METHOD FOR PRODUCING HIGH SILICON DUAL PHASE STEELS WITH IMPROVED DUCTILITY

Abstract

A method for producing a dual phase steel sheet is provided. The method includes providing a dual phase hot rolled steel sheet having a microstructure including ferrite and martensite and a composition including 0.1 to 0.3 wt. % C, 1.5 to 2.5 wt. % Si and 1.75 to 2.5 wt. % Mn. The steel sheet is annealed at a temperature from 750 to 875° C., water quenched to a temperature from 400 to 420° C. and subject to overaging at the temperature from 400 to 420° C. to convert the martensite in the hot rolled steel sheet to tempered martensite. The overaging is sufficient to provide the hot rolled steel sheet with a hole expansion ratio of at least 15%.

Description

FIELD OF THE INVENTION

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.

BACKGROUND OF THE INVENTION

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%.

SUMMARY OF THE INVENTION

The present invention provides 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.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the present invention will be elucidated with reference to the drawings, in which:

FIGS. 1a and 1b 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 to 2.5 wt. % in accordance with a preferred embodiment of the present invention;

FIGS. 2a 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;

FIGS. 3a 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;

FIGS. 4a 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.;

FIGS. 5a to 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;

FIGS. 6a to 6e plot the tensile properties versus annealing temperature for the samples of Table 4A;

FIG. 6f plots TE vs TS for the samples of Table 4A;

FIGS. 7a to 7e plot the tensile properties versus annealing temperature for the samples of Table 4B; and

FIG. 7f plots TE vs TS for the samples of Table 4B.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides 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) 2980 MPa with a total elongation (TE) 218%, for example. Another such product will have UTS 21180 MPa and TE 215%, for example.

In accordance with preferred embodiments, 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. %.

Examples

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.

TABLE 1
ID	C	Mn	Si	Nb	Mo	Al	P	S	N
15C0Si	0.15	1.77	0.01	0.019	0.15	0.037	0.008	0.005	0.0055
15C5Si	0.14	1.75	0.5	0.019	0.15	0.05	0.009	0.005	0.0055
15C10Si	0.15	1.77	0.98	0.019	0.15	0.049	0.009	0.004	0.0055
15C15Si	0.14	1.8	1.56	0.017	0.15	0.071	0.008	0.005	0.005
15C20Si	0.15	1.86	2.02	0.018	0.16	0.067	0.009	0.005	0.0053
15C25Si	0.14	1.86	2.5	0.018	0.16	0.075	0.008	0.005	0.0053
20C15Si	0.2	1.8	1.56	0.017	0.15	0.064	0.009	0.005	0.0061
20C20Si	0.21	1.85	1.99	0.018	0.16	0.068	0.008	0.005	0.0055
20C25Si	0.21	1.85	2.51	0.018	0.16	0.064	0.008	0.005	0.0056

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. FIGS. 1a and 1b 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. %. FIGS. 1a and 1b show the effect of Si addition on the balance between tensile strength and total elongation. The increase in Si content clearly enhances the ductility at the same level of tensile strength in both 0.15% C and 0.20% C steels. FIGS. 2a 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 wt. % Si and 2b at 2.5 wt. % Si. FIGS. 2a and 2b confirm that higher Si has more ferrite fraction at a similar level of tensile strength (TS about 1300 MPA). In addition, XRD results reveal no retained austenite in the annealed steels resulting in no TRIP effect by adding Si.

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 FIG. 1, further analysis of 0.2 wt. % C and 2.5 wt. % Si steel was performed.

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. FIGS. 3a 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. There is no major issue for cold mill load since both CTs have lower strength than GA DP T980. In addition, Mo addition is not required to produce DP microstructure with WQ-CAL. The composition without Mo will soften hot band strength in all ranges of CT. After mechanical grinding to remove the decarburized layers, the hot bands were cold rolled by about 50% on the laboratory cold mill.

TABLE 2
Grade	CT,	YS, Mpa	TS, Mpa	UE, %	TE, %	YPE,
0.2C—1.8Mn—2.5Si—0.15Mo—0.02Nb	580	451	860	9.9	17.7	0
	620	661	818	14.7	22.3	3.3

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. FIGS. 4a 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. Both YS and TS increase with AT at the cost TE. An annealing temperature of 800° C. with GJC 720° C. and OAT 400° C. can produce steel with a YS of about 950 MPa, TS of about 1250 MPa and TE of about 16%. It should be noted that this composition can produce multiple grades of steel at varying TS level from 980 to 1270 MPA: 1) YS=800MPA, TS=1080 MPa and TE=20%; and 2) YS=1040 MPa, TS=1310 MPa, and TE=15% (see Table 3). FIGS. 5a to 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. The sample annealed at AT 750° C. still contains undissolved cementites in a fully recrystallized ferrite matrix resulting in high TE and YPE. Starting from AT 775° C., it produces a dual phase microstructure of ferrite and tempered martensite. The sample processed at AT 800° C. contains a martensite fraction of about 40% and exhibits a TS of about 1180 MPa; similar to current industrial DP steel with TS of 980 with lower Si content that also contains about 40% martensite. A potential combination of higher TS and TE in high Si DP steels processed at AT of 825° C. and higher can be expected. Hole expansion (HE) and 90° free V bend tests were performed on the samples annealed at 800° C. Hole expansion and bendability demonstrated average 22% (std. dev. of 3% and based on 4 tests) and 1.1 r/t, respectively.

TABLE 3
AT,	Gauge,	YS,	TS,	UE,	TE,	YPE,
° C.	mm	MPa	MPa	%	%	%
725	1.5	698	814	15.3	25	4.6
725	1.5	712	819	14.9	24	5
750	1.5	664	797	15.8	26.5	4.2
750	1.5	650	790	15.1	27.2	2.7
775	1.5	808	1074	13	20.3	0
775	1.5	803	1091	12.5	20.1	0.3
800	1.5	952	1242	9.7	16.5	2.4
800	1.5	959	1250	9	15.8	0
825	1.5	1038	1307	8.3	14.8	0
825	1.5	1034	1314	8.4	15.1	0

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.

FIGS. 6a to 6e plot the tensile properties versus annealing temperature for the samples of Table 4A. FIG. 6f plots TE vs TS for the samples of Table 4A.

FIGS. 7a to 7e plot the tensile properties versus annealing temperature for the samples of Table 4B. FIG. 7f plots TE vs TS 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.

TABLE 4A
		AT,	OAT,
Serial	Si	C.	C.	Gauge	YS0.2	TS	UE	TE
301469	1.5	750	200	1.45	522	1032	11.7	16.9
301470	1.5	750	200	1.47	524	1021	11.6	17.2
300843	1.5	775	200	1.50	643	1184	8.8	13.7
300844	1.5	775	200	1.52	630	1166	8.9	13.5
300487	1.5	800	200	1.46	688	1197	7.7	11.8
300488	1.5	800	200	1.46	675	1195	7.9	13.8
300505	1.5	825	200	1.51	765	1271	7.7	12.4
300506	1.5	825	200	1.47	781	1269	7.1	12.0
300493	1.5	850	200	1.48	927	1333	5.7	9.9
300494	1.5	850	200	1.44	970	1319	5.2	8.6
300511	1.5	875	200	1.50	1066	1387	4.7	8.9
300512	1.5	875	200	1.50	1075	1373	4.6	9.0
301471	2	750	200	1.54	532	1056	13.1	19.5
301472	2	750	200	1.56	543	1062	12.6	19.2
300845	2	775	200	1.53	606	1173	10.3	16.1
300846	2	775	200	1.57	595	1148	10.3	15.9
300489	2	800	200	1.40	623	1180	9.2	13.2
300490	2	800	200	1.37	629	1186	9.6	14.7
300507	2	825	200	1.41	703	1268	8.4	13.2
300508	2	825	200	1.42	695	1265	8.7	13.2
300495	2	850	200	1.40	748	1257	6.4	10.7
300496	2	850	200	1.40	779	1272	7.4	12.0
300513	2	875	200	1.37	978	1366	5.7	9.0
300514	2	875	200	1.41	956	1335	4.9	8.4
301473	2.5	750	200	1.67	476	809	14.1	21.8
301474	2.5	750	200	1.45	481	807	12.6	19.9
300491	2.5	800	200	1.41	605	1168	10.2	15.3
300492	2.5	800	200	1.46	624	1184	10.6	16.6
300509	2.5	825	200	1.44	657	1237	9.2	14.3
300510	2.5	825	200	1.45	652	1235	9.9	15.8
300497	2.5	850	200	1.40	690	1245	9.3	15.0
300498	2.5	850	200	1.42	684	1233	8.9	14.6
300515	2.5	875	200	1.47	796	1285	7.6	12.8
300516	2.5	875	200	1.46	812	1305	6.2	9.6
300847	2.5	900	200	1.45	860	1347	7.2	12.3
300848	2.5	900	200	1.42	858	1347	6.9	11.6

TABLE 4B
		AT,	OAT,
Serial	Si	C.	C.	Gauge	YS0.2	TS	UE	TE
301451	1.5	750	420	1.57	780	976	11.0	19.7
301452	1.5	750	420	1.55	778	980	10.4	19.6
301453	1.5	775	420	1.42	868	1045	8.9	16.2
301454	1.5	775	420	1.44	834	1033	9.1	16.7
301455	1.5	800	420	1.44	989	1133	5.2	13.1
301456	1.5	800	420	1.42	1007	1135	5.2	13.2
301031	1.5	825	420	1.46	1060	1155	5.4	12.2
301032	1.5	825	420	1.46	1060	1146	5.5	12.1
301457	2	775	420	1.52	855	1065	9.8	17.3
301458	2	775	420	1.52	855	1068	10.3	19.4
301459	2	800	420	1.56	954	1120	8.7	17.2
301460	2	800	420	1.55	954	1118	8.7	15.6
301461	2	825	420	1.53	1043	1175	5.2	14.5
301462	2	825	420	1.54	1062	1184	5.2	16.4
301033	2	850	420	1.40	1111	1186	5.7	10.4
301034	2	850	420	1.37	1112	1194	5.8	11.1
301463	2.5	800	420	1.53	906	1118	9.6	17.6
301464	2.5	800	420	1.55	896	1097	9.7	17.5
301465	2.5	825	420	1.67	991	1154	8.3	15.7
301466	2.5	825	420	1.66	983	1147	8.8	16.6
301467	2.5	850	420	1.55	1071	1189	7.9	13.8
301468	2.5	850	420	1.54	1064	1183	7.8	13.1
301035	2.5	875	420	1.41	1120	1217	5.8	13.9
301036	2.5	875	420	1.46	1132	1225	6.0	13.7

Claims

1. A method for producing a dual phase steel sheet comprising the steps of: providing a dual phase hot rolled steel sheet having a microstructure including ferrite and martensite having a composition including: 0.1 to 0.3 wt. % C;1.5 to 2.5 wt. % Si; and1.75 to 2.5 wt. % Mn;annealing the hot rolled steel sheet at a temperature from 750 to 875° C.;water quenching the hot rolled steel sheet to a temperature from 400 to 420° C.; andoveraging the steel sheet at the temperature from 400 to 420° C. to convert the martensite in the hot rolled steel sheet to tempered martensite;the overaging sufficient to provide the hot rolled steel sheet with a hole expansion ratio of at least 15%.

2. The method for producing a dual phase steel sheet as recited in claim 1, further comprising the step of: grinding the hot rolled steel sheet to remove decarburized layers.

3. The method for producing a dual phase steel sheet as recited in claim 1, further comprising the step of: cold rolling the hot rolled steel sheet.

4. The method for producing a dual phase steel sheet as recited in claim 1, wherein said dual phase steel sheet has a hole expansion ratio of at least 20%.