JP5652110B2 - Steel plate excellent in laser cutting property and manufacturing method thereof - Google Patents

Description

  The present invention relates to a steel plate having a thickness of 6 mm or more that can be used for shipbuilding, civil engineering, construction, bridges, construction machinery, marine structures, storage tanks, pressure vessels, etc., and can obtain excellent cutting quality during laser cutting, and the like It relates to a manufacturing method.

  When hot rolled steel sheets are used in steel structures such as shipbuilding, civil engineering, architecture, bridges, construction machinery, offshore structures, storage tanks, pressure vessels, etc., it is common to assemble them after cutting them into the desired shape. It is. The ratio of cutting work in the machining process of steel structures is high, and there is a high demand for higher work efficiency and lower costs. In addition, the shape of the steel structure is complicated from the viewpoint of design and the like, and the shape of the cut surface is complicated because the steps after the cutting are omitted, and high accuracy is required for the cut surface.

  Conventionally, gas cutting and plasma cutting have been widely used as methods for cutting thick steel plates. Gas cutting is most widely used because the equipment is relatively simple and it is possible to cut a steel plate having a very large thickness. However, automatic work such as control and monitoring of the gas flame is difficult, and the cutting speed is relatively slow, so the workability is poor. Plasma cutting is capable of high-speed cutting up to a maximum thickness of about 50 mm, but has a torch life of only a few hours, has low workability due to frequent replacement work, and is difficult to be automated.

  On the other hand, laser cutting has been widely used since the cutting of thin steel plates, and in recent years, as the output of laser oscillators has increased and the price has been reduced, the range of application has also expanded to the cutting of thick steel plates. The features are that the torch life is long and the laser output is easy, so that the cutting operation can be fully automated easily. Further, the thermal deformation due to cutting is small, and the quality of the cut surface is good. For this reason, laser cutting can be said to be an ideal method for cutting thick steel plates from the viewpoint of workability and cutting quality.

  However, with the current laser output, the target plate thickness is limited to a maximum of about 25 mm, and when the plate thickness exceeds that, the stability of cutting sharply decreases. As for the capability of the laser cutting machine, further higher output is being promoted, but on the other hand, studies have been made to improve the laser cutting performance of the steel sheet itself (that no cutting failure occurs).

  In Patent Document 1, a steel material in which Cu, Ni, Ti, and Zr are added to a system in mass%, C: 0.20% or less, -Si: 0.1 to 1.0%, Mn: 2.5% or less. Is a steel plate in which a concentrated layer of Cu, Ni, Ti, Zr is formed at the interface between the base material and the scale layer by low temperature heating, low temperature rolling and accelerated cooling, and the concentrated layer efficiently uses the energy of the laser beam. In particular, a technique for improving the laser cutting property by absorbing it into the ground iron has been reported.

  In Patent Document 2, a steel material added with Si is subjected to high-pressure descaling before hot rolling, and then the scale thickness is controlled to 10 to 60 μm by a manufacturing method that combines ordinary rolling, accelerated cooling, and flatness adjustment processing. Technologies that improve laser cutting performance have been reported.

  In Patent Document 3, 0.03 to 0.06% C-0.05 to 0.3% Si-0.5 to 1. wt% adjusted for glossiness of the steel sheet surface and viscosity of the molten steel during laser cutting. 5% Mn-based steel material with Cu, Ni, Cr, etc. added to the composition, heated to a specific temperature, then subjected to high-pressure descaling multiple times during controlled rolling to remove scales and debris from the steel sheet surface There has been reported a technique for improving the laser cutting property by leaving no scale and making the scale unnecessarily thick.

JP 2008-195983 A JP 2008-95155 A Japanese Patent No. 3218166

  However, in the technique described in Patent Document 1, it is necessary to add an expensive alloy element, resulting in high cost. Further, due to low temperature heating-low temperature rolling, defects such as voids remain in the steel sheet or distortion occurs. Is a problem.

  In the technique described in Patent Document 2, strain is introduced into the steel sheet by accelerated cooling and flatness adjustment processing, and deformation at the time of cutting becomes a problem. In addition, the variation in scale composition) is remarkable and the standard deviation of the scale thickness is large, and it is difficult to stably ensure excellent laser cutting properties.

  In the technique described in Patent Document 3, not only is expensive alloy element required to be added and the cost becomes high, but also the rolling finishing temperature is low, so the time between rolling passes and between descaling passes becomes long. The variation of the scale form (scale thickness, scale composition) depending on the position in the steel sheet is remarkable, the standard deviation of the scale thickness is large, and it is difficult to ensure stable and excellent laser cutting ability.

  This invention is made | formed in view of these facts, and it aims at providing the steel plate excellent in economical efficiency, and the laser cutting property, and its manufacturing method. Here, being excellent in laser cutting property means that it is suitable for laser cutting. Specifically, it is excellent in workability with little deformation of the steel plate during cutting, and there is no notch in the cross section of the steel plate after cutting, and the back surface of the steel plate. Is excellent in cutting quality without adhesion of dross.

  In order to achieve the above-mentioned problems, the present inventors have ensured excellent laser cutting properties for thick steel plates, in order to ensure the composition of the steel plate, the manufacturing method, the scale form (scale thickness, scale composition), and the scale thickness. We conducted extensive research on various factors that determine the standard deviation and the viscosity of molten steel by laser cutting, and obtained the following findings.

  1. The thermal stress generated in the steel sheet by laser irradiation with high density energy is extremely large, and the scale of the surface layer is easily destroyed during laser irradiation. In order to prevent this, it is effective to improve the adhesion at the interface between the scale and the ground iron. For that purpose, it is important to reduce the average thickness of the scale layer and reduce the variation in the thickness of the scale layer.

  2. In order to achieve excellent laser cutting properties, it is necessary to efficiently convert laser energy into thermal energy and melt the laser irradiation part. In order to absorb the laser energy efficiently, that is, to increase the absorption ability, it is important to make the color tone of the scale generated on the surface of the steel sheet excellent in the absorption ability.

  3. In order to achieve the scale form described above, it is indispensable to improve the adhesion of the interface between the scale and the steel and to suppress the growth of the scale by managing the composition of the steel sheet. Furthermore, by strictly controlling the hot rolling conditions and the descaling conditions during hot rolling, the scales generated and grown before hot rolling, during hot rolling and after hot rolling are effectively peeled off, It is important to remove and inhibit growth.

  4). Moreover, the laser cutting performance is affected by the viscosity of the molten steel due to heating during laser cutting, and a component design that reduces the viscosity of the molten steel is effective in improving the laser cutting performance.

The present invention was made by further studying the obtained knowledge,
1. In mass%, C: 0.03 to 0.20%, Si: 0.01 to 0.60%, Mn: 0.1 to 2.5%, P: 0.016 to 0.050%, S: In a steel sheet containing 0.01% or less, Al: 0.07% or less, N: 0.01% or less, O: 0.0005 to 0.0050%, and the balance of Fe and inevitable impurities The Po shown by the following formula is −5 to 25, the average thickness of the scale layer on the steel sheet surface is 10 μm or less, and the scale layer contains 60 mass% or more of magnetite and has excellent laser cutting properties. Steel plate.

Po = (2.9Si−0.2Mn + 0.7Al) × (O−0.0023) ² × 10 ⁷
(Here, Si, Mn, Al, and O are contained (mass%))
2. In addition to the composition of the steel sheet, the mass is further Cu: 1.5% or less, Ni: 2.0% or less, Cr: 1.0% or less, Mo: 1.0% or less, Nb: 0.1% or less V: 0.1% or less, Ti: 0.03% or less, B: 0.005% or less, 1 or 2 or more types, The steel plate excellent in laser cutting property of 1 characterized by the above-mentioned.
3. The composition of the steel sheet further includes one or more of REM: 0.008% or less, Ca: 0.005% or less, and Mg: 0.005% or less in mass%. Or the steel plate excellent in the laser cutting property of 2.
4). The steel plate excellent in laser cutting property according to 1 to 3, wherein the standard deviation of the scale layer thickness on the steel plate surface is 4.0 μm or less.
After the slab or steel slab comprising the steel composition described in any one of 5.1 to 3 is reheated to 1000 to 1200 ° C, hot rolling is performed at 650 to 850 ° C, and 950 ° C to In laser cutting property, in the rolling pass between rolling end temperatures, water is sprayed on the front and back surfaces of the steel sheet to perform descaling 5 times or more and the time between passes of descaling is 10 to 50 seconds. Excellent steel plate manufacturing method.
6. During the rolling pass between 950 ° C. and the end temperature of rolling, accelerated cooling is performed once or twice at a water density of 4 m ³ / m ² min or more on the upper and lower surfaces of the steel sheet while passing the steel sheet through a hot rolling mill. 5. The method for producing a steel sheet excellent in laser cutting property according to 5, wherein the method is performed more than once.

  According to the present invention, in laser cutting, a steel plate having excellent workability and cutting quality is obtained, which greatly contributes to improvement in manufacturing efficiency and safety at the time of manufacturing a steel structure, and has a remarkable industrial effect. .

In the present invention, the form of the scale (scale thickness, scale composition) and the component composition of the steel sheet are specified. In the description,% is mass%.
[Scale form]
The average thickness of the scale layer on the surface of the steel sheet is 10 μm or less, and the scale composition includes 60% or more of magnetite in the scale layer.

  In order to obtain excellent laser cutting properties, the adhesion between the scale and the ground iron is increased to suppress the destruction of the scale during laser cutting, and the scale surface form has a high laser light absorption ability.

  In the present invention, the average thickness of the scale layer is strictly controlled in order to increase the adhesion between the scale and the ground iron. If the average thickness of the scale layer is thicker than 10 μm, the scale adhesiveness is lowered, so that the laser cutting property is inferior. Therefore, the average thickness of the scale is set to 10 μm or less. A method for obtaining the average thickness of the scale will be described later in Examples.

Furthermore, the composition of the scale is controlled so that the color tone is dark with high laser absorption. The scale generated on the surface of the steel sheet after hot rolling is mainly formed of wustite (FeO), magnetite (Fe ₃ O ₄ ), and hematite (Fe ₂ O ₃ ), but the composition of the scale layer is mainly composed of magnetite. Then, the color of the scale becomes black, the laser absorption ability is improved, and the laser cutting property is improved.

  In order to obtain such an effect, it is necessary to have 60% or more of magnetite in the scale layer. In addition, as the influence of the other scale composition, as the hematite increases, the color of the scale becomes closer to red and the laser absorptivity decreases, so it is preferable that it is as small as possible, but if it is less than 25%, the influence can be ignored. .

  In addition, wustite has little effect on the laser absorptivity, but is preferably as small as possible in order to reduce the adhesion with the ground iron, but if less than 15%, the influence can be ignored.

  In the present invention, in order to further improve the laser cutting property, in addition to the basic scale form, the standard deviation of the scale layer thickness on the steel sheet surface is set to 4.0 μm or less. When the standard deviation of the scale layer thickness exceeds 4.0 μm, the laser absorption on the steel plate surface becomes unstable and the laser cutting property is inferior. For this reason, the standard deviation of the scale layer thickness was set to 4.0 μm or less. A method for obtaining the standard deviation will be described later.

  However, the laser cutting property is affected by the viscosity of the molten steel in addition to the standard deviation of the scale form and scale layer thickness described above.

[Component composition of steel sheet]
In the present invention, 1. Scales generated and grown before hot rolling, during hot rolling and after hot rolling are easily peeled off by descaling during hot rolling, and the growth is suppressed. Sometimes the component composition of the steel sheet is defined so that the adhesion between the scale and the ground iron is high. In the description,% is mass%.

C: 0.03-0.20%
C is an element necessary for increasing the strength of steel and ensuring the necessary strength as a structural steel material. To obtain the effect, C is required to be contained in an amount of 0.03% or more. On the other hand, the content exceeding 0.20% deteriorates toughness and lowers weldability. For this reason, it limits to 0.03 to 0.20% of range. Preferably, it is 0.04 to 0.18%.

Si: 0.01-0.60%
Si acts as a deoxidizing material and is not only necessary for steelmaking, but also concentrates at the interface between the scale and the base iron to reduce the adhesion of the scale. This improves the peelability of the scale during descaling.

  Such an effect is particularly remarkable in a high temperature range of 650 to 950 ° C., and scales generated before and during rolling can be effectively removed.

  Further, Si concentrated at the interface between the scale and the base iron suppresses the growth of the scale generated on the surface of the steel sheet after hot rolling, and contributes to the improvement of the scale adhesion at the time of laser cutting. In order to acquire such an effect, 0.01% or more of content is required.

  On the other hand, if the content exceeds 0.60%, the toughness of the base material and the low temperature cracking property of the welded portion are remarkably deteriorated, so the content is limited to the range of 0.01 to 0.60%. Preferably, it is 0.02 to 0.55%.

Mn: 0.1 to 2.5%
Mn has the effect of increasing the hardenability of the steel, and the content of 0.1% or more is necessary to ensure the strength of the base material. On the other hand, if the content exceeds 2.5%, the toughness, ductility and weldability of the base material are significantly deteriorated, so the content is limited to the range of 0.1 to 2.5%. Preferably, it is 0.2 to 2.0%.

P: 0.016 to 0.050%
Even when P is added in a small amount, P is concentrated at the interface between the scale and the base iron, thereby reducing the adhesion of the scale. This improves the peelability of the scale during descaling.
Such an effect is particularly remarkable in a high temperature range of 650 to 950 ° C. When descaling is performed in this temperature range, the scale generated before and during rolling can be effectively removed.

  Further, P concentrated at the scale-base metal interface suppresses the growth of scale generated on the surface of the steel sheet after hot rolling, and contributes to the improvement of scale adhesion during laser cutting. In order to acquire such an effect, 0.016% or more needs to be contained. On the other hand, if the content exceeds 0.050%, the strength of the steel is increased and the toughness is remarkably deteriorated, so the content is limited to the range of 0.016 to 0.050%.

S: 0.01% or less Since S deteriorates the low temperature toughness and ductility of the base material, it is desirable to reduce the upper limit to 0.01%.

Al: 0.07% or less Al acts as a deoxidizer and is most commonly used in the molten steel deoxidation process of steel sheets. Further, N in the steel is fixed as AlN, which contributes to improvement of the toughness of the base material and the welded portion. On the other hand, if the content exceeds 0.07%, the toughness of the base material is lowered, and the toughness of the weld metal is deteriorated by being mixed in the weld metal part during welding, so the content is limited to 0.07% or less. .

N: 0.01% or less N is contained in steel as an inevitable impurity, and if it exceeds 0.01%, the toughness of the base metal and the welded portion is remarkably reduced, so it is limited to 0.01% or less. .

O: 0.0005 to 0.0050%
O is contained as an inevitable impurity and exists as an oxide in steel. During laser cutting, the oxide melts together with the base metal during the heating stage, but recrystallizes into the molten steel as an oxide such as Al or Si during the cooling process, thereby reducing the viscosity of the molten steel and improving laser cutting performance. Has the effect of In order to acquire such an effect, 0.0005% or more needs to be contained. On the other hand, if it exceeds 0.0050%, the oxide present in the steel becomes coarse, lowering the cleanliness and adversely affecting the toughness of the base metal, so it is limited to the range of 0.0005 to 0.0050%. .

Po (= (2.9Si−0.2Mn + 0.7Al) × (O−0.0023) ² × 10 ⁷ where Si, Mn, Al, and O are contained (mass%)): −5 to 25
This parameter formula Po is for lowering the viscosity of the molten steel of the present invention and imparting a characteristic excellent in laser cutting property, and is set to −5 to 25.

  In the present invention, it is not sufficient that each component simply satisfies the above composition range, and it is important that Po satisfies −5 to 25.

  The oxide present in the steel sheet is melted together with the base material in the heating stage of laser cutting and recrystallized as an oxide in the molten steel in the cooling process. At this time, the viscosity of the molten steel varies depending on the alloy element forming the oxide, and consequently affects the laser cutting property. When Po is in the range of −5 to 25, the viscosity of the molten steel is reduced, which is effective for improving laser cutting properties. For this reason, Po is limited to the range of -5-25.

  In the present invention, in addition to the above basic component system, one or more of Cu, Ni, Cr, Mo, Nb, V, Ti, B, REM, Ca and Mg are contained in order to further improve the characteristics. can do.

Cu: 1.5% or less, Ni: 2.0% or less, Cr: 1.0% or less, Mo: 1.0% or less, Nb: 0.1% or less, V: 0.1% or less, Ti: One or more of 0.03% or less, B: 0.005% or less Cu, Ni, Cr, Mo, Nb, V, Ti, B are all elements that contribute to improving the strength of steel, It can contain suitably according to the intensity | strength to desire.

  When adding Cu, it is preferable to set it as 0.1% or more, However, If it exceeds 1.5%, since hot brittleness will be produced and the surface property of a steel plate will deteriorate, it is set as 1.5% or less.

  When adding Ni, it is preferable to set it as 0.1% or more, but when it exceeds 2.0%, since an effect will be saturated and it will become economically disadvantageous, it shall be 2.0% or less.

  When adding Cr, it is preferable to make it 0.05% or more, but if it contains more than 1.0%, the base material toughness, ductility and weldability deteriorate significantly, so 1.0% or less. .

  When Mo is added, the content is preferably 0.05% or more. However, if it exceeds 1.0%, the base material toughness, ductility and weld crack resistance are adversely affected. .

  When Nb is added, the content is preferably 0.005% or more. However, if it exceeds 0.1%, the base metal toughness and ductility are deteriorated, so the content is made 0.1% or less.

  When V is added, the content is preferably 0.01% or more. However, if it exceeds 0.1%, the base metal toughness and ductility are deteriorated, so the content is made 0.1% or less.

  Ti is an additive element that has a strong affinity for N and precipitates as TiN during solidification, and contributes to high toughness by suppressing the coarsening of austenite grains in the weld heat affected zone. On the other hand, if added over 0.03%, TiN particles become coarse and deteriorate the base material and weld toughness, so the content is made 0.03% or less.

  B has the effect of increasing the strength of the steel through improving hardenability. On the other hand, if the content exceeds 0.005%, the hardenability is remarkably increased and the toughness and ductility of the base material are deteriorated, so the content is made 0.005% or less.

One or more of REM: 0.008% or less, Ca: 0.005% or less and Mg: 0.005% or less REM, Ca and Mg all contribute to toughness improvement, depending on the desired properties Select and add. When adding REM, it is preferable to set it as 0.002% or more, but even if it exceeds 0.02%, the effect is saturated, so 0.008% is made the upper limit.

  When adding Ca, it is preferable to make it 0.0005% or more, but since the effect is saturated even if it exceeds 0.005%, the upper limit is made 0.005%.

When adding Mg, it is preferable to set it as 0.001% or more, but since an effect will be saturated even if it exceeds 0.005%, 0.005% is made an upper limit.
The balance other than the above components is Fe and inevitable impurities.

  The steel sheet according to the present invention is preferably produced under the following production conditions. In the description, the “° C.” display relating to the temperature means a temperature at half the plate thickness.

Steel material heating temperature: 1000 ° C-1250 ° C
A steel material of a slab or steel slab having the above-described composition is prepared from molten steel by a generally known method such as a converter, electric furnace, vacuum melting furnace, etc., and reheated to 1000 ° C to 1250 ° C. If the reheating temperature is less than 1000 ° C., the deformation resistance in hot rolling is high, and the reduction amount per pass cannot be made large. Therefore, the number of rolling passes increases, and the rolling efficiency decreases, and the steel material ( In some cases, the casting defect in the slab cannot be crimped. On the other hand, when the reheating temperature exceeds 1250 ° C., surface flaws are likely to occur due to the scale during heating, the maintenance load after rolling increases, the crystal grains become coarse and the desired microstructure cannot be obtained, and local elongation occurs. Since the total elongation is reduced through a decrease in the temperature, the range is 1000 to 1250 ° C.

Hot rolling finish temperature: 650-850 ° C
When the end temperature of hot rolling exceeds 850 ° C., not only blisters are generated in the scale, but also the scale is excessively grown in the cooling process after the end of rolling, so that the desired scale form (scale thickness, scale composition) and The standard deviation of scale thickness cannot be obtained. On the other hand, when the rolling end temperature is lower than 650 ° C., the deformation resistance becomes too high, the rolling load increases, and the burden on the rolling mill increases. Further, in order to lower the rolling temperature, it is necessary to wait in the middle of rolling, which not only greatly hinders productivity but also excessive growth of the scale during standby, so that the desired scale form (scale thickness and Scale composition) cannot be obtained. Furthermore, since the strain accumulated in the steel plate becomes large, the steel plate is deformed during laser cutting, leading to a problem that the cutting accuracy is lowered or the cutting is stopped halfway. For this reason, it is set as the range of 650-850 degreeC.

  Although the cooling method after completion | finish of rolling is not prescribed | regulated especially, such as air cooling and water cooling, it is preferable to air-cool from a viewpoint of suppressing a slit distortion.

Between 950 ° C. and the end temperature of rolling, descaling is performed 5 times or more and the time between passes is 10 to 50 seconds. In order to stably achieve a desired scale form (scale thickness, scale composition) and standard deviation of scale thickness. In the present invention, it is important to strictly manage the number of descaling during rolling and the time between passes. When the number of descaling is less than 5, peeling of the scale that is generated and grows during rolling becomes insufficient, and a desired scale form (scale thickness, scale composition) and standard deviation of scale thickness cannot be obtained.

  In addition, if the time between passes of descaling is less than 10 seconds, not only does the production speed of the steel plate not realize because the conveying speed during rolling is too high, but it is sufficient for the scale generated and grown after the pass before descaling. Can not get a good descaling effect. On the other hand, if the time between passes of descaling exceeds 50 seconds, the scale grows excessively during standby, so that the desired scale thickness and scale composition cannot be obtained. For this reason, descaling is performed 5 times or more between 950 ° C. and the rolling end temperature, and the time between passes is set to a range of 10 to 50 seconds.

  In the description, the time between passes of descaling is the difference between the first descaling start time and the next descaling start time during reverse rolling, which is a manufacturing process of a thick steel plate. Further, as the descaling ability, the effect of the present invention can be exhibited if the injection pressure is 10 MPa or more.

Further, in addition to the above manufacturing conditions, in order to set the rolling end temperature to 650 to 800 ° C. while keeping the time between passes of descaling within 50 seconds, it is necessary to forcibly reduce the steel plate temperature, 950 ° C. Accelerated cooling is inserted once or twice at a water density of 4 m ³ / m ² min or more in the upper and lower surfaces of the steel sheet while passing the steel sheet through a hot rolling mill during the rolling pass between the rolling end temperature and the rolling end temperature. .

If the water density is less than 4 m ³ / m ² min, the effect of accelerated cooling becomes insufficient, the time between passes of descaling exceeds 50 seconds, and the scale grows excessively during standby, so the desired scale Thickness and scale composition cannot be obtained. For this reason, the accelerated cooling of the upper and lower surfaces of the steel sheet between 950 ° C. and the rolling end temperature is performed at a water density of 4 m ³ / m ² min or more.

  Steel slabs prepared with various component compositions shown in Table 1 by the converter-ladder refining-continuous casting method were made into steel plates with a thickness of 25 mm under various hot rolling conditions shown in Table 2. Steel No. in Table 2 1-1 to 1-7 are steel Nos. No. 1 with various hot rolling conditions changed, steel no. 2 to 8 are steel Nos. 2 to 8 respectively.

  A total of 6 samples of plate thickness × 20 mm from the rolling direction of ¼ width and ½ width, respectively, from the positions 500 mm from the front end in the longitudinal direction of each steel plate and 500 mm from the center and from the tail end. Collected. For each sample, five optical micrographs of 500 times magnification of the region including the ground iron and the scale layer were taken, and the thickness of the scale layer was measured using an image analyzer at any 10 locations in each photo. The average thickness of all six measurements was taken as the average thickness of the scale, and the standard deviation of all six measurements was taken as the standard deviation of the scale thickness.

In addition, the identification of the scale composition was obtained by the calibration curve method using the scale standard sample based on the result of the X-ray analysis method.
The laser cutting property was cut using a 6 kW carbon dioxide gas laser, and the presence or absence of notches in the cross section of the steel sheet after cutting and the presence or absence of dross on the back surface of the steel sheet were evaluated. The oxygen pressure was 0.3 kgf / cm ² and the laser cutting speed was 1100 mm / min. The cutting length was 500 mm.

The obtained results are shown in Table 2 together with the production conditions of the test steel plate.
Inventive examples (steel Nos. 1-1, 1-2, 2, 3, 4, 5, 6) have an average thickness of 10 μm or less and a volume fraction of magnetite contained in the scale layer of 60% or more. In addition, the viscosity of the molten steel during laser cutting was low, and the laser cutting property was very good.

  On the other hand, in the comparative example (steel No. 7) in which the amount of Si in the component composition is less than the range of the present invention and the comparative example (steel No. 8) in which P is less than the range of the present invention, the scale average thickness is increased. The amount of magnetite in the layer did not reach 60 mass%, and the laser cutting property was inferior.

  The comparative example (steel No. 9) in which the amount of O in the component composition is less than the scope of the present invention and Po is outside the scope of the present invention, and the comparative example (No. 10) where Po is outside the scope of the present invention, The viscosity of the molten steel was high and the laser cutting property was inferior.

  Comparative Example (Steel No. 1-3 in which the component composition is within the scope of the present invention, but the hot rolling and / or descaling conditions are outside the scope of the present invention, and the standard deviation of scale form and scale thickness is outside the scope of the present invention. 1-5, 1-6, 1-7, 1-8) were inferior in laser cutting property.

  Steel No. 1-4, the component composition and scale form are within the scope of the present invention, but the hot rolling finish temperature is low outside the scope of the present invention, and the accumulated strain in the steel sheet is large, so that the steel sheet deforms during laser cutting. It was forced to stop in the middle of cutting, workability was poor and laser cutting property was inferior.

Claims (6)

In mass%, C: 0.03 to 0.20%, Si: 0.01 to 0.60%, Mn: 0.1 to 2.5%, P: 0.016 to 0.050%, S: In a steel sheet containing 0.01% or less, Al: 0.07% or less, N: 0.01% or less, O: 0.0005 to 0.0050%, and the balance of Fe and inevitable impurities The laser cutting is characterized in that Po shown by the following formula is −5 to 25, the average thickness of the scale layer on the steel sheet surface is 10 μm or less, and magnetite is contained in the scale layer in the range of 60 mass% to 81 mass%. Steel sheet with excellent properties.
Po = (2.9Si−0.2Mn + 0.7Al) × (O−0.0023) ² × 10 ⁷
(Here, Si, Mn, Al, and O are contained (mass%))   In addition to the composition of the steel sheet, the mass is further Cu: 1.5% or less, Ni: 2.0% or less, Cr: 1.0% or less, Mo: 1.0% or less, Nb: 0.1% or less V: 0.1% or less, Ti: 0.03% or less, B: 0.005% or less, or one or two or more of them are contained. steel sheet.   The component composition of the steel sheet further includes one or more of REM: 0.008% or less, Ca: 0.005% or less, and Mg: 0.005% or less in mass%. Item 3. A steel sheet excellent in laser cutting properties according to item 1 or 2. Steel sheet excellent in laser cutting of any one of claims 1 to 3 standard deviations of the scale layer thickness of the steel sheet surface is characterized by comprising less 4.0 .mu.m.   The slab or steel slab comprising the component composition according to any one of claims 1 to 3 is reheated to 1000 to 1200 ° C, and then hot-rolled at 650 to 850 ° C, and 950 ° C to In laser cutting property, in the rolling pass between rolling end temperatures, water is sprayed on the front and back surfaces of the steel sheet to perform descaling 5 times or more and the time between passes of descaling is 10 to 50 seconds. Excellent steel plate manufacturing method. During the rolling pass between 950 ° C. and the rolling end temperature, accelerated cooling is performed once or twice at a water density of 4 m ³ / m ² min or more on the upper and lower surfaces of the steel sheet while passing the steel sheet through a hot rolling mill. The method for producing a steel sheet having excellent laser cutting properties according to claim 5, wherein the method is performed.