JP2002161336A - Super high tensile-strength cold-rolled steel sheet and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a super high tensile-strength cold-rolled steel sheet having a superior formability for extension flange with a hole extension rate of 75% or more, and having a tensile strength of 880-1,170 MPa, which can be manufactured industrially in an existing annealing furnace, and to provide a manufacturing method therefor. SOLUTION: The super high tensile-strength steel sheet with the tensile strength of 880-1,170 MPa, includes Mn, Cr and Mo of 1.6-2.5% in total, or B of 0.0005-0.0050%, or Mn, Cr and Mo of 1.6-2.5% in total together with B of 0.0005-0.0050% by wt.%, and a metal structure which is substantially a martensite single-phase (excluding a part within 10 μm deep from a surface layer of the steel sheet).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultra-high tensile strength cold-rolled steel sheet and a method for producing the same, and more particularly, it has properties suitable as a material for automobile parts and has a tensile strength of 880 to 1170.
The present invention relates to an ultra-high tensile strength cold-rolled steel sheet having a MPa of 980 to 1080 MPa, and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, in response to the need for lighter automobiles,
A cold rolled steel sheet having a tensile strength of 880 to 1170 MPa is applied to an automobile frame member, an automobile seat frame member, and the like.

[0003] Since members such as automobile frame members are formed by press working, their materials are required to have high stretch flangeability. However, conventionally, the tensile strength is 880 M
The so-called ultra-high-strength steel sheets of Pa or more are limited in use as bumper reinforcing materials, door impact beams, etc., and since the forming method of these members is sequential forming such as roll forming, high stretch flangeability is not necessarily required. The hole expansion ratio of the ultra-high strength steel sheet having a tensile strength of 880 to 1170 MPa, which is not required, is at most about 50%, and does not satisfy the high stretch flangeability required for the material of the automobile frame member and the like. Was.

[0004] For example, Japanese Patent Publication No. 5-10418 discloses a technique of "steel plate for laser processing excellent in stretch flangeability". It is improved by adjusting the components, and does not improve the general stretch flangeability of the punched or sheared end face during the molding of the automobile frame member or the like.

On the other hand, in parallel with the trend of increasing the strength of materials for automobile parts as described above, in the technique of assembling automobile parts, mechanical joining has begun to be used in place of conventionally used spot welding. . The term “mechanical joining” as used herein includes any method of joining a plurality of steel sheets by metal forming without applying heat, and a typical example thereof is TOX joining. In addition, there is also a joining method in which adhesion and the like are combined in an auxiliary manner.

[0006] Mechanical joining is a technique that enables a part manufacturing process, which was conventionally performed in a molding process and an assembly process by spot welding, to be performed in a single process of molding and joining. There is a manufacturing cost reduction effect. In addition, when welding ultra-high strength steel sheets, the phenomenon that the heat-affected zone is softened is inevitable, and there is a problem that high joint strength cannot be obtained by breaking from the heat-affected zone when a weld strength test is performed. The advantage of using a non-weld type mechanical joining that does not apply heat is extremely great in the technology of assembling parts made of ultra-high strength steel sheets.

[0007] However, although parts obtained by mechanically joining high-strength steel sheets are being applied to structural parts such as automobiles, the tensile strength of steel to be joined in such parts is 780 MPa.
a or less. The reason is that the conventional tensile strength of 7
When a high-tensile steel sheet of more than 80 MPa was joined by mechanical joining, cracks occurred at the joining portion, and the joint strength and fatigue strength were not sufficiently obtained. For this reason, in spite of the above-mentioned advantages, it has not been possible to use mechanical joining for assembling parts made of ultra-high-strength steel sheets.

[0008]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has been developed in accordance with the Iron and Steel Federation Standard (hereinafter referred to as "the Steel Federation") which is most suitable for press-forming automotive structural members, reinforcing members, and vehicle seat frame members. JFST1001-199
An existing continuous annealing furnace that has an excellent stretch flangeability with a hole expansion rate of 75% or more as specified in 6) and is most suitable to be applied to automotive parts and assembled by mechanical joining such as TOX joining. Industrially manufacturable, tensile strength of 880-1
An object of the present invention is to provide an ultra-high tensile strength cold-rolled steel sheet of 170 MPa, particularly 980 to 1080 MPa, and a method for producing the same.

[0009]

Means for Solving the Problems The present inventor has established a control of the structure for improving the stretch flangeability of a super-high tensile strength steel sheet having a tensile strength of 880 to 1170 MPa to a hole expansion ratio of 75% or more. We continued our research. as a result,
It has been found that by realizing a martensite single phase structure, the stretch flangeability of an ultra-high tensile strength steel sheet having a tensile strength of 880 to 1170 MPa can be dramatically improved.

Further, the present inventors have intensively studied the strength of the joint by mechanical joining. As a result, it was clarified that the damage of the metal material at the time of the mechanical joining process governed the joint strength thereafter. The degree of damage to the metal material during the processing of the mechanical joining is dominated by the influence of the metal structure. If two phases having different hardnesses such as martensite and ferrite are present, the workability of the two phases is reduced. Due to the difference, strain is concentrated at the two-phase interface and damage is increased, so it was found that it is necessary to use a single phase. By realizing the above-described martensitic single-phase structure, excellent mechanical bondability was obtained. Was obtained.

However, it is difficult to obtain a martensitic single-phase structure with an existing facility having a tensile strength of 880 to 1170 MPa industrially for the following reasons.

As shown in FIG. 1, a conventional ordinary continuous annealing furnace comprises, as shown in FIG. 1, a heating zone 1 for heating a steel sheet, a soaking zone 2 for keeping the heated steel sheet soaked, and a slow cooling of the steel sheet after the soaking. Quenching zone 3 for quenching the steel sheet after gradual cooling, and overaging (tempering) zone 5 for overaging (tempering) the quenched steel sheet.
The steel sheet S is supplied from the cold-rolled coil 7 on the entry side, and is passed through the heating zone 1, the soaking zone 2, the slow cooling zone 3, the rapid cooling zone 4, and the overaging (tempering) zone 5. The steel sheet S is continuously subjected to heating, soaking, holding, slow cooling, quenching, and overaging, and is temper-rolled as necessary by the temper rolling mill 6 on the delivery side. It is wound up. At this time, FIG.
As shown in the figure, since the slow cooling zone 3 is provided between the soaking zone 2 and the quenching zone 4, the plate temperature is inevitably 100 ° C.
After the decrease, the steel sheet S enters the rapid cooling zone 4. In order to obtain a martensitic single-phase structure in such a continuous annealing furnace, it is necessary to use an austenitic single-phase structure in the soaking zone 2 and a slow cooling zone 3.
Must be passed at a plate temperature of at least Ar ₃ points and quenched from there. However, at this time, a steel having a lower tensile strength, that is, a steel having a lower C equivalent, has a higher Ar ₃ point. Must pass at high temperature. However, in a normal continuous annealing furnace, there is a limit in increasing the temperature passing through the annealing zone 3 due to the temperature drop while passing through the annealing zone 3 and the like. Therefore, the conventional tensile strength of 880 to 1170 MPa In the case of the steel No. 3, the Ar ₃ point becomes higher than the entry temperature of the quenching zone 4, and it is not possible to suppress the formation of ferrite in the annealing zone 3, so that a martensitic single phase structure cannot be obtained.

Therefore, the present inventor has proposed that the tensile strength be 880-1.
Further studies were conducted to establish a technology for industrially producing an ultra-high-strength steel sheet of 170 MPa and having a martensitic single-phase structure in an existing continuous annealing furnace. As a result, M
at least one of n, Cr and Mo is 1.6 to
2.5%, B is 0.0005 to 0.00
50% or Mn, Cr and Mo
It has been found that it is effective to contain at least one of these in a total of 1.6 to 2.5% and B in an amount of 0.0005 to 0.0050%. Further, when B is added,
48/14 [N] to 3 ×
48/14 [N]% (where [N] is N amount (% by weight)
It is found that the effect of the addition of B can be enhanced by adding the compound in the range of (shown).

The present invention has been completed based on these findings and provides the following (1) to (9). (1) At least 1% by weight of Mn, Cr and Mo
It contains 1.6 to 2.5% of seeds in total, is substantially a martensite single phase (excluding a portion within 10 μm in depth from the surface of the steel sheet), and has a tensile strength of 880 to 1170 MPa. Ultra-high tensile cold rolled steel sheet.

(2) B is 0.0005 to 5% by weight.
An ultra-high tensile cold-rolled steel sheet containing 0.0050%, substantially a martensite single phase (excluding a portion within 10 μm in depth from the surface layer of the steel sheet), and having a tensile strength of 880 to 1170 MPa.

(3) Mn, Cr and Mo in weight%
At least one of 1.6 to 2.5% in total;
An ultra-high tensile cold-rolled steel containing 0005 to 0.0050%, substantially a martensite single phase (excluding a portion within a depth of 10 μm from the surface of a steel sheet) and a tensile strength of 880 to 1170 MPa. steel sheet.

(4) C: 0.01-0.0% by weight
7%, Si: 0.3% or less, P: 0.1% or less, S:
0.01% or less, Sol. Al: 0.01 to 0.1%,
N: 0.0050% or less, at least one of Mn, Cr, and Mo: 1.6 to 2.5% in total, the balance being substantially Fe, and substantially a martensitic single phase (steel plate) An ultra-high tensile cold-rolled steel sheet characterized by having a tensile strength of 880 to 1170 MPa, excluding a portion within a depth of 10 μm or less from the surface layer.

(5) In the above (4), in weight%,
B: An ultra-high tensile cold-rolled steel sheet further containing 0.0005 to 0.0050%.

(6) The above (2), (3) or (5)
In, by weight%, Ti: 48/14 [N] to 3 × 4
An ultra-high tensile cold-rolled steel sheet further containing 8/14 [N]% (where [N] indicates N content (% by weight)).

(7) An ultra-high tensile cold-rolled steel sheet according to any one of the above (1) to (6), further containing 0.001 to 0.04% by weight of Nb.

(8) An ultra-high tensile cold-rolled steel sheet according to any one of the above (1) to (7), wherein the hole expansion rate of the cut hole is 100% or more.

(9) A method for producing a cold-rolled steel sheet in which a steel slab having the composition described in any one of (1) to (7) is hot-rolled, cold-rolled, continuously annealed, and cooled. And heating to 800 to 890 ° C. by continuous annealing and holding,
Slowly cool at 20 ° C / sec or less, and from 680-750 ° C to 1
It is cooled to 200 ° C. or less at a cooling rate of more than 00 ° C./sec.
A method for producing an ultra-high tensile cold-rolled steel sheet.

Steels having a chemical composition and structure similar to the present invention have been disclosed in various prior arts. However, as in the present invention, a steel having a martensitic single phase structure and a tensile strength of 880-11 is used.
There is no one having an excellent stretch flangeability of 70 MPa and a hole expansion ratio of 75% or more and an excellent mechanical bonding property. The advantages of the present invention will be described below in comparison with such prior art.

Japanese Patent Publication No. 2- 1894 discloses a technique relating to a steel sheet having the same tensile strength level as the present invention. However, this technique has no suggestion as to the relationship between the metal structure and the stretch flangeability or mechanical bondability, and furthermore, since the C content is 0.10 to 0.20%, as in the present invention, the martensitic singlet is not used. It is extremely difficult to obtain a phase structure industrially.

Japanese Patent Publication No. 8-26401 and Japanese Patent No. 2528387 disclose "a steel sheet having a fine martensitic single phase structure" and "a technique for setting the martensite volume ratio to 80 to 97%", respectively. It has been disclosed. However, these techniques do not suggest that the stretch flangeability and mechanical bondability of the steel sheet become extremely good by forming a martensitic single-phase structure, and in addition to the steel sheet having a tensile strength of 1470 MPa or more. Therefore, the tensile strength of the target steel sheet is different from that of the present invention. Although it is relatively easy to obtain a martensite single phase structure in a steel sheet having a tensile strength of 1470 MPa or more, it is difficult to obtain a martensite single phase structure in a steel sheet having a tensile strength of 880 to 1170 MPa, which is the object of the present invention. It is extremely difficult, and this has been achieved industrially for the first time by the present invention. In addition, it is relatively easy to obtain a martensitic single-phase structure at a tensile strength of 1180 MPa or more, but such high strength is not suitable for molding applications because moldability is extremely reduced.

Furthermore, Japanese Patent No. 2826058 discloses a technique of "volume ratio of martensite of 70% or more". Examples of this publication, Table 1, steel No. 2
Describes a steel plate having a tensile strength of 880 to 1170 MPa and a martensite single phase structure. However, this technique does not suggest that the formation of a martensite single-phase structure results in extremely good stretch flangeability and mechanical joining properties of the steel sheet. Further, this technique has no consideration of industrially obtaining a martensitic single-phase structure having a tensile strength of 880 to 1170 MPa. This is because the steel No. The composition of No. 2 is not Mn + Cr + Mo ≧ 1.6% as in the present invention, and does not contain 0.0005 to 0.0050% of B, and is shown in Examples and Table 2 of this publication. Steel No. (2) The continuous annealing conditions are soaking temperature 900 ° C and quenching start temperature 850 ° C, which greatly deviate from the industrially achievable temperature conditions with ordinary continuous annealing equipment, and are inferior in industrial productivity. It is clear from In other words, this publication indicates that it is difficult to obtain a steel having a tensile strength of 880 to 1170 MPa and a martensitic single phase structure under industrially achievable continuous annealing conditions.

It is difficult to achieve the above continuous annealing conditions industrially with ordinary continuous annealing equipment for the following reasons. In the soaking zone 2 of the ordinary continuous annealing furnace shown in FIG. 1 described above, a combustible gas such as CO is burned in a heat-resistant steel tube called a radiant tube in order to generally keep the annealing atmosphere in a reducing state. And the radiant heat is used as a heat source. In order to maintain the plate temperature at 900 ° C. in such a tropical zone 2, the ambient temperature and the radiant tube temperature must be 900 ° C. or higher, and such severe conditions significantly shorten the life of the apparatus. Also, the quenching start temperature is set to 8
Although the temperature is set to 50 ° C., since the sheet temperature inevitably decreases by 100 ° C. or more in the slow cooling zone 3 (gas jet zone in the figure) provided between the soaking zone 2 and the quenching zone 4, the soaking temperature is set to 900
It is difficult to set the quenching start temperature to 850 ° C. when the temperature is set to 850 ° C., and in order to achieve such continuous annealing conditions, it is necessary to provide a uniform structure with a special structure capable of maintaining a high temperature and a short annealing zone. It is necessary to develop a new continuous annealing furnace having:
Furthermore, even if such a dedicated continuous annealing furnace was developed, from the viewpoint of the quality of the steel sheet, when heated and maintained at 900 ° C., the austenite crystal grain size was coarsened, so that the martensite structure after quenching was also coarsened, Since the bending property and toughness of the steel sheet are deteriorated, such continuous annealing conditions are not preferable, and materials that require such conditions are also not preferable.

[0028]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described specifically. The cold-rolled steel sheet according to the present invention contains Mn, C
at least one of r and Mo is 1.6 to 2.5 in total
% Or B is 0.0005 to 0.0050%
Containing or at least one of Mn, Cr and Mo
1.6 to 2.5% of the total seeds and 0.0005 to B
It is an ultra-high tensile cold-rolled steel sheet containing 0.0050%, is substantially a martensite single phase (excluding a portion within 10 μm in depth from the surface layer of the steel sheet), and has a tensile strength of 880 to 1170 MPa.

First, the steel composition will be described. Mn, C
at least one of r and Mo is 1.6 to 2.5 in total
The content is determined to lower the temperature of the Ar ₃ point, and this makes it possible to industrially obtain a martensitic single-phase structure in a normal continuous annealing furnace. Mn, Cr
If the total amount of Mo and Mo is less than 1.6%, the effect of lowering the temperature of the Ar ₃ point cannot be sufficiently obtained, while if the total amount of these elements exceeds 2.5%, the strength becomes 880 to 1170MPa.
Since it exceeds the range of a, at least one of Mn, Cr and Mo is set to 1.6 to 2.5% in total. In order to set the tensile strength of the cold-rolled steel sheet to 880 to 1170 MPa, according to the total amount of Mn, Cr and Mo,
It is necessary to adjust the content of C.

The reason why B is contained in an amount of 0.0005% to 0.0050% is to lower the temperature of Ar ₃ points.
This makes it possible to industrially obtain a martensite single-phase structure in a normal continuous annealing furnace. B is 0.0005%
If it is less than ₃ , the effect of lowering the temperature of the Ar ₃ point cannot be sufficiently obtained,
On the other hand, if the content exceeds 0.0050%, the deformation resistance of hot rolling increases, and the production becomes difficult.
5 to 0.0050%.

In the case where at least one of Mn, Cr and Mo is combined with B, the total amount of Mn and the like can be reduced as compared with the case where B is not contained. Therefore, the increase in tensile strength due to the above can be reduced, so that the upper limit of the amount of C is alleviated, thereby making it possible to avoid an increase in steelmaking cost.

It is preferable to add 0.001 to 0.04% of Nb. Nb has a function of suppressing coarsening of the austenitic structure in the solitary tropics, and the addition of Nb can prevent deterioration of the bendability and toughness of the steel sheet due to the coarsening of the austenitic structure. If it is less than 0.001%, the effect is not sufficient, and if it exceeds 0.04%, recrystallization is significantly delayed, which is not preferable. Therefore, when Nb is added, the amount of Nb added is set to 0.001 to 0.04%.

When B is contained, Ti is added in an amount of 48/1.
It is preferable to add 4 [N] to 3 × 48/14 [N]% (where [N] indicates the amount of N (% by weight)). Since the above-described effect of B can be obtained only when B is in a solid solution state, the effect is reduced when B is combined with N to form BN. At this time, if Ti is added and solid solution N is completely precipitated in advance as TiN, the amount of solid solution B increases and the effect of the addition of B can be maximized. If the effect of such Ti addition is less than 48/14 [N], the effect is insufficient. If it exceeds 3 × 48/14 [N], TiC is formed and ductility is reduced. N] ~ 3
× 48/14 [N].

As other components, C: 0.01 to 0.0
7%, Si: 0.3% or less, P: 0.1% or less, S:
0.01% or less, Sol. Al: 0.01 to 0.1%,
N: It is preferable to be 0.0050% or less.

C: 0.01 to 0.07% C is appropriately adjusted in accordance with the total amount of Mn, Cr and Mo in order to make the tensile strength of the cold-rolled steel sheet 880 to 1170 MPa. Is done. However, if it is less than 0.01%, the steel making cost is increased, and if it exceeds 0.07%, the tensile strength is 880 to 117 regardless of the total amount of Mn and the like.
Since the amount exceeds the range of 0 MPa, the C content is 0.01 to 0.0
7%. More preferably, it is 0.03 to 0.07%.

Si: 0.3% or less Si is a harmful element that raises the Ar ₃ point in the present invention, and it is desirable to reduce it as much as possible. If it exceeds 0.3%, it becomes difficult to obtain a martensitic single phase structure of substantially 880 to 1170 MPa.

P: 0.1% or less P may be added for the purpose of adjusting the strength. However, if it exceeds 0.1%, the toughness of the spot welded portion is deteriorated.
It needs to be 1% or less.

S: 0.01% or less S is treated as an impurity in the present invention. If it exceeds 0.01%, the stretch flangeability deteriorates due to MnS precipitates, so the content is made 0.01% or less.

Sol. Al: 0.01 to 0.1% Al is added as a deoxidizing agent. If it is less than 0.01%, the effect is not sufficient, and if it exceeds 0.1%, the effect is saturated and uneconomical. The Al content is set to 0.01 to 0.1%.

N: 0.0050% or less N is treated as an impurity in the present invention. 0.0050
%, 0.005% or less because the strength variation in the coil is caused.

The present invention relates to Cu, Ni, Zr, V, W, C
Even if impurity elements such as a, Sn, and O are contained in a normal range, the effect is not lost. Also, it is possible to intentionally add a small amount of these elements for the purpose of adjusting the strength and the like.

Next, the metal structure will be described. Substantially a martensitic single phase (depth 1 from steel sheet surface layer)
(Excluding the portion within 0 μm) is a very important component in the present invention, and by using such a structure, an extremely good stretch flangeability can be obtained. Here, the “substantially martensite single phase” means that the structure is quantitatively measured by an optical microscope, a scanning electron microscope, and an X-ray diffraction method, and that a total of 1% or more of ferrite, bainite, retained austenite, and the like is not contained. means. However, for example,
Precipitates formed by solute elements in steel, such as AlN, MnS, and TiN, and fine iron carbide precipitated by tempering martensite may be included. Ferrite may be formed on the outermost layer within 10 μm of the steel sheet surface due to decarburization of the surface layer, etc., but this does not affect the stretch flangeability and mechanical bonding properties, but rather improves the bendability. Ferrite may be contained in the surface layer of the steel sheet within 10 μm. For this reason, the metallographic structure is not limited to a portion within a depth of 10 μm from the surface layer of the steel sheet.

The cold-rolled steel sheet of the present invention has a tensile strength of 880 to 880.
1170 MPa. This is because, as described above, the cold-rolled steel sheet of the present invention is intended to be applied to a skeleton member for automobiles or the like which requires a strength in this range. Considering such applications, the tensile strength is preferably in the range of 980 to 1080 MPa.

The cold-rolled steel sheet of the present invention having the above-described steel composition and metal structure and having a tensile strength of 880 to 1170 MPa is manufactured according to the standards of the Iron and Steel Federation (JFST1001-199).
It has excellent stretch flangeability with a hole expansion rate of 75% or more as specified in 6), and is preferably formed by pressing this cold-rolled steel sheet into an automobile structural member, a reinforcing member, an automobile seat frame member and the like. be able to.

Further, the cold-rolled steel sheet of the present invention has excellent mechanical joining properties in which the hole expansion rate of the cut hole is 100% or more. It is possible to join with sufficient strength by mechanical joining. In addition, the hole expansion rate of the cut hole here means that a cut hole is formed in a test piece, and the following is based on the Iron and Steel Federation Standard (JFST1001-1).
This is a value obtained by performing a hole expansion test according to 996). If the hole expansion ratio of the cut hole is less than 100%, cracks occur in the steel plate inside the joint at the time of joining by mechanical joining, and the joining strength is remarkably reduced, so that excellent mechanical joining properties cannot be obtained (crack occurrence). For the actual example, see FIG. 4 described later.) In addition, a hole expansion test using a punched hole, which becomes a standard of the Iron and Steel Federation (JIST1001-19)
The hole expansion rate according to (96) is greatly affected by damage to the material due to punching, and even if this value is improved, the mechanical bondability is not necessarily improved.

Next, the manufacturing method will be described. More than
When manufacturing such cold-rolled steel sheets, first,
Steel slabs having the above composition
After heating, hot rolling is performed. Hot rolling is Ar ₃Temperature above the point
At a cooling rate of 30 ° C./sec or more.
Cool to 0 ° C or less and wind up at 620 ° C or less
Good. The scale is removed before cold rolling.
To a desired thickness, followed by heating to 800 to 890 ° C.
After holding, slowly cool at 20 ° C./sec or less,
At a cooling rate of over 100 ° C / sec.
To perform continuous annealing for cooling.

Hereinafter, the continuous annealing conditions will be described. Heating and holding at 800 to 890 ° C. If the heating and holding temperature is 800 ° C. or lower, it becomes difficult to set the quenching start temperature to the Ar ₃ point or higher, and a martensitic single phase structure cannot be obtained. On the other hand, if the temperature is 890 ° C. or higher, the austenite structure is coarsened, so that the bendability and toughness of the steel sheet are deteriorated, and the continuous annealing equipment is deteriorated. Therefore, the heating and holding temperature is 800 to
890 ° C.

Slow cooling at 20 ° C./sec or less In order to obtain a martensitic single-phase structure, it is necessary to pass the annealing zone at a plate temperature Ar of ₃ points or more. 20 ° C./sec or less. When the cooling rate in the slow cooling zone exceeds 20 ° C./sec, the sheet temperature becomes A in the slow cooling zone.
and ferrite occurs for less than r ₃ points, martensitic not single-phase structure is obtained.

· From 680 to 750 ° C to 100 ° C / sec
When the quenching start temperature is 680 ° C or lower, ferrite is generated and a martensitic single phase structure cannot be obtained. When the quenching start temperature is higher than 750 ° C, the plate shape is deteriorated. ~
750 ° C. In order to stably obtain a martensitic single phase structure, the quenching start temperature is preferably set to 700 ° C. or higher. On the other hand, the quenching cooling rate is
If the temperature is less than or equal to c, or if the quenching end temperature exceeds 200 ° C., the martensitic transformation becomes insufficient and the desired martensite single phase structure cannot be obtained, so the quenching cooling rate is set to 100 ° C./sec. The quenching end temperature is set to 200 ° C. or lower. More preferably, the cooling rate is more than 500 ° C./sec, and the quenching end temperature is 50 ° C. or less. At this time, the cooling method is not limited, but it is most preferable to quench in the jet water in order to suppress the variation of the material in the plate width direction and the longitudinal direction. Further, the quenching in the jet water causes a cooling rate of more than 500 ° C./sec and a quenching end temperature of 5 ° C.
Cooling below 0 ° C. can be easily achieved.

After quenching, to improve the toughness,
It is desirable to perform a tempering treatment at 00 to 250 ° C. for 3 minutes or more. If the temperature is less than 100 ° C. or less than 3 minutes, the effect of the tempering treatment is small, and if it exceeds 250 ° C., ductility is remarkably deteriorated due to low-temperature tempering embrittlement, which is not preferable.

Further, after performing the continuous annealing as described above, temper rolling may be performed. The temper reduction rate at this time is desirably 0.3% or more from the viewpoint of shape correction. Further, in order to prevent elongation of elongation, the temper rolling reduction is desirably 1.0% or less.

After the above-described steps, metal plating such as Zn plating and / or surface treatment for applying various lubricating films such as organic ones may be performed. It is included in the ultra-high tensile strength cold rolled steel sheet of the invention.

[0053]

Embodiments of the present invention will be described below. [Example 1] A steel slab having the chemical composition shown in Table 1 was produced by continuous casting, slab was reheated to 1250 ° C, and then hot-rolled to a thickness of 3.0 mm. Hot rolling was performed at a hot finishing temperature of about 870 ° C and a winding temperature of 560 to 600 ° C.
After pickling, cold rolling was performed to a sheet thickness of 1.2 mm. Subsequently, heat treatment was performed in a continuous annealing furnace. Heating and holding temperature is 85
0 ° C., the slow cooling rate in the slow cooling zone is 7 ° C./sec, the rapid cooling start temperature is 720 ° C., the rapid cooling stop temperature is about 40 ° C., and the rapid cooling is performed by water quenching in the jet water.
000 ° C./sec or more. The tempering process is 20
Performed by holding at 0 ° C. for about 10 minutes. afterwards,
Temper rolling was performed at an elongation of 0.5%.

The metallographic structure of the steel sheet thus obtained was observed in a cross section parallel to the longitudinal direction of the coil, mirror-polished and etched with nital, and scanned with a scanning electron microscope (SEM).
EM) A photograph was taken to determine the volume fraction of martensite. Table 1 also shows the determined volume fraction of martensite.

From the steel sheet obtained as described above, a JIS No. 5 test piece was sampled by cutting in a direction perpendicular to the rolling direction, and a tensile test was performed. In addition, the hole expansion ratio was measured according to the Iron and Steel Federation Standard (JFST1001-1996). Furthermore, 30 × 100m parallel to the rolling direction
m test piece cut out by cutting
180 ° bending was performed with a punch having a tip R having a pitch of mm, and a minimum bending radius at which no crack occurred was determined. For each steel plate, the YP (MP
Table 2 shows a), TS (MPa) and El (%), the hole expansion ratio (%), and the minimum bending radius (mm).

[0056]

[Table 1]

[0057]

[Table 2]

As shown in Table 2, the steel numbers 1 to 6 of the present invention have tensile strengths of 880 to 1170 MPa.
And a very good stretch flangeability with a hole expansion ratio of 75% or more, and a minimum bending radius of 1.
It has good bending characteristics of 0 mm or less. On the other hand, steel Nos. 7 to 10 as comparative examples were inferior in any of the properties. For example, in steel No. 7, since the total amount of Mn, Mo, and Cr was less than 1.6%, a structure of martensite single phase was not obtained, so that the hole expansion ratio was low and the stretch flangeability was poor. Steel No. 8 had an excessively high strength because the C content exceeded 0.07%, and therefore had a low hole expansion ratio, a large minimum bending radius, and poor stretch flangeability and bendability. Steel No. 9 has a C content of more than 0.07% and a Si content of more than 0.3%, so that a martensitic single phase structure cannot be obtained, and therefore a high hole expansion rate cannot be obtained, and stretch flangeability is not obtained. Was inferior. Steel No. 10 is M
Since the total amount of n, Mo and Cr exceeds 2.5%,
The tensile strength was too high, and therefore the hole expansion rate was low, the minimum bending radius was large, and the stretch flangeability and bendability were poor.

[Example 2] Using a steel slab having the same composition as steel numbers 1 to 3 of Example 1, cold rolling was performed under the same conditions as in Example 1, and then continuous under the conditions shown in Table 3. Annealing steps and temper rolling were performed to obtain steel sheets of test material symbols A to F which had been subjected to variously changed heat treatments. The obtained steel sheet is
Table 4 shows the results of the tensile test and the measurement of the hole expansion ratio and the minimum bending radius in the same manner as in Example 1.

[0060]

[Table 3]

[0061]

[Table 4]

As shown in Table 4, the test material symbols A to D of the present invention all have a tensile strength of 880 to 1170M.
It is Pa and has a very good stretch flangeability with a hole expansion ratio of 75% or more, and also has a good bending characteristic with a minimum bending radius of 1.0 mm or less. On the other hand, the test material symbols E and F as comparative examples were inferior in either property. For example, sample material symbol E has a soaking temperature of less than 800 ° C., so that the quenching start temperature becomes lower than 680 ° C. due to the unavoidable decrease of the plate temperature in the slow cooling zone. The structure was not obtained, so that the hole expansion rate was low and the stretch flangeability was poor.
For sample material symbol F, the quenching start temperature was lower than 680 ° C. because the slow cooling rate exceeded 20 ° C./sec, and a martensitic single phase structure was not obtained. Therefore, the hole expansion rate was low and the stretch flangeability was low. Was inferior.

Example 3 A steel slab having the chemical composition shown in Table 5 was produced by continuous casting, and was subjected to the same process as in Example 1 (however, the quenching cooling rate was about 2000 ° C.).
/ Sec) Each steel sheet was subjected to the same procedure as in Example 1 to determine the volume fraction of martensite and to carry out a tensile test. In addition, using a sample in which a hole of 10 mm in diameter was drilled by mechanical cutting in each steel plate, a hole expansion test in accordance with the Iron and Steel Federation Standard (JIST1001-1996) was performed, and the hole diameter at which a crack occurred was measured. By calculating the rate of change of this hole diameter from the initial hole diameter,
The hole expansion rate of the cut hole was determined.

Further, applicability to mechanical bonding that can be performed without heating, which has recently attracted attention, was evaluated based on peel strength measured by the method described below. The mechanical bondability was evaluated based on the peel strength, as described above, because the effect of material damage is dominant on the strength of the mechanically bonded joint, and the effect of the material appears significantly. is there.

First, two rectangular test pieces are overlapped so that their longitudinal directions are orthogonal to each other and intersect at the center, and FIG.
2 and a cylindrical punch (punch: 5.6 mm in diameter) shown in FIG.
(B) is press-formed using a die (die diameter: 8 mm, die depth: 1.2 mm) having a ring-shaped groove around the bottom. At this time, as shown in FIG. 2C, the two test pieces plastically flow into the grooves at the bottom of the die and are mechanically joined. Thereafter, the joint as shown in FIG. 3 is pulled in the direction perpendicular to the surface of the test piece to determine the strength at which the joint peels off. When the relationship between the peel strength and the geometric bondability was investigated in advance, it was found that the mechanical bondability was sufficient if the peel strength was 2.0 kN or more.

After the respective steel sheets were joined by TOX joining shown in FIG. 2, a peeling test was performed as shown in FIG. 3 to evaluate the joining strength. For each steel plate, the martensite volume ratio (%), the hole expansion ratio (%) of the cut hole,
YP (MPa), TS (MP
Table 6 shows a) and El (%), and the peel strength (kN).

[0067]

[Table 5]

[0068]

[Table 6]

As shown in Table 6, steel numbers 11 to 16 of the present invention have tensile strengths of 880 to 1170M.
When Pa is Pa and the hole expansion ratio of the cut hole is 100% or more, the joining strength is high and the mechanical joining property is extremely good. On the other hand, steel Nos. 17 to 20 as comparative examples were inferior in any of the properties. For example, steel number 17 is Mn + C
Since r + Mo was less than 1.6%, the structure of the martensite single phase was not obtained, and the joining strength by mechanical joining was inferior. FIG. 4 shows an enlarged view of the joint portion of the steel No. 17 by mechanical joining. As indicated by the arrow in FIG. 4, cracks occurred at the joints in steel No. 17, and it is considered that the joint strength was deteriorated due to the cracks. Steel No. 18 had a C content exceeding 0.07%, so the strength was too high and the elongation was extremely poor. In steel No. 19, the martensitic single phase structure was not obtained because the Si content exceeded 0.3%, and thus the bonding strength of mechanical bonding was inferior. Steel No. 20 is Mn + Cr + Mo
Exceeds 2.5%, the tensile strength was too high and the elongation was extremely poor.

[Example 4] Steel numbers 11 to 13 of Example 1
Using a steel slab having the same composition as in Example 3, cold rolling was performed under the same conditions as in Example 3, then a continuous annealing step and a temper rolling were performed under the conditions shown in Table 7, and variously changed heat treatments were performed. The steel sheets of the test material symbols G to N were obtained. For each of the obtained steel sheets, the martensite volume ratio and the hole expansion ratio of the cut hole were determined and the tensile test was performed in the same manner as in Example 3.
Table 8 shows the results of the measurement of the peel strength of mechanical bonding.

[0071]

[Table 7]

[0072]

[Table 8]

As shown in Table 8, the test material symbols G to J of the present invention all have a tensile strength of 880 to 1170M.
When Pa is Pa and the hole expansion ratio of the cut hole is 100% or more, the joining strength is high and the mechanical joining property is extremely good. On the other hand, the test material symbols K to N as comparative examples were inferior in any of the characteristics. For example, sample material K has a soaking temperature of less than 800 ° C., so that the plate temperature inevitably decreases in the slow cooling zone, the quenching start temperature becomes less than 680 ° C., and a martensitic single phase structure is obtained. Therefore, the joining strength of the mechanical joining was inferior. For sample material symbol L, since the cooling rate in slow cooling exceeds 20 ° C./sec, the quenching start temperature is less than 680 ° C., a martensitic single phase structure cannot be obtained, and the joining strength of mechanical joining is poor. Was. For sample material symbol M, quenching rate was 100 ° C./sec or less, ferrite was generated during cooling. For sample material symbol N, ferrite was generated during tempering because the quenching stop temperature exceeded 200 ° C.
Bainite was formed, and in each case, a martensitic single phase structure was not obtained, and the joining strength of mechanical joining was poor.

[0074]

According to the present invention, it has excellent stretch flangeability suitable for press molding and excellent mechanical bondability that can be bonded with high strength by mechanical bonding such as TOX bonding. Tensile strength of 880-1170 MPa, especially 980, which can be industrially manufactured in existing continuous annealing furnaces
It is possible to provide an ultra-high tensile cold-rolled steel sheet of a class of up to 1080 MPa.

By using the ultra-high tensile strength cold-rolled steel sheet of the present invention, it is possible to easily and inexpensively manufacture structural members and reinforcing members for automobiles, frame members of automobile seats and the like by press molding. By widely using such components, a significant reduction in the weight of an automobile can be achieved, and further, effects such as improvement in automobile fuel efficiency and reduction of CO ₂ emission can be expected.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing the configuration of an existing continuous annealing furnace.

FIGS. 2A and 2B are explanatory diagrams of mechanical joining, in which FIG. 2A shows a punch, FIG. 2B shows a die, and FIG.

FIG. 3 is an explanatory view of a peel strength test.

FIG. 4 is an enlarged view of a comparative example in which a crack has occurred in a joint portion.

Continuing from the front page (72) Inventor Akihide Yoshitake 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Inventor Hideyuki Tsurumaru 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Incorporated F term (reference) 4K037 EA02 EA05 EA11 EA15 EA16 EA17 EA18 EA19 EA23 EA25 EA27 EA31 EB06 EB08 FA03 FC04 FE02 FH01 FJ05 FJ06 FK02 FK03 FK06 FK08 FL01 FM02

Claims (9)

[Claims] Claims: 1. A martensitic single phase containing at least one of Mn, Cr and Mo in a weight percentage of 1.6 to 2.5% in total (parts having a depth of less than 10 μm from the surface layer of a steel sheet). Excluding) and a tensile strength of 880 to 1170MP
(a) an ultra-high tensile cold-rolled steel sheet; 2. B is 0.0005 to 0.00% by weight.
An ultra-high tensile cold-rolled steel sheet containing 50%, substantially a martensite single phase (excluding a portion within 10 μm in depth from the surface layer of the steel sheet), and having a tensile strength of 880 to 1170 MPa. 3. A total of 1.6 to 2.5% of at least one of Mn, Cr and Mo and B of 0.000% by weight.
An ultra-high tensile cold-rolled steel containing 5 to 0.0050%, substantially a martensitic single phase (excluding a portion within 10 μm in depth from the surface layer of the steel sheet), and having a tensile strength of 880 to 1170 MPa. steel sheet. 4. In% by weight, C: 0.01 to 0.07%, Si: 0.3% or less, P: 0.1% or less, S: 0.01% or less, Sol. Al: 0.01 to 0.1%, N: 0.0050% or less, at least one of Mn, Cr and Mo: 1.6 in total
~ 2.5%, the balance being substantially composed of Fe, a substantially martensitic single phase (excluding the portion within 10 µm in depth from the surface layer of the steel sheet), and a tensile strength of 880 to 1170 MPa. Ultra-high tensile cold rolled steel sheet characterized by the following. 5. B: 0.0005 to 0.00% by weight
The ultra-high tensile strength cold-rolled steel sheet according to claim 4, further comprising 50%. 6. Ti: 48/14 [N] to 3% by weight
The ultrahigh-density composition according to claim 2, further comprising × 48/14 [N]% (where [N] indicates N content (% by weight)). Tension cold rolled steel sheet. 7. Nb: 0.001 to 0.04 by weight%
%. The ultra-high tensile cold-rolled steel sheet according to any one of claims 1 to 6, further comprising: 8. The ultra-high tensile cold-rolled steel sheet according to claim 1, wherein a hole expansion ratio of the cut hole is 100% or more. 9. A method for producing a cold-rolled steel sheet, comprising hot rolling, cold rolling, continuous annealing, and cooling a steel slab having the component composition according to any one of claims 1 to 7. After heating to 800 to 890 ° C. by continuous annealing and holding,
Cool slowly at 0 ° C / sec or less, and
A method for producing an ultra-high tensile cold-rolled steel sheet having a tensile strength of 880 to 1170 MPa, characterized by cooling to a temperature of 200 ° C. or lower at a cooling rate of more than 0 ° C./sec.