JP4213815B2 - Cold-rolled steel sheet for post-press nitriding with excellent strength and low-temperature brittleness after nitriding and method for producing the same - Google Patents

Description

【００５０】
【表２】

【００５１】
【表３】

【００５２】
【発明の効果】
以上説明したように、本発明によれば、鋼組成、組織、及び製造条件を特定することにより、プレス成形し更に窒化した後に高い強度と低温靭性を有するプレス後窒化処理用冷延鋼板が製造できる。
【００５３】
このような鋼板を成形後窒化して用いることにより、寸法精度、強度、耐久性に優れた高強度一般構造用部品を低コストで製造することが可能となり、産業上極めて有益である。
【図面の簡単な説明】
【図１】本発明の実施例に係る鋼板のフェライトの平均結晶粒径と延性−脆性遷移温度の関係を示す図。[0001]
BACKGROUND OF THE INVENTION
The present invention can be applied to, for example, parts such as members and reinforcement in automobiles, parts that require shock absorption capability, parts that require dent resistance such as outer plates, and other parts for high-strength mechanical structures. relates to a press molded body steel sheet, it is possible to manufacture with good dimensional accuracy yet low cost excellent press molded product high strength and low temperature toughness by the use of this steel plate is nitrided after pressing, further nitrided press molding The present invention relates to a cold-rolled steel sheet for post-press nitriding that has excellent strength and low-temperature brittleness and a method for producing the same.
[0002]
[Prior art]
In recent years, weight reduction of a vehicle body weight is aimed at from the request | requirement of the fuel consumption improvement of a motor vehicle. In addition, from the viewpoint of collision safety, there is an increasing need for high strength and high rigidity of the body and shock absorption capability. In order to satisfy these needs, structural members such as members that require strength and reinforcement members such as reinforcement are switched from conventional soft steel plates to high-tensile steel plates with a TS (tensile strength) of 340 MPa or more. Is planned.
[0003]
However, since a high-tensile steel plate is elongated in comparison with a soft steel plate and is inferior in r value, it is a matter of course that formability is low and it is difficult to press-form complicated shapes. Furthermore, the springback increases due to an increase in YP (yield strength), and it is difficult to obtain good dimensional accuracy.
[0004]
For body panels, it is desirable to increase the yield strength of products from the viewpoint of ensuring dent resistance. However, increasing the strength of the material degrades the dimensional accuracy after pressing and also degrades the formability, which causes design limitations. In order to solve this problem, the inventors of the present invention have a steel plate controlled to a limited range in which components are excellent in high press formability and strength increase by nitriding treatment after press forming, and toughness after nitriding treatment. It was found to be excellent.
[0005]
However, the high-strength press-molded body produced in this way often undergoes shock deformation in a low-temperature environment, but there is a problem that the technique disclosed above does not necessarily have sufficient low-temperature toughness after nitriding. In addition, when the strain rate exceeds 10 ² / sec, there is a problem that the material breaks brittlely at the time of deformation by acting equivalently to the case of low temperature.
[0006]
Conventional nitriding steel plates having excellent nitriding or soft nitriding properties are disclosed in JP 54-21916, JP 55-76046, JP 1-96330, JP 8-35013, JP Several techniques are disclosed in Japanese Patent Publication No. 9-25517. Japanese Patent Laid-Open No. 6-136438 discloses a technique for hardening the inside of a plate by using precipitation strengthening of ε-Cu together with nitriding.
[0007]
[Problems to be solved by the invention]
However, Japanese Patent Application Laid-Open No. 54-21916 is intended for wear resistance and fatigue resistance, and the Ti concentration is too high for the purpose of machine structural parts intended by the present invention. It is inferior in fracture strength and toughness at low temperature because it is. Furthermore, this prior document does not show that the Ti concentration is markedly excellent in strength and toughness after nitriding in the scope of the present invention.
[0008]
In JP-A-55-76046 and JP-A-9-25517, Cr addition is essential, but Cr has the effect of increasing the hardness of the surface hardened layer, but it causes embrittlement not only at a low temperature but also at a normal temperature. Therefore, it must not be added to nitrided steel sheets for machine structural parts that require toughness. Moreover, since an additive element is expensive, it is not industrially preferable.
[0009]
Japanese Laid-Open Patent Publication No. 1-96330 is nitrided in the plate manufacturing stage and has a technical idea different from that of the present invention. Further, this prior document does not show that the strength and toughness after nitriding are particularly excellent in the limited range of C and Ti shown in the claims of the present invention. The technique disclosed in Japanese Patent Laid-Open No. 8-35013 is not excellent in strength after nitriding because the solid solution Ti amount contributing to the strength increase by nitriding is not sufficient as compared with the present invention.
[0010]
Furthermore, the Cu-added steel disclosed in JP-A-6-136438 has a remarkable occurrence of surface defects due to eutectic melting of Cu during hot rolling, and cannot be applied to parts that require good surface properties. . As described above, in the prior art, it is impossible to produce a steel sheet for nitriding treatment excellent in strength and toughness after nitriding, which is optimal for producing general structural parts by nitriding.
[0011]
An object of the present invention is to provide a cold-rolled steel sheet for post-press nitriding that is excellent in strength and low-temperature brittleness after nitriding, and further excellent in press formability and surface properties, and a method for producing the same.
[0012]
[Means for Solving the Problems]
In order to solve the above problems and achieve the object, the present invention uses the following means.
[0013]
(1) The steel sheet of the present invention is, by weight, C: 0.010% or less, Si: 0.05% or less, Mn: 0.1 to 1%, P: 0.1% or less, S: 0.02% or less, Sol. Al: 0.01-0.10%, Cu: 0.1% or less, N: 0.003% or less, Ti is defined by the following formula (1) Ti ^* : −0.01 to 0 contained in the range of .08%, the balance being Fe and unavoidable impurities, and wherein the average crystal grain size of the ferrite is 15μm or less, excellent in strength and low temperature brittleness after further nitrided press forming This is a cold-rolled steel sheet for nitriding after pressing .
[0014]
Ti ^* % = Ti% −48 / 14 × N% −48 / 12 × C% −48 / 32 × S% (1)
(2) The steel sheet of the present invention is characterized by containing, as a steel component, by weight%, and further B: 0.0002 to 0.001%, and further press-molded and nitrided according to (1) above . It is a cold-rolled steel sheet for post-press nitriding that has excellent strength and low-temperature brittleness.
[0015]
(3) The steel sheet of the present invention contains, as a steel component, by weight% and further contains Nb: 0.005 to 0.10%, press forming as described in (1) or (2) above and the intensity and cold rolled steel sheet for excellent press after nitriding to the low-temperature brittleness after further nitride.
[0016]
(4) The production method of the present invention is defined by a softening rate of 50% after hot rolling and cold rolling a steel having the composition described in any one of (1) to (3) above. that and performing recrystallization annealing in the range of recrystallization temperature ~ (recrystallization temperature + 50 ° C.), press molding was further cold rolled steel sheet excellent press after nitriding to the strength and low-temperature brittleness after nitriding It is a manufacturing method.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
In order to solve the above-mentioned problems, the present inventors diligently studied the mechanism of strength increase by nitriding. As a result, it is not chemically bonded to other elements in the steel sheet that contributes to the strength increase by nitriding. It was found that Ti is a solid solution Ti, and it is possible to produce a nitriding steel plate excellent in strength and toughness after nitriding by controlling this within a certain limited range.
[0018]
The solid solution Ti concentration is defined by the following formula, which is an amount obtained by excluding nitride, carbide, sulfide and complex thereof from the total addition amount of Ti.
[0019]
Ti ^* % = Ti% −48 / 14 × N% −48 / 12 × C% −48 / 32 × S%
Further, by limiting the chemical components to the component ranges as shown in claim 1 of the present invention, various properties necessary for industrial application of the steel sheet such as press formability and surface properties were satisfied.
[0020]
Furthermore, after nitriding, it has been found that it is extremely effective to reduce the ferrite structure (ferrite average crystal grain size) to 15 μm or less in order to suppress embrittlement under low temperature and high speed deformation while having high strength. It was.
[0021]
In addition, in order to obtain a steel sheet excellent in strength after nitriding and resistance to low temperature brittleness, the recrystallization annealing temperature after cold rolling is defined with a softening rate of 50% in the component range satisfying the above conditions. It has been found that a steel sheet excellent in strength and low-temperature brittleness can be obtained after nitriding by limiting the recrystallization temperature to the range of (recrystallization temperature + 50 ° C.) or less.
[0022]
Based on the above knowledge, the present inventors are defined by the amount of Ti, the amount of addition of nitride, carbide, sulfide and complex thereof in the ultra-low carbon steel of C: 0.010% or less. Nitriding with high strength and low temperature brittleness after nitriding by specifying the solid solution Ti concentration (Ti ^* ) and controlling the recrystallization annealing temperature after cold rolling within a certain range to refine the steel structure A steel sheet for treatment and a method for producing the same were found, and the present invention was completed.
[0023]
That is, the present invention provides a steel sheet for nitriding that is excellent in strength and low-temperature brittleness after nitriding, and further excellent in press formability and surface properties by limiting the steel composition, structure, and production conditions to the following ranges. Can do.
[0024]
The reasons for adding the components, the reasons for limiting the components, the reasons for limiting the steel structure, and the reasons for limiting the manufacturing conditions will be described below.
(1) Component composition range and steel structure Ti ^* : −0.01 to 0.08%, where Ti ^* % = Ti% −48 / 14 × N% −48 / 12 × C% −48 / 32 × S %
Ti is the most important chemical component of the present invention, and the control of the solid solution Ti excluding the nitrides, carbides, sulfides and their composites out of the total amount of Ti added is the strength after nitriding. Indispensable for balancing toughness. Solid solution Ti is defined by the formula Ti ^* % = Ti% −48 / 14 × N% −48 / 12 × C% −48 / 32 × S%, and this is in the range of −0.01 to 0.08%. It is necessary to control. Since this equation gives a solid solution Ti concentration that exists in equilibrium, Ti is present in a solid solution in a non-equilibrium state even if Ti ^* is −0.01% or more even in a negative region. Therefore, the strength increasing ability by nitriding is excellent. If it is less than -0.01%, the strength is not sufficiently increased by nitriding, and a high strength cannot be obtained. On the other hand, if it exceeds 0.08%, it becomes brittle after nitriding, and the low temperature toughness is remarkably deteriorated.
[0025]
C: 0.010% or less When the C concentration exceeds 0.010%, C forms TiC in the Ti-added steel, the strength increases, and the formability deteriorates remarkably. Therefore, C must be 0.010% or less.
[0026]
Si: 0.05% or less When Si exceeds 0.05%, the strength increases due to solid solution strengthening, and the moldability is reduced. Therefore, the upper limit of the content is 0.05%.
[0027]
Mn: 0.1 to 1%
Mn precipitates as MnS together with the solid solution S and suppresses surface defects caused by S. If it is less than 0.1%, the effect is not sufficient, and if it exceeds 1%, the strength increases due to solid solution strengthening and the moldability decreases. Therefore, the content is 0.1 to 1%.
[0028]
P: 0.1% or less P is a solid solution strengthening element like Mn and Si, and if it exceeds 0.1%, the formability is remarkably deteriorated. Therefore, the content is 0.1% or less.
S: 0.02% or less Since S causes surface flaws as described above, it must be controlled to 0.02% or less.
[0029]
Sol. Al: 0.01-0.10%
Sol. Al is added as a deoxidizing material. If it is less than 0.01%, the effect is not sufficient. Even if added over 0.10%, the effect is saturated and uneconomical, so it is 0.10% or less.
[0030]
N: 0.003% or less N is a solid solution N that prevents the formation of a texture suitable for improving the r value during annealing, generates stretcher strain marks during molding due to strain aging, and further contributes to strengthening during nitriding. In order to fix the molten Ti as TiN, it is desirable to reduce it as much as possible in order to reduce the effect of nitriding. If it exceeds 0.003%, the adverse effect becomes remarkable, so it is necessary to make it 0.003% or less.
[0031]
Cu: 0.1% or less Since Cu eutectic melts during hot rolling and the surface properties are remarkably deteriorated, it must be made 0.1% or less.
[0032]
In the present invention, in addition to the above-described elements, B and Nb may be further added in the following ranges as necessary.
B: 0.0002 to 0.001%
By adding 0.0002% or more of B, secondary work embrittlement during molding is suppressed. B also has the effect of lowering the ductile-brittle transition temperature of the material after nitriding by strengthening the grain boundaries. If the content is less than 0.0002%, the effect is not sufficient. If the content exceeds 0.001%, BN is formed in the plate manufacturing stage and the formability is deteriorated. Therefore, it is necessary to add 0.0002 to 0.001%. .
[0033]
Nb: 0.005 to 0.10%
Addition of 0.005 to 0.10% of Nb has the effect of improving the r value and elongation of the material. If the content is less than 0.005%, the effect is not obtained. If the content exceeds 0.10%, the effect is saturated and the cost is increased. There is a need to.
[0034]
Ferrite average grain size: 15 μm or less After nitriding, when deformed at a low temperature or an ultra-high strain rate exceeding 10 ² / sec, the higher the strength, the more likely to be brittle. Although the ductile-brittle transition temperature is evaluated by a Charpy impact test, it is industrially necessary that this value be −50 ° C. In order to make the ductile-brittle transition temperature -50 ° C. or lower without impairing the strength after nitriding, the average crystal grain size of ferrite needs to be 15 μm or lower. This is because the fracture mode is a grain boundary or cleavage, and the grain boundary becomes a resistance to crack propagation, and it is considered that embrittlement is suppressed with 15 μm as a critical point.
[0035]
By adjusting to the above component composition range and structure, it is possible to obtain a steel sheet for nitriding that is excellent in strength after nitriding and low-temperature brittleness, and further excellent in press formability and surface properties.
[0036]
A steel plate having such characteristics can be manufactured by the following manufacturing method.
[0037]
(2) Steel plate manufacturing process (manufacturing method)
The steel adjusted to the above component composition range is melted in a converter and made into a steel ingot, then hot-rolled and cold-rolled by a conventional method, and then redefined as a softening rate of 50%. Recrystallization annealing is performed in the range of the crystal temperature to (recrystallization temperature + 50 ° C.).
[0038]
a. Control of the annealing temperature after cold rolling is necessary in order to obtain a nitriding steel sheet having excellent annealing temperature and low temperature toughness. The annealing temperature must be not less than the recrystallization temperature defined by a softening rate of 50% and not more than (recrystallization temperature + 50 ° C.). If the annealing temperature is less than the recrystallization temperature, the formability of the steel sheet is insufficient and is not suitable for forming processing applications. When the annealing temperature exceeds (recrystallization temperature + 50 ° C.), the crystal grains become coarse and embrittlement is promoted. In addition, as the method of recrystallization treatment, either continuous annealing or box annealing may be used.
[0039]
A structural part having excellent strength and toughness can be produced by forming a steel sheet having the above components and structure into a predetermined shape and then performing nitriding or soft nitriding treatment. The effects of nitriding and soft nitriding here are the same for both gas nitriding and plasma (ion) nitriding. The nitriding conditions are the same as in the conventional case for the purpose of surface hardening, and a nitriding temperature of 500 to 600 ° C. and a nitriding time of about 30 minutes to 2 hours are appropriate.
[0040]
Examples of the present invention will be given below to prove the effects of the present invention.
[0041]
【Example】
A hot-rolled sheet having various chemical components and compositions as shown in Table 1 was cold-rolled to a thickness of 1.2 mm (AC: steel of the present invention, D: comparative steel). As a result of measuring the recrystallization temperature of these cold-rolled sheets, all were about 670 ° C. Annealing was performed at 700 ° C. using a continuous annealing furnace. Nitriding was performed by using a gas soft nitriding method in a mixed gas of NH ₃ , N ₂ , H ₂ and CO at 570 ° C. for 1 hour.
[0042]
The low temperature toughness was evaluated by measuring the absorbed energy by the Charpy impact test (JISZ2242) and measuring the ductile-brittle transition temperature defined as the temperature at which the absorbed energy becomes the median value between the ductile region and the brittle region. The test piece (JISZ2202) was made into a sub-size of No. 4 test piece by machining after two plates were attached by spot welding after nitriding.
[0043]
The average crystal grain size is determined by polishing a cross section perpendicular to the plate width direction, measuring the number of crystals per fixed length (I) (n) in parallel with the rolling direction and the plate thickness direction (about 100), The average grain size was measured as d = I / n , and the grain size in the rolling direction and the grain size in the plate thickness direction were geometrically averaged to obtain the average crystal grain size (JIS G0522 (steel ferrite grain size test method) cutting) Measurement method similar to the method (measuring particle size in JIS)).
[0044]
Table 2 shows the evaluation results of tensile properties and low temperature toughness before and after nitriding. The average crystal grain size of the ferrite after annealing is 15 μm or less. The inventive steels A to C exhibit a high tensile strength of 500 MPa or more after nitriding. The ductile-brittle transition temperature is very good at -50 ° C or lower. In contrast, Comparative Steel D with Ti ^* outside the scope of the present invention has high strength after nitriding, but has a high ductile-brittle transition temperature and is inferior in low-temperature brittleness.
[0045]
Table 3 shows that the test steel (invention steel) B was subjected to recrystallization annealing at various temperatures in a continuous annealing furnace, and the average crystal grain size of ferrite, the strength before and after nitriding, and the ductile-brittle transition temperature were investigated. A result is shown (No.2-4: example of this invention, No.1,5: comparative example). The recrystallization temperature of this steel (that is, the recrystallization temperature defined by 50% softening rate measured by Rockwell hardness) was 670 ° C.
[0046]
FIG. 1 shows the relationship between the average grain size of ferrite and the ductile-brittle transition temperature. It can be seen that when the particle size is 15 μm or less, the ductile-brittle transition temperature is as good as −50 ° C. or less, but when it exceeds 15 μm, it deteriorates.
[0047]
As shown in Table 3, Example No. of the present invention satisfying the production conditions of the present invention. Nos. 2 to 4 have high strength after nitriding, a ductile-brittle transition temperature of −50 ° C. or lower and extremely low temperature toughness.
[0048]
On the other hand, comparative example No. whose annealing temperature is less than recrystallization temperature. No. 1 has a low material elongation and is not excellent in moldability. Comparative Example No. Since No. 5 has a high annealing temperature, the average grain size of ferrite exceeds 15 μm, and the low-temperature brittleness is inferior.
[0049]
[Table 1]

[0050]
[Table 2]

[0051]
[Table 3]

[0052]
【The invention's effect】
As described above, according to the present invention, the steel composition, tissue, and by specifying the manufacturing conditions, cold-rolled steel sheet manufacturing press after nitriding with later high strength and low temperature toughness and further nitrided press forming it can.
[0053]
By using such a steel sheet after nitriding after forming, it becomes possible to manufacture a high-strength general structural component excellent in dimensional accuracy, strength, and durability at a low cost, which is extremely useful industrially.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between the average grain size of ferrite and the ductile-brittle transition temperature of a steel sheet according to an embodiment of the present invention.

Claims (4)

% By weight, C: 0.010% or less, Si: 0.05% or less, Mn: 0.1-1%, P: 0.1% or less, S: 0.02% or less, Sol. Al: 0.01-0.10%, Cu: 0.1% or less, N: 0.003% or less, Ti is defined by the following formula (1) Ti ^* : -0.01 to 0 contained in the range of .08%, the balance being Fe and unavoidable impurities, and wherein the average crystal grain size of the ferrite is 15μm or less, excellent in strength and low temperature brittleness after further nitrided press forming Cold rolled steel sheet for nitriding after pressing .
Ti ^* % = Ti% −48 / 14 × N% −48 / 12 × C% −48 / 32 × S% (1) The press excellent in strength and low-temperature brittleness after press forming and further nitriding according to claim 1, characterized in that the steel component further contains B: 0.0002 to 0.001% by weight%. Cold rolled steel sheet for post nitriding. It is excellent in strength and low-temperature brittleness after press forming and further nitriding according to claim 1 or 2, characterized by containing, as a steel component, by weight and further Nb: 0.005 to 0.10%. Cold rolled steel sheet for nitriding after pressing . After the steel having the composition according to any one of claims 1 to 3 is hot-rolled and cold-rolled, a recrystallization temperature defined by a softening rate of 50% to (recrystallization temperature + 50 ° C) and performing recrystallization annealing in the range, the production method of the strength and low-temperature brittleness to the excellent press after nitriding cold rolled steel sheet after further nitrided press molding.

Images