TW201402833A - High strength cold rolled steel sheet excellent in weldability and method for manufacturing the same - Google Patents

Abstract

Provided are a high-strength cold-rolled steel sheet which has a TS of 440MPa or above and excellent weldability and which neither forms humping beads even when subjected to high-speed plasma welding and nor brings about deterioration in the workability of a tailored blank, and a process for production of same.A A high-strength cold-rolled steel sheet which has a TS of 440MPa or above and excellent weldability, characterized by having a composition which contains by mass C: 0.0005 to 0.005%, Si: 0.1 to 1.0%, Mn: 1 to 2.5%, P: 0.01 to 0.2%, S: 0.015% or less, sol. Al: 0.05% or less, N: 0.007% or less, Ti: 0.01 to 0.1%, B: 0.0005 to 0.0020%, Cu: 0.05 to 0.5%, and Ni: 0.03 to 0.5% with the balance being Fe and unavoidable impurities and a ferrite single phase structure.

Description

High-strength cold-rolled steel sheet excellent in weldability and method of producing the same

The present invention relates to a high-strength cold-rolled steel sheet suitable for use in a structure such as a railway vehicle, an automobile, a ship, and the like, and having both workability and weldability, particularly a high-strength cold-rolled steel sheet having a tensile strength TS of 440 MPa or more and Production method.

With the progress of the degassing technology in the steel making process, the ultra-low carbon steel in which the amount of C in the steel is reduced to 0.0030% by mass or less can be mass-produced relatively inexpensively, and a nitrogen carbide forming element such as Ti or Nb is added thereto. It is an so-called IF (Interstitial Free) steel cold-rolled steel sheet, which is widely used in automotive parts and electrical equipment parts. Therefore, various types of IF steel-type cold-rolled steel sheets have been developed. For example, in Patent Document 1 and Patent Document 2, it is proposed to further add B to IF steel to which Ti and Nb are added, and to improve secondary work brittleness. A cold-rolled steel sheet excellent in workability. Further, in Patent Document 3, it is disclosed that Ni is further added to the IF steel to which Ti and Nb are added, and a steel sheet for deep drawing which imparts excellent brazed welding is obtained.

On the other hand, in recent years, in the steel sheet for automobiles, from the viewpoint of weight reduction and collision safety of the vehicle body, high strength is required, and the steps are shortened and the number of molds is reduced. There is a tailored blank material in which two or more steel sheets having different thicknesses and characteristics are welded and integrated. Therefore, there is a strong desire for a high-strength steel sheet having excellent workability and weldability, and particularly a high-strength cold-rolled steel sheet having a TS of 440 MPa or more.

Regarding the processability, although the IF steel type cold-rolled steel sheet as described above is desired, the weldability of the tailor-welded material using the IF steel type cold-rolled steel sheet has not been discussed. The welding property of the related tailor-welded material is proposed in Patent Document 4: a method of manufacturing a tailor-welded material by using a plasma welding material which is inexpensive, can be welded at a high speed, and is welded by plasma welding without using a welding material. In the case where the amount of the steel sheet C on the thick side is set to be 0.1% by mass or more, and the amount of Si is set to 0.8% by mass or more, the method of preventing the occurrence of welding defects of the so-called "humping bead" is prevented.

[Previous Technical Literature]

Patent Document 1: Japanese Patent Laid-Open No. 61-246344

Patent Document 2: Japanese Patent Laid-Open No. Hei 1-149943

Patent Document 3: Japanese Patent Laid-Open No. 2-232342

Patent Document 4: Japanese Patent Laid-Open Publication No. 2003-94170

However, in the method for producing a tailor-welded material described in Patent Document 4, it is necessary to set the amount of C of at least one of the steel sheets to 0.1% by mass or more, or to set the amount of Si to 0.8% by mass or more, which may result in a tailor-welded material. The problem of the processability is obviously deteriorated.

Further, if the welding speed of the plasma arc welding system is fast, the weld bead is easily formed, and thus the welding speed is difficult to increase in speed, that is, it is difficult to improve the productivity. For this issue, The object of the present invention is to improve the speed of welding by improving the steel sheet.

That is, an object of the present invention is to provide a high-strength cold-rolled steel sheet having excellent weldability of TS of 440 MPa or more without causing weld bead bulging even at high speed, without causing deterioration of workability of the tailor-welded material. And its manufacturing method.

The present inventors have dealt with the processability and weldability of a high-strength cold-rolled steel sheet having a TS of 440 MPa or more without causing weld bead bulging even if high-speed plasma welding is performed, and the workability of the tailor-welded material is not deteriorated. After intensive research, the following items were found.

i) By adding Cu to the IF steel, it is possible to suppress the occurrence of weld bead bulging during high-speed plasma welding, and to further limit the amount of O in the steel to an appropriate range, thereby further preventing weld beading during high-speed plasma welding. situation.

Ii) By setting the amount of C to 0.005 mass% or less and adding Ti to form an IF steel and forming a structure of a single phase of the ferrite core, excellent workability can be obtained even if a tailor welded material is formed.

The present invention has been completed based on the above findings, and a high-strength cold-rolled steel sheet having a TS of 440 MPa or more and excellent weldability is characterized by having C: 0.0005 to 0.005% by mass, Si: 0.1 to 1.0%, and Mn. : 1 to 2.5%, P: 0.01 to 0.2%, S: 0.015% or less, sol. Al: 0.05% or less, N: 0.007% or less, Ti: 0.01 to 0.1%, B: 0.0005 to 0.0020%, Cu: 0.05 ~0.5%, Ni: 0.03~0.5%, and the rest consists of Fe and unavoidable impurities, and has a structure consisting of a single phase of ferrite.

The high-strength cold-rolled steel sheet of the present invention preferably further contains at least one of O: 0.0025 to 0.0080%, Se: 0.0005 to 0.01%, and Te: 0.0005 to 0.01% by mass%.

The high-strength cold-rolled steel sheet according to the present invention is subjected to hot rolling after being subjected to hot rolling, and is subjected to hot rolling at a winding temperature of 680 ° C or lower, and after pickling, cold rolling is performed at a rolling ratio of 40% or more. Then, it can be manufactured by performing recrystallization annealing at a temperature range of 700 to 850 °C.

According to the present invention, it is possible to manufacture a high-strength cold-rolled steel sheet which is excellent in weldability in which TS does not cause weld bead bulging and which does not cause deterioration of workability of the tailor-welded material even if the welding speed is increased by 440 MPa or more. . Further, since the high-strength cold-rolled steel sheet of the present invention has excellent workability, it is suitable not only for automobile parts but also for electrical machine parts.

Hereinafter, the details of the present invention will be described. In addition, "%" indicating the amount of component elements means "% by mass" unless otherwise stated.

1) Composition

C: 0.0005~0.005%

If the amount of C is less than 0.0005%, the decarburization refining load in the steel making stage will increase, and the cost such as vacuum degassing will increase. On the other hand, if the amount of C exceeds 0.005%, the workability is deteriorated. Therefore, the C amount is set to 0.0005 to 0.005%. Preferably, the amount of C is made 0.003% or less.

Si: 0.1~1.0%

The Si system is an effective element for increasing the strength of steel. In order to obtain this effect, it is necessary to set the amount of Si to 0.1% or more. However, if the amount of Si exceeds 1.0%, it will cause iron embrittlement of the fat, and damage the strength-ductility balance. Therefore, the amount of Si is set to 0.1 to 1.0%. It is preferable to set the amount of Si to 0.7% or less.

Mn: 1~2.5%

Mn is an effective element for increasing the strength of steel. In order to obtain this effect, it is necessary to set the amount of Mn to be 1% or more. However, if the amount of Mn exceeds 2.5%, there is a problem that the center segregation in the slab is promoted or the workability of the final product is deteriorated. Therefore, the amount of Mn is set to 1 to 2.5%. In addition, from the viewpoint of preventing hot brittleness due to FeS formation, the amount of Mn is set to [Mn] and the amount of S is set so that Mn is bonded to the solid solution S in the steel to form MnS. In the case of [S], it is preferable to satisfy ([Mn]/55)/([S]/32)>100.

P: 0.01~0.2%

P is an effective element for the high strength of steel. In order to obtain this effect, it is necessary to set the amount of P to 0.01% or more. However, when the amount of P exceeds 0.2%, there is a concern that the grain boundary is broken in the HAZ portion, and the low-temperature toughness of the base material and the welded portion is deteriorated, and the impact resistance is deteriorated due to grain boundary segregation. Therefore, the amount of P is set to 0.01 to 0.2%.

S: 0.015% or less

When the amount of S exceeds 0.015%, it is like P, which causes deterioration of the low temperature toughness of the base material and the welded portion. Therefore, the amount of S is set to 0.015% or less, and the less the better. Further, as described above, it is preferable to satisfy ([Mn]/55)/([S]/32)>100.

sol.Al: 0.05% or less

Al is usually used as a deoxidizing element in the steel making stage. In the present invention, in order to control oxygen within a specific range, the amount of sol. Al is set to 0.05% or less. When the amount of sol.Al exceeds 0.05%, the amount of Al ₂ O ₃ increases, and in addition to deterioration in workability, the medium also causes the medium to become a starting point of the weld crack, and thus it is preferably avoided. Therefore, the amount of sol. Al is set to 0.05% or less.

N: 0.007% or less

If the amount of N exceeds 0.007%, workability and aging resistance are deteriorated. Therefore, the amount of N is set to be less than 0.007%, and the less the better.

Ti: 0.01~0.1%

The Ti system forms precipitates with C or N, and improves workability and aging resistance. In order to obtain this effect, it is necessary to set the amount of Ti to 0.01% or more. However, if the amount of Ti exceeds 0.1%, the alloy cost increases. Therefore, the amount of Ti is set to 0.01 to 0.1%. In addition, in order to effectively exhibit the effect of B below, when the amount of Ti is [Ti] and the amount of N is [N], it is preferable to satisfy [N]-(14/48)[Ti]≦0. .

B: 0.0005~0.0020%

If the B system exists in a solid solution state, the secondary work embrittlement resistance can be improved. In order to obtain this effect, it is necessary to set the amount of B to 0.0005% or more. However, if the amount of B exceeds 0.0020%, it will promote the fusion crack. Therefore, the amount of B is set to 0.0005 to 0.0020%.

Cu: 0.05~0.5%

Cu is an effective element for increasing the strength without deteriorating the workability and preventing the occurrence of bead bulging when the high-speed plasma is welded. In particular, when O coexists with steel in the range described later, the effect is improved. In order to obtain this effect, it is necessary to set the amount of Cu to 0.05% or more.

However, if the amount of Cu exceeds 0.5%, the effect is saturated and the surface properties are significantly deteriorated. Therefore, the amount of Cu is set to 0.05 to 0.5%. In addition, the reason why it is possible to prevent the occurrence of bead bulging at the time of high-speed plasma welding in the coexistence with Cu and O is not clear, but it is considered that the viscosity of the molten steel is properly adjusted at the time of welding, and the molten steel is improved. Due to the fluidity.

Ni: 0.03~0.5%

When the Cu content is set as described above, the surface properties are likely to deteriorate. In order to prevent this, it is necessary to add 0.03% or more of Ni. However, if the amount of Ni exceeds 0.5%, the effect is saturated. Therefore, the amount of Ni is set to 0.03 to 0.5%. Further, when the amount of Ni is [Ni] and the amount of Cu is [Cu], it is preferable to satisfy 0.25 × [Cu] ≦ [Ni] ≦ 0.75 × [Cu].

The remainder is Fe and unavoidable impurities, and it is preferable to contain at least one of O: 0.0025 to 0.0080%, Se: 0.0005 to 0.01%, and Te: 0.0005 to 0.01% for the following reasons.

O: 0.0025~0.0080%

As described above, in the coexistence with Cu, it is possible to further suppress the occurrence of bead bulging at the time of high-speed plasma welding. The reason is judged by the fact that in the coexistence with Cu, it is possible to make the molten steel viscous ‧ surface tension at the time of welding more appropriate. In order to obtain this effect, it is necessary to set the amount of O in the steel to 0.0025% or more. The better setting is more than 0.0040%. However, if the amount exceeds 0.0080%, in addition to the saturation of this effect, a large number of blow holes may occur in the continuously cast slab, resulting in an increase in the disposal cost of the slab surface or a deterioration in the workability of the steel sheet due to an increase in the medium. influences.

Se: 0.0005~0.01%, Te: 0.0005~0.01%

Se, Te is like O, and by the coexistence with Cu, the molten steel can be welded. Viscosity ‧ Surface tension is appropriate to prevent weld bead bulging during high-speed plasma welding. In order to obtain this effect, both the amount of Se and Te must be set to 0.0005% or more. However, if the amount of Se and Te exceeds 0.01%, the effect is saturated.

2) Organization

From the viewpoint of workability, it is necessary to set the structure of the ferrite-iron single phase. The "fertilizer iron single phase" may be any one of a polygonal ferrite phase and a bainitic ferrite phase, or may be a mixture of the two. Further, in order to secure a TS of 440 MPa and to prevent extreme softening of the welded portion, it is preferred that the ferrite phase has an average particle diameter of 50 μm or less.

3) Manufacturing conditions

The high-strength cold-rolled steel sheet according to the present invention is subjected to hot rolling after being subjected to hot rolling, and is subjected to hot rolling at a winding temperature of 680 ° C or lower, and after pickling, cold rolling is performed at a rolling ratio of 40% or more. Then, it can be manufactured by performing recrystallization annealing in a temperature range of 700 to 850 °C.

Winding temperature after hot rolling: 680 ° C or less

When the winding temperature exceeds 680 ° C, a compound such as Fe or Ti and P is easily formed, which hinders the development of a {111} aggregate structure which is excellent in deep drawability at the subsequent cold rolling-annealing. Therefore, the winding temperature is set to be 680 ° C or lower. More preferably 650 ° C or less.

Rolling rate of cold rolling: 40% or more

From the viewpoint of workability, the rolling rate is set to 40% or more. From the viewpoint of moldability and particularly deep drawability, the rolling rate is preferably set to 50% or more.

Recrystallization annealing temperature: 700~850 °C

In order to carry out recrystallization, it is necessary to set the annealing temperature to 700 ° C or higher. However, if it exceeds 850 ° C, the ferrite particles are coarsened, and the strength is lowered and the surface properties are deteriorated. Therefore, the recrystallization annealing temperature is set to 700 to 850 °C. From the viewpoint of sufficiently performing recrystallization, it is preferably maintained in a temperature region of 750 ° C or higher for 30 seconds or longer.

Other manufacturing conditions are applicable to the usual methods. That is, the steel is melted by a converter or an electric furnace, and continuous casting is performed to form a slab. Further, when the slab is subjected to hot rolling, the following method may be employed: a method in which rolling is performed after heating, a method in which rolling is performed without heating, a method in which a short-time heat treatment is performed, and then rolling is performed. . The hot rolling system may be carried out by rolling at a finishing temperature equal to or higher than the Ar ₃ transformation point in accordance with a usual method. The recrystallization annealing system may be any one of a box annealing method, a continuous annealing method, and the like. After annealing, skin rolling such as surface roughness adjustment, plate shape flattening, and the like can be performed. Further, surface treatment such as chemical conversion treatment or plating treatment may be performed later.

[Example 1]

The basic composition C of Cu, O, and Se shown in Table 1 is 0.002%, Si: 0.2%, Mn: 1.8%, P: 0.05%, S: 0.005%, sol. Al: 0.02%, and N: 0.003%. Steels No. 1 to 7 of Ti: 0.04% and B: 0.0010% were melted by a vacuum melting method, heated at 1200 ° C for 1 hr, and then subjected to rough rolling to obtain a sheet having a thickness of 35 mm. After the sheet was heated at 1250 ° C for 1 hr, the finish rolling was carried out in such a manner that the temperature at the inlet side was 900 ° C, and the heat treatment was performed at 580 ° C for 1 hr. Hot rolled sheet 4 mm thick. The hot-rolled sheet was removed by pickling, and cold-rolled at a rolling rate of 60% to obtain a cold-rolled sheet having a thickness of 1.6 mm, which was heated at 830 ° C for 180 sec using a salt bath. Recrystallization annealing is performed at a cooling rate of 10 ° C / sec, and then, After pickling, the salt adhered to the surface of the steel sheet was removed, and then the skin was rolled at a stretch ratio of 0.5%.

Then, for the steel plates of the same composition, the welding current: 60 A, the Ar gas flow rate for plasma: 0.6 L/min, the Ar gas flow rate for shielding: 10 L/min, and the nozzle diameter: 2.0 mm. The nozzle-sample distance: 3 mm is set to a certain value, but when the welding speed is changed within the range of 0.2 to 1.4 m/min, plasma welding is performed, and the occurrence of the weld bead is detected (×). (○).

The results are shown in Table 1.

It is known that in the conventional plasma welding, the boundary speed at which the welding can be performed without the occurrence of the bead bulging is about 0.2 to 0.4 m/min, whereas the present invention contains the sample of Cu (steel No. 3). Even if it is set to a high-speed welding speed of 1 m/min, even a sample containing more than O or Se (steel No. 4 to 7), even if it is set to a high-speed welding speed of 1 m/min or more, the bead bulging does not occur. It is excellent in high-speed plasma welding.

[Embodiment 2]

The steel No. A to F having the composition shown in Table 2 was melted, and the slab was formed by continuous casting, and then heated at 1200 ° C, and then subjected to finish rolling at a finishing temperature of 900 ° C. The winding temperature of °C was applied to form a hot rolled sheet having a thickness of 6 mm and 4 mm. After the hot-rolled sheet is pickled, cold rolling is performed at a rolling rate of 60% to form a cold-rolled sheet having a thickness of 2.4 mm and 1.6 mm, and continuous annealing is performed at an annealing temperature of 830 ° C, and then a tensile ratio of 0.5% is applied. The skin is rolled.

Then, the steel plates of the same composition are combined with the plate thickness shown in Table 3, and the welding current is 60 A, the flow rate of Ar gas for plasma is 0.6 L/min, the flow rate of Ar gas for shielding: 10 L/min, and the nozzle diameter: 2.0mm In the nozzle-sample distance: 3 mm, the welding speed: 1 m/min, plasma welding was performed, and (×) none (○) occurred in the investigation of the weld bead bulging. Further, the right angle direction TS, the total elongation El, and the average r value of the rolling direction of the obtained steel sheet were investigated using JIS No. 5 test piece.

The results are shown in Table 3. It is known that a steel sheet having a composition of the present invention has a TS of 440 MPa or more, and is excellent in workability, and the bead bulging does not occur even in high-speed plasma welding.

Claims (3)

A high-strength cold-rolled steel sheet having a tensile strength of 440 MPa or more and excellent weldability, comprising C: 0.0005 to 0.005% by mass, Si: 0.1 to 1.0%, and Mn: 1 to 2.5%, P. : 0.01 to 0.2%, S: 0.015% or less, sol. Al: 0.05% or less, N: 0.007% or less, Ti: 0.01 to 0.1%, B: 0.0005 to 0.0020%, Cu: 0.05 to 0.5%, Ni: 0.03 ~0.5%, the rest consists of Fe and unavoidable impurities, and has a structure consisting of a single phase of ferrite. A high-strength cold-rolled steel sheet having a tensile strength of 440 MPa or more and excellent weldability as in the first aspect of the patent application, wherein the amount of Ni is [Ni] and the amount of Cu is [Cu], which satisfies 0.25 × [Cu]≦[Ni]≦0.75×[Cu]. A method for producing a high-strength cold-rolled steel sheet having a tensile strength of 440 MPa or more and excellent weldability, wherein the steel slab having the composition of the first or second aspect of the patent application is subjected to hot rolling, and is subjected to a winding temperature of 680 ° C or lower. After winding, after pickling, cold rolling is performed according to a rolling rate of 40% or more, and then recrystallization annealing is performed at a temperature range of 700 to 850 °C.