JP6838422B2 - High-strength steel sheet and its manufacturing method - Google Patents

Description

The present invention relates to a material for a huge gear constituting a rotating mechanism of a large industrial machine represented by, for example, a rotary kiln (rotary kiln used for cement production), and particularly has a plate thickness of more than 200 mm. The surface layer and the thickness center are characterized in that the C-direction absorption energy of the central portion of the plate thickness at −40 ° C. is 20 J or more, the hardness of the surface layer is HB330 or more, and the hardness of the central portion of the plate thickness is HB300 or more. The present invention relates to a thick steel plate having excellent hardness of the portion and excellent low-temperature toughness at the center of the plate thickness, and a method for producing the same.

Huge gears are used in the rotating mechanism of large industrial machines such as rotary kilns. The steel plate used as the material is required to have a hardness of HB330 or higher on the surface layer and HB300 or higher at the center of the plate thickness from the viewpoint of fatigue resistance and durability of the gear. Toughness with C-direction absorbed energy ≥ 20J at ° C is required. The steel sheet used for the above purpose is subjected to strain relief annealing after gear processing, but the steel sheet used as the material must be tempered in advance at 500 ° C. or higher so that the material does not change at this time.

In recent years, with the aim of increasing the size of gears, there has been a demand for steel plates having a thickness of more than 200 mm, which has never existed before.
As the plate thickness increases, the cooling rate of the central portion of the plate thickness during quenching decreases, so that it becomes difficult to obtain the hardness of the central portion (even after tempering).
Ingredient design that simply increases the hardness causes a decrease in toughness. Therefore, in the case of an extra-thick material with a plate thickness of more than 200 mm, in order to secure surface hardness and central hardness, and also to ensure low-temperature toughness at -40 ° C. It is extremely difficult to adjust the component balance.

For example, in the method shown in Patent Document 1, wear-resistant steel up to a plate thickness of 120 mm is manufactured by specifying the C amount and the DI range, but the plate thickness exceeds 200 mm, which slows the cooling rate during quenching. The hardness of the thick center cannot be guaranteed. Further, the amount of C is larger than that of the present invention described later, and the toughness is limited to the test up to 0 ° C., and vE-40 ° C. ≥ 20J cannot be guaranteed.

Therefore, the hardness and / or toughness of the central part of the plate thickness is increased not only by adjusting the component balance but also by improving the manufacturing process, but it is effective in achieving both hardness and low temperature toughness at a plate thickness of more than 200 mm. No means have been established.

For example, in the method shown in Patent Document 2, a steel sheet is manufactured in consideration of the mechanical properties of the central portion of the sheet thickness by defining the water cooling immediately before rolling and the surface temperature during rolling, but the maximum sheet thickness is 100 mm. However, it is not possible to guarantee the hardness of the central portion of a steel sheet having a thickness of more than 200 mm, which slows down the cooling rate during quenching. Further, in the above document, low-temperature rolling with a surface temperature of 1000 ° C. or less during rolling is carried out, but rolling at a low temperature is disadvantageous for crimping internal voids due to an increase in deformation resistance, so a plate thickness of 200 mm is used. Not suitable for the production of extra-thick materials.

In the method shown in Patent Document 3, the mechanical properties are improved by holding the B-added steel for 1-2 hours before quenching, but the manufacturing method patent focuses on increasing the hardness due to boron, and the toughness is 0. Stay in the test up to ° C. Compared with the present invention described later, the amounts of Al and Al × N that contribute to the refinement of the crystal grains are low, and there is no step of dissolving in advance in order to finely precipitate AlN. It is not possible to guarantee the C-direction absorbed energy ≧ 20J at −40 ° C.

Japanese Unexamined Patent Publication No. 2014-25130 Japanese Unexamined Patent Publication No. 2-205629 Japanese Unexamined Patent Publication No. 62-235421

W. C. Leslie: The Physical Metallurgy of steels

Against the above background, the present invention has been difficult to manufacture in the past, especially when the plate thickness is more than 200 mm, the C-direction absorption energy at −40 ° C. at the center of the plate thickness is 20 J or more, and the surface layer. Provided are a thick steel sheet having a hardness of HB330 or more and a hardness HB at the center of the plate thickness of 300 or more, and a method for producing the same.

An object of the present invention is that the plate thickness is more than 200 mm, the absorption energy in the C direction at −40 ° C. at the center of the plate thickness is 20 J or more, the hardness of the surface layer is HB330 or more, and the hardness of the center of the plate thickness is HB. The purpose of providing a thick steel plate of 300 or more is to provide (1) mass%, C: 0.10% or more, 0.14% or less, Si: 0.00% or more, 0.40 or less, Mn: 0.90% or more, 1.50% 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, 1.50% 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: 0.0025% or more, 0.0070% or less, B: 0.0005% or more, 0.0020% or less, P: 0.000% or more, 0.0100% or less, S: 0. It contains 000% or more and 0.0020% or less, the balance is composed of Fe and unavoidable impurities, Al × N satisfies 2.0 × 10-4 or more, the following formula (1) is satisfied, and steel. The total area ratio of one or two types of tempered martensite and tempered bainite is 99% or more, and the remaining structure is ferrite, pearlite, retained austenite, non-tempered martensite, and non-tempered bainite. The three-point average of absorbed energy in the C direction at -40 ° C at the center of the plate thickness is 20 J or more, the hardness of the surface layer is 330 or more at HB, and the hardness at the center of the plate thickness is 300 or more at HB. A high-strength steel plate characterized by this.
Ceq = C + Mn / 6 + (Cu + Ni) / 15+ (Cr + Mo + V) / 5 ≧ 0.70 (1)
Here, C, Mn, Cu, Ni, Cr, Mo, and V are mass% of each element.
(2) 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% The high according to claim 1, wherein Mg: 0.0001% or more, 0.0030% or less, REM: 0.0001% or more, 0.0030% or less, and one or more of them are contained. Strong steel plate.
(3) In terms of mass%, C: 0.10% or more, 0.14% or less, Si: 0.00% or more, 0.40 or less, Mn: 0.90% or more, 1.50% 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, 1.50% 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: 0.0025% or more, 0.0070% or less,
B: 0.0005% or more, 0.0020% or less, P: 0.000% or more, 0.0100% or less, S: 0.000% or more, 0.0020% or less, and the balance is Fe and It consists of unavoidable impurities, and Al × N ^{satisfies 2.0 × 10 -4} or more.
Steel having a component satisfying the following formula (1) is heated to the AlN solid melting temperature Ts (° C.) or higher calculated by the following formula (2) and then tempered and treated at a temperature of more than 550 ° C. and less than Ac1. After the precipitation treatment, which heats the temperature T (° C.) and the treatment time tp (Hr) so as to satisfy the following formula (3), it is cooled to room temperature or raised as it is, reheated to 1000 ° C. or higher at the Ac3 transformation point, and water-cooled. By quenching treatment, tempering at 500 ° C or higher and 550 ° C or lower and cooling to room temperature , the plate thickness is more than 200 mm, and the metal structure is the total area ratio of one or two types of tempered martensite and tempered bainite. The remaining structure is ferrite, pearlite, retained austenite, non-tempered martensite, and non-tempered bainite.
A thick steel sheet with a three-point average of absorbed energy in the C direction at -40 ° C at the center of the plate thickness of 20 J or more, a surface hardness of 330 or more for HB, and a hardness of 300 or more for HB at the center of the plate thickness. A method for manufacturing a high-strength steel sheet, which comprises obtaining.
Ceq = C + Mn / 6 + (Cu + Ni) / 15+ (Cr + Mo + V) / 5 ≧ 0.70 (1)
Ts = 7400 / (1.95-Log ₁₀ ([Al] × [N])) -273 (2)
Log (tp [Hr]) + 0.012 × T [° C] ≧ 8.7 (3)
Here, C, Mn, Cu, Ni, Cr, Mo, V, Al, and N are mass% of each element.
(4) 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, 0.0030% or less, REM: 0.0001% or more, 0.0030% or less, the high strength according to claim 3. Steel plate manufacturing method.

According to the present invention, the three-point average of the absorbed energy in the C direction at −40 ° C. at the center of the plate thickness is 20 J or more, the hardness of the surface layer is 330 or more at HB, and the hardness at the center of the plate thickness is 300 or more at HB. Moreover, a thick steel plate having a thickness of more than 200 mm with a small difference in hardness between the surface layer and the central portion can be obtained, and its existence significance is great.

It is a figure which shows the relationship between Al and Al × N, and the toughness at the center of a plate thickness. It is a figure which shows the relationship between the precipitation treatment condition and the toughness of a plate thickness central part. It is a figure which shows the relationship between Ceq and the hardness at the center of a plate thickness. It is a figure which shows the relationship between tempering temperature and hardness.

The details will be described below.

First, in order to achieve both HB300 class hardness and -40 ° C toughness at the center with a plate thickness of more than 200 mm, it is necessary to add 0.050% or more of Al, and 2.0 × N of Al × N. It ^{should be 10 -4} or more. This is a component requirement for utilizing AlN as pinning particles for atomization.
Generally, fine graining of crystal grains is achieved by utilizing pinned particles or low-temperature rolling, but it is important to disperse the pinned particles because it is difficult to finely grain the crystal grains by rolling when the plate thickness exceeds 200 mm. It becomes.
As shown in FIG. 1, when Al is less than 0.050% or Al × N is ^{less than 2.0 × 10 -4} , the total amount of AlN is not sufficient, so the crystal grain size becomes more than 50 μm, and the plate thickness center. The C-direction absorption energy ≧ 20J at −40 ° C. in the part cannot be obtained. Further, when Al ≧ 0.085%, the C-direction absorption energy ≧ 20J at −40 ° C. at the center of the plate thickness cannot be secured due to the formation of coarse AlN, so the upper limit is limited to 0.085%. Will be done.

Secondly, in order to secure the hardness of the central portion when the plate thickness exceeds 200 mm, sufficient hardenability is required, and for that reason, it is necessary to satisfy 0.70 or more with the Ceq of the following formula (1). This is to avoid the formation of ferrite, which is a soft structure.
Ceq = C + Mn / 6 + (Cu + Ni) / 15+ (Cr + Mo + V) / 5 (1)
Each element symbol means a component composition and is mass%.

Thirdly, as a process requirement for obtaining the above-mentioned AlN pinning effect, the AlN solid solution temperature Ts (° C.) or higher expected by the following formula (2) is heated and then rolled to finely dissolve the solid solution AlN. In order to precipitate, the precipitation treatment is carried out at a temperature of more than 550 ° C. and less than Ac1 after rolling and before quenching and heating so that the treatment temperature T (° C.) and the treatment time tp (Hr) satisfy the following formula (3). There is a need.
Ts = 7400 / (1.95-Log ₁₀ ([Al] × [N])) -273 (2)
Log (tp [Hr]) + 0.012 × T [° C] ≧ 8.7 (3)
If solution formation 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 the precipitation treatment is reduced and a pinning effect can be obtained. It disappears. The solution temperature of the above formula (2) is a known one described in Non-Patent Document 1 and the like.

As shown in FIG. 2, in order to obtain the pinning action of AlN, it is necessary to carry out the precipitation treatment at an appropriate temperature and time. Under the treatment conditions where Log (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 and the toughness is not exhibited. Cannot be secured. On the other hand, when the temperature exceeds Ac1, it is retained in the α-γ two-phase region, so that Al and N are concentrated in the γ region, and AlN is locally coarsened, so that toughness cannot be ensured. The upper limit of the processing time is not particularly regulated from the viewpoint of mechanical properties, but from the viewpoint of industrial production efficiency, the upper limit is 5 days = 120 Hr.

FIG. 3 shows the relationship between Ceq and hardness with and without precipitation treatment. As shown in Patent Document 3, the precipitation treatment of the present invention contributes not only to the improvement of toughness but also to the increase of hardness by improving the hardenability of boron. The inventors have stated that in a steel sheet having a thickness of more than 200 mm, the hardness of the central portion is less than HB300 in the region of Ceq <0.70 even when the precipitation treatment is performed, and the lack of hardness is due to the formation of ferrite. It turned out that it was a thing. That is, it has been found that the Ceq defined by the formula (1) must be 0.70 or more as a condition that ferrite is not generated during quenching and cooling even for a thick steel sheet having a plate thickness of more than 200 mm.

Fourth, in order to secure the hardness of the surface layer and the central portion, it is necessary not only to optimize the components but also to satisfy the tempering temperature of 500 ° C. or higher and 550 ° C. or lower.

The tempering temperature must be 500 ° C. or higher due to the requirements for gear construction (preventing material deterioration during strain relief annealing). In addition, the tempering temperature must be 500 ° C. or higher in order to ensure sufficient tempering toughness of the structure. On the other hand, the present inventors examined the relationship between the tempering temperature and the hardness of the surface layer and the central portion, and as shown in FIG. 4, the hardness rapidly decreased when tempering over 550 ° C., and the surface layer HB330 and the central portion HB300. It was found that the tempering temperature needs to be 550 ° C. or lower because the tempering temperature cannot be secured.

Next, the component system will be described.
C: 0.10% or more, 0.14% or less C is an element effective for increasing the hardness of the hardened structure and improving the hardness, and the lower limit is 0.10%. On the other hand, excessive addition impairs toughness, so the upper limit is set to 0.14%.

Si: 0.00% or more, 0.40 or less Si is an effective element as a deoxidizing material and also for improving strength, but adding a large amount promotes temper brittleness and reduces toughness, so it is reduced. It is preferable that the upper limit is 0.40%. On the other hand, the lower limit may be 0.00%, but is preferably 0.05% or more from the viewpoint of deoxidizing efficiency and deoxidizing cost during molten steel refining.

Mn: 0.90% or more, 1.50% or less Mn is an element that is effective as a deoxidizing material and for improving hardenability and strength, but excessive addition promotes temper brittleness and reduces toughness. The upper limit is set to 1.50%. The lower limit is not particularly regulated, but 0.90% is added as the lower limit because Ceq can be secured at a lower cost than other alloys.

Cu: 0.00% or more, 0.40% or less Cu is an element that can increase the strength of steel without impairing low temperature toughness. The upper limit is set to 0.40% in order to reduce toughness due to precipitation or the like. Cu contributes to the suppression of ferrite by increasing Ceq, but it can be replaced by other alloying elements, and the lower limit is not particularly regulated. If it can be replaced, 0.00% may be used, but none by refining. It is an alloying element that is difficult to convert, 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. Moreover, since the addition of a large amount of Ni, which is an expensive alloy, causes deterioration of the manufacturing cost, the upper limit is set to 1.00% as the range in which industrial production can be established.

Cr: 0.60% or more, 1.50% or less, Mo: 0.60% or more, 1.00% or less Cr / Mo improves hardenability and increases core hardness, and precipitation hardening improves surface layer and center. It is an important element that raises the hardness of the part, and 0.60% or more is added to both Cr and Mo, but since adding a large amount rather lowers the toughness due to alloy carbide formation, the upper limit of Cr is 1.50%. , Mo has an upper limit of 1.00%.

V: 0.000% or more, 0.050% or less V improves the strength of the base metal by improving the formation and hardenability of carbides, but adding a large amount causes a decrease in toughness due to the formation of alloy carbides, so the upper limit is 0. It is set to 050%. Increasing Ceq contributes to the suppression of ferrite, but since V is an expensive alloy element and can be replaced by other alloys, the lower limit is not particularly regulated, and if it can be replaced, it may be 0.000%. However, it is an alloy element that is difficult to eliminate completely, and it is preferable that the lower limit is 0.003% as the amount contained as an unavoidable impurity.

Al: 0.050% or more, 0.085% or less,
Al is an effective element as a deoxidizing material, and at the same time, 0.050% or more is added in order to combine with N in steel to form AlN, contribute to fine graining of the structure, and contribute to ensuring toughness. Excessive addition causes a decrease in toughness and cracking of slabs due to coarse AlN, so the upper limit is set to 0.085%.

N: 0.0025% or more, 0.0070% or less,
N is added with 0.0025% as the lower limit because it forms a nitride / carbonitride with the alloying element, contributes to fine granulation, and contributes to ensuring toughness. On the other hand, when it is excessively dissolved in steel or when coarse nitrides or carbonitrides are formed, the toughness is lowered, so the upper limit is 0.0070%.

B: 0.0005% or more, 0.0020% or less B is an element that improves the hardenability of steel and improves the hardness of the center by adding a small amount, and accordingly reduces ΔHB, and 0.0005% or more is Is added. However, if the addition is excessive, coarse metal charcoal boride is formed and the toughness is lowered, so the upper limit is set to 0.0020%.

P: 0.0100% or less P is an impurity element contained in steel and is a harmful element that promotes grain boundary embrittlement and lowers toughness. Therefore, it is preferably as small as possible, up to 0.0100% or less. It will be reduced. The lower limit is preferably 0.000%, but is preferably 0.0010% from the viewpoint of increasing the refining cost and decreasing the productivity.

S: 0.0020% or less,
Since S is an impurity element contained in steel and is an element that lowers toughness through segregation and formation of sulfide, it is preferably as small as possible, and is reduced to 0.0020% or less. The lower limit is preferably 0.000%, but is preferably 0.0004% from the viewpoint of increasing the refining cost and reducing the productivity.

Further, depending on the purpose, the following selective elements may be added as elements that affect precipitates and inclusions.
Nb: 0.001% or more, 0.050% or less Nb is an element that forms a carbonitride and contributes to fine graining of the internal structure of steel, and can contain 0.001% or more. However, the upper limit of the coarse carbonitride produced by the addition of a large amount is set to 0.050% because it lowers the toughness.

Ti: 0.001% or more, 0.020% or less Ti is an element that forms a stable nitride and contributes to fine graining of the structure, and can contain 0.001% or more. However, since excessive addition of Ti causes a decrease in toughness due to coarse nitride, the addition amount is limited to 0.020%.

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,
Since Ca, Mg, and REM can all improve the mechanical properties of steel by combining with harmful impurities such as S and forming harmless inclusions, they can be contained in an amount of 0.0001% or more. However, if it is added excessively, not only the effect is saturated but also the melting damage of refractories such as casting nozzles is promoted, so the upper limit is set to 0.0030%.

Next, the organization will be described.
Structure: Tempering martensite and tempered bainite In the present invention, the structure of steel is tempered martensite and one or two types of tempered bainite in total area ratio of 99% or more, and the remaining structure is ferrite, pearlite, and residue. Austenite-Non-tempered martensite, non-tempered bainite.
Ferrite is a factor that lowers the hardness of steel materials. In particular, it tends to occur in the central portion of the plate thickness where the quenching and cooling rate is slow, which causes a hardness difference from the surface layer, and therefore it is preferable that it does not exist. Although pearlite is effective in ensuring hardness, it is preferable that it does not exist because it serves as a starting point for brittle fracture due to its hardness. Since pearlite is generated by thickening the C discharged at the time of ferrite precipitation, it is suppressed at the same time by avoiding ferrite precipitation.

Residual austenite and the structure that is not tempered serve as a starting point for brittle fracture and reduce the toughness of the steel material, so it is preferable that they do not exist. Since this steel material is tempered at 500 ° C. or higher, it basically does not occur.

It is desirable that the above-mentioned harmful structure and unavoidable structure (ferrite, pearlite, retained austenite, non-tempering martensite, non-tempering bainite) in this steel material are completely eliminated by the above-mentioned components and manufacturing method. The tissue fraction of retained austenite is volume% according to the measurement method, but for convenience, it is totaled to the area% of other tissues without any particular conversion.

Since it is difficult to distinguish between martensite and bainite after tempering, the area ratio is a combination of tempered martensite and tempered bainite. The method of observing each tissue will be described later.

The ingot that is the material of the thick steel sheet of the present invention is known as a continuous casting method or an ingot-incubation method after melting steel having the above-mentioned composition by a normal refining process such as a converter or an electric furnace. It can be manufactured by the above method, and there is no particular limitation.

Next, the manufacturing method will be described.

First, the slab having the above steel composition is continuously cast or cast by the ingot ingot method, and the obtained ingot is at a temperature of AlN solid solution temperature Ts or more and 1250 ° C. or less represented by the following formula 2. Heat. As the target heating temperature, since it is necessary to dissolve AlN in solid solution prior to the precipitation treatment as described above, the lower limit thereof is set to the temperature Ts represented by the following formula (2), but the temperature exceeds 1250 ° C. When heated, the scale of the steel sheet cannot be peeled off and the surface of the steel sheet is scratched. Therefore, the upper limit is set to 1250 ° C.

After rolling the heated steel ingot, it is heated at a temperature of more than 550 ° C. and less than Ac1 as described above so that the treatment temperature T (° C.) and the treatment time tp (Hr) satisfy the following formula (3). A quenching treatment of reheating to the Ac3 transformation point or higher and water cooling, and a tempering treatment of heating and cooling at a temperature of 500 ° C. or higher and 550 ° C. or lower are performed. The reason for reheating above the Ac3 transformation point during quenching is to have an austenite single-phase structure and to homogenize the structure and material of the steel sheet after quenching and tempering. Ac1 points are obtained from the empirical formula of the following formula (4), and Ac3 points are obtained from the empirical formula of the following formula (5).
Ts = 7400 / (1.95-Log ₁₀ ([Al] × [N])) -273 (2)
Log (tp [Hr]) + 0.012 × T [° C] ≧ 8.7 (3)
Ac1 = 720 -25 × C + 22 × Si-40 × Mn-30 × Ni + 20 × Cr + 25 × Mo ‥‥ (4)
Ac3 = 937.2-476.2 x C + 56 x Si-19.7 x Mn-16.3 x Cu-26.6 x Ni-4.9 x Cr + 38.1 x Mo + 124.8 x V + 198.4 x Al + 3315 x B-19.1 x Nb + 136.3 x Ti ... (5)

Comparison of the steel pieces obtained by melting the steels A1 to A10 and B1 to B23 having the chemical components shown in Table 1 with the steels of the present invention Nos. 1 to 10 shown in Table 2 and Nos. 11 to 40. Example Heat rolling and heat treatment were carried out under each condition to produce steel sheets having a thickness of 205 mm to 240 mm.

Then, a test piece for measuring Brinell hardness having a test surface parallel to the rolled surface was collected from the surface layer of all the steel sheets and the center of the plate thickness, and the Brinell hardness test specified in ASTM A370 was carried out. Regarding the surface hardness, in the present invention, the position where 0.7 to 1 mm was removed from the surface layer was used as the surface layer in order to avoid the decarburized layer, and the surface layer was subjected to a hardness test. As a judgment of the Brinell hardness test, those having a surface hardness of HB330 or more and a central hardness of HB300 or more were accepted.
In addition, the C-direction absorption energy at -40 ° C at the center of the plate thickness is measured by collecting Charpy impact test pieces specified in ASTM A370 in the C direction from the center of the plate thickness of all steel sheets. carried out. As a judgment of the Charpy impact test, the one in which the average value of the absorbed energies of the three bottles at −40 ° C. was 20 J or more was accepted.

Table 2 shows the above process conditions and the results of the mechanical test. The microstructure fraction was determined by collecting a test piece for observation from the center of the plate thickness and observing it from the width direction in the direction perpendicular to the rolling direction. Regarding the observation of multiple fields of view, the observation was performed while moving the sample in the longitudinal direction of rolling so that the fields of view did not overlap. The presence or absence of ferrite and pearlite among the structures was confirmed by observing the test pieces subjected to night-game etching at a magnification of 500 times with three visual fields in a region of about 250 μm × 350 μm. However, since this steel material has a very high hardenability, pearlite was at least 0% in all the examples by the optical microscope observation. For the residual γ, a test piece was taken from the same site as when the tissue was observed, and the volume fraction was measured by the X-ray diffraction method (integration method), which was used as the area fraction as it was. However, since this steel material is sufficiently tempered, the residual γ was detected in a very small amount in all the examples, so it was set to substantially 0% and is not shown 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. Underlined items in Tables 1 and 2 indicate that the components are outside the scope of the present invention. The tempered structure can be identified by the corrosion of the carbides precipitated in the crystal grains in the optical microscope observation after the corrosion of picric acid (when not tempered). Only the grain boundaries are corroded).

Test numbers 1 to 10 satisfy the chemical composition range of the present invention and suitable characteristic value conditions. All of these steels satisfy the targets in terms of surface hardness, center hardness, and absorbed energy.

In test numbers 11 and 12, C deviates from the chemical composition range of the present invention. In test number 11, C is low and the hardness at the time of quenching is not sufficient, so that the hardness at the center does not satisfy the target value. On the other hand, Test No. 12 is an example in which C is slightly off, and the absorbed energy is very low due to the influence of the precipitation of hard carbide which is the starting point of fracture.

In test number 13, Si is high outside the chemical composition range of the present invention, and although the hardness is sufficient, the absorbed energy does not satisfy the target due to the promotion of temper embrittlement by Si.

In test number 14, Mn is high outside the chemical composition range of the present invention, and the absorbed energy does not satisfy the target value due to the promotion of temper embrittlement.

In test number 15, P is high outside the chemical composition range of the present invention, and although the hardness is sufficient, the absorbed energy does not satisfy the target due to the embrittlement caused by P.

In test number 16, S is high outside the chemical composition range of the present invention, and the absorbed energy does not satisfy the target due to the formation of MnS which is an extension inclusion.

In test number 17, Cu is high outside the chemical composition range of the present invention, and the precipitated metallic Cu is the starting point of brittle fracture, so that the absorbed energy does not satisfy the target.

In test number 18, Ni is low outside the chemical composition range of the present invention, and the absorbed energy does not satisfy the target because it is less than the addition amount for improving toughness.

Test numbers 19 and 20 are examples in which Cr deviates from the chemical composition range of the present invention. In Test No. 19, Cr is slightly off, and a sufficient precipitation hardening action is not obtained. Therefore, the hardness of the surface layer and the center portion does not satisfy the target. On the other hand, in Test No. 20, Cr is out of the high range, and the absorbed energy does not satisfy the target value due to the effect of precipitation of coarse Cr carbide.

Test numbers 21 and 22 are examples in which Mo deviates from the chemical composition range of the present invention. In Test No. 21, Mo is slightly off, and sufficient precipitation hardening action is not obtained, so that the hardness of the surface layer and the center does not satisfy the target. On the other hand, in Test No. 22, Mo is out of the high range, and the absorbed energy does not satisfy the target value due to the effect of precipitation of coarse Mo carbide.

In test number 23, V is high outside the chemical composition range of the present invention, and the coarse carbide / nitride of V is the starting point of brittle fracture, so that the absorbed energy does not satisfy the target.

Test numbers 24 and 25 are examples in which Al deviates from the chemical composition range of the present invention. Test number 24 is an example in which Al deviates low, and AlN effective for pinning cannot be secured, and the absorbed energy does not satisfy the target. On the other hand, test number 25 is an example in which Al deviates to a high degree, and the absorbed energy does not satisfy the target because it becomes the starting point of brittle fracture due to the excessive coarsening of AlN.

Test numbers 26 and 27 are examples in which N deviates from the chemical composition range of the present invention, and test numbers 26 are examples in which N deviates to a low value, and the amount of nitride / carbonitride produced is insufficient. Since the pinning effect is weak, the absorbed energy does not meet the target due to the coarse graining of the crystal grains. On the other hand, test number 27 is an example in which N deviates high, and the absorbed energy does not satisfy the target due to excessive coarsening of the nitride / carbonitride.

In test numbers 28 and 29, B deviates from the chemical composition range of the present invention. Test number 28 is an example in which B is low, and as a result of being unable to secure the amount of solid solution B required for hardenability, the hardenability is insufficient even with Ceq ≥ 0.70, and the hardness at the center satisfies the target. Absent. On the other hand, Test No. 29 is an example in which B is excessively added, and the absorbed energy does not satisfy the target due to the precipitation of charcoal boride of a metal element.

In test numbers 30 and 31, Al × N was low outside the preferable range of the present invention, and AlN effective for pinning could not be secured, so that the absorbed energy did not satisfy the target.

Test numbers 32 and 33 have a component range and Al × N are within the range of the present invention, but Ceq is low outside the preferable range, and as a result of forming ferrite in the central portion due to a decrease in hardenability, the hardness of the central portion is high. Not meeting the goal.

In Test No. 34, 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 the unsolid solution coarse AlN remains, which serves as a brittle fracture starting point, and thus absorbs energy. Is not satisfied with the goal.

In Test No. 35, the component range is within the range of the present invention, but the precipitation treatment temperature deviates from the preferable range of the present invention. Test number 35 is an example in which the precipitation treatment temperature was low, and the absorption energy did not satisfy the target because sufficient AlN was not deposited and an effective AlN for pinning could not be secured.

In test numbers 37 and 38, although the component range was within the range of the present invention, the temperature and time of the precipitation treatment were below the preferable range of 8.7 of the present invention, and sufficient AlN precipitation was not performed and pinning was performed. Since the effective AlN could not be secured, the absorbed energy did not meet the target.

In Test No. 39, although the component range is within the range of the present invention, the tempering temperature is below the preferable range, and as a result of insufficient tempering of the structure, the absorbed energy does not satisfy the target.

In Test No. 40, although the component range was within the range of the present invention, the tempering temperature was higher than the preferable range, and the precipitation hardening action of Cr and Mo was sharply reduced, so that the surface layer and the center hardness satisfied the target. Absent.

Claims (4)

The plate thickness is more than 200 mm, and the steel component is mass%. C: 0.10% or more, 0.14% or less Si: 0.00% or more, 0.40 or less Mn: 0.90% or more, 1 .50% 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, 1.50% 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: 0.0025% or more, 0.0070% or less,
B: 0.0005% or more, 0.0020% or less P: 0.0100% or less,
S: 0.0020% or less,
Consisting of the balance Fe and unavoidable impurities
Al × N ^{satisfies 2.0 × 10 -4} or more,
Satisfy the following formula (1)
The metal structure is 99% or more in total area ratio of one or two types of tempered martensite and tempered bainite, and the remaining structure is ferrite, pearlite, retained austenite, non-tempered martensite, and non-tempered bainite. Yes,
The three-point average of the absorbed energy in the C direction at -40 ° C at the center of the plate thickness is 20 J or more, the hardness of the surface layer is 330 or more at HB, and the hardness at the center of the plate thickness is 300 or more at HB. A featured high-strength steel plate.
Ceq = C + Mn / 6 + (Cu + Ni) / 15+ (Cr + Mo + V) / 5 ≧ 0.70 (1)
Here, C, Mn, Cu, Ni, Cr, Mo, and V are mass% of each element. 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,
The high-strength steel sheet according to claim 1, wherein one or more of them are contained. When the steel component is mass%, C: 0.10% or more, 0.14% or less Si: 0.00% or more, 0.40 or less Mn: 0.90% or more, 1.50% 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, 1.50% 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: 0.0025% or more, 0.0070% or less,
B: 0.0005% or more, 0.0020% or less P: 0.0100% or less,
S: 0.0020% or less,
Made from the remaining portion Fe and unavoidable impurities,
Al × N ^{satisfies 2.0 × 10 -4} or more,
Steel with a component that satisfies the following formula (1) is heated to the AlN solid melt temperature Ts (° C) or higher calculated by the following formula (2) and then tempered, and the processing temperature is above 550 ° C and below Ac1. After the precipitation treatment in which T (° C.) and the treatment time tp (Hr) satisfy the following formula (3), the mixture is cooled to room temperature or heated as it is, reheated to 1000 ° C. or higher at the Ac3 transformation point, and cooled with water. By quenching, tempering at 500 ° C or higher and 550 ° C or lower and cooling to room temperature , the plate thickness is more than 200 mm, and the metal structure is tempered martensite, tempered bainite 1 or 2 in total area ratio. 99% or more, the remaining structure is ferrite, pearlite, retained austenite, non-tempered martensite, non-tempered bainite,
A thick steel sheet with a three-point average of absorbed energy in the C direction at -40 ° C at the center of the plate thickness of 20 J or more, a surface hardness of 330 or more for HB, and a hardness of 300 or more for HB at the center of the plate thickness. A method for manufacturing a high-strength steel sheet, which comprises obtaining.
Ceq = C + Mn / 6 + (Cu + Ni) / 15+ (Cr + Mo + V) / 5 ≧ 0.70 (1)
Ts = 7400 / (1.95-Log ₁₀ ([Al] × [N])) -273 (2)
Log (tp [Hr]) + 0.012 × T [° C] ≧ 8.7 (3)
Here, C, Mn, Cu, Ni, Cr, Mo, V, Al, and N are mass% of each element. 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,
The method for producing a high-strength steel sheet according to claim 3, wherein one or more of them are contained.