Method for manufacturing an ultra-high strength cold-rolled steel sheet with desirable delayed fracture resistance

FIELD OF THE INVENTION
The present invention relates to an ultra-high-strength cold-rolled steel sheet excellent in delayed fracture resistance and a method for manufacturing same.
BACKGROUND OF THE INVENTION
For the purpose of reducing the weight of an automobile or ensuring the safety of passengers, cold-rolled steel sheets having such a high tensile strength as to permit achievement of a higher strength and reduction of the weight of various structural members, are widely used as materials for protective components of an automobile such as a bumper reinforcement and a door guard bar. As a cold-rolled steel sheet having such a high tensile strength, ultra-high-strength cold-rolled steel sheets having a tensile strength of over 100 kgf/mm.sup.2 are proposed as follows:
(1) an ultra-high-strength cold-rolled steel sheet, disclosed in Japanese Patent Provisional Publication No. 61-3,843 published on Jan. 9, 1986, which consists essentially of:
carbon (C): from 0.02 to 0.30 wt. %,
silicon (Si): from 0.01 to 2.5 wt. %,
manganese (Mn): from 0.5 to 2.5 wt. %,
and
the balance being iron (Fe) and incidental impurities
(hereinafter referred to as the "prior art 1").
(2) an ultra-high-strength cold-rolled steel sheet, disclosed in Japanese Patent Provisional Publication No. 61-217,529 published on Sep. 27, 1986, which consists essentially of:
carbon (C): from 0.12 to 0.70 wt. %,
silicon (Si): from 0.4 to 1.0 wt. %,
manganese (Mn): from 0.2 to 2.5 wt. %,
soluble aluminum (Sol.Al): from 0.01 to 0.07 wt. %,
nitrogen (total N): up to 0.02 wt. %,
and
the balance being iron (Fe) and incidental impurities
(hereinafter referred to as the "prior art 2").
However, the prior arts 1 and 2 described above have the following problems:
It is true that the cold-rolled steel sheets of the prior arts 1 and 2 are excellent in workability and have a high tensile strength of over 100 kgf/mm.sup.2. An ultra-high-strength cold-rolled steel sheet having a tensile strength of over 100 kgf/mm.sup.2 is usually formed through the bending. In the cold-rolled steel sheets of the prior arts 1 and 2, however, when the tensile strength of the steel sheet becomes higher over 100 kgf/mm.sup.2, a fracture phenomenon (hereinafter referred to as the "delayed fracture") is suddenly caused by hydrogen penetrating into the interior of the steel sheet under the effect of a corrosion reaction taking place along with the lapse of time at a portion formed by the above-mentioned bending of the cold-rolled steel sheet. Therefore, even with a high tensile strength, a cold-rolled steel sheet susceptible to the delayed fracture, has a fatal defect as a material for protective components of an automobile, for example.
Under such circumstances, there is a strong demand for the development of an ultra-high-strength cold-rolled steel sheet excellent in the property inhibiting the occurrence of delayed fracture (hereinafter referred to as "delayed fracture resistance") and having a high tensile strength of over 100 kgf/mm.sup.2 and a method for manufacturing same, but such an ultra-high-strength cold-rolled steel sheet and a method for manufacturing same have not as yet been proposed.
An object of the present invention is therefore to provide an ultra-high-strength cold-rolled steel sheet excellent in delayed fracture resistance and having a high tensile strength of over 100 kgf/mm.sup.2 and a method for manufacturing same.
DISCLOSURE OF THE INVENTION
In accordance with one of the features of the present invention, there is provided an ultra-high-strength cold-rolled steel sheet excellent in delayed fracture resistance, which consists essentially of:
carbon (C): from 0.1 to 0.25 wt. %,
silicon (Si): up to 1 wt. %,
manganese (Mn): from 1 to 2.5 wt. %,
phosphorus (P): up to 0.020 wt. %,
sulfur (S): up to 0.005 wt. %,
soluble aluminum (Sol.Al): from 0.01 to 0.05 wt. %,
nitrogen (N): from 0.0010 to 0.0050 wt. %,
and
the balance being iron (Fe) and incidental impurities; and
said cold-rolled steel sheet satisfying the following formulae (1) and (2):
TS.gtoreq.320.times.(Ceq).sup.2 -155.times.Ceq+102 (1)
in said formula (1):
Ceq=C+(Si/24)+(Mn/6);
and
P.sub.DF .gtoreq.0 (2)
in said formula (2):
P.sub.DF =lnTS+exp[Rr/100]+2.95,
where, in said formulae (1) and (2):
P.sub.DF : delayed fracture resistance index,
TS: tensile strength (kgf/mm.sup.2), and
Rr: residual strength ratio (%) of a steel sheet as expressed by (bending/stretching tensile strength).div.(tensile strength) .times.100, when the steel sheet has been subjected to a 90.degree. V.-bending with a radius of 5 mm in a direction at right angles to the rolling direction.
The above-mentioned ultra-high-strength cold-rolled steel sheet may further additionally contain at least one element selected from the group consisting of:
niobium (Nb): from 0.005 to 0.05 wt. %,
titanium (Ti): from 0.005 to 0.05 wt. %,
and
vanadium (V): from 0.01 to 0.1 wt. %.
The above-mentioned ultra-high-strength cold-rolled steel sheets may further additionally contain at least one element selected from the group consisting of:
copper (Cu): From 0.1 to 1.0 wt. %,
nickel (Ni): From 0.1 to 1.0 wt. %,
boron (B): from 0.0005 to 0.0030 wt. %,
chromium (Cr): from 0.1 to 1.0 wt. %,
and
molybdenum (Mo): from 0.1 to 0.5 wt. %.
In accordance with another feature of the present invention, there is provided a method for manufacturing an ultra-high-strength cold-rolled steel sheet excellent in delayed fracture resistance, which comprises the steps of:
preparing a material having the chemical compositions as described above; then
subjecting said material to a hot rolling, a pickling and a cold rolling to prepare a cold-rolled steel sheet; and then
subjecting said cold-rolled steel sheet thus prepared to a continuous heat treatment which comprises the steps of: subjecting said cold-rolled steel sheet to a soaking treatment at a temperature within a range of from Ac.sub.3 to 900.degree. C. for a period of time within a range of from 30 seconds to 15 minutes, then quenching said cold-rolled steel sheet at a quenching rate of at least 400.degree. C./second from a temperature of at least a lower limit temperature (T.sub.Q) for starting quenching as expressed by the following formula to a temperature of up to 100.degree. C.: ##EQU1## and then, tempering said cold-rolled steel sheet at a temperature within a range of from 100.degree. to 300.degree. for a period of time within a range of from 1 to 15 minutes.

BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph illustrating the relationship between an evaluation of delayed fracture resistance and a delayed fracture resistance index (P.sub.DF) in an ultra-high-strength cold-rolled steel sheet;
FIG. 2 is a graph illustrating the effect of a residual strength ratio (Rr) and tensile strength (TS) on a delayed fracture resistance index (P.sub.DF) in an ultra-high-strength cold-rolled steel sheet;
FIG. 3 is a graph illustrating the effect of Ceq (=C+(Si/24)+(Mn/6)) on the lower limit value of tensile strength (TS) in an ultra-high-strength cold-rolled steel sheet;
FIG. 4 is a graph illustrating the effect of manufacturing conditions on a delayed fracture resistance index (P.sub.DF) in an ultra-high-strength cold-rolled steel sheet;
FIG. 5 is a schematic descriptive view illustrating the steps for measuring a residual strength ratio (R.sub.r) in an ultra-high-strength cold-rolled steel sheet; and
FIG. 6 is a schematic descriptive view illustrating the steps for preparing a test piece for evaluating delayed fracture resistance in an ultra-high-strength cold-rolled steel sheet.

DESCRIPTION OF PREFERRED EMBODIMENTS
From the above-mentioned point of view, extensive studies were carried out to develop an ultra-high-strength cold-rolled steel sheet excellent in delayed fracture resistance and having a high tensile strength of over 100 kgf/mm.sup.2 and a method for manufacturing same.
As a result, the following findings were obtained.
For an ultra-high-strength cold-rolled steel sheet having a high tensile strength of over 100 kgf/mm.sup.2 susceptible to the delayed fracture after the working, various factors having effects on delayed fracture resistance and the influence thereof were investigated. The investigation revealed that delayed fracture resistance of an ultra-high-strength cold-rolled steel sheet after the working was determined by tensile strength of the cold-rolled steel sheet and the degree of deterioration of the material of the cold-rolled steel sheet caused by the working.
More specifically:
(1) According as tensile strength of a cold-rolled steel sheet becomes larger, delayed fracture resistance of the cold-rolled steel sheet is deteriorated.
(2) According as the degree of deterioration of the material of a cold-rolled steel sheet caused by the working becomes larger, delayed fracture resistance of the cold-rolled steel sheet is deteriorated; and
(3) According as the uniformity of the structure of a cold-rolled steel sheet decreases, the degree of deterioration of the material of the cold-rolled steel sheet caused by the working becomes larger.
It is therefore possible to obtain an ultra-high-strength cold-rolled steel sheet excellent in delayed fracture resistance even after the working and having a high tensile strength of over 100 kgf/mm.sup.2, by increasing the uniformity of the structure of the steel sheet and specifying the degree of deterioration of the material of the steel sheet, which corresponds to tensile strength of the steel sheet.
The present invention was made on the basis of the above-mentioned findings. The ultra-high-strength cold-rolled steel sheet of the present invention excellent in delayed fracture resistance and having a high tensile strength of over 100 kgf/mm.sup.2 and the method for manufacturing same, are described below in detail.
The reasons of limiting the chemical composition of the cold-rolled steel sheet of the present invention within the above-mentioned ranges are described below.
(1) Carbon (C):
Carbon is an element having a function of increasing strength of a low-temperature transformation phase (for example, a martensitic structure or a bainitic structure). A carbon content of under 0.1 wt. % cannot however give a desired effect as described above. A carbon content of over 0.25 wt. % results on the other hand in a seriously decreased shock resistance to cause a deteriorated delayed fracture resistance of the steel sheet. The carbon content should therefore be limited within a range of from 0.1 to 0.25 wt. %.
(2) Silicon (Si):
Silicon is an element having a function of increasing ductility and temper-softening resistance of a steel sheet. A silicon content of over 1 wt. % causes however a considerable grain boundary oxidation in the surface portion of the steel sheet so that, upon the application of a stress to the steel sheet, the stress concentrates in the surface portion of the steel sheet, in which the grain boundary oxidation took place, thus resulting in the deterioration of delayed fracture resistance of the steel sheet. The silicon content should therefore be limited to up to 1 wt. %.
(3) Manganese (Mn):
Manganese is a low-cost element having a function of increasing hardenability of steel and giving a low-temperature transformation phase to steel. A manganese content of under 1 wt. % cannot however give a desired effect as described above. With a manganese content of over 2.5 wt. %, on the other hand, a banded structure caused by the segregation of manganese during the casting grows considerably in steel, deteriorating the uniformity of the structure of steel, and thus causes the deterioration of delayed fracture resistance of the steel sheet. The manganese content should therefore be limited within a range of from 1 to 2.5 wt. %.
(4) Phosphorus (P):
With a phosphorus content of over 0.020 wt. %, phosphorus segregates along grain boundaries of steel to cause the deterioration of delayed fracture resistance of the steel sheet. The phosphorus content should therefore be limited to up to 0.020 wt. %.
(5) Sulfur (S):
With a sulfur content of over 0.005 wt. %, a large amount of non-metallic inclusions (MnS) extending in the rolling direction are produced, and this causes the deterioration of delayed fracture resistance of the steel sheet. The sulfur content should therefore be limited to up to 0.005 wt. %.
(6) Soluble aluminum (Sol.Al):
Soluble aluminum is contained in steel as a residue of aluminum (Al) used as a deoxidizer. However, with a soluble aluminum content of under 0.01 wt. %, silicate inclusions remain in steel, thus causing the deterioration of delayed fracture resistance of the steel sheet. A soluble aluminum content of over 0.05 wt. % increases, on the other hand, surface flaws of the steel sheet to easily cause a delayed fracture of the steel sheet. The soluble aluminum content should therefore be limited within a range of from 0.01 to 0.05 wt. %.
(7) Nitrogen (N):
With a nitrogen content of under 0.0010 wt. %, there decrease nitrides in steel, leading to a coarser structure of steel, and hence to the deterioration of delayed fracture resistance of the steel sheet with a nitrogen content of over 0.0050 wt. %, on the other hand, nitrides in steel become coatset, thus resulting in the deterioration of delayed fracture resistance of the steel sheet. The nitrogen content should therefore be limited within a range of from 0.0010 to 0.0050 wt. %.
(8) The ultra-high-strength cold-rolled steel sheet of the present invention may further additionally contain, in addition to the above-mentioned chemical composition, at least one element selected from the group consisting of: from 0.005 to 0.05 wt. % niobium (Nb), from 0.005 to 0.05 wt. % titanium (Ti), and from 0.01 to 0.1 wt. % vanadium (V).
Niobium, titanium and vanadium have a function of forming carbon nitrides to achieve a finer structure of steel. For any of these elements, however, a content of under the respective lower limits cannot give a desired effect as described above with a content of over the respective upper limits, on the other hand, the above-mentioned desired effect is saturated, and at the same time, carbon nitrides becoming coarser cause the deterioration of delayed fracture resistance of the steel sheet. The respective contents of niobium, titanium and vanadium should therefore be limited within the above-mentioned ranges.
(9) The ultra-high-strength cold-rolled steel sheet of the present invention may further additionally contain, in addition to the above-mentioned chemical compositions, at least one element selected from the group consisting of: from 0.1 to 1.0 wt. % copper (Cu), from 0.1 to 1.0 wt. % nickel (Ni), from 0.0005 to 0.0030 wt. % boron (B), from 0.1 to 1.0 wt. % chromium (Cr) and from 0.1 to 0.5 wt. % molybdenum (Mo).
Copper, nickel, boron, chromium and molybdenum have, just as manganese, a function of increasing hardenability of steel. For any of these elements, with a content of under the respective lower limits, however, the desired effect as described above is not available. With a content of over the respective upper limits, on the other hand, the above-mentioned desired effect is saturated. The respective contents of copper, nickel, boron, chromium and molybdenum should therefore be limited within the above-mentioned ranges.
Now, the reason of specifying tensile strength (TS) of a cold-rolled steel sheet as expressed by the following formula (1) in terms of Ceq (=C+(Si/24)+(Mn/6)) is described below:
TS.gtoreq.320.times.(Ceq).sup.2 -155.times.Ceq+102 (1)
A high manganese content in steel promotes, as described above, formation of the banded structure in steel caused by the segregation of manganese during the casting, and thus causes the deterioration of delayed fracture resistance of the steel sheet. Formation of such a banded structure caused by the segregation of manganese is characterized in that: (1) formation of the banded structure is accelerated under the effect of coexistence of manganese with carbon (C) and silicon (Si), and (2) formation of the banded structure becomes more remarkable according as the structure of steel becomes composite (i.e., ferritic phase+low-temperature transformation phase). According as the structure of steel becomes more composite, furthermore, tensile strength of the cold-rolled steel sheet decreases.
It is therefore necessary to inhibit formation of the banded structure in steel caused by the segregation of manganese, which is accelerated under the effect of coexistence of manganese with carbon and silicon, and to prevent the structure of steel from becoming composite. More specifically, the structure of steel is prevented from becoming composite by means of Ceq (=C+(Si/24)+(Mn/6)) as determined by the contents of carbon, silicon and manganese.
Since tensile strength of the cold-rolled steel sheet decreases, as described above, along with the structure of steel becoming more composite, it is necessary "to control the lower limit value of tensile strength of the steel sheet by means of the above-mentioned formula (1) as expressed by Ceq, in order to ensure uniformity of the structure of steel.
Now, the delayed fracture resistance index (P.sub.DF) is described in the following paragraphs.
In order to obtain a cold-rolled steel sheet excellent in delayed fracture resistance even after the working, as described above, it is important to specify the degree of deterioration of the material of the steel sheet, which corresponds to tensile strength of the steel sheet. Experimental data derived from the research reveals that delayed fracture resistance of a cold-rolled steel sheet is improved when a delayed fracture resistance index (P.sub.DF) of the steel sheet as expressed by the following formula (2) takes a value of at least zero:
P.sub.DF =-lnTS+exp[Rr/100]+2.95 (2)
where,
TS: tensile strength (kgf/mm.sup.2),
Rr: residual strength ratio (%) of a steel sheet as expressed by (bending/stretching tensile strength)+(tensile strength) .times.100, when the steel sheet has been subjected to a 90.degree. V-bending with a radius of 5 mm in a direction at right angles to the rolling direction.
The first term of-the above-mentioned formula (2) (i.e., "-lnTS") represents the effect of tensile strength (TS) of the cold-rolled steel sheet on delayed fracture resistance of the steel sheet. A higher tensile strength (TS) of the cold-rolled steel sheet leads to a smaller P.sub.DF thereof.
The second term of the above-mentioned formula (2) (i.e., "exp[Rr/100]") represents the effect of the degree of deterioration of the material of the cold-rolled steel sheet caused by the working on delayed fracture resistance of the steel sheet. Deterioration of the material of the cold-rolled steel sheet caused by the working reduces the P.sub.DF of the steel sheet. The degree of deterioration of the material of the cold-rolled steel sheet caused by the working represents the degree of deterioration of the material of the steel sheet caused by the bending mainly used for forming an ultra-high-strength cold-rolled steel sheet. In the present invention, the degree of deterioration of the material of the steel sheet is represented by, as an index, a residual strength ratio (R.sub.r) of a steel sheet which has been subjected to a 90.degree. V-bending with a radius of 5 mm in a direction at right angles to the rolling direction. The direction at right angles to the rolling direction is selected because the material quality of an ultra-high-strength is poorer in the direction at right angles to the rolling direction than in a direction in parallel with the rolling direction, and evaluation is stricter in this direction. A 90.degree. V-bending is applied with a radius of 5 mm because this manner of working is a bending method most commonly used for an ultra-high-strength cold-rolled steel sheet.
Steps for measuring the residual strength ratio (R.sub.r) of a cold-rolled steel sheet is illustrated in FIG. 5. As shown in FIG. 5, the above-mentioned measuring steps comprise: subjecting a portion "a" of a test piece 1 cut out from a cold-rolled steel sheet to a 90.degree. V-bending with a radius of 5 mm in a direction at right angles to the rolling direction; then subjecting both sides "b" of the portion "a" of the test piece 1 to a bending with a radius of 6 mm to form a grip on each of the both end portions of the test piece 1; and then grasping the grips by means of a tensile testor to draw the test piece 1 in directions as indicated by "P" so as to determine a fracture stress at the moment of fracture of the test piece 1 at the portion "a". The thus determined fracture stress is referred to as the bending/stretching tensile strength, and the value calculated in accordance with a formula "(bending/stretching tensile strength)+(tensile strength before bending) .times.100", is adopted as the residual strength ratio (R.sub.r) (%) of the cold-rolled steel sheet.
The third term of the above-mentioned formula (2) (i.e., "+2.95") represents the correction for making the critical value of P.sub.DF zero.
Now, the reasons of limiting the manufacturing method of the present invention within the above-mentioned ranges are described below.
As described above in the findings, delayed fracture resistance of a cold-rolled steel sheet can be improved by increasing uniformity of the structure of the steel sheet and specifying the degree of deterioration of the material of the steel sheet, which corresponds to tensile strength of the steel sheet. In the manufacturing method of the present invention, therefore, it is important to make up for the deterioration of delayed fracture resistance of the cold-rolled steel sheet caused according as tensile strength of the steel sheet becomes larger, by uniforming the structure of the steel sheet to inhibit deterioration of the material of the steel sheet caused by the bending.
For this purpose, a material having a specific chemical composition is first hot-rolled and cold-rolled by the conventional methods to prepare a cold-rolled steel sheet, and then, the cold-rolled steel sheet thus prepared is subjected, in a continuous annealing, to a soaking treatment at a temperature within a range of from Ac.sub.3 to 900.degree. C. for a period of time within a range of from 30 seconds to 15 minutes when a soaking treatment is applied at a temperature of under Ac.sub.3, an as-rolled structure remains in the cold-rolled steel sheet to deteriorate uniformity of the structure of the steel sheet. Application of the soaking treatment to the cold-rolled steel sheet at a temperature of over 900.degree. C., on the other hand, gives rise to various operational problems, and, furthermore, the structure of steel becomes coarser to cause the deterioration of delayed fracture resistance of the steel sheet. Application of the soaking treatment to the cold-rolled steel sheet for a period of time of under 30 seconds makes it impossible to stably obtain an austenitic phase. When the soaking treatment is applied to the cold-rolled steel sheet for a period of time of over 15 minutes, on the other hand, the effect reaches saturation thereof. The conditions for the soaking treatment should therefore be limited within the ranges described above.
Then, the cold-rolled steel sheet, which has been subjected to the above-mentioned soaking treatment, is then slowly cooled to control the strength level thereof. The slow cooling rate should appropriately be within a range of from 1.degree. to 30.degree. C./second to minimize variations in the material quality in the width direction and the longitudinal direction of the steel sheet. After the completion of the above-mentioned slow cooling, the cold-rolled steel sheet is quenched. When the quenching starting temperature is low, the volume ratio of the precipitated ferritic phase increases, thus causing the deterioration of uniformity of the structure of the steel sheet. The quenching starting temperature should therefore be limited to at least a lower limit temperature (T.sub.Q) for starting quenching as expressed by the following formula: ##EQU2##
In the above-mentioned formula, the elements such as C and Si are represented in wt. % a as unit. In this formula, furthermore, the elements Si, Mo and Cr, which have a function of increasing the Ar.sub.3 transformation point, act to increase the T.sub.Q because they promote precipitation of the ferritic phase. The elements Mn, Cu, Ni and B, which have a function of decreasing the Ar.sub.3 transformation point, act to reduce the T.sub.Q because they inhibit precipitation of the ferritic phase. The element C, which has a function of reducing the Ar.sub.3 transformation point, just as Mn, Cu, Ni and B, has an effect on the T.sub.Q, unlike Mn, Cu, Ni and B. More specifically, even in a structure of steel having a ferritic phase of the same volume ratio, a higher C content leads to an increased difference in hardness between the low-temperature transformation phase and the ferritic phase, so that, upon the working, strain concentrates on the interface, resulting in a considerable deterioration of the material of the steel sheet. With a higher C content, therefore, it is necessary to inhibit precipitation of the ferritic phase.
Subsequently, the cold-rolled steel sheet is quenched at a quenching rate of at least 400.degree. C./second from a temperature of at least the above-mentioned lower limit temperature (T.sub.Q) for starting quenching to a temperature of up to 100.degree. C., to obtain a low-temperature transformation phase. When quenching is conducted at a cooling rate of under 400.degree. C./second, or to a temperature of over 100.degree. C., it is necessary to increase the contents of elements required for obtaining a desired high strength. This results in a higher manufacturing cost, and in addition, the mixed existence of the martensitic structure and the bainitic structure causes the deterioration of uniformity of the structure of the steel sheet. The quenching rate and the quenching stoppage temperature should therefore be limited within the above-mentioned ranges.
Then, the cold-rolled steel sheet is subjected to a tempering treatment, since an as-quenched martensitic phase of the steel sheet is brittle and thermally unstable. The tempering treatment is applied at a temperature within a range of from 100.degree. to 300.degree. C. for a period of time within a range of from 1 to 15 minutes. A tempering treatment at a temperature of under 100.degree. C. results in an insufficient tempering of the martensitic phase. A tempering treatment at a temperature of over 300.degree. C. causes, on the other hand, the precipitation of carbides on the crystal grain boudaries, and hence a serious deterioration of the material of the steel sheet caused by the working. A tempering treatment for a period of time of under one minute results in an insufficient tempering of the martensitic phase when a tempering treatment is applied for a period of time of over 15 minutes, the tempering effect is saturated.
Now, the ultra-high-strength cold-rolled steel sheet of the present invention excellent in delayed fracture resistance and the method for manufacturing same, are described further in detail by means of examples while comparing with examples for comparison.
EXAMPLES
Steels "A" to "Z" having chemical compositions within the scope of the present invention as shown in Table 1, and steels "a" to "j" having chemical compositions outside the scope of the present invention as shown also in Table 1, were tapped from a converter, and then, were continuously cast into respective slabs. The resultant slabs were then hot-rolled under conditions including a heating temperature of 1,200.degree. C., a finishing temperature of 820.degree. C. and a coiling temperature of 600.degree. C., to prepare hot-rolled steel sheets having a thickness of 3 mm. Then, the thus prepared hot-rolled steel sheets were pickled and cold-rolled to prepare cold-rolled steel sheets having a thickness of 1.4 mm. The thus prepared cold-rolled steel sheets were then subjected to a heat treatment in a combination-type continuous annealing line including a water-quenching apparatus and a roll-quenching apparatus under conditions as shown in Tables 2 and 4. The water quenching was applied at a cooling rate of about 1,000.degree. C./second, and the roll quenching was applied at a cooling rate of about 200.degree. C./second.
Thus, there were prepared samples of the cold-rolled steel sheets of the present invention, having chemical compositions within the scope of the present invention and subjected to heat treatments within the scope of the present invention (hereinafter referred to as the "samples of the invention") Nos. 1 to 3, 6 to 9, 11, 13, 15, 17 to 24, 26, 28, 29, 32 to 38, 40, 42, 43, 48, 50, 52 to 54, 56, 57, 59 to 64, 66, 68, 71, 72, 91, 92, 94 and 95, and, samples of the cold-rolled steel sheets having chemical compositions outside the scope of the present invention, and samples of the cold-rolled steel sheets, which, having chemical compositions within the scope of the present invention, were subjected to heat treatments outside the scope of the present invention (hereinafter referred to as the "samples for comparison") Nos. 4, 5, 10, 12, 14, 16, 25, 27, 30, 31, 39, 41, 44 to 47, 49, 51, 55, 58, 65, 67, 69, 70, 73 to 85, 93 and 96 to 98 were prepared.
For each of the above-mentioned samples of the invention and samples for comparison, tensile strength (TS), a residual strength ratio (R.sub.r) a delayed fracture resistance index (P.sub.DF) and delayed fracture resistance were investigated. The results are shown in Tables 3 and
TABLE 1__________________________________________________________________________Kindof Ac.sub.3Steel C Si Mn P S sol. Al N Nb Ti V Cu Ni B Cr Mo Ceq (.degree.C.)__________________________________________________________________________A 0.12 0.3 1.6 0.011 0.004 0.037 0.0023 0.40 828B 0.20 0.6 1.2 0.017 0.001 0.038 0.0039 0.1 0.43 836C 0.15 0.4 1.5 0.008 0.002 0.048 0.0033 0.015 0.42 829D 0.23 0.7 2.2 0.012 0.002 0.016 0.0028 0.020 0.63 793E 0.21 0.9 1.8 0.012 0.005 0.030 0.0016 0.55 824F 0.11 0.2 1.9 0.018 0.004 0.019 0.0048 0.44 815G 0.16 0.4 1.0 0.016 0.001 0.021 0.0031 0.006 0.5 0.3 0.34 840H 0.24 0.2 1.2 0.007 0.005 0.031 0.0036 0.9 0.45 783I 0.15 0.7 1.5 0.015 0.002 0.018 0.0011 0.43 835J 0.19 0.4 1.8 0.017 0.001 0.023 0.0048 0.048 0.51 806K 0.12 0.9 2.5 0.007 0.003 0.031 0.0021 0.031 0.02 0.57 822L 0.15 0.1 1.5 0.013 0.001 0.035 0.0036 0.020 0.005 0.1 0.40 813M 0.15 0.4 1.0 0.017 0.004 0.029 0.0031 0.9 0.33 829N 0.13 0.5 1.7 0.015 0.001 0.012 0.0021 0.015 0.008 0.43 823O 0.21 0.4 2.3 0.011 0.004 0.011 0.0018 0.09 0.61 778P 0.24 0.8 1.0 0.019 0.005 0.044 0.0029 0.5 0.44 863Q 0.10 0.2 2.0 0.010 0.001 0.041 0.0021 0.44 818R 0.23 0.9 1.2 0.015 0.002 0.030 0.0039 0.1 0.5 0.47 830S 0.10 0.2 1.1 0.019 0.004 0.027 0.0031 0.018 0.1 0.0005 0.29 844T 0.11 0.4 1.5 0.011 0.005 0.031 0.0029 0.048 0.38 836U 0.22 Tr. 1.1 0.007 0.002 0.018 0.0015 0.015 0.9 0.40 784V 0.15 Tr. 1.2 0.012 0.003 0.021 0.0028 0.35 812W 0.20 0.2 1.1 0.015 0.005 0.025 0.0031 0.39 816X 0.17 0.5 1.6 0.011 0.002 0.023 0.0024 0.030 0.0028 0.46 818Y 0.24 0.7 2.5 0.012 0.002 0.019 0.0030 0.031 0.69 783Z 0.22 0.9 2.4 0.010 0.003 0.023 0.0041 0.66 799a 0.20 0.4 2.5 0.012 0.001 0.031 *0.0008 0.63 783b 0.13 0.1 *2.7 0.011 0.004 0.025 0.0043 0.58 778c 0.13 *1.1 2.0 0.014 0.002 0.013 0.0037 0.51 841d 0.15 0.7 1.6 *0.022 0.004 0.047 0.0017 0.45 849e 0.21 0.3 1.1 0.007 *0.006 0.040 0.0027 0.41 818f *0.26 0.2 1.5 0.011 0.005 0.020 0.0031 0.52 786g 0.11 0.5 1.8 0.018 0.001 *0.052 0.0026 0.43 844h 0.18 0.1 2.2 0.012 0.002 0.030 0.0021 *0.060 0.55 783i 0.18 0.3 1.7 0.015 0.001 0.033 0.0012 *0.070 0.48 810j 0.12 0.9 2.1 0.014 0.004 0.011 0.0035 *0.11 0.51 831__________________________________________________________________________ Mark "*" shows outside the scope of the present invention. Ceq = C + Si/24 + Mn/6
TABLE 2__________________________________________________________________________ Lower limit Quench. Lower limit Kind Soaking temperature start Tempering Tempering of tensileSample of temperature for quench. temperature temperature time strengthNo. Steel Ceq (.degree.C.) start (.degree.C.) (.degree.C.) (.degree.C.) (sec.) (kgf/mm.sup.2)__________________________________________________________________________ 1 A 0.40 850 654 730 200 600 91 2 A 0.40 850 654 720 200 600 91 3 A 0.40 890 654 780 150 300 91 4 A 0.40 *802 654 660 240 180 91 5 B 0.43 850 737 *720 300 300 95 6 B 0.43 820 737 740 270 900 95 7 C 0.42 850 683 770 100 100 93 8 C 0.42 *800 683 750 220 800 93 9 C 0.42 850 683 710 220 700 9310 D 0.63 800 732 *700 120 520 13111 D 0.63 820 732 780 180 300 13112 D 0.63 820 732 750 *350 450 13113 D 0.63 850 732 740 260 120 13114 D 0.63 850 732 *680 260 120 13115 E 0.55 840 732 750 260 80 11416 E 0.55 840 732 *700 200 600 11417 E 0.55 840 732 740 200 510 11418 F 0.44 850 635 760 200 540 9619 G 0.34 850 716 770 110 700 8620 G 0.34 850 716 720 250 220 8621 H 0.45 820 753 770 100 600 9722 H 0.45 820 753 *750 290 600 9723 I 0.43 850 689 760 180 60 9524 I 0.43 850 689 700 240 900 9525 J 0.51 830 706 *700 *400 800 10626 J 0.51 830 706 750 180 800 10627 J 0.51 830 706 *680 200 800 10628 J 0.51 830 706 740 250 800 10629 J 0.51 830 706 745 250 500 10630 J 0.51 830 706 *610 250 500 10631 K 0.57 *800 639 720 200 500 11832 K 0.57 840 639 750 220 400 11833 K 0.57 840 639 720 130 400 11834 L 0.40 830 678 730 200 900 9135 L 0.40 850 678 710 260 500 9136 L 0.40 850 678 *660 200 800 9137 M 0.33 840 692 730 130 700 8638 M 0.33 840 692 710 130 700 8639 M 0.33 840 692 *680 130 700 8640 N 0.43 840 659 740 260 100 9541 O 0.61 840 707 750 *360 600 12742 O 0.61 840 707 750 270 900 12743 O 0.61 840 707 750 120 900 12744 O 0.61 790 707 *620 260 410 12745 P 0.44 880 784 *720 200 500 9646 P 0.44 880 784 *760 200 500 9647 P 0.44 880 784 800 *320 500 9648 Q 0.44 870 624 770 150 800 9649 R 0.47 840 762 *700 180 200 10050 R 0.47 840 762 770 260 300 10051 R 0.47 840 762 780 *310 400 10052 R 0.47 870 762 770 290 750 10053 S 0.29 850 648 740 200 100 8454 S 0.29 890 648 770 100 550 8455 S 0.29 *820 648 690 200 100 8456 T 0.38 840 651 720 250 500 8957 U 0.40 820 755 *710 260 700 9158 U 0.40 840 755 770 *400 800 9159 U 0.40 840 755 770 230 150 9160 V 0.35 820 684 770 100 500 8761 V 0.35 850 684 750 220 700 8762 W 0.39 850 731 760 *450 500 9063 W 0.39 850 731 760 260 700 9064 X 0.46 830 684 760 180 800 9865 X 0.46 *790 684 740 220 300 9866 X 0.46 850 684 710 200 300 9867 X 0.46 *800 684 *670 200 300 9868 Y 0.69 860 731 800 230 420 14769 Y 0.69 860 731 *728 230 420 14770 Y 0.69 820 731 *720 270 260 14771 Z 0.66 840 722 790 240 300 13972 Z 0.66 840 722 760 200 180 13973 Z 0.66 840 722 *700 200 180 13974 Z 0.66 870 722 *720 180 220 13975 a 0.63 830 693 760 120 500 13176 b 0.58 800 625 730 200 900 12077 c 0.51 850 666 750 270 100 10678 d 0.45 850 686 770 100 400 9779 e 0.41 820 741 750 230 800 9280 e 0.41 820 741 *700 200 600 9281 f 0.52 830 767 770 250 100 10882 g 0.43 860 644 770 180 500 9583 h 0.55 820 680 740 200 200 11484 i 0.48 840 699 760 110 700 10185 j 0.51 850 651 730 230 100 106__________________________________________________________________________ Ceq = C + Si/24 + Mn/6 Lower limit of tensile strength = 320 .times. (Ceq).sup.2 - 155 .times. Ceq + 102 Mark "*" shows outside the scope of the present invention.
TABLE 3__________________________________________________________________________ Delayed fracture Kind Tensile Residual resistanceSample of strength strength evaluationNo. Steel (kgf/mm.sup.2) ratio (%) P.sub.DF (points) Remarks__________________________________________________________________________ 1 A 113 95 0.808 5 Sample of the invention 2 A 102 72 0.379 4 Sample of the invention 3 A 129 73 0.165 4 Sample of the invention 4 A *82 33 -0.066 0 Sample for comparison 5 B 128 60 -0.080 0 Sample for comparison 6 B 140 81 0.256 4 Sample of the invention 7 C 143 95 0.573 5 Sample of the invention 8 C 122 63 0.024 3 Sample of the invention 9 C 103 96 0.927 5 Sample of the invention10 D 156 70 -0.086 0 Sample for comparison11 D 171 93 0.343 5 Sample of the invention12 D *125 40 -0.386 0 Sample for comparison13 D 142 85 0.334 5 Sample of the invention14 D *115 42 -0.273 0 Sample for comparison15 E 169 82 0.091 3 Sample of the invention16 E 140 68 -0.018 0 Sample for comparison17 E 151 79 0.136 4 Sample of the invention18 F 112 100 0.950 5 Sample of the invention19 G 150 95 0.525 5 Sample of the invention20 G 92 90 0.888 5 Sample of the invention21 H 178 85 0.108 3 Sample of the invention22 H 148 74 0.049 3 Sample of the invention23 I 145 96 0.585 5 Sample of the invention24 I 109 61 0.099 4 Sample of the invention25 J 115 53 -0.096 0 Sampel for comparison26 J 163 82 0.127 5 Sample of the invention27 J 123 52 -0.180 0 Sample for comparison28 J 130 82 0.353 5 Sample of the invention29 J 142 95 0.580 5 Sample of the invention30 J *87 35 -0.097 0 Sample for comparison31 K *107 30 -0.373 0 Sample for comparison32 K 121 96 0.766 5 Sample of the invention33 K 140 100 0.727 5 Sample of the invention34 L 135 91 0.529 5 Sample of the invention35 L 125 93 0.656 5 Sample of the invention36 L 118 67 0.134 5 Sample of the invention37 M 129 75 0.207 4 Sample of the invention38 M 116 71 0.230 3 Sample of the invention39 M 103 49 -0.052 0 Sample for comparison40 N 126 82 0.384 5 Samaple of the invention41 O 133 61 -0.100 0 Sample for comparison42 O 150 78 0.121 4 Sample of the invention43 O 166 90 0.298 5 Sample of the invention44 O *98 36 -0.202 0 Sample for comparison45 P 162 53 -0.439 0 Sampel for comparison46 P 178 80 -0.006 0 Sample for comparison47 P 173 67 -0.249 0 Sample for comparison48 Q 120 91 0.647 5 Sample of the invention49 R 145 42 -0.505 0 Sample for comparison50 R 170 92 0.323 4 Sample of the invention51 R 150 56 -0.310 0 Sample for comparison52 R 105 75 0.413 4 Sample of the invention53 S 105 96 0.908 5 Sample of the invention54 S 110 75 0.367 5 Sample of the invention55 S *83 29 -0.132 0 Sample for comparison56 T 105 83 0.589 5 Sample of the invention57 U 135 69 0.038 3 Sample of the invention58 U 136 50 -0.314 0 Sample for comparison59 U 158 96 0.499 5 Sample of the invention60 V 140 87 0.395 4 Sample of the invention61 V 120 93 0.697 5 Sample of the invention62 W 120 62 0.021 3 Sample of the invention63 W 142 98 0.659 5 Sample of the invention64 X 125 93 0.656 5 Sample of the invention65 X 114 42 -0.264 0 Sample for comparison66 X 140 96 0.620 5 Sample of the invention67 X *95 46 -0.020 0 Sample for comparison68 Y 172 90 0.262 5 Sample of the invention69 Y *143 62 -0.154 0 Sample for comparison70 Y *129 60 -0.088 0 Sample for comparison71 Z 163 85 0.196 4 Sample of the invention72 Z 145 76 0.112 4 Sample of the invention73 Z *104 40 -0.203 0 Sample for comparison74 Z *135 62 -0.096 0 Sample for comparison75 a 170 60 -0.364 0 Sample for comparison76 b 136 97 0.675 0 Sample for comparison77 c 130 88 0.493 1 Sample for comparison78 d 143 100 0.705 0 Sample for comparison79 e 160 100 0.593 0 Sample for comparison80 e 130 52 -0.236 0 Sample for comparison81 f 180 100 0.475 0 Sample for comparison82 g 118 100 0.898 1 Sample for comparison83 h 151 95 0.518 0 Sample for comparison84 i 155 100 0.625 0 Sample for comparison85 j 140 90 0.468 0 Sample for comparison__________________________________________________________________________ Mark "*" shows outside the scope of the present invention.
TABLE 4__________________________________________________________________________ Lower Low limit tem- tem- Lower perature Quench. perature limit Soaking for start holding of tensile Tensile Residual Delayed frac- Kind tem- quench. tem- tem- strength strength strength ture resist-Sample of perature start perature perature (kgf/ (kgf/ ratio ance evalua-No. Steel Ceq (.degree.C.) (.degree.C.) (.degree.C.) (.degree.C.) mm.sup.2) mm.sup.2) (%) P.sub.DF tion (points) Remarks__________________________________________________________________________91 B 0.43 850 737 750 320 95 107 68 0.251 3 Sample of the invention92 D 0.63 820 732 750 300 131 131 70 0.089 5 Sample of the invention93 D 0.63 820 732 *700 270 131 *125 62 -0.019 0 Sample for comparison94 J 0.51 850 706 760 340 106 113 63 0.100 5 Sample of the invention95 N 0.43 850 659 700 290 95 109 65 0.174 5 Sample of the invention96 O 0.61 840 707 720 300 127 *118 55 -0.087 0 Sample for comparison97 O 0.61 840 707 *650 250 127 *120 58 -0.051 0 Sample for comparison98 R 0.47 850 762 790 320 100 116 50 -0.155 0 Sample for comparison__________________________________________________________________________ Ceq = C + Si/24 + Mn/6 Lower limot of tensile strength = 32 .times. (Ceq).sup.2 - 155 .times. Ce + 102 Mark "*" shows outside the scope of the present invention.
The above-mentioned residual strength ratio (R.sub.r) of each of the samples of the invention and the samples for comparison was determined in accordance with the method described with reference to FIG. 5.
The above-mentioned delayed fracture resistance of each of the samples of the invention and the samples for comparison was evaluated in accordance with the following evaluation method.
More specifically, as shown in FIG. 6, a strip-shaped test piece 1 having dimensions of a thickness of 1.4 mm, a width (c) of 30 mm and a length (d) of 100 mm, and having grinding-treated edge faces, was cut out from each of the samples of-the invention and the samples for comparison. Then, a hole 2 was pierced in each of both end portions of the strip-shaped test piece 1. A center portion of the test piece 1 was then subjected to a bending with a radius of 5 mm. Then, a bolt 4 made of stainless steel was inserted into the above-mentioned two holes 2 through two washers 3 made of a tetrafluoroethylene resin, which washers inhibited formation of a local cell caused by the contact between different kinds of metal, to tighten the both end portions facing to each other of the test piece 1 by means of the bolt 4 until the distance (e) between the both ends of the test piece 1 became 10 mm, so as to apply stress to the bent portion of the test piece 1.
The strip-shaped test piece 1 of each of the samples of the invention and the samples for comparison thus applied with stress was immersed into 0.1 N hydrochloric acid to measure the time required before the occurrence of fractures in the bent portion of the test piece 1. Delayed fracture resistance of each of the samples of the invention and the samples for comparison was evaluated in the above-mentioned measurement by giving an evaluation of delayed fracture resistance of 0 point to the occurrence of fractures in the bent portion within 24 hours, 1 point to the occurrence of fractures within 100 hours, 2 points to the occurrence of fractures within 200 hours, 3 points to the occurrence of fractures within 300 hours, 4 points to the occurrence of fractures within 400 hours (400 hours not included), and 5 points to non-occurrence of fractures upon the lapse of 400 hours. Because the reduction in thickness of the test piece 1 and the production of local corrosion pits were serious after the lapse of 400 hours, the measurement was discontinued upon the lapse of 400 hours.
The above-mentioned test results of the residual strength ratio and the delayed fracture resistance are described further in detail with reference to FIGS. 1 to 4. FIG. 1 is a graph illustrating the relationship between an evaluation of delayed fracture resistance and a delayed fracture resistance index (P.sub.DF) in an ultra-high-strength cold-rolled steel sheet (i.e., each of the samples of the invention and the samples for comparison). In FIG. 1, the mark ".smallcircle." represents a sample comprising any one of steels "A" to "Z" having the chemical compositions within the scope of the present invention, which are free of niobium (Nb), titanium (Ti) and vanadium (V), and the mark ".circle-solid." presents a sample comprising any one of steels "A" to "Z" having the chemical compositions within the scope of the present invention, which contain at least one of niobium, titanium and vanadium. The mark ".smallcircle." and the mark ".circle-solid." represent not only the sample of the invention but also the sample for comparison. The mark ".tangle-solidup." represents the sample for comparison comprising any one of steel "a" to "j" having the chemical compositions outside the scope of the present invention.
As is clear from FIG. 1, all of the samples of the invention having a P.sub.DF (delayed fracture resistance index) of at least 0 show an evaluation of delayed fracture resistance of at least 3 points, and therefore, represent an excellent delayed fracture resistance. All of the samples for comparison show in contrast an evaluation of delayed fracture resistance of up to 1 point even with a P.sub.DF of at least 0, and therefore, represent a poor delayed fracture resistance.
FIG. 2 is a graph illustrating the effect of a residual strength ratio (R.sub.r) and tensile strength (TS) on a delayed fracture resistance index (P.sub.DF) in an ultra-high-strength cold-rolled steel sheet (i.e., each of the samples of the invention and the samples for comparison). In FIG. 2, the mark ".smallcircle." represents the sample of the invention having a P.sub.DF of at least 0, and the mark ".circle-solid." represents the sample for comparison having a P.sub.DF of under 0. As is clear from FIG. 2, all of the samples of the invention having a P.sub.DF of at least 0 show a residual strength ratio (R.sub.r) more excellent than that of the samples for comparison relative to the same tensile strength (TS). More specifically, the samples of the invention having a P.sub.DF of at least 0 show a residual strength ratio of at least 60%, and the samples of the invention having a high tensile strength of at least 140 kgf/mm.sup.2 show a high residual strength ratio of at least 70%. This suggests that the samples of the invention have a high tensile strength as well as an excellent delayed fracture resistance.
FIG. 3 is a graph illustrating the effect of Ceq (=C+(Si/24)+(Mn/6)) on the lower limit value of tensile strength (TS) in an ultra-high-strength cold-rolled steel sheet (i.e., each of the samples of the invention and the samples for comparison). In FIG. 3, the mark ".smallcircle." represents the sample of the invention having a P.sub.DF (delayed fracture resistance index) of at least 0, the mark ".circle-solid." represents the sample for comparison having a P.sub.DF of under 0, and the curve represents TS (tensile strength)=320.times.(Ceq).sup.2 -155.times.Ceq+102. As is evident from FIG. 3, all of the samples of the invention have a high P.sub.DF of at least 0 and a high TS of at least 320.times.(Ceq).sup.2 -155.times.Ceq+102. Some samples for comparison, in contrast, while having a high TS of at least 320.times.(Ceq).sup.2 -155.times.Ceq+102, have a low P.sub.DF of under 0, and the remaining samples for comparison have a low TS of under 320.times.(Ceq).sup.2 -155.times.Ceq+102 and a low P.sub.DF of under 0.
More specifically, it is possible, in the samples of the invention, to inhibit formation of the banded structure in steel caused by the segregation of manganese under the effect of the coexistence of manganese with carbon and silicon, and it is also possible to prevent the structure of steel from becoming composite, by using a value of Ceq (=C+(Si/24)+(Mn/6)) as determined by the contents of carbon, silicon and manganese, and controlling the lower limit value of tensile strength (TS) of the cold-rolled steel sheet in response to the value of Ceq.
FIG. 4 is a graph illustrating the effect of manufacturing conditions on the delayed fracture resistance index (P.sub.DF) in an ultra-high-strength cold-rolled steel sheet (i.e., each of the samples of the invention and the samples for comparison). In FIG. 4, the mark ".smallcircle." represents the sample of the invention, the soaking temperature and the tempering temperature of which are within the scope of the present invention as shown in Table 2, the mark ".circle-solid." represents the sample for comparison, the soaking temperature and/or the tempering temperature of which are outside the scope of the present invention also as shown in Table 2, and the mark ".tangle-solidup." represents the sample of the invention or the sample for comparison as shown in Table 4. As is clear from FIG. 4, in order that the P.sub.DF (delayed fracture resistance index) is at least 0, it is necessary to limit the quenching start temperature to at least the lower limit temperature (T.sub.Q) for starting quenching, in addition to the control of the soaking temperature and the tempering temperature.
According to the present invention, as described above in detail, it is possible to provide an ultra-high-strength cold-rolled steel sheet excellent in delayed fracture resistance and having a high tensile strength of over 100 kgf/mm.sup.2 and a method for manufacturing same, thus providing many industrially useful effects.

Number	Name	Date
3573898	Murai et al.	Apr 1971
3738874	Allten et al.	Jun 1973
4472208	Kunishige	Sep 1984

Number	Date	Country
61-3843	Jan 1986	JPX
61-217529	Sep 1986	JPX
62-86149	Apr 1987	JPX
2-236223	Sep 1990	JPX
2254134	Oct 1990	JPX

Method for manufacturing an ultra-high strength cold-rolled steel sheet with desirable delayed fracture resistance

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