EP0009050B1

EP0009050B1 - High strength steel and process of making

Info

Publication number: EP0009050B1
Application number: EP19790900260
Authority: EP
Inventors: Arnold G. Preban; Indra Gupta
Original assignee: Inland Steel Co
Current assignee: Inland Steel Co
Priority date: 1978-02-21
Filing date: 1979-09-11
Publication date: 1983-05-11
Also published as: WO1979000644A1; EP0009050A4; EP0009050A1; JPS55500221A; DE2965340D1

Abstract

A novel high strength steel especially adapted for automotive applications and the like is provided by cold rolling a low carbon aluminum-killed steel containing added phosphorus and silicon as ferrite strengtheners, continuously heating the cold rolled strip to an inter-critical temperature such that from about 5% to about 25% austenite is present, water quenching to obtain a dual phase ferrite-martensite microstructure having an average volume fraction of martensite of from about 5% to about 25%, and reheating to a sub-critical temperature to temper the steel. The resultant steel has good formability as evidenced by a minimum total elongation of 18%, is free from ductility loss due to room temperature aging, and after stamping followed by a typical paint bake cycle the formed part has a minimum yield strength of 550 MPa (80 ksi).

Description

This invention relates to a novel high strength steel and to a process for making such steel.
The urgent need of the automotive industry to meet fuel economy regulations while still producing roomy cars has intensified the search for improved high strength steels. Heretofore, the application of high strength steels with yield strengths of 550 MPa (80 ksi) in the formed part has been limited by inferior formability and also by the increased cost of such steels.
High strength in steels is readily obtained by suitable alloy additions, but the cost is usually prohibitive for automotive and other applications. It is known that martensitic steels possess high strength, but have poor ductility. It has also been proposed to quench renitrogenized steel from a temperature in the inter-critical region to obtain a ferrite-martensite microstructure which, upon subsequent straining and aging, has the desired mechanical properties. However, such steels have the disadvantage of somewhat restricted ductility which is further reduced by strain aging at room temperature which occurs between temper rolling in the steel plant and the time the steel sheet is used in a forming or stamping operation.
U.S.P.S. 4,050,959 discloses a process of making a cold reduced two phase steel in which the chemical composition of the steel is controlled within the following range in the steel making stage:
the steel is hot rolled, pickled and cold reduced, and the cold reduced steel strip so obtained is subjected to the following full continuous annealing process:

heated to a temperature in the range Ac, to 900° and held thereat for 5 to 180 seconds;
quenched to room temperature in a water jet stream;
re-heated from room temperature to a temperature of from 150 to 450°C and held thereat for from 5 to 300 seconds;
cooled from 150 to 450°C to room temperature and coiled.

However steels disclosed in U.S.P.S. 4,050,959 do not achieve the level of performance characteristics sort by applicants.
Accordingly, a primary object of the present invention is to provide, without the use of costly alloy additions, a novel and improved steel having good ductility which is suitable for use in sheet form for automotive applications and the like and which is not subject to room temperature aging but which has a high work hardening rate and age hardening response so that stamping and paint baking result in high strength in the finished part.
A more specific object of the invention is to provide a high strength low alloy steel of the foregoing character which exhibits a minimum total elongation of 18%, is free from ductility loss due to room temperature aging, and which, in the final fabricated part, exhibits a yield strength of at least 550 MPa (80 ksi).
These objects are achieved by providing a process for making a high strength steel with good ductility suitable for automotive applications and the like, which comprises:
providing a cold rolled strip of aluminium-killed steel containing added phosphorous and silicon as ferrite strengtheners, said steel comprising from 0.05 to 0.15 wt % carbon, from 0.30 to 0.60 wt % manganese, from 0.04 to 0.10 wt. % phosphorous, from 0.10 to 0.50 wt. % silicon, from 0.02 to 0.08 wt. % aluminium sufficient to tie up all the nitrogen present and to provide a fully killed steel and the balance essentially iron, and usual incidental elements in amounts not exceeding the normal amounts resulting from the steel making process;
heating said strip to an inter-critical temperature within a controlled narrow range between the A_i, and the A3 critical points such that from 5% to 25% austenite is present;
water quenching said strip at a rate in excess of the critical cooling rate so that all of the austenite present is converted to martensite, whereby to obtain a dual phase ferrite-martensite microstructure having an average volume fraction of martensite of from 5% to 25% and
tempering said strip by reheating said strip to a sub-critical temperature to effect tempering of the martensite;
the resultant steel having a minimum total elongation of 18% and a yield strength of from 400 to 500 MPa (58-73 ksi) and being free from ductility loss due to room temperature aging, and after straining and heating incident to fabrication of a finished part, said steel having a minimum yield strength of 550 MPa (80 ksi). The resultant sheet or strip has good formability; it is free from ductility loss due to room temperature ageing; and after straining and aging during fabrication of a formed part, the desired high strength level is developed. Specifically, the steel product exhibits a minimum of 18% total elongation prior to stamping and a minimum yield strength of 550 MPa (80 Ksi) after stamping followed by a typical automotive paint bake cycle. This combination of properties allows design engineers to take advantage of potential weight reductions by utilising the high strength of the steel while permitting part fabrication without extensive modification of existing dies.
High strength levels can be obtained in a ferrite-martensite microstructure by increasing the relative proportion of martensite, but the ductility and formability of the product suffers. In accordance with the present invention, the martensite content is kept low enough so that the cold rolled product has a minimum total elongation of 18%, but a moderately high strength level of from 400 to 500 MPa (58-73 Ksi), dependent upon the carbon content, is realised by including silicon and phosphorous in the steel as ferrite strengtheners. The dual phase product has a high strain hardening capacity, and the final fabricated part exhibits the desired minimum yield strength of 550 KPa (80 Ksi). As hereinafter described in more detail, in the present invention the dual phase microstructure is obtained by rapid quenching from an inter-critical temperature within a controlled narrow range at a cooling rate which is in excess of the critical cooling rate and which is high enough so that phosphorous does not migrate and relocate at the grain boundaries where it would cause poor ductility. Instead, the phosphorous remains within the ferrite grains, and the desired stiffening effect of phosphorous is realised while retaining acceptable ductility. Moreover, the cold rolled sheet product of the present invention is not subject to detrimental room temperature ageing due to nitrogen since the steel is aluminium-killled.
The steel composition of the present invention consists of the following with the balance essentially iron:
Expensive alloy additions are avoided, but the amounts of silicon and phosphorous added as ferrite strengtheners are greater than the residual amounts of these elements so that the steel can be regarded as a low alloy steel. Although appropriate additions of either phosphorous alone or silicon alone can provide high strength levels, there are adverse side effects in each case. Thus, if phosphorous is too high, the steel is too brittle and difficult to spot weld. If silicon is too high, the steel becomes more susceptible to surface defects making it unsuitable for exposed automotive application and also becomes more difficult to galvanise. Accordingly, the invention relies on the combined use of phosphorous and silicon within the specified ranges. The maximum silicon and manganese contents are such that rapid quenching with water, rather than air cooling, is necessary in order to obtain the desired ferrite-martensite microstructure. To eliminate room temperature ageing problems, the steel is fully killed with aluminum, as reflected by the above-listed range of aluminum content in the steel. However, the steel is still subject to carbon strain aging so that the desired high strength levels are obtained in automotive applications and the like after stamping and heating, as in paint baking.
The usual steelmaking practices may be followed. Typically, the hot metal from the blast furnace is refined in a basic oxygen converter. If desired, the hot metal may be subjected to conventional desulfurization, e.g. by calcium carbide injection, prior to being charged to the basic oxygen converter. The required additions of aluminum, silicon, and phosphorus may be carried out in the ladle prior to ingot casting or continuous casting.
The usual hot rolling and cold rolling practices may be used to provide cold rolled coils for subsequent continuous annealing in accordance with the invention. Typically, in the usual hot band sizes the finishing temperature may be from about 785°C to about 955°C, and the coiling temperature may be from about 480°C to about 705°C. In the cold rolling stage the percent cold reduction may range from about 40% to about 80%, but a relatively high degree of cold reduction of from about 50% to about 75% is preferred in order to obtain a fine grain size after the annealing step. The thickness of the cold rolled strip may be from about 0.3 mm to about 3 mm.
The cold rolled strip is processed, in accordance with the invention, in a continuous annealing line in which the strip is (1) heated in a soak section to a temperature between the A₁ and A3 critical points, (2) water quenched in a quench section at a rapid rate to obtain a dual phase ferrite-martensite microstructure, and (3) reheated in a tempering section to a subcritical temperature and cooled to ambient temperature. When necessary the strip is temper rolled for flatness.
As pointed out previously, a limited amount of martensite is required in the dual phase microstructure so as to achieve the desired combination of physical properties in the final steel strip. Consequently, the inter-critical soak temperature must be carefully controlled in the soak section of the continuous annealing line, preferably within ±10°C, for partially austenitizing the steel to the desired extent and thereby realizing the aim ductility and yield strength in the quenched and tempered product. Dependent upon the carbon content of the steel and also upon line speed and strip gauge, the cold rolled strip is heated to a narrow temperature range between the A₁ and the A3 critical points such that from 5% to 25% austenite is present. In general, the strip is heated at a soak temperature of from 745°C to 845°C for a period of from about 20 to about 120 seconds.
The partially austenitized strip passes from the soak furnace into a water quench zone of any suitable design capable of rapidly quenching the strip at a rate in excess of the critical cooling rate so that all of the austenite present is converted into martensite which is uniformly distributed in fine grain polygonal ferrite. The average volume fraction of martensite present in the quenched product is from about 5% to about 25%. Although the invention is not so limited, a preferred quench system utilizes submerged nozzles such as disclosed in Taylor et al U.S. Patent Nos. 3,360,202 and 3,410,734. The cooling rate will ordinarily be in excess of 1000°C/sec. As previously discussed, the rapid quench rate also has the advantage of avoiding relocation of phosphorus to the grain boundaries which would impair the ductility of the product.
In the tempering section of the continuous annealing line the quenched strip is reheated to a sub- critical temperature, e.g. from 150°C to 480°C for a period of from 5 to 300 seconds, in order to effect tempering of the relatively high carbon content martensite. The strip is then cooled to substantially ambient temperature and, when necessary, temper rolled for flatness in the conventional manner to obtain a cold reduction not in excess of 2%.
The resultant steel sheet or strip product has a minimum total elongation of 18% and a yield strength of from 400 to 500 MPa (58-73 ksi), dependent upon the carbon content. The product has excellent formability for automotive and other applications, and after stamping and a typical paint bake cycle the yield strength of the fabricated part exceeds the required minimum of 550 MPa (80 ksi) and in most cases is in excess of 620 MPa (90 ksi). The excess aluminum present in the killed steel ties up all the nitrogen present so that there is no loss of ductility due to room temperature aging. In addition, the uniformity of properties within a coil is excellent, e.g. the variation in yield strength being less than 40 MPa (6 ksi). The latter is an important property for a stamping die which is set up for springback control. Furthermore, the product has excellent spot weldability and compares favorably in this respect with aluminum-killed AISI 1006 steel.
For purposes of further illustrating the invention, but without limiting the same, the following specific example is presented.

Example

Two grades of aluminum-killed steel were made in a BOF shop with a nominal chemistry of 0.45/0.55% manganese, 0.25/0.35% silicon, 0.05/0.07% phosphorus, 0.03/0.06% aluminum and containing 0.07% carbon and 0.10% carbon, respectively. Low sulfur levels were achieved by calcium carbide injection to the hot metal in open ladles between the blast furnace and the BOF shop. The aluminum, silicon, and phosphorus additions were made to the ladle, and the steel was ingot cast using bottom pouring. Tests coils were produced from these steels in several different gauges using conventional hot rolling and cold rolling practices. The details of the hot and cold rolling steps and the actual steel chemistry are presented in Tables I and II, respectively. In Table II the chemistry of aluminum-killed AISI 1006 steel is shown for comparison.
The test coils were then processed in a continuous annealing line having in sequence a soak furnace, a water quench system with submerged nozzles, a tempering furnace, a final water cooling tank for cooling the strip to room temperature, and a temper mill. The line was operated at speeds ranging from 90 metres/min. for the 1.2 mm thick strip to 130 metres/min. for the 0.5 mm strip, with the heating times varying inversely with the line speed. For example, at a line speed of 110 metres/min. for the 0.8 mm strip, the heating times in the soak furnace and the tempering furnace were 70 and 65 seconds, respectively. The strip temperature in the soak furnace and in the tempering furnace, as determined from radiation pyrometer measurements, were about 788° and 260°C, respectively, and these temperatures were controlled within about ± 10°C.
A complete evaluation of the mechanical, microstructural, weldability, and formability characteristics of the steel was carried out on samples selected from various positions in the coils after completion of processing on the continuous annealing line. The determination of mechanical properties consisted of test sets which included tensile properties, cup height and hole expansion measurements, longitudinal and transverse stretchbend tests at a high R/t ratio, and r determination. Cup height tests, which measure the formed height of the cup at the onset of diffuse necking and are indicators of bulk formability, were performed using a Hille-Wallace hydraulic press to form 178 mm diameter blanks into dome shapes with a 50.8 mm diameter hemispherically nosed punch. Hole expansion tests, which determine the percent expansion of a 12.7 mm original diameter hole after the appearance of the first through-thickness crack, were performed using the same apparatus with a self-aligning punch to go through the initial hole. Stretch-bend tests consisted of applying force to the center of an edge clamped 50.8x209.5 mm test sample with a round end steel punch mounted in the movable platen of a hydraulic testing machine to produce a V-shaped specimen. The hole expansion and stretch-bend tests are measures of edge formability. An unsupported sample length of 139.7 mm with an R/t ratio of about 1.6 was employed for all stretch-bend tests. Tensile testing was performed using 50.8 mm gauge length specimens which were pulled at a cross head speed of 12.7 mm per minute on an Instron testing device.
The results of the tensile tests, averaged for each coil, are presented in Tables III and IV for the 0.07% carbon and the 0.10% carbon steels, respectively. In Tables III and IV the designation "A.R" refers to the "as-received" steel condition following the quench, temper and temper-roll treatment performed on the continuous annealing line, and the designation "S&A" refers to the steel properties which resulted from straining the as-received steel 2% in tension and aging for one hour at 204°C, which is a simulation of a stamping and pain-baking treatment. Formability and microstructural characterization parameters are presented in Table V. Included in this table are full curve "n" values, normal (r_m) and planar (Δr) anisotropy parameters, stretch-bend depths at an R/t ratio of 1.6, cup height and hole expansion data. In addition, the values of volume fraction martensite and ferrite grain size, which describe the optical microstructure, are also shown. In Tables III, IV, and V the designation L and T refer to longitudinal and transverse tests.
From Tables III and IV it is seen that for all coils total elongation in the as-received condition exceeded 18%, specifically ranging from 21 to 27% for the 0.07% carbon coils and ranging from 20 to 23% for the 0.10% carbon coils. The yield strength (0.2% offset) of the as-received steel was in the 416 to 448 MPa range for all gauges of the 0.07% carbon steel and in the 439 to 484 MPa range for the higher carbon grade. With proper attention to springback, these strength levels present no stamping press limitation problems. In addition, the as-received steel showed no yield point elongation and a yield strength to tensile strength spread averaging about 205 MPa (30 ksi).
From Table V it is seen that the plastic anisotropy values (r_m) are close to unity and that the L and T values of the bend-stretch data are essentially equal. Furthermore, both the L and T samples underwent 180° flat bends without edge cracking. These data show that the test steel behaved essentially isotropically. The slight deviation from isotropic behaviour, as shown by the marginally higher strengths in the longitudinal samples and the non-zero values of the planar anisotropy parameter Ar, do not seriously affect formability. By comparison with AISI 1006 (an aluminum-killed cold rolled steel), the values of cup height and hole expansion are only about 25 to 35% less for the test steel at any particular gauge, while the yield strength differences are greater by a factor of two or more.
The values of full curve "n" values and grain size, as shown in Table V, are relatively uniform while the strength coefficient K is somewhat dependent on carbon level. The average volume fraction of martensite ranges from about 13% to about 20%.
From Tables III and IV, it is also seen that the yield strengths of the formed and aged parts were well in excess of 550 MPa (80 ksi), being in most cases over 620 MPa (90 ksi). The strained and aged steel shows a much lower yield to tensile strength spread but still possesses high values of total elongation ranging from 16 to 21 %. Thus, in the case of subsequent unexpected deformation, e.g. an automobile collision, parts formed from the steel of the present invention would be able to withstand a large degree of plastic flow before fracture, an energy-absorbing characteristic not usually associated with steels of this strength level.
The attainment of the high part strength following forming and paint-baking is dependent on the ability of the steel to respond to the aging treatment. Since all portions of a formed part may not receive 2% strain and the paint cycle may not always consist of one hour at 204°C (400°F), the effects of lesser amounts of pre-strain and lower aging temperatures on strained and aged properties of the 0.8 mm test steel were also investigated. It was found that for an aging temperature of 204°C, the minimum strength of 550 MPa (80 ksi) is attained without any prestrain. This indicates that the strain imparted by the temper rolling is sufficient to produce the minimum required strain aging response in any stamping. At a lower aging temperature of 149°C, the test steel attains the minimum strength as long as the tensile pre-strain is in excess of 0.6%. If the tensile prestrain level is maintained at 2%, the minimum strength requirement is met even at aging temperatures as low as 121 °C. It was concluded from the data that the steel of the present invention is quite versatile and that, with proper die design to put additional strain into the flat areas of a part, even lower temperature paint-bake cycles are usable.
The weldability of the test steel was also evaluated along with that of AISI 1006 using a range of weld evaluation criteria. The welding lobe curves showed that the steel of the present invention is quite similar to plain carbon steel in terms of weld time-weld current flexibility, with no hold-time restrictions being required. Moreover, mechanical testing of the welds showed, in most cases, high strength levels commensurate with the base metal strength.

Claims

1. A process for making a high strength steel with good ductility suitable for automotive applications and the like, which comprises:

providing a cold rolled strip of aluminium-killed steel containing addec(phosphorous and silicon as ferrite strengtheners, said steel comprising from 0.05 to 0.15 wt % carbon, from 0.30 to 0.60 wt. % manganese, from 0.04 to 0.10 wt. % phosphorous, from 0.10 to 0.50 wt. % silicon, from 0.02 to 0.08 wt. % aluminium sufficient to tie up all the nitrogen present and to provide a fully killed steel and the balance essentially iron.

heating said strip to an inter-critical temperature within a controlled narrow range between the A₁ and the A3 critical points such that from 5% to 25% austenite is present;

water quenching said strip at a rate in excess of the critical cooling rate so that all of the austenite present is converted to martensite, whereby to obtain a dual phase ferrite-martensite microstructure having an average volume fraction of martensite of from 5% to 25% and

tempering said strip by reheating said strip to a sub-critical temperature to effect tempering of the martensite;

the resultant steel having a minimum total elongation of 18% and a yield strength of from 400 to 500 MPa (58-73 ksi) and being free from ductility loss due to room temperature aging, and after straining and heating incident to fabrication of a finished part, said steel having a minimum yield strength of 550 MPa (80 ksi).

2. The process of Claim 1 further characterised in that said heating, quenching, and tempering steps are carried out in a continuous annealing line having a soak furnace for heating the strip to a temperature of from 745°C to 845°C for a period of from 20 to 120 seconds and a tempering furnace for reheating the quenched strip to a temperature of from 150°C to 480°C for a period of from 5 to 300 seconds.

3. An improved steel especially adapted for automotive applications and the like comprising an aluminium-killed steel having a dual phase ferrite-martensite microstructure with an average volume fraction of martensite of from 5% to 25% and containing added phosphorous and silicon as ferrite strengtheners; said steel comprising from 0.05 to 0.15 wt.% carbon, from 0.30 to 0.60 wt.% manganese, from 0.04 to 0.10 wt.% phosphorous, from 0.10 to 0.50 wt.% silicon, from 0.02 to 0.08 wt.% aluminium sufficient to tie up all the nitrogen present and to provide a fully killed steel, and the balance essentially iron and said steel having a minimum total elongation of 18% and a yield strength of from 400 to 500 MPa (58 to 73 ksi) and being free from ductility loss due to room temperature aging, and after straining and heating incident to fabrication of a finished part, said steel having a minimum yield strength of 550 MPa (80 ksi).

4. The steel of Claim 3 further characterised in that said composition is as follows:

5. The steel of Claim 3 further characterised in that said composition is as follows:

6. The process of Claim 1 wherein said strip is heated in said heating step to a temperature of from 745°C to 845°C for a period of from 20 to 120 seconds.

7. The process of Claims 1, 2 or 6, wherein after said tempering step the strip is cooled to substantially ambient temperature and thereafter temper rolled to impart a pre-strain sufficient to provide a predetermined minimum strain hardening and aging response.

8. The process of Claim 7 wherein said temper rolling step imparts a pre-strain in excess of 0.6% and up to 2%.

9. The process of Claim 8 wherein said temper rolling step imparts a pre-strain of 2%.