EP0070082B1

EP0070082B1 - A high strength hot rolled steel sheet having excellent flash butt weldability

Info

Publication number: EP0070082B1
Application number: EP19820301325
Authority: EP
Inventors: Hiroshi Hashimoto; Isao Takahashi; Toshiyuki Kato; Masatoshi Shinozaki; Toshio Irie
Original assignee: Kawasaki Steel Corp
Current assignee: JFE Steel Corp
Priority date: 1981-03-19
Filing date: 1982-03-16
Publication date: 1986-12-17
Also published as: EP0070082A2; JPS57155347A; CA1200124A; EP0070082A3; JPS6221069B2; DE3274748D1

Description

The present invention relates to high strength hot-rolled steel sheets which have excellent flash butt weldability and which are suitable for the manufacture of articles such as wheel rims.
Recently, in the field of automobile production, positive efforts have been made to reduce the weight of cars in order to decrease the fuel cost per running distance. For this purpose, the use of high strength steel sheets instead of conventional mild steel sheets have been considered. More particularly, the use of high strength steel sheets for wheel rims has been studied but a variety of problems remain to be solved.
The production of a wheel rim generally comprises the following steps. Strip-shaped thin steel sheets of a given dimension are cut off from a slit steel strip and formed into a loop by roll bending. The butted end surfaces are then flash- butt-welded to form a cylinder. The weld beads fomed in this step are removed and the cylinder is then processed, by means of a subsequent cold forming stage, into the final desired shape. The wheel rim is then subjected to a test, whereby the rims are classified into acceptable ones and defective ones.
A problem encountered in such a production process concerns the formability of the flash butt welded portion at the cold forming stage. In this cold forming stage the edges are subjected to stretch flange forming and the elongation is as severe as 20-25%.
When it is attempted to use high strength steel sheets for wheel rims instead of conventional mild steel sheets, the following problems occur.

1) The high strength steel sheets have a high notch sensitivity at the sheared edges and the formability of the material itself is low.
2) The high strength steel sheets usually contain a larger amount of alloying elements, such as Si, Mn, etc., than the mild steel sheets, so that the hardness at the weld interface is increased by the flash-butt- welding and hence there is a deterioration in formability at the welded portion.

Heretofore, as steel sheets for the production of wheel rims, mild steel sheets, such as SAPH 32 of JIS G 3113 and the like have been used and the number found to be defective in the cold forming step is less than 1%. However, when high strength steel sheets are used as mentioned above, there are many problems and if the number found to be defective is taken into consideration, it is substantially impossible to use the high strength steel sheets for wheel rim manufacture in practice.
The inventors have made a large number of investigations and found that (1) if the S content is reduced as far as possible or, in addition to that, an appropriate amount of Ca and REM is added, the strength flange forming ability of high strength steel sheets can be improved and (2) the deterioration in the formability of the welded portion can be prevented by intentionally adding P, the content of which has ordinarily been kept as low as possible because of the fear of increasing the brittleness, and even if precipitation hardening elements, such as Nb, Ti or V are added, the desired properties are substantially maintained. Thus the inventors have succeeded in providing high tensile strength hot rolled steel sheets which can be applied to wheel rim manufacture which has been heretofore considered to be impossible.
In US-A-3 827 924 there are described rolled steel sheets of high strength. The rolled steel sheets may be hot rolled sheets or cold rolled sheets obtained by subjecting the hot rolled sheets to cold rolling, recrystallising annealing and skin pass rolling. In accordance with the preferred embodiment of US-A-3 827 924 the sheet is a cold rolled steel sheet for inter alia automobile applications and, as such, the sheet has excellent drawability, high strength, little susceptibility to embrittlement and excellent corrosion resistance. Sample E of Table 3 of US-A-3 827 924 discloses such a cold rolled steel sheet formed from a composition comprising 0.086% C, 0.25% Si, 0.108% P, 0.61 % Mn, 0.011% S, and 0.084% AI. This composition contains a higher sulphur content than the composition of the steel used in accordance with the present invention and there is no suggestion in US-A-3 827 924 that hot rolled sheets formed from the steel used in accordance with the present invention would-have improved flash butt weldability.
Accordingly the present invention provides a high strength hot rolled steel sheet having excellent flash butt weldability consisting of from 0.04 to 0.15% of C, from 0.5 to 2.3% of Mn, not more than 0.5% of Si provided that the amount of Mn plus the amount of Si is from 0.5 to 2.3%, from 0.05 to 0.15% of P, from 0.005 to 0.100% of Al, not more than 0.010% of S, optionally at least one of Ti, Nb and V in a total amount of from 0.01 to 0.1 %, and optionally at least one of Ca and REM in a total amount of from 0.001 to 0.05%, the remainder being Fe and incidental impurities.
For a better understanding of the invention, and to show how the same may be carried out, reference will now be made, by way of example to the accompanying drawings, wherein:

Fig. 1 is a diagrammatic view showing a cross-section of a side bend test jig;
Fig. 2 is a vertical view showing the shape of a test piece after applying the side bend test;
Fig. 3 is a graph showing the relationship between the side bend elongation and the number of defective rims;
Fig. 4 is a graph showing the relationship between the side bend elongation of the flash butt welded joints and the contents of Si+Mn and P; and
Fig. 5 is a graph showing the relationship between the side bend elongation of the flash butt welded joints and the P content.

The cold forming stage of wheel- rim manufacture involves a process wherein the edges are subjected to stretch flange forming. The "side bend test" is suitable for assessing this property accurately. The side bend test is carried out as follows. As shown in Fig. 1, a rectangular test piece 2 having sheared edges (width: 40 mm, length: 170 mm) is mounted on a test holder 1 and pressed down with a punch 3 in a direction perpendicular to the test piece and the pressing is continued until the test piece 2 is deformed and a crack A is formed as shown in Fig. 2. The elongation d' between the marked points at the curved outer surface is measured and compared with the original distance d (50 mm). The elongation obtained in this test is referred to as "side bend elongation").
The side bend elongations of so called mild steel sheet having a strength of lower than 40 kgf/mm²; of a conventionally produced solid solution hardened high strength steel sheet containing C, Si or Mn; and of a precipitation hardened high strength steel sheet containing Nb, Ti or V were measured using the above described test method. When the results were compared with the number of defective rims which were formed when the rim producing experiment was actually effected, it was found that there was a high correlation between the results as shown in Fig. 3 the side bend elongation of the welded joint must be more than 20% in order to obtain a number of defective rims which was less than 1% which is the range desired for commercial manufacture of wheel rims. In Fig. 3, the open circles representthe mild steel sheet and the solid circles represent the high tensile strength steel sheets. Only the mild steel sheet results in a number of defective rims substantially near 0.
The inventors continued their detailed study of the problem based on this discovery and found that a steel sheet obtained by adding an appropriate amount of P to high strength steel sheets containing a large amount of Si and Mn, exhibits little- deterioration in formability at the welds formed by flash butt welding. That is, when aluminium killed high strength hot rolled steel sheets consisting of 0.07 to 0.1 % of C, 1.6 to 2.4% of Si+Mn, about 0.015% (not added) or about 0.08% (added) of P, and 0.004 to 0.007% of S with the remainder being substantially Fe were flash butt welded and the side bend test of the welded joints was carried out, it was found that, as shown in Fig. 4, th side bend elongation necessary for satisfactory rim forming (that is an elongation of higher than 20%) was exhibited only by the steels with added P. However, it is apparent from Fig. 4 that even with a steel with added P, when the content of Si+Mn exceeds 2.3%, the strength of the base material becomes too high and the side bend elongation of the welded joint suddenly lowers. Accordingly, it has been found that the addition of P is an essential requirement and that Si+Mn must be at most 2.3%.
In order to examine the influence of the P content upon the side bend elongation of the welded joints, aluminium killed high strength hot rolled steel sheets consisting of 0.07 to 0.1 % of C, about 1.9% of Si+Mn, 0.015 to 0.2% of P, and 0.004 to 0.007% of S with the remainder being substantially Fe were flash butt welded and the side bend elongation of the welded joints was measured. It was found that the side bend elongation corresponding to feasible rim forming (that is an elongation of higher than 20%) was obtained when the P content was not less than 0.05%. The reason why the side bend elongation at flash butt welded portions of high strength steel sheets is considerably improved by the addition of P of not less than 0.05%, is not presently clear but is believed to be as follows.
In general, a flash butt-weld has a structure containing bainite or martensite with increased amounts of Si+Mn and the hardness becomes greater. However it has been found that when P is added to steel having such a structure, there is an increase in ferrite at the weld and the ferrite grains at the heat affected zone are apt to become larger. Accordingly, the difference in hardness between the base metal and the weld presumably becomes small in the case of P added steel and hence the formability of the flash butt weld is improved.
A first aspect of the present invention arises as a consequence the above described discovery and resides in a high strength hot rolled steel sheet having excellent flash butt weldability and consisting from 0.04 to 0.15% of C, from 0.5 to 2.3% of Mn, not more than 0.5% of Si provided that the amount of Mn plus the amount of Si is from 0.5 to 2.3%, from 0.05 to 0.15% of P, and from 0.005 to 0.100% of AI, wherein the S content is reduced to less than 0.010% and the remainder is Fe and incidental impurities.
. When a higher strength is required, at least one of Ti, Nb and V, which are precipitation hardening elements, is added to the above composition in a total amount of from 0.01 to 0.1 %. Additionally, or alternatively, when the formability is to be improved, at least one of Ca and REM, which are the sulfide forming elements, is added to the above described composition in a total amount of from 0.001 to 0.005%. In a preferred embodiment, one or more of the above described precipitation hardening elements and one or more of the above described sulfide forming elements are added.
Explanation will now be made with respect to the reason for limiting the fundamental component composition of the first aspect of the present invention.
The C content needs to be at least 0.04% for ensuring the strength while, when the C content exceeds 0.15%, the deterioration in formability increases, so the C content is limited within the range of 0.04 to 0.15%.
The Mn content needs to be at least 0.5% for obtaining high tensile strength but when said content exceeds 2.3%, the formability is considerably deteriorated. Thus the Mn content is limited within the range of 0.5 to 2.3%.
Si is an active element for increasing the tensile strength but when the Si content exceeds 0.5%, the phosphate treating ability, which is a pretreatment upon coating, is deteriorated, so that the addition of Si is at most 0.5%.
When the Mn+Si content exceeds 2.3%, the side bend elongation of the welded joint is lower than 20%, so the upper limit of Mn+Si content is 2.3%. The lower limit is 0.5% which is necessary for obtaining high tensile strength.
As shown in Fig. 5, unless the P content is not less than 0.05%, a side bend elongation of more than 20% can not be obtained. If said content exceeds 0.15%, the effect is maintained but the brittleness becomes noticeable so the upper limit is 0.15%.
AI is an element used for deoxidation but when the AI content is less than 0.005%, the deoxidation is insufficient and the formability deteriorates because of oxide inclusions. On the other hand, when the AI content exceeds 0.10%, alumina inclusions increase, the surface state deteriorates and further the formability deteriorates. Thus the Al content is limited to the range of 0.005 to 0.10%.
The S content is preferred to be low as far as possible. In high strength steel sheet, the notch sensitivity of the sheared edge is high and the deterioration in formability increases. Thus it is necessary to .reduce the sulfide inclusions as far as possible. For this purpose, S must be at most 0.01% and is preferably not more than 0.007%. Explanation will now be made with respect to the second aspect of the present invention wherein the high strength hot rolled steel sheet having excellent flash butt weldability in accordance with the first aspect of the invention . additionally contains at least one of Ti, Nb and V in a totai amount of from 0.01 to 0.1 %.
In general, when a high strength steel sheet is produced, a solid solution hardening element, such as C, Mn or Si, is included. However, when a high strength steel sheet having a higher strength is required, it is usual to add a precipitation hardening element, such as Nb, Ti or V, together with these solid solution hardening elements. When the inventors, examining the affect of added P on the side bend elongation, added at least one of the above described precipitation hardening elements to the composition in accordance with the first aspect of the present invention containing 0.05 to 0.15% of P, it was found that the presence of from 0.01 to 0.1% in total of at least one of Ti, Nb and V improved the tensile strength without deleteriously affecting the improvement in the side bend elongation caused by the addition of P.
The reason why the total amount of at least one of Ti, Nb and V is limited within the range of 0.01 to 0.1% is as follows. When the content is less than 0.01 %, it is difficult to obtain high strength and when the content exceeds 0.1 %, the improvement in the strength does not increase in proportion to the increase in the added amount.
Explanation will now be made with respect to the third aspect of the present invention, wherein the high strength hot rolled steel sheet having excellent flash butt weldability in accordance with the first aspect of the invention additionally contains at least one of Ca and REM in a total amount of from 0.001 to 0.05%.
As mentioned above when giving the reason for limiting the S content in accordance with the first aspect of the present invention, the smaller the amount of sulfides in the high strength steel sheets, the higher is the formability of the steel. However, it has been known that, even for a given amount of sulfide, there is a difference in formability depending upon the state of the sulfide in the steel. When the sulfide is present as MnS, said sulfide is generally in the elongated state in the rolling direction, so that formability in the direction perpendicular to the rolling direction is poor. In order to improve this defect, it is effective to add Ca or REM so that the sulfide is in the form of small globules.
When the inventors, examining the affect of added P on the side bend elongation, added at least one of the above described elements Ca and REM to the composition in accordance with ths first aspect of the present invention containing 0.05 to 0.15% of P, it was found that the presence of from 0.001 to 0.05% in total of at least one of Ca and REM improved the side bend elongation of the welded joint and the side bend elongation of the base metal.
The reason why the amount of at least one of Ca and REM is limited within the range of from 0.001 to 0.05'% in total is as follows. Ca or REM combines with S to make globular inclusions whereby the formability is improved, so these elements are added in an amount-of no less than 0.001% alone or in combination. However the addition of more than 0.05% does not increase the affect and oxide inclusions are formed and the formability deteriorates somewhat. Thus the amount is at most 0.05%.
Explanation will now be made with respect to the fourth aspect of the present invention wherein the high strength hot rolled steel sheet having excellent flash butt weldability in accordance with the first aspect of the invention additionally contains from 0.01 to 0.1 % in total of at least one of Ti, Nb and V and from 0.001 to 0.05% in total of at least one of Ca and REM, concurrently.
Experiments have been made in the same manner as mentioned with respect to the second and third aspects of the present invention and it has been found that the steels of the fourth aspect of the invention have improved side bend elongation due to the addition of P and improved strength and also are improved in regard to the side bend elongation of the base metal.
In general, when the content of Mn and Si is increased, the oxides of Si and Mn remain in the weld interface and may cause weld defects. When the Mn/Si ratio is from 4 to 12, the melting point of the oxides formed during the welding is lowered and therefore said oxides are readily removed when upsetting and the properties of the weld are improved.
The steels of the present invention can be produced by means of known techniques using conventional installations and therefore their production will only be explained in outline.
Steels refined in a conventional refining furnace, such as a converter, are cast by means of a slab making process or a continuous casting process. In this case, it is necessary to make the S content less than 0.010%, so it is preferable for the S content to be made low after the iron melting step. The steel ingot or the continuously cast slab is formed, by means of the usual hot rolling installation and technique, into a hot coil. The thickness of the sheet is in many cases 2 to 4 mm.
The following Examples are given for the purpose of illustration of this invention and are not intended as limitations thereof.
Steels having 12 different chemical compositions, as shown in Table 1a, were refined in a converter and continuously cast. The cast steel slabs were reheated at a temperature of 1,250 to 1,300°C and hot rolled at a finishing temperature of 850°±20°C and a cooling temperature of 580°±20°C before being passed through a general pickling line or a shot line to obtain a hot rolled steel sheet having a thickness of about 2.6 mm. The resultant steel sheets were tested with respect to the properties as shown in Table 1b. The "percent defective in wheel rim forming" shown in the following Table 1 was determined by carrying out a production test on the rim forming line using the above described steel sheets.
As can be seen from Table 1b, the side bend elongation of aluminium killed high strength hot rolled steel sheets (A-X) of the present invention containing the defined amounts of C, Si, Mn, P and S exhibits little deterioration in the flash butt welded joints (the reduction being only from 2 to 4%) and has a value higher than 20% which indicates that wheel rim forming is feasible.
In steel I (which is a comparative steel where the P content is as low as 0.015%) the side bend elongating percent of the base metal is high but that of the welded joint is as low as 19%. In steel J (which is a comparative steel where the P content is as high as 0.183%) the side bend elongating percent of the welded joint is as high as 25% but there is noticeable brittleness so steels having this amount of P are excluded from the present invention. In steel K (which is a comparative steel where Si+Mn is as high as 2.48%) the side bend elongation of the welded joint is as low as 11 %. In steel L (which is a comparative steel where the S content is as high as 0.012%) the side bend elongation of the base metal is low and the side bend elongation of the welded joint is as low as 18%.
As mentioned above, the steels of the present invention exhibit little deterioration in the side bend elongation of the flash butt weld and have properties enabling their application to the commercial manufacture of wheel rims. Hitherto the use of high strength hot rolled steel sheets for this purpose has been considered to be impractical. Thus, the present invention can contribute to a reduction in the fuel cost per . running distance because it enables the car weight to be decreased.
The steels of the present invention are also applicable to the manufacture of other articles involving working after flash butt welding as well as wheel rims.

Claims

1. A high strength hot rolled steel sheet having excellent flash butt weldability consisting of from 0.04 to 0.15% of C, from 0.5 to 2.3% of Mn, not more than 0.5% of Si provided that the amount of Mn plus the amount of Si is from 0.5 to 2.3%, from 0.05 to 0.15% of P, from 0.005 to 0.100% of Al, not more than 0.010% of S, optionally at least one of Ti, Nb and V in a total amount of from 0.01 to 0.1%, and optionally at least one of Ca and REM in a total amount of from 0.001 to 0.05%, the remainder being Fe and incidental impurities.

2. A high strength hot rolled steel sheet as claimed in claim 1, wherein the value of Mn content/Si content is from 4 to 12.

3. A high strength hot rolled steel sheet as claimed in claim 1 or 2 wherein the S content is not more than 0.007%.

4. A method of manufacturing an article which comprises flash butt welding and cold forming a high strength hot rolled steel sheet as claimed in any one of the preceding claims.

5. A method as claimed in claim 4 wherein the article is a wheel rim.