KR20200022387A - Steel plate and its manufacturing method - Google Patents

Abstract

In the steel sheet according to one aspect of the present invention, the chemical component is within a predetermined range, and the total area ratio of martensite and bainite is 99% or more in the sheet thickness center portion, and the former austenite particle diameter is in the sheet thickness center portion. The average value of is less than 80 µm, Ceq is 0.750 to 0.800%, Al x N is 2.0 × 10 ⁻⁴ or more, Ti / N is 3.4 or less, 4 × f / g ≧ 9.00 is satisfied, and the plate thickness The 3-point average of C direction Charpy in -20 degreeC in a center part is 47 J or more, the hardness of a surface layer and the said plate | board thickness center part is 350 or more in HB, and plate | board thickness is more than 200 mm.

Description

Steel plate and its manufacturing method

TECHNICAL FIELD This invention relates to a steel plate and its manufacturing method.

A huge cogwheel (gear) is used for the rotating mechanism of a large industrial machine represented by a rotary kiln. The steel plate made of a raw material is required for hardness and toughness from the viewpoint of fatigue resistance and durability of the gear. In recent years, it has become demanded for the steel plate which becomes HB350 or more and vE- ₂₀ degreeC≥47J of sheet thickness center part in surface layer and sheet thickness center part. This is due to the fact that the characteristics of the sheet thickness center are important because the gears are produced by scraping the steel to the sheet thickness center.

In addition, in recent years, in order to increase the size of the gear, a steel sheet having a plate thickness of more than 200 mm has not been required. With increase of plate | board thickness, the cooling rate of the plate | board thickness center part at the time of quenching falls. Therefore, in the steel plate more than 200 mm in thickness, hardness of a center part becomes difficult to obtain even after tempering. On the other hand, the component design aimed at simply raising the hardness causes a decrease in toughness. Usually, hardness and toughness are inversely related. Therefore, in the extremely thick material of more than 200 mm of plate | board thickness, the component balance adjustment for ensuring surface layer hardness and center part hardness, and also securing toughness is extremely difficult.

Moreover, for the purpose of improving weldability, the request which made carbon equivalent Ceq by main containing element into 0.800% or less arises. When Ceq exceeds 0.800%, load, such as raising the preheating temperature at the time of welding, increases with demand. In the welding operation of an extremely thick material such as this steel, the number of welding passes is very large, and the increase in welding load is also large. In addition, Ceq is represented by following formula (1), for example. The element symbol contained in Formula (1) represents content (mass%) of each element in the chemical component of steel materials.

Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5: Formula (1)

Conventionally, there was no steel material with a plate thickness of more than 200 mm that ensures Ceq ≦ 0.800%, central hardness ≧ HB350, and guarantees the low temperature toughness described above at -20 ° C. In addition, the steel plate which is a raw material must be tempered at 500 degreeC or more so that a material may not change by stress relief annealing after gear processing. The fact that tempering is necessary also made the situation unfavorable for achieving the hardness target of steel.

Patent document 1 is a huge gear material used for the rotating mechanism of a large industrial machine, The board thickness is more than 200 mm, The objective of providing a thick steel plate with a small difference in hardness between a surface layer and a center, and its manufacturing method is set as the subject, The three-point average of the C-direction Charpy at -20 ° C is 20J or more, the hardness of the surface layer is 330 or more in HB, the hardness of the plate thickness center is 300 or more in HB, and the hardness difference ΔHB between the surface layer and the center of the plate thickness. The thick steel plate which is 30 or less is provided. However, Patent Document 1 is not intended to stably set the hardness of the sheet thickness center portion to HB350 or more.

Japanese Patent Publication No. 2017-186592

Under such circumstances, in the present invention, the plate thickness is more than 200 mm, and in particular, Ceq is 0.850% or less, Ceq is 0.750% or more for the convenience of securing the hardness of the plate thickness center, and the surface layer and the plate thickness center are A steel sheet having a hardness of at least HB350 and an absorption energy at −20 ° C. at a sheet thickness center of at least 47 J, and a method of manufacturing the same.

The gist of the present invention is as follows.

(I) As for the steel plate which concerns on one aspect of this invention, chemical composition is unit mass%, C: 0.16 to 0.20%, Si: 0.50 to 1.00%, Mn: 0.90 to 1.50%, P: 0.010% or less, S: 0.0020% or less, Cu: 0 to 0.40%, Ni: 0.20 to 1.00%, Cr: 0.60 to 0.99%, Mo: 0.60 to 1.00%, V: 0 to 0.050%, Al: 0.050 to 0.085%, N: 0.0020 to 0.0070%, B: 0.0005 to 0.0020%, Nb: 0 to 0.050%, Ti: 0 to 0.020%, Ca: 0 to 0.0030%, Mg: 0 to 0.0030%, REM: 0 to 0.0030% and the balance: Fe and impurities In the sheet thickness center part, the total area ratio of martensite and bainite is 99% or more, and the average value of the old austenite particle diameter in the said plate thickness center part is less than 80 micrometers, and is represented by Formula (1) represents a Ceq is 0.750 to 0.800%, Al × N is 2.0 × 10 ^-4 or more, and Ti / N is 3.4 or less, and the value g shown by the value f and the equation (3) represented by the formula (2), 4 satisfying f / g ≧ 9.00, the -20 ° C Charpy absorbed energy measured in the C direction in the sheet thickness center is 47 J or more, the hardness of the surface layer and the sheet thickness center is HB350 or more, and the plate thickness is 200 mm. Excess.

Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5: Formula (1)

f = 4 × C + Si + 2 × Mn + Ni + 2 × Cr + 5 × Mo: Formula (2)

g = 2 × Cr + 3 × Mo + 5 × V: Formula (3)

Here, the element symbol described in each formula means content in the unit mass% of the element with respect to each element symbol.

(II) The manufacturing method of the steel plate which concerns on another aspect of this invention is a manufacturing method of the steel plate as described in said (I), The process of heating a slab, and hot-rolling the slab and obtaining a steel plate whose plate | board thickness is more than 200 mm And a step of cooling the steel sheet, a step of depositing the steel sheet, a step of quenching the steel sheet, and a step of tempering the steel sheet, wherein the chemical component of the slab is in unit mass%, C: 0.16 to 0.20%, Si: 0.50 to 1.00%, Mn: 0.90 to 1.50%, P: 0.010% or less, S: 0.0020% or less, Cu: 0 to 0.40%, Ni: 0.20 to 1.00%, Cr: 0.60 To 0.99%, Mo: 0.60 to 1.00%, V: 0 to 0.050%, Al: 0.050 to 0.085%, N: 0.0020 to 0.0070%, B: 0.0005 to 0.0020%, Nb: 0 to 0.050%, Ti: 0 to 0 0.020%, Ca: 0 to 0.0030%, Mg: 0 to 0.0030%, REM: 0 to 0.0030% and the balance: Fe and impurities, and the formula of the slab Ceq represented by (1) is 0.750 to 0.800%, Al × N of the slab is 2.0 × 10 ⁻⁴ or more, Ti / N of the slab is 3.4 or less, and the value f represented by formula (2) of the slab And the value g represented by Formula (3) satisfies 4xf / g≥9.00, and the slab heating temperature in the step of heating the slab is equal to or higher than the AlN solid solution temperature Ts (° C) calculated by Formula (4). The step of depositing the steel sheet is performed by heating the steel sheet to a precipitation processing temperature Tp (° C.) of less than 550 ° C. and less than Ac1, and then maintaining the precipitation steel at the temperature for the precipitation processing time tp (hour). Tp (占 폚) and precipitation treatment time tp (time) satisfy Equation (5), and Ac1 is represented by Equation (7), and the step of quenching the steel sheet is performed by quenching the steel sheet at 900 to 950 占 폚. It heats to holding temperature Tq (degreeC), and the quenching holding time tq (minute) shown by Formula (6) at this temperature. The above step is carried out by holding for a period of time, followed by water cooling, and the step of tempering the steel sheet is performed by heating the steel sheet to a tempering temperature of 500 to 550 ° C, and then cooling to 150 ° C or less.

Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5: Formula (1)

f = 4 × C + Si + 2 × Mn + Ni + 2 × Cr + 5 × Mo: Formula (2)

g = 2 × Cr + 3 × Mo + 5 × V: Formula (3)

Ts = 7400 / (1.95-log ₁₀ (Al × N))-273: Equation (4)

Log ₁₀ (tp) + 0.012 x Tp ≥ 8.7: (5)

tq = 0.033 × (950-Tq ) 2 + (1.5 × f) 2/10: (6)

Ac1 = 750-25 × C + 22 × Si-40 × Mn-30 × Ni + 20 × Cr + 25 × Mo: Formula (7)

Here, the element symbol described in each formula is content by the unit mass% of the element with respect to each element symbol.

(III) In the manufacturing method of the steel plate as described in said (II), cooling end temperature in the process of cooling the said steel plate may be 150 degrees C or less.

According to the present invention, even in a steel sheet having a sheet thickness of more than 200 mm, it is possible to provide a steel sheet which is excellent in the hardness of the surface layer and the sheet thickness center portion and the impact absorption energy performance of the sheet thickness center portion, and suppresses Ceq to 0.800% or less. It can be used for the rotary mechanism of large industrial machines represented by rotary kilns.

1: is a schematic diagram of the cross section perpendicular | vertical to a rolling direction of the steel plate which concerns on this embodiment.
FIG. 2: is a figure which shows the relationship between C amount and plate | board thickness center hardness, and the relationship between C amount and plate | board thickness center toughness (vE- ₂₀ degreeC).
3 is a diagram illustrating a relationship between Ceq and central hardness.
4 is a diagram illustrating a relationship between 4xf / g and sheet thickness center toughness.
FIG. 5 is a diagram showing a relationship between the precipitation treatment temperature Tp and the precipitation treatment time Log ₁₀ (tp) which were tested using the component A4 of the example.
6A is a diagram showing the relationship between the quench holding temperature Tq, the quench holding time tq, and the central hardness obtained as a result of experimenting with the component A6 of the example.
6B is a diagram showing the relationship between the quench holding temperature Tq, the quench holding time tq, and the central hardness obtained as a result of experimenting with the component A2 of the example.
7 is a flowchart showing a method for manufacturing a steel sheet according to the present embodiment.

In the steel sheet according to the present embodiment, mechanical properties of both the sheet thickness center part (sometimes referred to simply as "center part") of the steel plate and the surface layer of the steel plate (sometimes referred to simply as "surface layer") are controlled. As shown in FIG. 1, the plate thickness center part 11 of the steel plate 1 has a depth of 3/8 of the plate thickness T of the steel plate 1 from the rolling surface 13 which is the outermost surface of the steel plate 1. It is an area | region between the surface and the surface of the 5/8 depth of the plate | board thickness T from the rolling surface 13. The center plane of the plate thickness center part 11 of the steel plate 1, and the center plane of the steel plate 1 will correspond. The surface layer 12 of the steel plate 1 is an area | region between the surface of 1 mm in depth and the surface of 5 mm in depth from the rolling surface 13 of the steel plate 1. In this embodiment, the area | region from the outermost surface of the steel plate 1 to a depth of 1 mm is excluded from the surface layer 12 of the steel plate 1. This is because this region corresponds to a decarburized layer and a portion to be removed during processing. In addition, from the edge part of the longitudinal direction and the width direction of a steel plate, as a general rule, the test piece for mechanical test, micro structure observation, etc. is taken from the part spaced apart more than plate | board thickness.

In the steel sheet according to the present embodiment, the following (1) to (7) have important meanings. In the sheet thickness center part of the steel plate which has a chemical composition of Ceq <= 0.800%, as a requirement for achieving both the hardness of HB350 grade and vE _-20 degreeC≥47J, component parameter formula (3) and precipitation treatment ( 5) is important.

(1) Upper and lower limits of the amount of C to achieve both central hardness and central toughness (under the conditions described below)

In general, when the central hardness is HB350 or more, the surface layer can also be secured to HB350 or more.

(2) Ceq lower limit for securing center hardness

(3) The lower limit of the parametric formula 4 × f / g for securing the core toughness

(4) Lower limit of parametric Al × N for securing central toughness

(5) Solution treatment and precipitation treatment before temperature to secure central hardness and toughness (temperature and time)

(6) Quenching conditions (temperature and time) towards securing the hardness of the core

(7) Upper and lower limits of the tempering temperature to secure hardness and toughness in the center

It will be described in detail below.

(1) Upper and lower limits of the amount of C to achieve both central hardness and central toughness (under the conditions described below)

As a 1st item, in order to raise both the hardness of a plate | board thickness center part, and toughness under the conditions mentioned later, C needs to satisfy 0.16 to 0.20% as a component composition (mass%) of the said steel. In order to ensure both toughness and hardness in the sheet thickness center part of the steel plate more than 200 mm in thickness, it is necessary to suppress generation | occurrence | production of the carbide which becomes a starting point of brittle fracture. C must be 0.20% or less in order to suppress the formation of carbides and to achieve vE _{-20 ° C} (ave.)? On the other hand, the decrease in C greatly reduces the hardness of the steel. Therefore, in order to make hardness of a center part HB350 or more even after tempering of 500 degreeC or more, as shown in FIG. 2, C needs to be 0.16% or more similarly.

(2) Regulation of Ceq Lower Limit for Securing Center Hardness

As a 2nd item, in order to ensure the hardness of a center part in the steel plate more than 200 mm in thickness, sufficient quenchability is needed. Therefore, after performing the precipitation process mentioned later, Ceq calculated by following formula (1) needs to satisfy 0.750% or more. This is to avoid formation of ferrite which is a soft structure at the time of quenching, and to form the structure mainly comprised from bainite and martensite. In addition, it is not necessary to set an upper limit to Ceq from the viewpoint of having the hardness and toughness of the central portion. However, increasing Ceq makes it easy to generate weld cracks. When Ceq exceeds 0.800%, welding work efficiency will deteriorate remarkably, for example, the need to raise the preheating temperature before welding in order to avoid welding cracking will arise. Therefore, Ceq in the steel plate which concerns on this embodiment shall be 0.800% or less. Ceq may be 0.790% or less, 0.785% or less, or 0.780% or less.

Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5: Formula (1)

The element symbol contained in Formula (1) shows content (mass%) of each element in the chemical component of a steel plate.

As shown in FIG. 3, in the steel plate more than 200 mm in thickness, when the Ceq was less than 0.750%, the present inventors discovered that even if precipitation process was performed, the hardness of the center part of plate thickness will be less than HB350. When Ceq is less than 0.750%, the reason why the hardness of the sheet thickness center part is insufficient is considered to be because the ferrite which is a soft structure was produced | generated. Ceq may be 0.755% or more, 0.760% or more, or 0.770% or more. In addition, although Ceq is 0.750% or more, the steel which lacks the hardness of the sheet thickness center part is plotted in FIG. The reason why the hardness of the sheet thickness center part is insufficient in this steel is that precipitation treatment is not performed.

(3) The lower limit of the parameter expression "4 x f / g" for securing the toughness of the center portion

As a third item, in order to ensure the hardness ≥ HB350 at the center portion and to achieve the toughness of vE _{-20 ° C} ≥ 47J at the center of the sheet thickness while maintaining Ceq≤0.800% in the steel sheet having a sheet thickness of more than 200 mm, the following formula ( It is necessary for the parameter f defined by 2) and the parameter g defined by the following formula (3) to satisfy the relationship of 9 × f / g or more.

f = 4 × C + Si + 2 × Mn + Ni + 2 × Cr + 5 × Mo: Formula (2)

g = 2 × Cr + 3 × Mo + 5 × V: Formula (3)

The element symbol contained in Formula (2) and Formula (3) shows content (mass%) of each element in the chemical component of a steel plate.

As shown in Fig. 4, in a steel sheet having a plate thickness of more than 200 mm, Ceq ≦ 0.800%, and a hardness of the plate thickness center portion of HB350 or more, the toughness of the plate thickness center portion when 4 × f / g is less than 9.00. The inventors found out that it could not be secured. The element which concerns on parameter f is an element which improves the quenchability of a steel plate by solid-solution in a matrix at the time of quenching. On the other hand, the element which concerns on parameter g is an element which reduces the toughness of a steel plate by forming a precipitate at the time of tempering. That is, these elements improve the quenchability and lower the toughness by forming precipitates during tempering. When 4xf / g is large, it shows that quenchability is improving, reducing the element which precipitates at the time of tempering.

In addition, Cr precipitates, Mo precipitates, and V precipitates at the time of tempering in the steel plate which concerns on this embodiment are so fine that it cannot be observed unless it is a transmission electron microscope. Therefore, it is industrially unrealistic to define the distribution state of the above-mentioned precipitate itself. From this, it is possible to understand the usefulness of controlling the precipitate by the parameter formula 4xf / g.

4xf / g may be 9.20 or more, 9.50 or more, or 9.80 or more. Although the upper limit of 4xf / g does not need to be specifically defined, it is good also as 11.00, 10.70, 10.50, 10.00, or 9.90, for example.

(4) Lower limit of parametric Al × N for securing central toughness

In order to ensure both hardness and low-temperature toughness in the center of the steel plate with a plate thickness of more than 200 mm, the Al content needs to be 0.050% or more, and Al x N (Al content (mass%) and N content (mass) %) Product) needs to be ^{2.0x10 <-4>} or more. This is a requirement for utilizing the pinning effect of AlN, which contributes to the grain size of the steel sheet. When Al is less than 0.050% or Al x N is less than 2.0 x 10 &lt; ^{-4 &} gt ;, the old austenite grain size is coarsened, and the low temperature toughness at the center of the steel sheet is degraded. This is considered to be because the total amount of AlN was insufficient.

In addition, since AlN which acts as the pinning particle in the steel plate which concerns on this embodiment is very fine, it is difficult to observe. Therefore, it is industrially unrealistic to define the distribution state of AlN itself which acts as pinned particles. From this, the usefulness of controlling AlN which acts as a pinned particle | grain by the parameter Al * N can be understood.

Al * N may be 2.2x10 <^-4> or more, 2.5x10 <^-4> or more, or 3.0 * 10 <^-4> or more. Although it is not necessary to specifically define the upper limit of Al * N, 5.95 * 10 <^-4> which is a product of the upper limit of each of Al content and N content mentioned later may be made into the upper limit of Al * N. Al × N may be 5.7 × 10 ⁻⁴ or less, 5.5 × 10 ⁻⁴ or less, 5.2 × 10 ⁻⁴ or less, or 4.8 × 10 ⁻⁴ or less.

(5) Solving treatment and precipitation treatment (temperature and time) before quenching to secure central hardness and toughness

As a process requirement for obtaining the pinning effect of AlN, there are a solution treatment and a precipitation treatment. In the solution treatment, the slab is heated up to AlN solid solution temperature Ts or more calculated by the following formula (4). Hot rolling is performed after solution treatment. In the precipitation treatment, in order to finely precipitate Al and N dissolved in the matrix as AlN by the above solution, the hot rolled steel sheet is heated to the precipitation treatment temperature Tp which is a temperature of more than 550 ° C and less than Ac1 after hot rolling and before quenching heating. The temperature is maintained by the precipitation treatment time tp at this precipitation treatment temperature Tp. Here, it is necessary to perform a precipitation process so that precipitation process temperature Tp and precipitation process time tp may satisfy following formula (5).

Ts = 7400 / (1.95-log ₁₀ (Al × N))-273: Equation (4)

Log ₁₀ (tp) + 0.012 x Tp ≥ 8.7: (5)

Here, Ts in Formula (4) is the solid solution temperature (degreeC) of AlN, and "Al" and "N" are each content of Al and N (mass%). "Tp" in Formula (5) is precipitation process temperature (degreeC), and "tp" is precipitation process time (hour).

In addition, slight temperature fluctuations are allowed during the temperature retention of the precipitation treatment. Moreover, the temperature fluctuation may arise in actual operation. Therefore, the precipitation process temperature Tp is until the temperature of the sheet thickness center part of a steel plate exceeds "maximum temperature-40 degreeC of the sheet thickness center part of the steel plate in precipitation process" until the steel plate is extracted from a heat treatment furnace, It is defined as the average temperature of the steel sheet in the center of the plate thickness. Specifically, precipitation process temperature Tp is a value computed by the following formula (8).

Tp = {∫ [t _A → t _B ] T (t) dt} / (t _B -t _A ): Equation (8)

t _A : The point in time when the temperature of the sheet thickness center part of the steel plate last exceeded "maximum temperature -40 degreeC of the plate thickness center part of the steel plate during precipitation process"

t _B : time point at which the steel sheet is extracted from the heat treatment furnace

T (t): Time-dependent change in temperature at the center of the sheet thickness of the steel sheet (time history of temperature)

∫ [t _A → t _B ] T (t) dt: Change over time in the center of the plate thickness of the steel plate, integral value from t _A to t _B

In addition, the precipitation process time tp is a time until the steel plate is extracted from a heat treatment furnace after the temperature of the sheet thickness center part of a steel plate finally exceeds "the maximum temperature of 40 degreeC of the plate thickness center part of the steel plate in precipitation process." (That is, "t _B -t _A "). The precipitation process temperature Tp obtained by applying the time history of the temperature at the time of the precipitation process of the plate | board thickness center part of a steel plate to Formula (8) mentioned above is more than 550 degreeC, and is less than Ac1, and precipitation process temperature Tp and precipitation process time tp If it satisfy | fills Formula (5), it is judged that the suitable precipitation process was performed.

If the solution is not formed before hot rolling, the coarse AlN generated at the time of casting of the steel remains in the steel, and the total amount of AlN in the steel decreases. Therefore, fine AlN obtained by the precipitation process decreases, and the pinning effect cannot be obtained.

The present inventors measured vE- ₂₀ degreeC of the steel plate manufactured by applying various precipitation process time tp and precipitation process temperature Tp to the steel which has component A4 of the Example mentioned later. The result is shown in FIG. It can be seen from FIG. 5 that the precipitation treatment needs to be performed at an appropriate precipitation treatment temperature Tp and the precipitation treatment time tp in order to obtain the pinning action of AlN.

Specifically, FIG. 5 plots each steel plate with the horizontal axis being the precipitation process temperature Tp of each steel plate, and the vertical axis being Log ₁₀ (tp) of each steel plate. The unit of tp is time Hr. In FIG. 5, the steel plate plotted by the X mark has become vE- ₂₀ degreeC less than 47J, and the steel plate plotted by the o mark has vE- ₂₀ degreeC being 47J or more. It can be seen from FIG. 5 that toughness cannot be secured under processing conditions of Log ₁₀ (tp) + 0.012 × T <8.7. This is presumably because AlN was not sufficiently precipitated in the precipitation treatment and the pinning effect could not be exhibited. On the other hand, even when precipitation processing temperature Tp exceeds Ac1, it turns out that toughness is not securable. This is because when the precipitation treatment temperature Tp exceeds Ac1, the precipitation treatment is a temperature retention in the α-γ two-phase region, so that the concentration of Al and N in the γ region occurs, and coarsening of AlN is locally performed. It is assumed that it occurs. In addition, the upper limit of precipitation processing time tp is not specifically regulated from a mechanical property viewpoint. However, from the viewpoint of industrial production efficiency, 5 days = 120 hours may be the upper limit of the precipitation treatment time tp.

(6) Quenching conditions (temperature and time) towards securing the hardness of the core

As a sixth item, in order to make the hardness of the sheet thickness center part more than HB350 in the component range of the steel plate which concerns on this embodiment, after a sufficient precipitation of AlN is produced by the said precipitation process, the quenching on predetermined conditions is carried out. It needs to be done. Specifically, the hot rolled steel sheet is reheated to a quenching holding temperature Tq of 900 ° C or higher and 950 ° C or lower, and the hot rolled steel sheet is held at this temperature for a quenching holding time tq (minutes) or more represented by the following formula (6), and then It is necessary to perform a quenching process for water-cooling the hot rolled steel sheet.

tq = 0.033 × (950-Tq ) 2 + (1.5 × f) 2/10: (6)

In Formula (6), Tq is a quenching holding temperature (degreeC) and f is a value obtained by Formula (2) mentioned above. In addition, quenching holding temperature Tq shows the temperature of the plate | board thickness center part of a steel plate, not the set temperature of a heat processing furnace.

In addition, slight temperature fluctuations are allowed during quenching of temperature. Moreover, the temperature fluctuation may arise in actual operation. Therefore, the quenching holding temperature Tq is until the temperature of the sheet thickness center part of the steel plate exceeds "the maximum temperature -40 degreeC of the sheet thickness center part of the steel plate in ching" last, until the steel plate is extracted from a heat treatment furnace. It is defined as the steel plate average temperature at the center of the plate thickness. Specifically, the quenching holding temperature Tq is a value calculated by the following formula (9).

Tq = {∫ [t ₁ → t ₂ ] T (t) dt} / (t ₂ -t ₁ ): Equation (9)

t _1: "? -40 ℃ maximum temperature in the center of plate thickness of steel sheet called" the temperature of the center of plate thickness of the steel sheet of the last time exceeds the

t ₂ : time point at which the steel sheet is extracted from the heat treatment furnace

T (t): Temperature change over time in the center of the plate thickness of the steel sheet (time history of temperature)

∫ [t ₁ → t ₂ ] T (t) dt: Integral value from t ₁ to t ₂ of the change over time in the center of the plate thickness of the steel sheet

Hereinafter, in order to distinguish it from Tq as an operation target value mentioned later, the value computed by Formula (8) may be described as "performance Tq." In addition, the quenching retention time of the steel plate as the performance value is that the steel plate is extracted from the heat treatment furnace after the temperature of the plate thickness center part of the steel plate finally exceeds the "maximum temperature -40 degreeC of the plate thickness center part of the steel plate in quenching". It is defined as the time until it becomes (that is, "t _2- t ₁ "). The quenching holding time of the steel sheet as the performance value defined below as "t ₂ -t ₁ " may be described as "performance tq." In addition, the quenching holding time tq computed from Formula (6) may be described as "necessary tq." It is required as a manufacturing condition of the steel plate which concerns on this embodiment that earnings tq is more than required tq.

The quench holding temperature Tq may be controlled based on the measured value by inserting the thermocouple near the center of the sheet thickness of the steel sheet, or based on the estimated value by the heat conductivity calculation based on the furnace temperature and the sheet thickness. May be controlled.

An example of an actual quenching method is shown below. For example, before the quenching process, a quenching holding temperature (target Tq) and a quenching holding time (target tq) as target values, such as satisfying equation (6), are predetermined. The steel sheet is inserted into a heat treatment furnace, and the steel sheet is heated to a temperature range within the target Tq ± 20 ° C and maintained at that temperature. After maintaining the temperature of a steel plate within the range of target Tq +/- 20 degreeC at least between target tq, the cooling process for quenching is performed. Thereafter, the performance Tq is calculated by applying the time history T (t) of the performance temperature (actual value or estimated value) of the sheet thickness center part of the steel sheet to the above-mentioned formula (8). Moreover, the elapsed time from the time point t ₁ when the temperature of the sheet thickness center part of a steel plate exceeded "maximum temperature -40 degreeC of the sheet thickness center part of the steel plate during ching" to the time point t _{2 with} which the steel plate was extracted from the heat treatment furnace Let t be the performance tq. Subsequently, the performance Tq is substituted into Tq of Formula (6), and required tq is calculated. When the performance tq is not smaller than the required tq (that is, in the case of the performance tq≥necessary tq), it is determined that an appropriate quenching process has been performed.

In addition, also in precipitation process, determination in the same procedure is required.

6A shows the result of experiment using the steel which has component A6 of the Example mentioned later, and FIG. 6B shows the result of experiment using the steel which has component A2 of the Example mentioned later.

MEANS TO SOLVE THE PROBLEM The present inventors apply various temperature holding time (time when the temperature of the center part of a hot-rolled steel plate was isothermally maintained at the quenching holding temperature Tq), and quenching holding temperature Tq, and manufactures various steel plates, Measured. 6A and 6B plot each steel plate with the horizontal axis being the quenching holding temperature Tq of each steel plate, and the vertical axis being the temperature holding time of each steel plate. 6A and 6B, the steel plate plotted by the X mark has a central hardness of less than 350 HB, and the steel plate plotted by the o mark has a central hardness of 350 HB or more.

6A is a steel sheet whose temperature holding time is shorter than the quenching holding time tq represented by Formula (6) mentioned above (steel plate plotted below the curve in FIG. 6A and FIG. 6B), and the central hardness becomes less than HB350. FIG. And FIG. 6B. This is considered to be due to the inability to secure the quenchability because the alloy for improving the quenchability was not sufficiently dissolved in the matrix. In addition, the quench holding time tq becomes a function of f because the larger the amount of alloy, the more time is required for their solid solution.

When the quenching holding temperature Tq is less than 900 ° C, solid solution of the alloying element is not sufficiently performed. Therefore, quenchability cannot be secured and HB350 cannot be achieved in the center of a steel plate. On the other hand, when the quenching holding temperature Tq exceeds 950 ° C, AlN is partially dissolved and advantageous N is associated with B in the steel. Thereby, the quenchability improvement effect of B is inhibited, and HB350 cannot be achieved in the center part of a steel plate.

(7) Upper and lower limits of the tempering temperature to secure hardness and toughness in the center

In addition, as a seventh item, in consideration of the construction requirements of the gear (prevention of reduction of the material in the stress relief annealing), the tempering temperature needs to be 500 ° C or higher. In order to secure the toughness of a steel plate by adding and tempering a structure sufficiently, tempering temperature needs to be 500 degreeC or more. On the other hand, the steel sheet which concerns on this embodiment may fall hardness rapidly by tempering exceeding 550 degreeC. From this, tempering temperature needs to be 550 degrees C or less. After this tempering, the steel plate is cooled to 150 degrees C or less.

Next, the structure of the steel plate which concerns on this embodiment is demonstrated. In the steel sheet according to the present embodiment, the total area ratio of martensite and bainite is 99% or more. Although the remainder of the structure is not specifically defined, for example, ferrite, pearlite, residual austenite, and the like can be considered. These other structures are acceptable as long as they are less than 1 area%.

The above structure is achieved by quenching under the condition that ferrite does not come out, and by tempering at a sufficient high temperature. Specifically, the steel sheet having a Ceq ≧ 0.750% or more component is achieved by performing quenching under the above-described conditions and tempering under the above-described conditions after the precipitation treatment under the above-described conditions.

Ferrite is a factor of lowering the hardness of steel materials. In particular, ferrite tends to occur in the center of the plate thickness with a slow quenching rate. In order to ensure the hardness of the core, the amount of ferrite must be made as low as possible.

Although pearlite is effective for securing hardness, the pearlite has a brittle fracture starting point due to its rigidity. For this reason, the amount of pearlite must also be as low as possible. Perlite is produced by the concentration of C discharged during ferrite precipitation. Therefore, pearlite formation is also suppressed at the same time by avoiding precipitation of ferrite.

Residual austenite becomes a brittle fracture starting point, and reduces the toughness of steel materials. Therefore, the amount of retained austenite must be as low as possible. If tempering is performed at a tempering temperature of 500 ° C. or higher, the formation of residual austenite is suppressed.

As mentioned above, generation | occurrence | production of the ferrite, pearlite, residual austenite, etc. which are harmful structures in the steel plate which concerns on this embodiment need to be suppressed as much as possible. Even considering production due to micro segregation and operational fluctuations, the structures of neither of these martensite and bainite must be reduced to less than 1%.

Next, the various component ranges in the steel plate which concerns on this embodiment are demonstrated. Content unit "%" of an alloying element means the mass%.

C: 0.16 to 0.20%

Since C increases the hardness of the quenched structure, C is an element effective for improving the hardness. Based on the experimental results shown in FIG. 2 described above, 0.16% is taken as the lower limit of the C content. On the other hand, an excessive amount of C impairs the toughness of the steel sheet and also causes a difference in hardness between the surface layer and the central portion. Therefore, based on the experiment result shown in FIG. 2 mentioned above similarly, the upper limit of C content is made into 0.20%. The C content may be 0.17% or more, 0.18% or more, or 0.19% or more. The C content may be 0.19% or less, 0.18% or less, or 0.17% or less.

Si: 0.50 to 1.00%

Si has a deoxidation effect. In addition, Si is an effective element in order to improve the strength of a steel sheet, and can improve quenchability without raising Ceq. Therefore, content of Si is made into 0.50% or more. However, a large amount of Si promotes temper brittleness and lowers the toughness of the steel sheet. Therefore, it is preferable to reduce Si content, and the upper limit shall be 1.00%. Si content may be 0.60% or more, 0.65% or more, or 0.70% or more. Si content may be 0.90% or less, 0.85% or less, or 0.80% or less.

Mn: 0.90 to 1.50%

Mn has a deoxidation effect. In addition, Mn is an element which improves quenchability and is effective for improving the strength of a steel sheet. Therefore, Mn content becomes 0.90% or more. On the other hand, excessive Mn encourages temper brittleness and lowers the toughness of the steel sheet. Therefore, the upper limit of Mn content is made into 1.50%. The Mn content may be 1.00% or more, 1.05% or more, or 1.10% or more. The Mn content may be 1.40% or less, 1.35% or less, or 1.30% or less.

P: 0.010% or less

P is an impurity element contained in steel. P is a harmful element which promotes grain embrittlement and reduces the toughness of the steel sheet. Therefore, it is preferable that P content is as few as possible. Therefore, P content is reduced to 0.010% or less. Since P is not needed by the steel plate which concerns on this embodiment, the minimum of P content is 0%. However, from a viewpoint of refining cost and productivity, P content may be prescribed | regulated to 0.001% or more. The P content may be 0.002% or more, 0.003% or more, or 0.005% or more. The P content may be 0.008% or less, 0.007% or less, or 0.006% or less.

S: 0.0020% or less

S is an impurity element contained in steel. S is an element that lowers the toughness of the steel sheet through the formation of segregation and sulfides. Therefore, it is preferable that S content is as few as possible. Therefore, S content is reduced to 0.0020% or less. Since S is not needed by the steel plate which concerns on this embodiment, the minimum of S content is 0%. However, S content may be 0.0004% or more from a viewpoint of refining cost and productivity. The S content may be 0.0005% or more, 0.0006% or more, or 0.0007% or more. The S content may be 0.0018% or less, 0.0015% or less, or 0.0010% or less.

Cu: 0 to 0.40%

Cu is an element capable of increasing the strength of steel without impairing low temperature toughness. However, a large amount of Cu may generate a crack in a steel plate at the time of hot working. In addition, a large amount of Cu may reduce the toughness of the steel sheet through deposition of metal Cu. For this reason, the upper limit of Cu content is made into 0.40%. Cu contributes to the suppression of ferrite by increasing Ceq, but it is not essential to the steel sheet according to the present embodiment because replacement by other alloying elements is possible. For this reason, the minimum of Cu content shall be 0%. However, since reduction of Cu requires cost, in view of refining cost, 0.01% or 0.02% may be the lower limit of the Cu content. The Cu content may be 0.03% or more, 0.05% or more, or 0.10% or more. Cu content may be 0.35% or less, 0.30% or less, or 0.20% or less.

Ni: 0.20 to 1.00%

Ni is an effective element for improving the strength and toughness of steel. Therefore, Ni content becomes 0.20% or more. On the other hand, even if the amount of Ni is excessive, the effect is saturated, and a large amount of Ni, which is an expensive alloy, causes deterioration of manufacturing cost. Therefore, the upper limit of Ni content is made into 1.00%. The Ni content may be 0.25% or more, 0.30% or more, or 0.40% or more. The Ni content may be 0.90% or less, 0.80% or less, or 0.70% or less.

Cr: 0.60 to 0.99%

Mo: 0.60 to 1.00%

Cr and Mo have the effect | action which improves quenchability and raises central hardness. Moreover, Cr and Mo also have the effect of raising the hardness of a surface layer and a center part by precipitation hardening. Therefore, content of Cr and Mo is made into 0.60% or more. However, excessive amounts of Cr and Mo may reduce toughness due to alloy carbide formation. For this reason, the upper limit of Cr content is made into 0.99%, and the upper limit of Mo content is made into 1.00%. The Cr content may be 0.65% or more, 0.70% or more, or 0.75% or more. The Cr content may be 0.95% or less, 0.90% or less, or 0.80% or less. The Mo content may be 0.65% or more, 0.70% or more, or 0.75% or more. The Mo content may be 0.95% or less, 0.90% or less, or 0.80% or less.

V: 0 to 0.050%

V improves the base material strength through the formation of carbides. However, a large amount of V causes a drop in toughness due to alloy carbide formation. Therefore, the upper limit of V content is made into 0.050%. Although V contributes to the suppression of ferrite by increasing Ceq, V is an expensive alloying element and can be replaced by another alloy, and thus is not essential to the steel sheet according to the present embodiment. For this reason, the minimum of V content shall be 0%. However, from the viewpoint of refining cost, 0.003% may be the lower limit of the V content. The V content may be 0.005% or more, 0.010% or more, or 0.015% or more. The V content may be 0.045% or less, 0.040% or less, or 0.035% or less.

Al: 0.050 to 0.085%,

Al is an effective element as a deoxidizer. Al also associates with N in the steel to form AlN and contributes to the refinement of the structure. In addition, Al becomes AlN in the precipitation treatment and contributes to the decomposition of BN, thereby also stabilizing the quenchability exhibited by B. Therefore, Al content is made into 0.050% or more. However, excess Al forms coarse AlN, resulting in a decrease in toughness and cracking of the cast piece. Therefore, the upper limit of Al content is made into 0.085%. The Al content may be 0.055% or more, 0.060% or more, or 0.065% or more. The Al content may be 0.080% or less, 0.075% or less, or 0.070% or less.

N: 0.0020 to 0.0070%,

N forms nitride element and nitride carbonitride with an alloying element, and contributes to the structure refinement of a steel plate. Therefore, 0.0020% is made into the minimum of N content. On the other hand, when N is excessively dissolved in steel and when N forms coarse nitride, carbonitride, etc., the toughness of the steel sheet is lowered. Therefore, N content is made into 0.0070% an upper limit. The N content may be 0.0025% or more, 0.0030% or more, or 0.0035% or more. N content may be 0.0065% or less, 0.0060% or less, or 0.0050% or less.

As mentioned above, it is necessary to make Al * N (the product of Al content and N content) into 2.0 * 10 <^-4> or more. This is to utilize the pinning effect of AlN, which contributes to the grain size of the steel sheet.

B: 0.0005 to 0.0020%

B is an element which improves the quenchability of steel and improves strength. Therefore, B content is made into 0.0005% or more. However, when B becomes excess, metal carbide will be formed and quenchability will fall. Therefore, the upper limit of B content is made into 0.0020%. The B content may be 0.0007% or more, 0.0008% or more, or 0.0010% or more. The B content may be 0.0018% or less, 0.0016% or less, or 0.0015% or less.

Moreover, as an optional element, the following element content which affects toughness is prescribed | regulated. However, since the steel plate which concerns on this embodiment is not essential for the following selection elements in order to solve the subject, the lower limit of content of each selection element is 0%.

Nb: 0 to 0.050%

Nb is an element which contributes to refinement of the internal structure of steel by forming carbonitride and affects toughness. Therefore, 0.001% or more of Nb can be contained. However, coarse carbonitride produced by a large amount of Nb lowers toughness. Therefore, the upper limit of Nb content is made into 0.050%. The Nb content may be 0.002% or more, 0.005% or more, or 0.008% or more. The Nb content may be 0.045% or less, 0.040% or less, or 0.035% or less.

Ti: 0 to 0.020%

Ti / N≤3.4

Ti is an element which contributes to the refinement of a structure by forming a stable nitride, and affects toughness. Therefore, 0.001% or more of Ti can be contained. However, excess Ti causes a drop in toughness due to coarse nitride. Therefore, Ti content makes 0.020% an upper limit. The Ti content may be 0.002% or more, 0.005% or more, or 0.008% or more. The Ti content may be 0.018% or less, 0.016% or less, or 0.012% or less.

In addition, when Ti content exceeds the stoichiometric ratio of TiN, specifically, when Ti / N> 3.4, excess Ti will form carbide and it will reduce toughness. Therefore, it is preferable to set Ti / N ≦ 3.4. Ti / N may be 3.3 or less, 3.2 or less, or 3.0 or less. Although it is not necessary to define the lower limit of Ti / N, since the lower limit of Ti content is 0%, the lower limit of Ti / N may be defined as 0%. Ti / N may be 0.2 or more, 0.5 or more, or 1.0 or more.

Ca: 0 to 0.0030%,

Mg: 0% to 0.0030%,

REM: 0-0.0030%,

Ca, Mg and REM all bind to harmful impurities such as S and form harmless inclusions. Thereby, Ca, Mg, and REM can all improve mechanical properties, such as toughness of steel. Therefore, content of Ca, Mg, and REM can be made into 0.0001% or more. However, when the content of Ca, Mg, and REM becomes excessive, the effect is not only saturated, but also facilitates melting of refractory materials such as casting nozzles. Therefore, the upper limit of content of Ca, Mg, and REM is made into 0.0030%. It is good also considering content of Ca, Mg, and REM as 0.0002% or more, 0.0005% or more, or 0.0010% or more. The content of each of Ca, Mg and REM may be 0.0025% or less, 0.0020% or less, or 0.0015% or less. In addition, the term "REM" refers to a total of 17 elements consisting of Sc, Y and lanthanoid, and "content of REM" means the total content of these 17 elements.

Remainder of the chemical component of the steel plate which concerns on this embodiment contains iron and an impurity. An impurity is a component mixed with raw materials, such as ore or scrap, or various factors of a manufacturing process, when manufacturing steel materials industrially.

The average value of the old austenite grain size in the sheet thickness center part of the steel plate which concerns on this embodiment is less than 80 micrometers. When the old austenite particle diameter in the sheet thickness center part is 80 micrometers, a sheet thickness center part will have high toughness. The old austenite grain size in the center of the sheet thickness may be 76 µm or less, 73 µm or less, 70 µm or less, or 68 µm or less. The refinement of the old austenite particle diameter in the sheet thickness center part of the steel plate which concerns on this embodiment is mainly achieved by the pinning effect by fine AlN as mentioned above.

The plate | board thickness of the steel plate which concerns on this embodiment is more than 200 mm. The steel plate whose plate | board thickness is more than 200 mm can be used as a raw material of the huge gear used for the rotating mechanism of the large industrial machine represented by a rotary kiln, and therefore industrial application is high. However, the steel plate concerning this embodiment has favorable hardness and low-temperature toughness even if plate | board thickness is 200 mm or less. The sheet thickness of the steel sheet may be 205 mm or more, 210 mm or more, or 220 mm or more. Although the upper limit of the plate | board thickness of a steel plate is not specifically limited, You may make plate | board thickness 250 mm or less, 240 mm or less, or 230 mm or less.

-20 degreeC Charpy absorption energy (vE- ₂₀ degreeC) measured in the C direction of the plate | board thickness center part of the steel plate which concerns on this embodiment is 47 J or more. Here, Charpy absorbed energy is a 3-point average of the values measured based on ASTM (American Society for Testing and Materials) A370-2017a. VE- ₂₀ degreeC measured in the C direction is vE- ₂₀ degreeC obtained using the Charpy impact test piece collected along the C direction (direction perpendicular | vertical to a rolling direction and a plate thickness direction). The steel sheet to which the above-mentioned requirements are satisfied with respect to the Charpy absorbed energy has a high low temperature toughness even in the center of the plate thickness which is difficult to control mechanical characteristics normally. The -20 ° C Charpy absorption energy measured in the C direction of the sheet thickness center part of the steel sheet according to the present embodiment may be 50J or more, 55J or more, or 60J or more. Although it is not necessary to define the upper limit of the -20 degreeC Charpy absorbed energy measured in C direction of the sheet | seat thickness center part of the steel plate which concerns on this embodiment, you may define this as 400J, 380J, or 350J, for example.

The hardness of the surface layer and sheet thickness center part of the steel plate which concerns on this embodiment is HB350 or more. Here, the hardness of the steel sheet according to the present embodiment is a five-point average of HBW10 / 3000 (indenter diameter 10 mm, test force 3000 kgf) specified in JIS Z 2243-1: 2018. The steel sheet which satisfies the above-mentioned requirements with respect to hardness has a high hardness even in a sheet thickness center part where it is difficult to secure a high hardness, and the hardness of the surface layer is not excessive. The surface layer hardness of the steel sheet according to the present embodiment may be HB360 or more, HB370 or more, or HB380 or more. The hardness of the plate thickness center part of the steel plate which concerns on this embodiment may be HB360 or more, HB370 or more, or HB380 or more. Although it is not necessary to define the upper limit of the surface layer hardness of the steel plate which concerns on this embodiment, you may define this as HB450, HB420, or HB400, for example. Although it is not necessary to define the upper limit of the hardness of the sheet thickness center part of the steel plate which concerns on this embodiment, you may define this as HB450, HB420, or HB400, for example.

Next, the measuring method of each component of the steel plate which concerns on this embodiment is demonstrated below.

In order to exclude the influence of surface decarburization, segregation, etc., the component of a steel plate is normal in 1 / 4T part (the depth position of 1/4 of thickness T of steel plate, and its vicinity from the rolling surface of a steel plate). Measured by the method. Based on this measured value, Ceq, AlxN, Ti / N, and 4xf / g of a steel plate are computed. If the molten steel analysis value of the slab used as a material of a steel plate is known, you may consider it as a chemical component of a steel plate.

The -20 ° C Charpy absorbed energy (vE _{-20 ° C} ) measured in the C direction in the sheet thickness center part is measured according to ASTM A370-2017a. The test piece shall be a V notched test piece. Three test pieces are extract | collected from the plate thickness center part of a steel plate. At the time of collection of the test piece, the longitudinal direction of the test piece and the C direction (the direction perpendicular to the rolling direction and the sheet thickness direction) of the steel sheet are made to coincide. The average value of vE- ₂₀ degreeC of these three test pieces is made into -20 degreeC Charpy absorbed energy measured in the C direction in the sheet thickness center part of a steel plate.

The surface layer hardness of the steel sheet and the hardness of the sheet thickness center portion of the steel sheet are measured based on JIS Z 2243-1: 2018. HBW10 / 3000 is obtained by setting the indenter diameter to 10 mm and the test force to 3000 kgf. The measurement of the surface layer hardness is performed by press-fitting the indenter to the surface formed by removing the area | region to the depth of at least 1 mm from the rolling surface of a steel plate. The average value of the measurement result of surface hardness in 5 points | pieces is made into the surface layer hardness of a steel plate. Hardness measurement of the sheet thickness center part of a steel plate is performed by indenting an indenter with respect to the location corresponded to plate thickness center part in the surface formed by cutting a steel plate parallel with a rolling surface. The average value of the measurement result of the plate | board thickness center part hardness in 5 points | pieces is made into the hardness of the plate thickness center part of a steel plate.

The measuring method of the area ratio of martensite and bainite in plate center part is as follows. An observation surface is made into the surface parallel to the rolling direction of a steel plate, and it grind | polish and nital-etch it. This observation surface is observed with a 500 times optical microscope. Based on the optical micrograph, the sum of the area ratios of martensite and bainite can be measured. The total area of the observation visual field is 0.300 mm 2 or more.

The average value measuring method of the old austenite particle diameter in plate center part is as follows. An observation surface is made into the surface parallel to the rolling direction of a steel plate, and this is polished and picric acid etching. The average slice length is measured by the slice method, and the average slice length is taken as the average sphere γ particle size. However, the section length at the time of a measurement shall be 1000 micrometers (1 mm) or more. Although it is not necessary to specifically determine the upper limit of a fragment length, it is not necessary to measure by the fragment length exceeding 2000 micrometers (2 mm), and the upper limit may be 2000 micrometers (2 mm).

Next, the preferable manufacturing method of the steel plate which concerns on this embodiment is demonstrated. According to the findings of the present inventors, according to the manufacturing conditions described below, the steel sheet according to the present embodiment can be obtained. However, even if it is a steel plate obtained by conditions other than the manufacturing conditions demonstrated below, it corresponds to the steel plate which concerns on this embodiment as long as the requirements mentioned above are satisfied.

The manufacturing method of the steel plate which concerns on this embodiment is a process S1 of heating a slab, the process S2 of hot-rolling a slab, obtaining a steel plate, the process S3 of cooling a steel plate, and depositing a steel plate, as shown in FIG. The process S4 to process, the process S5 which quenches a steel plate, and the process S6 which temper a steel plate are provided. The manufacturing conditions in these processes are as follows.

TABLE 1

In the step S1 of heating the slab, the slab having the components of the steel sheet according to the present embodiment described above is cast and then heated to a temperature of AlN solid solution temperature Ts or more calculated by the above formula (4). The technical significance of AlN solid solution temperature Ts is as above-mentioned.

The slab component not only satisfies the upper and lower limits of the alloy elements, but also has a Ceq of 0.750 to 0.800%, Al x N of 2.0 x 10 ^-4 or more, Ti / N of 3.4 or less, and 4, similarly to steel sheets. Xf / g needs to be 9.00 or more. The preferable numerical ranges of content of each alloying element, Ceq, Al * N, Ti / N, and 4 * f / g are the same as those of a steel plate. When the molten steel analysis value of a slab is known, you may consider that value as a chemical component of a slab.

The step S2 of hot rolling the heated slab is not particularly limited. In this embodiment, since the plate | board thickness is for the manufacture of the steel plate which is more than 200 mm, the plate | board thickness of the steel plate (hot rolled steel plate) obtained by hot rolling becomes more than 200 mm.

In step S3 of cooling the steel sheet, it is preferable to complete the transformation from the austenite to other structures in the structure of the steel sheet by cooling the steel sheet to 500 ° C. or lower, preferably 150 ° C. or lower.

In step S4 of precipitation-treating a steel plate, a steel plate is heated to precipitation process temperature Tp, and temperature is maintained at this temperature T. FIG. Precipitation process temperature Tp is temperature more than 550 degreeC and less than Ac1, and satisfy | fills Formula (5) mentioned above. Precipitation processing time tp also satisfy | fills Formula (5) mentioned above. The technical significance of the precipitation treatment conditions is as described above. In addition, after temperature retention in the step S4 of the precipitation treatment, the steel sheet may be cooled to 500 ° C. or lower, preferably 150 ° C. or lower (for example, room temperature), or may be heated up as it is for subsequent quenching. .

In step S5 of quenching the steel sheet, the steel sheet is heated to a quenching holding temperature Tq (° C.) of 900 ° C. or more and 950 ° C. or lower, and during the quenching holding time tq (minutes) or more shown in the above formula (6), The temperature is maintained, followed by water cooling. The technical significance of the quench holding temperature Tq and the quench holding time tq is as described above. In the quenching treatment, the cooling means of the steel sheet after the completion of temperature holding is obtained by water cooling or a cooling rate at the same level as this. Quenching end temperature is 150 degrees C or less, for example.

In process S6 which tempers a steel plate, after tempering at tempering temperature of 500 degreeC or more and 550 degrees C or less, it is preferable to cool to 150 degrees C or less. The technical significance of the tempering temperature is as described above.

Example

Steel slabs obtained by dissolving steels of A1 to A10 and B1 to B24 having chemical components shown in Table 2-1 are the steels of the present invention Nos. 1 to 10 and Nos. 11 to 10 shown in Tables 3-1 to 3-3. Comparative Examples of 43 The heating, rolling, and heat treatment were carried out under the respective conditions to prepare a steel sheet having a plate thickness of 210 mm to 230 mm. In addition, the manufacturing conditions which are not described in a table | surface are as follows. All the chemical components of Table 2-1 are molten steel analysis values.

End temperature of cooling after hot rolling: 150 degrees C or less in all the examples and the comparative example

Cooling means in quenching: Water cooling (cooling up to 150 ° C or less)

End temperature of cooling in tempering: 150 degrees C or less in all the examples and the comparative example

The surface layer hardness of the steel sheet and the hardness of the sheet thickness center portion of the steel sheet were measured based on JIS Z 2243-1: 2018. HBW10 / 3000 was calculated | required by making an indenter diameter 10 mm and test force 3000 kgf. The measurement of surface layer hardness was performed by indenting the indenter to the surface formed by removing the area | region to the depth of at least 1 mm from the rolling surface of a steel plate. The average value of the measurement result of surface layer hardness in 5 points | pieces was made into the surface layer hardness of the steel plate (Table 4 "HB surface layer"). The hardness measurement of the sheet thickness center part of the steel plate was performed by indenting the indenter to the location corresponding to a sheet thickness center part in the surface formed by cutting a steel plate parallel to a rolling surface. The average value of the measurement result of the plate | board thickness center part hardness in 5 points | pieces was made into the hardness of the plate thickness center part of steel plate (Table 4 "HB center part"). In addition, the test piece of the steel plate was extract | collected in the site | part spaced apart more than plate | board thickness from the edge part of the width direction and the longitudinal direction of the steel plate.

_-20 degreeC Charpy absorbed energy (vE- ₂₀ degreeC) measured in C direction in the sheet thickness center part was measured based on ASTM A370-2017a. Three test pieces were extract | collected from the plate | board thickness center part of the steel plate. At the time of collection of the test piece, the longitudinal direction of the test piece and the C direction (direction perpendicular to the rolling direction and the plate thickness direction) of the steel sheet were made to coincide. The average value of vE- ₂₀ degreeC of these three test pieces was made into -20 degreeC Charpy absorbed energy measured in C direction in the plate | board thickness center part of a steel plate (Table 4 "vE- ₂₀ degreeC").

The method of measuring the area ratio of martensite and bainite in the sheet thickness center part was as follows. Observation surface was made into the surface parallel to the rolling direction of a steel plate, and it grind | polished and nital etching. This observation surface was observed with the optical microscope of 500 times. Based on the optical micrograph, the sum of the area ratios of martensite and bainite was measured. The total area of the observation visual field was 0.300 mm 2 or more.

The average value measuring method of the old austenite grain size in the sheet thickness center part was as follows. The observation surface was made into the surface parallel to the rolling direction of a steel plate, and it grind | polished and the picric acid etching. The average section length was measured by the sectioning method (section length: 1000 micrometers or more and 2000 micrometers or less), and the average section length was made into the average value of the old austenite particle diameter in Table center part (Table 4 "sphere gamma particle size").

Hereinafter, a component is shown in Table 2-1 and Table 2-2, a manufacturing method is shown in Tables 3-1 to 3-3, a material, evaluation, etc. are shown in Table 4. "Precipitation process temperature Tp" shown in a table | surface is a value obtained by applying the heat history at the time of a precipitation process to said Formula (8). The "precipitation processing time tp" described in the table is, until the steel sheet is extracted from the heat treatment furnace after the temperature of the sheet thickness center part of the steel sheet finally exceeds the "maximum temperature -40 degreeC of the sheet thickness center part in precipitation process". It's time. "? Ching holding temperature Tq" shown in a table | surface is a value obtained by applying the heat history at the time of a quenching process to said Formula (9). The "actual quench holding time" described in the table indicates that the steel sheet is extracted from the heat treatment furnace after the temperature of the sheet thickness center portion of the steel sheet finally exceeds the "maximum temperature -40 ° C of the sheet thickness center portion of the sheet steel during quenching". Time to be (ie, performance tq). "? Ching holding time tq" shown in a table | surface is the value obtained by substituting the latching holding temperature Tq and f shown in the table | surface for Formula (6) mentioned above. In the Al × N column of Table 2-2, for example, 2.2E-04 means 2.2 × 10 ⁻⁴ . In addition, in Table 3-2, it is determined using the precipitation process time threshold value obtained using the following formula (5 ') whether precipitation process temperature Tp and precipitation process time tp satisfy | fill Formula (5). have. When precipitation process time tp is more than a precipitation process time threshold value, precipitation process temperature Tp and precipitation process time tp satisfy | fill Formula (5).

(Precipitation Processing Time Threshold) = 10 ^{(-0.012 × Tp + 8.7)} : Formula (5 ')

TABLE 2-1

Table 2-2

Table 3-1

Table 3-2

Table 3-3

TABLE 4

Test Nos. 1 to 10 satisfy the chemical composition range and suitable manufacturing conditions of the present invention. All of these steels had a total area ratio of martensite and bainite of 99% or more, and the average value of the old austenite grain size in the center portion was 80 µm or less, and both the surface hardness, the center hardness, and the center impact absorption energy met the target. I'm making it.

In Test Nos. 11 and 12, C deviated from the chemical component range of the present invention. In the test number 11, since C was insufficient and the hardness at the time of quenching was not enough, the hardness did not satisfy a target value after tempering. On the other hand, Test No. 12 is an example in which C is excessive, and is a position where the impact absorbing energy is low due to the influence of hard carbide precipitation, which becomes the starting point of fracture.

In the test numbers 13 and 14, Si deviated from the chemical component range of this invention. In the test number 13, since the Si was insufficient and the quenchability could not be secured, the central hardness did not satisfy the target value. On the other hand, Test No. 14 is an example in which Si is excessively high, but the hardness is sufficient, but the shock absorbing energy does not satisfy the target due to the promotion of temper embrittlement by Si.

In the test numbers 15 and 16, Mn deviates from the chemical component range of this invention. In the test number 15, since the Mn was insufficient and the hardness at the time of quenching was not enough, the center hardness did not satisfy the target value even after tempering. On the other hand, Test No. 16 is an example in which Mn is excessive, and the shock absorbing energy does not satisfy the target value due to the promotion of temper embrittlement.

In the test number 17, although P content is high beyond the chemical component range of this invention, and hardness is sufficient, impact absorption energy does not satisfy | fill the target by the embrittlement resulting from P.

In the test number 18, S content is high beyond the chemical component range of this invention. Therefore, in the test number 18, the impact absorption energy does not satisfy the target by generation of MnS which is an extension inclusion.

In the test number 19, Cu content was high beyond the chemical component range of this invention, and the precipitated metal Cu became a brittle fracture starting point. Therefore, in the test number 19, shock absorption energy does not satisfy a target.

In the test number 20, Ni content is low beyond the chemical component range of this invention and does not account for the quantity which improves toughness. Therefore, in the test number 20, shock absorption energy does not satisfy | fill a target.

Test numbers 21 and 22 are examples in which Cr deviated from the chemical component range of the present invention. In the test number 21, Cr is insufficient and sufficient quenchability and precipitation strengthening action are not obtained. From this, in the test number 21, central hardness cannot satisfy | fill a target, and shock absorption energy does not achieve the target. On the other hand, in the test number 22, Cr was excess and the influence of precipitation of coarse Cr carbide was remarkable. As a result, in the test number 22, the shock absorbing energy does not satisfy the target.

Test numbers 23 and 24 are examples in which Mo deviated from the chemical component range of the present invention. In the test number 23, Mo was insufficient and sufficient quenchability and precipitation strengthening action were not obtained. From this, in the test number 23, central hardness did not satisfy | fill the target, and shock absorption energy did not achieve the target. On the other hand, in the test number 24, the influence of precipitation of Mo and excess and coarse Mo carbide was remarkable. As a result, in the test number 24, the impact absorption energy does not satisfy the target value.

In the test number 25, V was high beyond the chemical component range of this invention, and the coarse carbide, nitride, etc. of V became a brittle fracture starting point. From this, in the test number 25, shock absorption energy does not satisfy a target.

Test numbers 26 and 27 are examples in which Al deviated from the chemical component range of the present invention. Test number 26 is an example in which Al is insufficient, and it is not possible to secure AlN effective for pinning, and the excess N is associated with B to reduce the quenchability improving effect. For this reason, in the test number 26, structures other than martensite and bainite became excess, and the particle size of the retained austenite became coarse. As a result, in test number 26, the central hardness and the shock absorbing energy did not satisfy the target. On one side, Test No. 25 is an example in which Al is excessive, and AlN excessively coarsens, resulting in a brittle fracture starting point. For this reason, in the test number 25, shock absorption energy does not satisfy | fill a target.

Test numbers 28 and 29 are examples in which N deviated from the chemical component range of the present invention. Test No. 28 is an example in which N is insufficient and Al x N is less than a predetermined range, and the pinning effect is weak in that the amount of nitride and carbonitride is insufficient, and granulation of crystal grains occurs. As a result, in test number 28, the energy of absorbing shock does not satisfy the target. On the other hand, Test No. 29 is an example in which N is excessive, and excessive coarsening of nitrides and carbonitrides occurs. As a result, in the test number 29, the shock absorbing energy does not satisfy the target.

In Test Nos. 30 and 31, B deviated from the chemical component range of the present invention. Test number 30 is an example of a lack of B, and it is no longer possible to secure the amount of employment B necessary for identification. As a result, in the test number 30, structures other than martensite and bainite become excess, and central hardness and impact absorption energy do not satisfy the target. On the other hand, Test No. 31 is an example in which B is excessively contained, and the impact absorbed energy does not satisfy the target because the carbide of the metal element precipitates.

In the test number 32, although the component range of each alloying element is in the range of this invention, Ceq is low beyond the suitable range of this invention. In the test number 32, as a result of ferrite which generate | occur | produced in the structure | tissue by deterioration of quenchability, center hardness and impact absorption energy do not satisfy | fill the target.

In Test Nos. 33 and 34, although the component range and Ceq of the individual alloy elements are within the scope of the present invention, the parametric formula 4xf / g is lower than the suitable range of the present invention. In the test numbers 33 and 34, the hardening effect by the precipitation element became large compared with the improvement of quenchability. Therefore, in the test numbers 33 and 34, the shock absorption energy does not satisfy a target.

Test number 35 indicates that although various indices derived from the component range and chemical composition of individual alloy elements are within the scope of the present invention, the heating temperature before rolling is less than the solid solution temperature Ts. In the test number 35, unemployed coarse AlN remained and it became a brittle fracture starting point. For this reason, in the test number 35, the old austenite particle diameter coarsened, and absorption energy does not satisfy | fill the target.

In Test Nos. 36 and 37, although various indices derived from the component range and chemical composition of individual alloy elements are within the scope of the present invention, the precipitation treatment temperature deviates from the suitable range of the present invention. Test No. 36 is an example in which the precipitation treatment temperature was low, and sufficient AlN was not precipitated and AlN effective for pinning could not be secured. As a result, in Test No. 36, the old austenite grain size coarsened, and the absorbed energy did not satisfy the target. On the other hand, Test No. 37 is an example in which the precipitation treatment temperature exceeded Ac1, and caused coarsening of AlN locally by holding in the α-γ two-phase region. For this reason, in test number 37, absorption energy does not satisfy | fill a target.

Test No. 38 indicates that although various indexes derived from the component range and chemical composition of individual alloying elements are within the scope of the present invention, the temperature and time of the precipitation treatment do not satisfy the formula (5), which is a suitable range of the present invention. Yes. In Test No. 38, sufficient AlN was not precipitated and AlN effective for pinning could not be secured. From this, in Test No. 38, the old austenite grain size coarsened, and the absorbed energy did not satisfy the target.

Test number 39 is an example in which the quenching temperature is lower than the suitable range of the present invention, although various indexes derived from the component ranges and chemical components of individual alloy elements are within the scope of the present invention. In the test number 39, since the solid solution of the alloying element was not fully performed, quenchability was low and excess ferrite was produced | generated. As a result, in the test number 39, the central hardness and the absorbed energy did not satisfy the target.

Test number 40 is an example in which the quench holding temperature Tq exceeds the suitable range of the present invention although the various indexes derived from the component ranges and chemical components of individual alloy elements are within the scope of the present invention. In test number 40, excessive coarsening of grains occurred. From this, in Test No. 40, the old austenite grain size coarsened, and the absorbed energy did not satisfy the target.

Test number 41 indicates that although various indexes derived from the component range and chemical composition of the individual alloy elements are within the scope of the present invention, the actual quench holding time is less than the quench holding time tq which is a suitable range of the present invention. It is an example in which the solid solution of an alloying element is not fully made. From this, in the test number 41, there was low quenchability and excess ferrite was produced | generated. As a result, in test number 41, the central hardness and the absorbed energy did not satisfy the target.

Test number 42 is an example in which various indexes derived from component ranges and chemical components of individual alloy elements are within the scope of the present invention, but the tempering temperature is less than the suitable range. In the test number 42, temper embrittlement generate | occur | produced. From this, in test number 42, absorbed energy does not satisfy a target.

Test number 43 is an example in which the tempering temperature exceeds the suitable range although the various indexes derived from the component range and chemical composition of the individual alloy elements are within the scope of the present invention. In the test number 43, the precipitation hardening effect | action of alloy carbide reduced. Therefore, in the test number 43, central hardness does not satisfy the target.

1: steel plate
11: plate thickness in the center
12: surface layer
13: rolled surface

Claims (3)

Chemical component is unit mass%,
C: 0.16 to 0.20%,
Si: 0.50 to 1.00%,
Mn: 0.90 to 1.50%,
P: 0.010% or less,
S: 0.0020% or less,
Cu: 0 to 0.40%,
Ni: 0.20 to 1.00%,
Cr: 0.60 to 0.99%,
Mo: 0.60 to 1.00%,
V: 0 to 0.050%,
Al: 0.050 to 0.085%,
N: 0.0020 to 0.0070%,
B: 0.0005 to 0.0020%,
Nb: 0 to 0.050%,
Ti: 0 to 0.020%,
Ca: 0 to 0.0030%,
Mg: 0% to 0.0030%,
REM: 0-0.0030%, and
Balance: consisting of Fe and impurities,
In the center of sheet thickness, the total area ratio of martensite and bainite is 99% or more,
The average value of the old austenite particle diameter in the said plate | board thickness center part is less than 80 micrometers,
Ceq represented by Formula (1) is 0.750 to 0.800%,
Al × N is at least 2.0 × 10 ⁻⁴ ,
Ti / N is 3.4 or less,
Moreover, the value f shown by Formula (2) and the value g represented by Formula (3) satisfy | fill 4xf / g≥9.00,
-20 degreeC Charpy absorbed energy measured in C direction in the said plate | board thickness center part is 47J or more,
The hardness of the surface layer and the sheet thickness center is at least HB350,
Plate thickness greater than 200mm
Steel sheet, characterized in that.
Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5: Formula (1)
f = 4 × C + Si + 2 × Mn + Ni + 2 × Cr + 5 × Mo: Formula (2)
g = 2 × Cr + 3 × Mo + 5 × V: Formula (3)
Here, the element symbol described in each formula means content in the unit mass% of the element with respect to each element symbol. It is a manufacturing method of the steel plate of Claim 1,
Heating the slab,
Hot rolling the slab to obtain a steel sheet having a plate thickness greater than 200 mm;
Cooling the steel sheet;
Precipitating the steel sheet;
A step for quenching the steel sheet,
Tempering the steel sheet
Equipped,
The chemical component of the slab is, in unit mass%, C: 0.16 to 0.20%, Si: 0.50 to 1.00%, Mn: 0.90 to 1.50%, P: 0.010% or less, S: 0.0020% or less, Cu: 0 to 0.40 %, Ni: 0.20 to 1.00%, Cr: 0.60 to 0.99%, Mo: 0.60 to 1.00%, V: 0 to 0.050%, Al: 0.050 to 0.085%, N: 0.0020 to 0.0070%, B: 0.0005 to 0.0020% , Nb: 0 to 0.050%, Ti: 0 to 0.020%, Ca: 0 to 0.0030%, Mg: 0 to 0.0030%, REM: 0 to 0.0030%, and the balance: Fe and impurities, and the formula of the slab (1 Ceq represented by) is 0.750 to 0.800%, Al x N of the slab is 2.0 × 10 ^-4 or more, Ti / N of the slab is 3.4 or less, the value f and the formula represented by the formula (2) of the slab The value g represented by (3) satisfies 4xf / g≥9.00,
Slab heating temperature in the process of heating the said slab is AlN solid solution temperature Ts (degreeC) or more computed by Formula (4),
The step of depositing the steel sheet is performed by heating the steel sheet to a precipitation treatment temperature Tp (° C.) of less than 550 ° C. and less than Ac1, and then maintaining the precipitation steel at this temperature for a precipitation treatment time tp (hour). (° C) and the precipitation treatment time tp (time) satisfy the formula (5), wherein Ac1 is represented by the formula (7),
In the step of quenching the steel sheet, the steel sheet is heated to a quenching holding temperature Tq (° C.) of 900 to 950 ° C., and held at this temperature for at least the quenching holding time tq (minutes) shown in formula (6), It is then performed by water cooling,
The step of tempering the steel sheet is carried out by heating the steel sheet to a tempering temperature of 500 to 550 ° C. and then cooling it to 150 ° C. or less.
Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5: Formula (1)
f = 4 × C + Si + 2 × Mn + Ni + 2 × Cr + 5 × Mo: Formula (2)
g = 2 × Cr + 3 × Mo + 5 × V: Formula (3)
Ts = 7400 / (1.95-log ₁₀ (Al × N))-273: Equation (4)
Log ₁₀ (tp) + 0.012 x Tp ≥ 8.7: (5)
tq = 0.033 × (950-Tq ) 2 + (1.5 × f) 2/10: (6)
Ac1 = 750-25 × C + 22 × Si-40 × Mn-30 × Ni + 20 × Cr + 25 × Mo: Formula (7)
Here, the element symbol described in each formula is content by the unit mass% of the element with respect to each element symbol. The method for producing a steel sheet according to claim 2, wherein the cooling end temperature in the step of cooling the steel sheet is 150 ° C or less.

Images