EP2060646A1

EP2060646A1 - Stainless steel sheet for structural members excellent in impact -absorbing characteristics

Info

Publication number: EP2060646A1
Application number: EP07831178A
Authority: EP
Inventors: Junichi Hamada; Haruhiko Kajimura; Fumio Fudanoki; Toshio Tanoue; Ken Kimura
Original assignee: Nippon Steel and Sumikin Stainless Steel Corp
Current assignee: Nippon Steel Stainless Steel Corp
Priority date: 2006-12-27
Filing date: 2007-10-30
Publication date: 2009-05-20
Anticipated expiration: 2027-10-30
Also published as: CN101410543A; US20100233015A1; JP5165236B2; WO2008078457A1; CN101410543B; EP2060646B1; EP2060646A4; KR20080106200A; JP2008163358A

Abstract

This invention provides a steel sheet for structural components excellent in impact absorption property comprising, in mass%, C: 0.005 to 0.05%, N: 0.01 to 0.30%, Si: 0.1 to 2%, Mn: 0.1 to 15%, Ni: 0.5 to 8%, Cu: 0.1 to 5%, Cr: 11 to 20%, Al: 0.01 to 0.5%, and a balance of Fe and unavoidable impurities, wherein Md₃₀ value given by equation (A) is 0 to 100 °C, and total impact energy absorption in dynamic tensile testing is 500 MJ/m³ or greater:

{Md}_{30} = 551 - 462 (C + N) - 9.2 Si - 8.1 Mn - 13.7 Cr - 29 (Ni + Cu)

Description

FIELD OF THE INVENTION

This invention relates to a stainless steel sheet used chiefly in structural components requiring strength and impact absorption capability, and particularly to a stainless steel sheet for automobile and bus impact absorption components such as front side members, pillars and bumpers, and for structural components such as vehicle suspension members and rims, railcar bodies and the like.

DESCRIPTION OF THE RELATED ART

Environmental concerns have in recent years made improvement of the fuel economy of cars, motorcycles, buses, railcars and other means of transport a critical issue. One aggressively-pursued approach to boosting fuel economy has been car body weight reduction. Car body weight reduction relies heavily on lowering the weight of the materials used to fabricate the body components, specifically on reducing the thickness of sheet steels. However, sheet metal thickness reduction has the undesirable effect of degrading rigidity and collision (crash) safety performance. As strength enhancement of the steels used for component' fabrication is an effective way to increase collision safety, ordinary steels and high-strength steels are utilized in automobile impact absorption components. Ordinary steels are, however, poor in corrosion resistance and multi-coat coating is essential for their use. They cannot be used for unpainted or lightly painted components, and multi-coat painting increases cost. Although ordinary steels can be imparted with high strength by various methods such as solution hardening, precipitation hardening, dual phasing, and deformation-induced transformation, all of the methods are disadvantageous in the point that the strengthening is accompanied by a marked decline in ductility. As ductility declines, fabrication into the structural component becomes increasingly difficult, so that the degree of structural freedom is greatly degraded.
Cr-containing stainless steels are far superior to ordinary steels in corrosion resistance and are therefore viewed as having the potential to reduce weight by lowering the corrosion margin (extra thickness to compensate for expected corrosion) and to eliminate the need for painting. In addition, austenitic stainless steels are excellent in strength-ductility balance and are considered capable of achieving high strength in combination with high ductility through chemical composition adjustment. Moreover, as regards collision safety improvement, utilizing a steel having high impact absorption capability in the vehicle frame makes it possible, for example, to absorb crash impact by component collapse deformation and thus to lessen the impact on passengers during a collision. In other words, considerable merits can be realized regarding fuel economy improvement through body weight reduction, painting simplification and safety enhancement.
Austenitic stainless steels such as SUS301L and SUS304 are used in the structural components of railcars, for instance, because they are excellent in corrosion resistance, ductility and formability. Japanese Patent Publication (A) No. 2002-20843 teaches an austenitic stainless steel with high strain rate and excellent impact absorption capability that is intended for use mainly in structural components and reinforcing materials for railcars and ordinary vehicles. This is a steel containing 6 to 8% Ni and having an austenite structure that achieves high strength during high-speed deformation owing to the formation of deformation-induced martensite phase. This prior art defines the deformation strengths under dynamic deformation and static deformation, maximum strength, work-hardening index and other properties of the steel. However, it is inadequate on the point of impact energy absorption, which is the most important aspect from the viewpoint of safety at the time of sustaining a high-velocity impact, and even though the difference between dynamic deformation strength and static deformation strength may be great, collision performance may be inferior if the static deformation strength is low. The dynamic/static ratio is defined as the ratio between the maximum dynamic and static strengths. But strength, e.g., yield strength, in the relatively low strain range is strongly affected by the impact absorption property at the time of collision, so the definition based on the maximum strength ratio may become a problem in some cases. Moreover, when deformation occurs during a collision, not only strength but also steel ductility may be a contributing factor, and this has necessitated a design taking heavy deformation reaching the point of destruction into consideration as an absorbed energy property. In other words, the teaching of Japanese Patent Publication (A) No. 2002-20843 is insufficient regarding safety performance at the time of collision, namely, impact absorption property. In addition, the inclusion of a relatively large amount of Ni makes cost high, so that application to automobiles, motorcycles, buses and other ordinary transportation vehicles has been difficult.
Further, martensitic stainless steel sheets imparted with high strength by quenching (e.g., SUS420) have very low ductility and are extremely poor in weld toughness. Since automobiles, buses and railcars have many welded structures, their structural reliability is greatly impaired by poor weld toughness. On the other hand, ferritic stainless steel sheets (e.g., SUS430) are low in strength and not suitable for members requiring strength, and they are incapable of improving collision safety performance owing to their low impact energy absorption at the time of high-velocity deformation.

SUMMARY OF THE INVENTION

Thus no technology has been available for enabling a vehicle structural component made of stainless steel sheet to achieve good collision safety performance by improving its impact energy absorption during high-speed deformation, while simultaneously ensuring good formability of the stainless steel sheet. The present invention is directed to overcoming the foregoing issues by providing a stainless steel sheet that is both high in strength and excellent in impact absorption property during high-speed deformation.
The inventors carried out a study on metal structure in relation to deformation mechanism at the time of sustaining high-speed deformation. As a result, they discovered a technique that enables improvement of impact energy absorption during high-speed deformation of an austenitic stainless steel while simultaneously achieving excellent sheet workability. Specifically, for increasing deformation resistance during ultra-high speed deformation of a strain rate of 10³/sec, deformation-induced transformation is positively exploited to increase work hardenability, thereby increasing impact energy absorption through a dramatic improvement in strength and ductility when the component collides. Therefore, a vehicle body fabricated using the steel sheet absorbs the impact at the time of a collision and minimizes body collapse, thereby markedly increasing the safety of passengers.
The gist of the present invention is as set out in the following.
A steel sheet for structural components excellent in impact absorption property comprising, in mass%, C: 0.005 to 0.05%, N: 0.01 to 0.30%, Si: 0.1 to 2%, Mn: 0.1 to 15%, Ni: 0.5 to 8%, Cu: 0.1 to 5%, Cr: 11 to 20%, Al: 0.01 to 0.5%, and a balance of Fe and unavoidable impurities, wherein Md₃₀ value given by equation (A) is 0 to 100 °C, and total impact energy absorption in dynamic tensile testing is 500 MJ/m³ or greater: ${Md}_{30} = 551 - 462 (C + N) - 9.2 Si - 8.1 Mn - 13.7 Cr - 29 (Ni + Cu)$

(2) The steel sheet for structural components excellent in impact absorption property according to (1), wherein dynamic/static ratio of yield strength is 1.4 or greater.
(3) The steel sheet for structural components excellent in impact absorption property according to (1) or (2), wherein tensile strength is 600 MPa or greater and elongation at break is 40% or greater in static tensile testing.
(4) A steel sheet for structural components excellent in impact absorption property comprising, in mass%, C: 0.005 to 0.05%, N: 0.01 to 0.30%, Si: 0.1 to 2%, Mn: 0.1 to 15%, Ni: 0.5 to 8%, Cu: 0.1 to 5%, Cr: 11 to 20%, Al: 0.01 to 0.5%, and a balance of Fe and unavoidable impurities, wherein Md₃₀ value given by equation (A) is 0 to 100 °C, and impact energy absorption to 10% strain in dynamic tensile testing is 50 MJ/m³ or greater: ${Md}_{30} = 551 - 462 (C + N) - 9.2 Si - 8.1 Mn - 13.7 Cr - 29 (Ni + Cu)$
(5) The steel sheet for structural components excellent in impact absorption property according to (4), wherein dynamic/static ratio of yield strength is 1.4 or greater.
(6) The steel sheet for structural components excellent in impact absorption property according to (4) or (5), wherein tensile strength is 600 MPa or greater and elongation at break is 40% or greater in static tensile testing.
(7) The steel sheet for structural components excellent in impact absorption property according to (4) or (5), wherein tensile strength is 700 MPa or greater and elongation at break is 5% or greater in static tensile testing.

"Total impact energy absorption in dynamic tensile testing" is defined as the impact energy absorption up to break when a high-velocity tensile test is conducted at a strain rate of 10³/sec corresponding to that at the time of a vehicle collision, and "impact energy absorption to 10% strain" is defined as the impact energy absorption up to the 10% strain region in the high-velocity tensile test. The static tensile test is a tensile test conducted at the usual strain rate (strain rate of 10^-3 to ^-2/sec).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the relationship between Md₃₀ value and total impact energy absorption in high-speed tensile testing.
FIG. 2 is a diagram showing the relationship between Md₃₀ value and impact energy absorption to 10% strain in high-speed tensile testing.

DETAILED DESCRIPTION OF THE INVENTION

The reasons for the limitations of the invention are explained in the following.
The important point in the present invention is the impact absorption upon incurring a high-speed impact. The impact force at the time of a vehicle collision is applied to structural components of the vehicle. The impact absorption capability of the steel constituting the components is therefore important. Up to now, no attempt has been made to provide a stainless steel that takes into account the impact energy absorption at high strain rate and high speed, nor has vehicle design with this in mind been carried out. Most vehicle structural components have angular cross-sections as typified by hat-shaped formed components. Although the strain region that absorbs impact differs among different structural components, what is important at locations that collapse during collision is the impact energy absorption up to material destruction. Total impact energy absorption is therefore used as an index. Total impact energy absorption improves as both strength and ductility are higher during high-speed deformation. However, conventional high-strength steel sheet, while high in strength, is low in fracture ductility and is therefore limited in total energy absorption.
The present invention improves collision safety performance to the utmost from the material standpoint by utilizing high ductility and high work hardenability property during deformation to dramatically improve total energy absorption. Moreover, since some locations need to absorb impact up to the 10% strain region, i.e., a relatively low strain rate region, impact energy absorption to strain rate of 10% is adopted as an index. Although this depends on the component shape, it applies to automobile front side member regions and the like, as indicated in " Report on Research Group Results Regarding High-Speed Deformation of Automotive Materials" (compiled by The Iron and Steel Institute of Japan, p12).
The larger is the ratio between yield strength in static tensile testing and yield strength in dynamic tensile testing, the more preferable for an impact absorption structural member. Moreover, a steel with high ductility is preferable for fabrication into vehicle structural components. The elongation at break in static tensile testing was therefore used as a general material index.
The inventors carried out a study based on the foregoing indexes, by which they learned that that the optimum stainless steel in terms of excellent impact absorption property is an austenitic stainless steel utilizing work hardening by deformation-induced transformation. They further learned that desired impact energy absorption during high-speed deformation can be achieved by adjusting the various constituents to control austenite so that deformation-induced martensite transformation occurs suitably during high-speed deformation.
Austenite stability constituting an index of deformation-induced martensite transformation is calculated based on Md₃₀ value shown below (from the Stainless Steel Handbook compiled by the Japan Stainless Steel Association). The Md₃₀ value is the temperature at which 50% of martensite is formed at the time of imparting tensile strain to a true strain of 0.3. When impact energy absorption was assessed using this value, it was found that the excellent impact energy absorption prescribed by the present invention could be obtained. ${Md}_{30} = 551 - 462 (C + N) - 9.2 Si - 8.1 Mn - 13.7 Cr - 29 (Ni + Cu) - 18.5 Mo - 68 Nb .$
When Mo and Nb are not contained, the foregoing Md₃₀ becomes that of the following equation (A): ${Md}_{30} = 551 - 462 (C + N) - 9.2 Si - 8.1 Mn - 13.7 Cr - 29 (Ni + Cu)$
Explanation will be made first regarding the steel composition.
C must be added to a content of 0.005% or greater to achieve high strength. On the other hand, C content is defined as 0.05% or less, because addition of a large amount degrades formability and weldability. Taking refining cost and grain boundary corrosion property into account, the more preferable content range is 0.01 to 0.02%.
N, like C, is effective for strength enhancement and beneficial for improving impact energy absorption. For these purposes, it must be added to a content of 0.01% or greater. On the other hand, N content is defined as 0.30% or less, because excessive addition degrades formability and weldability. Taking refining cost, manufacturability and grain boundary corrosion property into account, the more preferable content range is 0.015 to 0.025%.
Si is a deoxidizing element that is also a solution hardening element effective for achieving high strength. For these purposes, it must be added to a content of 0.1% or greater. On the other hand, Si content is defined as 2% or less, because addition of a large amount degrades formability and markedly lowers the dynamic/static ratio. Taking manufacturability into account, the more preferable content range is 0.2 to 1%.
Mn is a deoxidizing element and a solution hardening element effective for achieving high strength. Mn also promotes work hardening of austenite phase during high-speed deformation. For these purposes, it must be added to a content of 0.1% or greater. On the other hand, Mn content is defined as 15% or less, because when added in a large amount, deformation-induced martensite is not formed and formation of MnS, which is a water-soluble inclusion, degrades corrosion resistance. Taking descaling property in the manufacturing process into account, the more preferable content range is 1 to 10%.
Ni is an element that improves corrosion resistance. For this, and for austenite phase formation, Ni must be present at a content of 0.5% or greater. On the other hand, Ni content is defined as 8% or less, because when added in a large amount, raw material cost is markedly higher and deformation-induced martensite is not formed. Taking manufacturability, stress corrosion cracking and the like into account, the more preferable content range is 1.5 to 7.5%.
Cu improves formability and contributes to dynamic/static ratio improvement. It is added to a content of 0.1% or greater. Cu also produces its effects when included from scrap or the like in the composition adjustment process. When added in excess of 5%, however, deformation-induced martensite formation no long occurs, so the content is defined as 5% or less. The more preferable range is 0.1 to 4%.
Cr is an important element that must be added to a content of 11% or greater from the viewpoint corrosion resistance. On the other hand, the upper limit of Cr addition is defined as 20%, because excessive addition necessitates addition of large amounts of other elements for structure regulation. The content range is preferably 14 to 18%.
Al is added as a deoxidizing element and also because it renders sulfides harmless and contributes to improvement of workability aspects such as hole expandability during component processing. These effects appear at an Al content of 0.01% or greater, so the lower limit of content is defined as 0.01%. The upper content limit is defined as 0.5%, because addition in excess of this level leads to surface flaw occurrence and manufacturability degradation. Taking cost and the like into account, the more preferable content range is 0.1 to 0.5%.
When the material is impacted, it manifests deformation-induced transformation that transforms austenite phase into martensite phase, thereby effectively giving rise to work hardening during deformation. The efficient formation of martensite phase during deformation causes high strengthening and also prevents necking, thereby contributing to ductility improvement. Since martensite transformation is affected by strain and temperature, martensite formation is inhibited by the heat of deformation generated during high-speed deformation. However, in the stainless steel sheet of the present invention, it was found that martensite formation at the initial stage of deformation is sometimes promoted more during dynamic deformation than during static deformation. This is attributable to the strain rate dependence of transformation dependent on composition and the effect thereof dramatically improves impact energy absorption during high-speed deformation.
Various stainless steel sheets (thickness; 1.5 mm) were subjected to dynamic tensile testing at a strain rate of 10³/sec. The effect of Md₃₀ value on total impact energy absorption and impact energy absorption to 10% strain at this time are shown in FIG.s 1 and 2, respectively.
It can be seen that within the range of the present invention both total impact energy absorption and impact energy absorption to 10% strain exhibit excellent values. When Md₃₀ value is too high, ductility is thought to be lowered because cracking occurs at the boundary between austenite phase and martensite phase owing to excessive formation of martensite during deformation. Heretofore, total impact energy absorption at the time of high-speed deformation of high-strength steel has been thought to be on the order of less than 400 MJ/m³ (see, for example, CAMP-ISIJ, Vol9 (1996), P1101, Fig. 4 and Symposium on Automobile Materials, Japan Stainless Steel Association, 1997, p71).
The present invention provides a steel having much higher impact absorption property than the conventional high-strength steel, wherein the total impact energy absorption is defined as 500 MJ/m³ or greater and, from FIG.s. 1 and 2, the range of Md₃₀ value is defined as 0 to 100 °C. In the Md₃₀ value range of the present invention, the impact energy absorption to 10% strain obtained is 50 MJ/m³ or greater. Studies conducted by the inventors showed that if impact energy absorption of 50 MJ/m³ can be obtained, that is adequate as the impact absorption property in the relatively low strain region. So the impact energy absorption to 10% strain is defined as 50 MJ/m³ or greater. No upper limit value is defined for the impact energy absorption because the effect of the present invention can be realized without defining one.
The dynamic/static ratio is an index representing the deformation rate dependence of work hardening. It is the ratio of yield strength in dynamic tensile testing to yield strength in static tensile testing and is here defined specifically as (yield strength in dynamic tensile test when conducting dynamic tensile testing at strain rate of 10³/sec) / (yield strength when conducting static tensile testing at strain rate of 10^-2/sec). Since the dynamic/static ratio indicates the degree of hardening at the time of deformation at high speed as in an automobile collision, the suitability of a steel for use in an impact absorption structural component increases in proportion as the value of the dynamic/static ratio increases. For example, " Report on Research Group Results Regarding High-Speed Deformation of Automotive Materials" (compiled by The Iron and Steel Institute of Japan, 2001, p12, Fig.6) gives dynamic/static ratios for conventional steels, with the dynamic/static ratio of a steel having a tensile strength of 600 MPa or greater shown as 1.3 or less. The present invention defines the dynamic/static ratio as 1.4 or greater and provides a steel of high strength and high dynamic/static ratio unattainable by conventional steels. No upper limit value is defined for the dynamic/static ratio because the effect of the present invention can be realized without defining one.
The stainless steel of the present invention is intended for fabrication into structural components. It is therefore important for it to have good formability. As pointed out earlier, most vehicle structural components have angular cross-sections as typified by hat-shaped formed components. As the fabrication involves bending and drawing, the steel requires ductility. A study was carried out regarding methods of fabricating impact absorption components. It was found with regard to steel for which tensile strength was 600 MPa or greater in static tensile testing, adequate forming was possible if elongation at break was 40% or greater. Elongation at break in static tensile testing was therefore defined as 40% or greater.
Some components require high strength of 700 MPa or greater. Such high-strength steels are adjusted in strength by cold rolling and annealing followed by temper rolling. Although no upper limit of strength is necessary from the material aspect, the upper limit is defined as 1600 MPa in view of manufacturing and practical concerns. When temper rolling is conducted, the reduction can be set in accordance with the required strength level. However, taking manufacturability into consideration, it is preferably around 1 to 70%. The steel sheet manufactured in this manner is reduced in elongation at break in static tensile testing. However, the elongation at break in static tensile testing of a steel sheet of the foregoing tensile strength level is required to be 5% or greater. It is therefore defined as 5% or greater and is preferably 10% or greater.
The method of manufacturing the steel sheet of the present invention is not particularly defined and the product thickness can be decided based on requirements. The hot rolling conditions, hot rolled sheet thickness, hot rolled sheet and cold rolled sheet annealing temperature and atmosphere, and other matters can be suitably selected. No special equipment is required in connection with the pass schedule, cold rolling reduction and roll diameter in cold rolling, and efficient use of existing equipment suffices. Use/non-use of lubricant during temper rolling, the number of temper rolling passes and the like are also not particularly specified. If desired, shape correction utilizing a tension leveler can be applied after cold rolling and annealing or after temper rolling. Although the product structure is fundamentally austenite, formation of a second phase, such as of ferrite or martensite, is also acceptable.

EXAMPLES

The present invention will be concretely explained in the following with reference to working examples.
Steels having the chemical compositions shown in Table 1 were produced and cast into slabs. Each slab was hot rolled, annealed, pickled, cold rolled to a thickness of 1.5 mm, annealed, pickled, and temper rolled to obtain a product sheet. The so-obtained product sheet was subjected to the aforesaid static tensile test and dynamic tensile test.
Table 1 includes examples corresponding to claims 1 to 6. The steels having chemical compositions prescribed by the present invention were superior to the comparison steels in both total impact energy absorption to destruction and impact energy absorption in the low strain region to 10% strain, so that that they were excellent in impact absorption property. Such steels are suitable for use in impact absorption components at risk of experiencing relatively large deformation The steels were also suitable for formation into complex structural members, as evidenced by their high elongation at break and excellent ductility in static tensile testing.
Table 2 includes examples corresponding to claim 7. The invention examples, whose temper rolling reduction was adjusted to achieve tensile strength of 700 MPa or greater and elongation at break is 5% or greater, exhibited high impact energy absorption to 10% strain of 50 MJ/m³ or greater in dynamic tensile testing, as well as a dynamic/static ratio of 1.4 or greater, making them suitable for use in high-strength members required to absorb impact in the low strain region.
As is clear from the foregoing explanation, the present invention enables provision of a high-strength stainless steel sheet excellent in impact absorption capability even without addition of large amounts of alloying elements. The stainless steel sheet manifests outstanding industrial usefulness, including environmental protection through weight reduction and improved collision safety, especially when utilized in the structural components of transport means such as automobiles, buses and railcars.

Claims

A steel sheet for structural components excellent in impact absorption property comprising, in mass%:
C: 0.005 to 0.05%,

N: 0.01 to 0.30%,

Si: 0.1 to 2%,

Mn: 0.1 to 15%,

Ni: 0.5 to 8%,

Cu: 0.1 to 5%,

Cr: 11 to 20%,

Al: 0.01 to 0.5%, and

a balance of Fe and unavoidable impurities,

wherein Md₃₀ value given by equation (A) is 0 to 100 °C, and total impact energy absorption in dynamic tensile testing is 500 MJ/m³ or greater: ${Md}_{30} = 551 - 462 (C + N) - 9.2 Si - 8.1 Mn - 13.7 Cr - 29 (Ni + Cu)$
The steel sheet for structural components excellent in impact absorption property according to claim 1, wherein dynamic/static ratio of yield strength is 1.4 or greater.
The steel sheet for structural components excellent in impact absorption property according to claim 1 or 2, wherein tensile strength is 600 MPa or greater and elongation at break is 40% or greater in static tensile testing.
A steel sheet for structural components excellent in impact absorption property comprising, in mass%:
C: 0.005 to 0.05%,

N: 0.01 to 0.30%,

Si: 0.1 to 2%,

Mn: 0.1 to 15%,

Ni: 0.5 to 8%,

Cu: 0.1 to 5%,

Cr: 11 to 20%,

Al: 0.01 to 0.5%, and

a balance of Fe and unavoidable impurities,

wherein Md₃₀ value given by equation (A) is 0 to 100 °C, and impact energy absorption to 10% strain in dynamic tensile testing is 50 MJ/m³ or greater: ${Md}_{30} = 551 - 462 (C + N) - 9.2 Si - 8.1 Mn - 13.7 Cr - 29 (Ni + Cu)$
The steel sheet for structural components excellent in impact absorption property according to claim 4, wherein dynamic/static ratio of yield strength is 1.4 or greater.
The steel sheet for structural components excellent in impact absorption property according to claim 4 or 5, wherein tensile strength is 600 MPa or greater and elongation at break is 40% or greater in static tensile testing.
The steel sheet for structural components excellent in impact absorption property according to claim 4 or 5, wherein tensile strength is 700 MPa or greater and elongation at break is 5% or greater in static tensile testing.