U.S. patent application number 10/558579 was filed with the patent office on 2007-01-11 for high strength thin steel sheet excellent in resistance to delayed fracture after forming and method for preparation thereof , and automobile parts requiring strength manufactured from high strength thin steel sheet.
Invention is credited to Nobuhiro Fujita, Toshiki Nonaka, Hirokazu Taniguchi.
Application Number | 20070006948 10/558579 |
Document ID | / |
Family ID | 33485770 |
Filed Date | 2007-01-11 |
United States Patent
Application |
20070006948 |
Kind Code |
A1 |
Nonaka; Toshiki ; et
al. |
January 11, 2007 |
High strength thin steel sheet excellent in resistance to delayed
fracture after forming and method for preparation thereof , and
automobile parts requiring strength manufactured from high strength
thin steel sheet
Abstract
Steel sheets containing residual austenite of not more than 7
vol.%, crystallized and/or precipitated compounds with particle
diameters of 0.01 to 5.0 .mu.m of 100 to 100000 particle/mm.sup.2
and C of 0.05 to 0.3 mass %, Si of not more than 3.0 mass %, Mn of
0.01 to 3.0 mass %, P of not more than 0.02 mass %, S of not more
than 0.02 mass %, Al of 0.01 to 3.0 mass %, N of not more than 0.01
mass % and Mg of 0.0002 to 0.01 mass %, with the remainder
comprising iron and unavoidable impurities.
Inventors: |
Nonaka; Toshiki; (Aichi,
JP) ; Fujita; Nobuhiro; (Chiba, JP) ;
Taniguchi; Hirokazu; (Aichi, JP) |
Correspondence
Address: |
KENYON & KENYON LLP
ONE BROADWAY
NEW YORK
NY
10004
US
|
Family ID: |
33485770 |
Appl. No.: |
10/558579 |
Filed: |
May 27, 2003 |
PCT Filed: |
May 27, 2003 |
PCT NO: |
PCT/JP03/06617 |
371 Date: |
November 28, 2005 |
Current U.S.
Class: |
148/603 ;
148/320 |
Current CPC
Class: |
C22C 38/02 20130101;
C22C 38/16 20130101; C22C 38/12 20130101; C21D 2211/00 20130101;
C22C 38/04 20130101; C22C 38/004 20130101; C22C 38/14 20130101;
C21D 2211/004 20130101; C22C 1/06 20130101; C22C 38/06 20130101;
C21D 9/46 20130101 |
Class at
Publication: |
148/603 ;
148/320 |
International
Class: |
C22C 38/00 20060101
C22C038/00 |
Claims
1. High-strength steel sheet having excellent post-forming
delayed-fracture resistance, characterized by: containing, in mass
%, C : 0.05 to 0.3%, Si: not more than 3.0%, Mn: 0.01 to 3.0%, P :
not more than 0.02%, S : not more than 0.02%, Al: 0.01 to 3.0%, N :
not more than 0.01% and Mg: 0.0002 to 0.01%, with the remainder
comprising iron and unavoidable impurities, having the residual
austenite in the structure of steel being not more than 7 vol. %,
including one or more of the oxides, sulfides, composite
crystallized products and composite precipitates of Mg having means
particle diameter d in the range of 0.01 to 5.0 .mu.m, density
.rho. in the range of 100 to l00000/mm.sup.2, and distribution
satisfying the ratio between the standard deviation a from mean
particle diameter and mean particle diameter d,
.sigma./d.ltoreq.1.0, and having the volume percentage V.gamma.(%)
of residual austenite and tensile strength TSI (MPa) satisfying
equation (A)
1000(V.gamma.-0.1).sup.-5.5+.alpha.(Mg-40).sup.2-50(d-0.2).sup.2+1.1
lnp+700(TS-680).sup.0.9.gtoreq.10 Equation (A): where
.alpha.=-0.005(Mg.ltoreq.40), .alpha.=-0.002(Mg>40) V.gamma.:
volume percentage of residual austenite (%) Mg: the amount of Mg
(mass ppm) d : particle diameter (.mu.m) .rho.: density
(particle/mm.sup.2) TS: tensile strength (MPa) and, furthermore,
(i) 1000(V-0.1).sup.-5.5=10 when 1000(V-0.1).sup.-5.5.gtoreq.10,
(ii) 2.ltoreq.Mg.ltoreq.100 ppm (iii) (d-0.2).sup.2=0.2 when
0.01.ltoreq.d.ltoreq.5.0 .mu.m and (d-0.2).sup.2=0.2 (iv)
100.ltoreq..rho..ltoreq.100000 particle/mm.sup.2 and (v) 780
MPa.ltoreq.TS
2. High-strength steel sheet having excellent post-forming
delayed-fracture resistance described in claim 1, characterized by:
containing, furthermore, in mass %, one or more of V : 0.005 to 1
mass %, Ti: 0.002 to 1 mass %, Nb: 0.002 to 1 mass % and Zr: 0.002
to 1 mass %.
3. High-strength steel sheet having excellent post-forming
delayed-fracture resistance described in claim 1, characterized by:
containing, furthermore, in mass %, one or more of Cr: 0.005 to 5
mass %, Mo: 0.005 to 5 mass % and W : 0.005 to 5 mass %.
4. High-strength steel sheet having excellent post-forming
delayed-fracture resistance described in any of claims 1,
characterized by: containing, furthermore, in mass %, Cu: 0.005 to
2.0 mass %.
5. High-strength steel sheet having excellent post-forming
delayed-fracture resistance described in claim 1, characterized by:
containing, furthermore, in mass %, one or more of Ni: 0.005 to 2.0
mass % and Co: 0.005 to 2.0 mass %.
6. High-strength steel sheet having excellent post-forming
delayed-fracture resistance described in claim 1, characterized by:
containing, furthermore, in mass %, B: 0.0002 to 0.1 mass %.
7. High-strength steel sheet having excellent post-forming
delayed-fracture resistance described in claim 1, characterized by:
containing furthermore, in mass %, one or more of REM: 0.0005 to
0.01 mass %, Ca: 0.0005 to 0.01 mass % and Y : 0.0005 to 0.01 mass
%.
8. High-strength steel sheet having excellent post-forming
delayed-fracture resistance described in claim 1, characterized by
that: the high-strength steel sheet is hot-rolled or cold-rolled
steel sheet.
9. High-strength steel sheet having excellent post-forming
delayed-fracture resistance described in claim 1, characterized by
that the high-strength steel sheet is zinc-coated on the
surface.
10. High-strength steel sheet having excellent post-forming
delayed-fracture resistance described in claim 8, characterized by
that: the high-strength steel sheet is also film-laminated.
11. Method for manufacturing high-strength steel sheet having
excellent post-forming delayed-fracture resistance, characterized
by comprising the steps of: preparing slab of the composition
described in claim 1, hot rolling said slab with a finishing
temperature not lower than the Ar.sub.3 point, coiling the
hot-rolled strip at a temperature between 500.degree. C. and
800.degree. C., cold rolling with a draft of 30 to 80% after
applying pickling, applying recrystallization annealing by soaking
at not lower than 600.degree. C. and not higher than 950.degree.
C., and then applying temper rolling.
12. Method for manufacturing high-strength steel sheet having
excellent post-forming delayed-fracture resistance described in
claim 11, characterized by comprising, furthermore, the step of:
holding the strip in the temperature range of 200 to 700.degree. C.
for 1 minute to 10 hours after annealing.
13. Automotive structural member, characterized by being
manufactured of high-strength steel sheet having the excellent
post-forming delayed-fracture resistance described in claim 1.
Description
TECHNICAL FIELD
[0001] The present invention relates to high-strength steel sheets,
which inhibit delayed failures and delayed fractures that lead to
problems, particularly with high-strength steel sheets, a method of
manufacturing such steel sheets, and high-strength automotive part
manufactured of such steel sheets.
BACKGROUND ART
[0002] While high-strength steels are often used for bolts,
pre-stressed concrete (PC) wires, line pipes and other uses, it has
been known that penetration of hydrogen into steel leads to delayed
fractures when the strength exceeds 780 MPa.
[0003] Meanwhile, there has been little awareness of
delayed-fracture problems because (i) the penetrated hydrogen
escapes from within steel in a short time because the sheet
thickness is small and (ii) steel sheets whose strength is greater
than 780 MPa were seldom used because of low workability.
[0004] Recently, however, the need to reduce the weight and
increase the pre-collision passive safety of automobiles have been
rapidly increasing the use of ultra-high strength steel sheets
having a tensile strength of 780 MPa or more for bumpers, impact
beams and other reinforcing members, sheet rails, etc., with press
forming, pipe forming, bending, end-face machining or bore
expanding applied. Therefore, it is urgently necessary to develop
ultra-high strength steel sheets having high delayed-fracture
resistance.
[0005] Most of the conventional technologies to improve
delayed-fracture resistance have been developed for bolts, bars,
shapes, plates and other steels that are used as they are and where
applied forces are less than the yield strength or stress
thereof.
[0006] For example, the development of steels for bars, shapes and
bolts have been centered on tempered martensite. "New Developments
in Elucidation of Delayed Fracture" (Iron and Steel Institute of
Japan, Ad-hoc Group on Structures and Characteristics of Materials,
Study Group on Delayed Fracture of High-strength Steels, January
1997) reports that addition of such elements as Cr, Mo and V that
exhibit resistance to temper softening is effective in improving
resistance to delayed fracture.
[0007] This technology changes the morphology of delayed fracture
from intergranular to transgranular by precipitating alloy carbides
and using the precipitated alloy carbides as hydrogen trap
sites.
[0008] Containing 0.4% or more C and large quantities of alloying
elements, however, such steels do not have the workability and
weldability required of steel sheets. In addition, the need to
apply hours of precipitation heat treatment to separate alloy
carbides presents a problem in productivity.
[0009] Japanese Unexamined Patent Publication No. 11-293383
discloses that oxides consisting primarily of Ti and Mg are
effective in preventing the occurrence of hydrogen defects.
[0010] However, this technology is for steel plates. While delayed
fracture after welding with high heat input is considered, no
consideration is given to the effects of high-level forming and
generation of burrs resulting from end-face machining that are
often applied to steel sheets.
[0011] Furthermore, no consideration is given to workability that
is a basic property of steel sheets.
[0012] Regarding the delayed fracture of steel sheets, meanwhile,
the furtherance, due to residual austenite, of delayed fractures
resulting from working-induced transformation has been reported
(Ex. Yamazaki et al. CAMP-ISIJ vol. 5, p. 1839-1842 (1992)).
[0013] While considering the forming of steel sheets, this paper
reports the quantity control of residual austenite for the purpose
of precluding the deterioration of delayed-fracture resistance.
[0014] That is to say, this paper concerns the high-strength steel
sheets having certain specific structures, but does not concern any
fundamental measures for improving delayed-fracture resistance.
SUMMARY OF THE INVENTION
[0015] As described above, hardly any countermeasures have been
developed against delayed fractures caused by pre-use forming and
other working while considering, in particular, service
environments and productivity on existing equipment and securing
the intrinsic formability.
[0016] Against such backgrounds, the inventors discovered measures
to fundamentally improve resistance to delayed fractures while
giving adequate consideration to service environments of steel
sheets and manufacturing processes with existing equipment.
[0017] That is to say, the inventors discovered that
delayed-fracture resistance after forming of high-strength steel
sheets can be improved without deteriorating the formability
thereof by forming compounds or composite compounds of Mg and
controlling the shape of such compounds.
[0018] In addition, the inventors discovered effective
manufacturing methods for high-strength steel sheets using existing
manufacturing equipment (such as hot-rolling, continuous annealing,
hot-dip galvanizing and electrolytic equipment). The details are as
described below:
[0019] (1) High-strength steel sheet having excellent post-forming
delayed-fracture resistance, characterized by:
[0020] containing, in mass %, [0021] C : 0.05 to 0.3%, [0022] Si:
not more than 3.0%, [0023] Mn: 0.01 to 3.0%, [0024] P : not more
than 0.02%, [0025] S : not more than 0.02%, [0026] Al: 0.01 to
3.0%, [0027] N : not more than 0.01% and [0028] Mg: 0.0002 to
0.01%,
[0029] with the remainder comprising iron and unavoidable
impurities,
[0030] having the residual austenite in the structure of steel
being not more than 7 vol. %,
[0031] including one or more of the oxides, sulfides, composite
crystallized products and composite precipitates of Mg having means
particle diameter d in the range of 0.01 to 5.0 .mu.m, density
.rho. in the range of 100 to 100000/mm.sup.2, and distribution
satisfying the ratio between the standard deviation from mean
particle diameter and mean particle diameter d,
.sigma./d.ltoreq.1.0, and
[0032] having the volume percentage V.gamma.(%) of residual
austenite and tensile strength TS (MPa) satisfying equation (A)
1000(V.gamma.-0.1).sup.5.5+.alpha.(Mg-40).sup.2-50(d-0.2).sup.2+1.1
lnp+700(TS-680).sup.-0.9.gtoreq.10 Equation (A): where
.alpha.=-0.005(Mg.ltoreq.40), .alpha.=-0.002(Mg>40) V.gamma.:
volume percentage of residual austenite (%) Mg: the amount of Mg
(mass ppm) d : particle diameter (.mu.m) .rho.: density
(particle/mm.sup.2) TS: tensile strength (MPa)
[0033] and, furthermore,
(i) 1000 (V-0.1).sup.-5.5=10 when
1000(V-0.1).sup.-5.5.gtoreq.10,
(ii) 2.gtoreq.Mg.gtoreq.100 ppm
(iii) (d-0.2).sup.2=0.2 when 0.01.gtoreq.d.gtoreq.5.0 .mu.m and
(d-0.2).sup.2.ltoreq.0.2
(iv) 100.gtoreq..rho..gtoreq.100000 particle/mm.sup.2 and
(v) 780 MPa.gtoreq.TS
[0034] (2) High-strength steel sheet having excellent post-forming
delayed-fracture resistance described in (1), characterized by:
[0035] containing, furthermore, in mass %, one or more of
[0036] V : 0.005 to 1 mass %,
[0037] Ti: 0.002 to 1 mass %,
[0038] Nb: 0.002 to 1 mass % and
[0039] Zr: 0.002 to 1 mass %.
[0040] (3) High-strength steel sheet having excellent post-forming
delayed-fracture resistance described in (1) or (2), characterized
by: [0041] containing, furthermore, in mass %, one or more of
[0042] Cr: 0.005 to 5 mass %,
[0043] Mo: 0.005 to 5 mass % and
[0044] W : 0.005 to 5 mass %.
[0045] (4) High-strength steel sheet having excellent post-forming
delayed-fracture resistance described in any of (1) to (3),
characterized by: [0046] containing, furthermore, in mass %,
[0047] Cu: 0.005 to 2.0 mass %.
[0048] (5) High-strength steel sheet having excellent post-forming
delayed-fracture resistance described in any of (1) to (4),
characterized by: [0049] containing, furthermore, in mass %, one or
more of
[0050] Ni: 0.005 to 2.0 mass % and
[0051] Co: 0.005 to 2.0 mass %.
[0052] (6) High-strength steel sheet having excellent post-forming
delayed-fracture resistance described in any of (1) to (5),
characterized by: [0053] containing, furthermore, in mass %,
[0054] B: 0.0002 to 0.1 mass %.
[0055] (7) High-strength steel sheet having excellent post-forming
delayed-fracture resistance described in any of (1) to (6),
characterized by:
[0056] containing furthermore, in mass %, one or more of
[0057] REM: 0.0005 to 0.01 mass %,
[0058] Ca: 0.0005 to 0.01 mass % and
[0059] Y : 0.0005 to 0.01 mass %.
[0060] (8) High-strength steel sheet having excellent post-forming
delayed-fracture resistance described in any of (1) to (7),
characterized by that:
[0061] the high-strength steel sheet is hot-rolled or cold-rolled
steel sheet.
[0062] (9) High-strength steel sheet having excellent post-forming
delayed-fracture resistance described in any of (1) to (7),
characterized by that the high-strength steel sheet is zinc-coated
on the surface.
[0063] (10) High-strength steel sheet having excellent post-forming
delayed-fracture resistance described in (8) or (9), characterized
by that:
[0064] the high-strength steel sheet is also film-laminated.
[0065] (11) Method for manufacturing high-strength steel sheet
having excellent post-forming delayed-fracture resistance,
characterized by comprising the steps of: [0066] preparing slab of
the composition described in any of (1) to (7), [0067] hot-rolling
said slab with a finishing temperature not lower than the Ar.sub.3
point, [0068] coiling the hot-rolled strip at a temperature between
500.degree. C. and 800.degree. C., [0069] cold-rolling with a draft
of 30 to 80% after applying pickling, [0070] applying
recrystallization annealing by soaking at not lower than
600.degree. C. and not higher than 950.degree. C., and then [0071]
applying temper rolling.
[0072] (12) Method for manufacturing high-strength steel sheet
having excellent post-forming delayed-fracture resistance described
in (11), characterized by comprising, furthermore, the step of:
[0073] holding the strip in the temperature range of 200 to
700.degree. C. for 1 minute to 10 hours after annealing.
[0074] (13) Automotive structural member, characterized by being
manufactured of high-strength steel sheet having the excellent
post-forming delayed-fracture resistance described in any of (1) to
(7).
BRIEF DESCRIPTION OF THE DRAWINGS
[0075] FIG. 1 shows the relationship between equation (A) and
delayed-fracture time.
[0076] FIG. 2 shows the relationship between equation (A) and
residual austenite.
[0077] FIG. 3 shows the relationship between equation (A) and Mg
content.
[0078] FIG. 4 shows the relationship between equation (A) and
density.
THE MOST PREFERRED EMBODIMENT
[0079] It is considered that delayed fracture in tempered
martensite steel starts from the voids and other defects resulting
from the accumulation of hydrogen in prior austenite grain
boundaries or other regions.
[0080] Therefore, if the trap site of hydrogen is uniformly and
finely dispersed and hydrogen is trapped therein, the concentration
of diffusible hydrogen and, as a result, the sensibility to delayed
fracture, drop.
[0081] As disclosed in said Japanese Unexamined Patent Publication
No. 11-293383, it is known that resistance to hydrogen-induced
delayed fracture is improved by controlling the dispersion pattern
of oxides in steel plates to which Mg and Ti are added in
combination.
[0082] In steel sheets, however, if high residual stresses and
end-face burrs are generated as a result of forming, resistance to
delayed fracture inevitably deteriorates and accompanying
deterioration of delayed-fracture property cannot be
compensated.
[0083] Thus, few studies have been made of delayed-fracture
property with consideration given to the use pattern of steel
sheets and the problem of the deterioration of delayed-fracture
property in steel sheets cannot be solved by the shape control of
Mg and Ti oxides alone. Also, solid dispersion of trap site
involves the possibility of deteriorating ductility which is a
basic property of steel sheets.
[0084] Against the above background, the inventors studied the
influences of various crystallized products and precipitates, and
the strength and structure of steel sheets, in order to secure or
improve the delayed-fracture resistance thereof after forming in
the service environment thereof.
[0085] The studies led to the discovery of technology to improve or
secure the delayed-fracture resistance of steel sheets in the
service environment thereof, even under high residual stresses or
in the presence of end-face burrs. That is to say, it is possible
to make compatible ductility and delayed-fracture resistance after
forming by effectively dispersing the compounds or composite
crystallized products or precipitates of Mg, which are trap sites
for hydrogen, by controlling (i) the dispersion pattern of oxides
or sulfides containing Mg and the composite crystallized or
precipitated compounds therewith,
[0086] (ii) the quantity of residual austenite in the
microstructure of steel sheet, and
[0087] (iii) the strength of steel sheet.
[0088] Then, equation (A) was defined as the condition to satisfy
the above compatibility (Equation (A) will be discussed in detail
later.).
[0089] The presence in the crystal grains (except the phase
boundaries in the microstructure, such as prior-austenite grain
boundaries) of the oxides or sulfides containing Mg and the
composite crystallized or precipitated compounds therewith
described in (i) is more effective in the improvement of
delayed-fracture property.
[0090] The parameters described in (i), (ii) and (iii) can be
effectively controlled by limiting the manufacturing conditions so
that the shape of the crystallized or precipitated products, such
as oxides, nitrides and sulfides, of various elements, is
controlled so as to form the trap sites of hydrogen.
[0091] The present invention secures adequate post-forming
delayed-fracture resistance in high-strength steel sheets by
satisfying equation (A).
[0092] This is considered to be due to the difference between the
dislocation and residual stress field induced by forming and the
interaction of the particles forming the trap sites in steel sheets
and the dislocation and residual stress field induced by
hot-rolling and cooling after welding and the interaction of the
particles forming the trap sites in steel plates. This is also
considered to be due to the difference between the heat treatments
applied to steel sheets and plates.
[0093] Said parameters (i) and (ii) are limited as described
below.
[0094] Quantity of residual austenite: The upper limit of residual
austenite is limited to 7 vol. % because residual austenite
increases the susceptibility to delayed fracture when it changes to
martensite by working induced transformation.
[0095] Mean particle diameter: The mean particle diameter is
limited to between 0.01 .mu.m and 5.0 .mu.m. The particles to form
the hydrogen trap sites must have substantial sizes. Besides, the
presence of fine particles in large quantities is unfavorable for
securing the ductility of steel sheets and makes difficult the
manufacture thereof.
[0096] Therefore, the lower limit of the mean particle diameter was
set at 0.01 .mu.m, and the upper limit was set at 5.0 .mu.m because
coarse particles do not form trap sites and can sometimes become
the starting point of fracture.
[0097] Particle density: The particle density was limited to
between 100 and 100000/mm.sup.2. Lower particle densities mean few
trap sites, which, in turn, means that adequate post-forming
delayed-fracture property cannot be secured. Therefore, the lower
limit was set at 100/mm.sup.2.
[0098] The upper limit was set at 100000/mm.sup.2 because higher
particle densities deteriorate ductility and formability and
saturate the delayed-fracture resistance improving effect.
[0099] Particle distribution: The particle distribution was defined
so that the ratio between the standard deviation .sigma. from the
mean particle diameter and the mean particle diameter d satisfies
.sigma./d.ltoreq.1.0. If .sigma./d>1.0, particles are
widespread, which, in turn, reduces the delayed fracture improving
effect and thereby deteriorates ductility and increases the number
of fracture starting points. Therefore, the upper limit of
.sigma./d was set at 1.0.
[0100] Here, measurement of particles containing Mg compounds will
be discussed. Particles are measured by observing membranes or
sampled replicas through a scanning or transmission electron
microscope, with a magnification of 5000 to 100000, in at least 30
visual fields.
[0101] The particle diameter is evaluated by the circle equivalent
diameter obtained by image analysis. In determining density, each
composite precipitated or crystallized compound is counted as
one.
[0102] While composition analysis is done by using energy
dispersive x-ray (EDX) analysis and electron energy loss
spectroscopy (EELS), structural analysis is done by analyzing
diffraction patterns.
[0103] The composite compounds are compounds (such as carbides,
nitrides, oxides and sulfides) of Mg and other alloying additives
(such as Ti, Nb, V, Cr, Mo, REM, and Ca).
[0104] More details of the present invention are described
below.
[0105] The present invention relates to high-strength steel sheets
and primarily to steel sheets having a tensile strength of not
lower than 780 MPa and a thickness in the range of 0.5 to 4.0
mm.
[0106] Next, equation (A) will be explained. Equation (A) was
derived from FIG. 1 as described below, based on the understanding
that the volume percentage, mean particle diameter, density, Mg
content and tensile strength of residual austenite are the factors
involved in delayed-fracture resistance.
1000(V.gamma.-0.1).sup.-5.5+.alpha.(Mg-40).sup.2-50(d-0.2).sup.2+1.1
lnp+700(TS-680).sup.-0.9.gtoreq.10 Equation A: where
.alpha.=-0.005(Mg.gtoreq.40), .alpha.=-0.002(Mg>40) [0107]
V.gamma.: volume percentage of residual austenite (%) [0108] Mg:
the quantity of Mg (mass ppm) [0109] d : particle diameter (.mu.m)
[0110] .rho.: density (particle/mm.sup.2) [0111] TS: tensile
strength (MPa) and
[0112] (i) 1000(V.gamma.-0.1).sup.-5.5=10 when
1050(V.gamma.-0.1).sup.-5.5.gtoreq.10
[0113] (ii) 2.ltoreq.Mg.ltoreq.100 ppm
[0114] (iii) (d-0.2).sup.2=0.2 when 0.01.ltoreq.d.ltoreq.5.0 .mu.m
and (d-0.2).sup.2.gtoreq.0.2
[0115] (iv) 100.ltoreq..rho..ltoreq.100000/mm.sup.2
[0116] (v) 780 MPa.ltoreq.TS
[0117] When the left side of equation (A) is set as function f
(V.gamma., Mg, d, .rho., TS), delayed-fracture resistance
remarkably improves if the value of f (V.gamma., Mg, d, p, TS) is
greater than 10.
[0118] FIGS. 2 to 4 show the effects of the individual variables on
delayed-fracture resistance. In the figures, .largecircle. shows
good delayed-fracture resistance and x shows poor delayed-fracture
resistance.
[0119] FIG. 2 shows the relationship between f(V.gamma.) and volume
percentage of residual austenite V.gamma.. It is assumed that Mg
content is 300 ppm, mean particle diameter is 0.4 .mu.m, density is
1500 particle/mm.sup.2, and tensile strength is 1480 MPa.
[0120] While delayed-fracture resistance deteriorates if V.gamma.
is high, steels according to the present invention with high
f(V.gamma.) exhibit good delayed-fracture resistance when V.gamma.
is not higher than 7%.
[0121] Even if V.gamma. is not higher than 7%, delayed-fracture
resistance in compared steels marked with .times. deteriorates
because Mg content, particle diameter and density are outside the
range specified by the present invention and, therefore,
f(V.gamma.)<10.
[0122] FIG. 3 shows the relationship between f(Mg) and the quantity
of Mg added. It is assumed that the volume percentage of residual
austenite is 3.0%, mean particle diameter is 0.4 gm, density is
1500 particle/mm.sup.2, and tensile strength is 1480 MPa.
[0123] Where Mg content is 20 to 70 ppm, there is a region where
delayed-fracture resistance is particularly good. In steels marked
with x and the Mg content is not higher than 100 ppm,
delayed-fracture resistance deteriorates because the quantity of
residual austenite, particle diameter and density are outside the
ranges specified by the present invention and, therefore,
f(Mg)<10.
[0124] FIG. 4 shows the relationship between f(.rho.) and the
density of crystallized and precipitated compounds. It is assumed
that the volume percentage of residual austenite is 3.0%, Mg
content is 30 ppm and tensile strength is 1380 MPa. If density is
low, delayed-fracture resistance is poor.
[0125] In steels marked with .times., though density .rho. is
within the range specified by the present invention,
delayed-fracture resistance deteriorates because the quantity of
residual austenite, Mg content and particle diameter are outside
the range of the present invention and, therefore, f(.rho.) is
<10.
[0126] Thus, excellent fracture resistance is obtainable when said
parameters satisfy equation (A).
[0127] Next, the reasons why the present invention limits the
chemical composition of steel will be explained. In addition, %
means mass %.
[0128] C is an element that increases the strength of steel sheets.
C is particularly necessary for increasing strength as it forms
hard phases such as martensite and austenite. In order to obtain
780 MPa or greater strength, C of not less than 0.05% is necessary.
If, however, the C content is too high, the amount of cementite,
which becomes the starting point of brittle fracture, increases,
thereby causing hydrogen brittleness. Therefore, the upper limit is
set at 0.3%.
[0129] Si is a substitutional solid solution strengthening element
that greatly hardens steel. Si effectively increases the strength
of steel sheets and inhibits the precipitation of cementite. If the
Si content exceeds 3.0%, scale removal in the hot-rolling process
becomes costly and prone to economic disadvantage. Therefore, the
upper limit is set at 3.0%.
[0130] In order to improve coatability, Si content should
preferably be not more than 0.6% because too much Si addition
deteriorates coatability.
[0131] Mn is an element that is effective for increasing the
strength of steel sheets. As this effect is unobtainable if Mn
content is less than 0.01%, the lower limit is set at 0.01%. On the
other hand, too much Mn addition not only promotes joint
segregation with P and S but also deteriorates workability.
Therefore, the upper limit is set at 3.0%.
[0132] P is an element that promotes intergranular fracture by
intergranular segregation. While a lower P content is preferable,
too low an addition is unfavorable from the viewpoint of production
cost. As P deteriorates corrosion resistance, the upper limit is
set at 0.02%.
[0133] S is an element that promotes hydrogen absorption in
corrosive environments. While a lower content is preferable, it is
unpreferable from the viewpoint of production cost to reduce S
content too much. The upper limit is set at 0.02% because a lower
content is preferable, particularly for enhancing workability.
[0134] Al, at not less than 0.01%, is added for deoxidation.
However, too much addition increases alumina and other inclusions,
thereby deteriorating workability and weldability. Therefore, the
upper limit is set at 3.0%. Addition of not less than 0.2% Al is
preferable for inhibiting the formation of residual austenite.
[0135] N contributes to deterioration of workability and formation
of blowholes during welding. Therefore, a lower N content is
preferable. The upper limit is set at 0.01% because an addition in
excess thereof deteriorates workability.
[0136] Mg is a necessary element because compounds of Mg
effectively improve delayed-fracture resistance. Mg is also
necessary for producing composite crystallized or precipitated
compounds with other elements and controlling the shape thereof in
such a manner as to contribute to improvement of delayed-fracture
resistance. Thus, not less than 0.0002% Mg is added.
[0137] When added in excess of 0.01%, however, Mg forms coarse
oxides and sulfides, thereby losing effectiveness in shape control
and lowering formability fundamentally required of steel sheets.
Therefore, the upper limit is set at 0.01%.
[0138] Next, V, Ti, Nb and Zr are strong-carbide-forming elements
that improve strength and delayed-fracture resistance by forming
precipitates and inclusions.
[0139] Furthermore, V is effective for increasing steel strength
and refining particle size.
[0140] As, however, said effect is unobtainable when V content is
less than 0.005%, the lower limit is set at 0.005%. On the other
hand, when V content exceeds 1%, carbonitrides precipitate so much
that ductility drops significantly. Therefore, the upper limit is
set at 1%.
[0141] Ti is an element that effectively increases steel strength
and refines particle size. The lower limit is set at 0.002% because
the number of precipitates decreases therebelow. On the other hand,
the upper limit is set at 1% because coarse precipitated or
crystallized compounds are formed thereabove, which, in turn, lower
workability and the delayed-fracture resistance.
[0142] Nb also effectively increases steel strength and refines
particle size. The lower limit is set at 0.002% as said effect is
unobtainable therebelow. On the other hand, the upper limit is set
at 1% because carbonitride precipitation increases and, as a
result, workability and delayed-fracture resistance drop
thereabove.
[0143] Furthermore, Zn is an element that effectively increases
steel strength and refines particle size. However, the lower limit
is set at 0.002% because the number of precipitates decreases
therebelow. On the other hand, the upper limit is set at 1% because
coarse precipitated or crystallized compounds are formed
thereabove, which, in turn, lowers workability and delayed-fracture
resistance.
[0144] Next, Cr, Mo and W are elements that form carbides and
exhibit resistance to temper softening and are necessary for the
improvement of strength and delayed-fracture resistance.
[0145] Cr is effective for increasing steel strength. The lower
limit is set at 0.005% because said effect is unobtainable
therebelow. On the other hand, the upper limit is set at 5% because
workability drops thereabove.
[0146] Mo not only increases hardenability and stably forms
martensite in continuous annealing lines but also strengthens grain
boundaries and inhibits the occurrence of hydrogen brittleness. The
lower limit is set at 0.005% because said effects are unobtainable
therebelow. The upper limit is set at 5% because said effects
saturate thereabove.
[0147] W is an element that increases steel strength. The lower
limit is set at 0.005% because said effect is unobtainable
therebelow. On the other hand, the upper limit is set at 5% because
workability drops thereabove.
[0148] Next, not less than 0.005% Cu is added because Cu is
effective for strengthening and fine precipitation thereof
contributes to the improvement of delayed-fracture resistance. The
upper limit is set at 2.0% because excessive addition brings about
deterioration of workability.
[0149] Next, Ni and Co are strengthening elements that increase
hardenability.
[0150] Ni has effects to improve delayed-fracture property by
forming Ni sulfides and, thereby, inhibiting hydrogen penetration
and increases the strength of steel sheets by enhancing the
hardenability thereof.
[0151] The lower limit is set at 0.005% because said effects are
unobtainable therebelow, whereas the upper limit is set at 2%
because workability drops thereabove.
[0152] As Co increases strength effectively, not less than 0.005%
is added. The upper limit is set at 2.0% because excessive addition
brings about deterioration of workability.
[0153] B is an element effective for increasing the strength of
steel sheets. The lower limit is set at 0.0002% because said effect
is unobtainable therebelow, whereas the upper limit is set at 0.1%
because hot workability deteriorates thereabove.
[0154] Next, REM (rare-earth metals), Ca and Y are effective for
the shape control of inclusions and conducive to delayed-fracture
resistance. While the lower limit is set at 0.0005%, the upper
limit is set at 0.01% because excessive addition deteriorates hot
workability.
[0155] Next, manufacturing methods will be described.
[0156] First, slabs having specified compositions are hot-rolled.
Here, finish rolling is carried out at a temperature not lower than
the Ar.sub.3 point in order to prevent the excessive straining of
ferrite particles and the lowering of workability.
[0157] If the finish rolling temperature is too high, the size of
recrystallized particles and composite precipitated and
crystallized compounds of Mg after annealing becomes unnecessarily
coarse. Therefore, the finish rolling temperature should preferably
be not higher than 940.degree. C.
[0158] Coiling at higher temperatures promotes recrystallization
and particle growth and improves workability. At the same time,
however, coiling at higher temperatures promotes the growth of
scale formed during hot rolling and, thereby, lowers pickling
efficiency. Therefore, the coiling temperature is set at not higher
than 800.degree. C.
[0159] If the coiling temperature is too low, steel sheets harden
and receive higher loads during cold rolling. Therefore, the
coiling temperature is set at not lower than 500.degree. C.
[0160] If the draft of cold rolling after pickling is low, profile
shape straightening of steel sheets becomes difficult. Therefore,
the lower limit of the draft is set at 30%. If the draft exceeds
80%, sheet edge cracks and profile shape irregularities tend to
occur. Therefore, the upper limit is set at 80%.
[0161] If the continuous annealing temperature is too low,
recrystallization is undone and the steel structure hardens. If the
continuous annealing temperature is too high, on the other hand,
crystal grains become coarse and surface roughening sometimes
occurs in the subsequent pressing process. Therefore, the
continuous annealing temperature is set at not lower than
600.degree. C. and not higher than 950.degree. C. Annealing is done
by using continuous annealing equipment or box annealing
equipment.
[0162] If necessary, annealed steel sheets may be held in a
temperature range between 200.degree. C. and 700.degree. C. for 1
minute to 10 hours and, then, cooled. This heat treatment causes
precipitation of alloy carbides or nitrides (such as carbonitrides
containing V, Cr, Mo and W).
[0163] The precipitates thus formed serve as new hydrogen trap
sites and further improve delayed-fracture resistance. If the
temperature is low and the time is short, adequate precipitation
does not occur. If the temperature is high and the time is long,
precipitated compounds become coarse. As the precipitates fail to
serve as trap sites in both cases, the temperature and time are
limited to the ranges described above.
[0164] If the slab casting speed is fast, Mg compounds become
excessively fine. If the slab casting speed is slow, Mg compounds
become coarse and the number thereof decreases. In both cases,
therefore, Mg compounds sometimes fail to achieve adequate effect
in delayed-fracture control.
[0165] While the preferable slab casting speed is 0.05 to 20.0
m/minute, the speed between 1.0 m/minute and 3.0 m/minute is more
preferable for stable use of the delayed fracture improving effect
of Mg compounds.
[0166] The steel sheets according to the present invention may be
hot-rolled, cold-rolled or metal-coated. Metal coating may be
ordinary zinc-coating, Al coating, etc. Coating may be provided by
either hot-dip process or electrolytic process. Post-coating
alloying heat treatment or multi-layer coating may be applied,
too.
[0167] Film-laminated uncoated or coated steel sheets are also
within the scope of the present invention.
[0168] High-strength automotive parts (such as bumpers, door impact
beams and other reinforcing members) manufactured of high-strength
steel sheets according to the present invention (such as steel
sheets with strength of not lower than 780 MPa) also maintain
excellent properties (such as strength and rigidity) and exhibit
good shock absorption and delayed-fracture resistance.
EXAMPLES
[0169] Next, the present invention will be described based on
embodiments thereof.
[0170] Steels having compositions given in Table 1 were prepared
and continuously cast into slabs by conventional method. Reference
characters A to J designate steels whose compositions are according
to the present invention, whereas reference characters K to M
designate steels whose compositions are outside the scope of the
present invention.
[0171] The steels were heated in the heating furnace at
temperatures between 1160.degree. C. and 1250.degree. C.,
hot-rolled with finishing temperatures between 870.degree. C. and
900.degree. C., and coiled at temperatures between 650.degree. C.
and 750.degree. C.
[0172] The steels, except the one marked with H, were then made
into steel sheets by applying cold rolling after pickling,
recrystallization annealing and 0.4% temper-rolling.
[0173] The steels marked with I and J were alloyed galvanized steel
sheets with a coating weight of 50 g/m.sup.2 on each side. The
steel marked with J was further subjected to film laminating
treatment. Table 2 shows the manufacturing methods and properties
of the steel sheets.
[0174] Table 3 shows evaluations of the delayed-fracture resistance
of the steel sheets. Evaluations were made by bending 80 mm by 30
mm rectangular specimens, fitting a waterproof strain gage on the
surface thereof, dipping the specimens in a 0:5 mol/l sulfuric
acid, electrolyzing the solution, and causing hydrogen
penetration.
[0175] Then, occurrences of cracks were evaluated. While bending
was done to radiuses of 5 mm, 10 mm and 15 mm, stresses were
applied with forces of 60 MPa and 90 MPa.
[0176] As shown in Tables 2 and 3, the steels marked with 1, 2, 3,
5 and 7 to 12 exhibited high enough tensile strength and ductility
for use as automotive reinforcing members, took long time before
cracks occurred and showed excellent delayed-fracture
resistance.
[0177] By comparison, the steels marked with 4, 6 and 13 to 15,
which were tested for the purpose of comparison, were outside the
scope of the present invention in respect of either composition or
annealing temperature.
[0178] The steels marked with 4 and 6 deviated from the scope of
the present invention in respect of the value of equation (A) and
it did not take long before cracks occurred. The steels marked with
13 to 15 deviated from the scope of the present invention in
respect of chemical composition and did not take long time before
crack occurred because the number of crystallized or precipitated
compounds serving as hydrogen trap sites was few or too much
hydrogen was trapped. Obviously, the delayed-fracture resistances
of these steels were different from those obtained by the present
invention. TABLE-US-00001 TABLE 1 Reference Character
Classification C Si Mn P S Al N Mg Ti Nb V A Steel of the 0.15 0.50
2.50 0.016 0.006 0.035 0.006 0.0042 -- -- -- invention B Steel of
the 0.12 0.62 2.60 0.017 0.006 0.032 0.005 0.0038 0.050 -- --
invention C Steel of the 0.15 0.50 2.90 0.015 0.004 0.035 0.004
0.0039 0.050 -- -- invention D Steel of the 0.14 0.44 2.60 0.015
0.005 0.034 0.006 0.0052 0.100 -- 0.042 invention E Steel of the
0.15 0.50 2.60 0.007 0.002 0.030 0.003 0.0028 0.050 0.012 --
invention F Steel of the 0.16 1.03 2.30 0.011 0.001 0.054 0.004
0.0055 0.054 -- invention G Steel of the 0.16 1.52 2.33 0.012 0.003
0.325 0.005 0.0033 -- -- 0.131 invention H Steel of the 0.21 0.52
1.51 0.011 0.002 0.312 0.004 0.0032 0.011 -- -- invention I Steel
of the 0.16 0.02 2.21 0.008 0.003 0.721 0.001 0.0048 0.055 0.051
0.051 invention J Steel of the 0.15 0.01 2.55 0.009 0.003 1.211
0.003 0.0054 -- 0.088 0.041 invention K Steel for 0.15 0.50 2.50
0.016 0.006 0.035 0.006 -- 0.051 -- -- comparison L Steel for 0.12
0.48 2.33 0.015 0.005 0.035 0.005 0.0012 -- -- 1.311 comparison M
Steel for 0.18 0.52 2.10 0.011 0.003 0.035 0.002 -- -- 0.008 --
comparison Sheet Reference Thickness Character Cr Mo W Cu Ni Co B
REM Ca Y (mm) Steel Type A -- -- -- -- -- -- -- -- -- -- 1.2
Cold-rolled steel sheet B -- -- -- -- -- -- -- -- -- -- 1.4
Cold-rolled steel sheet C -- 0.300 -- -- -- -- -- -- -- -- 1.2
Cold-rolled steel sheet D 0.01 -- -- 0.01 -- -- -- -- -- -- 1.0
Cold-rolled steel sheet E -- -- 0.02 0.01 0.02 -- -- -- -- -- 0.8
Cold-rolled steel sheet F -- 0.052 -- 0.01 -- 0.01 0.0005 -- -- --
1.6 Cold-rolled steel sheet G -- 0.061 -- 0.11 -- 0.009 0.0005 --
-- 0.0016 1.4 Cold-rolled steel sheet H -- -- -- -- -- -- -- -- --
-- 3.4 Hot-rolled steel sheet I -- 0.285 -- -- -- -- -- -- -- --
1.4 Galvanized steel sheet J 0.02 0.286 0.013 -- 0.02 -- -- 0.0012
0.0022 -- 1.8 Galvanized steel sheet K -- -- -- -- -- -- -- -- --
-- 1.2 Cold-rolled steel sheet L -- -- -- 2.12 -- -- 0.012 -- -- --
1.4 Cold-rolled steel sheet M -- -- -- -- -- -- -- 0.0011 0.0015 --
1.6 Cold-rolled steel sheet
[0179] TABLE-US-00002 TABLE 2 Manufacturing Conditions Tensile
Casting Heating Finishing Coiling Annealing Properties Test
Reference Speed Temperature Temperature Temperature Temperature TS
EI Number Character Classification (m/min) (.degree. C.) (.degree.
C.) (.degree. C.) (.degree. C.) (MPa) (%) 1 A Steel of the 1.5 1180
880 650 850 1410 8 invention 2 B Steel of the 1.4 1190 870 700 820
1160 12 invention 3 C Steel of the 2.1 1240 880 650 820 1380 8
invention 4 Steel for 1.7 1190 880 550 550 1610 2 comparison 5 D
Steel of the 1.5 1230 900 600 840 1360 9 invention 6 Steel for 1.6
1210 870 550 970 1310 10 comparison 7 E Steel of the 1.3 1200 880
600 820 1410 8 invention 8 F Steel of the 1.5 1150 890 700 830 1480
8 invention 9 G Steel of the 1.8 1160 880 600 800 1420 7 invention
10 H Steel of the 1.7 1230 900 550 -- 1400 8 invention 11 I Steel
of the 1.6 1200 900 650 810 1390 8 invention 12 J Steel of the 1.5
1220 880 600 820 1530 8 invention 13 K Steel for 1.7 1180 890 600
840 1410 8 comparison 14 L Steel for 1.8 1190 890 600 850 1390 4
comparison 15 M Steel for 1.2 1220 890 600 830 1470 8
comparison
[0180] TABLE-US-00003 TABLE 3 Time to Crack Occurrence Bend Radius
Bend Radius Bend Radius Percentage 15 mm 10 mm 5 mm of Particle
Stress Stress Stress Stress Stress Stress Test Residual .gamma.
Diameter Density Equation 60 90 60 90 60 90 Number Classification
(%) (.mu.m) (Particle/mm.sup.2) (A) kgf/mm.sup.2 kgf/mm.sup.2
kgf/mm.sup.2 kgf/mm.sup.2 kgf/mm.sup.2 kgf/mm.sup.2 1 Steel of the
2.6 0.2 1000 15.92 .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. invention 2 Steel of the
3.7 0.18 1550 11.62 .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. invention 3 Steel of the
4.2 0.12 2500 10.63 .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. invention 4 Steel for 3.0
0.45 1000 8.82 .largecircle. X .largecircle. X X X comparison 5
Steel of the 2.7 0.2 12000 17.24 .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. .largecircle. invention 6
Steel for 3.3 0.12 300 9.45 X X X X X X comparison 7 Steel of the
3.1 0.2 1600 11.63 .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. invention 8 Steel of the
2.7 0.18 3400 15.61 .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. invention 9 Steel of the
2.4 0.16 5400 21.13 .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. invention 10 Steel of the
2.8 0.14 2600 14.46 .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. invention 11 Steel of the
3.1 0.13 2200 12.11 .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. invention 12 Steel of the
2.4 0.12 1200 19.02 .largecircle. .largecircle. .largecircle.
.largecircle. .largecircle. .largecircle. invention 13 Steel for
2.5 0.19 1000 9.55 .largecircle. X X X X X comparison 14 Steel for
4.2 0.2 1200 6.21 X X X X X X comparison 15 Steel for 3.2 0.2 1000
3.31 X X X X X X comparison
INDUSTRIAL APPLICABILITY
[0181] As described above, the high-strength steel sheets according
to the present invention effectively disperse Mg compounds or
composite crystallized or precipitated compounds, which function as
hydrogen trap sites, and thereby make ductility compatible with
delayed-fracture resistance after forming.
[0182] The high-strength automotive members prepared by forming the
high-strength steel sheets according to the present invention (such
as bumpers, door impact beams and other reinforcing members) also
maintained excellent properties and exhibited good shock absorption
and delayed-fracture resistance.
* * * * *