DE2424685A1 - HIGH-STRENGTH, COLD-ROLLED STEEL SHEET WITH EXCELLENT OVERLAP RESISTANCE WELDABILITY - Google Patents

Description

High-strength, cold-rolled sheet steel with excellent Overlap resistance weldability ^

The invention relates to high strength, cold rolled steel sheets and steel strips, particularly high strength, cold rolled ones Steel sheets with excellent workability and overlap resistance weldability.

Heretofore, high-strength, cold-rolled steel sheets have been used to a limited extent, for example for hard plate. For this reason, no value was placed on the weldability. Therefore, a conventional steel was used with a content of 0.10 to 0.20 i »C after partial Bekristallisieren or double cold reduction. If increased hardness was required, the content of elements such as P and N was increased somewhat, as described in JA-OS 70-34015. Also in JA-0Sen 71-9541 and 71-9542, there is a method of manufacturing a high-strength cold-rolled steel sheet having a tensile strength of 60 to 120 kg / mm ² by quenching a steel containing 0.04 to 0.25 # C and 0.60 ί> or less Mn. The materials obtained in these processes

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however, have poor processability and lap resistance weldability on. Considering the quality of the product in terms of constant conditions during the manufacturing process carefully examining how, for example, the processes for partial recrystallization and quenching, the result is that here the quality of the product is impaired by fluctuations in the process conditions and not qualitatively consistently good product is achieved.

Recently, however, the problem of internal security in the manufacture of automobiles has come to the fore more and more and for this reason, there is an increasing demand for high strength cold rolled steel sheets with excellent weldability and processability (press formability). In particular, the weldability of these materials was used as reinforcements a serious problem for safety bodies. Since in these cases mostly spot resistance welding or the When seam resistance welding is used, the steel must have excellent lap resistance weldability.

The object of the invention is to create a high-strength, cold-rolled steel sheet which meets the above-mentioned requirements.

Another object of the invention is to provide a high strength cold rolled steel sheet excellent in press formability O

Another object of the invention is to provide a high strength cold rolled steel sheet with excellent lap resistance weldability.

The invention will now be further explained with reference to the following description and the drawings:

In the drawings:

- 3 - ■

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Fig. 1 is a graphical representation of the relationship between: the tensile strength and overall ductility of various high strength cold rolled steel sheets; I.

Fig. 2 is a graph showing the hardness of the welding formed lump and the characteristics of the! Crack formation as a function of the carbon content j of thin steel sheets, obtained with a shear! try and

3 is a graph showing the influence of the value of Nb / C on the tensile strength of the base sheet and the spot welded part.

According to a preferred embodiment of the invention

a steel according to the invention with a content of 0.05 to 0.15, 0.20 to 0.50 # Si, 0.30 to 1.30 * Mn and up to 0.030 * S and an addition of Nb as a carbide-forming component in one j amount of 0.3 ^ fib / C £ 3.0 subjected to the normal process steps of j hot rolling, cold rolling and tempering, whereby a j high-strength, cold-rolled steel sheet with a tensile strength j of 45 to 60 kg / am ² or a yield stress of 30 up to 45 kg / | mm ² with excellent lap resistance weldability j is obtained.

The invention provides a steel sheet which is excellent in both tensile strength and lap resistance weldability. The improved overlap drag tail Availability is an essential E property for a useful high-strength cold-rolled steel. With regard to processability, FIG. 1 shows the dependence of the tensile strength and the overall ductility of various high strength cold rolled steel sheets. From this figure results that the steel sheets according to the invention have superior properties

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shafts in terms of overall extensibility and processability have compared to the known steel sheets, although due to differences in the manufacturing processes of the range the strength is different.

Generally recognized methods for evaluating spot weldability are described in regulation Z 3137 of JIS (Japanese Industrial Standard). The spot weldability is determined by the absolute values of tensile strength and transverse tensile strength around the ratio between these two, namely the weld stretchability ratio of the weld point (transverse tensile strength / tensile strength) according to the above-mentioned test methods. If the obtained values are equal to or larger than the SPCC material (cold rolled metal plate according to G3141 of JIS), the weldability is good. The transverse extensibility is particularly important, as it reflects the degree to which the weld flakes off in the event of impacts. In the steel according to the invention, Nb is added in an amount of 0.3 * Nb / C »3.0, whereby the above-mentioned properties of the steel sheet become better than those of the SPVC material. In addition, if S is added in an amount of = 0.015 $> , the transverse tensile strength and the elongation ratio of the weld are greatly improved.

In Fig. 2 is the relationship between the maximum hardness of the natural when welding in the spot welded part formed metal lump and the carbon content and you can see how the carbon content increases the properties of Affects steels that do not contain any additives. The drawing shows that at a content of C £ 0.12 # a Cracking occurs in the base metal and, if the content of C> 0.12 #, hot in the metal lump formed during welding uni also occurs during a tensile test the ductility of the weld deteriorates. If against

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Nb in an amount- © η 0.3 = Nb / C = 3.0 to such a steel at a level of $ 00.12 is added, cracking occurs in the base metal and the transverse tensile strength and ductility are impaired of the weld are considerably improved, although the maximum hardness of the metal lump formed during welding increases (see. The steels which are provided with dots in Fig. 2).

The content of individual elements in the steel according to the invention will now be further explained below:

Since the content of Nb and S are characteristic features of the steel according to the invention, an individual is made detailed explanation based on the example. The other main components are explained below:

In the steel according to the invention, the upper limit of the carbon content is $ 0.15, since it has a higher carbon content an addition of larger amounts of Nb is required, causing practical difficulties and little improvement in Extensibility of the weld result. If the carbon content is less than $ 0.05, there will be insufficient strength achieved. Si and Mn, which are mainly used as preliminary deoxidizing agents, also serve for improvement in strength, however, they cause little, if any, reduction in spot weldability. With regard to cold workability the maximum salary for Si to be preferably $ 0.50 and the maximum salary for Mn $ 1.30. With regard To maintain the strength, the minimum content of Si should be $ 0.20 and the minimum content of Mn should be $ 0.30. The maximum salary at P is said to be $ 0.050 in terms of toughness, penalty workability and weldability of the product. The dissolved Al used as a deoxidizer to reduce Inclusions and also to improve the effects of Nb should be used in an amount between $ 0.003 and $ 0.08.

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The invention will now be further explained using the following example:

example

Nb-containing steel with the composition listed in Table 1 was melted in a furnace for steel production, divided into 12-ton batches and rolled flat. The flat-rolled product was then hot-rolled at a finish temperature 830 to 880 ⁰ C and then wound ribbon at a temperature between 500 and 55O ⁰ C, a strip was obtained having a thickness of 2.8 mm. After descaling the strip was cold-rolled to a thickness of 1.0 mm and then annealed to Eekristallisation for 5 hours at a temperature of 700 ⁰ C and tempergewalzt 1.5 #, where a finished product was obtained.

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Table 1

Vsxand-
Artfvteil C. Si Chemical composition, (#) P. S. Sol.
Al. Nb Nb / C of \ 0.13 0.28 0.012 0.022 0.035 tr 0 StahlV 0.10 0.21 Mn 0.013 0.025 0.038 0.013 0.13 A. 0.11 0.35 1.16 0.011 0.019 0.045 0.035 0.32 B. 0.12 0.31 1.03 0.010 0.026 0.066 0.093 0.78 * C 0.09 0.25 0.96 0.015 0.030 0.030 0.213 2.37 * D 0.11 0.40 0.83 0.017 0.022 0.056 0.502 4.57 * E 0.09 0.36 0.75 0.012 0.011 0.048 tr 0 F. 0.08 0.27 0.90 0.015 0.010 0.051 0.017 0.21 G 0.11 0.28 0.81 0.014 0.007 0.029 0.039 0.35 H 0.12 0.30 1.01 0.011 0.008 0.018 0.088 0.73 * I 0.10 0.36 0.79 0.015 0.006 0.055 0.235 2.35 * J 0.10 0.35 0.85 0.016 0.008 0.038 0.551 5.51 * K 0.81 L. 0.99

Notes: * Steels with a composition within the range according to the invention.

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The steels A - F in the table above serve to illustrate, how the Nb content affects the properties of steels with a common content of S (0.015 to 0.030 #). Of these steels, steels C, D and E are in the invention Composition area.

The steels G to L serve to illustrate how the Nb content influences the properties of steels that are up to Desulfurized an S content of $ 0.015 or less. from of these steels, steels I, J and K are in the composition range according to the invention.

The mechanical properties and spot weldability properties of steels with the composition according to Table 1 are listed in Table 2 below:

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Tabel

\ Property
a "\ xen • Stretching
grenzeo
(kg / miir Mechanical properties and spot welding properties - Gesami
stretchy ^ -Hardness
. (Hv) Durehm. d.
Naturally"
Core v,
melted
zenem Me
tall (mm) , Tensile strength
speed, (k _g 7
Sweat
Point ) Transverse pull
strength,
(kg / sweat
Point) Stretchy
ability d.
Sweat
sung ■ maximum
Hardness d.
of course 1. core
v.geschmol
zenem metal ArX \
of \
StählsV 30.5 Tensile strength
speed ρ 37.8 125 6.2 1030 835 80.8 356 A · 35.1 41.6 35.7 147 6.7 1090 820 75.3 379 B. 38.6 44.7 32.3 151 6.9 1150 885 77.0 405 * C 41.9 47.9 30.8 155 7.1 1180 905 76.8 412 * D 41.6 50.1 29.5 155 7.2 1205 955 79.3 425 * E 42.3 51.6 29.1 156 7.0 1200 815 68.1 435 F. 30.5 50.7 38.1 124 6.3 1025 865 84.7 367 G 33.9 41.3 36.0 143 6.6 1080 870 80.5 369 H 38.1 43.1 32.5 152 6.9 1145 915 80.1 412 * I 43.5 47.3 30.9 153 7.2 1170 1020 86.7 405 * J ' 42.8 50.6 30.1 159 7.3 1200 1070 89.2 396 y * K 43.0 51.2 29.6 159 7.1 1210 900 74.7 393 pounds L. 49.8 σ
OC cn

Notes: * steels whose composition range corresponds to the invention,

In general, it is considered expedient to limit the welding current during spot welding and to increase the welding pressure, corresponding to the increase in the tensile strength of the basic element. The results obtained in Table 2 above were carried out by Applying a welding current and welding pressure suitable for the tensile strength of the steel. The remaining Welding conditions were as follows:

Electrode JIS type Cr-Gu Current period 10 periods Initial hold time 15 periods

If the value of the welding current is greater than the appropriate value, surface evaporation occurs (splashing of the molten metal). If the value is too small, it becomes the diameter of the metal lump formed during welding decreases and there is a decrease in tensile strength. It is therefore necessary to carry out the welding accordingly for everyone Steel to carry out the given tensile strength conditions.

As can be seen from Table 2 and FIG. 3, the tensile strength and transverse tensile strength increase as the content of Nb increases. However, the transverse tensile strength decreases if the value of Nb / C is more than 3. ' If the value of Nb / C = 3, the value of the transverse tensile strength increases more than that of the tensile strength. It is therefore clear that a steel with the composition range defined according to the invention, ie 0.3 = Nb / C << 3 _{t, has} a transverse tensile strength and weld ductility which is equal to or greater than that of normal mild steel. The question of the sulfur content can be left out of consideration if it remains below the generally accepted level of O, O3O? 6. If the steel is but desulfurized to a sulfur content of 0.015 i »S or less, the transverse tensile strength is considerably improved.

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The invention creates a consistently high-strength cold-rolled steel sheet with a tensile strength of 45-60 kg /
mm or a yield point of 30-45 kg / mm as well as an equally good or even better overlap resistance than normal mild steel.

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Claims (1)

Claim: High strength cold rolled steel sheet excellent in lap resistance weldability, made by hot rolling, cold rolling and recrystallization annealing of a steel containing 0.05 to 0.15 # C, 0.20 to 0.50 # Si, 0.30 to 1.30 # Mn , up to 0.030 fi S, 0.3 = Wb = C, remainder iron and unavoidable impurities.