JP7144173B2 - Steel sheet with excellent base material toughness and surface properties and its manufacturing method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a steel sheet having excellent base material toughness and surface properties, and a method for producing the same.

Steel sheets produced by hot rolling form scales during heating and hot rolling. Although the scale is removed by descaling during hot rolling, there are cases where the scale is not sufficiently removed. In this case, the thickness of the scale layer on the surface of the steel sheet tends to be uneven (uneven scale). If reduction is performed in a state in which this scale unevenness has occurred, variations in characteristic values such as the strength of the steel sheet may occur. In addition, if controlled cooling is performed in a state in which scale unevenness has occurred, cooling unevenness and temperature unevenness after controlled cooling may occur, and in this case also variations may occur in the characteristic values of the steel sheet.

As a technique for suppressing scale unevenness on the steel plate surface, for example, Patent Document 1 can be cited. Patent Document 1 relates to a method for manufacturing a yield strength controlled steel plate for construction, and hot rolling is performed by rough rolling and controlled rolling starting from a temperature range of 950 ° C. or less for a billet satisfying a predetermined chemical composition. In the step of rolling and then accelerated cooling, the steel billet is passed through the rolling mill only by descaling without reduction, and after one second or more has passed after descaling, without reduction again or without reduction It is disclosed that the rolling and descaling steps are performed at least one or more times between passes just prior to controlled rolling or at the beginning of controlled rolling.

JP-A-10-212521

Steel sheets used for structures such as buildings and bridges are also required to have excellent base material toughness. However, Patent Document 1 does not refer to ensuring excellent base material toughness. Further, in the manufacturing method of Patent Document 1, it is difficult to say that the productivity is sufficiently high because a pass for descaling without reduction is essential.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a steel sheet that achieves both excellent base material toughness and excellent surface properties, and a method for producing the same.

In aspect 1 of the present invention, the component composition is
C: 0.03 to 0.17% by mass,
Si: 0.15 to 0.60% by mass,
Mn: 1.0 to 2.0% by mass,
P: more than 0% by mass, 0.030% by mass or less,
S: more than 0% by mass, 0.005% by mass or less,
Al: 0.010 to 0.080% by mass,
N: 0.0010 to 0.0100% by mass,
Ti: more than 0% by mass, 0.03% by mass or less, and Ca: more than 0% by mass, 0.01% by mass or less, the balance being iron and unavoidable impurities,
Ceq determined by the following formula (1) is 0.300% by mass or more and 0.420% by mass or less,
The scale layer on the surface of the steel sheet is formed in the order of a powdery scale layer and a non-powderous scale layer from the surface side, or is formed only by the non-powderous scale layer, and the thickness of the powdery scale layer is 10 μm or less. suppressed, and
It is a steel plate with excellent base material toughness and surface properties, with a Charpy impact absorption energy of 120 J or more at −40° C. at the position of plate thickness/4.
Ceq=[C]+[Mn]/6+[Si]/24+[Ni]/40+[Cr]/5+[Mo]/4+[V]/14 (1)
provided that [C], [Mn], [Si], [Ni], [Cr], [Mo] and [V] are C, Mn, Si, Ni, Cr, Mo and The content of V is shown, and elements not included are set to 0.

Aspect 2 of the present invention further comprises
Cu: more than 0% by mass, 1.5% by mass or less,
Ni: more than 0% by mass, 1.5% by mass or less,
Cr: more than 0% by mass, 1.5% by mass or less,
Mo: more than 0% by mass, 1.5% by mass or less,
V: more than 0% by mass and 0.1% by mass or less,
The steel sheet according to aspect 1, containing one or more elements selected from the group consisting of Nb: more than 0% by mass and 0.06% by mass or less, and B: more than 0% by mass and 0.005% by mass or less. .

Aspect 3 of the present invention further comprises
3. The steel sheet according to aspect 1 or 2, comprising one or more elements selected from REM: more than 0% by mass and 0.01% by mass or less, and Zr: more than 0% by mass and 0.010% by mass or less.

Aspect 4 of the present invention is a method for producing the steel plate according to any one of aspects 1 to 3,
Using a slab having the chemical composition according to any one of aspects 1 to 3, a heating step, a first hot rolling step, a second hot rolling step in which rolling is performed in a plurality of rolling passes, and a controlled cooling step are performed in this order. including
After the heating step and before the first hot rolling step; and during rolling in the first hot rolling step; performed one or more times, and
This is a method for producing a steel sheet excellent in base material toughness and surface properties, in which the second hot rolling step is performed so as to satisfy all of the following (a) to (c).
(a) 10 seconds or more before the start of the second hot rolling or between at least one of a plurality of rolling passes (excluding between the final pass and the immediately preceding rolling pass) Air cooling is performed for 120 seconds or less, and the temperature at the plate thickness/4 position is set to 900° C. or higher and 940° C. or lower.
(b) Rolling is performed in a temperature range of 900° C. or higher and 940° C. or lower at the thickness/4 position so that the cumulative rolling reduction is 20% or higher and 67% or lower.
(c) performing a second descaling in the rolling pass after the air cooling;

INDUSTRIAL APPLICABILITY The present invention can provide a steel sheet having both excellent surface properties and base material toughness, and a method for producing the same. Since the steel sheet of the present invention has excellent surface properties, it is possible to suppress the occurrence of cooling unevenness and the like in the manufacturing process, and it is possible to suppress variations in the characteristic values of the steel sheet.

FIG. 1 is a graph showing the relationship between the thickness of the powdery scale layer and the difference from the average cooling stop temperature. FIG. 2 is a graph showing the relationship between the air cooling time and the cumulative rolling reduction in the temperature range of 900° C. or more and 940° C. or less at the plate thickness/4 position. FIG. 3 is a graph showing the relationship between the cumulative reduction rate in the temperature range of 900° C. or higher and 940° C. or lower at the position of thickness/4 and the Charpy impact absorption energy at -40° C. at the position of thickness/4. be. FIG. 4A is a diagram showing the results of measuring the difference from the average cooling stop temperature at each location in the width direction of the steel sheet when descaling was not performed after air cooling. FIG. 4B is a diagram showing the results of measuring the difference from the average cooling stop temperature at each location in the width direction of the steel sheet when descaling was performed after air cooling. FIG. 5 is a diagram illustrating thickness measurement positions of the powdery scale layer in the example.

As for the characteristics of steel sheets, there is knowledge to improve both the surface texture and the toughness of the base material, but it is difficult to achieve both excellent surface texture and toughness of the base material. The present inventors have made extensive studies to obtain a steel sheet that achieves both excellent surface properties and base material toughness, and a method for producing the same. As a result, first of all, the excellent surface quality means that the scale layer on the surface of the steel sheet is formed in the order of a powder scale layer and a non-powder scale layer from the surface side, or is formed only by a non-powder scale layer. , the thickness of the powdery scale layer is suppressed to 10 μm or less.

The surface texture will be described below. Reduction and cooling in a state in which scale unevenness occurs as described above can be cited as a cause of deterioration of the surface properties of the steel sheet, specifically flatness and material homogeneity. Therefore, it is important to suppress scale unevenness in order to obtain excellent surface properties. Regarding this scale unevenness, the present inventors have found that (i) the scale layer formed on the surface of the steel sheet can be roughly divided into a powdery scale layer and a non-powderous scale layer, and (ii) as the thickness of the powdery scale layer increases, Furthermore, it was found that scale unevenness is likely to occur, causing cooling unevenness in the manufacturing process, etc. Therefore, it was found that it is necessary to suppress the formation of a powdery scale layer among the scale layers formed on the surface of the steel sheet.

In addition, it was found that the powdery scale was formed by the growth of the scale remaining unevenly after descaling, which was pulverized by subsequent rolling. Therefore, in order to suppress the generation of this powdery scale, it is necessary to perform appropriate descaling. A specific descaling method will be described in detail in the manufacturing method described later. Powdery scale and non-powderous scale can be distinguished by observation with an optical microscope, as shown in Examples below.

In Patent Document 1, red scale and light black ash scale are classified, but instead of classifying by the color of the scale in this way, the patent document refers to non-powder scale and powder scale in order from the base iron side. From the viewpoint of achieving both excellent surface properties and base material toughness, it is important to judge the scale properties by a classification different from 1 and to control the thickness of the powdery scale.

FIG. 1 is a graph showing the relationship between the thickness of the powdery scale layer and the difference from the average cooling stop temperature, which was obtained as follows. That is, Test No. of Examples described later. Using the steel plate of E, the temperature of the steel plate surface immediately after accelerated cooling and before air cooling to room temperature and the scale of the steel plate obtained by air cooling to room temperature were observed at a pitch of 100 mm in the width direction of the same steel plate. The thickness of the scale layer was measured. In the present invention, the temperature of the steel plate surface was measured with a radiation thermometer. The thickness of the powdery scale layer was measured as shown in Examples below. Then, the average value of the temperatures at each measurement position in the width direction is defined as the "average cooling stop temperature", and the absolute value of the difference between the measured temperature at each measurement position and the average cooling stop temperature is defined as the "average cooling stop temperature." difference.

As shown in FIG. 1, as the thickness of the powdery scale layer increases, the difference from the average cooling stop temperature increases. This means that as the thickness of the powdery scale layer increases, the temperature deviation after accelerated cooling increases, that is, the cooling stop temperature unevenness occurs after accelerated cooling. When cooling stop temperature unevenness occurs after accelerated cooling, deterioration of material homogeneity and deterioration of flatness after cooling are likely to occur. From FIG. 1, it can be seen that it is important to reduce the thickness of the powdery scale layer in order to reduce the difference from the average cooling stop temperature.

Further, from FIG. 1, it can be seen that when the thickness of the powdery scale layer exceeds 10 μm, the difference from the average cooling stop temperature tends to increase significantly. Therefore, in the present invention, the thickness of the powdery scale layer is set to 10 μm or less. The thickness of the powdery scale layer is preferably 8 μm or less, more preferably 5 μm or less, and most preferably the thickness of the powdery scale layer is 0 μm, that is, the scale layer on the surface of the steel sheet has no powdery scale, The reason is that the scale layer on the surface of the steel sheet is formed of only a non-powder scale layer.

In addition, when heat treatment such as normalizing, quenching, and tempering is performed, the scale properties change due to exposure to high temperatures during the heat treatment. Shall specify scale properties.

In addition to the excellent surface properties of the steel sheet, as mentioned above, it is also necessary to ensure excellent base metal toughness. Therefore, in the present invention, the Charpy impact absorption energy at -40 ° C. at the position of plate thickness / 4, specifically, the absorption energy when performing the Charpy impact test at -40 ° C. at the position of t / 4 of the steel plate ( Hereinafter, it may simply be referred to as “absorbed energy at −40° C.”) is also specified to achieve 120 J or more. The absorbed energy is preferably 150 J or more, more preferably 200 J or more.

Next, the chemical composition of the steel sheet of the present invention will be explained.

C: 0.03 to 0.17% by mass
C is an essential element for ensuring the strength of the base material and the weld zone, and should be contained in an amount of 0.03% by mass or more. The amount of C is preferably 0.04% by mass or more, more preferably 0.05% by mass or more. On the other hand, if the amount of C is too large, the base metal toughness and weldability deteriorate. Therefore, the amount of C should be 0.17% by mass or less. It is preferably 0.16% by mass or less, more preferably 0.15% by mass or less.

Si: 0.15 to 0.60% by mass
Si is an element that has a deoxidizing effect and is effective in improving the strength of the base metal and the weld zone. In order to obtain these effects, the amount of Si is set to 0.15% by mass or more. The amount of Si is preferably 0.20% by mass or more, more preferably 0.25% by mass or more. However, if the amount of Si is too large, weldability and toughness deteriorate. On the other hand, if the amount of Si is excessive, the adhesion between the scale and the base material increases, making it difficult to control the scale. Therefore, the Si content should be suppressed to 0.60% by mass or less. The Si content is preferably 0.55% by mass or less, more preferably 0.50% by mass or less.

Mn: 1.0 to 2.0% by mass
Mn is an element effective for improving the strength of the base material and the weld zone, and is contained in an amount of 1.0% by mass or more in the present invention. The Mn content is preferably 1.05% by mass or more, more preferably 1.10% by mass or more. However, if the amount of Mn is too large, not only is MnS generated and the toughness of the base material deteriorates, but also the toughness of the HAZ (HEAT-AFFECTED ZONE) formed when the steel plate of the present invention is welded. Weldability deteriorates. Therefore, the upper limit of the amount of Mn is set to 2.0% by mass. The Mn content is preferably 1.9% by mass or less, more preferably 1.8% by mass or less, and even more preferably 1.70% by mass or less.

P: more than 0% by mass and 0.030% by mass or less P is an element that is unavoidably contained in steel materials. Therefore, in the present invention, the amount of P is suppressed to 0.030% by mass or less. The amount of P is preferably 0.025% by mass or less, more preferably 0.020% by mass or less.

S: more than 0% by mass, 0.005% by mass or less S is an element that generates a large amount of MnS and significantly deteriorates the toughness of the base material if it is too much, so in the present invention, the upper limit of the S amount is 0.005% by mass. and The S content is preferably 0.003% by mass or less.

Al: 0.010 to 0.080% by mass
Al is a strong deoxidizing element and is an element necessary for deoxidizing molten steel. In the present invention, Al must be 0.010% by mass or more. The Al content is preferably 0.015% by mass or more, more preferably 0.020% by mass or more. On the other hand, if the Al content is too high, Al oxides are formed in clusters, resulting in deterioration of the toughness of the base material. Therefore, the Al content should be 0.080% by mass or less. The Al content is preferably 0.070% by mass or less, more preferably 0.060% by mass or less.

N: 0.0010 to 0.0100% by mass
N is an element that precipitates as TiN in steel and suppresses coarsening of austenite grains during heating to improve the toughness of the base material. In addition, the precipitation of TiN suppresses coarsening of austenite grains in the HAZ, promotes ferrite transformation, and improves the toughness of the HAZ. In order to obtain these effects, it is necessary to contain 0.0010% by mass or more of N. The amount of N is preferably 0.0020% by mass or more, more preferably 0.0030% by mass or more, and still more preferably 0.0040% by mass or more. However, if the amount of N is too large, the presence of solute N deteriorates the toughness of the base material, so the amount of N must be 0.0100% by mass or less. The N content is preferably 0.0080% by mass or less, more preferably 0.0070% by mass or less.

Ti: more than 0% by mass and 0.03% by mass or less Ti is an element that precipitates as TiN in steel and suppresses coarsening of austenite grains during heating to improve the toughness of the base material. In addition, the precipitation of TiN suppresses coarsening of austenite grains in the HAZ, promotes ferrite transformation, and improves the toughness of the HAZ. In order to obtain these effects, the amount of Ti should be more than 0% by mass. The amount of Ti is preferably 0.003% by mass or more, more preferably 0.005% by mass or more, and still more preferably 0.010% by mass or more. On the other hand, if the amount of Ti is excessive, the toughness of the base material deteriorates due to the presence of Ti in solid solution and the precipitation of TiC. The Ti content is preferably 0.020% by mass or less, more preferably 0.015% by mass or less.

Ca: more than 0% by mass and 0.01% by mass or less Ca is an element effective in suppressing the formation of MnS through desulfurization and increasing the toughness of the base material. In order to obtain this effect, the amount of Ca is made more than 0% by mass. The amount of Ca is preferably 0.0003% by mass or more, more preferably 0.0010% by mass or more. On the other hand, if Ca is contained excessively, coarse inclusions are formed and the toughness of the base material deteriorates, so the upper limit of the amount of Ca is made 0.01% by mass. The amount of Ca is preferably 0.0050% by mass or less, more preferably 0.0030% by mass or less, and even more preferably 0.0025% by mass or less.

The higher the carbon equivalent Ceq, the higher the strength. In the steel sheet of the present invention, Ceq is adjusted in addition to each component composition being within the above content range. If the Ceq is too low, it becomes difficult to stably ensure the strength required for the structure, so the Ceq is made 0.300% by mass or more. It is preferably 0.310% by mass or more, more preferably 0.320% by mass or more, and still more preferably 0.330% by mass or more. On the other hand, if Ceq is too high, hardening of the base material and HAZ occurs, leading to deterioration of base material toughness and HAZ toughness. It is preferably 0.410% by mass or less, more preferably 0.400% by mass or less.

The above Ceq can be calculated by the following formula (1).
Ceq=[C]+[Mn]/6+[Si]/24+[Ni]/40+[Cr]/5+[Mo]/4+[V]/14 (1)
provided that [C], [Mn], [Si], [Ni], [Cr], [Mo] and [V] are C, Mn, Si, Ni, Cr, Mo and The content of V is shown, and elements not included are set to 0.

The basic components of the steel sheet of the present invention are as described above, and the balance is iron and unavoidable impurities (eg, O, Sb, etc.). Unavoidable impurities are elements brought in depending on the conditions of raw materials, materials, manufacturing equipment, and the like. For example, there are elements, such as P and S, whose content is generally preferably as low as possible and thus are unavoidable impurities, but whose composition range is separately defined as described above. For this reason, in this specification, "inevitable impurities" constituting the balance exclude elements whose composition range is separately defined.

The steel sheet of the present invention may further contain the following elements as necessary.

Cu: more than 0% by mass, 1.5% by mass or less,
Ni: more than 0% by mass, 1.5% by mass or less,
Cr: more than 0% by mass, 1.5% by mass or less,
Mo: more than 0% by mass, 1.5% by mass or less,
V: more than 0% by mass and 0.1% by mass or less,
One or more elements selected from the group consisting of Nb: more than 0% by mass and 0.06% by mass or less and B: more than 0% by mass and 0.005% by mass or less

All of these elements are elements that improve hardenability.

Cu is an element effective in improving hardenability and increasing strength. In order to obtain this effect, the Cu content is preferably more than 0% by mass, more preferably 0.01% by mass or more. The Cu content is more preferably 0.05% by mass or more, still more preferably 0.10% by mass or more. However, if the amount of Cu exceeds 1.5% by mass, the toughness deteriorates, so the amount of Cu is preferably 1.5% by mass or less. The Cu content is more preferably 1.0% by mass or less, still more preferably 0.50% by mass or less, and even more preferably 0.25% by mass or less.

Ni is an element effective in improving the strength and toughness of the base material and weld zone. In order to obtain this effect, the Ni content is preferably more than 0% by mass, more preferably 0.01% by mass or more. The amount of Ni is more preferably 0.05% by mass or more, still more preferably 0.10% by mass or more. However, if a large amount of Ni is contained, the adhesion between the scale and the base material increases, making it difficult to control the scale. The Ni content is more preferably 1.0% by mass or less, still more preferably 0.50% by mass or less, and even more preferably 0.30% by mass or less.

Cr is an element effective in improving strength, and in order to obtain this effect, the Cr content is preferably more than 0% by mass, more preferably 0.01% by mass or more. The Cr content is more preferably 0.05% by mass or more, and still more preferably 0.10% by mass or more. On the other hand, when the amount of Cr exceeds 1.5% by mass, the toughness of the base material deteriorates. Therefore, the Cr content is preferably 1.5% by mass or less. The Cr content is more preferably 1.0% by mass or less, still more preferably 0.50% by mass or less, and even more preferably 0.30% by mass or less.

Mo is an element effective in improving the strength and toughness of the base material. To obtain this effect, the Mo content is preferably more than 0% by mass, more preferably 0.01% by mass or more. Mo content is more preferably 0.05% by mass or more, still more preferably 0.10% by mass or more. However, if the amount of Mo exceeds 1.5% by mass, the base metal toughness and weldability deteriorate. Therefore, the Mo content is preferably 1.5% by mass or less, more preferably 1.0% by mass or less, still more preferably 0.50% by mass or less, and even more preferably 0.25% by mass or less.

V is an element effective in improving strength, and in order to obtain this effect, the amount of V is preferably more than 0% by mass, more preferably 0.003% by mass or more, and still more preferably 0.010% by mass. % or more. On the other hand, if the V content exceeds 0.1% by mass, weldability and base metal toughness deteriorate. Therefore, the V content is preferably 0.1% by mass or less, more preferably 0.08% by mass or less.

Nb is an effective element for increasing strength and base metal toughness without deteriorating weldability. To obtain this effect, the Nb content is preferably more than 0% by mass, more preferably 0.002% by mass or more. The Nb content is more preferably 0.005% by mass or more. However, if the amount of Nb exceeds 0.06% by mass, the toughness of the base metal and HAZ deteriorates. Therefore, in the present invention, it is preferable to set the upper limit of the Nb amount to 0.06% by mass. The Nb content is more preferably 0.050% by mass or less, still more preferably 0.040% by mass or less, still more preferably 0.035% by mass or less, and particularly preferably 0.030% by mass or less.

B has the effect of increasing the hardenability and increasing the strength of the base material and the weld zone. Furthermore, during welding, B combines with N to precipitate BN in the process of cooling the heated HAZ, and promotes ferrite transformation from within the austenite grains, so it also has the effect of improving the toughness of the HAZ. In order to obtain these effects, the amount of B is preferably more than 0% by mass, preferably 0.0002% by mass or more. The amount of B is more preferably 0.0005% by mass or more, and still more preferably 0.0010% by mass or more. However, if the B content is excessive, the toughness of the base metal and the weldability are deteriorated, so the B content is preferably 0.005% by mass or less. The amount of B is more preferably 0.004% by mass or less, still more preferably 0.0030% by mass or less.

One or more elements selected from REM: more than 0% by mass and 0.01% by mass or less and Zr: more than 0% by mass and 0.010% by mass or less

These elements are elements that control the formation of inclusions and contribute to the toughness of the base material.

REM (rare earth element) is an element effective in suppressing the formation of MnS and increasing the toughness of the base material through desulfurization. In order to exhibit such an effect, the content of REM is preferably more than 0% by mass, more preferably 0.0002% by mass or more. The REM amount is more preferably 0.0005% by mass or more, and even more preferably 0.0010% by mass or more. On the other hand, even if REM is contained in a large amount, the effect is saturated. Therefore, the upper limit of the amount of REM is preferably 0.01% by mass. From the viewpoint of suppressing clogging of the submerged nozzle during casting and improving productivity, the amount of REM is more preferably 0.0050% by mass or less, still more preferably 0.0030% by mass or less, and even more preferably 0.0050% by mass or less. 0020% by mass or less. In the present invention, REM means lanthanoid elements (15 elements from La to Lu), Sc (scandium) and Y.

Zr is an element that contributes to the improvement of base material toughness by forming oxides by deoxidizing action and finely dispersing them. In order to exhibit these effects, the Zr content is preferably more than 0% by mass, more preferably 0.0002% by mass or more. The Zr content is more preferably 0.0003% by mass or more, still more preferably 0.0005% by mass or more, and particularly preferably 0.0010% by mass or more. On the other hand, if Zr is added excessively, coarse inclusions are formed and the toughness of the base material deteriorates. Therefore, the Zr content is preferably 0.010% by mass or less. The Zr content is more preferably 0.0050% by mass or less, still more preferably 0.0030% by mass or less, and even more preferably 0.0020% by mass or less.

The steel sheet of the present invention has a thickness of, for example, 10 mm or more and 40 mm or less, and further 35 mm or less. When the plate thickness is within the above range, the steel sheet defined in the present invention is easily obtained by the manufacturing method defined in the present invention.

The steel sheet of the present invention having the properties described above is suitably used for shipbuilding, construction, bridges, offshore structures and the like.

The manufacturing method of the present invention is a method for manufacturing the steel sheet, wherein a billet having the chemical composition is used, and the second hot rolling is carried out in a heating step, a first hot rolling step, and a plurality of rolling passes. and a controlled cooling step in that order,
After the heating step and before the first hot rolling step; and during rolling in the first hot rolling step; one or more times, and
It is characterized in that the second hot rolling step is performed so as to satisfy all of the following (a) to (c).
(a) 10 seconds or more before the start of the second hot rolling or between at least one of a plurality of rolling passes (excluding between the final pass and the immediately preceding rolling pass) Air cooling is performed for 120 seconds or less, and the temperature at the plate thickness/4 position is set to 900° C. or higher and 940° C. or lower.
(b) Rolling is performed in a temperature range of 900° C. or higher and 940° C. or lower at the thickness/4 position so that the cumulative rolling reduction is 20% or higher and 67% or lower.
(c) performing a second descaling in the rolling pass after the air cooling;

Hereinafter, the method for manufacturing a steel sheet according to the present invention will be described in the order of steps.

(Heating process)
The heating temperature of the slab in the heating process can be appropriately selected according to the type of steel sheet. For example, heating at 1000 to 1250°C can be mentioned.

(First hot rolling step)
In the first hot rolling process (rough rolling process), the slab heated in the heating process is subjected to reciprocating or unidirectional rolling. After the heating step and before the first hot rolling step; During rolling in the first hot rolling step; Scale one or more times. A detailed description will be given below.

In order to remove the scale generated in the heating step, descaling is performed once or more at a slab surface temperature in the range of 1000 to 1200° C. before the first hot rolling. Furthermore, during rolling in the first hot rolling step, descaling is performed once or more while the slab surface temperature is in the range of 1000 to 1200°C. In any descaling, it is preferable to set the collision pressure to 5.0 MPa or more. The collision pressure is more preferably 7.0 MPa or higher. The descaling of the collision pressure includes descaling at high water pressure.

The number of times of descaling before the first hot rolling is preferably one from the viewpoint of productivity. Rolling in the first hot rolling step can be performed one or more times. That is, rolling can be performed in a plurality of rolling passes, and in this case, descaling during rolling in the first hot rolling step can be performed between rolling passes. The upper limit of the number of times of descaling during rolling can be, for example, 5 times.

Conditions other than the above in the first hot rolling step are not particularly limited, and conventional conditions such as rolling reduction can be adopted. For example, rolling is performed at a temperature range of 1000° C. or higher and 1200° C. or lower at a thickness/4 temperature range with a cumulative rolling reduction of 10 to 90%.

(Second hot rolling step)
In the second hot rolling process (finish rolling process), the steel sheet that has undergone the first hot rolling process is rolled in a plurality of rolling passes. In order to achieve both the suppression of powdery scale and the securing of excellent base material toughness, the second hot rolling step must be carried out so as to satisfy all of the following (a) to (c).
(a) 10 seconds or more before the start of the second hot rolling or between at least one of a plurality of rolling passes (excluding between the final pass and the immediately preceding rolling pass) Air cooling is performed for 120 seconds or less, and the temperature at the plate thickness/4 position is set to 900° C. or higher and 940° C. or lower.
(b) Rolling so that the thickness/4 position (hereinafter sometimes referred to as “plate thickness/4 position”) is in the temperature range of 900 ° C. or higher and 940 ° C. or lower, and the cumulative rolling reduction is 20% or higher and 67% or lower I do.
(c) Descaling is performed in the rolling pass after the air cooling.

Each of the conditions (a) to (c) will be described below.

(a) 10 seconds or more before the start of the second hot rolling or between at least one of a plurality of rolling passes (excluding between the final pass and the immediately preceding rolling pass) Air cooling is performed for 120 seconds or less, and the temperature at the plate thickness/4 position is set to 900° C. or higher and 940° C. or lower.

In order to ensure excellent base material toughness, as described later, the cumulative rolling reduction (hereinafter sometimes simply referred to as “cumulative rolling reduction”) in the temperature range of 900 ° C. or higher and 940 ° C. or lower at the plate thickness / 4 positions It is necessary to secure more than a certain amount. In order to lower the temperature at the plate thickness/4 position to the temperature range of 900° C. or higher and 940° C. or lower after the first hot rolling step, it is necessary to provide air cooling in the second hot rolling step.

Air-cooling time shall be 10 seconds or more and 120 seconds or less. By setting the air-cooling time to 10 seconds or more, the temperature at the plate thickness/4 position can be cooled to 940° C. or less, and the cumulative draft described later can be secured. The air cooling time is preferably 15 seconds or longer, more preferably 20 seconds or longer. On the other hand, when the air-cooling time exceeds 120 seconds, the scale grows, so even if descaling is performed after air-cooling, appropriate scale control cannot be performed and powdery scale remains. The air cooling time is preferably 115 seconds or less, more preferably 110 seconds or less.

FIG. 2 is a graph showing the relationship between the air cooling time and the cumulative rolling reduction in the temperature range of 900° C. or more and 940° C. or less (thickness/4 positions). FIG. 2 shows the cumulative rolling reduction in the temperature range of 900° C. or higher and 940° C. or lower when rolling is performed with the same number of passes after performing air cooling for various air cooling times. As shown in FIG. 2, when the air-cooling time is less than 10 seconds, the rolling is performed in a state where the temperature of the steel sheet does not drop sufficiently, that is, rolling at a high temperature exceeding 940 ° C. The cumulative rolling reduction rate in the temperature range of ℃ to 940℃ cannot be ensured.

Air cooling for the predetermined time is performed at least once. When air cooling is performed a plurality of times, the temperature at the position of plate thickness/4 should be 900° C. or more and 940° C. or less in the second and subsequent air coolings.

The timing of air cooling is as follows. That is, from the viewpoint of ensuring the toughness of the base material by performing reduction in a predetermined temperature range after air cooling, air cooling is performed before the start of the second hot rolling, that is, after the end of the first hot rolling and the second hot rolling. or during at least one of a plurality of rolling passes except between the final pass and the immediately preceding rolling pass. In other words, after air cooling, it undergoes at least two rolling passes.

(b) Cumulative rolling reduction in the temperature range of 900° C. or higher and 940° C. or lower at the plate thickness/4 position is 20% or higher and 67% or lower

In order to ensure excellent base metal toughness, it is necessary to control the rolling reduction in controlled rolling. In the present invention, by setting the cumulative reduction rate in the temperature range of 900° C. to 940° C. in the range of 900° C. to 940° C. in thickness/4 positions, the crystal grain size can be made finer, and excellent base material toughness can be ensured. FIG. 3 is a graph showing the relationship between the cumulative reduction rate in the temperature range of 900 ° C. or higher and 940 ° C. or lower at the plate thickness / 4 position and the Charpy impact absorption energy at -40 ° C. at the plate thickness / 4 position, The results of the examples described later are arranged. From FIG. 3, it can be seen that the absorbed energy of 120 J or more can be achieved by setting the cumulative reduction ratio to 20% or more. The cumulative rolling reduction is preferably 25% or more, more preferably 30% or more.

On the other hand, if the cumulative rolling reduction is too large, the steel sheet structure is extremely flattened and the properties tend to be anisotropic. Therefore, in the present invention, the cumulative rolling reduction is set to 67% or less. The cumulative rolling reduction is preferably 60% or less, more preferably 55% or less.

The temperature at the steel plate/4 position was calculated using a process computer by a method suitable for calculation such as the finite difference method. In the present invention, since the toughness of the base material is evaluated by the thickness of the steel sheet/the Charpy impact absorption energy at the 4 position, the rolling reduction was also controlled at the temperature at the position of 1/4 of the thickness.

In the second hot rolling step, rolling is performed in a plurality of rolling passes. For example, 6 to 10 rolling passes can be used. In the second hot rolling step, examples of combinations of one air cooling and a reduction (rolling pass) for ensuring the cumulative reduction rate include the following modes (I) and (II).
(I) Reduction in one or more passes not limited to a predetermined temperature range → Air cooling → Reduction in two or more passes in a predetermined temperature range (II) Air cooling → Reduction in two or more passes in a predetermined temperature range

(c) Implementation of the second descaling in the rolling pass after air cooling When air cooling is performed, scale grows during air cooling, and powdery scale is generated in the subsequent rolling, and the temperature after controlled cooling (accelerated cooling) Unevenness increases. Therefore, in order to suppress the growth of scale, the second descaling is performed after air cooling.

In the second descaling, the collision pressure is preferably 1.0 MPa or more. The collision pressure is more preferably 1.5 MPa or higher. The descaling of the collision pressure includes descaling at high water pressure. The number of times of the second descaling can be one or more. The second descaling may be performed after the first rolling pass after air cooling. Preferably, the descaling is performed once in the rolling pass immediately after air cooling, that is, in the first rolling pass after air cooling.

4A and 4B are graphs showing the effect of the presence or absence of descaling after air cooling on the difference from the average cooling stop temperature of each part (the center of the width and both ends of the width) in the width direction of the steel plate. FIG. 4B shows the case without descaling after air cooling, and the case with descaling after air cooling. From the comparison of FIGS. 4A and 4B, when descaling was not performed after air cooling, powdery scale was generated and the surface texture became uneven, and the temperature unevenness immediately after accelerated cooling was large, and the temperature unevenness in the width direction of the steel sheet. reached about 60°C. On the other hand, when descaling was performed after air cooling, the generation of powdery scale was suppressed, so the temperature unevenness immediately after accelerated cooling was reduced, and the temperature unevenness in the steel sheet width direction was suppressed to less than 20°C.

If the temperature of the steel sheet is only adjusted during rolling in order to ensure the reduction ratio in a certain temperature range for the purpose of ensuring the toughness of the base material, the scale will grow unevenly, resulting in excellent surface properties and toughness of the base material. It was difficult to achieve both. However, in the present invention, by performing the second hot rolling (finish rolling) so as to satisfy the above (a) to (c), both excellent surface properties and base material toughness can be achieved.

(Controlled cooling process)
Controlled cooling is performed after the second hot rolling. As a condition for controlled cooling, for example, cooling to a temperature range of 400 to 600° C. immediately after finish rolling at an average cooling rate of 5 to 25° C./sec, followed by air cooling to room temperature can be mentioned.

After the controlled cooling step, the steel sheet of the present invention can be obtained through, for example, hot straightening and air cooling as conventionally performed.

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. The present invention is not limited by the following examples, and can be implemented with appropriate modifications within the scope that can match the spirit described above and below. subsumed in

A slab was obtained by melting and casting a steel having the chemical composition shown in Table 1 below. Then, after reheating the slab to 1050 to 1250 ° C., before rolling in the first hot rolling process (rough rolling process), the slab surface temperature is in the range of 1000 to 1200 ° C. and the collision pressure is 5.0 MPa or more. was performed once. Rough rolling was then carried out. During this rough rolling, descaling was performed once at a surface temperature in the range of 1000 to 1200° C. and a collision pressure of 5.0 MPa or more.

After that, 6-pass or 8-pass hot reverse rolling was further performed in the second hot rolling step (finish rolling step). In the finish rolling, rolling, air cooling, and descaling were performed under the conditions shown in Table 2. Then, finish rolling is completed when the surface temperature is in the range of 830 to 900 ° C., cooled to a temperature range of 400 to 600 ° C. at an average cooling rate of 5 to 25 ° C./sec, and then air-cooled to a thickness of 20 mm. Got a steel plate.

"Before the first pass" in "Timing of air cooling" in Table 2 means that air cooling was performed before the start of finish rolling. In Table 2, "Presence or absence of descaling after air cooling" indicates "yes" when descaling was performed at a collision pressure of 1.0 MPa or more after air cooling during finish rolling, and "yes" when the descaling was not performed. I said "no". The timing of the descaling was a rolling pass immediately after air cooling at the air cooling timing shown in Table 2. For example, when the air cooling timing was "before the first pass", descaling was performed during the first pass rolling.

In Table 2, Test No. E corresponds to a conventional example in which air cooling for 10 seconds or more is not provided in the finish rolling process. Test no. The air cooling time of 7 seconds in E indicates the time between rolling passes.

In Table 2, the cumulative rolling reduction in the temperature range of 900° C. or higher and 940° C. or lower at the plate thickness/4 position was calculated using the following formula (2).
Cumulative rolling reduction in the temperature range of 900 ° C or higher and 940 ° C or lower at the position of plate thickness / 4 [%]
=(H1-H2)/H1×100 (2)
However, H1: Rolling start plate thickness when the position of plate thickness/4 is 940 ° C. [mm]
H2: Plate thickness when the plate thickness/4 position is 900 ° C. [mm]

Using the obtained steel sheet, the scale properties on the surface of the steel sheet were observed in order to evaluate the surface properties of the steel sheet, and the thickness of the steel sheet/4 positions was measured in order to evaluate the toughness of the base material. Charpy impact absorption energy at -40°C was determined.

[Observation of scale properties]
After embedding a steel slab in resin so that the cross-section of the steel plate in the plate thickness direction can be observed, a sample was prepared by mirror-polishing and not corroding (etching), and observed using an optical microscope. Then, a photograph including the powdery scale portion was taken at an observation magnification of 400 so that the scale was parallel to the horizontal direction of the photograph. The powdery scale layer is observed as black because the surface of the scale powder that is partially unpolished is not perpendicular to the incident light, so that the reflected light hardly reaches the eyepiece lens. Taking advantage of this effect, a photograph of the scale portion was binarized using image analysis software, and the black portion was determined to be powdery scale. An example of the photograph taken is shown in FIG. In FIG. 5, X is the powdery scale layer, R is the non-powder scale layer, and Q is the steel plate. As shown in FIG. 5, the thickness of the powdery scale layer is, in order from the left end of the width of the photograph, the first measurement line 1 on the 1/4 part, the second measurement line 2 on the 1/2 part, and the third measurement line on the 3/4 part. Three lines of the measurement line 3 were drawn, the thickness of the powdery scale layer on each line was obtained, and the thickness of the three points was averaged.

The scale layer on the surface of the steel sheet is formed in the order of a powdery scale layer and a non-powderous scale layer from the surface side, or is formed only by the non-powderous scale layer, and the thickness of the powdery scale layer is 10 μm or less. When the thickness of the powdery scale layer exceeded 10 μm, it was evaluated as “X”. Table 2 shows the results. In addition, Test No. in Table 2. E was a comparative example in which the Charpy absorbed energy obtained by the following Charpy impact test was low and the toughness of the base material was deteriorated, so the scale properties were not evaluated.

[Charpy impact test]
The Charpy impact test was performed at a test temperature of -40°C according to JIS Z2242 or ASTM A370 to determine the Charpy absorbed energy at -40°C. In this example, the values of Charpy absorbed energy at −40° C. of a total of three sampled test pieces were determined, and the case where the average value of these three test pieces was 120 J or more was evaluated as excellent in base material toughness. In Table 2, the data marked with * are the results of testing by the ASTM method, and the other data are the results of testing by the JIS method. Table 2 shows the results.

From Tables 1 and 2 the following can be observed.

Test no. In A to D, the chemical compositions and manufacturing conditions were within the ranges specified in the present invention, so the obtained steel sheets exhibited excellent surface properties and excellent base metal toughness. On the other hand, Test No. In E, no air cooling was provided for 10 seconds or more, and reduction was started in a high temperature range of 940°C or higher at the plate thickness/4 position. As a result, the base metal toughness deteriorated because the cumulative rolling reduction in the temperature range of 900° C. or higher and 940° C. or lower (thickness/4 positions) was insufficient. Also test no. In F, air cooling was performed as specified in the present invention, but since descaling was not performed after air cooling, the powdery scale layer became as thick as 20 μm and the surface properties of the steel sheet deteriorated.

Claims (4)

Ingredient composition
C: 0.03 to 0.17% by mass,
Si: 0.25 to 0.60% by mass,
Mn: 1.0 to 2.0% by mass,
P: more than 0% by mass, 0.030% by mass or less,
S: more than 0% by mass, 0.005% by mass or less,
Al: 0.010 to 0.080% by mass,
N: 0.0010 to 0.0100% by mass,
Ti: 0.005% by mass or more and 0.03% by mass or less, Ca: 0.0010% by mass or more and 0.0025 % by mass or less, the balance being iron and unavoidable impurities,
Ceq determined by the following formula (1) is 0.300% by mass or more and 0.420% by mass or less,
The scale layer on the surface of the steel sheet is formed in the order of a powdery scale layer and a non-powderous scale layer from the surface side, or is formed only by the non-powderous scale layer, and the thickness of the powdery scale layer is 10 μm or less. suppressed, and
A steel plate with excellent base material toughness and surface properties, with a Charpy impact absorption energy of 235 J or more at -40°C at the position of plate thickness/4.
Ceq=[C]+[Mn]/6+[Si]/24+[Ni]/40+[Cr]/5+[Mo]/4+[V]/14 (1)
provided that [C], [Mn], [Si], [Ni], [Cr], [Mo] and [V] are C, Mn, Si, Ni, Cr, Mo and The content of V is shown, and elements not included are set to 0. Furthermore,
Cu: more than 0% by mass, 0.50% by mass or less,
Ni: more than 0% by mass, 0.50% by mass or less,
Cr: more than 0% by mass, 0.50% by mass or less,
The steel sheet according to claim 1, comprising one or more elements selected from the group consisting of Mo: more than 0 mass% and 0.50 mass% or less, and Nb: more than 0 mass% and 0.06 mass% or less. Furthermore,
The steel sheet according to claim 1 or 2, comprising one or more elements selected from REM: more than 0% by mass and 0.01% by mass or less and Zr: more than 0% by mass and 0.010% by mass or less. A method for producing a steel plate according to any one of claims 1 to 3,
Using a slab having the chemical composition according to any one of claims 1 to 3, the heating process, the first hot rolling process, the second hot rolling process in which rolling is performed in a plurality of rolling passes, and the controlled cooling process are performed. including, in order,
After the heating step and before the first hot rolling step; and during rolling in the first hot rolling step; performed one or more times, and
A method for producing a steel plate having excellent base material toughness and surface properties, wherein the second hot rolling step is performed so as to satisfy all of the following (a) to (c).
(a) 10 seconds or more before the start of the second hot rolling or between at least one of a plurality of rolling passes (excluding between the final pass and the immediately preceding rolling pass) Air cooling is performed for 120 seconds or less, and the temperature at the plate thickness/4 position is set to 900° C. or higher and 940° C. or lower.
(b) Rolling is performed in a temperature range of 900° C. or higher and 940° C. or lower at the thickness/4 position so that the cumulative rolling reduction is 20% or higher and 67% or lower.
(c) performing a second descaling in the rolling pass after the air cooling;

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