JP2008045154A - High-hardness hot-rolled steel plate having excellent weldability, workability and penetration resistance to high-speed collision against high hardness missile, and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hot-rolled steel plate having penetration resistance to a high-speed collision against a high-hardness missile, weldability and workability. <P>SOLUTION: The steel plate has a chemical composition comprising 0.22 or more but less than 0.30% C, 0.15 to 0.50% Si, 0.10 or more but less than 0.60% Mn, 0.005% or less P, 0.0020% or less S, 2.5 to 4.5% Ni, 0.20 or more but less than 1.00% Mo, 0.005 to 0.030% Nb, 0.01 to 0.10% Al, 0.006% or less N, optionally one or two elements selected from among 0.10 to 0.80% Cr, 0.01 to 0.20% V, 0.003 to 0.030% Ti and 0.0005 to 0.0030% B, and remaining Fe and unavoidable impurities, while satisfying a PCE value of 0.60% or less. The hot-rolled steel plate has such a composition, has a microstructure that includes a martensitic structure in an amount of 95% or higher by fraction which contains crystal grains of former austenite with an average grain size of 6 μm or less, and has a Brinell hardness of 470 to 560. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a high-hardness hot-rolled steel sheet excellent in weldability, workability, and high-speed collision penetration resistance to a high-hardness flying object used for a structural member, and a method for manufacturing the same.

  When a high-speed flying object collides with a steel plate, generally, the higher the hardness of the steel plate, the more difficult the flying object penetrates. Therefore, a hard steel plate is sometimes used as a so-called armor plate that protects human life and equipment from bullets flying at high speeds exceeding the speed of sound. The use of armor plates varies, but when used for structural members such as vehicles, not only penetration resistance but also good weldability is required, and depending on the applied member, workability such as bending is good It is required at the same time.

  Projectiles that collide, that is, bullets, are roughly classified into those having low hardness whose core is lead, and those having high hardness whose core is high-hardness steel or tungsten. Since the penetrating force of the latter high-hardness flying object is remarkably larger than that of the low-hardness flying object, the thickness of the steel used for defense must be considerably increased. However, since the power performance of the vehicle is limited, it is necessary to reduce the weight of the protective steel plate as a structural member of the vehicle as much as possible, and it is desirable to obtain the necessary penetration resistance performance with the smallest possible thickness.

When welding, a special molten material or a welding method is required, or high temperature preheating is required, the range of members to be applied and welding repair are limited. Therefore, for example, it is desirable that normal CO ₂ welding using a general molten material is possible without preheating.

  The use site is also limited when bending is difficult. Therefore, it is desirable to have a workability such that, for example, bending can be performed without defects by bending 180 degrees at 5 t (bending radius 5 times the plate thickness).

  Further, considering economic efficiency, it is desirable to use a hot-rolled steel sheet having a relatively small production load rather than a forged steel having a large production load.

  The present inventor previously disclosed in Patent Document 1 and Patent Document 2 regarding a steel material having a high resistance to high-speed collision penetration. The invention described in this document assumes penetration resistance when a low-hardness flying object (lead bullet) collides at a high speed exceeding 900 m / s. As for weldability, Patent Document 2 considers weldability on the premise of preheating, but does not consider weldability without preheating. Further, regarding the bending workability, neither Patent Document 1 nor Patent Document 2 is considered.

  For these reasons, a hot-rolled steel sheet that has high-speed impact penetration resistance to high-hardness flying objects, weldability, and workability and can be manufactured economically has not been known so far.

JP 11-264050 A JP 2005-264275 A

  Accordingly, an object of the present invention is to provide a hot-rolled steel sheet that has high-speed impact penetration resistance to a high-hardness flying object, weldability, and workability and can be manufactured economically.

  The inventor of the present invention previously obtained the knowledge for improving the high-speed collision penetration performance with respect to the low-hardness flying object in the invention described in Patent Document 2. However, since the penetrating phenomenon is greatly different when the flying object becomes high in hardness, various new factors that affect the penetration and non-penetration of the flying object at the time of repeated experiments with a high-hardness flying object colliding with the steel sheet Was analyzed.

  As a result, firstly, it was found that there is an optimum hardness range for the high-speed collision penetration resistance of steel plates to high-hardness flying objects. To improve penetration resistance even for high-hardness flying objects, a certain degree of hardness is required and a martensite structure is advantageous. However, the higher the hardness, the lower the deformability and the higher the hardness. There is a tendency that the back surface is easily cracked or peeled off, and the relationship between hardness and penetration resistance is not linear.

  FIG. 1 shows a steel plate having a thickness of 18 mm × length of 300 mm × width of 300 mm (C: 0.18 to 0.37%, Si: 0.18 to 0.25%, Mn: 0.35 to 0.51%, P : 0.002 to 0.004%, S: 0.0004 to 0.0008%, Ni: 3.1 to 3.7%, Cr: 0 to 0.5%, Mo: 0.30 to 0.56 %, Nb: 0.013 to 0.017%, Al: 0.06 to 0.07%, N: 0.003 to 0.005%, all in mass%, and the average particle size of prior austenite is 6 μm or less, and The relationship between the Brinell hardness of the martensite structure fraction of 95% or more and the non-penetration limit speed V50 is shown. Here, V50 means the speed of the flying body where penetration and non-penetration are about 50%. Specifically, flying of Hv700 (hardened carbon steel) having a diameter of about 13 mm, a weight of about 25 g, and a conical tip. Measure the shooting speed at a distance of 2m before the test body using the body, change the shooting speed to collide with the steel plate perpendicularly, and damage to the 0.5mm-thick aluminum plate installed on the back of the steel plate due to flying objects or fragments of the steel plate Is determined to be non-penetrating when no non-penetration occurs, and shooting is performed until one non-penetrating and one penetrating are obtained within a speed range of 15 m / s, and the average value of the two shooting speeds is obtained.

  As shown in FIG. 1, high penetration resistance is obtained when the Brinell hardness is in the range of 470 or more and 560 or less. When the Brinell hardness of the steel sheet is lower than 470, the high-hardness flying object itself penetrates at an impact speed exceeding V50. On the other hand, when the Brinell hardness of the steel plate is higher than 560, even if the high-hardness flying object itself does not penetrate, cracks occur on the back surface, or a part of the back surface peels off, so that fragments are scattered, May be determined, resulting in a lower V50. Therefore, in the present invention, the Brinell hardness (HB) is set to 470 to 560.

  Secondly, as an important finding of the present invention, the penetration resistance performance can be greatly improved by making the prior austenite grain size of the martensite structure significantly smaller than conventional martensitic steels. I found.

  FIG. 2 shows a test steel having a plate thickness of 18 mm × length of 300 mm × width of 300 mm and a Brinell hardness of about 400 (C: 0.18%, Si: 0.22%, Mn: 0.41%, P: 0.00). 004%, S: 0.0005%, Ni: 3.3%, Mo: 0.33%, Nb: 0.017%, Al: 0.07%, N: 0.004%, all by mass, HB: 398 to 411) and a test steel having a Brinell hardness of about 500 (C: 0.24%, Si: 0.25%, Mn: 0.51%, P: 0.003%, S: 0.00). 0006%, Ni: 3.4%, Mo: 0.35%, Nb: 0.015%, Al: 0.07%, N: 0.004%, all in mass%, HB492-508) The relationship between prior austenite grain size and non-penetration limit speed V50 is shown. V50 here is also measured by the same method as described above, and is the velocity of the flying object at which penetration and non-penetration are approximately 50%.

  From FIG. 2, it can be seen that the penetration resistance to the high-hardness flying object is remarkably improved when the prior austenite grain size of the martensite structure is 6 μm or less. In particular, when the Brinell hardness is about 500 and the prior austenite grain size is 6 μm or less, very excellent penetration resistance is shown.

  When the flying speed of the flying object increases, the protrusion on the rear surface side of the steel sheet increases, and cracks and delamination occur at a certain limit protrusion amount. Comparing the behavior of steel plates of the same hardness at the time of collision with a high-hardness flying object, when the particle size is 6 μm or less, the particle size is more than 6 μm even if the protruding amount on the back side is the same at the same impact speed. As compared with, cracks are less likely to occur, and as a result, a tendency to increase V50 is recognized. From this, it is considered that the deformability of the steel sheet against high-speed deformation is improved by the refinement of the prior austenite grain size. The lower limit of the prior austenite grain size is not particularly specified, but if it is less than 1 μm, it is difficult to realize industrially, and 1 μm or more is preferable.

  As described above, since the inventors obtained the knowledge that the penetration resistance performance against high-hardness flying objects can be greatly improved by refining the prior austenite grain size of the martensite structure to 6 μm or less, The hot-rolled steel sheet manufacturing process to obtain a stable microstructure was repeated.

  As a result, firstly, the austenite grain size during heating is greatly influenced by the structure before heating, and the optimum structure before heating for the refinement of austenite is coarse MA (mixed structure of martensite and austenite) or retained austenite. It has been found that there are few fine bainite structures. The purpose of making a fine bainite structure is to increase the number of transformation nucleation sites during quenching and reheating. Coarse MA and residual austenite remain in the orientation relationship with the previous structure, and the orientation is uniform. Therefore, austenite produced with these as nuclei during reheating tends to coalesce with the surrounding austenite, and in particular, the heating temperature becomes Ac3. When near, the average grain size may increase because some of the crystal grains become abnormally coarse. The purpose of suppressing the formation of coarse MA and residual austenite is to avoid such a mixed grain structure and to obtain fine and uniform heated austenite.

  Specifically, the steel to which an appropriate amount of Nb is added is subjected to controlled rolling to ensure a cumulative reduction ratio of 40% or more at 840 ° C. or lower and 790 ° C. or higher, and immediately after completion of hot rolling, at a cooling rate of 3 ° C./s or higher. Accelerated cooling is performed to a temperature not higher than 300 ° C. and not lower than 300 ° C. The controlled structure is refined by controlling rolling while suppressing recrystallization with Nb, and further accelerated cooling is performed to suppress the progress of recrystallization after rolling to obtain a fine bainite structure. By setting the cooling stop temperature to 550 ° C. or lower and 300 ° C. or higher, generation of coarse MA and residual austenite is suppressed.

  Secondly, it has been found that there is a high correlation between the austenite grain size during quenching heating and the quenching heating temperature, and the lower the quenching heating temperature, the finer the austenite structure during heating. Since the lower limit of the heating temperature is the Ac3 transformation point, the quenching heating temperature can be set lower by reducing the Ac3 transformation point of the steel material by adjusting the chemical composition of the alloy, and the heating austenite structure can be further refined. Can do.

  In order to greatly reduce the Ac3 transformation point, there is a method of adding a large amount of austenite former. Both Ni and Mn are strong austenite formers, but Ni improves the toughness and therefore has the effect of suppressing cracking upon impact, whereas Mn decreases the toughness when added in large amounts, so the Ac3 transformation Ni is given priority as an alloy to be added to adjust the point.

  FIG. 3 shows the relationship between the Ac3 transformation point, the quenching heating temperature, and the prior austenite grain size when the steel sheet subjected to the above-described appropriate controlled rolling and accelerated cooling is reheated and quenched. Various test steel plates having a thickness of 20 mm (C: 0.23 to 0.28%, Si: 0.17 to 0.25%, Mn: 0.37 to 0.49%, P: 0.002%, S: 0.0005 to 0.0008%, Ni: 1.5 to 4.1%, Cr: 0 to 0.5%, Mo: 0.24 to 0.41%, Nb: 0.011 to 0.001. 017%, N: 0.003 to 0.005%, and mass%) is controlled rolling to ensure a cumulative reduction of 40% or more at 840 ° C. or lower and 790 ° C. or higher for all the test steel plates, and further heat Immediately after the end of rolling, the steel sheet was cooled to a temperature of 550 ° C. or lower and 300 ° C. or higher at a cooling rate of 3 ° C./s or higher. Further, these steel plates were quenched by varying the quenching heating temperature Tq, and the prior austenite grain size of the steel plates after quenching was measured. As shown in FIG. 3, when the quenching heating temperature Tq is higher than the Ac3 temperature + 40 ° C., recrystallization proceeds and fine austenite grains of 6 μm or less are not obtained. Further, when Tq is less than Ac3 transformation point + 10 ° C., the average prior austenite grain size may be increased instead of being mixed, and therefore, stable prior austenite grains of 6 μm or less cannot be obtained. Further, when the quenching heating temperature is high, recrystallization is likely to proceed. Even if Tq is in the range of Ac3 transformation point + 10 ° C. to + 40 ° C. and exceeds 810 ° C., fine old austenite grains of 6 μm or less are used. May not be obtained. When the Ac3 transformation temperature of the steel material exceeds 800 ° C., Tq cannot be set to Ac3 transformation point + 10 ° C. and 810 ° C. or less, so it is essential that the Ac3 transformation temperature of the steel material is at least 800 ° C. or less. .

  Therefore, in order to stably obtain fine prior austenite grains of 6 μm or less, the Ac3 transformation point is lowered to at least 790 ° C. or less by adding Ni or the like, and the quenching heating temperature is set to Ac3 transformation point + 10 ° C. to + 40 ° C. And, it is a necessary condition that the temperature be 810 ° C. or lower.

  As described in Patent Document 2, it is very important to avoid the plugging phenomenon in which the steel sheet is punched into a plug shape by adiabatic shearing force, as described in Patent Document 2, for improving the high-speed impact penetration performance for a low-hardness flying object. . Since the penetrating force of a high-hardness flying object is large, the steel material used for defense is relatively thick, so the so-called plugging phenomenon in which the entire thickness is punched out is not seen so much. However, when the hitting speed is relatively high, there is a phenomenon in which a part of the back surface is peeled off and scattered, and as a result of metallurgical analysis, it was found that adiabatic shearing force is still acting.

  In order to avoid this backside peeling phenomenon, as in the case of avoiding the plugging phenomenon, (a) addition of Mo and Nb for fixing the high-density dislocations in the martensite structure and increasing the resistance when the temperature rises, (B) Refining the prior austenite grain size and (c) increasing the temperature of the Ac3 transformation point are effective. In the present invention, it is essential to lower the Ac3 transformation temperature for refinement of prior austenite, so the measure of (c) cannot be taken. However, if the finer grain size can be obtained even if the Ac3 transformation point is lowered (b) This is advantageous for the penetration resistance performance. However, excessive reduction in the temperature of Ac3 tends to cause backside peeling due to adiabatic shearing, which may lower the penetration resistance.

  Since the steel of the present invention has a fine crystal grain size, it has relatively good weldability despite its high hardness. As for weldability, it is possible to suppress the alloying elements, specifically, PCE = C + Si / 30 + Mn / 12 + Ni / 50 + Cr / 15 + Mo / 6 + V / 8 + 25P + 30S + 15B, if the PCE is reduced to 0.60 or less, it is possible to weld without cracking without preheating. Confirmed that it was possible. FIG. 4 shows the results of investigating the relationship between PCE and weldability. Test steel plate having a thickness of 20 mm (C: 0.22 to 0.29%, Si: 0.17 to 0.27%, Mn: 0.30 to 0.51%, P: 0.001 to 0.005 %, S: 0.0005 to 0.0018%, Ni: 2.7 to 4.4%, Cr: 0 to 0.6%, Mo: 0.24 to 0.79%, Nb: 0.009 to 0.018%, Ti: 0 to 0.017%, B: 0 to 0.0052%, N: 0.003 to 0.005%, all in mass%, the average grain size of prior austenite is 6 μm or less, martense Using a site structure fraction of 95% or more and Brinell hardness of 470 or more and 560 or less, a welding test according to JIS Z 3154 (heat input 2.0 kJ / mm in CO2 welding, molten metal YM-28 ( 1.2mmφ), atmosphere temperature 20 ° C, humidity 60%, no preheating) It was used as a pass. FIG. 4 shows that if PCE is 0.60% or less, welding without cracking is possible without preheating.

  The microstructure of the steel sheet is preferably as close to full martensite as possible. Since the hardness decreases when the martensite fraction of the structure decreases and the retained austenite fraction increases, the martensite structure fraction is desirably 95% or more.

  This invention is made | formed based on these knowledge, The place made into the summary is as follows.

(1) By mass%, C: 0.22% or more, less than 0.30%, Si: 0.15% or more, 0.50% or less, Mn: 0.10% or more, less than 0.60%, P : 0.005% or less, S: 0.0020% or less, Ni: 2.5% or more, 4.5% or less, Mo: 0.20% or more, less than 1.00%, Nb: 0.005% or more 0.030% or less, Al: 0.01% or more, 0.10% or less, N: 0.006% or less, other chemical components composed of Fe and inevitable impurities, and represented by the following formula Steel having a PCE value of 0.60% or less, consisting of a microstructure of 95% or more of martensite structure whose average grain size of prior austenite is 6 μm or less, and Brinell hardness of 470 or more and 560 or less For weldable, workable and high-hardness flying objects characterized by High-hardness hot-rolled steel sheet with excellent high-speed impact penetration performance.
PCE = [C] + [Si] / 30 + [Mn] / 12 + [Ni] / 50 + [Cr] / 15 + [Mo] / 6 + [V] / 8 + 25 [P] +30 [S] +15 [B] where [C], [Si], [Mn], [Ni], [Cr], [Mo], [V], [P], [S], and [B] are C, Si, Mn, Ni, It is the mass% of Cr, Mo, V, P, S, B.

  (2) By mass%, Cr: 0.10% or more, 0.80% or less, V: 0.01% or more, 0.20% or less, Ti: 0.003% or more, 0.030% or less B: One or more of 0.0005% or more and 0.0030% or less are contained, weldability, workability and high-speed impact penetration resistance as described in the above item (1) High-hardness hot-rolled steel sheet with excellent performance.

  (3) A hot slab in which the steel slab or slab having the chemical composition described in the above (1) or (2) is heated to 1100 ° C. or higher and a cumulative rolling reduction of 40% or higher is ensured at 840 ° C. or lower and 790 ° C. Rolling is performed to obtain a thick steel plate, and accelerated cooling is performed immediately after hot rolling to a temperature of 550 ° C. or lower and 300 ° C. or higher at a cooling rate of 3 ° C./s or higher. Furthermore, after reheating this to a temperature of Ac3 transformation point + 10 ° C. to Ac3 transformation point + 40 ° C. and 810 ° C. or less, a heat treatment is performed to cool to 100 ° C. or less at a cooling rate of 5 ° C./s or more. A method for producing a high-hardness hot-rolled steel sheet having excellent weldability, workability, and high-speed collision penetration resistance to a high-hardness flying object.

  According to the present invention, it is possible to obtain a high-hardness hot-rolled steel sheet that is excellent in weldability, workability, and high-speed impact penetration resistance to a high-hardness flying object while having a relatively low alloy amount. Moreover, according to the manufacturing method of this invention, the said hot-rolled steel plate can be economically manufactured by converter melting and hot rolling.

  Hereinafter, the present invention will be described in detail.

First, the reasons for limiting the steel components of the present invention will be described.
C is an important element that determines the hardness of martensite. In order to obtain the Brinell hardness of 470 or more and 560 or less necessary for obtaining the desired penetration resistance in the present invention, the range of C is made 0.22% or more and less than 0.30%.

  Si is added as a deoxidizing element. However, if Si is extremely low, backside peeling due to adiabatic shear tends to occur, and the penetration resistance tends to be reduced. For this reason, the lower limit of the Si addition amount is set to 0.15%. Further, excessive addition raises the Ac3 temperature of the steel, so the quenching heating temperature cannot be lowered, which hinders austenite refinement. For this reason, the upper limit of Si addition amount is set to 0.50%.

  Mn suppresses toughness reduction due to segregation of S grain boundaries by forming MnS. For this purpose, an addition of 0.10% or more is necessary. Mn is an austenite former and is effective for lowering the Ac3 transformation point, but because toughness is lowered by addition of a large amount, Ni is given priority as an alloy to be added to lower the Ac3 transformation point. On the premise that a certain amount of Ni is added, 0.60% or more of Mn excessively lowers the Ac3 transformation temperature to cause backside peeling due to adiabatic shearing, which may lower the penetration resistance. Therefore, the Mn content is 0.10% or more and less than 0.60%.

  P is an inevitable impurity and is a harmful element that deteriorates weldability and bending workability. Therefore, the upper limit is set to 0.005%, and the PCE is regulated so as not to exceed 0.60 according to the amount of other elements added.

  S is a very harmful element that deteriorates weldability and bending workability, easily causes cracks on the back surface of the steel sheet when it is hit, and lowers penetration resistance. Particularly, in order to obtain the desired bending workability and penetration resistance in the present invention, the content is restricted to 0.0020% or less.

  Ni is an important element in the present invention in order to lower the Ac3 transformation point and lower the quenching temperature as an austenite former and to refine the prior austenite. In addition, it increases the toughness, and is effective in avoiding cracks during impact. In particular, addition of 2.5% or more is necessary to make the Ac3 transformation point 800 ° C. or less. However, an excessive decrease in the Ac3 transformation temperature tends to cause backside peeling due to adiabatic shearing at the same time, so that the penetration resistance may be lowered. Therefore, the upper limit of addition is set to 4.5%. Therefore, the amount of Ni added is 2.7% to 4.5%.

  Mo, when present in a solid solution state in the martensite structure, fixes high-density dislocations and increases resistance when the temperature rises. It is an effective element for improving. However, when a large amount is added, weldability may be reduced. In order to achieve both penetration resistance and weldability, the addition amount of Mo is 0.20% or more and less than 1.00%.

  Nb precipitates finely as Nb (CN) during hot rolling, suppresses recrystallization of austenite, and also suppresses crystal grain growth during quenching heating, so the old austenite grain size of the martensite structure is reduced to a fine grain. It is a very important element to make it. For this purpose, the amount of Nb added must be 0.005% or more. However, if added excessively, fine Nb (CN) cannot be obtained, and a sufficient fine graining effect cannot be obtained. Nb has the effect of improving penetration resistance when it exists in a solid solution state as in Mo, but in the present invention, importance is attached to the refinement of prior austenite grains, and the upper limit of the addition amount is 0.030%. To do.

  Al is added in an amount of 0.01% or more as a deoxidizing element or an inclusion form controlling element. When B is added, 0.05% or more may be added for the purpose of fixing N in order to secure free B necessary for improving hardenability. In either case, excessive addition may reduce toughness, so the upper limit is made 0.10%.

  N is a harmful element that lowers toughness when contained in excess, so the amount is preferably as small as possible. Desirably, it is 0.006% or less.

  The above are the basic chemical components of steel in the present invention. In addition, in the present invention, one or more of Cr, V, Ti, and B can be added in addition to the above chemical components.

  Since Cr improves hardenability, it is added in order to obtain a sufficient martensite structure fraction to the center of the plate thickness, particularly when the steel plate is thick. If it is less than 0.10%, those effects are small. Conversely, if it exceeds 0.80%, the Ac3 transformation point becomes high and the quenching temperature is restricted, so the Cr content is 0.10 to 0.80%. Is desirable.

  V is effective for improving hardenability. If the content is less than 0.01%, these effects are small. Conversely, if the content exceeds 0.20%, coarse precipitates are formed, which is harmful to toughness. Therefore, the V content is desirably 0.01% or more and 0.20% or less.

  Ti is added in an amount of 0.003% or more in order to secure free B necessary for improving the hardenability so as not to form BN by fixing N as TiN when adding B. Since such addition inhibits toughness, the upper limit of addition is 0.030%.

  B increases the hardenability, so it is added to make it easier to obtain a martensite structure, particularly when the steel sheet is thick. In order to exert the effect, 0.0005% or more is necessary, but if added over 0.0030%, weldability and toughness may be lowered, so the B content is 0.0005% or more, 0 0030% or less.

  As described above, with regard to weldability, if alloying elements are suppressed, specifically, PCE = C + Si / 30 + Mn / 12 + Ni / 50 + Cr / 15 + Mo / 6 + V / 8 + 25P + 30S + 15B is reduced to 0.60 or less, there is no preheating. It is possible to weld without cracking.

  As described above, according to the present invention, high penetration resistance can be obtained by making the microstructure as full martensite as much as possible in addition to the steel components. The microstructure is mainly martensite or a mixed structure of martensite and retained austenite, and the martensite has a structure fraction of 95% or more. The retained austenite is morphologically similar to the martensite structure, and it is not easy to distinguish retained austenite and martensite from observation of the microstructure. The structural fraction of martensite and retained austenite can be quantitatively determined from the ratio of the integrated strength of ferrite and the integrated strength of austenite by X-ray analysis. In addition, the upper limit of the structure fraction of martensite is not particularly defined, and includes the case where structures other than martensite cannot be measured.

  Next, a manufacturing method will be described.

  First, a steel slab or slab of the above steel components is heated and hot rolled.

  The heating temperature is 1100 ° C. or higher in order to dissolve Nb.

  Hot rolling ensures a cumulative reduction of 40% or more in a temperature range of 840 ° C. or lower and 790 ° C. or higher. This is for precipitating Nb (CN) and obtaining a sufficient controlled rolling effect. If the controlled rolling is over 840 ° C. or the cumulative rolling reduction in the temperature range of 840 ° C. or lower and 790 ° C. or higher is less than 40%, a sufficient processed structure cannot be obtained and a fine bainite cannot be obtained. Austenite of 6 μm or less may not be obtained during reheating. In addition, when rolling is performed at a temperature lower than 790, the texture of the steel sheet may become strong and may be easily peeled off during impact.

  Immediately after hot rolling, accelerated cooling is performed to a temperature of 550 ° C. or lower and 300 ° C. or higher at a cooling rate of 3 ° C./s or higher. The purpose is to suppress the formation of coarse MA and residual austenite in the structure before heating. If the cooling rate after hot rolling is less than 3 ° C./s or the end temperature of accelerated cooling exceeds 550 ° C., the bainite transformation becomes insufficient and MA tends to be generated. Further, even when the end temperature of accelerated cooling is less than 300 ° C., it is partly martensitic and easily forms MA, and in either case, austenite of 6 μm or less may not be obtained at the time of reheating. .

  The steel sheet after hot rolling is reheated to a temperature of Ac3 transformation point + 10 ° C. to Ac3 transformation point + 40 ° C. and 810 ° C. or less. Thereby, as described above, reheated austenite having an average particle size of 6 μm or less can be obtained. Furthermore, by cooling this to 100 ° C. or less at a cooling rate of 5 ° C./s or more, the fraction of the martensite structure can be 95% or more, and the Brinell hardness can be 470 or more and 560 or less. When the cooling rate is less than 5 ° C./s or the end temperature of accelerated cooling exceeds 100 degrees, the fraction of martensite structure does not become 95% or more, and as a result, a Brinell hardness of 470 or more and 560 or less cannot be obtained. There is. Tempering heat treatment is not particularly required, but the properties of the steel sheet do not depart from the present invention even if heat treatment is performed at a temperature of 300 ° C. or lower.

  Since the steel obtained by such a manufacturing method has a fine structure, it exhibits excellent bending workability while having high hardness. Moreover, the steel component and impurity amount of the present invention can be produced by melting in a converter and can be produced by hot rolling, so that the production cost is relatively low and the economy is excellent.

  As an armor application, an application as a member of a car body, a hull, or a building can be considered, and it may be used as a thick plate or a thin plate according to each application. It is possible to have the hardness of the present invention and the prior austenite grain size over the entire thickness up to a plate thickness of about 50 mm at which a quenching cooling rate of 8 ° C./s can be obtained to the center of the plate thickness by water cooling, but more than that plate Even a thick steel plate has high penetration resistance and can be applied to armor applications.

  A single steel material may be used, or a plurality of steel materials may be laminated. Furthermore, it is good also as a shape which bonds another material and another steel plate, such as a bulletproof fiber, aluminum, and titanium, to the surface, a back surface, or both surfaces.

  The steel slab obtained by melting steels A to AB having the chemical components shown in Table 1 was heated to 1200 ° C., and then each of the production conditions of Examples 1 to 31 and Comparative Examples shown in Table 2 Thus, a steel plate having a thickness of 20 mm was manufactured.

  These steel plates were examined for penetration resistance against high-speed collisions of high-hardness flying objects. Specifically, using a flying body of Hv700 (hardened carbon steel) having a diameter of about 13 mm and a weight of about 25 g, the tip is measured at a distance of 2 m before the test body, and the shooting speed is about 750 m / s (739 m / s). s to 758 m / s), and it was judged to be bulletproof when no damage was caused to the 0.5 mm thick aluminum plate placed on the back side of the steel plate due to penetration of flying objects or fragments of the steel plate. The case where two impacts were made by separating each landing position 10 cm or more on one steel sheet surface and two shots were not passed was accepted, and the case where the aluminum plate was damaged even by one shot was rejected. The thickness of all the test steel plates was 20 mm.

  Evaluation of weldability was carried out according to the content according to JIS Z 3154.

  Bending workability is the method specified in JIS Z2248, bending 180 degrees in the C direction at 5 times the plate thickness (5t) according to the test piece JIS1 and after the bending test, cracks and other defects on the outside of the bent portion If it does not occur, it was accepted.

Toughness was evaluated by the average value of the three absorbed energy values per cm ² in the Charpy impact test at −40 ° C. The test piece was sampled from the center of the plate thickness at right angles to the rolling direction, and used as a JIS Z 2201 No. 4 Charpy test piece having a width of 5 mm. If the toughness is low, problems such as processing may occur, and cracking is likely to occur during impact, which may reduce the high-speed impact penetration performance. Martensite and residual austenite volume fraction Vγ (%) were measured using X-rays under the conditions of a Cr-Kα target, a tube voltage of 30 kV, and a tube current of 20 mA, using a microfocus X-ray stress measurement device PSPC-M / SF manufactured by Rigaku Corporation. Thus, the integral intensity ratio between the ferrite (211) surface and the austenite (220) surface was obtained and calculated by the equation (1).

Vγ (%) = {1 / [(K × 2Ia / Ir) +1]} × 100 (1)
Where Ia: integral strength of ferrite,
Ir: integrated intensity of austenite,
K: Correction coefficient (= 0.30)
For the prior austenite grain size, the average grain size was measured in accordance with the steel grain size microscope test method specified in JIS G0551.

  In addition, the numerical value which underlined in the table | surface shows the thing with insufficient chemical components, temperature conditions, and characteristics outside this invention.

  In Invention Examples 1 to 9 in Table 2, all passed the above penetration resistance test, weld crack test, and bending test. On the other hand, in Comparative Examples 10 to 29 that deviate from the chemical component range limited by the present invention, although the manufacturing method is the method of the present invention, it is one of penetration resistance, weldability, and bending workability. It is rejected by the above. Further, even if the chemical composition is within the range of the steel of the present invention, Comparative Examples 30 to 37 deviating from the production method of the present invention also fail in one or more of penetration resistance, weldability and bending workability.

It is a figure which shows the relationship between Brinell hardness and non-penetration limit speed V50. It is a figure which shows the relationship between the prior austenite grain size and the non-penetration limit speed V50 of the steel plates of Brinell hardness 400 and 500. It is a figure which shows the relationship between an Ac3 transformation point, quenching heating temperature, and a prior-austenite particle size. It is a figure which shows the relationship between PCE value and the crack presence or absence in a JIS Z3154 weld crack test.

Claims (3)

% By mass
C: 0.22% or more, less than 0.30%,
Si: 0.15% or more, 0.50% or less,
Mn: 0.10% or more, less than 0.60%,
P: 0.005% or less,
S: 0.0020% or less,
Ni: 2.5% or more, 4.5% or less,
Mo: 0.20% or more, less than 1.00%,
Nb: 0.005% or more, 0.030% or less,
Al: 0.01% or more, 0.10% or less,
N: In steel containing 0.006% or less, other chemical components composed of Fe and inevitable impurities, and having a PCE value of 0.60% or less represented by the following formula, the average grain size of prior austenite is High-speed collision resistance against weldability, workability and high-hardness flying bodies, characterized by comprising a microstructure with a martensite structure fraction of 6% or less and a microstructure of 95% or more and a Brinell hardness of 470 or more and 560 or less. High-hardness hot-rolled steel sheet with excellent penetration performance.
PCE = [C] + [Si] / 30 + [Mn] / 12 + [Ni] / 50 + [Cr] / 15 + [Mo] / 6 + [V] / 8 + 25 [P] +30 [S] +15 [B]
Here, [C], [Si], [Mn], [Ni], [Cr], [Mo], [V], [P], [S], and [B] are C, Si, and Mn, respectively. , Ni, Cr, Mo, V, P, S, B mass%. In mass%,
Cr: 0.10% or more, 0.80% or less,
V: 0.01% or more, 0.20% or less,
Ti: 0.003% or more, 0.030% or less,
B: It contains 1 type or 2 types or more of 0.0005% or more and 0.0030% or less, It is excellent in the weldability, workability, and high-speed impact penetration performance of Claim 1 characterized by the above-mentioned. High hardness hot rolled steel sheet.   A steel slab or slab having the chemical composition according to claim 1 or 2 is heated to 1100 ° C or higher, and hot rolled to ensure a cumulative reduction ratio of 40% or higher at 840 ° C or lower and 790 ° C or higher. A steel plate is used, and accelerated cooling is performed immediately after hot rolling to a temperature of 550 ° C. or lower and 300 ° C. or higher at a cooling rate of 3 ° C./s or higher. Furthermore, after reheating this to a temperature of Ac3 transformation point + 10 ° C. to Ac3 transformation point + 40 ° C. and 810 ° C. or less, a heat treatment is performed to cool to 100 ° C. or less at a cooling rate of 5 ° C./s or more. A method for producing a high-hardness hot-rolled steel sheet having excellent weldability, workability, and high-speed collision penetration resistance to a high-hardness flying object.

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