EP0750049A1 - Ferritic steel and its manufacture and use - Google Patents

Abstract

A ferritic steel contains (wt. %) 0.05-0.3 C, 0.8-3.0 Mn, 0.4-2.5 Al, < 0.2 Si, < 0.08 P, < 0.05 S, balance Fe and unavoidable impurities. Ceq = %C + <1>/20%Mn + <1>/20%Cr + <1>/15%Mo = 0.1-0.325. Al ≥ 7.6Ceq - 0.36 wt. %. Also claimed is a process for producing the above steel having high strength, good cold workability and surface quality in the hot-rolled state and good cold-rollability with a structure consisting predominantly of pre-eutectic ferrite and smaller proportions of martensite and/or bainite and/or residual austenite, which is cast into a blank, hot rolled with a starting temperature of above 1000 degrees C and an end temperature (ET) in the region Ar3 - 50 degrees C < ET < Ar3 + 100 degrees C, cooled from the ET at a rate of 15-70 K/s to the coiling temperature which is below 500 degrees C, and coiled.

Description

The invention relates to a ferritic steel, a method for producing this steel with a predominantly polygonal-ferritic structure and one or more carbon-enriched second phases, and a preferred use of this steel. The steel should have high strength and good formability as well as improved surface quality after hot forming in the last production stage.

Dual-phase steels are known which have a structure, e.g. B. from up to 80 vol .-% of polygonal relatively soft ferrite and the rest of carbon-rich martensite. The carbon-rich second phase, which is present in smaller quantities, is embedded in the island in the pre-eutectoid ferritic phase. Such a steel has good mechanical properties and favorable cold formability.

Known steels with predominantly polygonal ferrite in the structure and martensite embedded therein consist of (in mass%) 0.03 to 0.12% C, up to 0.8% Si and 0.8 to 1.7% Mn (DE 29 24 340 C2) or 0.02 to 0.2% C, 0.05 to 2.0% Si, 0.5 to 2% Mn, 0.3 to 1.5% Cr and up to 1% Cu, Ni and Mo (EP 0 072 867 B1). Both steels are calmed with aluminum and contain soluble residual contents of less than 0.1% Al. Silicon in these steels promotes ferrite transformation. In combination with manganese and if necessary, chromium is suppressed to form pearlite. This ensures sufficient accumulation of carbon in the second phase and the formation of polygonal ferrite in predominant relation to the second phase. However, these known alloys have the disadvantage that an inhomogeneous surface structure is formed during hot rolling, which is visible through tongues of red scale. After pickling, bumps remain on the surface. For many applications, such material is not salable. So far, it has not been possible to improve the surface quality of these hot-rolled steels. There is also a need for steels which have both high strength and good cold forming properties. These requirements can be characterized by the product of tensile strength and elongation Rm · A5. This should be over 16,000 N / mm ² ·% in both the longitudinal and transverse directions.

This leads to the task of developing a steel with a predominantly polygonal ferritic structure, which has the outstanding range of mechanical properties of known steels at least in the same size, with tensile strength values Rm> 500 N / mm ² and elongation values A5> 16000 / Rm in% is just as good cold formable as the known steels, but has a better surface structure than the known steels after the production by hot forming in the last product stage.

To solve this problem, a steel with (in mass%)
0.05 to 0.3% carbon
0.8 to 3.0% manganese
0.4 to 2.5% aluminum
0.01 to 0.2% silicon
less than 0.08% phosphorus
less than 0.05% sulfur
Balance iron including unavoidable impurities
proposed with a structure consisting predominantly of polygonal ferrite and smaller proportions of martensite and / or bainite and / or residual austenite, which with a carbon _equivalent (C _eq. ) of greater than 0.1 to 0.325 $${\text{C.}}_{\text{equ.}}\text{=\% C + 1/20\% Mn + 1/20\% Cr + 1/15\% Mo}$$

Aluminum in an amount of% by mass $${\text{Al ≧ 7.6 · C}}_{\text{equ.}}\text{- contains 0.36.}$$

The desired conversion to bainite or martensite in a previously formed ferrite matrix results in a favorable residual stress state of the structure with a positive influence on the cold forming ability. At the same time, the level of tensile strength is increased compared to a ferritic-pearlitic structure, as is the case in the known hot-rolled structural steels (St 37 to St 52). With similar suitability as the known structural steels for direct processing into geometrically sophisticated formed end products, the higher strength offers the possibility of reducing the thickness and thus saving weight.

Such a steel not only achieves the good strength level of known silicon-alloyed dual-phase steels, but also has improved surface quality after the hot forming, as is e.g. for wheel disks of motor vehicles, which are produced by cold forming the hot-rolled steel.

In addition, the following additional elements can be added to the steel up to the specified amounts (in mass%):
up to 0.05% titanium
up to 0.8% chromium
up to 0.5% molybdenum
up to 0.8% copper
up to 0.5% nickel.

Such a steel alloyed with aluminum instead of silicon achieves an elongation at break A ₅ > 34% with a tensile strength value R _m = 500 N / mm ² and an elongation at break A ₅ > 24% with a tensile strength value of 700 N / mm ² , ie the product Rm · A5 is certainly over 16,000 N / mm ² ·% in both the transverse direction and in the longitudinal direction.

Characteristic of the steel according to the invention is the aluminum content, which is considerably increased compared to known steels with 0.4-2.5%. For this purpose, the silicon content was limited to less than 0.2% according to the invention.

In contrast, known steels of this type mostly had silicon contents of over 1%. The steels alloyed with aluminum according to the invention have the desired pearlite-free two- or multi-phase structure and have excellent strength properties. Above all, the surface quality of the thermoformed product is much better than that of previously known silicon alloyed steels. Aluminum ensures an extensive formation of globular ferrite with a content in the range of 0.4 to 2.5%. The formation of pearlite is delayed more than that of silicon-alloyed steels and can be safely avoided if the claimed process parameters are observed.

The carbon content of 0.05 to 0.3% is within the normal range for generic steels.

Manganese is added in an amount of 0.8 to 3.0% in order to avoid the formation of pearlite and to enrich the austenite in addition to carbon. Manganese has a solidifying effect and increases the strength level. The contents of carbon and manganese are interchangeable under the aspects of pearlite avoidance and effects on ferrite formation within the framework set by the carbon equivalent. The carbon equivalent is determined as: $${\text{C.}}_{\text{aqu.}}\text{=\% C + 1/20\% Mn + 1/20\% Cr + 1/15\% Mo}$$

Carbon equivalence values higher than 0.1% result in higher aluminum contents. According to the invention, the intersection of the carbon equivalent value and the corresponding aluminum value should lie in the shaded area in FIG. 1 in order to ensure a ferrite content of over 70% and suppression of pearlite formation under large-scale production conditions. The carbon equivalent value should ensure a max. 0.325 can be limited.

An addition of titanium of up to 0.05% ensures nitrogen removal and prevents the formation of elongated manganese sulfides.

Chromium in an amount of up to 0.8% can be added to improve the martensite resistance and to prevent pearlite formation.

In an amount of up to 0.5%, molybdenum increases the range of successful cooling rates.

Copper and nickel in an amount of up to 0.5% each can help lower the transition temperature and prevent pearlite.

To influence the formation of sulfides, treatment of the molten metal with calcium silicon is advisable.

liegen.The hot rolling end temperature ET should be in the range of $$\text{Ar3 - 50 ° C <ET <Ar3 + 100 ° C}$$

lie.

The Ar3 temperature, which should be in the range of 750 to 950 ° C, is calculated for Al contents up to 1% $$\text{Ar3 [° C] = 900 + 60\% Al - 60\% Mn - 300\% C}$$

For aluminum contents over 1 to 2.5%: $$\text{Ar3 [° C] = 900 + 100\% Al - 60\% Mn - 300\% C}$$

When hot strip is produced from the steel according to the invention, elevated hot rolling end temperatures are predominantly only permissible up to 850 ° C. Rolling at higher final roll temperatures has a positive influence on the hot strip profile. Rolling can be done with less force, and the rolling speed can be increased. There is no need to swing the supporting strip to cool down before the finishing relay. Overall, this results in a productivity gain.

The cooling of the hot rolling end temperature to the reel temperature between room temperature and 500 ° C is accelerated with a cooling rate of 15 to 70 K / s.

When cooling the hot rolling end temperature, the process according to the invention can further promote the formation of ferrite in the range from Ar3 to Ar3 - 200 ° C. by taking a cooling break of 2 to 30 s, in which the cooling rate is below 15 K / s.

Fig. 2 shows a schematic representation of the production of hot strip coupled with the cooling process of the steel according to the invention during and after hot rolling.

From this it can be seen that the undesired entry into the pearlite area can be reliably avoided if the specified conditions for the hot rolling end temperature, the cooling rate and the reel temperature are observed.

example 1

A steel A according to the invention with the values according to Table 1 was hot-rolled to a final strip thickness of 3.7 mm with a hot-rolling end temperature of 875 ° C. The cooling from this temperature was carried out at 30 K / s to the reel temperatures (HT) given in Table 2. The properties of this steel A according to the invention were determined on flat tensile specimens in accordance with DIN EN 10002.

The values for the yield strength, tensile strength, elongation and the yield ratio for the layers along and across the rolling direction are given in Table 2.

A sample was coiled at higher temperature (HT = 685 ° C). This was not pearlite-free and did not achieve the required properties.

For comparison, the corresponding strength properties of a steel B known from DE 34 40 752 C2 with the composition according to table 1 were also entered in table 2.

For steel A according to the invention, the reel temperature was varied between 80 ° C and 350 ° C. The strength values determined in each case make it clear that the steel according to the invention has very good properties in the entire reel area, which at least correspond to those of the known silicon-alloyed comparison steel B.

Table 2 also shows the mechanical properties of a steel C according to the invention of the composition according to Table 1. The results were determined on a round tensile specimen with a diameter of 4 mm. The hot rolling was simulated by a flat compression test. The values were measured in the longitudinal direction (material flow direction). The reel temperature was 200 ° C for the first sample and 400 ° C for the second sample. This steel also has the favorable range of mechanical properties; but also better surface quality than steel B.

The results reported in Table 2 make it clear that the yield ratio in the entire range of the reel temperature is below 0.8. Table 1 (Chemical composition) stole C% Mn% Si% P% Al% Cr% N% S% C _equ A 0.076 1.45 0.053 0.019 1.23 0.35 0.002 <0.001 0.16 B * 0.090 0.38 0.71 0.013 0.025 0.62 0.006 0.009 0.14 C. 0.090 1.51 0.03 <0.005 1.19 0.50 0.005 0.004 0.19 D 0.20 1.49 0.04 <0.005 1.99 0.02 0.005 0.004 0.27 *) Comparative steel

Table 2 stole Location to the Walzrichtg. ET [° C] HT [° C] Rp _0.2 ; Roe deer [N / mm ² ] R _m [N / mm ² ] A ₅ (%) Rp _0.2 / R _m Reh / R _m RmA5 N / mm ² % A L 860 80 372 639 30.3 0.58 19361.7 A Q 860 80 405 642 27.3 0.63 17526.6 A L 880 200 379 641 32.5 0.59 20832.5 A Q 880 200 402 640 25.6 0.63 16384 A L 880 280 320 588 36.3 0.54 21344.4 A Q 880 280 395 592 28.4 0.67 16812.8 A L 880 350 362 545 34.9 0.66 19020.5 A Q 880 350 363 542 34.8 0.67 18861.6 A ** L 880 685 331 477 29.9 0.69 14262.3 A ** Q 880 685 376 497 34.7 0.76 17245.9 B * L 200 368 579 28.5 0.64 16501.5 B * Q 200 388 570 26.4 0.68 15048 C. L 910 400 380 506 37 0.48 18722 C. L 880 350 417 524 33 0.72 17292 D L 910 350 447 569 35.5 0.79 20199.5 D L 880 400 440 561 37 0.78 20757 Explanation of table 2
*) Comparative steel **) Outside the stressed range (HT> 500 ° C) Determination of the properties according to DIN EN 10002 on flat tensile samples
HT: reel temperature
Rp _0.2 : 0.02% proof stress
R _m : tensile strength
A ₅ : Elongation at break
L: longitudinal /
Q: Cross

Claims (5)

Ferritic steel, with (in mass%)
0.05 to 0.3% carbon
0.8 to 3.0% manganese
0.4 to 2.5% aluminum
less than 0.2% silicon
less than 0.08% phosphorus
less than 0.05% sulfur
Rest of iron including unavoidable
Impurities,
with a carbon equivalent of greater than 0.1 to 0.325 $${\text{C.}}_{\text{equ.}}\text{=\% C + 1/20\% Mn + 1/20\% Cr + 1/15\% Mo}$$

Aluminum in an amount of $${\text{Al ≧ 7.6 · C}}_{\text{aqu.}}\text{- contains 0.36\% by mass.}$$

A method for producing a steel according to claim 1 with high strength, good cold formability and surface quality in the hot-rolled state and good cold-rollability with a structure consisting predominantly of pre-eutectoid ferrite and smaller proportions of martensite and / or bainite and / or residual austenite, which is cast in the strand, with a hot rolling start temperature of over 1000 ° C and with a hot rolling end temperature (ET) in the range of $$\text{Ar3 - 50 ° C <ET <Ar3 + 100 ° C}$$

is hot-rolled, then cooled and coiled from the hot rolling final temperature (ET) at a speed of 15 to 70 K / s to the coiling temperature in the range below 500 ° C. Method according to claim 2,
characterized in that the steel additionally with (in mass%)
up to 0.05% titanium
up to 0.8% chromium
up to 0.5% molybdenum
up to 0.5% copper
up to 0.8% nickel
is alloyed individually or in groups. Method according to claim 2,
characterized in that in the temperature range between Ar3 and Ar3 -200 ° C there is a cooling break for a period of 2 to 30 s during which the cooling rate is less than 15 K / s. Use of a steel according to claim 1 as a material for the production of cold-formed wheel disks.

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