EP0040553A1

EP0040553A1 - Process for producing a dual-phase steel

Info

Publication number: EP0040553A1
Application number: EP81302260A
Authority: EP
Inventors: Roger Charles Hudd; Alun Jones
Original assignee: British Steel Corp
Current assignee: British Steel Corp
Priority date: 1980-05-21
Filing date: 1981-05-21
Publication date: 1981-11-25
Also published as: JPS579831A

Abstract

Dual-phase steel produced by intercritically annealing strip exhibiting a bainitic or acicular ferrite structure, the composition of the steel comprising 0.03% to 0.25% carbon, 0.3% to 2.5% manganese, up to 1.5% silicon, up to 0.25% molybdenum and up to 2% chromium.

Description

This invention relates to steels displaying a high uniform and total elongation combined with a high ultimate tensile strength. More particularly, the invention is concerned with steel strip having such mechanical properties and having a structure which consists predominantly of ferrite and martensite.
In general, attempts to improve the tensile strength of a steel intended to have good drawing characteristics produces a decrease in ductility and makes the steel less satisfactory for producing components by drawing or by stamping. In, for example, the case where components having high tensile strength are to be produced by stamping from strip, any attempt to increase the tensile strength of the strip tends to result in localised necking produced during the stamping operation as a result of insufficient ductility.
It is an object of the present invention to produce steel strip in which a combination of high tensile strength and good ductility is achieved.
According to one aspect the present invention provides a process for producing a dual-phase steel in which steel strip is hot rolled and coiled at a temperature of between about 350°C and about 580^oC, and in which the strip is subsequently continuously annealed in the two phase ferrite austenite field and cooled to transform part or all of the austenite to martensite, the composition of the steel comprising,.by weight, 0.03% to 0.25% carbon, 0.3% to 2.5% manganese, up to 1.5% silicon, up to 0.25% molybdenum and up to 2% chromium, the remainder being i_lon except for incidental impurities and residuals.
To achieve the properties of the invention, namely, high tensile strength and good ductility, the structure should comprise a significant volume fraction of colonies of substantially martensite in a mainly ferrite matrix, eg up to 60%, and for good mechanical characteristics the martensite should be finely dispersed within the ferrite matrix.
More particularly, the steel strip may be coiled at between about 350°C and about 540°C whereby to produce a substantially bainitic structure throughout. Alternatively, coiling temperatures of between about 540°C and 580°C may be adopted whereby to produce a substantially acicular ferrite structure throughout. Whereas reference is made here to 'substantially' bainitic and 'substantially' acicular ferrite, it is to be understood that in the former case there will be a certain proportion of acicular ferrite structure and likewise with acicular ferrite there will be a certain proportion of bainite.The proportion of bainite and acicular ferrite for any given composition depends not only on the coiling temperature but also on the finishing temperature and details of the cooling cycle between finishing and coiling, the latter depending on the water spray system used and the thickness of the strip.
The acicular ferrite structure gives improved consistency and will require less rigid control during annealing but in general the ductility will be less than that achieved with bainite.
A more selective coiling temperature of between about 540⁰C and up to 565°C may be chosen for the acicular ferrite structure.
Likewise a more restrictive composition may be employed, namely, between 0.03% and 0.15% carbon, between 0.8% and 2.5% manganese, up to 1% silicon, between 0.05% to 0.25% molybdenum and up to 1.5% chromium.
Ideally, the steel of the present invention contains less than 0.02% sulphur, less than 0.02% phosphorus and less than 0.015% nitrogen.
Carbide or nitride forming elements, such as niobium, vanadium or titanium are not essential to produce the good combination of strength and ductility of the steel of the invention but will normally be present in concentrations of less than 0.01%. Residual type elements such as copper, nickel and tin will normally be present in amounts depending on the steelmaking practice.
The hot band may be produced from slab derived from any conventionally cast ingot or from a slab which is continuously cast. In either case the slab reheating temperature while not critical should be high enough to ensure that the steel is substantially fully austenitic throughout the hot rolling operation. To ensure this, the hot rolling should be at a finishing temperature which is preferably not lower than the Ar3 temperature.
After coiling the strip is continuously annealed and cooled to produce the necessary concentration of martensite. Preferably, the strip is continuously annealed at a temperature lying within the range of 760°C to 800°C, although the temperature may be lower than 760°C or indeed most temperatues between the Acl and the Ac3 temperatures would be suitable.
After annealing the strip may be cooled at a rate sufficient to achieve the martensite-polygonal ferrite structure which depends on the composition used. This cooling rate suitably is maintained until the temperature of the strip has been reduced to about 400°C but may be extended beyond this range. A cooling rate between 3°C and 10°C per second would be suitable for the composition of Example 1 and 2 below. Faster cooling rates would also be suitable but a higher strength product would be produced.
Clearly, the strip may be cold rolled to any desired gauge before being continuously annealed.
Some embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which:-

Figure 1 is a graph of uniform elongation v tensile strength for one specific composition coiled at different temperatures to produce different structures;
Figures 2(a) to 2(d) are graphs of ultimate tensile strength and uniform elongation v annealing temperature for steels with these structures.

Example 1

A steel produced by conventional refining and having the following composition was cast into ingots and was subsequently slabbed.
The slab was reheated to a temperature of about 1270°C and was subsequently hot rolled at a finishing.temperature of approximately 900°C. The hot band emerging from the last stand of the mill was cooled by conventional water flow incident upon the strip surface so that at coiling, the hot band'temperature was within the range 500°C to 540°C and typically 520°C. At this stage the structure of the steel was found to be substantially bainitic.
After coiling, the hot reduced strip was intercritically annealed by immersion in a salt bath maintained at a temperature of 760°C, 790°C or 830°C. The strip was maintained at these annealing temperaturesfor a time between 1½ to 3 minutes and was subsequently cooled naturally or by a forced air blast directed at the strip surface. The force cooling increased the cooling rate from 3.5°C to 6°C per second during the period that the strip was cooling to a temperature of 450°C.
Mechanical tests carried out on the steel of the invention typically displayed, after one set of annealing conditions, a tensile strength of 640 Newtons/mm2 coupled with a uniform elongation of 26% and a total elongation of 35%, whereas conventional dual-phase steel of this tensile strength would have a uniform elongation of approximately 19%.

Example 2

A steel produced by conventional refining and having the following composition was cast into ingots and was subsequently slabbed.
The slab was reheated to a temperature of about 127₀°_C and subsequently hot rolled at a finishing temperature of approximately 900°C. The hot band emerging from the last stand of the mill was cooled by conventional water flow incident upon the strip surface so that at coiling, the hot band temperature was within the range 540°C to 58C°C and typically at 560°C. At this stage, the steel was found to have a substantially acicular ferrite structure.
After coiling, the hot reduced strip was intercritically annealed by immersion in a salt bath maintained at temperatures of 760°C, 790⁰C or 830°C. The strip was maintained at these annealing temperatures for a time between 1½ to 3 minutes and was subsequently cooled naturally or by a forced air blast directed at the strip surface. The force cooling increased the cooling rate from 3.5°C to 6⁰C per second during the period that the strip was cooling to a temperature of 450°C.
Mechanical testc carried cut on the steel of the invention, after one set of annealing conditions, typically displayed a tensile strength of 640 Newtons/mm2 coupled with a uniform elongation of 23.5% and a total elongation of 34%.

Example 3

A steel having the following composition was cast into ingots and subsequently slabbed.
The slab was treated in the same manner as described above but two coils were produced, one being coiled at a temperature of between 520°C and 560°C and the other at a temperature of between 560°C and 630°C; the lower temperatures being that at the trailing end and the higher temperatures that at the leading end.
With this arrangement the trailing end of the first coil exhibited a substantially bainitic structure; the leading end of the first coil and the trailing end of the second coil exhibited a substantially acicular ferrite structure whilst the leading end of the second coil was conventional ferrite/pearlite.
After coiling the hot reduced strip was annealed as before.
Referring now to Figures 1 and 2 representations are shown of the characteristics of this latter steel.
In Figure 1 it can be seen that the bainite starting structure produces the most favourable combinations of percentage elongation and tensile strength after annealing. These combinations are inferior for the acicular ferrite structure but these are still substantially better than for the ferrite/pearlite structure. Note that the elonga- ti.on'is that relevant to uniform elongation only; the equivalent total elongation would be of the order of an additional 10% on the figures shown.
The bainitic structure therefore gives the best ductility/strength properties but reference to Figure 2 illustrates.the fact that the acicular ferrite structure requires less stringent control in the annealing step to give sufficiently reproducible properties. In particular, Figure 2(a) shows the treatment of the bainite_'structure where it can be seen that small differences in the annealing temperatures, particularly in the lower range 760°C to 790°C produce very marked changes in both the tensile strength and the elongation characteristics. In contrast, with the ferrite/pearlite structure Figure 2(b) small differences in the annealing temperature produce very little changes in the strength and elongation characteristics but one suffers here of course, from diminished levels in both of these qualities. The values obtained are those normally obtained previously for dual-phase steels.
The acicular ferrite structures, Figures 2(c) and 2(d), each show a less marked change than bainite in these qualities with changes in the annealing temperature and whilst not as stable as ferrite/pearlite, the acicular ferrite does show enhanced levels of both strength and elongation. More consistent properties may thus be obtained with acicular ferrite. More benefit is obtained when the structure consists substantially of bainite or acicular ferrite or bainite and acicular ferrite, but lower levels of these transformation products will also give useful benefit. In the latter case the benefits increase as the proportion of bainite or acicular ferrite or bainite and acicular ferrite increases, and useful benefits are expected when the proportion of acicular ferrite is greater than 50% of the whole structure or when the proportion of bainite and acicular ferrite is greater than 30%.
The examples shown are not intended to be exhaustive and many other compositions within the broad range quoted may be used. For example, a molybdenum-free composition having an enhanced chromium content of 0.6 with the same sort of range of other elements as in Example 2 may be used. Alternatively, the molybdenum may be replaced by an enhanced manganese content without the need to increase the chromium. The silicon content may also be varied to change the tensile strength of the steel. Alternatively, the strength of the steel may be controlled by altering the carbon content. In addition steels with a lower carbon content, say 0.05%, and containing 1.4% manganese and 0.66% chromium can be cold rolled and continuously annealed to give a product with a tensile strength in the range of 400 to 500 Newtons/mm². All compositions give improved properties in the annealed strip provided that the structure prior to annealing contains a significant proportion of bainite and/or acicular ferrite. The dispersion of the martensite in the ferrite matrix following annealing is significantly less coarse than that of martensite in conventionally annealed steels and it is this aspect which renders these dual-phase steels superior.

Claims

1. A process for producing dual-phase steel, characterised in that steel strip is hot rolled and coiled at a temperature of between about 350°C to about 580°C, and in which the strip is subsequently continuously annealed in the two-phase ferrite austenite field and cooled to transform at least the bulk of the austenite to martensite, the composition of the steel comprising, by weight, 0.03% to 0.25% carbon, 0.3% to 2.5% manganese, up to 1.5% silicon, up to 0.25% molybdenum and up to 2% chromium the remainder being iron except for incidental impurities and residuals in amounts depending on the steelmaking practice.

2. A process according to Claim 1, characterised in that the steel strip is coiled at between 350 C to about 540°C whereby to produce a substantially bainitic structure throughout.

3. A process according to claim 1, characterised in that the steel strip is coiled at between about 540°C and 580°C whereby to produce a substantially acicular ferrite structure throughout.

4. A process according to Claim 3, characterised in that the steel strip is coiled at between about 540°C and 565°C.

5. A process according to Claim 1, characterised in that the steel strip is coiled at between about 400°C and 560°C whereby to produce a mixed structure throughout substantially of bainite and acicular ferrite.

6. A process according to any one of claims 1 to 5, characterised in that the hot rolled strip is subsequently cold rolled before annealing.

7. A process according to any one of claims 1 to 6, characterised in that the strip is annealed at temperatures between 760°C and 830°C.

8. A process according to claim 7, characterised in that the annealing temperature is between 760 C and 790°C.

9. A process according to any one claims 1 to 8, characterised in that the anneal is terminated by forced or natural air cooling.

10. A process according to any one of claims 1 to 9, characterised in that the composition of the steel comprises, by weight, 0.05% to 0.15% carbon, 0.8% to 2.5% manganese, up to 1% silicon, 0.05% to 0.25% molybdenum and up to 1.0% chromium.

11. A process according to claim 10, characterised in that the composition of the steel comprises, by weight, about 0.1% carbon, between 1.3% to 1.4% manganese, between 0.5% and 0.6% silicon, about 0.1% molybdenum and about 0.04% chromium.

12. A process according to any one of claims 1 to 11, characterised in that the steel includes chromium or molybdenum but not both.

13. A dual-phase steel produced by a process according to any one of the preceding claims.