Patent Application
20140147329
The present invention relates generally to dual phase (DP) steels. More specifically the present invention relates to DP steel having a high silicon content ranging between 0.5-3.5 wt. %. Most specifically the present invention relates to high Si bearing DP steels with improved ductility through water quenching continuous annealing.
As the use of high strength steels increases in automotive applications, there is a growing demand for steels of increased strength without sacrificing formability. Dual phase (DP) steels are a common choice because they provide a good balance of strength and ductility. As martensite volume fraction continues to increase in newly developed steels, increasing strength even further, ductility becomes a limiting factor. Silicon is an advantageous alloying element because it has been found to shift the strength-ductility curve up and to the right in DP steels. However, silicon forms oxides which can cause adhesion issues with zinc coatings, so there is pressure to minimize silicon content while achieving the required mechanical properties.
Thus, there is a need in the art for DP steels having an ultimate tensile strength greater than or equal to about 980 MPa and a total elongation of greater than or equal to about 15%.
The present invention is a dual phase steel (martensite+ferrite). The dual phase steel has a tensile strength of at least 980 MPa, and a total elongation of at least 15%. The dual phase steel may have a total elongation of at least 18%. The dual phase steel may also have a tensile strength of at least 1180 MPa.
The dual phase steel may include between 0.5-3.5 wt. % Si, and more preferably between 1.5-2.5 wt. % Si. The dual phase steel may further include between 0.1-0.3 wt. % C, more preferably between 0.14-0.21 wt % C and most preferably less than 0.19 wt. % C, such as about 0.15 wt. % C. The dual phase steel may further include between 1-3 wt. % Mn, more preferably between 1.75-2.5 wt % Mn, and most preferably about 1.8-2.2 wt % Mn.
The dual phase steel may further include between 0.05-1 wt % Al, between 0.005-0.1 wt. % total of one or more elements selected from the group consisting of Nb, Ti, and V, and between 0-0.3 wt. % Mo.
a and 1 b plot TE vs TS for 0.15C-1.8Mn-0.15Mo-0.02Nb-XSi and 0.20C-1.8Mn-0.15Mo-0.02Nb-XSi for varied silicon between 1.5-2.5 wt. %;
a and 2b are SEM micrographs from 0.2% C steels having similar TS of about 1300 MPa at two Si levels. 2a at 1.5% Si and 2b at 2.5% Si;
a and 3b are SEM micrographs of hot bands at CTs of 580° C. and 620° C., respectively from which the microstructures of the steels may be discerned;
a and 4b plot the tensile properties strength (both TS and YS) and TE, respectively, as a function of annealing temperature (AT) with a Gas Jet Cool (GJC) temperature of 720° C. and an Overage (OA) temperature of 400° C.;
a-5d are SEM micrographs of samples annealed at: 5a=750° C., 5b=775° C., 5c=800° C. and 5d=825° C., showing the microstructure of the annealed samples;
a- 6e plot the tensile properties versus annealing temperature for the samples of Table 4A;
f plots TE vs TS for the samples of Table 4A;
a- 7e plot the tensile properties versus annealing temperature for the samples of Table 4B; and
f plots TE vs TS for the samples of Table 4B.
The present invention is a family of Dual Phase (DP) microstructure (ferrite+martensite) steels. The steels have minimal to no retained austenite. The inventive steels have a unique combination of high strength and formability. The tensile properties of the present invention preferably provide for multiple steel products. One such product has an ultimate tensile strength (UTS) 980 MPa with a total elongation (TE) 18%. Another such product will have UTS 1180 MPa and TE 15%.
Broadly the alloy has a composition (in wt %) including C: 0.1-0.3; Mn: 1-3, Si: 0.5-3.5; Al: 0.05-1, optionally Mo: 0-0.3, Nb, Ti, V: 0.005-0.1 total, the remainder being iron and inevitable residuals such as S, P, and N. More preferably the carbon is in a range of 0.14-0.21 wt %, and is preferred below 0.19 wt. % for good weldability. Most preferably the carbon is about 0.15 wt % of the alloy. The manganese content is more preferably between 1.75-2.5 wt %, and most preferably about 1.8-2.2 wt %. The silicon content is more preferably between 1.5-2.5 wt %.
WQ-CAL (water quenching continuous annealing line) is utilized to produce lean chemistry based martensitic and DP grades due to its unique water quenching capability. Therefore, the present inventors have focused on DP microstructure through WQ-CAL. In DP steels, ferrite and martensite dominantly govern ductility and strength, respectively. Therefore, strengthening of both ferrite and martensite is required to achieve high strength and ductility, simultaneously. The addition of Si effectively increases the strength of ferrite and facilitates a lower fraction of martensite to be utilized to produce the same strength level. Consequently, the ductility in DP steels is enhanced. High Si bearing DP steel has therefore been chosen as the main metallurgical concept.
In order to analyze the metallurgical effects of high Si bearing DP steels, laboratory heats with various amounts of Si have been produced by vacuum induction melting. Chemical composition of the investigated steels is listed in Table 1. The first six steels are based on 0.15C-1.8Mn-0.15Mo-0.02Nb with Si content ranging from 0-2.5 wt. %. The others have 0.2% C with 1.5-2.5 wt. % Si. It should be noted that although these steels contain 0.15 wt. % Mo, Mo addition is not required to produce a DP microstructure through WQ-CAL. Thus Mo is an optional element in the alloy family of the present invention.
After hot rolling with aim FT 870° C. and CT 580° C., both sides of the hot bands were mechanically ground to remove the decarburized layers prior to cold rolling with a reduction of about 50%. The full hard materials were annealed in a high temperature salt pot from 750 to 875° C. for 150 seconds, quickly transferred to a water tank, followed by a tempering treatment at 400/420° C. for 150 seconds. A high overaging temperature has been chosen in order to improve the hole expansion and bendability of the steels. Two JIS-T tensile tests were performed for each condition.
Annealing Properties of 2.5% Si Bearing Steel
Since 0.2% C steel with 2.5 wt. % Si achieves useful tensile properties, as shown in
Hot/Cold Rolling
Two hot rolling schedules with different coiling temperatures (CT) of 580 and 620° C. and the same aim finishing temperature (FT) of 870° C. have been conducted using a 0.2 wt. % C and 2.5 wt. % Si steel. Tensile properties of the generated hot bands are summarized in Table 2. Higher CT produces higher YS, lower TS and better ductility. Lower CT promotes the formation of bainite (bainitic ferrite) resulting in lower YS, higher TS and lower TE. However, the main microstructure consists of ferrite and pearlite at both CTs.
Annealing
Annealing simulations were performed on full hard steels produced from hot bands with CT 620° C., using salt pots. The full hard materials were annealed at various temperatures from 775 to 825° C. for 150 seconds, followed by a treatment at 720° C. for 50 seconds to simulate gas jet cooling and then quickly water quenched. The quenched samples were subsequently overaged at 400° C. for 150 seconds. High OAT of 400° C. was chosen to improve hole expansion and bendability.
Table 4A presents the tensile properties of alloys of the present invention having the basic formula 0.15C-1.8Mn-Si-0.02Nb-0.15Mo, with varied Si between 1.5-2.5 wt. %. The cold rolled alloy sheets were annealed at varied temperatures between 750-900° C. and overage treated at 200° C.
Table 4B presents the tensile properties of alloys of the present invention having the basic formula 0.15C-1.8Mn-Si-0.02Nb-0.15Mo, with varied Si between 1.5-2.5 wt. %. The cold rolled alloy sheets were annealed at varied temperatures between 750-900° C. and overage treated at 420° C.
a- 6e plot the tensile properties versus annealing temperature for the samples of Table 4A.
a- 7e plot the tensile properties versus annealing temperature for the samples of Table 4B.
As can be seen, the strength (both TS and YS) increase with increasing annealing temperature for both 200 and 420° C. overaging temperature. Also, the elongation (both TE and UE) decrease with increasing annealing temperature for both 200 and 420° C. overaging temperature. On the other hand, the Hole Expansion (HE) does not seem to be affected in any discernable way by annealing temperature, but the increase in the OA temperature seems to raise the average HE somewhat. Finally, the different OA temperatures do not seem to have any effect on the plots of TE vs TS.
It is to be understood that the disclosure set forth herein is presented in the form of detailed embodiments described for the purpose of making a full and complete disclosure of the present invention, and that such details are not to be interpreted as limiting the true scope of this invention as set forth and defined in the appended claims.
This Application claims the benefit under 35 U.S.C. 119(e) of U.S. Provisional Application No. 61/629,757 filed Nov. 28, 2011.
