JP2019173081A - High hardness steel plate excellent in hardness at plate thickness center part and low temperature toughness and having plate thickness of over 200 mm and method of producing the same - Google Patents

Abstract

To provide a high hardness steel plate having a plate thickness of over 200 mm, Ceq of 0.80 or less, a hardness of a surface layer and a plate thickness center part of HB350 or more, an absorption energy at -20°C of the plate thickness center part of 47 J or more, and a method of producing the same.SOLUTION: The high hardness steel plate is composed of a prescribed components, has a tempered martensite and/or tempered bainite of 99% or more by area percentage, other structures such as ferrite, pearlite, retained austenite and not tempered structure of less than 1 area%, Ceq represented by the formula (1) satisfying 0.750 to 0.800, a value f represented by the formula (2) and a value g represented by the formula (3) satisfying 4×f/g of 9.00, a 3 point average of C direction Charpy at -20°C in the plate thickness center part of 47 J or more, and a hardness of the surface layer and the plate thickness center part of 350 HB or more. Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 (1) f=4C+Si+2Mn+Ni+2Cr+5Mo (2) g=2Cr+3Mo+5 V (3).SELECTED DRAWING: None

Description

  The present invention relates to a high-hardness steel plate having a thickness of more than 200 mm and excellent in low-temperature toughness and excellent in the hardness and low-temperature toughness used in a rotating mechanism such as a gear of a large-scale industrial machine such as a rotary kiln and a method for producing the same.

  A huge gear (gear) is used for a rotation mechanism of a large industrial machine represented by a rotary kiln. The steel plate used as the material is required to have hardness and toughness from the viewpoint of the fatigue resistance and durability of the gear. 47J is now required. This is because gears are manufactured by cutting the steel material to the center of the plate thickness, and thus the characteristics of the center of the plate thickness are emphasized.

  In addition, in recent years, with the aim of increasing the size of gears, a steel plate having a thickness of more than 200 mm, which has not been heretofore, has been demanded. As the plate thickness increases, the cooling rate of the central portion of the plate thickness during quenching decreases, so that it is difficult to obtain the hardness of the central portion even after tempering. Ingredient design that simply increases the hardness causes a decrease in toughness, so it is extremely difficult to adjust the component balance to ensure layer hardness and center hardness and toughness with an extremely thick material with a plate thickness exceeding 200 mm. .

Further, for the purpose of improving the weldability, there has been a demand for the carbon equivalent Ceq due to the main contained elements to be 0.800 or less. When this exceeds 0.800, a load such as increasing the preheat temperature at the time of welding increases at the consumer. An extremely thick material such as this steel material has a very large number of welding passes, so the increase in welding load is also large. Ceq is represented by the following formula (1), for example.
Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (1)

  Conventionally, there has been no steel material that has a plate thickness of over 200 mm, ensures Ceq ≦ 0.800, has a center hardness ≧ HB350, and guarantees the low-temperature toughness at −20 ° C. Moreover, the steel plate used as a raw material had to be tempered at 500 ° C. or higher so that the material did not change due to strain relief annealing after gear processing, which was a disadvantageous condition for achieving hardness.

  As the current technology, for example, as disclosed in Patent Document 1, it is an extremely thick high hardness steel with a plate thickness of over 200 mm, HB 330 grade and −40 ° C., utilizing precipitation treatment before quenching—solid solution B securing by AlN precipitation, low Although the hardenability is improved even with Ceq (about 0.70), the upper limit of Ceq is not regulated and C is set to 0.14 or less. Therefore, only when Ceq> 0.800, Combines hardness HB350 and toughness. For this reason, it is not possible to reduce loads such as increasing the residual heat temperature during welding at the consumer.

  Patent Document 2 is an ultra-thick high-hardness steel having a thickness of more than 200 mm, HB330 grade and -20 ° C toughness, and tempering conditions are specified in order to suppress the hardness difference between the surface and the central part. The part hardness is HB300 grade, and even if Ceq is increased to 0.850 in the examples, the center part hardness ≧ HB350 cannot be achieved.

Japanese Unexamined Patent Publication No. 2017-028135 JP 2017-186592 A

  Under such circumstances, in the present invention, in particular, the plate thickness is over 200 mm, Ceq shown by the following formula is 0.800 or less, and Ceq is set to 0.750 or more for the convenience of securing the hardness of the central portion of the plate thickness. The present invention provides a high-hardness steel plate having a hardness of HB 350 or more at the surface layer and the center portion of the plate thickness, and an absorption energy at −20 ° C. of the plate thickness center portion of 47 J or more, and a method for producing the same.

In order to solve the above problems, the steel components are in mass%, C: 0.16% or more, 0.20% or less, Si: 0.50% or more, 1.00 or less, Mn: 0.90% or more, 1.50% or less, P: 0.000% or more, 0.010% or less, S: 0.000% or more, 0.002% or less, Cu: 0.00% or more, 0.40% or less, Ni: 0.20% or more, 1.00% or less, Cr: 0.60% or more, 1.00% or less, Mo: 0.60% or more, 1.00% or less, V :: 0.000% or more, 0 0.050% or less, Al: 0.050% or more, 0.085% or less, N: 0.0020% or more, 0.0070% or less, B: 0.0005% or more, 0.0020% or less, residual Fe and It consists of inevitable impurities, and tempered martensite and / or tempered bainite is 99% or more in area ratio. Thus, other structures such as ferrite, pearlite, retained austenite, and non-tempered structure are less than 1 area%, and Ceq represented by the following formula (1) satisfies 0.750 or more and 0.800 or less, and the following formula The value f shown in (2) and the value g shown in the following formula (3) satisfy 4 × f / g of 9.00 or more, and 3 points of C-direction Charpy at −20 ° C. at the center of the plate thickness A high-strength steel sheet having a thickness of more than 200 mm and excellent in hardness and low-temperature toughness in the thickness center, wherein the average is 47 J or more and the hardness of the surface layer and the thickness center is 350 or more in HB. .
Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (1)
f = 4C + Si + 2Mn + Ni + 2Cr + 5Mo (2)
g = 2Cr + 3Mo + 5V (3)
Here, each element symbol in each formula means mass% of the component.

  In addition to the above components, Nb: 0.001% or more, 0.050% or less, Ti: 0.001% or more, 0.020% or less, Ca: 0.0001% or more, 0.0030% or less, Mg: 0.0001% or more and 0.0030% or less, REM: 0.0001% or more and 0.0030% or less, and when containing Ti and containing Ti, Ti / N ≦ 3. 4 is also preferably satisfied.

Further, the steel components are in mass%, C: 0.16% or more, 0.20% or less, Si: 0.50% or more, 1.00 or less, Mn: 0.90% or more, 1.50% or less, P: 0.000% or more, 0.010% or less, S: 0.000% or more, 0.002% or less, Cu: 0.00% or more, 0.40% or less, Ni: 0.20% or more, 1.00% or less, Cr: 0.60% or more, 1.00% or less, Mo: 0.60% or more, 1.00% or less, V :: 0.000% or more, 0.050% or less, Al : 0.050% or more, 0.085% or less, N: 0.0020% or more, 0.0070% or less, B: 0.0005% or more, 0.0020% or less, component composed of residual Fe and inevitable impurities The steel is cooled to a temperature equal to or higher than the AlN solid solution temperature Ts calculated by the following formula (4), cooled after hot rolling, and further After the precipitation treatment in which the treatment temperature T and the treatment time tp are heated so that the treatment temperature T and the treatment time tp satisfy the following formula (5) at a temperature higher than 50 ° C. and less than Ac1, the temperature is cooled to room temperature or is heated as it is, In the production method, a quenching treatment in which the steel is reheated to a quenching holding time H or more and is cooled with water as shown in the following formula (6), and tempered at 500 ° C. or more and 550 ° C. or less and cooled to room temperature is preferable.
Ts = 7400 / (1.95-log ₁₀ [Al] [N]) (4)
Log ₁₀ (tp) + 0.012 × T ≧ 8.7 (5)
H = 0.033 (950−Tq) 2+ (1.5f) 2/10 (6)
Here, Ts is a solid solution temperature (° C.) of AlN, [Al] and [N] are mass% of each element, T is a precipitation treatment temperature (° C.), tp is a precipitation treatment time (Hr), and H Is the quenching holding time (min), Tq is the quenching holding temperature (° C.), and f is a value obtained by the above equation (2).

  In addition to the above components, Nb: 0.001% to 0.050%, Ti: 0.001% to 0.020%, Ca: 0.0001% to 0.0030%, Mg : 0.0001% or more and 0.0030% or less, REM: 0.0001% or more and 0.0030% or less, and when Ti is contained and Ti / N ≦ 3.4 The manufacturing method which satisfy | fills is preferable.

  According to the present invention, even a steel plate having a thickness of more than 200 mm can provide a steel plate having excellent hardness from the surface layer to the center of the plate thickness and impact absorption energy performance at the center of the plate thickness and having Ceq suppressed to 0.800 or less. It can be used for the rotation mechanism of large industrial machines represented.

It is a figure which shows the relationship between C amount, plate | board thickness center part hardness, and plate | board thickness center part toughness (vE-20 degreeC). It is a figure which shows the relationship between Ceq and center part hardness. It is a figure which shows the relationship between 4xf / g and the board thickness center part toughness which were experimented using component A4 of the Example. Precipitation treatment temperature and precipitation treatment time is a diagram showing a relationship between _{Log 10 (tp [Hr])} . It is a figure which shows the relationship between quenching holding temperature and quenching holding time, (a) is the result of experimenting using component A6 of the example, and (b) is the result of experimenting using component A2 of the example. .

In the present invention, the following (1) to (5) are important. In particular, the component parameter formula (3) and the precipitation treatment (4) are particularly important as requirements for achieving HB350 grade hardness at the center of the plate thickness and vE-20 ° C. ≧ 47 J at Ceq ≦ 0.800. It is.
(1) Upper and lower limits of C amount to achieve both center hardness and center toughness (under the conditions described later) Generally, when the center hardness is HB350 or higher, the surface layer can also be secured to HB350 or higher. It is. Here, the surface layer is a position excluding the decarburized layer and the portion removed during processing, and is generally in the range of 1 to 10 mm from the outermost layer.
(2) Ceq lower limit for securing center hardness (3) Lower limit of parameter formula f / g for securing center toughness (4) Pre-quenching precipitation treatment (temperature and temperature) for securing center hardness time)
(5) Quenching conditions (temperature and time) for securing the center hardness
Details will be described below.

(1) Upper and lower limits of C amount for achieving both center hardness and center toughness (under conditions described later);
As the first item, in order to achieve both hardness and toughness at the center of the thickness under the conditions described later, C satisfies 0.16% or more and 0.20% or less as the component composition (mass%) of the steel. There is a need to. In order to ensure toughness and hardness at the center of the plate thickness exceeding 200 mm, it is necessary to suppress the carbide that becomes the starting point of brittle fracture, and as shown in FIG. ave.) ≧ 47 J, C must be 0.20% or less. On the other hand, the decrease in C greatly reduces the hardness of the steel material. Therefore, in order to secure HB350 at the center even when tempering at 500 ° C. or higher, as shown in FIG. It is necessary to.

(2) Ceq lower limit for securing center hardness;
As a second item, in order to ensure the hardness of the central portion with a plate thickness of more than 200 mm, sufficient hardenability is necessary, and after performing the precipitation treatment described later, the Ceq of the following formula (1) is 0. It is necessary to satisfy 750 or more. This is to avoid the formation of ferrite, which is a soft structure during quenching, and to form a bainite-martensite structure that is the basis of tempered martensite and tempered bainite in the product. In addition, there is no upper limit for Ceq in order to combine the hardness and toughness of the center part, but the increase in Ceq tends to cause weld cracking. When Ceq exceeds 0.800, preheating before welding is performed to avoid cracking. Ceq in the present invention is set to 0.80 or less in order to remarkably deteriorate the work efficiency such as the necessity of raising the temperature.
Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (1)
Each element symbol means a component composition (%).

  As shown in FIG. 2, it was found that the hardness of the central portion of the plate thickness is less than HB350 when the Ceq is less than 0.750 in the steel plate having a plate thickness of over 200 mm even when the precipitation treatment is performed. This is because the generation of ferrite, which is a soft tissue, was avoided.

(3) The lower limit of the parameter formula f / g for ensuring the toughness of the center part;
As a third item, in order to secure the hardness of the central portion ≧ HB350 while setting Ceq ≦ 0.800 at a plate thickness of over 200 mm and to achieve the toughness of vE−20 ° C. ≧ 47 J at the plate thickness center portion, The relationship between the parameter formula f shown in Formula 2 and the parameter formula g shown in Formula 3 below requires that 4 × f / g is 9.00 or more.
f = 4C + Si + 2Mn + Ni + 2Cr + 5Mo (2)
g = 2Cr + 3Mo + 5V (3)
Here, each element symbol in each formula means mass% of the component.

  As shown in FIG. 3, when the plate thickness exceeds 200 mm, Ceq ≦ 0.800, and the hardness at the center of the plate thickness is HB350 or more, 4 × f / g using the parameter formula g is 9.00. In the case of less than this, it was found that the toughness at the center of the plate thickness could not be secured. The element that forms the parameter formula f is an element that enhances hardenability by being dissolved in quenching. On the other hand, the element forming the parameter formula g is an element that forms precipitates during tempering, and these additions improve the hardenability while reducing the toughness due to the formation of precipitates during tempering. A large f / g indicates that the hardenability is improved while reducing the elements precipitated during tempering. In addition, since the precipitates at the time of tempering in the scope of the present invention are fine enough to be observed with a transmission electron microscope, it is industrially impractical to define the distribution state of the precipitates themselves. The usefulness of organizing by the parameter formula f / g can be understood.

(4) Process conditions for ensuring the toughness of the central part Solution → Roll → Precipitation treatment;
As a process requirement for obtaining the pinning effect of the above AlN, in order to finely precipitate the solid solution AlN by rolling after solution treatment at the AlN solid solution temperature Ts or higher calculated by the following formula (4), after rolling Before quenching and heating, it is necessary to carry out the precipitation treatment at a temperature higher than 550 ° C. and less than Ac1, so that the treatment temperature T and the treatment time tp satisfy the following formula (5).
Ts = 7400 / (1.95-log ₁₀ [Al] [N]) (4)
Log ₁₀ (tp [Hr]) + 0.012 × T [° C.] ≧ 8.7 (5)
Here, Ts is the solid solution temperature (° C.) of AlN, [Al] and [N] are mass% of each element, T is the precipitation treatment temperature (° C.), and tp is the precipitation treatment time (Hr).

  If solution treatment is not performed before rolling, coarse AlN generated during casting remains, and the total amount of AlN in the steel is reduced, so that the fine AlN obtained by precipitation treatment is reduced, and a pinning effect can be obtained. Disappear. The solution temperature is a known one.

FIG. 4 shows the results of experiments using the component A4 of Examples described later. In order to obtain the pinning action of AlN, it is necessary to perform the precipitation treatment at an appropriate temperature and time. Under the processing conditions of Log ₁₀ (tp [Hr]) + 0.012 × T [° C.] <8.7 indicated by x in the figure, AlN is not sufficiently precipitated, so that the pinning effect cannot be exhibited. Toughness cannot be ensured. On the other hand, when the temperature exceeds Ac1, the α-γ two-phase region is maintained, so that Al and N are concentrated in the γ region, and AlN is coarsened locally, so that toughness cannot be ensured. The upper limit of the treatment time is not particularly restricted from the viewpoint of mechanical properties, but 5 days = 120 Hr is the upper limit from the viewpoint of industrial production efficiency.

(5) Quenching conditions (temperature and time) for securing the center hardness;
As a fourth item, in the component range of the steel materials shown in (2) and (3) above, sufficient precipitation of AlN and BN of the BN are sufficient in the above-described precipitation treatment in order to satisfy the hardness at the center of the plate thickness ≧ HB350. In addition to causing decomposition, a quenching treatment is performed by reheating and quenching with water at a temperature of 900 ° C. to 950 ° C. at the time of quenching under conditions that satisfy the quenching holding time H (minutes) shown in the following formula 6. There is a need to.
H = 0.033 (950-Tq) 2+ (1.5f) 2/10 (6)
Here, Tq is a quenching holding temperature, and f is a value obtained by the above-described equation (2). The quenching holding temperature and time indicate the temperature and time of the steel plate thickness center, not the heat treatment furnace. These are preferably measured by inserting a thermocouple or the like, but may be calculated by heat conduction based on the furnace temperature and plate thickness.

  FIG. 5 (a) shows the result of the experiment using the component A6 of the example described later, and FIG. 5 (b) shows the result of the experiment using the component A2 of the example described later. When the quenching holding time is less than the value of the quenching holding time H (minute) shown in the above-described component parameter formula 2 and the above formula 6 calculated using the same, the center hardness is less than HB350. This is because the added alloy was not sufficiently dissolved and hardenability could not be secured. The reason why the above equation is a function of f is that as the amount of the alloy increases, it takes time to dissolve them.

  When the temperature Tq is less than 900 ° C., the alloy elements are not sufficiently dissolved, so that the hardenability cannot be ensured and HB350 cannot be achieved. On the other hand, when the quenching temperature exceeds 950 ° C., AlN partially dissolves, and the liberated N is combined with B in the steel to inhibit the effect of improving the hardenability of B, so that HB350 cannot be achieved.

(6) Upper and lower limits of tempering temperature to ensure the hardness and toughness of the center part;
In addition, as the item (6), the tempering temperature needs to be 500 ° C. or more due to the construction requirements of the gears (preventing deterioration of the material by strain relief annealing). In addition, the structure is sufficiently tempered toughness. In order to ensure this, the tempering temperature must be 500 ° C. or higher. On the other hand, since the hardness of this steel material rapidly decreases when tempering above 550 ° C., the tempering temperature needs to be 550 ° C. or lower. After this tempering, it is cooled to room temperature.

  Next, the structure in the present invention will be described. Tempered martensite and / or tempered bainite is 99% or more in area ratio, and other structures such as ferrite, pearlite, retained austenite, and structure not tempered are less than 1 area%. The above is achieved by quenching under conditions where no ferrite is produced, and tempering at a sufficiently high temperature. Specifically, the component of item (2) (Ceq ≧ 0.750 or more) is quenched under the conditions shown in item (5) after the precipitation treatment in items (3) and (4), and under the conditions shown in item (6). This is achieved by tempering.

  As another structure, ferrite is a factor that decreases the hardness of steel. In particular, in order to make it difficult to obtain the hardness of the central portion, which is likely to occur in the central portion of the plate thickness where the quenching and cooling rate is slow, it must be eliminated by the requirement described in the above item (2).

  Although pearlite is effective in securing hardness, it is a brittle fracture starting point because of its hardness, so it must be eliminated. Since pearlite is generated by concentrating C discharged during ferrite precipitation, it is simultaneously suppressed by avoiding ferrite precipitation.

  The retained austenite and the structure that has not been tempered become brittle fracture starting points and reduce the toughness of the steel material, so all must be eliminated. Since this steel material is tempered at 500 ° C. or higher, it basically does not occur.

  As described above, the ferritic, pearlite, retained austenite, and non-tempered structures that are harmful structures in this steel material must be completely eliminated by the above-mentioned components and manufacturing methods. Must also be reduced to less than 1%. The structure is determined by observing from the width direction in a direction perpendicular to the rolling direction. Regarding the observation of a plurality of visual fields, the observation is performed while moving the sample in the rolling longitudinal direction so that there is no overlapping of visual fields. Regarding the ferrite and pearlite in the structure, the presence or absence is confirmed by performing three views of a region of about 250 μm × 350 μm with a 500 × optical microscope observation of the test piece subjected to nital etching. For residual γ, a test piece is taken from the same site as that during tissue observation, and the volume fraction is measured by the X-ray diffraction method (integration method), and this is used as the area ratio as it is. Those that have not been tempered can be identified by the corrosion of the carbides precipitated in the crystal grains in the observation with an optical microscope after picric acid corrosion. Corroded).

  Next, various component ranges in the steel sheet of the present invention will be described.

C: 0.16% or more and 0.20% or less C is an element that increases the hardness of the quenched structure and is effective for improving the hardness, and the lower limit is 0.16% from FIG. On the other hand, excessive addition impairs toughness and causes a difference in hardness between the surface layer and the central portion, so the upper limit is 0.20% from FIG.

Si: 0.50% or more, 1.00% or less Si is an element effective as a deoxidizer and also for improving the strength, and enhances the hardenability without increasing Ceq. In order to promote temper brittleness and reduce toughness, it is preferable to reduce the toughness, and the upper limit is made 1.00%. On the other hand, the lower limit may be 0.00%, but it is preferably 0.05% or more from the viewpoint of deoxidation efficiency at the time of molten steel refining and deoxidation cost.

Mn: 0.90% or more, 1.50% or less Mn is an element effective as a deoxidizing material, improving hardenability and improving strength, and is added in an amount of 0.90% or more. In order to promote brittleness and reduce toughness, the upper limit is made 1.50%.

P: 0.000% or more, 0.010% or less,
P is an impurity element contained in the steel and is a harmful element that promotes grain boundary embrittlement and lowers toughness. Therefore, P is preferably as small as possible, and is reduced to 0.010% or less. The lower limit is preferably 0.000%, but is preferably 0.001% from the viewpoint of increasing refining costs and reducing productivity.

S: 0.000% or more, 0.002% or less,
S is an impurity element contained in the steel, and is an element that lowers toughness through segregation and sulfide formation, so it is preferably as small as possible and is reduced to 0.002% or less. The lower limit is preferably 0.000%, but is preferably 0.0004% from the viewpoint of an increase in refining costs and a decrease in productivity.

Cu: 0.00% or more and 0.40% or less Cu is an element that can increase the strength of steel without impairing the low-temperature toughness. The upper limit is made 0.40% in order to reduce toughness by precipitation or the like. Although Cu contributes to the suppression of ferrite by increasing Ceq, it can be replaced by other alloy elements, and the lower limit is not particularly restricted, and if it can be replaced, 0.00% may be used. It is an alloy element that is difficult to be formed, and the lower limit is preferably 0.02%.

Ni: 0.20% or more, 1.00% or less Ni is an element effective for improving the strength and toughness of steel, and 0.20% or more is added, but the effect is saturated by excessive addition. In addition, since a large amount of Ni, which is an expensive alloy, causes a deterioration in manufacturing cost, the upper limit is desirably set to 1.00% as a range in which industrial production can be achieved.

Cr: 0.60% or more, 1.00% or less Mo: 0.60% or more, 1.00% or less Cr / Mo improves the hardenability and increases the central hardness, and the surface layer and the central part by precipitation hardening. Is an important element that raises the hardness of Cr and Mo. Each of Cr and Mo is added in an amount of 0.60% or more, but these are elements that lower the toughness by forming alloy carbides. And

V: 0.000% or more and 0.050% or less V improves the strength of the base metal through the formation of carbides, but adding a large amount causes a decrease in toughness due to the formation of alloy carbides, so the upper limit is made 0.050%. Increasing Ceq also contributes to suppression of ferrite, but V is an expensive alloy element and can be replaced by other alloys, so the lower limit is not particularly restricted, and if it can be replaced, 0.000% However, it is an alloy element that is difficult to eliminate at all, and the amount contained as an inevitable impurity is preferably set to 0.003% as a lower limit.

Al: 0.050% or more, 0.085% or less,
Al is an element that is effective as a deoxidizing material, and is combined with N in the steel to form AlN and contribute to the refinement of the structure. In addition, it becomes AlN in the precipitation treatment and contributes to the decomposition of BN. Since 0.05% or more is added because it also has the effect of stabilizing the hardenability of the steel, excessive addition causes a decrease in toughness and cracking of the slab due to coarse AlN, so the upper limit is made 0.085%.

N: 0.0020% or more, 0.0070% or less,
N forms alloy elements and nitrides / carbonitrides and contributes to fine graining, so 0.0020% is added as the lower limit. On the other hand, when it is excessively dissolved in steel and when coarse nitrides / carbonitrides are formed, the toughness is lowered, so 0.0070% is made the upper limit.

B: 0.0005% or more and 0.0020% or less B is an element that improves the hardenability of steel by adding a small amount and improves the strength, and 0.0005% or more is added. However, if the addition is excessive, metal borate is formed and the hardenability is lowered, so the upper limit is made 0.0020%.

  Furthermore, the following elements that affect toughness are defined as selective elements.

Nb: 0.001% or more, 0.050% or less Nb is an element that forms carbonitrides and contributes to the refinement of the internal structure of steel and affects the toughness, and may contain 0.001% or more. I can do it. However, since the coarse carbonitride produced by adding a large amount lowers the toughness, the upper limit is made 0.050%.

Ti: 0.001% or more, 0.020% or less Ti / N ≦ 3.4
Ti is an element that forms a stable nitride, contributes to the refinement of the structure and affects the toughness, and can be contained in an amount of 0.001% or more. However, excessive addition of Ti causes toughness reduction due to coarse nitrides, so the addition amount is made 0.020% as an upper limit. Further, when Ti is added, when it is added exceeding the stoichiometric ratio of TiN, specifically when Ti> 3.4N, excessive Ti forms carbides and reduces toughness. It is preferable to restrict to Ti ≦ 3.4N.

Ca: 0.0001% or more, 0.0030% or less,
Mg: 0.0001% or more, 0.0030% or less,
REM: 0.0001% or more, 0.0030% or less,
Ca, Mg, and REM can be combined with harmful impurities such as S and form harmless inclusions to improve mechanical properties such as toughness of steel. Can do. However, if the addition is excessive, not only the effect is saturated, but also melting damage of refractories such as casting nozzles is promoted, so the upper limit is made 0.0030%.

  Next, the preferable manufacturing method of the steel plate of this invention is demonstrated.

  After casting the steel slab of the above components, it is cooled after heating and hot rolling to a temperature equal to or higher than the AlN solid solution temperature Ts calculated by the above formula (4), and further at a temperature of more than 550 ° C. and less than Ac1, and the processing temperature After the precipitation treatment in which T and the treatment time tp are heated so as to satisfy the above-mentioned formula (5), cooling to room temperature or raising the temperature as it is, quenching shown in the above-mentioned formula (6) at a temperature of 900 ° C. or higher and 950 ° C. or lower It is preferable that the holding time H is re-heated to a cooling time of H or more and water-cooled.

  The steel pieces obtained by melting the steels A1 to A10 and B1 to B24 having the chemical components shown in Table 1 are compared with No. 1 to 10 of the present invention steel shown in Table 2 and No. 11 to 45. Heat rolling and heat treatment were carried out under the conditions of each of the examples, and steel plates having a thickness of 210 mm to 230 mm were produced.

Thereafter, specimens were collected from the surface of all the steel plates and the central portion of the plate thickness, and the Brinell hardness test specified in ASTM A370 was performed. About surface layer hardness, in this invention, the position which removed 0.7-1 mm from surface layer was made into surface layer in order to avoid a decarburized layer, and it used for the hardness test. For the center hardness, a test piece was taken so that the hardness from the Z direction (that is, the plane parallel to the steel plate surface) could be measured. As the determination of the Brinell hardness test, the surface layer hardness and the sheet thickness center portion hardness of HB350 or more were accepted.
In addition, the C direction absorbed energy at −40 ° C. at the center of the plate thickness was obtained by collecting Charpy impact test pieces specified in ASTM A370 in the C direction from the center of the plate thickness of all steel plates. Carried out. As a judgment of the Charpy impact test, a sample having an average value of three absorbed energy at −40 ° C. of 47 J or more was regarded as acceptable.

  The structure was determined by observing from the width direction in a direction perpendicular to the rolling direction. Since this steel material has a very high hardenability, the pearlite was 0% in all the examples in at least the observation with the optical microscope. In addition, since the steel material was sufficiently tempered, the residual γ was an extremely small amount detected in all the examples, so it was substantially 0% and is not listed in the table. In addition, since this steel material is sufficiently tempered, martensite that is not tempered and bainite that is not tempered are 0% in all the examples, so this is also not shown in the table.

  Table 1 shows the components, and Table 2 shows the production method, material, evaluation and the like.

  Test numbers 1 to 10 satisfy the chemical component range of the present invention as well as suitable production conditions. All these steels satisfy the targets for surface hardness, center hardness, and shock absorption energy.

  In test numbers 11 and 12, C deviates from the chemical component range of the present invention. In Test No. 11, C is slightly lower and the hardness at the time of quenching is not sufficient, so the hardness does not satisfy the target value even after tempering. On the other hand, test number 12 is an example in which C is deviated to a higher level, and the impact absorption energy is low due to the influence of hard carbide precipitation that becomes the starting point of fracture.

  In test numbers 13 and 14, Si deviates from the chemical component range of the present invention. In Test No. 13, Si was removed at a low level, and the hardenability could not be ensured, so the center hardness did not satisfy the target value. On the other hand, Test No. 14 is an example in which Si is deviated to a higher level. Although the hardness is sufficient, the impact absorption energy does not satisfy the target due to the promotion of temper embrittlement by Si.

  Test numbers 15 and 16 have Mn deviating from the chemical component range of the present invention. In Test No. 15, Mn is slightly lower and the hardness at the time of quenching is not sufficient, so that the hardness at the center does not satisfy the target value even after tempering. On the other hand, the test number 16 is an example in which Mn is deviated to a high level, and the impact absorption energy does not satisfy the target value due to the promotion of temper embrittlement.

  In Test No. 17, P is high exceeding the chemical component range of the present invention and the hardness is sufficient, but the impact absorption energy does not satisfy the target due to embrittlement caused by P.

  In Test No. 18, S is high and deviates from the chemical component range of the present invention, and the impact absorption energy does not satisfy the target due to the formation of MnS which is an extension inclusion.

  In Test No. 19, Cu was high deviating from the chemical component range of the present invention, and the precipitated metal Cu became the brittle fracture starting point, so the impact absorption energy did not satisfy the target.

  In Test No. 20, Ni deviates from the chemical component range of the present invention, and the impact absorption energy does not satisfy the target because the Ni content is low and less than the addition amount for improving toughness.

  Test numbers 21 and 22 are examples in which Cr deviates from the chemical component range of the present invention. In Test No. 21, since Cr is slightly lower and sufficient hardenability and precipitation strengthening action are not obtained, the center hardness does not satisfy the target. On the other hand, in Test No. 22, Cr is not high and impact absorption energy does not satisfy the target due to the precipitation effect of coarse Cr carbide.

  Test numbers 23 and 24 are examples in which Mo deviates from the chemical component range of the present invention. In Test No. 23, Mo is deviated slightly, and since sufficient hardenability and precipitation strengthening action are not obtained, the center hardness does not satisfy the target. On the other hand, in Test No. 24, Mo is not high and the impact absorption energy does not satisfy the target value due to the precipitation effect of coarse Mo carbide.

  In Test No. 25, V is higher than the chemical component range of the present invention, and since the coarse carbide / nitride of V has become a brittle fracture starting point, the impact absorption energy does not satisfy the target.

  Test numbers 26 and 27 are examples in which Al deviates from the chemical component range of the present invention. Test No. 26 is an example in which Al was removed at a low level, and AlN effective for pinning could not be secured, and the effect of improving hardenability was reduced by combining excess N with B. Energy is not meeting the target. On the other hand, Test No. 25 is an example in which Al deviates to a high level, and since AlN becomes excessively coarse and becomes a brittle fracture starting point, the impact absorption energy does not satisfy the target.

  Test Nos. 28 and 29 are examples in which N deviates from the chemical composition range of the present invention. Test No. 26 is an example in which N deviates slightly. Nitride / carbonitride generation is insufficient. Therefore, the pinning effect is weak, and the impact absorption energy does not satisfy the target due to the coarsening of crystal grains. On the other hand, test number 29 is an example in which N deviates to a high level, and the impact absorption energy does not satisfy the target due to excessive coarsening of nitrides / carbonitrides.

  In test numbers 30 and 31, B deviates from the chemical component range of the present invention. Test No. 30 is an example in which B deviates slightly, and as a result of being unable to secure the solid solution B amount necessary for hardenability, the center hardness and impact absorption energy do not satisfy the targets. On the other hand, test number 31 is an example in which B is added excessively, and the impact absorption energy does not satisfy the target due to the precipitation of a metal element boride.

  In Test No. 32, although individual component ranges are within the range of the present invention, Ceq is low outside the preferred range of the present invention, and as a result of ferrite forming in the structure due to a decrease in hardenability, the hardness of the central portion and the impact absorption Energy is not meeting the target.

  Although test numbers 33 and 34 are within the individual component ranges and Ceq is within the scope of the present invention, the parameter formula 4 × f / g is lower than the preferred range of the present invention, compared to the improvement in hardenability. The impact absorption energy does not satisfy the target because the hardening effect by the precipitated elements has increased.

  Test No. 35, although the component range is within the range of the present invention, the heating temperature before rolling is lower than the solid solution temperature Ts, and since the undissolved coarse AlN remains and becomes a brittle fracture starting point, the absorbed energy Is not meeting the goal.

  In Test Nos. 36 and 37, although the component range is within the range of the present invention, the precipitation treatment temperature deviates from the preferred range of the present invention. Test No. 36 is an example in which the deposition temperature was low, and the absorbed energy did not satisfy the target because sufficient AlN was not deposited and AlN effective for pinning could not be secured. On the other hand, test number 37 is an example in which the precipitation treatment temperature exceeded Ac1, and the absorbed energy did not satisfy the target because AlN was locally coarsened by holding in the α-γ two-phase region.

  Test No. 38 has a component range within the range of the present invention, but the temperature and time of the deposition process are below the preferred range of the present invention, and sufficient AlN is not deposited, so that AlN effective for pinning can be secured. The absorbed energy did not meet the target because there was no.

  In Test No. 39, although the component range is within the range of the present invention, the quenching temperature is lower than the preferred range of the present invention, and the alloy elements are not sufficiently dissolved, so that the hardenability is low and the formation of ferrite. As a result, the center hardness and the absorbed energy are not meeting the target.

  In test No. 40, although the component range is within the range of the present invention, the quenching temperature is higher than the preferred range of the present invention, and the crystal grains are excessively coarsened, so the absorbed energy does not satisfy the target. .

  Test No. 41 has a component range within the range of the present invention, but the quenching retention time is less than the preferred range of the present invention, and the alloy elements are not sufficiently dissolved, so the hardenability is low, and the ferrite As a result, the core hardness and absorbed energy are not meeting the target.

  In Test No. 42, although the component range is within the range of the present invention, the tempering temperature is lower than the preferred range and temper embrittlement occurs, so the absorbed energy does not satisfy the target.

  In Test No. 44, although the component range is within the range of the present invention, the tempering temperature is higher than the preferred range, and the precipitation hardening action of the alloy carbide is reduced, so that the center hardness cannot satisfy the target.

Claims (4)

Steel component is mass%,
C: 0.16% or more, 0.20% or less,
Si: 0.50% or more, 1.00 or less,
Mn: 0.90% or more, 1.50% or less,
P: 0.000% or more, 0.010% or less,
S: 0.000% or more, 0.002% or less,
Cu: 0.00% or more, 0.40% or less,
Ni: 0.20% or more, 1.00% or less,
Cr: 0.60% or more, 1.00% or less,
Mo: 0.60% or more, 1.00% or less,
V :: 0.000% or more, 0.050% or less,
Al: 0.050% or more, 0.085% or less,
N: 0.0020% or more, 0.0070% or less,
B: 0.0005% or more, 0.0020% or less,
Consisting of residual Fe and inevitable impurities,
Tempered martensite and / or tempered bainite is 99% or more in area ratio, and other structures such as ferrite, pearlite, retained austenite, and structure not tempered are less than 1 area%,
Ceq represented by the following formula (1) satisfies 0.750 or more and 0.800 or less, and the value f represented by the following formula (2) and the value g represented by the following formula (3) are 4 × f / g is 9 0.003 or more, the three-point C-direction Charpy at −20 ° C. at the center of the plate thickness is 47 J or more, and the hardness of the surface layer and the center of the plate thickness is 350 or more in HB. A high-strength steel plate having a thickness of more than 200 mm and excellent in hardness and low-temperature toughness at the center of the plate thickness.
Ceq = C + Mn / 6 + (Cu + Ni) / 15 + (Cr + Mo + V) / 5 (1)
f = 4C + Si + 2Mn + Ni + 2Cr + 5Mo (2)
g = 2Cr + 3Mo + 5V (3)
Here, each element symbol in each formula means mass% of the component. In addition to the above ingredients,
Nb: 0.001% or more, 0.050% or less,
Ti: 0.001% or more, 0.020% or less,
Ca: 0.0001% or more, 0.0030% or less,
Mg: 0.0001% or more, 0.0030% or less,
REM: 0.0001% or more, 0.0030% or less,
2 or more, and when Ti is contained, Ti / N ≦ 3.4 is satisfied, and the thickness of the sheet thickness center portion according to claim 1 excellent in hardness and low-temperature toughness High-strength steel sheet exceeding 200 mm. Steel component is mass%,
C: 0.16% or more, 0.20% or less,
Si: 0.50% or more, 1.00 or less,
Mn: 0.90% or more, 1.50% or less,
P: 0.000% or more, 0.010% or less,
S: 0.000% or more, 0.002% or less,
Cu: 0.00% or more, 0.40% or less,
Ni: 0.20% or more, 1.00% or less,
Cr: 0.60% or more, 1.00% or less,
Mo: 0.60% or more, 1.00% or less,
V :: 0.000% or more, 0.050% or less,
Al: 0.050% or more, 0.085% or less,
N: 0.0020% or more, 0.0070% or less,
B: 0.0005% or more, 0.0020% or less,
The steel composed of residual Fe and inevitable impurities is heated to a temperature equal to or higher than the AlN solid solution temperature Ts calculated by the following formula (4), cooled after hot rolling, and further processed at a temperature higher than 550 ° C. and lower than Ac1. After the precipitation treatment in which the temperature T and the treatment time tp are heated so as to satisfy the following formula (5), it is cooled to room temperature or heated as it is, and is quenched and held at a temperature of 900 ° C. or more and 950 ° C. or less as shown in the following formula (6). It is subjected to quenching treatment that is reheated for time H or more and cooled with water, tempered at 500 ° C. or more and 550 ° C. or less and cooled to room temperature, and has a thickness of more than 200 mm excellent in hardness and low temperature toughness at the center of the thickness. Manufacturing method of high strength steel sheet.
Ts = 7400 / (1.95-log ₁₀ [Al] [N]) (4)
Log ₁₀ (tp) + 0.012 × T ≧ 8.7 (5)
H = 0.033 (950−Tq) 2+ (1.5f) 2/10 (6)
Here, Ts is a solid solution temperature (° C.) of AlN, [Al] and [N] are mass% of each element, T is a precipitation treatment temperature (° C.), tp is a precipitation treatment time (Hr), and H Is the quenching holding time (min), Tq is the quenching holding temperature (° C.), and f is a value obtained by the above equation (2). In addition to the above ingredients,
Nb: 0.001% or more, 0.050% or less,
Ti: 0.001% or more, 0.020% or less,
Ca: 0.0001% or more, 0.0030% or less,
Mg: 0.0001% or more, 0.0030% or less,
REM: 0.0001% or more, 0.0030% or less,
4 or more, and when Ti is contained, Ti / N ≦ 3.4 is satisfied, and the plate thickness of 200 mm excellent in hardness and low temperature toughness and weld toughness according to claim 3 Manufacturing method of super high strength steel sheet.