JPS592734B2 - High-strength cold-rolled steel plate with excellent impact resistance and spot weldability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a high-Si-based high-strength cold-rolled steel sheet that has excellent impact resistance and spot weldability, particularly spot weldability without deteriorating impact secondary workability.

Recently, there has been an extremely strong demand from various fields for the development of high-strength steel sheets with excellent press workability, weldability, etc.

For example, in response to the tightening of safety regulations to protect automobile occupants due to the increase in car-to-vehicle accidents, high-strength steel sheets are being used to strengthen car bodies, as they have a high impact energy absorption capacity and are less deformed during collisions. On the other hand, with regard to the installation of new anti-pollution equipment in response to stricter exhaust gas regulations, efforts are being made to reduce weight by thinning steel plates in order to avoid an increase in vehicle weight and the resulting increase in fuel consumption. For this purpose, high-strength steel plates with high strength are required.

In addition, such thin steel sheets are generally formed by press working and assembled by welding to be used as an integral structure, so they can be formed into complex shapes with good dimensional accuracy without breakage or damage. It can be molded and has sufficient workability to maintain a certain toughness after molding, and when assembled as a welded structure in a subsequent assembly process,
Since it is necessary to have good strength, impact resistance, fatigue strength, etc., it is required to have excellent weldability, especially spot weldability.

In response to such demands, high-manganese (Mn)-based high-strength cold-rolled steel sheets have been proposed, but these are not necessarily sufficient in terms of workability, strength-ductility balance, and the like.

For this reason, the present inventors first proposed to provide a high-Si type high-strength cold-rolled steel sheet.
No. 48, etc.), and has responded to the above request.

The above-mentioned high-Si cold-rolled steel sheet far exceeds conventional high-strength steel in terms of performance such as workability, strength-ductility balance, and weldability, but it has the ability to cope with increasingly severe usage conditions. In order to achieve this goal, there remains room for further study and improvement of the strength-ductility balance, impact secondary workability, etc.

Based on this recognition, the present inventors have conducted intensive research to provide an even better high-strength cold-rolled steel sheet, and as a result have developed the 8 i/M
While adjusting the n (weight) ratio to 1 or more, P is set to 0 along with Mo.
．． If more than 0.03% is actively added, the spot weldability (tensile shear strength) will be further improved than only by adjusting the S i /M n ratio without deteriorating the impact secondary workability (see Table 2 below). ), and have completed the present invention.That is, the present invention is based on C0.03 to C0.03.
2%, Si1.0-3.0%, Mn 0.2-2.0
%, Po, more than 0.3% and less than 0.2%, Mo0.10%
more than 0.6% and preferably NbO, 01-0
, 1st Section, Vo, 01~0.1%, T i 0.02~0
．． 2% and one or more of ZrO, 01 to 0.2%, the balance being iron and inevitable impurities, and having a Si/Mn (weight) ratio of 1 or more. The purpose of the present invention is to provide a high-strength cold-rolled steel sheet with excellent weldability and spot weldability.

Next, the present invention will be explained in detail.

C is generally not an effective element for increasing the strength of steel, and also has a large effect on improving secondary impact workability, and the amount of C added is determined from this point of view.

If the amount added is less than 0.03%, not only the above effects will be insufficient, but also the spot weldability will be poor;
If it exceeds C0.03~0, a huge cementite will be generated and cause deterioration of press formability and spot weldability.
．． It shall be 20%.

A range of 0.03 to 0.15% is particularly suitable.

Si is a main element for imparting strength, and the steel of the present invention is basically a high 3i steel plate.

The rate of increase in tensile strength with respect to the amount of Si added is about twice as high as that of Mn in high Mn steel.

Compared to the case where strength is imparted by Mn, this can reduce the amount of alloy added by half and reduce costs, and can eliminate difficulties in manufacturing technology associated with the addition of a large amount of alloy.

In addition, compared to high-Mn steel sheets, high-Si steel sheets have less hardness reduction during hot rolling and cold rolling-annealing, maintain high strength, have an excellent strength-ductility balance, and have excellent stretch flangeability and deep It also has excellent squeezing properties.

By adding a certain amount of Si in this way, it is possible to significantly improve workability represented by high strength, total elongation, stretch flangeability, deep drawability, etc., and the addition amount is 1.0
~3.0 section is required.

If it is less than 1.0%, the strength is insufficient, while 3.0%
Addition of a large amount exceeding the above range will cause problems in steelmaking operations and will also result in significant deterioration of cold workability.

In order to fully utilize the features of such high-Si steel sheets, S
It is necessary that the weight ratio of i and Mn (S i/Mn ) be 1 or more.

Further, this Si can also sufficiently function as a deoxidizing agent when added in an amount within the above range.

Mn is an element effective in imparting strength and preventing red brittleness during hot rolling.

Furthermore, when attempting to increase strength by adding only Si, the transformation point may rise excessively, which may cause various troubles during hot rolling or cause deterioration of spot welds. , certain restrictions are placed on the amount of Si added.

Therefore, strength is supplemented by solid solution strengthening of Mn, and C
It is necessary to add at least a coefficient of 0.2 or more for the purpose of refining the structure of the spot weld in coexistence with the above.

However, it is not necessary to exceed 2.0, especially between 0.2 and 1.
Section 5 is preferable.

P is an extremely effective element for improving workability and strength by adding a certain amount to a substrate covering Si and Mn as main reinforcing elements.

In particular, it has been found that spot weldability (tensile shear strength) can be improved in high-Si steels with a Si/Mn ratio of 1.0 or more (see Table 2).

For this purpose, it is necessary to add more than 0.03% of Kokichi, and as the amount added increases, the strength and workability improve significantly.

However, if it exceeds 0.2%, impact secondary workability deteriorates.

Therefore, it is more than 0.03% and less than 0.2%, preferably 0.
．． It shall be more than 0.3% and less than 0.15%.

Mo has Si and Mn as basic reinforcing elements, and P
By adding an appropriate amount to high-strength cold-rolled steel with improved workability etc., the strength-ductility balance, strength-stretch flange balance, and impact secondary workability are significantly improved.

In particular, the tendency for impact secondary workability to deteriorate due to the addition of a large amount of P is a serious problem, and attempts have been made to deal with it by adjusting the amount of added C, At, Si, etc., but there are no sufficient solutions. It has not become possible.

The effect of improving impact secondary workability by adding a certain amount of Mo is extremely remarkable, and although the improvement mechanism has not yet been fully elucidated, the present inventors have found that MO improves the structure after annealing. It is believed that it greatly contributes to improving the properties of high-strength cold-rolled steel sheets by having a finer graining effect and favorably changing the precipitation state of carbides.

In addition to the above-mentioned improvement effect, Mo also has the effect of improving yield strength and tensile strength due to its ferrite strengthening effect.

In order to effectively exhibit the above-mentioned effects of adding Mo, the amount added should be more than 0.10% and less than 0.6%.

If the coefficient is less than 0.1, the effect of improving secondary impact workability is insufficient; on the other hand, if the coefficient exceeds 0.6, not only will the improvement effect of secondary impact processability become slow, but recrystallization will occur. Although it causes a rise in temperature and the strength increase effect is not large,
The degree of deterioration in workability is large, which only unnecessarily increases costs. Therefore, the upper limit of Mo should be 0.6%.

Particularly preferred is a range of 0.15 to 0.40%.

In addition to the above elements, Nb, V, and Ti as necessary.

Precipitation-strengthening elements such as Zr can be added.

When particularly strong strength is required, for example, tensile strength of 60 to 9
In order to obtain a high strength cold rolled steel sheet exceeding /-, one or more of the above precipitation strengthening elements should be added in addition to the solid solution strengthening elements of Si and Mn, which are added as basic strengthening elements. This makes it possible to significantly improve strength.

To strengthen with the above elements, for example, Nb0.01
%~0.1 ratio, Vo, 01~0.1%, T i O, 0
It is effective to add one or more of 2 to 0.2% and Zr0901 to 0.2%.

If the amount of each element added is less than the above, the effect of adding each element will be insufficient, and even if the amount of each added above is exceeded,
The increase in effectiveness is slow and there is a cost disadvantage.

Note that A4 can be added as appropriate as a deoxidizing element.

Due to its deoxidizing effect, At
In addition to contributing to improving the addition yield of Zt and the like, it also suppresses or prevents the tendency for impact secondary workability to deteriorate due to the addition of P.

From this point of view, it is effective to set the amount of A4 added to 0.01% or more, but on the other hand, if the amount added is too large, the amount of alumina produced will increase, leading to surface defects such as sliver scratches.
Its upper limit should be 0.15%.

Especially when the surface quality of the product is strictly required, 0.0
It is desirable to specify it at 8% or less.

In addition, most of the oxygen in steel exists as nonmetallic inclusions,
If the amount increases, impact secondary workability deteriorates, and at the same time, C, Si, Mn, P added as strength imparting elements
It is desirable that the amount of oxygen is 0.015% or less, since this reduces the effects such as.

In addition, impurities such as S that are unavoidably mixed in deteriorate the material properties represented by stretch flangeability, so it is desirable that it be as low as possible; for example, S is preferably 0.03% or less.

Next, the properties of the high-strength cold-rolled steel sheet according to the present invention will be described with reference to examples.

The test steel having the chemical composition shown in Table 1 was made into a 2.7u thick hot-rolled plate through the usual melting, blooming and hot rolling processes, and then pickled and cold rolled to a 0.0mm thickness. An 8u thick cold rolled steel plate was obtained.

The steel plate was annealed at 700° C. for 3 hours and then subjected to about 1 inch of temper rolling to obtain a product.

Table 2 shows the mechanical properties, workability and spot weldability of the product.

In the same table, the spot weldability is expressed as the cross tensile shear strength (Kq) of the welded part, and the impact secondary workability is expressed as the molded cup (blank diameter 70mm, product cup diameter 33mm It was formed by step drawing.

), a test method was carried out in which the cup edge was expanded, and the temperature at which the incidence of longitudinal cracking was 50 was defined as the longitudinal cracking transition temperature, and this was used as an index representing the impact secondary workability.

The classification of groups (I) to (Iv) in Tables 1 and 2 is P content of 0.01, 0.03, and 0.08, respectively.
% and 0.12%.

In addition, group ■ has various P contents of the above groups, and also has N
b, V, Ti and/or Zr.

If the balance between tensile strength and ductility is illustrated for each group, the first
As shown in the figure.

In the figure, I, II. ■ and ■ are the above groups (1
), (n), (4) and (5).

As is clear from the figure, the addition of P can improve tensile strength without reducing elongation, and the effect of improving the strength-ductility balance becomes more pronounced as the amount of P added increases. Is recognized.

In addition, as for those of group (V), as is clear from Table 2, the tensile strength is further improved.

However, as is clear from each group in Tables 1 and 2, an increase in the amount of P added is accompanied by an increase in the transition temperature, that is, a deterioration in the impact secondary workability.

However, the deterioration of impact secondary workability due to the addition of P is
This can be solved by adding o.

For example, 111) (P O, 12%
), the transition temperature decreases (i.e.,
It is recognized that the impact secondary workability is improved.

This is illustrated in FIG. 2, from which it can be seen that Mo0.
Although the effect is insufficient when the content is less than 1%, it is clearly recognized that the transition temperature decreases rapidly when the content exceeds 0.1%.

Regarding spot weldability, looking at the cross tensile shear stress in Table 2 above, it is clear that the steel of the present invention is much better than the other test steels.

Regarding spot weldability, it has been generally believed that as the Si content increases, it deteriorates significantly and that 0.6% Si is the limit, but as is clear from the table, Si1. It is recognized that even in high-Si systems exceeding Ol, significant improvements can be made by adding P and Mo.

In addition, the appropriate C content in order not to inhibit the spot weldability of high-strength cold-rolled steel sheets whose basic components are Si + Mn is 0.0.
3 to 0.2, and even when P and Mo are added simultaneously, it is desirable to maintain this appropriate content range.

As described above, the steel of the present invention has a basic composition of Si and Mn, and by adding a certain amount of P and Mo to this, the steel has improved properties such as strength, workability, weldability, and impact secondary workability. This is an improved product that can fully meet the severe demands for quality improvement as a high-strength cold-rolled steel sheet for applications such as automotive steel sheets.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the strength-ductility balance using P% as a parameter, and FIG. 2 is a graph showing the influence of Mo on the transition temperature.

Claims (1)

[Claims] I C0.03-0.2%, S i 1.0-3.0
%, Mn0.2-2.0%, Po, over 03% and 0.
Section 2 and below, M. High impact resistance and spot weldability, characterized by having a coefficient of more than 0.10% and less than 0.6%, the balance being iron and unavoidable impurities, and a Si /Mn (weight) ratio of 1 or more. Strength cold rolled steel plate. 2 C0.03~0.2%, Si 1.0~3.0
%, Mn0.2-2.0%, Po, over 0.3% and 0.2
% or less, M. More than 0.10% and less than 0.6% and NbO, 01 to 0
．． Section 1, Vo, Section 01-0.1, Ti0.02-0.2
The S
A high-strength cold-rolled steel sheet with excellent impact resistance and spot weldability, characterized by an i/Mn (weight) ratio of 1 or more.