JP2002363650A - Method for producing ultrahigh strength cold rolled steel sheet having excellent seam weldability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing an ultrasonic strength cold rolled steel sheet which is provided with high strength of >=950 MPa, and has excellent seam weldability. SOLUTION: Steel having chemical components containing, by mass, 0.05 to 0.15% C and 0.1 to 0.5% Si also so as to satisfy 0.5 to 1.0% 5×C+Si, containing 1.5 to 2.5% Mn, <=0.02% P, <=0.005% S, 0.02 to 0.06% Al and 0.005 to 0.3% Mo, and the balance Fe with inevitable impurities is subjected to hot rolling so as to be finished at a finishing temperature of the Ar3 point or higher. The steel sheet is coiled at 500 to 650 deg.C, is thereafter pickled, and is cold-rolled at a draft of 30 to 70%. After that, the steel sheet is heated and annealed at an annealing temperature within the temperature range of 820 to 920 deg.C, and is slowly cooled to a rapid cooling starting temperature within the temperature range of 700 to 600 deg.C. Successively, the steel sheet is rapidly cooled to a rapid cooling finishing temperature of <=100 deg.C at a cooling rate of >=100 deg.C/s, and is thereafter subjected to overaging treatment at an overaging temperature within the temperature range of 180 to 300 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing an ultra-high strength cold rolled steel sheet having a tensile strength of 950 MPa or more and excellent in seam weldability.

[0002]

2. Description of the Related Art With the increasing demand for weight reduction for improving the safety of automobiles and reducing fuel consumption, high-strength cold-rolled steel sheets having good workability have been used as automotive steel sheets. With the spread of hot controlled rolling technology and continuous annealing technology, such a cold rolled steel hill has a strength-ductility balance in which a ferrite phase and a hard low-temperature transformation forming phase such as martensite and bainite coexist. High-strength composite microstructure steel sheets have been widely used.

[0003] In recent years, demands for higher strength have become more severe, and ultra-high strength cold rolled steel sheets having a tensile strength of 950 MPa or more have come to be used.
Such an ultra-high strength cold rolled steel sheet is disclosed in, for example,
As described in JP-A-2-99417, JP-A-3-277743, and JP-A-7-197183, the structure is mainly composed of ferrite and martensite. Si is added for the purpose of improving ductility due to formation of ferrite. Production of these ultra-high strength cold rolled steel sheets, as described in the above-mentioned publication, after hot rolling, pickling, cold rolling,
After that, recrystallization annealing and quenching are performed to change the structure to a structure mainly composed of ferrite and martensite, and in some cases, an overaging treatment is performed.

[0004]

Recently, even in such ultra-high-strength cold-rolled steel sheets, not only bending or spot welding but also seam welding between steel sheets is being performed. However, it has become apparent that when ultra-high strength cold-rolled steel sheets are seam-welded to each other, they tend to break at the nugget portion (melt solidified portion). Seam welding is a type of resistance welding, similar to spot welding, but spot welding is a method in which the proper nugget is formed by melting and solidifying the material between the electrodes by pressing and energizing the electrode tip at a certain point. In contrast, seam welding in which a large-diameter electrode ring is welded while rotating has a disadvantage that a nugget is difficult to form. That is, in the case of seam welding, since the large-diameter electrode ring is energized while rotating, the current path does not have a symmetrical shape as in spot welding, and the current flows backward from the center, resulting in an asymmetrical current path. Are formed, and because the contact area of the electrode ring is large, not only is it difficult to obtain an appropriate pressing force, but also a current path is formed at a location where no pressing is applied. Therefore, the electrode
The contact pressure between the plates and between the plates becomes insufficient, and this is not the case between the Cu electrode and the plates, but especially between the plates, the contact electric resistance increases, and a discharge-like current is applied. The energy is concentrated only in the vicinity of the plate surface layer to be formed, an appropriate molten portion is not formed, and after cooling, the limited nugget portion has an extremely high hardness. For this reason, when a tensile stress is applied, peeling and breaking occur, or the inside of the nugget breaks, so that sufficient strength cannot be obtained. In order to improve these, there is a method of increasing the current and the pressing force in the welding work, but the former merely makes it easier to generate dust, and the latter has equipment limitations. Such a problem becomes worse when the amount of alloying elements is increased in order to produce a high-strength steel sheet.

The inventor of the present invention has conducted intensive studies on the reason for the deterioration of the strength of the nugget portion formed by seam welding of an ultra-high strength cold-rolled steel sheet. Was identified as the cause. That is, when a large amount of Si is added to the steel sheet, the electric resistance of the steel sheet itself increases, and in particular, a Si oxide layer is formed on the steel sheet surface, thereby increasing the contact resistance between the disk electrode and the steel sheet, Since heat will be generated here,
The formation of the nugget between the steel sheets tends to be insufficient. Therefore, it is necessary to increase the heat input in order to sufficiently form the nugget portion. However, in this case, the molten metal portion becomes narrow and the temperature becomes high.
The nugget portion formed by solidification of such a molten metal portion increases in hardness and becomes brittle, so that the nugget portion falls below the strength of the base material and breaks easily occur here.

The present invention has been made in view of such a problem, and has as its object to provide an ultra-high-strength cold-rolled steel sheet having high strength of 950 MPa or more and excellent in seam weldability.

[0007]

SUMMARY OF THE INVENTION In view of the cause of deterioration in seam weldability of an ultra-high strength cold rolled steel sheet, the present inventor has determined the hardness and component amount of a nugget formed by seam welding (particularly ensuring the strength of the steel sheet). As a result of intensive research on the relationship between C to be formed and the content of Si for ensuring ductility) and manufacturing conditions (particularly, the quenching start temperature and overaging temperature that affect the structure of the steel sheet), these conditions were specified in a specific range. By doing so, it was found that the increase in hardness of the nugget portion could be suppressed, and the strength was higher than the base material strength, and the present invention was completed.

That is, in the method for producing an ultra-high-strength cold-rolled steel sheet according to the present invention, the chemical component is mass%, and C: 0.05-0.
15%, Si: 0.1 to 0.5% and 5 × C + Si:
0.5 to 1.0%, Mn: 1.5 to 2.5%, P:
0.02% or less, S: 0.005% or less, Al: 0.
02-0.06%, Mo: 0.005-0.3%, or further Cr: 0.1-0.5%, Ni: 0.1-
After hot rolling of steel containing 0.5% of one or two kinds, the balance being Fe and unavoidable impurities, at a finishing temperature of at least ₃ points of Ar and winding at 500 to 650 ° C, After pickling and cold rolling at a reduction of 30 to 70%, 82
Annealing by heating to an annealing temperature in the temperature range of 0 to 920 ° C, then gradually cooling to a quenching start temperature in the temperature range of 700 to 600 ° C, and subsequently 100 ° C at a cooling rate of 100 ° C / s or more After quenching to the following quenching end temperature, 18
The overaging treatment is performed at an overaging temperature within a temperature range of 0 to 300 ° C.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, a method for producing an ultra-high strength cold rolled steel sheet according to the present invention will be described in detail. First, the reasons for limiting the chemical components of the steel used in the present invention will be described. Hereinafter, the unit is mass%.

C: 0.05% to 0.15% C is necessary for mainly producing martensite by quenching after annealing and soaking to obtain the strength of the steel sheet.
In order to obtain a sufficient amount of martensite to secure a strength of 950 MPa or more, it is necessary to add at least 0.05%. However, when the addition amount exceeds 0.15%, the ductility decreases, and even when Si is restricted to the range described later, the hardness of the nugget portion is increased, so that the nugget portion becomes brittle and the seam weldability deteriorates. Therefore, the upper limit of the addition amount is set to 0.15%.

Si: 0.1-0.5%, 5 × C + Si (element symbol indicates the content of the element): 0.5-1.0% Si strengthens steel and improves ductility. Therefore, at least 0.1% is necessary. However, when the amount of Si is 0.5
%, The electric resistance value at the time of seam welding increases, the current concentrates on the tip of the electrode, and the calorific value increases. Therefore, the molten metal portion becomes narrow and high in temperature. For this reason, the hardness of the nugget part after cooling increases and becomes brittle, and when a tensile stress is applied, it breaks from the nugget part,
The rupture strength of the weld decreases. Therefore, the upper limit is 0.5%
And Furthermore, even if the amount of Si is set to 0.1 to 0.5% and the amount of C is set to 0.05 to 0.15%, in a region where both the amount of Si and the amount of C are large, the nugget is formed by the synergistic action of Si and C. In a region where both the Si content and the C content are small, the hardness of the nugget portion can be suppressed, but the strength of the steel sheet itself decreases and it becomes impossible to secure an ultra-high strength of 950 MPa or more. Therefore, the lower limit of 5 × C + Si is set to 0.5% and the upper limit is set to 1.0%.

Mn: 1.5 to 2.5% Mn enhances the partition ratio of C to the austenite phase in the two-phase state of austenite + ferrite during annealing, soaking, and slow cooling to improve the stability of the austenite phase. In the quenching process, the formation of martensite is facilitated, and the C content in the ferrite phase necessarily decreases, thereby improving ductility. If the content is less than 1.5%, the effect is too small. On the other hand, if the content exceeds 2.5%, the above effect is saturated and the workability is deteriorated due to segregation. And

P: not more than 0.02% P has the effect of strengthening steel, but the ductility is reduced due to embrittlement, so the upper limit is made 0.02%.

S: 0.005% or less S forms sulfide-based inclusions and degrades workability and weldability.

Al: 0.02 to 0.06% Al is added for the purpose of deoxidation. If it is less than 0.02%, its effect is too small. On the other hand, if it exceeds 0.06%, it is added. Since the effect is saturated, the upper limit is made 0.06%.

Mo: 0.05 to 0.3% Mo improves the austenite phase stability without forming an oxide film on the surface of the steel sheet and without increasing the electric resistance, and is effective for increasing the strength of martensite. Has the effect of promoting the creation of sites. If the content is less than 0.05%, the effect is too small. On the other hand, if the content exceeds 0.3%, the effect is saturated and the cost increases. Therefore, the upper limit is set to 0.3%.

The steel sheet of the present invention contains the above components, and the balance F
e and unavoidable impurities, but may further contain one or more of the following Cr and Ni as necessary.

Cr: 0.1-0.5% Cr enhances the stability of the austenite phase in the steel and promotes the formation of martensite, which is effective for increasing the strength. 0.01
When the amount is less than 0.5%, the effect is too small. On the other hand, when the amount exceeds 0.50%, the effect becomes saturated. Therefore, the lower limit is set to 0.01% and the upper limit is set to 0.50%.

Ni: 0.1 to 0.5% Ni, like Cr, enhances the stability of the austenite phase and promotes the formation of martensite. If the content is less than 0.1%, the effect is too small. On the other hand, if it exceeds 0.50%, the effect is saturated, and the cost is increased.
The lower limit is 0.1% and the upper limit is 0.5%.

Next, the manufacturing conditions will be described. The outline of the production method of the present invention, after melting the steel of the above chemical composition,
A slab is manufactured by continuous casting or ingot, hot-rolled, wound, pickled, cold-rolled, then annealed, and austenite is mainly converted to martensite in a cooling process after annealing. After the transformation, an overaging treatment is performed. Hereinafter, the invention conditions in each step will be described in detail.

The slab heating temperature in the hot rolling is heated to about 1100 to 1250 according to a conventional method. This heating may be once cooled to around room temperature after casting and re-heated,
Further, it may be inserted into a heating furnace at a high temperature and heated. Furthermore, after casting, it may be subjected to rolling at the same temperature.

In the hot rolling, the rolling is completed by setting the finishing temperature to at least _three points of Ar so as to avoid a sharp increase in deformation resistance.
The cooling rate from rolling to winding is not particularly limited. It may be cooled at about 30 to 100 ° C./s according to a conventional method. Thereafter, the film is wound at a winding temperature of 500 to 650 ° C. 500
If the temperature is lower than ℃, the steel sheet becomes hard and cold rolling becomes difficult. On the other hand, when the temperature exceeds 650 ° C., the crystal grain size becomes coarse and a non-uniform structure is easily generated. In addition, the concentration of Si in the surface layer is caused, and thus Si oxide is generated, thereby increasing the hardness of the nugget part during seam welding.

The hot rolled sheet thus obtained is subjected to cold rolling after pickling. The rolling reduction in cold rolling is
30% to 70%. If the thickness is less than 30%, the thickness of the hot-rolled sheet must be reduced to obtain a thin sheet of about 1 mm or less, and it becomes difficult to control the thickness of the hot-rolled sheet. Load becomes excessive.

After cold rolling, the cold rolled sheet is continuously annealed. The annealing temperature of the continuous annealing is performed at a temperature in the range of 820 to 920 ° C. If the temperature is lower than 820 ° C., an appropriate amount of austenite that becomes martensite by rapid cooling cannot be obtained, and the desired strength cannot be obtained, and the structure becomes non-uniform and the workability decreases. On the other hand, when the temperature exceeds 920 ° C., a complete austenite single phase is formed, and the formation of a ferrite phase in the slow cooling process becomes insufficient, and the workability is reduced. In addition, Si is concentrated on the surface layer, and Si oxide is generated. The annealing time may be set as long as the structure of the cold-rolled sheet is ferrite + austenite, and depends on the sheet thickness.
It takes only a few minutes.

After annealing, it is gradually cooled to a rapid cooling start temperature in the range of 700 to 600 ° C. Cooling rate is 1-30 ° C / se
A degree of c is preferred. By this slow cooling, an appropriate amount of ferrite and austenite in which C is enriched by the formation of ferrite are obtained, transformation of austenite to martensite is promoted, and a composite structure having good workability is obtained.

The quenching start temperature is an important condition for determining the amount of martensite. If the temperature exceeds 700 ° C., the amount of martensite becomes excessive and ductility decreases, while if it is less than 600 ° C., the amount of ferrite becomes excessive and the amount of martensite decreases. Becomes less than 50 area%, and the strength is deteriorated. The quenching cooling rate is 100 ° C./sec or more so that a martensitic structure can be obtained from austenite as much as possible. As a cooling method, an appropriate cooling method such as water quenching, water-cooled roll cooling, and air-water cooling can be adopted. The quenching stop temperature is 1 to transform austenite to martensite as much as possible.
It should be below 00 ° C.

Thereafter, overaging treatment is performed at a temperature in the range of 130 to 300 ° C. to remove C dissolved in ferrite.
To improve ductility. If the temperature is lower than 130 ° C., an effect of improving ductility cannot be expected, while if it is higher than 300 ° C., tempering of martensite occurs to cause deterioration in strength. The time for the overaging process may be about several minutes.

Under the above manufacturing conditions, the tensile strength is 950 M
An ultra-high-strength steel sheet containing Pa or more and containing martensite in an area ratio of 50% or more and the balance substantially consisting of ferrite is obtained. Hereinafter, the present invention will be described more specifically with reference to Examples, but the present invention is not construed as being limited to such Examples.

[0029]

EXAMPLE Steel slabs having the chemical compositions shown in Table 1 were used in the range of 1200 to 12
After heating to 20 ° C., hot rolling was performed to a thickness of 2.4 mm at a finishing temperature of 880 to 900 ° C., and 550 to 600 ° C. (Table 2).
No. 1-21), 710 ° C (No. 22 in Table 2), 935 ° C
(Table 2, No. 23). After pickling, cold rolling was performed to a sheet thickness of 1.2 mm, and continuous annealing was performed. At this time, the annealing temperature was 840 to 880 ° C., and the annealing time was about 90 seconds. After annealing, it was cooled at a rate of 10 ° C./s by gas jet cooling to the quenching start temperature shown in Table 2, water-quenched from the same temperature to room temperature, and then maintained at the overageing temperature shown in the same table for about 3 minutes. Aging treatment was applied.

[0030] Tensile test specimens were collected from the obtained cold-rolled steel sheets and examined for mechanical properties. In addition, a welded test piece (base material) was sampled, the ends of the base material were overlapped and seam-welded under the following conditions, and the Vickers hardness (Hv) of the nugget portion was measured. They were pulled together to examine the fracture site. The results of these surveys are also shown in Table 2. FIGS. 1 and 2 show graphs in which the relationship between the C amount and the Si amount, the tensile strength, and the hardness of the nugget part is arranged for Sample Nos. 1 to 16. The area surrounded by the solid line in the figure indicates the content range of the C content and the Si content of the present invention (however, errors are not taken into account). Each plot shows the tensile strength (FIG. 1) and the nugget portion. The measurement value of the hardness (FIG. 2) is attached.・ Seam welding conditions Electrode width: 6 mm, current: 30 kA, pressing force: 3.5 ton,
Speed: 8m / min

[0031]

[Table 1]

[0032]

[Table 2]

According to Table 2, FIG. 1 and FIG.
2, 3, 5 to 7, 9 to 11, 17, 19, and 21 all have a tensile strength of 950 MPa or more, a hardness of a nugget part of 420 Hv or less, and perform seam welding with a large heat input. No break occurred in the nugget part. In contrast, Nos. 1, 4, 8, 12 to 1 of the comparative examples
In Nos. 6, 18, and 20, the chemical components are out of the range of the present invention. In No. 1, the intended strength could not be secured, and in other samples, the hardness of the nugget part was too high, and the nugget part was broken. did. Further, even if the components of the present invention were satisfied, high strength could not be ensured in Nos. 22 and 23 where the production conditions were unsuitable, or the hardness of the nugget portion was increased and the nugget portion was broken.

[0034]

As described above, according to the present invention,
Despite having a tensile strength of 950 MPa or more, it has excellent seam weldability and can provide an ultra-high-strength thin steel sheet suitable as a strength member such as a bumper of an automobile. This can contribute to weight reduction.

[Brief description of the drawings]

FIG. 1 shows C content and S in a cold rolled steel sheet according to an example.
It is a graph which shows the relationship between i amount and tensile strength.

FIG. 2 shows C content and S in a cold rolled steel sheet according to an example.
It is a graph which shows the relationship between i amount and hardness (Vickers hardness) in a nugget part.

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Kuniyasu Araga 2-14-19 Minamieki Minamieki, Nakamura-ku, Nagoya City, Aichi Prefecture Sumitomo Life Nagoya Building Kobe Steel, Ltd. Nagoya Branch Office F-term (reference) 4K037 EA01 EA05 EA06 EA11 EA15 EA16 EA17 EA21 EA23 EA25 EA27 EB05 EC00 EC01 FA02 FA03 FB00 FD04 FE01 FE02 FG00 FH01 FJ05 FJ06 FK02 FK03 FK06 FK08 FL01

Claims (2)

[Claims] 1. The chemical component is mass% and C: 0.05 to
0.15%, Si: 0.1-0.5% and 5 × C + S
i: 0.5 to 1.0%, Mn: 1.5 to 2.5%, P
: 0.02% or less, S: 0.005% or less, Al:
0.02-0.06%, Mo: 0.005-0.3%,
Containing steel and the balance consisting of Fe and inevitable impurities
r Finish hot rolling at a finishing temperature of ₃ points or more, and
After winding at 50 ° C., pickling, cold rolling at a reduction of 30 to 70%, heating to an annealing temperature within a temperature range of 820 to 920 ° C., and thereafter 700 to 600 ° C
Slowly cooled to the quenching start temperature within the temperature range
After rapidly cooling to a quenching termination temperature of 100 ° C or less at a cooling rate of 00 ° C / s or more, an overaging treatment is performed at an overaging temperature within a temperature range of 180 to 300 ° C. Manufacturing method of excellent super high strength cold rolled steel sheet. 2. The chemical composition further includes Cr: 0.1 to
The method for producing an ultra-high-strength cold-rolled steel sheet according to claim 1, comprising one or two of 0.5% and Ni: 0.1 to 0.5%.