JP7589130B2 - Steel plate and its manufacturing method - Google Patents

Description

This disclosure relates to steel plates and manufacturing methods thereof, and in particular to steel plates that can be processed into a desired shape by laser cutting and manufacturing methods thereof.

Heavy steel plates and other steel plates are used in large quantities in steel structures, including shipbuilding, architecture, industrial machinery, and bridges. Many of these structures are manufactured by cutting steel plates into a specified shape and assembling them by welding or other methods. Gas cutting, plasma cutting, and laser cutting are used as methods for cutting steel plates. Among these, laser cutting is characterized by a small heat-affected zone and excellent processing precision. In recent years, with the practical application of high-power laser cutting machines, laser cutting can also be applied to cutting thicker steel plates. For this reason, laser cutting is being used more widely than ever before in cutting steel plates.

Many steel plates, such as thick steel plates, are oxidized by the atmosphere during the hot rolling process of the slab, forming a scale (oxide film) on their surface. The scale contributes to obtaining good laser cuttability because it has the effect of suppressing excessive temperature rise of the steel at the laser spot. Therefore, in laser cutting of steel plates with scale on their surface, if the scale peels off the surface of the steel plate or is peeled off by the laser during cutting, the steel plate cannot be cut, or cutting defects such as self-burning due to excessive heat input to notches or corners, which are abnormal cuts that are gouged out of the cut surface, and cuts may not be stable. In other words, adhesion of the scale is essential to improving laser cuttability.

Therefore, in order to obtain stable laser cutting performance, steel sheets with improved scale adhesion have been developed.
Patent Document 1 discloses a thick steel plate in which the scale thickness and scale composition are controlled to improve adhesion between the scale and the base steel (steel portion).
Patent Document 2 discloses a thick steel plate in which the scale thickness and the thickness of the Fe-Si oxide layer in the scale are controlled to improve adhesion between the scale and the base steel.
Patent Document 3 also discloses a steel sheet in which the scale thickness and scale composition are controlled to improve adhesion between the scale and the base steel.

JP 2020-114938 A JP 2021-095614 A Patent No. 5652110

However, when attempting to cut into more complex shapes using laser cutting, there is a problem that the width of the cutting groove on the surface of the steel sheet increases, making it easier for self-burning, in which dross is seen to adhere to the back surface of the steel sheet, to occur. Even if the steel sheets described in Patent Documents 1 to 3 are used, there is a risk that self-burning that occurs when laser cutting into complex shapes cannot be sufficiently suppressed and excellent laser cuttability cannot be obtained. Furthermore, as can be seen in many of the examples in Patent Documents 1 to 3, conventional steel sheets that have increased adhesion between the scale and the base steel in order to improve laser cuttability contain expensive additive elements such as one or more selected from Cu, Ni, Cr, Mo, Nb, V, and Ti, which increases the cost.

The present disclosure has been made in consideration of these circumstances, and aims to provide a steel sheet and a manufacturing method thereof that has excellent laser cuttability even when cut to obtain complex shapes, and does not contain expensive additive elements such as one or more selected from Cu, Ni, Cr, Mo, Nb, V, and Ti.

A first aspect of the present invention is a steel plate which contains C: 0.10 mass% or more and 0.20 mass% or less, Si: 0.30 mass% or more and 0.60 mass% or less, Mn: 0.30 mass% or more and 1.50 mass% or less, P: 0.030 mass% or less (including 0 mass%), S: 0.010 mass% or less (including 0 mass%), Al: 0.005 mass% or more and 0.050 mass% or less, N: 0.0060 mass% or less (including 0 mass%), with the balance consisting of Fe and inevitable impurities, in which the Si content and the Mn content satisfy the following formula (1), which has a scale layer with a thickness of 10 μm or more and 60 μm or less on the surface, and which has an interface roughness index defined by the following formula (2) of 1.33 or more.

0.29≦[Si]/√[Mn] (1)
Here, [Si] and [Mn] are the contents of Si and Mn expressed in mass %, respectively.

Interface roughness index=((interface contact length between base steel and scale layer)/(straight-line length of interface between base steel and scale layer)) ² (2)

Aspect 2 of the present invention includes a step of preparing a steel material containing C: 0.10 mass% or more and 0.20 mass% or less, Si: 0.30 mass% or more and 0.60 mass% or less, Mn: 0.30 mass% or more and 1.50 mass% or less, P: 0.030 mass% or less (including 0 mass%), S: 0.010 mass% or less (including 0 mass%), Al: 0.005 mass% or more and 0.050 mass% or less, N: 0.0060 mass% or less (including 0 mass%), with the balance being Fe and inevitable impurities, and the Si content and the Mn content satisfying the following formula (1): and a hot rolling step including a step of performing hot rolling, in which descaling with high pressure water is performed once or more and at least three passes of rolling are performed from the final descaling, and a cumulative reduction rate after final descaling, which is the sum of the reduction rates in each pass from the first rolling pass to the final rolling pass after the final descaling, is set to 80.0% or more and a finish rolling temperature is set to 870°C or higher.

0.29≦[Si]/√[Mn] (1)
Here, [Si] and [Mn] are the contents of Si and Mn expressed in mass %, respectively.

Interface roughness index=((interface contact length between base steel and scale layer)/(straight-line length of interface between base steel and scale layer)) ² (2)

According to one embodiment of the present invention, it is possible to provide a steel sheet and a manufacturing method thereof that has excellent laser cuttability even when cut to obtain complex shapes and does not contain expensive additive elements such as one or more selected from Cu, Ni, Cr, Mo, Nb, V, and Ti.

FIG. 1A is a schematic cross-sectional view showing an early stage of scale formation. FIG. 1B is a schematic cross-sectional view showing a state in which the generation of scale has progressed to a certain extent and the inner oxide and the outer oxide have combined. FIG. 2 is a schematic cross-sectional view illustrating the "interface contact length between the base steel and the scale layer" and the "linear length of the interface between the base steel and the scale layer" used to determine the interface roughness index. FIG. 3 is a schematic plan view showing a laser cut portion.

As a result of intensive research into improving the adhesion of the scale, the inventors have found that laser cuttability can be improved by setting the thickness of the scale layer within a predetermined range and setting the interface roughness index, which is the square of the ratio of the contact length of the interface between the base steel and the scale layer to the linear length of the interface between the base steel and the scale layer, to a predetermined value or more. In order to achieve the interface roughness index to a predetermined value or more without using expensive additive elements, in addition to setting each element in the composition within a predetermined range, the ratio of the Si content to the square root of the Mn content is set to a predetermined value or more. Then, using a steel material of such a composition, the inventors have found that the above-mentioned predetermined interface roughness index can be achieved by descaling under controlled conditions, i.e., descaling at least once during hot rolling, setting the number of rolling passes and cumulative reduction rate after final descaling to a predetermined value or more, and setting the finish rolling temperature to a predetermined temperature or more, thereby arriving at a steel sheet according to an embodiment of the present invention.
The details of the embodiment of the present invention are given below.

<1. Chemical composition>
The steel plate according to the embodiment of the present invention has the following characteristics: C: 0.10 mass % or more and 0.20 mass % or less; Si: 0.30 mass % or more and 0.60 mass % or less; Mn: 0.30 mass % or more; 50% by mass or less, P: 0.030% by mass or less (including 0% by mass), S: 0.010% by mass or less (including 0% by mass), Al: 0.005% by mass to 0.050% by mass The alloy contains N: 0.060 mass % or less (including 0 mass %), with the balance being Fe and inevitable impurities. Furthermore, the Si content and the Mn content satisfy the following formula (1).

0.29≦[Si]/√[Mn] (1)
Here, [Si] and [Mn] are the contents of Si and Mn expressed in mass %, respectively.

The content of each element and formula (1) will be described in detail below.

[1. Chemical Composition]
(C: 0.10% by mass or more and 0.20% by mass or less)
C is an element that increases the strength of steel, and in order to ensure the desired high strength, it is necessary to include 0.10 mass% or more of C. The C content is preferably 0.12 mass% or more. On the other hand, if the C content exceeds 0.20 mass%, the toughness of the steel plate is deteriorated. The upper limit of the C content is preferably 0.18 mass%.

(Si: 0.30 mass% or more and 0.60 mass% or less)
Silicon is an element that increases the interface roughness index and improves scale adhesion. Silicon forms _Fe2SiO4 (fayalite ₎ as an inner oxide, and as the scale grows on the steel sheet, it dissolves the outer oxide FeO/Fe. By combining with the _{SiO2 eutectic} compound, it promotes the unevenness of the interface between the base steel and the scale. In order to fully obtain this effect and suppress scale peeling during laser cutting, the lower limit of the Si content is set to 1. The Si content is set to 0.30% by mass. The Si content is preferably 0.33% by mass or more. On the other hand, an excessive Si content leads to poor descaling during rolling, and it is necessary to control the scale layer thickness. Therefore, the upper limit of the Si content is set to 0.60 mass%, and the Si content is preferably 0.55 mass% or less.

(Mn: 0.30 mass% or more and 1.50 mass% or less)
Mn can inexpensively increase the strength of a steel sheet. In order to ensure sufficient strength as a steel sheet, the Mn content is set to 0.30 mass% or more, and preferably 0.35 mass% or more. Excessive addition of Mn deteriorates weldability, reduces the interface roughness index, and deteriorates laser cuttability. Therefore, the Mn content is set to 1.50 mass% or less. The Mn content is preferably 1.40 mass% or less. % or less.

(P: 0.030% by mass or less (including 0% by mass))
P is inevitably present as an impurity and deteriorates the toughness of steel, so its content is desirably reduced as much as possible. Therefore, the upper limit of the P content is set to 0.030 mass%. It is difficult to make the P content 0% by mass in the manufacturing process, and it is usually contained at about 0.002% by mass or more. In this specification, "containing 0% by mass" does not mean that it is intentionally added. The term "contains" is intended to include embodiments in which the above-mentioned steps are not performed and the content is below the impurity level.

(S: 0.010% by mass or less (including 0% by mass))
S, like P, is inevitably present as an impurity. A high S content deteriorates the toughness of the steel and reduces scale adhesion, so the upper limit is set to 0.010 mass%. It is difficult to reduce the S content to 0 mass% in the manufacturing methods usually used industrially, and the S content is usually about 0.001 mass% or more.

(Al: 0.005% by mass or more and 0.050% by mass or less)
While Al is an element added as a deoxidizer, if it is added in excess of 0.050 mass%, the amount of alumina-based nonmetallic inclusions increases, lowering the cleanliness of the steel. The content is set to 0.005 to 0.050% by mass. The lower limit is preferably 0.010% by mass, and the upper limit is preferably 0.045% by mass.

(N: 0.0060% by mass or less (including 0% by mass))
N is also inevitably present as an impurity, but if its content is 0.0060 mass% or less, it does not adversely affect weldability, cast quality, etc. Therefore, the N content is set to 0.0060 mass% or less (including 0 mass%).

(Remainder)
The balance is iron and inevitable impurities. As inevitable impurities, elements brought in due to the conditions of raw materials, materials, manufacturing facilities, etc. are allowed to be mixed in. Representative examples of inevitable impurities include As, Sn, Sb, and H.
In addition, there are elements such as P and S that are usually treated as inevitable impurity elements, but whose composition ranges are separately defined as described above. Therefore, in this specification, the "unavoidable impurities" constituting the balance are a concept excluding elements whose composition ranges are separately defined.
As described above, the steel sheet according to the embodiment of the present invention contains only C, Si, Mn, and Al, other than the remaining iron and inevitable impurities. That is, it is characterized by not containing expensive additive elements such as Cu, Ni, Cr, Mo, Nb, V, and Ti. Note that "not containing additive elements" means that the target elements are not intentionally added, and allows a state in which they are not intentionally added but are contained at an impurity level.

(Relationship between Si content and Mn content)
In addition to limiting the ranges of the contents of the individual elements described above, the Si content and the Mn content satisfy the following formula (1): The inventors have found that by setting the ratio of the Si content to the square root of the Mn content ([Si]/√[Mn]) to 0.29 or more as defined in formula (1), the interface roughness index, the details of which will be described later, can be increased to a sufficiently large value.

0.29≦[Si]/√[Mn] (1)
Here, [Si] and [Mn] are the contents of Si and Mn expressed in mass %, respectively.

The parameter [Si]/√[Mn] is preferably 0.35 or more.

1A and 1B, the mechanism by which the interface roughness index can be increased by satisfying formula (1) will be described below. Note that this mechanism was considered by the inventors based on the knowledge currently available, but it is not intended to limit the technical scope of the present invention.
Fig. 1A is a schematic cross-sectional view showing an early stage of scale formation, and Fig. 1B is a schematic cross-sectional view showing a state in which scale formation has progressed to a certain extent and internal and external oxides have combined. In the early stage of scale formation, as shown in Fig. 1A, an internal oxide 2 which is _Fe2SiO4 is formed in a base steel 1, an external oxide, an FeO _{/ Fe2SiO4} eutectic compound ₃ , is formed on the surface of the base steel 1, and further wustite (FeO) ₄ is formed outside the FeO/ _Fe2SiO4 eutectic compound 3.

Then, as scale formation progresses further, as shown in Fig. 1B, the grown inner oxide 2 combines with the FeO/ _Fe2SiO4 eutectic compound ₃ to form scale 5 (which in Fig. 1B includes wustite 4, eutectic compound 3, and inner oxide 2), causing irregularities at the interface with the base steel 1. At this time, if the Si content is sufficiently high, the formation and growth of the inner oxide 2 and its combination with the FeO/ _Fe2SiO4 eutectic compound ₃ , which is the outer oxide, are promoted, and as a result, more irregularities can be produced at the interface between the base steel 1 and the scale 5.

On the other hand, Mn, which exists in the oxide by substituting part of the Fe site, has a stronger affinity with O than Fe. For this reason, increasing the Mn content inhibits the inward diffusion of O in the oxide, slowing down the growth of the internal oxide 2 and inhibiting the occurrence of irregularities at the interface between the base steel 1 and the scale 5. Therefore, it is desirable to reduce the Mn content.

The parameter that indicates both the sufficient Si content and the reduced Mn content described above is [Si]/√[Mn], and by satisfying formula (1), it is possible to generate a large amount of unevenness at the interface between the base steel 1 and the scale 5, as shown, for example, in schematic form in Figure 2, the details of which will be described later, and thus obtain a sufficiently large interface unevenness index.

2. Scale layer thickness
The presence of the scale layer suppresses the temperature rise of the steel during laser cutting, and the effect of reducing the occurrence of cutting defects such as notches and self-burning increases as the thickness of the scale layer on the steel sheet surface increases. Since a sufficient temperature rise suppression effect cannot be obtained when the scale layer thickness is less than 10 μm, the lower limit of the scale layer thickness is set to 10 μm.
On the other hand, the thicker the scale layer of the steel plate, the more significant the scale spalling during laser cutting. Even if the steel plate has the above-mentioned composition, if the scale layer thickness exceeds 60 μm, it becomes difficult to suppress the scale spalling during laser cutting, so the upper limit of the scale layer thickness is set to 60 μm. Such a scale layer thickness of 10 μm or more or 60 μm or more can be obtained by using the manufacturing method described later. The scale layer thickness is preferably 20 μm or more and 50 μm or less, and more preferably 20 μm or more and 40 μm or less.

In this specification, the "scale layer" includes an FeO/Fe2SiO4 eutectic compound formed at the interface between the base steel and an iron oxide layer such as wustite (FeO), magnetite ( _{Fe3O4 )} , and hematite ( _{Fe2O3 )} (e.g., wustite (FeO), magnetite ( _{Fe3O4 )} , and hematite ( _{Fe2O3 )} ) in _{Si- added} steel. In other words, the "scale layer thickness" means the total thickness of the iron oxide layer and the FeO/ _{Fe2SiO4 eutectic} compound layer.
Furthermore, when evaluating the scale layer thickness of a steel plate, the sample to be evaluated is taken, if possible, from a portion that is 300 mm or more away from the end in the plate width direction and 1000 mm or more away from the end in the longitudinal direction (rolling direction) so that the scale layer thickness of a representative portion of the steel plate can be evaluated.

<3. Interface roughness index＞
It is believed that scale peeling during laser cutting occurs when the thermal stress generated during cutting is greater than the adhesion force between the steel substrate and the scale. The adhesion force between the steel substrate and the scale is proportional to the adhesion area. Based on this, the inventors have determined that it is possible to suppress scale peeling and achieve laser cutting by controlling the interface roughness index, which is a parameter proportional to the adhesion area between the base steel and the scale and is defined by the following formula (2): It has been found that this improves the laser cutting properties, particularly when cutting out complex shapes. To fully obtain this effect, the interface roughness index is set to 1.33 or more. The interface roughness index is preferably set to 1.40 or more. and more preferably, 1.45 or more.

Interface roughness index=((interface contact length between base steel and scale layer)/(straight-line length of interface between base steel and scale layer)) ² (2)

As can be seen from formula (2), the interface roughness index is the square of the ratio of the "interface contact length between the base steel and the scale layer" to the "linear length of the interface between the base steel and the scale layer". FIG. 2 is a schematic cross-sectional view explaining the "interface contact length between the base steel and the scale layer" and the "linear length of the interface between the base steel and the scale layer" used to determine the interface roughness index. The interface roughness index is measured using a scanning electron microscope (SEM) photograph of a cross section including the interface between the base steel and the scale layer. Here, the "interface contact length between the base steel and the scale layer" refers to the length (total length) on the SEM photograph of the interface 10 between the base steel 1 and the scale layer 5, which is shown diagrammatically as a zigzag straight line in FIG. 2. The "linear length of the interface between the base steel and the scale layer" refers to the lateral length of the interface 10 between the base steel 1 and the scale layer 5 on the SEM photograph, as indicated by "L" in FIG. 2.
Furthermore, when evaluating the interface roughness index of a steel plate, the sample to be evaluated is taken, if possible, from a portion that is at least 300 mm away from the end in the plate width direction and at least 1000 mm away from the end in the longitudinal direction (rolling direction) so that the interface roughness index of a representative portion of the steel plate can be evaluated.

<4. Manufacturing method>
As described in detail below, the steel plate according to the embodiment of the present invention can be manufactured by preparing a steel material such as a cast slab having a predetermined composition, heating this steel material to a temperature of 1000°C or more and 1250°C or less, and then performing descaling with high-pressure water one or more times during hot rolling, performing at least three or more passes of rolling from the final descaling, setting the "cumulative reduction after final descaling", which is the sum of the reductions in each pass from the first rolling pass to the final rolling pass after the final descaling, to 80.0% or more, and setting the finish rolling temperature to 870°C or more.

[3-1. Preparation of steel material having a specified chemical composition]
A steel material having the composition shown in "1. Chemical composition" above is prepared for hot rolling in the next rolling step. The steel material may be a steel material that is normally used for hot rolling of thick plates. An example of such a steel material is a cast piece. Examples of cast pieces include a slab obtained by a continuous casting method and an ingot obtained by an ingot casting method using a mold. If necessary, these slabs and ingots may be subjected to surface treatment, heat treatment, processing treatment, etc. to prepare a steel material for rolling.

[3-2. Rolling]
The above-mentioned steel material is hot rolled.
(heating)
The steel material is heated to 1000° C. or more and 1250° C. or less. From the viewpoint of dissolving predetermined alloy elements in the steel material, the lower limit of the heating temperature is set to 1000° C. On the other hand, if the heating temperature exceeds 1250° C., the deformability of the scale during hot rolling decreases, deteriorating the scale properties, so the upper limit of the heating temperature is set to 1250° C.

(Hot rolling)
During hot rolling, descaling with high pressure water is performed one or more times. When scale broken into powder during rolling becomes hematite (Fe ₂ O ₃ ) and turns into red scale, which deteriorates laser cuttability, descaling with high pressure water is performed one or more times, and rolling is performed while removing the generated scale. The number of descaling times is at least one, i.e., it may be performed only once, or it may be performed multiple times. For example, water with a jet pressure of 10 MPa or more, preferably 15 MPa or more may be used as the high pressure water.

The properties of the scale formed on the final steel sheet product are mainly determined by the subsequent passes including the final pass, particularly the last three passes. If descaling is performed in one or more of the last three passes and the surface temperature of the steel sheet decreases, the growth of Fe ₂ SiO ₄ , which is an internal oxide, is inhibited, and a sufficiently high interface roughness index cannot be obtained. Therefore, after the last descaling of the above one or more descaling passes, at least three passes of rolling (hot rolling) are performed.

In addition, the cumulative reduction rate after final descaling, which is the sum of the reduction rates in each pass from the first rolling pass to the last rolling pass after the above-mentioned final descaling is performed, is set to 80.0% or more. The scale layer that forms on the steel sheet increases the interface unevenness index by being pushed into the base steel during rolling. By setting the cumulative reduction rate after final descaling as 80.0% or more, it is possible to make the interface unevenness index 1.33 or more.

The cumulative reduction is the sum of the reduction in each rolling pass, and can be calculated by adding up the reduction in each rolling pass calculated by the following formula (4).

Reduction rate (%) = (H ₀ - H ₁ )/H ₀ ×100 (4)
Here, _H0 is the entry thickness (mm) of the rolling pass, and _H1 is the exit thickness (mm) of the rolling pass.

Furthermore, the rolling temperature in the final pass, i.e., the finish rolling temperature, is set to 870°C or higher. If rolling is performed at a low temperature without descaling, the scale layer will break down into powder and turn into red scale, so the rolling temperature in the final pass is set to 870°C or higher. This allows the scale layer thickness of the resulting steel plate to be 10 μm or more and 60 μm or less.

In addition, the cooling after rolling is preferably performed by air cooling to a temperature of, for example, 300° C. or lower in order to more reliably suppress peeling of the scale layer formed during rolling. In the present invention, air cooling means cooling by leaving the rolled steel sheet in the air and allowing the heat to be removed by the surrounding air. The cooling rate may change depending on the season, but this does not affect the present invention.
Furthermore, if heat treatment is performed on the rolled steel sheet, the scale layer formed during rolling may peel off due to thermal stress generated by heating the steel sheet surface. Therefore, it is preferable not to perform heat treatment after hot rolling.

The temperature of the steel material described in the rolling process described above may be measured using a non-contact thermometer such as a radiation thermometer, or a contact thermometer such as a thermocouple. It may also be confirmed by simulation, etc.

The present invention will be described in more detail below with reference to examples. The present invention is not limited to the following examples, and can be modified as appropriate within the scope of the above and below-described aims, and all such modifications are within the technical scope of the present invention.

1. Sample Preparation Steels having the composition shown in Table 1 were melted in a converter. These molten steels were used to obtain slabs by continuous casting. Table 1 also lists the value of [Si]/√[Mn], which is the right side of formula (1). In Table 1, values that are outside the scope of the embodiment of the present invention are underlined.

The obtained slabs were heated to the heating temperatures shown in Table 2 and then hot-rolled to obtain steel plate samples (thick plates) having the plate thicknesses shown in Table 2.
More detailed hot rolling conditions, such as the number of descalings using high-pressure water, the number of rolling passes after final descaling (i.e., the number of rolling passes after the final descaling), the cumulative reduction after final descaling (i.e., the sum of the reductions in the passes from the first rolling pass to the last rolling pass after the final descaling) and the finish rolling temperature, are shown in Table 2. Furthermore, the cooling method to 300° C. after rolling is also shown in Table 2.
In Table 2, conditions that deviate from the conditions of the embodiment of the present invention are underlined.

The above heating temperature was the average temperature at the center of the slab when the slab was extracted, which was determined through prior investigation, and the time in the furnace (residence time) was set to be longer than the time required to achieve a finish rolling temperature of 870°C. The finish rolling temperature was the steel plate surface temperature immediately before the final pass of hot rolling, and was measured using a radiation thermometer.

2. Sample Evaluation The obtained steel sheet samples were subjected to measurement of scale layer thickness, measurement of interface unevenness index, and evaluation of laser cuttability by the methods detailed below.

<Measurement of scale layer thickness>
Samples for measuring the scale layer thickness were taken from a portion of each steel plate sample that was 300 mm or more away from the end in the plate width direction and 1000 mm or more away from the end in the longitudinal direction (rolling direction). An observation surface was prepared so that a 20 mm wide area could be observed in a cross section perpendicular to the rolling direction, and reflected electron images were taken from three fields of view (area of one field of view: 75 μm × 110 μm or more) at 1000x magnification using a scanning electron microscope (SEM).
In each visual field, the scale layer thickness of a representative portion was measured, and the average of the measured values in the three visual fields was taken as the scale layer thickness of each steel sheet sample. As described above, in this specification, the "scale layer" includes the FeO/ _Fe2SiO4 eutectic compound formed at the interface between the base steel and an iron oxide layer such as wustite (FeO), magnetite ( _{Fe3O4 )} , and hematite ( _Fe2O3 ) _, in Si _- added steel. The "scale layer thickness" means the total thickness of the iron oxide layer and the FeO/ _{Fe2SiO4 eutectic} compound layer.
The scale layer thickness of each steel plate sample is shown in Table 3. Among the scale layer thicknesses shown in Table 3, those that deviate from the embodiment of the present invention are underlined.

<Measurement of interface roughness index>
Using the three fields of backscattered electron images at 1000x magnification of each steel sheet sample used in the above "Measurement of scale layer thickness", the interfacial contact length between the base steel and the scale layer and the linear length of the interface between the base steel and the scale layer were measured as described above with reference to Figure 2, and the interface unevenness index for each field of view was determined using formula (2). The average value of the interface unevenness index for the three fields of view was then taken as the interface unevenness index for each steel sheet sample.
The interface unevenness index of each steel sheet sample is shown in Table 3. Among the interface unevenness indices shown in Table 3, those that deviate from the embodiment of the present invention are underlined.

<Laser cuttability evaluation>
The laser cuttability was tested using a 6kW- _CO2 laser cutting machine. A sample for evaluating laser cuttability was prepared by cutting each steel plate sample to a size of 300 mm long x 200 mm wide. The surface state of the sample was left black. FIG. 3 is a schematic plan view showing the laser cut portion. As shown in FIG. 3, the laser cutting was performed by moving 120 mm in the length direction (vertical direction in FIG. 3) from the piercing position A of the sample for evaluating laser cuttability (100), then changing the direction and moving 20 mm in the width direction (horizontal direction in FIG. 3), then changing the direction and returning 120 mm in the length direction, moving or returning in the length direction a total of seven times, and moving in the width direction a total of six times, to cut to position B.
In this case, as shown in FIG. 3, the bending portion when changing direction from the first to fourth times was 5R, the bending portion when changing direction from the fifth to eighth times was 2.5R, and the bending portion when changing direction from the ninth to twelfth times was 1R.

The laser cutting conditions were as follows according to the plate thickness:
Plate thickness 16 mm: Output 3.80 kW, Frequency 850 Hz, Duty 78%, Cutting speed 880 mm/min. Plate thickness 19 mm: Output 4.25 kW, Frequency 850 Hz, Duty 78%, Cutting speed 820 mm/min. Plate thickness 25 mm: Output 4.50 kW, Frequency 850 Hz, Duty 82%, Cutting speed 650 mm/min.

Samples in which self-burning occurred, with the width of the cut groove on the steel plate surface expanding to 2 mm or more and dross adhering to the back surface, were deemed to have poor laser cuttability and were marked with an "X" in Table 3. On the other hand, samples that did not suffer from such cutting defects were deemed to have good laser cuttability and were marked with an "O" in Table 3.

From Tables 1 to 3, the following can be considered.
Steel plate samples No. 1 to 6 all satisfied all of the requirements for chemical composition and manufacturing conditions stipulated in the embodiment of the present invention. As a result, as shown in Table 3, the scale layer thickness and interface roughness index satisfied the values stipulated in the embodiment of the present invention, and good laser cuttability was obtained.

On the other hand, the steel plate samples Nos. 7 to 11 do not satisfy the requirements according to the embodiment of the present invention in the following respects, and therefore, sufficient laser cuttability was not obtained.
Steel plate sample No. 7 has an insufficient Si content and does not satisfy formula (1). Furthermore, the cumulative reduction rate after final descaling is also insufficient. Furthermore, the interface roughness index is insufficient.

Steel plate sample No. 8 has an insufficient Si content and does not satisfy formula (1). The cumulative reduction rate after final descaling is also insufficient. Furthermore, the interface roughness index is insufficient.
Steel sheet sample No. 9 has an insufficient Si content and does not satisfy formula (1). Furthermore, the cumulative reduction rate after final descaling is also insufficient. Furthermore, the interface roughness index is insufficient.

Steel plate sample No. 10 has an insufficient Si content and does not satisfy formula (1). Furthermore, the finish rolling temperature is too low, and the cumulative reduction rate after final descaling is also too small. Furthermore, the scale layer thickness and interface roughness index are too small.
Steel sheet sample No. 11 has an insufficient Si content and does not satisfy formula (1). Furthermore, the finish rolling temperature is too low, and the cumulative reduction rate after final descaling is also too small. Furthermore, the interface irregularity index is too small.

1 Base iron 2 Inner oxide 3 FeO/ _{Fe2SiO4 eutectic} compound
4 Wustite (FeO)
5 Scale layer 10 Interface between base steel and scale layer 100 Laser cuttability evaluation sample

Claims (2)

C: 0.10% by mass or more and 0.20% by mass or less,
Si: 0.30% by mass or more and 0.60% by mass or less,
Mn: 0.30% by mass or more and 1.50% by mass or less,
P: 0.030% by mass or less (including 0% by mass),
S: 0.010% by mass or less (including 0% by mass),
Al: 0.005% by mass or more and 0.050% by mass or less,
N: 0.0060% by mass or less (including 0% by mass),
The balance is Fe and unavoidable impurities,
The Si content and the Mn content satisfy the following formula (1),
A scale layer having a thickness of 10 μm or more and 60 μm or less on the surface,
A steel sheet having an interface unevenness index of 1.33 or more, as defined by the following formula (2):

0.29≦[Si]/√[Mn] (1)
Here, [Si] and [Mn] are the contents of Si and Mn expressed in mass %, respectively.

Interface roughness index=((interface contact length between base steel and scale layer)/(straight-line length of interface between base steel and scale layer)) ² (2) C: 0.10% by mass or more and 0.20% by mass or less,
Si: 0.30% by mass or more and 0.60% by mass or less,
Mn: 0.30% by mass or more and 1.50% by mass or less,
P: 0.030% by mass or less (including 0% by mass),
S: 0.010% by mass or less (including 0% by mass),
Al: 0.005% by mass or more and 0.050% by mass or less,
N: 0.0060% by mass or less (including 0% by mass),
and the balance being Fe and unavoidable impurities, and the Si content and the Mn content satisfy the following formula (1):
The steel material is heated to a temperature of 1000° C. or more and 1250° C. or less, and then hot-rolled. Descaling with high-pressure water is performed once or more, and at least three passes of rolling are performed from the final descaling. and a cumulative reduction rate after final descaling, which is the sum of the reduction rates in each pass from the first rolling pass to the last rolling pass after the final descaling, is set to 80.0% or more. and performing hot rolling at a finish rolling temperature of 870° C. or higher;
Including ,
However, for at least the first three stands from the entry side of the finishing rolling mill row, cooling shall be performed at a position within 2 m in the longitudinal direction of the steel plate from the roll bite at the entry side of each stand and within 4 s immediately before the steel plate is bitten by the roll. The injection of cooling water is started, and at the delivery side of each stand, the injection of cooling water is ended within 1 m in the longitudinal direction of the steel sheet from the roll bite and within 1.5 s immediately after the steel sheet is bitten by the rolls. A method for manufacturing a steel sheet having a scale layer having a thickness of 10 μm or more and 60 μm or less on a surface thereof, and an interface roughness index defined by the following formula (2) of 1.33 or more.

0.29≦[Si]/√[Mn] (1)
Here, [Si] and [Mn] are the contents of Si and Mn expressed in mass %, respectively.

Interface roughness index=((interface contact length between base steel and scale layer)/(straight-line length of interface between base steel and scale layer)) ² (2)

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