KR20110025877A - High-strength cold-rolled steel sheet excellent in weldability and process for production of same - Google Patents

Abstract

Provided is a high strength cold rolled steel sheet having excellent weldability of TS of 440 MPa or more, which does not generate humping beads even when high-speed plasma welding is performed, and does not cause deterioration of workability of the tailored blank material, and a method of manufacturing the same. In 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% Hereinafter, a structure containing 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%, having a composition consisting of residual Fe and unavoidable impurities, and further comprising a ferrite single phase. A high strength cold rolled steel sheet having excellent weldability, characterized by having a TS of 440 MPa or more.

Description

High strength cold rolled steel sheet with excellent weldability and manufacturing method thereof {HIGH-STRENGTH COLD-ROLLED STEEL SHEET EXCELLENT IN WELDABILITY AND PROCESS FOR PRODUCTION OF SAME}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high strength cold rolled steel sheet having workability and weldability suitable for use in structures of railroad cars, automobiles, ships, and the like, in particular, a high strength cold rolled steel sheet having a tensile strength (TS) of 440 MPa or more, and a method of manufacturing the same.

With advances in degassing techniques in steelmaking processes, ultra-low carbon steels having a reduced amount of C in steel to 0.0030% by mass or less can be produced in large quantities at a relatively low cost, thereby forming a nitride-forming element such as Ti or Nb. The so-called IF (Interstitial Free) steel type cold rolled steel sheet, which has been added with excellent workability, has been widely used in automobile parts and electrical appliance parts. Therefore, cold rolled steel sheets of various IF steel types have been developed. For example, in Patent Document 1 and Patent Document 2, B is further added to IF steel containing Ti or Nb to improve secondary work brittleness. The cold rolled steel sheet which is excellent in the workability which aimed at this is proposed. In addition, Patent Document 3 discloses a thin steel sheet for deep drawing processing in which Ni is added to IF steel to which Ti or Nb is added to provide excellent soldering weldability.

On the other hand, in recent years, steel sheets for automobiles have been strengthened from the viewpoint of weight reduction of the vehicle body and crash safety, and welding two or more steel sheets having different plate thicknesses and properties for the purpose of shortening the process and reducing the number of molds. Thus, a tailored blank material was used. Therefore, the demand for the high strength steel plate which has the outstanding workability and weldability, especially the high strength cold rolled steel plate whose TS is 440 Mpa or more, is becoming strong.

As for the workability, a cold rolled steel sheet of the IF steel type as described above is desired, but hardly the weldability of the tailored blank material using the cold rolled steel sheet of the IF steel type is studied. Regarding the weldability of the tailored blank material, Patent Document 4 discloses a steel sheet having a different plate thickness in a method of producing a tailored blank material by plasma welding in which equipment cost is low and high-speed welding is possible, and also no welding material is used. The method of preventing the generation of a welding defect called a humping bead is proposed by making C amount of the steel plate of the side of a thick plate thickness into 0.1 mass% or more, or Si amount into 0.8 mass% or more.

Japanese Laid-Open Patent Publication No. 61-246344 Japanese Patent Laid-Open No. 1-149943 Japanese Patent Application Laid-open No. Hei 2-232342 Japanese Laid-Open Patent Publication 2003-94170

However, in the manufacturing method of the tailored blank material of patent document 4, it is necessary to make C amount of at least one steel plate 0.1 mass% or more or Si amount 0.8 mass% or more, and the problem that the workability of a tailored blank material is remarkably inferior. There is.

In addition, the plasma arc welding has a problem that it becomes difficult to form the humping beads when the welding speed is increased, so that the welding speed is high, that is, the productivity is difficult to improve. With respect to this problem, the present invention aims to achieve higher welding speed by improving the steel sheet.

That is, the present invention provides a high-strength cold rolled steel sheet having excellent weldability of TS of 440 MPa or more, which does not generate humping beads even when plasma welding is performed at high speed, and does not cause deterioration of workability of the tailored blank material, and a method of manufacturing the same. It aims to do it.

MEANS TO SOLVE THE PROBLEM The present inventors earnestly examined the workability and weldability of the high strength cold-rolled steel sheet whose TS is 440 Mpa or more, which does not generate a humping bead even if it speeds up plasma welding, and does not cause deterioration of the workability of a tailored blank material, We found the following knowledge.

I) By adding Cu to the IF steel, suppression of the hump bead generation during high-speed plasma welding, and by further defining the amount of O in the steel in an appropriate range, it is possible to further prevent the generation of the hump bead during high-speed plasma welding.

Ii) By setting the amount of C to 0.005 mass% or less, adding Ti to an IF steel, and forming a ferrite single phase structure, excellent workability can be obtained even as a tailored blank material.

The present invention has been made based on the above findings, and in 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, 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% to 0.5%, and the balance Fe and inevitable Provided is a high strength cold rolled steel sheet excellent in weldability having a composition made of impurities and having a structure composed of a ferrite single phase, having a TS of 440 MPa or more.

It is preferable that the high strength cold-rolled steel sheet of this invention further contains at least 1 sort (s) of O: 0.0025-0.0080% or Se: 0.0005-0.01% and Te: 0.0005-0.01% by mass.

The high strength cold-rolled steel sheet of the present invention is wound up at a winding temperature of 680 ° C. or lower after hot rolling the slab having the composition as described above, after pickling, cold rolled to 40% or more of a reduction ratio, and subsequently to a temperature of 700 to 850 ° C. It can manufacture by the method of performing recrystallization annealing in the range.

According to the present invention, it is possible to manufacture a high strength cold rolled steel sheet having excellent weldability of TS of 440 MPa or more, which does not generate humping beads and cause deterioration of workability of the tailored blank material even when the plasma welding is speeded up. Moreover, since the high strength cold rolled sheet steel of this invention is equipped with the outstanding workability, it is suitable not only for automobile parts but also for electrical equipment components.

The detail of this invention is demonstrated below. In addition,% which shows the quantity of a component element means the mass% unless there is particular notice.

1) Composition

C: 0.0005% to 0.005%

When the amount of C is less than 0.0005%, the load of decarburization and refining in the steelmaking stage becomes high, and the cost of vacuum degassing and the like also increases. Moreover, when C amount exceeds 0.005%, deterioration of workability will be caused. Therefore, the amount of C is made into 0.0005 to 0.005%. Preferably, the amount of C is made into 0.003% or less.

Si: 0.1% to 1.0%

Si is an element effective for increasing the strength of steel. In order to acquire such an effect, Si amount needs to be 0.1% or more. However, if the amount of Si exceeds 1.0%, it will cause embrittlement of ferrite and damage the strength-ductility balance. Therefore, the amount of Si is made into 0.1 to 1.0%. Preferably, the amount of Si is made into 0.7% or less.

Mn: 1 to 2.5%

Mn is an element effective for increasing the strength of steel. In order to acquire such an effect, Mn amount needs to be 1% or more. However, when Mn amount exceeds 2.5%, there exists a problem of encouraging center segregation in a slab, or degrading the workability of a final product. Therefore, the amount of Mn is made into 1 to 2.5%. In addition, from the viewpoint of preventing hot brittleness due to FeS formation, when Mn is set to [Mn] and S is set to [S] in order to form MnS in combination with solid solution S in steel, ] / 55) / ([S] / 32)> 100 is preferable.

P: 0.01% to 0.2%

P is an element effective for high strength of steel. In order to acquire such an effect, P amount needs to be 0.01% or more. However, when the P content exceeds 0.2%, not only the grain boundary breakdown in the HAZ portion and the low temperature toughness of the base material and the weld portion are degraded, but also the grain boundary segregation causes the impact resistance to deteriorate. Therefore, P amount is made into 0.01 to 0.2%.

S: 0.015% or less

When the amount of S exceeds 0.015%, the low temperature toughness of a base material and a weld part will be degraded similarly to P. Therefore, the amount of S is made into 0.015% or less, and it is so preferable that there is little. Moreover, it is preferable to satisfy ([Mn] / 55) / ([S] / 32)> 100 as mentioned above.

sol.Al: 0.05% or less

Al is usually used as a deoxidation element in the steelmaking stage. In the present invention, the amount of sol.Al is made 0.05% or less in order to control oxygen in a specific range. When the amount of sol.Al exceeds 0.05%, Al ₂ O ₃ increases, and in addition to deteriorating workability, inclusions may be a starting point of weld cracking, which is not preferable. Therefore, the amount of sol.Al is made into 0.05% or less.

N: 0.007% or less

When the amount of N exceeds 0.007%, deterioration of workability and aging resistance is caused. Therefore, the amount of N is made into 0.007% or less, but smaller is more preferable.

Ti: 0.01% to 0.1%

0 을 만족시키는 것이 바람직하다. Ti forms precipitates with C or N to improve workability and age resistance. In order to acquire such an effect, Ti amount needs to be 0.01% or more. However, when Ti amount exceeds 0.1%, an alloy cost will increase. Therefore, Ti amount may be 0.01 to 0.1%. In order to effectively exhibit the effects of B described below, when the Ti amount is [Ti] and the N amount is [N], [N]-(14/48) [Ti]

It is desirable to satisfy zero.

B: 0.0005% to 0.0020%

When B exists in solid solution, the secondary processing brittleness is improved. In order to acquire such an effect, the amount of B needs to be 0.0005% or more. However, when the amount of B exceeds 0.0020%, a welding crack is encouraged. Therefore, the amount of B is made into 0.0005 to 0.0020%.

Cu: 0.05% to 0.5%

Cu can achieve high strength without degrading workability and is an effective element for preventing the generation of humping beads during high-speed plasma welding. In particular, the effect becomes high under coexistence with O in the steel of the range mentioned later. In order to acquire such an effect, Cu amount needs to be 0.05% or more.

However, when the amount of Cu exceeds 0.5%, the effect will be saturated and a significant deterioration of surface property will be caused. Therefore, the amount of Cu is made into 0.05 to 0.5%. In addition, although the reason which can prevent generation of humping bead at the time of high-speed plasma welding in the presence of Cu and O is not necessarily clear, it is thought that it is because the viscosity of molten steel at the time of welding is optimized and the flowability of molten steel improves.

Ni: 0.03 to 0.5%

[Ni]

0.75×[Cu] 를 만족시키는 것이 바람직하다. When content of Cu is carried out as mentioned above, surface property will deteriorate easily. In order to prevent it, 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, Ni amount is made into 0.03 to 0.5%. In addition, when Ni amount is [Ni] and Cu amount is [Cu], 0.25x [Cu]

[Ni]

It is preferable to satisfy 0.75x [Cu].

The balance is Fe and an unavoidable impurity. Further, for the following reasons, at least one of O: 0.0025 to 0.0080%, Se: 0.0005 to 0.01% and Te: 0.0005 to 0.01% is preferably contained.

O: 0.0025% to 0.0080%

As mentioned above, generation | occurrence | production of the humping bead at the time of high speed plasma welding in coexistence with Cu can be suppressed further. It is thought that the viscosity and surface tension of molten steel at the time of welding are further optimized under coexistence with Cu. In order to acquire such an effect, O amount in steel needs to be 0.0025% or more. More preferably, you may be 0.0040% or more. However, when the amount exceeds 0.0080%, the effect is not only saturated, but also blowholes of the continuous casting slab increase, and the cost of the slab surface is increased or the inclusions increase, which adversely affects the workability of the steel sheet.

Se: 0.0005 to 0.01%, Te: 0.0005 to 0.01%

Se and Te, like O, optimize the viscosity and surface tension of molten steel at the time of coexistence with Cu, and prevent generation of humping beads at the time of high speed plasma welding. In order to acquire such an effect, Se and Te amount should be 0.0005% or more. However, when Se or Te amount exceeds 0.01%, the effect will be saturated.

2) organization

It is necessary to make a ferrite single phase structure from a workability viewpoint. Here, the ferrite single phase may be either a polygonal ferrite phase or a bainitic ferrite phase, or both may be mixed. In order to secure a TS of 440 MPa and to prevent extreme softening of the welded portion, the average particle diameter of the ferrite phase is preferably 50 µm or less.

3) manufacturing condition

The high strength cold-rolled steel sheet of the present invention is wound up at a winding temperature of 680 ° C. or lower after hot rolling the slab having the composition as described above, after pickling, cold rolled to 40% or more of a reduction ratio, and subsequently to a temperature of 700 to 850 ° C. It can manufacture by the method of performing recrystallization annealing in the range.

Winding temperature after hot rolling: 680 ℃ or less

When the coiling temperature exceeds 680 ° C., compounds of Fe, Ti, and P are liable to occur, which inhibits the development of {111} texture that is suitable for deep drawing workability during subsequent cold rolling and annealing. Therefore, the coiling temperature is at most 680 ° C. More preferably, it is 650 degrees C or less.

Rolling rate of cold rolling: 40% or more

From the viewpoint of workability, the reduction ratio is 40% or more. It is preferable to make a rolling reduction into 50% or more from a viewpoint of moldability, especially a deep drawing property improvement.

Recrystallization Annealing Temperature: 700 ~ 850 ℃

In order to recrystallize, annealing temperature needs to be 700 degreeC or more, but when it exceeds 850 degreeC, a ferrite particle will coarsen and it will lead to a fall of strength and surface property deterioration. Therefore, recrystallization annealing temperature shall be 700-850 degreeC. It is preferable to hold | maintain for 30 second or more in the temperature range 750 degreeC or more from a viewpoint to fully recrystallize.

Other manufacturing conditions can apply a conventional method. That is, steel is melted in a converter or an electric furnace, and it casts continuously and is made into a slab. In addition, in order to hot-roll a slab, it can carry out by the method of rolling after a heating, the method of rolling directly, without heating, the method of performing a short-time heat processing, and rolling. Hot rolling, when rolling is finished at a temperature above Ar ₃ transformation point in a conventional manner. Recrystallization annealing can be performed by either box annealing or continuous annealing. After annealing, skin pass rolling for the purpose of adjusting the surface roughness, flattening the plate shape, and the like can be performed. Moreover, after that, surface treatment such as chemical conversion treatment or plating treatment can also be performed.

Example 1

Basic composition containing Cu, O, Se shown in Table 1 0.002% C-0.2% Si-1.8% Mn-0.05% P-0.005% S-0.02% sol.Al-0.003% N-0.04% Ti-0.0010% Steel No. 1-7 of B was solvent-processed by the vacuum dissolution method, and after 1-hr heating at 1200 degreeC, the rough rolling was performed and the sheet bar of 35 mm of thickness was produced. After heating this sheet bar at 1250 degreeC for 1 hr, it finish-rolled so that the final rolling side temperature might be 900 degreeC in 7 passes, and it carried out the winding equivalent heat processing of 1 hr at 580 degreeC, and it was set as the hot-rolled sheet of 4 mm thickness. . The hot rolled sheet was descaled by pickling, cold rolled at a reduction ratio of 60% to form a cold rolled sheet having a thickness of 1.6 mm, and then cooled at 10 ° C./sec after heating for 180 sec at 830 ° C. using a salt bath. After recrystallization annealing to cool by, followed by pickling to remove the salt adhering to the surface of the steel sheet, skin pass rolling of 0.5% elongation was performed.

And steel sheets of the same composition were welded: 60 A, flow rate of Ar gas for plasma: 0.6 l / min, flow rate of Ar gas for shield: 10 l / min, nozzle diameter: 2.0 mmφ, nozzle-sample distance: The conditions of 3 mm were made constant, the welding speed was changed to 0.2-1.4 m / min, plasma welding was performed, and the presence or absence (x) of humping bead generation was investigated.

The results are shown in Table 1.

In the conventional plasma welding, the limit speed that can be welded without generating a humping bead was about 0.2 to 0.4 m / min, whereas the sample (steel No. 3) containing Cu of the present invention was a high-speed welding of 1 m / min. Even if it is a speed, it can be seen that a sample (steel Nos. 4 to 7) containing O or Se further does not generate humping beads even at a high welding speed of 1 m / min or more and is excellent in high-speed plasma weldability. .

Example 2

After steel No.A-F of the component composition shown in Table 2 was made into a slab by continuous casting, after heating at 1200 degreeC, it finish-rolled at the finishing temperature of 900 degreeC, and it wound up at the winding temperature of 580 degreeC. It wound up and it was set as the hot rolled sheet of plate thickness 6mm and 4mm. After pickling, the hot rolled sheet was cold rolled at a reduction ratio of 60% to form a cold rolled sheet having a sheet thickness of 2.4 mm and 1.6 mm, continuously annealed at an annealing temperature of 830 ° C., and subjected to skin pass rolling with an elongation rate of 0.5%.

And the steel plates of the same composition are combined with the plate thickness shown in Table 3, and welding current: 60 A, flow volume of Ar gas for plasma: 0.6 L / min, flow rate of Ar gas for shield: 10 L / min, nozzle diameter: Plasma welding was carried out under the conditions of 2.0 mmφ, nozzle-sample distance: 3 mm, welding speed: 1 m / min, and the presence or absence (*) of humping bead generation was investigated. In addition, TS, total elongation E1, and average r value of the direction orthogonal to the rolling direction of the obtained steel plate were investigated using the JIS No. 5 test piece.

The results are shown in Table 3. As for the steel plate which has the component composition of the example of this invention, TS of 440 Mpa or more is obtained, it is excellent in workability, and it turns out that a humping bead does not generate | occur | produce at the time of high speed plasma welding.

Claims (4)

In 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% Hereinafter, Ti: 0.01 to 0.1%, B: 0.0005 to 0.0020%, Cu: 0.05 to 0.5%, Ni: 0.03 to 0.5%, and have a composition consisting of the remaining Fe and unavoidable impurities, and further, a ferrite single phase High strength cold rolled steel sheet excellent in weldability whose tensile strength is 440 Mpa or more, It has a structure which consists of a). The method of claim 1,
Furthermore, the high strength cold-rolled steel sheet which is excellent in the weldability whose tensile strength is 440 Mpa or more characterized by having a composition containing O: 0.0025 to 0.0080% by mass%. The method according to claim 1 or 2,
Furthermore, the high strength cold-rolled steel sheet excellent in the weldability whose tensile strength is 440 Mpa or more characterized by having the composition which contains at least 1 sort (s) in Se: 0.0005 to 0.01% and Te: 0.0005 to 0.01% by mass%. The slab which has a composition as described in any one of Claims 1-3 is wound up at the winding temperature of 680 degreeC or less after hot rolling, cold-rolled to 40% or more of reduction ratio after pickling, and then 700-850 A method for producing a high strength cold rolled steel sheet excellent in weldability, wherein the tensile strength is 440 MPa or more, characterized by performing recrystallization annealing at a temperature range of ° C.