JPH02115352A - Worked parts having weld zone excellent in strength characteristic - Google Patents

Abstract

PURPOSE:To obtain worked parts in which steel plate is used as a base material and which has superior cruciform joint tensile strength by providing a worked product of the prescribed plate thickness prepared by subjecting a steel plate with a specific composition to spot welding, reducing absolute hardness in a nugget zone, and providing a part increased in hardness in comparison with the hardness of a base material to a heat-affected zone on the periphery of the nugget. CONSTITUTION:A worked product of 0.4-10.0mm plate thickness prepared by subjecting a steel plate which has a composition containing, by weight, <=0.015% C, 0.05-1.5% Mn, <=0.030% S, 0.8-3.0% Cu, <=0.100% P, <=1.0% Si, <=0.0050% N, and 0.002-0.10% Sol.Al and further containing, if necessary, one or more kinds among 0.01-0.2% Ti, <=0.0030% B, and 1.0% Ni to spot welding is provided. Further, when X and Y represent Vickers hardness in the above worked zone and Vickers hardness in the nugget zone, respectively, the hardness in a heat-affected zone located at a distance equal to plate thickness from the outside periphery of the nugget is regulated to >=1/4.(3X+Y). By this method, the worked parts showing extremely superior cruciform tensile strength and having a weld zone excellent in strength characteristics can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a processed part made of a high-Cu type high-strength thin steel plate and having a spot welded part with excellent static and dynamic strength.

[Prior Art] Recently, there has been an extremely strong demand from users for the development of high-strength steel sheets with excellent press workability and spot weldability.

For example, in response to the tightening of safety regulations to protect automobile occupants in the event of a collision, the use of high-strength steel sheets as copper plates, which have a high ability to absorb collision energy and are less likely to deform during a collision, is being considered. In order to avoid the increase in vehicle weight and the resulting increase in fuel consumption when installing new equipment, efforts are being made to reduce the weight of steel plates by reducing their thickness.For this purpose, high strength steel plates are being used. requested.

In addition, since such thin steel sheets are generally formed by press working and assembled by welding and used as an integral structure, they have sufficient workability to be formed into complex shapes with good dimensional accuracy without breakage or constriction. It also has good strength when assembled as a welded structure in the subsequent assembly process. Impact resistance. Due to the need to have fatigue strength, etc.
It is required to have excellent weldability, especially spot weldability.

However, conventional high-strength thin steel sheets for processing have a carbon content of approximately 0.03
96 or higher, and is manufactured by adding solid solution strengthening elements such as Mn, Si, P, etc., or by utilizing precipitation strengthening with carbonitrides such as Ti and Nb, in addition to strengthening the structure by quenching using carbon. Normally, when steel plates are made to have high strength using this method, there is a problem that the strength of the spot welds of the steel plates, especially the cross tensile strength of static strength, decreases as the steel plate strength increases. The use of high-strength thin steel plates was restricted.

[Problem to be solved by the invention] Taking the example of an automobile body, which is a typical example of a thin steel plate structure,
Automobile bodies and the parts that constitute them are basically press-formed steel plates and assembled by spot welding. The number of spot welding points per passenger car is as large as about 4,000, and the shape of the joint used is basically as shown in FIG. 1.

In general, the static strength of a joint in a thin steel plate structure is tensile shear strength and cross tensile strength, and the dynamic strength is fatigue strength. Impact strength. Creep strength is an issue.

As shown in FIG. 2, the tensile shear strength of a spot welded joint is approximately proportional to the tensile strength of the base material. However, as shown in FIG. 3, the cross tensile strength is not proportional to the tensile strength of the base material, and may decrease in some cases. These strength characteristics indicate that even if high-strength steel plates are used to reduce the weight of the car body, the strength of the cross-tension joints cannot be expected to be improved. This is the point.

The present invention has focused on this problem and has developed a technology that uses high-strength steel plates as a base material to provide finished parts with excellent cross-tension joint strength.

[Means for Solving the Problems] A processed part according to the present invention is configured as follows. That is, C O. with weight t. 015 96 or less Mn O. 05 ~L5 5k S 0.030 $ or less Cu O. 8-3.0'*P O. 100 N or less Si 1.0 or less N O. 0050% or less Sol. AQ O. 002~0.10'! Contains
Furthermore, if necessary, Ti of 0.01-0.24 and 0,
0030! A steel plate with a thickness of 0.4 to 1 spot welded containing one or more of the following B and Ni below Ot, with the remainder consisting of unavoidable impurities and Fe.
For a 0.0mm processed product, the hardness of the base material (Vickers hardness) is X, and the hardness of the nugget part (Vickers hardness) is Y.
If Y≦270 and the hardness (Vickers hardness) of the heat-affected zone at a distance equal to the plate thickness from the outer periphery of the nugget is
This is a patented processed part characterized in that it is (3X+Y) or more.

The contents of the present invention will be explained below.

As is well known, spot welding concentrates a large current on the welding area,
The Joule heat generated by this is used as a heat source to heat the metal, and at the same time applies a large pressure to join metals under high current and pressure. At this time, the temperature of the weld zone reaches above the melting point of the materials, and a heat-affected zone is formed around the nugget where the weld is fused and where the microscopic structure and mechanical properties have changed, although they are not melted. be done. FIG. 4 shows an example of temperature distribution in a spot weld.

The welding conditions are as shown in Table 1, and the thickness of the sample material is 2.0 mm.

Table 1 Spot welding conditions An example of the hardness distribution when welding conventional high-strength hot rolled steel plates (0.15!IiC - 0.7 Wood Mn) under these spot welding conditions is also shown in Figure 4. did. What is characteristic in FIG. 4 is that in the case of a conventional high-strength steel plate, the hardness of the nugget portion is locally extremely hard. This phenomenon is not a problem in tensile shear tests of welds, but in cross tension tests it is considered to be a cause of reducing the strength of welds for the following reasons.In other words, in tensile shear tests of spot welds, Since the welded part sometimes rotates, the welded part immediately before rupture has a shape as shown in FIG. 5(a).

In this case, the load mainly acts between ■ and ■, and the influence of a kind of notch between ■ and ■ and between ■ and ■ becomes small on the transmission of force to the welding part. Therefore, the fracture occurs not in the hard nugget portion but in the soft base material. In contrast, in the cross tension test, the force applied to the weld is applied to the entire periphery of the weld. As shown in FIG. 5(b), this acts as a large concentrated stress on the end of the welded portion through the notches between ■ and ■ and between ■ and ■. Fracture occurs within the nugget at low loads when the resistance of the metal within the nugget and the heat affected zone to this load is low. This means that in conventional high-strength steel plates, as the tensile strength of the base metal increases, the tensile shear strength of the weld increases as shown in Figure 2 above, and the cross tensile strength increases as shown in Figure 3 above. This is thought to be the reason for the decrease as shown.

As a result of intensive studies based on the above situation, the inventors of the present invention have found that, as shown in Fig. 4, the absolute hardness of the nugget is kept low compared to the conventional one, and the heat-affected zone around the nugget is By providing a portion whose hardness increases relative to the hardness of the base metal, it is possible to obtain a welded portion with excellent not only tensile shear strength but also cross tensile strength, which has not been available in the past.

The experimental results that formed the basis of the present invention will be explained below. In order to suppress the hardness of the nugget portion, it is necessary to control elements that improve hardenability, such as C and Mn. The maximum temperature reached by the heat affected zone on the outer periphery of the nugget varies depending on the distance from the nugget, but ranges from approximately 1000°C to 700"C.
It is cooled from this temperature to room temperature in an extremely short time. As a result of various studies on elements that can be precipitated at this temperature and time to maintain the hardness of the heat-affected zone on the outer periphery of the nugget at a predetermined level, it has been found that Cu is optimal.

The cross-sectional hardness and cross tensile strength of spot welds were investigated when the contents of C, Mn, and Cu were varied in AM-killed hot-rolled steel sheets. Note that the welding conditions are the same as in Table 1. For these spot welds, the relationship between the hardness of the nugget part (Vickers hardness) and the cross tensile strength was summarized, and as shown in Figure 6, the cross tensile strength is the same as that of the heat affected zone on the outer periphery of the nugget. It is correlated with the relative relationship between the hardness, the hardness X of the base material part, and the hardness Y of the nugget part, and the distance from the outer periphery of the nugget is 1111, which is the same as the plate thickness! It has been found that extremely excellent cross tensile strength can be obtained by setting the hardness of the heat-affected zone to 1.(3X+Y) or more.

In other words, if the hardness of the heat-affected zone at a distance equal to the plate thickness from the outer periphery of the nugget does not exceed I (3X + Y), as the hardness of the nugget increases, the cross tensile strength decreases;
It was found that when the hardness of the heat-affected zone at a distance equal to the plate thickness from the outer periphery of the nugget is %・(3X+Y) or more, the cross tensile strength increases as the hardness of the nugget increases in the range of Y≦270. . As a result of further research, we found that by spot welding steel plates with a specified composition using a conventional method, we were able to create a welded part with excellent strength properties with the above-mentioned hardness distribution after spot welding. I was able to get the parts.

Next, the reason for limiting the composition range of the thin steel plate that is the raw material for the processed parts in this invention will be explained.

C is an element that increases hardenability, and it is necessary to reduce it as much as possible in order to keep the hardness of the nugget portion low. At the same time, it increases ductility during processing and maximizes the effect of Cu addition. 0.015 for that! must be as follows. If C exceeds 0.01596, the hardness of the nugget portion exceeds 270, so the upper limit is set to 0.015%.

Mn is an element that improves hardenability.1. In order to keep the hardness of the nugget portion as low as 270 or less, it is necessary to keep the hardness to 1.5 or less. If the Mn content becomes too low, surface flaws will easily occur on the steel plate, so the lower limit is set to 0.05 t.

In order to improve the workability of the steel plate, it is preferable that S be lower, and the upper limit is 0.03! Totosu.

Cu is an important element for providing the heat-affected zone on the outer periphery of the nugget with an area whose hardness increases relative to the hardness of the base material. Cu is kept in a solid solution state after processing and before spot welding, and the heat input during welding causes Cu to precipitate in the heat affected zone to increase the hardness. If it is less than 0.8, the increase in hardness of the heat-affected zone that leads to improvement in cross tensile strength is insufficient, and the hardness of the heat-affected zone at a distance equal to the plate thickness from the outer periphery of the nugget is
Y), the lower limit is set to 0.8t. On the other hand, 3
．． 0! If it exceeds k, the surface quality of the steel plate will deteriorate, so the upper limit is 3.0! Let it be J.

P is effective as an element for increasing the strength and corrosion resistance of steel sheets, but when it is not necessary, the amount of P may be 0.03% or less. On the other hand, in order to improve the strength and corrosion resistance of the steel plate, it is preferable to add 0.06 to 0.10% of P.

However, if it exceeds 0.1 ON, the nugget portion becomes extremely brittle and the welding strength decreases, so the upper limit is set to O, IO'4.

Si is usually 0.03 as an impurity! The following elements are included in 1. depending on the required strength level as elements that increase the strength of steel sheets. Ot or less, preferably 0.3 to 1.0! e
Add i. However, 1. If it exceeds H, the surface quality of the steel plate will deteriorate, so the upper limit is set to 1. Let it be H.

The amount of N is preferably lower in order to improve workability, and is preferably 0.
00509 & below.

Sol and Al are elements necessary for deoxidation, and if Sol and A are less than 0.002%, deoxidation will be insufficient, and if they are too empty, the amount of alumina produced will increase, which will have a negative impact on the surface quality of the steel sheet. Therefore, the upper limit is set to 0.1096.

Ti is added as necessary, and Ti is added to 0, O
When added in the range of 1 to 0.2 t, C and N are fixed by Ti, so the resulting steel plate becomes a non-aging steel plate with improved workability and can be processed into parts with more complex shapes. becomes. 0.0 to obtain high press workability.
It is necessary to add more than 0.01%, and on the other hand, adding more than 0.2% is disadvantageous in terms of cost.

Ni is effective in maintaining high quality of the surface of the processed parts of the present invention. 1. as necessary. H or less may be added.

However, 1.0! If it exceeds Ii, the effect of Ni will be saturated, so the upper limit is set at 1.0t.

B is effective in maintaining high quality surface quality of the processed parts of the present invention. 0.0030 if necessary! The following may be added to. However, 0.0030! If it exceeds k, not only the effect of B will be saturated, but also the hardness of the nugget will become hard and the cross tensile strength will decrease, so the upper limit is set at 0.0030 mm.

A steel plate having the composition described above can be spot-welded in a conventional manner after processing to produce a processed part having a welded part with excellent strength characteristics.An example of spot-welding conditions will be described below.

When setting welding conditions, you can basically use the RWMA recommended conditions as the main one, but if you want to eliminate differences in welding conditions due to changes in plate thickness as much as possible, other conditions except welding current should be set based on the plate thickness. It is preferable to determine it as a function of . That is, when the plate thickness is t (mIll), the energization time T = 10
t+2 (AC cycle), pressure P = 200 to 400
・t (kgf) etc. Note that the cooling rate of the weld zone differs depending on the length of the electrode holding time, but in order to cause sufficient Cu precipitation in the heat-affected zone, a slower cooling rate is preferable, and a shorter electrode holding time is preferable. preferable.

The reason why the plate thickness is limited to 0.4 to 10.0 m+a in the present invention is as follows. In the present invention, the materials used are hot-rolled steel plates and cold-rolled copper plates, and the plate thickness is 0.4 m.
In addition to the fact that materials thinner than 0.4 mm are not often used for the applications targeted by the present invention, when the plate thickness becomes thinner than 0.4 mm, the cooling rate after welding increases significantly, resulting in the hardness of the nugget part. Since the thickness exceeds 270, the lower limit of the plate thickness is set to 0.
．． The length shall be 4 mm. On the other hand, if the plate thickness exceeds 10.0 mm, it is rarely used for the purpose of the present invention, so the upper limit is set at 10.0 mm.

[Example] Examples of the present invention will be shown below along with comparative examples.

(Example 1) Hot-rolled steel plate with the composition shown in Table 2 (thickness: 2.0 mm)N
o, 1 to 16 are spot welded under the conditions shown in Table 1,
The hardness distribution of the weld was measured, and the cross tensile strength, tensile shear strength, and surface quality were evaluated. As is clear from Table 2, the materials in the component range of this invention are No.
It is clear that the Lemospot welds in Examples 1, 2, 8 to 15 have excellent strength properties and excellent surface quality. Comparative steel No. 3 has a high C content, but in this case, the nugget portion is hard and the cross tensile strength is poor. Comparative steel N004 has a low Mn content and poor surface quality. Comparative steel No. 5 has a high Mn content, has a hard nugget portion, and is inferior in cross tensile strength. The comparative steel N086 has a low Cu content, insufficient hardness in the heat affected zone, and poor cross tensile strength. The comparison material of steel N097 has a high Cu content;
Good cross tensile strength but poor surface quality. Comparative steel No. 16 is made of a conventional C--Mn hot-rolled steel plate, has high hardness in the nugget portion, and is significantly inferior in cross tensile strength. The method for manufacturing the hot-rolled steel sheet used as the material of the present invention is as follows. 105 pieces of steel with the composition shown in Table 2
After heating at 0℃~1250℃, 860℃~930℃
The hot rolling was completed at 350 to 150° C., and the product was manufactured by being pickled.

(Example 2) Cold-rolled steel plate with the composition shown in Table 3 (thickness: 0.8 mm) No.
, 17, and 18 were spot welded under the conditions shown in Table 4, and the hardness distribution of the welded part was measured, as well as the cross tensile strength,
Tensile shear strength and surface quality were evaluated. As is clear from Table 3, material steel No. 17 in the composition range of this invention
has excellent strength properties and surface quality of spot welds. That is clear.

Although the comparative steel No. 18 has a high C content, in this case, the hardness of the nugget portion is hard and the cross tensile strength is poor. In addition, the manufacturing method of the material cold-rolled steel sheet of the present invention No. 17 is as follows. After heating the steel slabs having the compositions shown in Table 3 at 1050°C, hot rolling was completed at 880°C and coiled at 250°C to obtain hot rolled steel plates with a thickness of 4.0II1 m. Thereafter, it was pickled, cold rolled at a rolling reduction of 80 tons, and then continuously annealed. Continuous annealing conditions are 800℃ and 1
Recrystallization annealing was performed for 1 minute, followed by cooling to room temperature at a cooling rate of 100° C./sec.

[Effects of the Invention] As described above, according to the present invention, by solving the conventional problems of spot welded joints, the use of high-strength steel plates in automobiles is promoted, and passenger safety and economy are improved. Contribute.

[Brief explanation of the drawing]

Figures 1 (a) and (b) are diagrams showing the basic structure of a spot welded joint, and Figure 2 is a diagram showing the relationship between the tensile shear strength of the spot weld and the tensile strength of the base metal. Figure 3 is a diagram showing the relationship between the cross tensile strength of the spot weld and the tensile strength of the base metal, Figure 4 is a diagram showing an example of the temperature distribution and hardness distribution of the spot weld, and Figure 5 is a diagram showing an example of the temperature distribution and hardness distribution of the spot weld. (a), (b
) is a diagram showing the shape of a welded part directly at fracture, and Figure 6 shows the relationship between cross tensile strength and nugget hardness, corresponding to the hardness of the heat-affected zone with respect to the hardness of the base metal and nugget. FIG.

Claims (1)

[Claims] 1. In weight%, C 0.015% or less Mn 0.05-1.5% S 0.030% or less Cu 0.8-3.0% P 0.100% or less Si 1.0% or less N 0.0050 % or less Sol. Contains 0.002 to 0.10% Al as an essential component, and the remainder is unavoidable impurities and F.
The plate thickness is 0.4 to 10.
0mm processed product, where the hardness of the base material (Vickers hardness) is X, and the hardness of the nugget part (Vickers hardness) is Y, then Y≦270 and the same distance as the plate thickness from the outer periphery of the nugget. The hardness of the heat affected zone (Vickers hardness) is 1/4.
A processed part having a welded part with excellent strength characteristics, characterized in that the strength is (3X+Y) or more. 2. C0.015% or less Mn 0.05-1.5% S 0.030% or less Cu 0.8-3.0% P 0.100% or less Si 1.0% or less N 0.0050% or less Sol. Contains 0.002-0.10% Al, further 0.01-0.2% Ti, and 0.00% Ti.
Contains one or more of 30% or less B and 1.0% or less Ni, with the remainder being unavoidable impurities and Fe.
Plate thickness 0.4 to 10.0 made by spot welding steel plates made of
mm processed product, the hardness of the base material (Vickers hardness)
If X is the hardness of the nugget (Vickers hardness), then Y≦270 and the hardness (Vickers hardness) of the heat-affected zone at a distance equal to the plate thickness from the outer periphery of the nugget is 1/4・(
A processed part having a welded part with excellent strength characteristics, characterized in that the strength is 3