JP5382203B2 - Steel for thermal cutting using oxygen - Google Patents

Description

  The present invention relates to a steel material for thermal cutting using oxygen, and more particularly to a steel material for thermal cutting using oxygen that can be cut at high speed with a laser, plasma, or gas. In particular, the present invention relates to a steel material on which a surface mill scale (black skin) generated during steel material rolling is formed.

  Due to the remarkable technological progress of the laser cutting machine in recent years, the laser cutting machine can be applied not only to a thin plate having a thickness of 3.2 mm or less but also to cutting a thick plate having a thickness of up to 30 mm. Therefore, laser cutting machines are widely used for cutting thick plates. The laser cutting machine has less safety problems than the gas cutting machine, and is suitable for use in unattended operation at night. To perform unattended operation at night with a laser cutting machine, work using large steel materials (hereinafter, steel materials may be described as “steel plates”) may be used to ensure sufficient work. It is desirable.

  However, in an unattended operation at night when there are no workers to handle the cutting trouble, the trouble will be neglected, and cutting will stop, or defective products with broken cutting surfaces will be mass-produced. Problems can arise. In particular, since the disturbance of the cut surface is a factor that impairs the product value of the product, it is necessary to rework the cut surface, resulting in an increase in cost. In addition, products with significantly disturbed cut surfaces may have to be scrapped as defective.

  The above-mentioned cutting trouble is roughly classified into two types, one having a cause on the cutting machine side and the other having a cause on the steel plate side.

  There are various factors that can cause trouble on the cutting machine side. One example is the change in laser power due to voltage fluctuations at night and the vibration of the cutting machine. Various ideas have been made to eliminate such trouble factors, but it is impossible to completely eliminate them.

  Therefore, in recent years, there has been a demand for a high-quality steel plate that can be cut stably even if some problems such as fluctuations in laser output and vibration occur on the cutting machine side.

  On the other hand, the technological progress of plasma cutting has been remarkable, and the cutting surface quality and cutting allowance have been rapidly improved. Plasma cutting has a high cutting speed and can cut a steel plate thicker than laser cutting. Furthermore, the plasma cutting machine has an advantage that maintenance is easier than a laser cutting machine. However, the plasma cutting machine has a drawback that the life of the consumable is short. In particular, cutting thick steel plates with high-power plasma shortens the life of consumables. Therefore, there is a great need for a technique for stably cutting a steel plate at a high speed at a low output.

  Until now, it has been pointed out that the surface properties of steel sheets greatly affect laser cutting properties. In particular, it has been considered that improving the adhesion of the scale on the surface of the steel sheet is effective in improving the laser cutting property.

As a method for improving the scale adhesion, Patent Document 1 discloses a steel plate excellent in laser cutting property in which a scale mainly composed of Fe ₃ O ₄ (magnetite) is formed and a method for producing the same.

  Patent Document 2 discloses a scale characterized in that after rolling is completed at a temperature of 850 to 720 ° C., water is sprayed on the front and back surfaces of the steel plate to cool the steel plate to a temperature of 600 to 700 ° C., and then air-cooled. A method for producing a steel sheet having excellent adhesion and laser cutting properties has been proposed.

  As another method for improving the scale adhesion, a method of adding an alloy element such as Cu or Ni to the steel sheet is also disclosed. Patent Document 3 discloses a steel sheet for laser cutting, characterized in that the surface roughness of the scale is 3.0 μm or less in terms of centerline average roughness (Ra) and Cu + Ni + Cr is contained in an amount of 0.3% by mass or more. Is disclosed.

In addition to Cu and Ni, Al is oxidized during rolling to form an Al ₂ O ₃ -containing layer at the interface between the base iron and the scale, thereby improving the adhesion of the scale. A thick steel plate excellent in laser cutting property that satisfies ≦ Al + Cu + Ni ≦ 2.0 mass% and a method for producing the same are disclosed.

  From the idea that Cr, Al, Cu and Ni only need to be concentrated at the interface to improve the adhesion of the scale, Patent Document 5 describes that Cr, Al, A thick steel sheet having a concentrated layer in which one or more of Cu and Ni are concentrated and having a thickness of 1.0 μm or more and excellent in laser cutting properties is disclosed.

JP 2003-221640 A JP-A-8-218119 JP-A-8-3692 Japanese Patent Laid-Open No. 11-323478 Japanese Patent Laid-Open No. 11-343541

  As in the method of Patent Document 1, it is difficult to stably perform laser cutting because scale adhesion is insufficient only by using a scale as a main component of magnetite. Moreover, it is difficult to perform the process for magnetizing the scale in the manufacturing process of the steel sheet.

  In the invention described in Patent Document 2, a thin scale is formed by high-pressure water descaling and low-temperature rolling, and the scale composition is controlled by cooling. However, this method makes the scale too thin. In laser cutting of thick steel plates, oxygen is used as an assist gas. The upper part of the steel plate where the laser beam is concentrated is cut by the laser, and the lower part of the steel plate is cut by the oxidation reaction heat of the steel plate. The scale on the surface of the steel plate suppresses the oxidation reaction at the top of the steel plate. Without the scale, an oxidation reaction starts on the surface of the steel sheet, melting proceeds in the surface direction, not in the cross-sectional direction, and causes cutting defects such as cutting notches and gouging. Therefore, if there is a portion where the scale is cracked or peeled off, a cutting notch or gouging occurs at that portion. If the scale becomes too thin, even if the adhesion is good, the effect of suppressing abnormal oxidation due to the scale is lost, so the laser cutting performance improvement effect is small.

  As in the method of Patent Document 3, it is not easy to reliably manage the surface roughness of the scale so that Ra is 3.0 μm or less, and it is for stable laser cutting in long-time unattended operation at night. Further improvement is desired to solve advanced problems.

  As in the methods of Patent Documents 4 and 5, the adhesion of the scale is improved by adding various elements, but there is no provision for the scale itself that greatly affects the laser cutting ability, and stable laser cutting ability. Is not guaranteed.

  As shown in the above patent documents, in order to improve laser cutting performance, conventionally, emphasis has been placed on improving scale adhesion, and complicated heat treatment or addition of expensive alloy elements has been attempted. It was. In the present invention, by taking the chemical composition of the steel material within an appropriate range without considering the scale adhesion, cutting troubles such as cutting interruption when using a high-power laser cutting machine can be generated at any plate thickness. An object of the present invention is to provide a steel material that can be prevented as much as possible and that can be cut at high speed using a plasma cutting machine and a gas cutting machine.

  In order to solve the above-mentioned problems, the present inventors conducted various investigations on the behavior of steel materials in the thermal cutting of steel materials using oxygen such as high-power laser cutting, plasma cutting, and gas cutting. As a result, first, the following findings (a) and (b) were obtained.

  (A) Cutting failure during cutting using a high-power laser cutting machine, especially the major factor causing cutting interruption, is the position of the steel relative to the focal point of the laser due to swells or scratches (including scale) of the steel. It is a so-called “defocus” that fluctuates. Therefore, when the steel material is insensitive to laser defocus, in other words, when the allowable variation range that can be cut from the laser focus position with respect to the steel material is large, the cutting speed can be increased and the thickness can be increased. Even in the case of a large steel plate, cutting becomes possible. In addition, stable cutting is possible regardless of the scale condition.

(B) The eutectic point of Fe ₂ SiO ₄ (firelite) generated by oxidation of steel at the time of cutting, that is, the melting point of the oxide, is closely related to the cutting property of the steel material. Then, by lowering the eutectic point of Fe ₂ SiO ₄ , Fe (or iron oxide) melted at the time of cutting is easily discharged from the cut part, so that the laser cutting property is improved. The decrease in the eutectic point of Fe ₂ SiO ₄ facilitates the discharge of molten iron to the upper part of the cutting part (the side to which heat enters), so there are problems due to abnormal combustion that can occur when cutting at high speed with high heat input The so-called “burning” phenomenon can be suppressed.

Then, next, the technique for lowering the eutectic point of Fe ₂ SiO ₄ was examined, and as a result, the following findings (c) to (k) were obtained.

(C) As an element that lowers the eutectic point of Fe ₂ SiO ₄ , the effect of P is particularly great. P is effective in lowering the eutectic point by being contained in Fe ₂ SiO ₄ .

  (D) The mechanical properties and weldability of the steel material can be deteriorated simply by containing the above P in the steel material. However, if the P content is optimized by the ratio to the Si content in the steel material, the cutting performance of the steel material is significantly improved without adversely affecting the performance of the steel material, so that the cutting speed can be increased.

  (E) When cutting heat input is increased in order to increase the laser cutting speed, a burning phenomenon is likely to occur. However, in the cutting of a steel material appropriately containing P as described in (d), the burning phenomenon is unlikely to occur. Therefore, it is possible to cut at an extremely high speed. In order to increase the laser cutting speed, it is necessary that the duty of the laser pulse (output time% per pulse oscillation time) exceeds 65%. In the steel material of the present invention in which P / Si is controlled, it is easy to discharge the surface melt by the kinetic energy of oxygen gas. Therefore, burning does not occur even in a situation where the duty is increased, and the applied heat is efficiently cut. Used for.

  (F) In the cutting of the steel material in which the P content is optimized in the ratio to the Si content described above, the allowable variation range that can be cut from the laser focus position is increased, and the focus shift in the cutting process is prevented. Since it becomes insensitive, the cutting failure or cutting interruption resulting from the position fluctuation | variation of steel materials can be prevented. This is partly because the surface melt can be easily discharged as described above.

  (G) Molten iron is easily discharged at the time of cutting, making it difficult for heat to be transferred to the steel material at the time of cutting, thereby suppressing an increase in the temperature of the steel material, cutting displacement due to thermal deformation, and fine processing at the corner (10 seconds) It is possible to shorten the time of cutting) while cooling the steel material by performing a degree of piercing.

(H) In plasma cutting using oxygen, the eutectic point of Fe ₂ SiO ₄ influences the cutting performance basically as in laser cutting. The influence is remarkable in the cutting of a thick steel plate (for example, 20 mm or more) that requires oxygen plasma cutting (cutting by an oxidation reaction with an assist gas) that cannot be cut only by the plasma output. Further, if it is necessary to increase the heat input for cutting in order to increase the plasma cutting speed, the cutting surface quality is lowered and the life of the consumables is shortened. However, the content of P in the ratio to the Si content in (d) above. In the cutting of a steel material that is suitable, since heat input for obtaining the same cutting speed can be suppressed, the quality of the cut surface is not deteriorated and the life of the consumables is extended.

  (I) In plasma cutting, the reason is not clear, but the addition of Ti, V, etc., which lowers the eutectic temperature, conversely decreases the cutting speed, and the cutting surface has a wave shape. There is a case where a so-called “ripple” phenomenon is caused. This is presumed to be due to the different influence ranges of laser and plasma from the steel surface at the time of cutting and the mechanism of molten iron discharge.

  (J) In order to apply steel materials having the same composition to thermal cutting using any oxygen such as gas cutting, laser cutting, and plasma cutting, it was found that the composition of the alloy elements must be adjusted to an appropriate range. .

  (K) With respect to the scale, it is possible to ensure a stable cutting property without considering the scale adhesion that has been studied conventionally. By adjusting the elemental composition of the steel material, a Si concentrated region where Si is concentrated and an Al concentrated region having a high Al / Si ratio are formed in the scale. By setting the Si in the Si-concentrated region to a certain content or more and the Al / Si ratio in the Al-concentrated region located outside the Si-concentrated region to a certain value or more, stable cutting properties are achieved. I found it.

This invention is completed based on said knowledge, and makes a summary the steel material for thermal cutting using the oxygen shown to following (1)-( 7 ).

( 1 ) By mass%, C: 0.02 to 0.20%, Si: 0.06 to 0.20%, Mn: 0.20 to 1.60%, P: 0.015 to 0.033% , S: 0.015% or less, Al: 0.03-0.08 %, Cu: 0.01% or more and less than 0.5%, Ni: 0.01-0.5% and N: 0.009% include the following, with P / Si ≧ 0.12, and satisfies Al / Si ≦ 0.60, have a chemical composition the balance being Fe and impurities, a steel having a scale on the surface,
In the scale, there is a layer of Si-concentrated regions in which Si is 0.4% or more, and an Al-concentrated region in which the Al / Si ratio is 0.3 or more exists in layers on the surface layer side of the Si-concentrated region. thermal cutting steel material with oxygen you characterized.

( 2 ) The chemical composition of the steel material is characterized in that, instead of a part of Fe, by mass%, Ti or 0.05% or less and V: 0.05% or less are contained. The steel material for thermal cutting using the oxygen as described in said ( 1 ).

( 3 ) The oxygen as described in ( 1 ) or ( 2 ) above, wherein the chemical composition of the steel material contains, in place of a part of Fe, mass% and further Cr: 2.0% or less. Steel material for thermal cutting using

( 4 ) The chemical composition of the steel material is selected from mass%, Mo: 0.5% or less, W: 0.4% or less, and Nb: 0.04% or less, instead of part of Fe. The steel material for thermal cutting using oxygen according to any one of ( 1 ) to ( 3 ), comprising at least one kind.

( 5 ) Any one of ( 1 ) to ( 4 ) above, wherein the chemical composition of the steel material contains, in place of a part of Fe, mass% and further B: 0.003% or less Steel material for thermal cutting using oxygen described in 1.

( 6 ) The chemical composition of the steel material is selected from mass%, Ca: 0.005% or less, Mg: 0.005% or less, and REM: 0.005% or less, instead of part of Fe. The steel material for thermal cutting using oxygen according to any one of ( 1 ) to ( 5 ), comprising at least one kind.

( 7 ) The chemical composition of the steel material is mass% instead of a part of Fe, further contains Sn: 0.50% or less, Cu: less than 0.1%, and the Cu / Sn ratio is 1. The steel material for thermal cutting using oxygen according to any one of ( 1 ) to ( 6 ), which is 0.0 or less.

  The steel material of the present invention is a heat-cutting steel material using oxygen as shown above. It is a steel material for cutting using oxygen as an assist gas, in other words, an oxidation reaction, and can be used for laser cutting, plasma cutting, gas cutting, and the like.

  In the case of laser cutting, the steel material of the present invention is insensitive to laser defocus due to swells or scratches, so that the allowable range of variation that can be cut from the laser focus position increases, and the cutting heat input It has extremely good laser cutting property that the burning phenomenon hardly occurs even when the height is increased. Therefore, if the steel material of the present invention is used, it is possible to cut at a very high speed using a high-power laser cutting machine, and it is possible to cut a complicated shape without defects without performing scale control by special heat treatment. Is possible.

  Further, since thermal deformation at the time of cutting is also suppressed, a “bridge” for ensuring cutting dimensional accuracy due to distortion can be minimized. That is, since it is possible to ultimately shorten the time for the person to gas-cut the bridge portion, productivity can be improved. Further, in plasma cutting, it is possible to cut at a high speed by suppressing the wavy phenomenon of the cut surface.

Cutting shape in an embodiment of the present invention Photograph showing burning phenomenon Cross-sectional photograph showing the wavy phenomenon

  Hereinafter, each requirement of the present invention will be described in detail. In addition, “%” of the content of each element means “mass%”.

(A) About the basic chemical composition of steel:
Si: 0.06-0.20%
Si is an element that oxidizes during thermal cutting using oxygen to form Fe ₂ SiO ₄ in the scale, and largely governs thermal cutting properties using oxygen.

The Si content greatly affects the amount of Fe ₂ SiO ₄ formed. That is, the eutectic temperature of Fe ₂ SiO ₄ is 1173 ° C., which is much lower than the melting point of FeO (1369 ° C.). Therefore, if Fe ₂ SiO ₄ is present in the iron oxide generated during thermal cutting using oxygen, the liquid phase is maintained up to 1173 ° C., so that the molten Fe or iron oxide is discharged in a low temperature range. Becomes easier and the cutting property is improved. If the Si content in the base material is less than 0.06%, the amount of Fe ₂ SiO ₄ produced is reduced and it becomes difficult to discharge the molten Fe or iron oxide. The phenomenon called “Egure” occurs. Further, as will be described later, the Si content is closely related to the presence of Fe ₂ SiO ₄ in the scale, so that the Si content in the Si concentrated region of the scale is 0.4% or more. It is necessary to make the Si content in the steel material 0.06% or more.

On the other hand, if the content of Si exceeds 0.20%, a sufficient amount of Fe ₂ SiO ₄ is generated, but since it is necessary to contain a large amount of P described later, mechanical properties and weldability, etc. The properties of the steel material will deteriorate.

  Accordingly, the Si content is 0.06 to 0.20%. The upper limit with preferable Si content is 0.15%. The preferred lower limit is 0.08%.

P: 0.010 to 0.033%, P / Si ≧ 0.12
P is an element that is most likely to be contained in Fe ₂ SiO ₄ at the time of cutting and effectively lowers the eutectic point, and makes it easy to discharge molten Fe or iron oxide in a low temperature range, and thermal cutting using oxygen Has the effect of enhancing the sex. Furthermore, since the discharge of iron oxide is facilitated, an increase in the steel sheet temperature during cutting is suppressed, and thermal distortion during cutting is suppressed. For this reason, the content of P becomes an important factor like Si.

P needs to be contained in a sufficient amount with respect to Si. When the P / Si ratio is less than 0.12, the effect of lowering the eutectic point of Fe ₂ SiO ₄ is not sufficiently exhibited, so the P / Si ratio needs to be 0.12 or more. The P / Si ratio is preferably 0.15 or more. However, when the P content is less than 0.010%, the Si content has to be reduced to make the P / Si ratio 0.15 or more, and the Fe ₂ SiO ₄ amount is reduced. For this reason, the stability of laser cutting is reduced, and the limit cutting speed is reduced in plasma cutting. On the other hand, the cutting performance improves with an increase in the P content. However, if it exceeds 0.033%, the improvement shows a saturation tendency, and the properties of the steel material such as mechanical properties and weldability deteriorate. Therefore, in order to obtain excellent cutting properties, the P content needs to be 0.010 to 0.033%.

Al: more than 0.02% and 0.08% or less, Al / Si ≦ 0.60
Since Al has a deoxidizing action, it needs to be contained. Further, Al, like P, is easily contained in Fe ₂ SiO ₄ present in the iron oxide generated during thermal cutting using oxygen, and has the effect of lowering the eutectic temperature. Therefore, the molten Fe or iron oxide can be easily discharged in a low temperature range, and the thermal cutting property (laser cutting property) is improved. Furthermore, in order to set the Al / Si ratio in the Al-concentrated region in the scale described later to 0.3 or more, Al needs to be contained in an amount exceeding 0.02%. On the other hand, if Al exceeds 0.08%, hard island martensite is generated in the welded portion, and the toughness deteriorates. Therefore, the Al content is more than 0.02% and not more than 0.08%. The upper limit with preferable Al content is 0.05%. A preferred lower limit is 0.03%.

When Al is excessively contained in the steel material with respect to Si, Al ₂ O ₃ (alumina) having a high melting point is generated in addition to Fe ₂ SiO ₄ at the time of thermal cutting using oxygen, and the cutting performance deteriorates. For this reason, the content of Al is set to Al / Si ≦ 0.60 in relation to the content of Si.

(B) About suitable chemical composition of steel:
In addition to having the chemical composition described in the above item (A), the steel material for thermal cutting using oxygen according to the present invention preferably contains the following amount of elements.

C: 0.02 to 0.20%
Since C is an element that increases the strength, it is preferable to contain 0.02% or more. However, if it exceeds 0.20%, the toughness of the steel sheet may be deteriorated. For this reason, the C content is preferably 0.02 to 0.20%. C is an inexpensive element, and since the effect of improving the cutting property by reaction heat between O (oxygen) and C in steel can be expected at the time of thermal cutting using oxygen, the lower limit of the C content is 0.05%. More preferably.

Mn: 0.20 to 1.60%
Since Mn is an element effective for ensuring the strength of the steel material, it is preferably contained in an amount of 0.20% or more. However, when the content of Mn exceeds 1.60%, deterioration of toughness and thermal cutting ability using oxygen may be caused. Therefore, the Mn content is preferably 0.20 to 1.60%. The lower limit of the Mn content is more preferably 0.30%. The upper limit of the Mn content is more preferably 1.30%.

S: 0.015% or less S is present in steel as an impurity and has little influence on the thermal cutting property using oxygen, but if its content is large, mechanical properties such as toughness of the steel material May adversely affect properties. Therefore, the S content is preferably suppressed to a certain amount or less, and is preferably 0.015% or less. The S content is more preferably 0.010% or less.

Cu: 0.01% or more and less than 0.5% Cu is an element that improves corrosion resistance. In order to acquire this effect, it is necessary to make it contain 0.01% or more. The Cu content is more preferably 0.02% or more. On the other hand, when the content is large, there is a concern of causing Cu checking. For this reason, it is good to suppress Cu content to less than a fixed amount, it is preferable to set it as less than 0.5%, and it is more preferable to set it as 0.4% or less.

Ni: 0.01 to 0.5%
Ni is also an element that improves the corrosion resistance. In order to acquire this effect, it is necessary to make it contain 0.01% or more. The Ni content is more preferably 0.02% or more. On the other hand, when the Ni content is large, there is a risk of adversely affecting the quality of the slab. Therefore, the content of Ni is preferably suppressed to a certain amount or less, preferably 0.5% or less, and more preferably 0.4% or less.

N: 0.009% or less N is present in steel as an impurity, and when its content is large, the weldability and the quality of the slab may be adversely affected. For this reason, the N content is preferably suppressed to a certain amount or less, and is preferably 0.009% or less.

  For the above reasons, the steel material for thermal cutting using oxygen according to the present invention is mass%, C: 0.02 to 0.20%, Si: 0.06 to 0.20%, Mn: 0.20. To 1.60%, P: 0.010 to 0.033%, S: 0.015% or less, Al: more than 0.02% to 0.08% or less, Cu: 0.01% to 0.5% %, Ni: 0.01 to 0.5%, and N: 0.009% or less, and the balance has a steel composition consisting of Fe and impurities.

  In addition, an impurity means the component mixed by various factors of raw materials such as ores and scraps and manufacturing processes, and is allowed within a range that does not adversely affect the present invention.

The steel material for thermal cutting using oxygen according to the present invention may further contain one or more selected from the elements listed in the following (a) to (f) as necessary.
(B) One or more of Ti: 0.05% or less and V: 0.05% or less (b) Cr: 2.0% or less (c) Mo: 0.4% or less, W: 0.4 % Or less and Nb: one or more of 0.04% or less (d) B: 0.003% or less (e) Ca: 0.005% or less, Mg: 0.005% or less, and REM: 0.005 % Or less of (%) or less (f) Sn: 0.50% or less

  Hereinafter, the above elements will be described.

One or more of Ti: 0.05% or less and V: 0.05% or less Ti and V are both elements that lower the eutectic point of Fe ₂ SiO ₄ , and are formed of molten Fe or iron oxide. It has the effect of facilitating discharge in a low temperature range and enhancing the thermal cutting property using oxygen. For this reason, in cutting using oxygen, particularly laser cutting, performance is improved. However, with regard to plasma cutting, since the critical cutting speed may be reduced, Ti is preferably 0.05% and V is preferably 0.05% as the upper limit, and 0.04% as the upper limit. More preferably. In order to obtain the above effects, the Ti and V contents are each preferably 0.005% or more, and more preferably 0.01% or more.

Cr: 2.0% or less Cr has the effect of increasing the strength of the steel sheet. However, when the content exceeds 2.0%, Cr oxide having a high melting point is formed to deteriorate the flowability of the molten metal. In the case of deterioration of the roughness of the thermal cutting surface using oxygen and laser cutting. May lead to cut notch formation. Therefore, the Cr content in the case of inclusion is 2.0% or less. The Cr content is more preferably 1.5% or less. In order to obtain the above effect, the Cr content is preferably 0.02% or more, and more preferably 0.03% or more.

One or more of Mo: 0.5% or less, W: 0.4% or less, and Nb: 0.04% or less Mo, W, and Nb have the effect of increasing the strength. You may contain said element. This will be described in detail below.

  Mo has the effect | action which raises the intensity | strength of a steel plate by solid solution strengthening. However, if Mo is contained in a large amount exceeding 0.5%, it is disadvantageous in terms of cost, and weldability may be impaired. Therefore, the Mo content in the case of inclusion is 0.5% or less. The Mo content is more preferably 0.4% or less. In order to acquire said effect, it is preferable to contain Mo 0.1% or more, and it is more preferable to contain 0.15% or more.

  W also has the effect of increasing the strength of the steel sheet by solid solution strengthening. However, if W is contained in a large amount exceeding 0.4%, it is disadvantageous in terms of cost and weldability may be impaired. Therefore, if W is included, the W content is 0.4% or less. The W content is more preferably 0.3% or less. In order to acquire said effect, it is preferable to contain W 0.05% or more, and it is more preferable to contain 0.08% or more.

  Nb has the effect | action which raises the intensity | strength of a steel plate by precipitation strengthening. However, containing a large amount of Nb exceeding 0.04% is disadvantageous in terms of cost and may deteriorate the toughness of the welded portion. Accordingly, the Nb content in the case of inclusion is 0.04% or less. The Nb content is more preferably 0.03% or less. In order to obtain the above effect, the Nb content is preferably 0.005% or more, and more preferably 0.01% or more.

B: 0.0030% or less B has an effect of improving hardenability. However, when B is contained in excess of 0.0030%, the weldability may be deteriorated. Therefore, when B is included, the content of B is set to 0.0030% or less. The B content is more preferably 0.0020% or less. In order to obtain the above effect, B is preferably contained in an amount of 0.0005% or more, and more preferably 0.0008% or more.

One or more of Ca: 0.005% or less, Mg: 0.005% or less, and REM: 0.005% or less Ca, Mg, and REM are in the heat affected zone (hereinafter referred to as “HAZ”). Since it has the effect | action which improves toughness, in order to acquire this effect, you may contain said element. This will be described in detail below.

  Ca has the effect | action which improves HAZ toughness. However, if the Ca content exceeds 0.005%, the thermal cutting property using oxygen may be impaired. Therefore, when Ca is contained, the content of Ca is set to 0.005% or less. The Ca content is more preferably 0.004% or less. In order to acquire said effect, it is preferable to contain Ca 0.001% or more, and it is more preferable to contain 0.002% or more.

  Mg has the effect | action which improves HAZ toughness. However, if the Mg content exceeds 0.005%, the thermal cutting property using oxygen may be impaired. Therefore, when Mg is contained, the content of Mg is set to 0.005% or less. The Mg content is more preferably 0.004% or less. In order to acquire said effect, it is preferable to contain Mg 0.001% or more, and it is more preferable to contain 0.002% or more.

  REM has the effect | action which improves HAZ toughness. However, if the content of REM exceeds 0.005%, the thermal cutting property using oxygen may be impaired. Therefore, the content of REM in the case of inclusion is 0.005% or less. The REM content is more preferably 0.004% or less. In order to acquire said effect, it is preferable to contain REM 0.001% or more, and it is more preferable to contain 0.002% or more.

  Note that REM is a generic name for a total of 17 elements of Sc, Y, and lanthanoid, and the content of REM means the total amount of the above elements.

Sn dissolves as Sn ²⁺ and has an action of inhibiting corrosion by an inhibitor action in an acidic chloride solution. Further, rapidly to reduce the Fe ^3+, by having an effect of reducing Fe ³⁺ concentration as oxidizing agent, since inhibit corrosion promoting effect of Fe ^3+, thereby improving the weather resistance in high airborne salt environments. Moreover, Sn has the effect | action which suppresses the anodic dissolution reaction of steel and improves corrosion resistance. These effects are saturated when the Sn content exceeds 0.50%. Therefore, the Sn content when contained is 0.50% or less. The Sn content is preferably 0.30% or less. In order to acquire said effect, it is preferable to contain Sn 0.03% or more, and it is more preferable to contain 0.05% or more.

  In the case of containing Sn, the Cu content is less than 0.1% and the Cu / Sn ratio is 1.0 or less. If the Cu content is 0.1% or more, or the Cu / Sn ratio exceeds 1.0, the corrosion resistance may decrease due to the Cu content, and further, the cause of rolling cracks when manufacturing the steel sheet It becomes. The upper limit of the Cu content when Sn is contained is preferably 0.09%.

(C) About the scale:
The scale is an oxide layer formed by oxidizing the steel surface during the manufacture of the steel material. The composition of the scale is not uniform over the entire area of the cross section. In the present invention, the cutting property is stabilized by forming the Si concentrated region and the Al concentrated region in a scale in the scale. In addition, the Si-concentrated region and the Al-concentrated region do not need to be formed as a complete layer on the entire surface of the steel sheet, and even if there is a region that does not satisfy the requirements of the present invention in part of the layer, the cutting performance is greatly affected. Never give.

The Si enriched region is formed by the concentration of Si in the vicinity of the scale / steel interface, and is a region relatively rich in Fe ₂ SiO ₄ . In the Si concentration region, it is necessary to concentrate 0.4% or more of Si by mass%. By forming such a concentrated region, it is considered that cutting defects such as burning are suppressed as a low melting point component at the initial stage of cutting.

  Further, an Al concentrated region is formed in a layered manner on the surface layer side from the Si concentrated region. By setting the Al / Si ratio in the Al-concentrated region to 0.3 or more, discharge of molten oxide at the time of cutting becomes easy.

  The Al-enriched region is also formed relatively near the scale / steel interface. Each concentrated layer exists in the order of the scale / steel interface, the Si concentrated region, and the Al concentrated region from the steel material side. Such a configuration can reliably prevent cutting defects such as burning.

The quantitative analysis of each element in the scale can be easily measured by a glow discharge luminescence surface analyzer, and the presence of Fe ₂ SiO ₄ can be confirmed by a technique such as oblique angle X-ray diffraction.

(D) Other:
The thickness of the steel material for thermal cutting using oxygen of the present invention is not particularly limited. Naturally, thin-walled materials can be thermally cut using oxygen, but even those having a thickness of 12 mm or more can be sufficiently heat-cut using oxygen. However, in the case of laser cutting, although depending on the output characteristics, the upper limit of the steel material thickness is preferably 30 mm. In the case of plasma cutting, the upper limit of the steel material thickness is preferably 100 mm, although it depends on the output characteristics.

  Further, the thickness of the scale is not particularly limited, but it is a scale generated in a normal steel material manufacturing process, and a steel material having a scale of 3 to 100 μm on the surface has a remarkable effect of suppressing cutting defects. When the scale exceeds 100 μm, the unevenness of the surface scale tends to become severe, which is not preferable.

  Even if iron rust components (FeOOH: iron oxyhydroxide) other than the so-called rolling scale are mixed on the surface, there is no serious problem, but the mixing rate should be 20% or less by mass%.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples. Needless to say, both laser and plasma cutting are controlled by oxide control, and the steel of the present invention can also be applied to gas cutting.

  Steel No. 1 having the chemical composition shown in Table 1. The slabs 1 to 22 were heated to 1120 ° C., the initial scale was removed with a descaler, and then rolled to a plate thickness of 16 mm, 25 mm, and 32 mm at a finishing temperature of 750 to 900 ° C. to obtain a type 3 steel material. Then, after waiting for the growth of the scale, the flatness was corrected with a leveler after water cooling. Scale remained on the surface of the manufactured steel.

  In Table 1, the steel No. 1 to 18 are steels according to examples of the present invention whose chemical composition is within the range defined by the present invention. On the other hand, Steel No. 19-22 are steels of comparative examples that deviate from the conditions defined in the present invention. Among these, steel No. About the steel material of 22, since the crack generate | occur | produced, the measurement shown below was not performed.

  Using the Marcus-type high-frequency glow discharge luminescence surface analyzer GD-Profiler2 manufactured by HORIBA, the generated scale was measured for the element profile from the surface at a discharge area: 4 mmφ, RF output: 35 W, argon pressure: 600 Pa, Fe in the scale, Main components such as O, Al, Si, and P were analyzed. The scale thickness was determined from the O profile. Based on these data, the Si concentrated area and the Al concentrated area in the scale were confirmed, and the Si content in the Si concentrated area and the Al / Si ratio in the Al concentrated area were calculated.

In addition, the steel sample with the scale formed was subjected to an oblique angle X-ray diffraction experiment (incident angle of 5 °, CoKα ray) to confirm the presence or absence of the Fe ₂ SiO ₄ component. Although it is a small peak, steel no. On the scales of 1 to 20 steel materials, a peak of Fe ₂ SiO ₄ was observed in the vicinity of 55 ° to 56 ° (JCPDS 34-0178, d = 1.922A). On the other hand, Steel No. No Fe ₂ SiO ₄ peak was observed on the scale of 21 steel.

Laser cutting machine used for cutting is a CO ₂ laser output 6kW made Koikesansokogyo Corporation. Cutting was performed at an output of 5000 W and a frequency of 1000 Hz. The oxygen gas pressure was 0.05 MPa on the inside and 0.03 MPa on the outside.

  For steel with a plate thickness of 16 mm, in order to evaluate the critical cutting speed, first focus the laser on the steel surface, and after piercing (penetration), square steel of 50 mm x 50 mm (however, corner R is 3 mm). Cutting, Duty: 50%, cutting speed was increased from 900 mm / min, cutting speed was increased at a pitch of 25 mm / min, and it was determined whether or not notch phenomenon, noro sticking and burning cutting failure occurred. The maximum cutting speed at which none of the notch phenomenon, adhesion of sticking and burning was observed was taken as the limit cutting speed.

  The cutting margin (cuttable range) is measured by first determining the laser focus on the surface of the steel material, and then conducting a test under conditions where the nozzle tip position is away from the steel material surface at a pitch of 0.25 mm. The determination was made based on whether or not the notch phenomenon and cutting failure due to adhesion of the noro occurred. Then, the maximum deviation of the nozzle tip position where both the notch phenomenon and the adhesion of the nozzle were not observed was measured, and the allowable variation range that can be cut from the laser focus position was obtained. The cutting speed was 900 mm / min.

  In addition, in order to evaluate the stability of the operation, a shape as shown in FIG. 1 (herein referred to as Type 1 and Type 2) from a steel material having a plate thickness of 25 mm and approximately 200 mm × 200 mm is cut at a cutting speed of 700 mm / min and 20% Duty. The number of sheets in which the burning phenomenon occurred (resulting rate) was evaluated. An example of the burning phenomenon is shown in FIG. The part where the cutting groove is wide is the part where the burning phenomenon occurs.

  Plasma cutting was performed using a Super 400 manufactured by Koike Oxygen Industry Co., Ltd. A steel material having a width of 150 mm and a thickness of 32 mm was cut at a pitch of 50 mm. At 134A, the speed was increased from 1150 mm / min to 5% pitch (57.5 mm / min) by using the overdrive function of the cutting machine, and it was determined whether or not cutting failure due to back surface adhesion occurred. The maximum cutting speed at which no cutting failure (no adhesion) was observed was taken as the limit cutting speed. Further, the surface at the maximum speed when cutting was possible was evaluated, and a case where a wavy phenomenon as shown in FIG. 3 was observed was evaluated as x, and a case where a smooth surface quality was observed without a waved phenomenon was evaluated as ◯. Table 2 summarizes the survey results.

  From the laser cutting test results shown in Table 2, the test No. of the present invention example that satisfies the conditions specified in the present invention is shown. In Nos. 1 to 18, the limit cutting speed is 1350 mm / min or more, and the allowable variation range for laser cutting is 4.75 mm or more, which is insensitive to laser defocus and excellent in cutting performance. .

  In contrast, Test No. of the comparative example. In 19-21, the cuttable range is 2.5 mm or less. Test No. Although the limit cutting speed increased at 21, the cuttable range was very narrow. That is, even if cutting is possible at high speed, a cutting failure may occur due to slight fluctuations in conditions.

  It is clear that the steel material of the present invention can be cut without causing a burning phenomenon in a complicated cut shape.

  From the results of the plasma cutting test shown in Table 2, the test No. of the present invention example that satisfies the conditions specified in the present invention is shown. In Nos. 1 to 18, a thick steel plate having a thickness of 32 mm can be cut at 1800 mm / min or more, has excellent plasma cutting properties, and has good surface quality and no wavy phenomenon.

  In contrast, test No. of the comparative example. In 19-21, the limit cutting speed was about 1550-1610 mm / min, and even if it was able to cut, there was a problem in the surface quality.

  The steel material of the present invention is insensitive to defocusing in the case of laser cutting, and has a very good laser cutting property that the burning phenomenon hardly occurs even when the cutting heat input is increased. Therefore, it is possible to cut a complicated shape without any trouble without controlling the scale by special heat treatment. Further, in plasma cutting, it is possible to cut at a high speed by suppressing the wavy phenomenon of the cut surface.

Claims (7)

In mass%, C: 0.02 to 0.20%, Si: 0.06 to 0.20%, Mn: 0.20 to 1.60%, P: 0.015 to 0.033%, S: 0.015% or less, Al: 0.03 to 0.08%, Cu: 0.01% or more and less than 0.5%, Ni: 0.01 to 0.5% and N: 0.009% or less in P / Si ≧ 0.12, and satisfies Al / Si ≦ 0.60, have a chemical composition the balance being Fe and impurities, a steel having a scale on the surface,
In the scale, there is a layer of Si-concentrated regions in which Si is 0.4% or more, and an Al-concentrated region in which the Al / Si ratio is 0.3 or more exists in layers on the surface layer side of the Si-concentrated region. thermal cutting steel material with oxygen you characterized. Chemical composition of the steel material, instead of a part of Fe, by mass%, further, Ti: 0.05% or less and V: claim 1, characterized in that it contains one or both of 0.05% or less Steel material for thermal cutting using oxygen described in 1. 3. The heat using oxygen according to claim 1 or 2 , wherein the chemical composition of the steel material contains, by mass%, Cr: 2.0% or less instead of part of Fe. Steel for cutting. The chemical composition of the steel material is one or more selected from mass% instead of part of Fe, and further selected from Mo: 0.5% or less, W: 0.4% or less, and Nb: 0.04% or less. thermal cutting steel using oxygen according to any one of claims 1 to 3, characterized in that it contains. The oxygen composition according to any one of claims 1 to 4 , wherein the chemical composition of the steel material contains, by mass%, B: 0.003% or less instead of part of Fe. Steel material for thermal cutting using The chemical composition of the steel material is one or more selected from mass% instead of part of Fe, and further selected from Ca: 0.005% or less, Mg: 0.005% or less, and REM: 0.005% or less. thermal cutting steel using oxygen according to any one of claims 1 to 5, characterized in that it contains. The chemical composition of the steel material is mass% instead of part of Fe, and further contains Sn: 0.50% or less, Cu: less than 0.1%, and the Cu / Sn ratio is 1.0 or less. thermal cutting steel using oxygen according to any one of claims 1 to 6, characterized in that it.

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