JP4374350B2 - High-hardness hot-rolled steel sheet excellent in weldability, workability, and high-speed impact penetration performance and method for producing the same - Google Patents

Description

  The present invention relates to a high-hardness hot-rolled steel sheet excellent in weldability, workability, and high-speed impact penetration performance used for structural members, and a method for producing 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. For this reason, hard steel plates are sometimes used in equipment that protects human lives and equipment from bullets and the like. Equipment for which protective steel plates are used varies, but when used for structural members such as vehicles and ships, not only high-speed collision penetration resistance but also good weldability is required. At the same time, good workability such as bending is required.

  The level of anti-high-speed impact penetration assumed here is the NIJ level III (an evaluation index of elasticity of National Institute of Justice, a research institution of the US Department of Justice) that is generally used as an evaluation standard for penetration resistance to bullets in the United States and the like. , NIJ level III corresponds to, for example, a level that is bullet resistant to 838 m / s of a lead bullet with a mass of 9.7 g). It is very important that a material for protection when used for a structural member such as a vehicle or a ship is practically lightweight, for example, a relatively thin plate thickness of 10 mm or less, and an intended high-speed collision-resistant penetration. It is necessary to have performance. Therefore, the target level of the high-speed collision penetration resistance is assumed to be non-penetration in a test equivalent to NIJ level III with a steel plate having a thickness of 8 mm.

  When welding, a special molten material is required or high temperature preheating is required, application range and welding repair are limited. Therefore, for example, it is desirable that normal CO2 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 thinner the plate thickness, the higher the strength required to have high-speed collision penetration resistance against such high-speed impacts. On the other hand, the higher the hardness, the lower the toughness, and the more easily cracking occurs due to the shock wave when hit. In addition, the higher the hardness, the more rapidly the local adiabatic temperature rises due to the large shearing force generated around the bullet when it is hit, and the strength of the steel sheet decreases, so the steel sheet is punched into a plug and penetrates. That is, the so-called plugging phenomenon is likely to occur. Therefore, it is not easy to achieve both high hardness and avoidance of cracks and penetration. In general, the higher the hardness, the lower the workability such as weldability and bending.

  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 high-speed collision penetration resistance when the speed of the flying object exceeds 900 m / s. Therefore, the amount of alloy is large, such as the C content being 0.3% or more. 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 equivalent to NIJ level III, weldability, and workability and can be manufactured economically has not been known so far.

JP 11-264050 A JP 2005-264275 A

  Therefore, the present invention provides a hot-rolled steel sheet having a high-speed collision penetration resistance equivalent to NIJ level III, a weldability and workability at a plate thickness of 10 mm or less, and a method for manufacturing the same. It is the purpose.

  In order to solve such problems, the present inventor further added bending workability at the same time in addition to the knowledge for providing high-speed collision penetration performance and weldability obtained in the invention described in Patent Document 2. We studied earnestly on how to combine the two. Since Patent Document 2 assumes high-speed collision penetration performance when the speed of the flying object exceeds 900 m / s, a martensite structure having a high dislocation density after setting the C amount to 0.3% or more and However, in this case, since the hardness is increased, it is difficult to ensure bending workability. In the present invention, since the assumed high-speed collision penetration performance is equivalent to NIJ level III, the C amount can be reduced. As a result of examining the relationship between the bending workability and the C content, it is necessary to lower the C content to less than 0.3% in order to obtain bending workability that does not cause cracking in a 180 degree bending test at 5 tons. The knowledge that it is. Furthermore, bending workability can be improved by making the prior austenite grain size of the martensite structure fine.

  It was studied to improve the high-speed collision penetration resistance as much as possible within the range of C content less than 0.30%. As a result, in order to obtain high-speed impact penetration resistance equivalent to NIJ level III, Brinell hardness of at least 470 or more is necessary, and in order to obtain this hardness, the C amount should be 0.22% or more. It became clear that is necessary. Further, as in the invention described in Patent Document 2, it is important to avoid a plugging phenomenon in which the steel sheet is punched into a plug shape by adiabatic shearing force. In order to avoid this, as described in Patent Document 2, it is effective to add Mo and Nb, which increase the resistance when the high-temperature dislocations in the martensite structure are fixed and the temperature rises. However, if Mo and Nb are added in a large amount, the weldability is deteriorated, so the addition amount must be limited in the present invention. As a result of earnestly examining the means for avoiding plugging without adding a lot of alloying elements, the inventors have made the transformation temperature at the time of temperature rise as high as possible and refined the prior austenite grain size of the martensite structure. I have gained new knowledge that it is effective. The reason for the effect of increasing the transformation temperature at the time of temperature rise is that the plugging is a phenomenon in which the structure loses resistance due to transformation from martensite to austenite due to rapid temperature rise. We believe that suppression works effectively. Specifically, it is effective to reduce the austenite former such as Mn or Ni as much as possible. Further, regarding the effect of refining the crystal grain size, it is considered that the shear force is dispersed by the grain boundary sliding deformation. Specifically, the NIJ level III equivalent test in FIG. 1 (steel composition: 0.26). % C-0.22% Si-0.48% Mn-0.002% P-0.0008% S-0.23% Mo-0.04% Al-0.22% Nb-0.013% Ti -0.0011% B-0.003% N, steel plate thickness: 8 mm) When the Brinell hardness is 470 or more, if the prior austenite average grain size is 20 μm or less, the higher high-speed impact penetration resistance It was found that performance was obtained.

Refining the prior austenite average grain size can reduce the unit of fracture by refining the lath length of the martensite structure after quenching, thus improving the toughness of the heat affected zone and welding. It is also effective in avoiding cracks. As for weldability, steel materials having various alloy compositions satisfying the condition that the fraction of the martensite structure in which the average grain size of prior austenite is 20 μm or less is 95% or more and the Brinell hardness is 470 or more. Was investigated. That is, the investigation was carried out by a JIS Z 3154 weld cracking test. The test conditions were CO ₂ welding, heat input: 2.0 kJ / mm, molten material: YM-28 (1.2 mmφ), atmosphere: temperature 20 ° C., humidity 60 %, Test steel composition range: C: 0.23-0.33%, Si: 0.09-0.68%, Mn: 0.29-0.88%, P: 0.001-0 .010%, S: 0.004 to 0.0043%, Ni: 0 to 2.0%, Cr: 0 to 2.5%, Mo: 0.19 to 1.34%, V: 0 to 0. 0. It was 29% and B: 0.0007 to 0.0052%. In all the test steel sheets, the fraction of the martensite structure in which the average grain size of the prior austenite was 20 μm or less was 95% or more and the Brinell hardness was 470 or more.
As a result, in the steel material satisfying these conditions, the alloy elements are further suppressed, specifically, as shown in FIG. 2, PCE = C + Si / 30 + Mn / 12 + Ni / 50 + Cr / 15 + Mo / 6 + V / 8 + 25P + 30S + 15B It was confirmed that if the heat resistance is 0.55% or less, it is possible to weld without cracking without preheating in CO ₂ welding with a heat input of about 2.0 kJ / cm.

  The microstructure of the steel sheet is preferably as close to full martensite as possible. When the martensite fraction of the structure is lowered and the retained austenite fraction is increased, the hardness is lowered and plugging is likely to occur, and penetration is facilitated. Therefore, it is desirable that the martensite structure fraction is 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, 0.30% or less, Si: 0.03% or more, 0.5% or less, Mn: 0.10% or more, less than 0.60%, P : 0.005% or less, S: 0.002% or less, Mo: 0.15% or more, less than 1.0%, Nb: 0.01% or more, 0.10% or less, Ti: 0.003% or more 0.05% or less, Al: 0.001% or more, 0.1% or less, B: 0.0005% or more, 0.003% or less, and other composition including Fe and inevitable impurities, And a steel having a PCE value of 0.55% or less, represented by the following formula, comprising a microstructure with a martensite fraction with an average grain size of prior austenite of 20 μm or less, and a Brinell hardness Weldability, workability, and high-speed impact penetration resistance, characterized by a thickness of 470 or more High hardness hot-rolled steel sheet excellent.
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%.

  (2) In mass%, Cr: 0.2% or more, 2.0% or less, V: 0.03% or more, 0.20% or less, Ni: 0.2% or more, 1.0% or less A high-hardness hot-rolled steel sheet excellent in weldability, workability and high-speed impact penetration performance as described in (1) above, comprising one or two of the above.

  (3) Hot rolling to heat a steel slab or cast slab having the composition described in (1) or (2) above to 1100 ° C or higher and ensure a cumulative reduction of 40% or higher at 860 ° C or lower and 800 ° C or higher. A thick steel plate, and heat treatment is performed to cool the steel plate from Ac3 transformation point temperature + 20 ° C. to Ac3 transformation point temperature + 50 ° C. to 100 ° C. or less at a cooling rate of 10 ° C./sec or more. A method for producing a high-hardness hot-rolled steel sheet having excellent workability and high-speed impact penetration performance.

  According to the present invention, a hot-rolled steel sheet having a relatively low alloy amount, having a plate thickness of 10 mm or less and having high-speed collision penetration resistance equivalent to NIJ level III, excellent weldability, and excellent configuration is obtained. It is done. 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 element that determines the hardness of martensite, which is important for high-speed impact penetration performance. In order to obtain the Brinell hardness of 470 or more necessary for obtaining the desired high-speed impact penetration performance in the present invention, addition of 0.22% or more is necessary, but if added over 0.30%, welding is performed. Since the properties are lowered and the welding conditions are limited, the content is made 0.22% or more and 0.30% or less.

  Si is a deoxidizing element necessary for steelmaking, and 0.03% or more is added for that purpose. However, excessive addition may reduce the toughness of the steel material, so the upper limit is made 0.5%.

  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. However, Mn is an austenite former, and when added in an amount of 0.60% or more, the transformation temperature is lowered to cause plugging, thereby reducing the high-speed collision penetration resistance. Therefore, the 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 0.55 according to the amount of other elements added.

  S is a very harmful element that deteriorates weldability and bending workability, easily cracks, and lowers high-speed impact penetration performance. In particular, in order to obtain the desired bending workability and high-speed impact penetration performance in the present invention, the content is made 0.002% or less, and the PCE is regulated not to exceed 0.55 depending on the amount of other elements added. .

  Mo is an effective element for improving the high-speed collision penetration resistance since Mo exists in a solid solution state in the martensite structure to fix high-density dislocations and increase the resistance when the temperature rises. However, when a large amount is added, weldability may be reduced. In order to achieve both penetration resistance and weldability, the amount of Mo is 0.15% or more and less than 1.0%.

  Nb precipitates finely during hot rolling, suppresses recrystallization of austenite, has the effect of refining the prior austenite grain size of the martensite structure, and improves the high-speed impact penetration resistance like Mo It is a very useful element. In order to exhibit this effect, addition of 0.01% or more is essential. However, if added over 0.10%, the Nb precipitates become coarse, so the effect of suppressing recrystallization is lost, the prior austenite grain size becomes coarse, and the toughness and weldability may deteriorate. For this reason, Nb content shall be 0.01 or more and 0.10% or less.

  Ti is fixed to N as TiN so that BN is not formed, and 0.003% or more is added in order to secure free B necessary for improving hardenability, but excessive addition of coarse TiN As a result, the toughness is lowered and the bending workability may be lowered. Therefore, the addition amount of Ti is set to 0.003% or more and 0.05% or less.

  Al is contained as 0.001% or more and 0.1% or less as a deoxidizing element or inclusion form control element.

  B increases hardenability, makes it easy to obtain a martensite structure, and increases hardness. In order to exert the effect, 0.0005% or more is necessary, but if added over 0.003%, the weldability may be lowered, so the content of B is 0.0005% or more, 0.003 % Or less.

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

  Ni enhances toughness, so that it is effective in avoiding cracks during impact, particularly when the steel sheet is about 10 mm or less. In order to obtain this effect, addition of 0.2% or more is necessary. However, Ni is an austenite former, and if it is added in a large amount, the transformation temperature is lowered, so that plugging occurs and the high-speed collision penetration resistance is lowered. For this reason, the addition amount of Ni shall be 0.2% or more and 1.0% or less.

  Cr is effective for improving hardenability. If it is less than 0.2%, those effects are small. Conversely, if it exceeds 2.0%, the weldability is lowered. Therefore, the Cr content is desirably 0.2% or more and 2.0% or less.

  V is effective for improving hardenability. If it is less than 0.03%, those effects are small, and if it exceeds 0.20%, coarse precipitates are formed, which is harmful to toughness. Therefore, the V content is preferably 0.03% or more and 0.20% or less.

  In addition to the above components, N is an unavoidable impurity that is a harmful element that reduces toughness. Desirably, N is 0.01% or less.

  As described above, in the present invention, in addition to the steel composition, a high-speed collision penetration resistance can be obtained by making the microstructure a full martensite structure as much as possible. The microstructure is mainly 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.

  Next, a manufacturing method will be described.

  First, hot rolling is performed by heating a steel slab or slab having the above steel component composition. The heating temperature may be a normal 1100 ° C. or higher. The upper limit of the heating temperature is not particularly limited, but it is realistic to set it to about 1300 ° C. from the viewpoint of heating cost. In hot rolling, in order to precipitate Nb (CN) and obtain a controlled rolling effect, a cumulative rolling reduction of 40% or more is ensured in a temperature range of 860 ° C. or lower and 800 ° C. or higher. After rolling, the steel is once cooled below the Ar3 transformation point by cooling or accelerated cooling, and then reheated to a range of Ac3 transformation point +20 to 50 ° C. to perform quenching heat treatment. The meaning of this heating temperature range is to ensure sufficient hardenability and to refine austenite. In quenching, in order to obtain a steel having a martensite fraction of 95% or more and a Brinell hardness of 470 or more, a cooling rate of 10 ° C./sec or more is required to 100 ° C. or less. 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.

  The steel obtained by such a manufacturing method has a microstructure of 95% or more of martensite structure in which the average grain size of prior austenite is 20 μm or less, and exhibits high high-speed impact penetration resistance and excellent bending workability. Moreover, the steel composition 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 is relatively inexpensive and the economy is excellent.

  The steel plate of the present invention may be used as a vehicle body, a hull, a building member, or the like, and may be used as a thick plate or a thin plate depending on each use.

  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.

  Steel pieces obtained by melting steels A to AA having the compositions shown in Table 1 were heated to 1200 ° C., and then manufactured under the respective production conditions of Examples 1 to 31 and Comparative Examples shown in Table 2. A steel plate having a thickness of 8 mm to 16 mm was manufactured.

  About these steel plates, the high-speed impact penetration performance was investigated by the ballistic test equivalent to NIJ level III. Specifically, using a lead bullet with a diameter of 7.62 mm and a mass of 9.7 g, the shot was shot perpendicularly to the steel plate at a velocity of 838 m / s or more and 857 m / s or less, and the thickness 0.5 mm installed on the back surface of the steel plate. The steel plate was judged to be bulletproof when there was no damage due to penetration or debris. The case where each landing position was separated by 10 cm or more on one steel plate surface and fired 5 times to make it all bulletproof was accepted, and the case where the aluminum plate was damaged by penetration or debris was rejected. The plate thickness of the test steel plate was set to 8 mm as it was when the original plate thickness was 8 mm, and when it was thicker than 8 mm, it was all reduced by cutting.

The weldability was evaluated by a weld crack test in accordance with JIS Z 3154. In an atmosphere with an atmospheric temperature of 20 ° C. and a humidity of 60%, CO ₂ welding with a heat input of 2.0 kJ / mm using 1.2 mmφ YM-28 as a melt was performed, and the case where no cracks occurred was determined to be acceptable.
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 retained 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 type 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 with the underline in the table | surface shows that the component value outside this invention, temperature conditions, and a characteristic are inadequate.

  In Invention Examples 1 to 7 in Table 2, all passed the above-mentioned NIJ level III equivalent test, weld crack test, and bending test. On the other hand, in Comparative Examples 8 to 27 deviating from the chemical composition range limited by the present invention, although the manufacturing method is the method of the present invention, the high-speed impact penetration resistance, weldability, and bending workability are One or more of them are rejected. Even in the case of the steel of the present invention, Comparative Examples 28 to 31 that deviate from the production method of the present invention also fail in one or more of the high-speed collision penetration performance, weldability, and bending workability.

It is a figure which shows the relationship between a Brinell hardness and a prior-austenite particle size, and high-speed collision penetration resistance. It is a figure which shows the relationship between a PCE value and the crack generation presence or absence in a JIS Z3154 weld crack test.

Claims (3)

% By mass
C: 0.22% or more, 0.30% or less,
Si: 0.03% or more, 0.5% or less,
Mn: 0.1% or more, less than 0.60%,
P: 0.005% or less,
S: 0.002% or less,
Mo: 0.15% or more, less than 1.0%,
Nb: 0.01% or more, 0.10% or less,
Ti: 0.003% or more, 0.05% or less,
Al: 0.001% or more, 0.1% or less,
B: Steel containing 0.0005% or more and 0.003% or less, having a composition composed of other Fe and inevitable impurities, and having a PCE value of 0.55% or less represented by the following formula, Weldability, workability and high-speed collision resistance, characterized by comprising a microstructure with a martensite structure fraction of 95% or more and an average particle size of prior austenite of 20 μm or less and a Brinell hardness of 470 or more 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: 0.2% or more, 2.0% or less,
V: 0.03% or more, 0.20% or less,
Ni: One or two of 0.2% or more and 1.0% or less is contained, and the high excellent weldability, workability and high-speed impact penetration performance according to claim 1 Hardness hot-rolled steel sheet.   A steel plate or slab having the composition according to claim 1 or 2 is heated to 1100 ° C or higher and hot rolled to ensure a cumulative reduction of 40% or higher at 860 ° C or lower and 800 ° C or higher. And heat treatment is performed by cooling from Ac3 transformation point temperature + 20 ° C. to Ac3 transformation point temperature + 50 ° C. to 100 ° C. or less at a cooling rate of 10 ° C./sec or more. A manufacturing method for high-hardness hot-rolled steel sheets with excellent high-speed impact penetration performance.

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