JP2005248313A - Extremely-low-carbon steel sheet excellent in spot weldability - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel sheet which is excellent in spot weldability without worsening its formability. <P>SOLUTION: The extremely-low-carbon steel sheet comprises, by mass%, 0.0005-0.003% C, ≤0.1% Si, ≤0.5% Mn, ≤0.05% P, ≤0.02% S, ≤0.01% Al, ≤0.005% N, 0.001-0.01% O (oxygen), La and/or Ce, provided that the total amount of La and/or Ce satisfies the relation: La+Ce≥S/4, and the balance being Fe and unavoidable impurities. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a forming steel sheet that exhibits superiority in applications that use spot welding for assembly, such as transportation equipment such as automobiles and home appliances.

  For transportation equipment such as automobiles and home appliances, a process of assembling press-molded parts using spot welding is often used. In this case, from the viewpoint of press formability and cost, it is common to use a steel plate having excellent formability as the material. Above all, ultra low carbon steel sheets that can be applied to formability to a simple shape are widely used in automobile bodies and the like where the quality of design occupies a large specific gravity for evaluation in the market.

  On the other hand, even in such a field, depending on the part, in order to achieve the purpose of reducing the weight by reducing the plate thickness and achieving the purpose of obtaining the rigidity at the time of collision, the use of a high strength steel plate is simultaneously performed.

  In this way, even in the case of automobiles, as a steel plate used, a wide range of materials from soft materials such as ultra-low carbon steel plates to high-strength steel plates coexist, and in order to increase the efficiency of the assembly process, There is a strong demand for steel sheets of high strength to be capable of spot welding for assembly under similar conditions.

  It has been pointed out that there is a problem with spot weldability of ultra-low carbon steel sheets when comparing ultra-low carbon steel sheets with high-strength steel sheets or low-carbon steel sheets.

  For example, it is known that the amount of current required to obtain an appropriate strength is larger than that of the other steel plates described above, so that the load on the current cable or the like is large and the life is shortened.

  This is because the ultra-low carbon steel plate is softer than other steel plates. That is, when both are compared, in the ultra-low carbon steel plate, the contact area between the steel plate and the welding electrode during welding is relatively large, and the current density is low for the same amount of current. As a result, the volume that can be melted with the same amount of current (called a nugget after welding) decreases, so the welding strength decreases. Therefore, if an attempt is made to compensate for this and obtain an appropriate welding strength (nugget diameter), a large amount of current is required.

  Several techniques are known for solving these problems.

  For example, Patent Document 1 discloses an ultra-low carbon steel sheet having a predetermined chemical component and having a carbon enriched layer that satisfies a specific condition in the surface layer portion. Patent Document 2 proposes a technique for ensuring the static strength of a welded portion by placing boron (B) and nitrogen (N) among steel components under specific conditions. Further, although related to the plated steel sheet, Patent Document 3 proposes a steel sheet that has a nickel (Ni) oxide layer on its surface to improve spot weldability.

  All of these prior arts have high industrial value and are considered to be able to obtain a certain effect, but are not necessarily sufficient. In the technique of Patent Document 1, a process of carburizing is necessary for the formation of the carbon enriched layer, and there is a concern about an increase in manufacturing cost. It is also conceivable that the mechanical properties are affected by carburization and the moldability is deteriorated.

  The steel sheet of Patent Document 2 essentially contains boron in an amount of 0.0010 to 0.0030% by mass, and a decrease in formability such as elongation that is impaired thereby cannot be ignored depending on the application. On the other hand, the proposal according to Patent Document 3 is intended to produce a nickel oxide layer by first performing nickel plating and then oxidizing the layer by a method such as anodic oxidation. It is conceivable that applicable applications are limited.

JP 7-197187 A Japanese Patent Laid-Open No. 10-8195 JP-A-1-119651

  In this way, no solution has been found that does not involve an increase in manufacturing costs, such as the addition of new processes, and there is no concern about the deterioration of the excellent formability inherent in ultra-low carbon steel sheets. Has been.

  This invention is made | formed in view of such a condition, and provides the steel plate excellent in spot weldability, without impairing the outstanding formability as an ultra-low carbon steel plate.

  The present inventors investigated in detail the process of the deformation | transformation which a welding electrode exerts on a to-be-welded material (steel plate) at the time of spot welding. An extremely thin thermocouple was placed on the surface of the steel plate in the vicinity of the target welding position and on the inside of the steel plate, and experiments with varying welding conditions (current value, number of energization cycles, pressure between electrodes, etc.) were conducted with high accuracy. The relationship between deformation and the nugget diameter formed was examined from various viewpoints.

  As a result, it is immediately before the temperature at which the transformation from the ferrite phase to the austenite phase occurs that most affects the deformation amount of the steel sheet, and if the resistance to deformation in that temperature range can be increased, the problem can not be solved. I came up with the idea of being there.

  As a means for this, the temperature at which the press molding is performed, that is, the mechanical properties at room temperature, should not be affected at all, or it should be hardly affected. We thought that it would be effective to disperse the substance, and repeated the experiment including a method of forming the fine substance in the steel sheet.

  As a result, while keeping the amount of aluminum (Al) low, one or two of lanthanum (La) and cerium (Ce) are contained, and the total amount of one or two of La and Ce is sulfur. It has been found that by making a predetermined relationship to the amount of (S), spot weldability can be improved without any loss of mechanical properties.

  Further, the gist of the present invention which has been completed through the optimization by experiments is as follows.

(1) In mass%,
C: 0.0005 to 0.003%,
Si ≦ 0.1%,
Mn ≦ 0.5%,
P ≦ 0.05%,
S ≦ 0.02%,
Al ≦ 0.01%,
N ≦ 0.005%,
O (oxygen): 0.001 to 0.01% is contained, and further one or two of La and Ce are contained so as to satisfy La + Ce ≧ S / 4, and the balance is Fe and inevitable impurities. An ultra-low carbon steel sheet with excellent spot weldability.
(2) Further, Ti: 0.01 to 0.1%,
Nb: 0.002 to 0.01%,
One or both of these, The ultra-low carbon steel plate excellent in the spot weldability as described in said (1) characterized by the above-mentioned.
(3) Further, B: 0.0005 to 0.0020%
The ultra-low carbon steel sheet excellent in spot weldability as described in (1) or (2) above.

  According to the present invention, an ultra-low carbon steel sheet having the same formability as a conventional ultra-low carbon steel sheet and excellent in spot weldability can be obtained. Moreover, since no new process is required for this purpose, the influence on the manufacturing cost is extremely small.

  Various experiments were conducted with the aim of dispersing extremely fine substances in the steel. We have extensively studied oxides produced in the deoxidation process, hot rolling of steel sheets, and carbonitrides precipitated in the winding process. The present invention has been completed through such efforts, and the reasons for limitation will be described below. In addition,% means the mass%.

C: 0.0005 to 0.003%
C is the most important element that determines the formability of the steel sheet. In order to obtain excellent formability, it is necessary to be 0.003% or less. On the other hand, if it is less than 0.0005%, an excessive load is applied to the steel making process, so 0.0005% is the lower limit. And

Si ≦ 0.1%
Si can be added in a small amount as a solid solution strengthening element. However, when it exceeds 0.1%, deterioration of moldability becomes a problem. Therefore, it is set to 0.1% or less, preferably 0.05% or less. The lower limit is not particularly defined, but if it is less than 0.003%, the effect of addition is poor, and therefore it is preferably made 0.003% or more.

Mn ≦ 0.5%
Mn can be added as a strengthening element or a deoxidizing element. However, if added over 0.5%, the ductility is lowered, so the content is made 0.5% or less. The lower limit of Mn is not particularly defined, but if less than 0.02%, the effect of addition is poor, so 0.02% or more is preferable.

P ≦ 0.05%
P is an impurity, and P segregated at the grain boundary causes grain boundary embrittlement. P is also an element that causes secondary processing cracks. However, 0.05% or less is an acceptable range.

S ≦ 0.02%
S is an impurity, and if it is too much, it not only causes cracking during hot rolling, but also causes deterioration of ductility, so it should be reduced as much as possible, but it is acceptable if it is 0.02% or less.

Al ≦ 0.01%
Al can be used as a deoxidizing element. However, as described in the Examples, if it exceeds 0.01%, no effect of improving spot weldability is observed, so 0.01% or less. There is no particular lower limit for Al. A method of not adding it as a deoxidizing element is also acceptable.

N ≦ 0.005%
N, which is an impurity, is an element that deteriorates the formability of a steel sheet when it exists in a solid solution state as in C, and therefore its content is set to 0.005% or less.

O: 0.001 to 0.01%
O is an essential element for oxide formation. The effect of improving spot weldability is recognized at 0.001% or more. On the other hand, when it exceeds 0.01%, the occurrence of surface scratches due to coarse oxides is inevitable. For these reasons, the O content is set to 0.001 to 0.01%.

La + Ce ≧ S / 4
La and Ce are both present in the steel as oxides or inclusions containing them, and are considered to play a central role in improving spot weldability. However, it does not have to contain only one or two of La and Ce, but as shown in the examples described later, the effect is obtained when the above relational expression is satisfied in relation to S. Was found to be prominent.

  The reason is not necessarily clear, but among the contained La and Ce, oxides formed by La and Ce that are not consumed for the formation of sulfides (including inclusions including them) are spots. It can be estimated that it contributes essentially to the improvement of weldability.

  La and Ce are desirably added to molten steel using Misch metal as a raw material. Although it is possible to add each of the pure substance raw materials, it is not preferable because it only increases the manufacturing cost. From such an idea, the amount of La or Ce added alone or two of them was not specifically limited, and the relationship between the total amount of one or two elements and the amount of S was defined.

  On the other hand, if the amount of La + Ce exceeds 0.02%, the oxides are coalesced and coarsened, which may cause surface scratches. Therefore, the upper limit is preferably 0.02%.

Ti: 0.01 to 0.1%
Ti forms precipitates with C, N, and S to fix those elements and to improve the formability. The effect is recognized by addition of 0.01% or more. On the other hand, excessive addition leads to an increase in cost, so 0.1% or less, preferably 0.07% or less is good.

Nb: 0.002 to 0.01%
Nb, as well as Ti, fixes C, N, and S as precipitates and contributes to improvement of formability. At least 0.002% is necessary for the expression of the effect, but if it exceeds 0.01%, the recrystallization temperature rises remarkably. . Therefore, it is limited to the above range.

B: 0.0005 to 0.0020%
B is added as necessary to prevent secondary cracking of the steel sheet. The effect is not recognized if it is less than 0.0005%, and is saturated even if added over 0.0020%, so it is limited to 0.0005 to 0.0020%.

  In the present invention, the component other than the above is Fe, but the inclusion of inevitable impurities such as scrap mixed from the melting raw material is allowed.

  Moreover, although only the Example using a cold-rolled steel plate is demonstrated below, this invention is not limited to a cold-rolled steel plate, It can apply also to a hot-rolled steel plate.

  A manufacturing method for obtaining excellent moldability will be described.

Reheating temperature: 1000-1200 ° C
The reheating temperature of the slab must be determined in consideration of a lower limit value derived from the purpose of reducing the rolling load and an upper limit value for suppressing the crystal grain size from being coarsened by heating. If this temperature is less than 1000 ° C., the load required for rolling becomes large, and depending on the capability of the rolling mill, there is a possibility that the slab width must be restricted.

  On the other hand, if the temperature exceeds 1200 ° C., the particle size before the start of rolling becomes too coarse, and it becomes difficult to achieve fine graining after hot rolling that is advantageous for obtaining a high r value. Therefore, the slab heating temperature is set to 1000 ° C. or more and 1200 ° C. or less.

  The hot rolling rate is not particularly limited, but is preferably 60 to 95% for the purpose of obtaining a more uniform structure.

Finish rolling temperature: 900-980 ° C
It is necessary to perform finish rolling in the austenite region as much as possible. This is because if the rolling is performed in a temperature range including ferrite, the processed structure tends to remain, which leads to deterioration of formability. For this purpose, the finish rolling temperature needs to be 900 ° C. or higher. On the other hand, if it exceeds 980 ° C., the hot-rolled plate particle size tends to be coarse. Therefore, it is necessary to set the temperature to 980 ° C. or lower.

Cooling rate: The steel sheet after hot rolling at 10 to 80 ° C./second needs to be cooled quickly for the purpose of suppressing the coarsening of crystal grains as much as possible. For this purpose, a cooling rate of 10 ° C./second or more is necessary. On the other hand, if the cooling rate exceeds 80 ° C./second, the plate-passability and material non-uniformity of the steel plate in the cooling zone cannot be overlooked.

Winding temperature: 400-700 ° C
The rolled steel sheet is wound up through a cooling zone, and a final structure is formed in this process. In order to obtain excellent moldability, it is necessary to have a recrystallized structure (partially, a recovery structure is allowed), so 400 ° C. or higher is necessary. On the other hand, when it exceeds 700 ° C., the hot-rolled scale grows vigorously, increasing the load of the subsequent pickling process. Therefore, it is necessary to set the temperature to 700 ° C. or lower.

  When used as a hot-rolled steel sheet, skin pass rolling may be performed following pickling for the purpose of shape correction, change in surface roughness, and the like.

Cold rolling rate: 60-95%
In order to obtain a high r value, it is necessary to develop a specific texture by annealing (heat treatment) after cold rolling. For this purpose, it is first necessary to develop a texture having a specific orientation in cold rolling and to accumulate a necessary amount of strain. In order to satisfy this condition, a cold rolling rate of 60 to 95% is required.

Annealing temperature: 700-890 ° C
In order to obtain high moldability, it is necessary to obtain a uniform recrystallized structure. For that purpose, it is necessary to anneal at 700 ° C. or higher. On the other hand, when the temperature exceeds 890 ° C., the crystal grain size becomes too large, which causes rough skin after molding. Therefore, the upper limit is 890 ° C.

  Skin annealing may be performed after annealing.

  Examples will be described below together with comparative examples.

  Steels a to j having chemical components described in Table 1 were melt cast. It was reheated to 1150 ° C. and hot-rolled to obtain a 4 mm hot-rolled steel sheet. The rolling rate was 87%, the finishing temperature was 920 to 930 ° C, and the heat treatment corresponding to winding was held at 630 ° C for 1 hour and then cooled in the furnace. Next, they were pickled, cold-rolled (cold rolling rate of 85%) and annealed at 800 ° C. to obtain 0.8 mm cold-rolled steel sheets. Furthermore, 1% skin pass rolling was performed.

  A JIS No. 5 test piece was collected from the obtained cold-rolled steel sheet and examined for mechanical properties. Table 2 shows the mechanical properties.

  On the other hand, the appropriate welding current was evaluated as follows.

  First, a plurality of 50 × 50 mm cold-rolled steel sheets were prepared as one set. They were welded using a 30 ° conical headed 4.5 mmφ electrode. The energization time and the holding time after energization were both fixed at 0.2 seconds (10 cycles), and the applied pressure was fixed at 1.96 kN. Then, the current value was changed, and a current-nugget diameter curve as illustrated in FIG. 1 was obtained for each steel. The nugget diameter was determined by microscopic observation of a polished and etched cross section after welding.

  The current value at which the nugget diameter becomes 5√t (t is the plate thickness, that is, 0.8 mm) was obtained using the curve of each steel, and the value was determined as an appropriate current value.

  Appropriate current values are also shown in Table 2. As is apparent from Table 2, the steel of the present invention is excellent in ductility and r value, which are indicators of formability, and the appropriate current value is also lower than that of the comparative steel.

  Steels k to z having chemical components described in Table 3 were melt cast. From these, a 0.8 mm cold-rolled steel sheet was obtained under the same conditions as in Example 1.

  Evaluation of mechanical properties by JIS No. 5 test piece and determination of appropriate welding current by the same method as in Example 1 were performed. As a result, in any steel, the tensile strength was 280 to 295 MPa, the elongation was 52% or more, the r value was 2.3 or more, and it was clear that the steel had excellent formability. There was a significant difference between the steels.

  In FIG. 2, the appropriate electric current value of each steel is shown with respect to (La + Ce) -S / 4. As is apparent from the figure, the appropriate current value of the steel of the present invention is on the low current side as compared with the comparative steel, and it is understood that the spot weldability is excellent.

  As described above, according to the present invention, an ultra-low carbon steel sheet excellent in formability and spot weldability can be provided. Further, according to the present invention, the manufacturing cost of the ultra-low carbon steel sheet does not increase, so the present invention has a great industrial applicability.

It is a figure which shows an example of the electric current-nugget diameter curve for calculating | requiring an appropriate electric current value. It is a figure which shows the appropriate electric current value of each steel with respect to (La + Ce) -S / 4.

Claims (3)

In mass%
C: 0.0005 to 0.003%,
Si ≦ 0.1%,
Mn ≦ 0.5%,
P ≦ 0.05%,
S ≦ 0.02%,
Al ≦ 0.01%,
N ≦ 0.005%,
O (oxygen): containing 0.001 to 0.01%,
An ultra-low carbon steel sheet excellent in spot weldability, characterized in that the total of one or two of La and Ce is contained so as to satisfy La + Ce ≧ S / 4, and the balance is composed of Fe and inevitable impurities. further,
Ti: 0.01 to 0.1%,
Nb: 0.002 to 0.01%,
One or both of these are contained, The ultra-low carbon steel plate excellent in spot weldability of Claim 1 characterized by the above-mentioned. further,
B: 0.0005 to 0.0020%
The ultra-low carbon steel plate excellent in spot weldability according to claim 1 or 2, characterized by comprising:

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