Nishinosu, Oita-shi Nippon Steel Co., Ltd. Oita Works (72) Inventor Toshiaki Haji 1 Nishinozu, Oita-shi, Oita-shi Nippon Steel Co., Ltd. Oita Works

Claims (5)

[Claims] 1. A rolled steel sheet having a structure having a thickness of 10 μm or less, a Fe ₃ O ₄ composition ratio of 70% or more, and a texture colony aspect ratio of 10 or more. Young's modulus at right angles to the direction is 230
Structural thick steel plate with good scale adhesion and high Young's modulus, which is characterized in that it is at least 00 kgf / mm ² . 2. A steel plate average temperature of 1200 ° C. to Ar in a step of rolling a structural steel slab that has been rolled from Ac ₃ ° C. or higher at a rolling reduction of 50% or higher at less than Ar ₃ points in a two-phase rolling process.
The relationship between the cooling time and the total rolling time between ₁ ° C is the cooling time /
Total rolling time> 0.2, surface cooling rate is 2 ° C /
A method for producing a structural thick steel sheet having good scale adhesion and high Young's modulus, which is characterized by rolling while cooling at a cooling rate of at least 2 seconds. 3. A structure having good scale adhesion and a high Young's modulus as set forth in claim 2, characterized in that after the rolling is completed, it is subsequently acceleratedly cooled to a temperature of 650 ° C. or lower at a cooling rate of 5 ° C./sec or higher. Manufacturing method of thick steel plate. 4. The scale adherence according to claim 2, wherein the scale adhesion is accelerated and cooled to a temperature of 650 ° C. or lower at a cooling rate of 5 ° C./sec or more within 10 seconds after the rolling is completed.
Manufacturing method of structural steel plate with high Young's modulus. 5. The method for manufacturing a structural thick steel sheet having good scale adhesion and a high Young's modulus according to claim 2, wherein quenching and tempering are performed after rolling is completed.