JP2503338B2 - Good workability and high strength cold rolled steel sheet with excellent fatigue strength of spot welds - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength cold-rolled steel sheet which has good formability and is excellent in fatigue strength of spot welds used for automobile panels and home electric appliances.

[0002]

2. Description of the Related Art Many attempts have heretofore been made to increase the strength of cold-rolled steel sheets. Especially tensile strength is 35-5
In the case of a steel sheet having 0 kgf / mm ² , Si, Mn, Cr
Alternatively, P has been added to increase the strength. Further, due to recent technological advances, it becomes easy to melt ultra-low carbon steel, and Ti or Nb is added to this to secure workability. On the other hand, this type of steel plate is spot-welded when assembling parts formed by press working or the like, and many techniques have been proposed for this spot welding.

For example, Japanese Patent Laid-Open No. 63-310939 discloses an ultra-low carbon steel sheet to which Ti and / or Nb is added, and the recrystallization annealing temperature and time are selected to obtain a cross-sectional structure ratio of 5
It is disclosed that the joint fatigue strength of the spot welded portion is improved by optimizing the hardness distribution of the nugget portion and its periphery by leaving ˜30% of the non-recrystallized structure. However, a steel sheet having such a structure has a low elongation (El) and an average Langfoud value (r value), and has a problem in formability. In addition, manufacturing conditions are narrow, and strict temperature control is required.

In Japanese Patent Laid-Open No. 63-317647, the yield strength (YP) is increased by precipitating a Ti compound having a grain size of 0.05 μm or less in an ultra low carbon steel sheet to which Ti and Nb are added at the same time. It is described that the contact resistance during spot welding is increased to improve the strength and toughness of the welded portion. In this type of steel sheet, fine precipitates tend to cause deterioration of El, and increasing YP impairs the shape fixability at the time of pressing, which is the basic performance of the ultra-low carbon steel sheet.

Further, in Japanese Laid-Open Patent Publication No. 63-317648, the surface of Ti-Nb ultra-low carbon steel sheet is dulled to increase the surface roughness, thereby reducing the contact area between the steel sheets and the contact during welding. An invention has been proposed in which good resistance to spot welding is obtained by increasing the resistance.
Japanese Patent No. 7649 discloses that by adding Ti, Nb, and B in appropriate amounts, YP can be increased to increase contact resistance, and the structure after welding can be made fine to increase the strength of the welded portion. However, in the former case, the manufacturing cost is increased to increase the surface roughness, and in the latter case, when the tensile strength (TS) is increased to increase the strength, YP is further increased, and there is a fear of surface strain.

Recently, ultra-low carbon steel or Nb
And / or B added, Al / N
It is possible to improve the fatigue resistance of spot welded joints by adjusting the balance and O content appropriately.
Although it is proposed by Japanese Patent Publication No. 23, there is no disclosure about a Ti-containing ultra-low carbon steel sheet, and since it contains a large amount of Al, there is a problem in manufacturing cost.

[0007]

A desirable high-strength cold-rolled steel sheet used for an automobile panel or the like is a steel sheet which does not have a very high yield strength, is significantly work-hardened, and has a paint bake hardening ability. In addition, when YP-El is 0.2% or less during press molding, stretcher strain does not occur, excellent workability such as average r value (deep drawing property) and elongation (overhang property), and secondary processing. It also needs to be excellent. Furthermore, spot weldability must be good when assembling press-formed parts. Although each of the above-mentioned prior arts discloses the spot weldability, none of the above-mentioned prior arts has the properties required for the work forming and the spot welds have excellent fatigue strength.

According to the present invention, Mn and Cr, which have not been used as solid solution strengthening elements in the past, are added to ultra-low carbon steel to make effective use of them, and further, ultra-low carbon containing at least one of Ti and Nb. A cold-rolled steel sheet having a yield strength (YP) of 15 to 25 kgf / mm ² and a yield ratio (YR = YP /
TS, TS: Tensile strength) 0.55 or less, Yield point elongation (YR-El) 0.2% or less during press molding, Work hardening amount (WH) 4 kgf / mm ² or more, Bake hardening during coating baking Quantity (BH) is 2-5kgf / mm ² , and WH + BH and 6kg
f / mm ² or more, average r value of 1.5 or more, T.I. El is 35%
As described above, the ductility-brittleness transition temperature is -50 ° C or less, the secondary workability is excellent, the surface shape property after molding is excellent, the dent resistance is excellent, and the fatigue strength of the spot weld is excellent. It is intended to provide a high strength cold rolled steel sheet.

[0009]

In order to achieve the above object, the present invention has the following structures. That is, by weight, (1) C: 0.0003 to 0.01%, Si: 0.005 to 0.5%, Mn: 0.3 to 3.0%, Cr: 0.01 to 3.0% , And Mn + Cr to be 0.5% or more, P: 0.001 to 0.12%, S: 0.001 to 0.015%, Al: 0.005 to 0.10%, B: 0.0001 to 0.002% Further, one or two of Ti: 0.003 to 0.10% and Nb: 0.003 to 0.10%, Ti ^* = T. When Ti−3.42N, 2 ≦ (Ti ^* + 0.
52 Nb) / C ≦ 20, and is a good workability high strength cold rolled steel sheet excellent in fatigue strength of spot welds, characterized in that it comprises the balance Fe and unavoidable impurities. In addition, by weight B: 0.0001-
It is a good workability high strength cold rolled steel sheet excellent in formability and fatigue strength of spot welds, which is characterized by containing 0.002%.

[0010]

The present inventors have obtained a cold-rolled steel sheet having a TS of 35 to 50 kgf / mm ² grade and excellent workability, and a workability-improving Ti or Nb single or composite electrode added. Through intensive research based on low carbon steel, we obtained new knowledge as described below.

First, Si, P, Mn, and Cr, which are typical solid solution strengthening elements, are added to the above base steel and cold rolled,
The tensile properties after annealing and temper rolling, especially the yield behavior and work hardening phenomenon were investigated in detail. As a result, Si and P, which are conventionally frequently used as solid solution strengthening elements, are as follows: a) First, the addition of a very small amount significantly increases the yield strength, and b) As a result, the work hardening rate in a low strain region is significantly reduced. There was found.

On the other hand, when Mn and Cr, which are not conventionally used as solid solution strengthening elements, are added, a) yield strength is hardly increased, b) tensile strength is increased, and c) as a result,
We obtained a very important new finding that the work hardening rate in the low strain region is increased by these additions. As a result of investigating these mechanisms, a) the yield strength is determined by the difference in atomic radius between the Fe element and the added X element, and increases as the difference in atomic radius increases. B) Work hardening rate is dislocation It was deeply related to the slip behavior of the above, and the basic principle was established that, when the stacking fault energy was lowered by the addition of the X element, the cross slip of dislocations became difficult and the work hardening rate increased.
According to this, the atomic radii of Si and P are much smaller than that of Fe, and hence the difference in atomic radii becomes larger, so that the yield strength increases remarkably. The atomic radii of Mn and Cr are very close to those of Fe, so that the yield strength changes almost. It can be understood that it was not made. On the other hand, although the influence on the stacking fault energy is not always clear, from the observation result of the dislocation structure after the initial work hardening, Si and P have almost no influence on the stacking fault energy within the investigated addition amount range, It became clear for the first time that Mn and Cr tended to reduce this.

According to the above mechanism, it is considered that when Mn and Cr are added, the yield strength hardly changes, the work hardening rate increases, and the tensile strength increases. Such characteristic behavior is due to the above-mentioned object of the present invention, that is, low YP and high W.
In order to achieve the characteristics of H, it means that it is difficult to add Si and P in the related art, and the addition of Mn and Cr is essential. Therefore, in the present invention, the positive use of Mn and Cr is the basic means of the invention. However, since the desired strength may not be obtained and the manufacturing cost may be increased only by adding Mn and Cr, the combined use with Si and P is also considered.

Secondly, the present inventors have obtained a new finding that coating bake hardenability is also improved by positively adding Mn and Cr. In the present invention, Ti and Nb are used to secure workability.
Is used as a base, but in this case, since C and N are generally fixed to Ti and Nb, there is no bake hardenability, or there is little.
This is because the affinity between C and N and Ti and Nb is very strong, so that (Ti + N)
b) When the amount is larger than the (C + N) amount, the solid solution (C
+ N) is unlikely to remain. However, even in such a case, the new fact that the BH property can be imparted by positively adding Mn and Cr suggests that the solubility products of TiC and NbC have changed. That is,
Since Mn and Cr have an interaction of attractive force with C, TiC
It is considered that the solid solution C in the matrix in equilibrium with NbC and NbC is further stabilized and the solubility product of these precipitates is increased, and as a result, the amount of remaining solid solution C is increased and the BH property is improved. Therefore, addition of Mn and Cr can be utilized as a new means for imparting paint bake hardenability.

The solid solution C which contributes to the baking property of the coating is B
Since the grain boundaries are strengthened similarly to the above, it is also effective as a means for preventing secondary work embrittlement, which is known as a drawback of ultra-low carbon steel. It is well known that secondary working embrittlement is promoted by the presence of P segregated at the grain boundaries.
Has an interaction between P and attractive force, and reduces the amount of segregated P, so that it was found that it is also effective in preventing embrittlement in secondary working.

In order to assemble a press-molded part such as an automobile, the welding work must be carried out efficiently, and for that purpose, it is important that the spot weldability is excellent. In general, it is known that the spot weldability is superior as the lower limit of the welding aptitude current is lower. It is also well known that the lower limit of the welding aptitude current is required to be lower as the yield strength of the steel sheet is higher. This is because when the yield strength of the steel sheet is high, local deformation is less likely to occur due to the pressure applied from the electrode during spot welding, so that the contact area between the electrode and the steel sheet becomes small. As described above, the steel of the present invention is designed to reduce YP as much as possible from the viewpoint of surface strain resistance during pressing. However, since the WH is very high, the yield strength after press forming is about the same as that of the high-strength cold-rolled steel sheet using ordinary P or Si, and both spot weldability and surface strain resistance can be achieved. .

Further, in the spot welded portion, fatigue fracture may occur due to repetitive stress when the vehicle is running,
Excellent fatigue strength after spot welding is also a very important issue. In the steel of the present invention, since Mn and Cr are positively added, the transformation structure by spot welding becomes fine, and it has been found that the fatigue strength of the spot welded portion is remarkably excellent. On the other hand, in a conventional high-strength steel sheet to which P or Si is added, spot weldability and fatigue strength after welding are extremely low. In such a steel sheet, although the yield strength is high, it is considered that the main cause of the characteristic deterioration is that the crystal grains are remarkably coarsened by heating during welding.

Nowadays, with the progress of rust prevention of automobile bodies and the like, the trend toward the use of surface-treated steel sheets is remarkable. Surface treatment methods for steel sheets are roughly classified into electroplating and hot dip coating. The former generally has good plating adhesion regardless of the type of steel sheet, but has a problem that it is difficult to make a thick basis weight and the cost becomes high. On the other hand, the latter does not have the drawback of the former, but depending on the type of steel sheet, the plating adhesion is deteriorated or the Fe-Zn alloying reaction is greatly affected. The present inventors have found that the steel of the present invention which suppresses the addition amounts of Si and P, which are often used as strengthening elements in conventional steel, and utilizes Mn and Cr, is alloyed and melted by a continuous hot dip galvanizing process of a Zenzimer system. In the manufacture of galvanized steel sheets, we also gained new knowledge with the following advantages. That is, since Si and P suppress the alloying reaction between Zn and Fe, when producing a steel sheet containing a large amount of these elements, the line speed must be reduced and productivity must be reduced. Further, the addition of Si deteriorates the adhesion of the plating and causes various problems during press molding. On the other hand, it has been found that the addition of Mn and Cr does not have such an adverse effect. Although the mechanism is not always clear, a) Mn segregated on the surface changes the type and morphology of the oxide remaining after reduction to improve plating adhesion, and b) Mn.
It is presumed that Cr and Cr have a role of reducing the amount of P segregated at the grain boundaries through the interaction with P and normalizing the alloying reaction. This point was also utilized as a solution to the problems of the conventional method. The present invention is constructed based on such an idea and new knowledge.

The reasons for limiting the components and characteristic values in the present invention as described above will be further described below. C content: C is a very important element that determines the material properties of the product. The present invention is premised on a vacuum degassed ultra low carbon steel, but if C is less than 0.0003%, the grain boundary strength decreases, secondary work embrittlement occurs, and the manufacturing cost significantly increases. . On the other hand, if the C content exceeds 0.01%, the strength increases, but the formability remarkably decreases, so the range is made 0.0003% to 0.01%.

Si amount: Si is known as an element that increases the strength at a low cost, and the addition amount may be changed according to the target strength level, and the lower limit may be 0.005. On the other hand, when the addition amount is increased, YP rises too much and surface strain occurs during pressing. Further, problems such as deterioration of chemical conversion treatment property and adhesiveness of hot dip galvanizing, and deterioration of productivity due to delay of alloy reaction occur. It also deteriorates the fatigue resistance of the spot welds. Therefore, the upper limit is set to 0.8%.

Mn content: Mn is an effective solid solution strengthening element that does not significantly increase the yield strength and increases the strength, imparts bake hardenability, prevents secondary work brittleness, and further spot welding. Since it has the effect of improving the fatigue resistance of the part and improving the chemical conversion treatment property and the hot dip galvanizing property, it is positively added in the present invention. However, if added in an excessively large amount, the yield strength remarkably increases and the ductility decreases, so the upper limit is made 3.0%. The lower limit is C
It is determined in consideration of the r content, but 0.3% or more is necessary.

Cr content: Cr and Mn are both effective elements that increase the yield strength with almost no increase in yield strength.
In addition to imparting bake hardenability and preventing secondary work brittleness, it improves fatigue strength after spot welding, so it is actively used in the present invention. However, if the amount of Cr is large, the pickling property of the hot-rolled sheet deteriorates and the chemical conversion treatment property of the product sheet deteriorates, so the upper limit is made 3%. M mentioned above
n and Cr are added for the same function purpose, but in order to obtain appropriate characteristics by these elements, Mn + Cr should be 0.5
% Or more is preferable.

P content: P is known as an element that increases the strength at a low cost, and is often used in conventional solid solution strengthened high strength steel sheets. However, if the addition amount exceeds 0.12%, even if Mn and Cr are positively added as in the present invention, a) the yield strength is excessively increased to cause a surface shape defect during pressing, or b). Secondary processing embrittlement may occur. Further, c) the alloying reaction during continuous hot-dip galvanizing becomes extremely slow, and the productivity is reduced. Furthermore, d) the fatigue strength after spot welding is significantly reduced. Therefore, the upper limit value is set to 0.12%.

S amount: S is preferably low, but 0.00
If it is less than 1%, the production cost increases, so this is set as the lower limit. On the other hand, if it exceeds 0.015%, MnS or T
Since a large number of sulfides such as iS precipitate and workability deteriorates, this is set as the upper limit.

Al amount: Al is used for deoxidation adjustment,
If it is less than 0.005%, the yield of Ti addition is reduced. On the other hand, if it exceeds 0.1%, the cost increases.

Amount of Ti, Nb: Ti, Nb is TiC, TiN, or Nb (C
By fixing as N), it has a role of ensuring workability and non-aging property of the ultra-low carbon steel. Therefore, Ti ^* =
T. When Ti−3.42N, 2 ≦ (Ti ^* ++
It is preferable that 0.52 Nb) / C ≦ 20. At that time, it is preferable that the content range of each element is 0.003 to 0.15%, and if it is less than 0.005%, the addition effect does not appear, while if it is 0.1% or more, the alloy cost increases significantly. Because it invites.

The N content is not particularly specified, but a lower content is preferable. If the amount increases, excessive addition of Ti or Al becomes necessary, or workability deteriorates.
060% or less. In the present invention, B may be added as necessary, and when N is fixed in advance at this time, it segregates at the grain boundaries and is effective in preventing secondary work embrittlement. However, if it is less than 0.0001%, the effect is insufficient, and if it exceeds 0.0020%, it causes an increase in addition cost and slab cracking. Therefore, it is preferable to set this range when adding.

According to the present invention, the high-strength cold-rolled steel sheet having the above-mentioned composition is manufactured by the usual methods such as hot rolling, cold rolling and annealing. However, when hot rolling the slab, the finishing temperature, from the viewpoint of securing the workability of-products plate and Ar ₃ -100 ° C. or more, and preferably set to 750 ° C. The coiling temperature from room temperature.

Thus, according to the present invention, 35-50
kgf / mm ² tensile strength (TS) and yield strength (Y
P) of 15 to 25 kgf / mm ² , yield ratio (YR = YP / T
S) is low YR of 0.55 or less, the average r value is 1.5 or more, the elongation (T.El) is 35% or more, and the YP-El during press molding is 0.2% or less, and Excellent secondary workability that does not cause cracking due to impact during or after processing,
Moreover, the bake hardening amount (BH) during baking is ² to 5 kgf / mm ²
Thus, it is possible to obtain a high-strength cold-rolled steel sheet having good surface shape after forming, excellent dent resistance, and excellent fatigue strength of spot welds.

[0030]

EXAMPLE Steels having the compositions shown in Table 1 were melted and hot-rolled at a slab heating temperature of 1150 ° C., a finishing temperature of 910 ° C., and a winding temperature of 650 ° C.
% Cold-rolled, then annealed at 840 ° C. and temper-rolled to obtain a product. A JIS No. 5 tensile test piece was sampled from this cold rolled steel sheet and subjected to a tensile test. In addition, the lower limit value of the welding suitability current when performing spot welding was evaluated using a material that was given a tensile deformation of 2%. The welding strength was evaluated by tensile shear strength after welding under the conditions of welding time: 8 cycles, applied pressure: 190 kgf, and current 8 kA. Furthermore, as for fatigue characteristics, the nugget diameter is all 5.0 by adjusting spot welding conditions.
A tensile shear fatigue test piece was prepared so as to have a thickness of mm, and this was subjected to a fatigue test of perfect swing. The obtained results are shown in Table 2.

[0031]

[Table 1]

[0032]

[Table 2] As is clear from Table 2, the steel sheet of the present invention has a lower yield strength (YP), a higher elongation (El), and a higher average r value than the conventional materials. Further, it is also excellent in BH property and secondary workability. Further, it can be seen that the spot weldability and the fatigue strength after spot welding are also excellent. On the other hand, the comparative steels out of the range of the present invention were inferior in each mechanical property, spot weldability and fatigue resistance after welding.

[0033]

As is apparent from the above description, according to the present invention, by betting a Ti / Nb-containing ultra-low carbon steel sheet in which Mn, Cr, etc. are effectively utilized, a spot welded portion with unprecedented workability can be obtained. A high-strength cold-rolled steel sheet having excellent fatigue strength can be obtained. Further, the steel sheet of the present invention is excellent in BH property and secondary workability and can be widely used as a material for automobile frames and containers.

 ─────────────────────────────────────────────────── --Continued from the front page (56) References JP-A-2-163323 (JP, A) JP-A-63-317649 (JP, A) JP-A-63-317648 (JP, A) JP-A-63- 310939 (JP, A) JP-A-2-163318 (JP, A)

Claims (2)

(57) [Claims] 1. By weight, C: 0.0003 to 0.01%, Si: 0.005 to 0.8%, Mn: 0.3 to 3.0%, Cr: 0.01 to 3.0%. And Mn + Cr to be 0.5% or more, P: 0.001 to 0.12%, S: 0.001 to 0.015%, Al: 0.005 to 0.10%, and Ti: 0.003 to 0.10% and Nb: 0.003 to 0.10%, one or two of which Ti ^* = T. When Ti−3.42N, 2 ≦ (Ti ^* + 0.
52 Nb) / C ≦ 20, and the balance Fe and unavoidable impurities make the workability and high strength cold rolled steel sheet excellent in fatigue strength of spot welds. 2. By weight, C: 0.0003 to 0.01%, Si: 0.005 to 0.5%, Mn: 0.3 to 3.0%, Cr: 0.01 to 3.0%. , And Mn + Cr to be 0.5% or more, P: 0.001 to 0.12%, S: 0.001 to 0.015%, Al: 0.005 to 0.10%, B: 0.0001 to 0.002% Further, one or two of Ti: 0.003 to 0.10% and Nb: 0.003 to 0.10%, Ti ^* = T. When Ti−3.42N, 2 ≦ (Ti ^* + 0.
52 Nb) / C ≦ 20, and the balance Fe and unavoidable impurities make the workability and high strength cold rolled steel sheet excellent in fatigue strength of spot welds.