CN105392906A - High Strength Steel Exhibiting Good Ductility and Method of Production via In-Line Heat Treatment Downstream of Molten Zinc Bath - Google Patents

Abstract

Steel with high strength and good formability is produced with compositions and methods for forming austenitic and martensitic microstructure in the steel. Carbon, manganese, molybdenum, nickel copper and chromium may promote the formation of room temperature stable (or meta-stable) austenite by mechanisms such as lowering transformation temperatures for non-martensitic constituents, and/or increasing the hardenability of steel. Thermal cycles utilizing a rapid cooling below a martensite start temperature followed by reheating may promote formation of room temperature stable austenite by permitting diffusion of carbon into austenite from martensite.

Description

The high-strength steel showing good ductility and the production method passed through at molten zinc plating groove downstream burning optimization on line

This application claims that the sequence number submitted on May 17th, 2013 is 61/824,699, name is called the temporary patent application of " High-StrengthSteelExhibitingGoodDuctilityandMethodofProd uctionviaIn-LinePartitioningTreatmentDownstreamofMoltenz incBath "; With the sequence number that on May 17th, 2013 submits to be 61/824,643, name is called the right of priority of the temporary patent application of " High-StrengthSteelExhibitingGoodDuctilityandMethodofProd uctionviaIn-LinePartitioningTreatmentbyZincBath ".By patent application serial numbers be by reference 61/824,699 and 61/824,643 disclosure be incorporated to herein.

Background technology

The steel that preparation has high strength and good formability characteristic is required.But the circumscribed factor of the hot-work ability of property and industrial production line needed for such as relatively low alloying additive, the business preparation showing the steel of such characteristic is difficult.The present invention relates to steel compositions and working method, described method uses galvanizing/galvanneal (HDG) technique to prepare steel, thus makes the steel of gained show high strength and property capable of cold forming.

Summary of the invention

Steel of the present invention uses the HDG technique of composition and improvement to prepare, and the two has prepared the usual microstructure be made up of martensite and austenite (except other composition) of gained together.In order to realize such microstructure, said composition comprises specific alloying additive, and HDG technique comprises specific process modification, and it all relates at least in part, and to order about austenitic transformation be martensite, subsequently at room temperature partially stabilized austenite.

Brief Description Of Drawings

Be bonded to this specification sheets and form this specification sheets part illustrate embodiment, and with generality provided above, the effect playing the principle explaining present disclosure together with the detailed description of embodiment provided below is described.

Fig. 1 describes the schematic diagram carrying out the HDG temperature distribution of distributing (partition) step after zinc-plated/galvanneal.

Fig. 2 describes the schematic diagram of the HDG temperature distribution of carrying out allocation step in zinc-plated/galvanneal process.

Fig. 3 describes the graphic representation of the Rockwell hardness relative to rate of cooling drafting of an embodiment.

Fig. 4 describes the graphic representation of the Rockwell hardness relative to rate of cooling drafting of another embodiment.

Fig. 5 describes the graphic representation of the Rockwell hardness relative to rate of cooling drafting of another embodiment.

Fig. 6 describes six Photomicrographs of the embodiment of the Fig. 3 obtained by the sample cooled with different rate of cooling.

Fig. 7 describes six Photomicrographs of the embodiment of the Fig. 4 obtained by the sample cooled with different rate of cooling.

Fig. 8 describes six Photomicrographs of the embodiment of the Fig. 5 obtained by the sample cooled with different rate of cooling.

Fig. 9 describe for several embodiment with the stretching data plot of austenitizing temperature change.

Figure 10 describe for several embodiment with the stretching data plot of austenitizing temperature change.

Figure 11 describe for several embodiment with the stretching data plot of quenching temperature change.

Figure 12 describe for several embodiment with the stretching data plot of quenching temperature change.

Describe in detail

Fig. 1 illustrates the indicative icon for realizing the thermal cycling of high strength and property capable of cold forming in the steel sheets with specific chemical constitution (hereafter describing in further detail).Especially, Fig. 1 illustrates typical galvanizing or galvanneal heat distribution (10), and wherein process modification is shown in broken lines.In one embodiment, described technique comprises austenitizing usually, be quickly cooled to the quenching temperature of specifying to change austenite fraction into martensite subsequently, and to allow carbon to diffuse out and in remaining austenite by martensite under remaining on the temperature of raising and dispense temperature, at room temperature stable austenite thus.In some embodiments, heat distribution shown in Figure 1 can use together with the continuous hot-dipping galvanizing of routine or galvanneal production line, but such production line is not needs.

As can be seen in Figure 1, first steel sheets is heated to peak metal temperatures (12).Peak metal temperatures (12) in illustrated embodiment is depicted as at least higher than austenite transformation temperature (A ₁) (such as two-phase: austenite+ferrite area).Thus, under peak metal temperatures (12), steel at least partially will change austenite into.Although Fig. 1 illustrates that peak metal temperatures (12) is only higher than A ₁but should be understood that, peak metal temperatures can also comprise and changes austenitic temperature (A completely into higher than ferrite in some embodiments ₃) temperature of (such as single-phase: austenite region).

Next, steel sheets experience cooling fast.Along with steel sheet cools down, some embodiments can comprise of short duration interruption, for zinc-plated or galvanneal when cooling.Using in zinc-plated embodiment, owing to being derived from the heat of fused zinc zinc bath, steel sheets can maintain steady temperature (14) momently.In other embodiments, galvanneal technique can also be used, and the temperature of steel sheets can be increased to a little the galvanneal temperature (16) can carrying out galvanneal technique.But, zinc-plated or galvanneal technique can be omitted completely in other embodiments, and can continuous coo1ing steel sheets.

Demonstrate at the martensite start temperature (M lower than steel sheets _s) time, steel sheets continues to be quickly cooled to predetermined quenching temperature (18).Should be understood that, to M _srate of cooling can highly must be enough to by under peak metal temperatures (12) formed austenite at least some change martensite into.In other words, rate of cooling can must be enough to austenitic transformation to become martensite soon, instead of other non-martensite composition changed under relatively low rate of cooling, such as ferrite, perlite or bainite.

As shown in FIG. 1, quenching temperature (18) is lower than M _s.Quenching temperature (18) and M _sbetween difference can be depending on used steel sheets individuality composition and change.But, in many embodiments, quenching temperature (18) and M _sbetween difference can must be enough to greatly to be formed the martensite of sufficient quantity thus serve as carbon source when final cooling in order to stable austenite with avoid producing excessive " fresh " martensite.In addition, quenching temperature (18) can highly must be enough to avoid in initial quench process, consume too much austenite (such as given embodiment, avoiding being greater than the austenitic excess carbon enrichment for the carbon enrichment required for stable austenite).

In many embodiments, quenching temperature (18) can be from about 191 DEG C to about 281 DEG C, but such restriction is not needs.In addition, quenching temperature (18) can be calculated for given steel composition.For such calculating, quenching temperature (18) corresponds to has room temperature M after the distribution _sthe retained austenite of temperature.The method calculating quenching temperature (18) is known in the art, and be described in J.G.Speer, A.M.Streicher, D.K.Matlock, F.Rizzo and G.Krauss, " QuenchingAndPartitioning:AFundamentallyNewProcesstoCreat eHighStrengthTripSheetMicrostructures, " AusteniteFormationandDecomposition, 505-522 page, 2003; With ProceedingsoftheInternationalConferenceonAdvancedHighStr engthSheetSteelsforAutomotiveApplications, A.M.Streicher in 2004, J.G.J.Speer, D.K.Matlock, and B.C.DeCooman, in " QuenchingandPartitioningResponseofaSi-AddedTRIPSheetStee l ", by reference its theme is incorporated to herein.

Quenching temperature (18) can lowly obtain (relative to M _s) be enough to be formed sufficient quantity martensite to serve as carbon source when final quenching in order to stable austenite with avoid producing excessive " fresh " martensite.Alternately, quenching temperature (18) can highly must be enough to avoid in initial quench process, consuming too much austenite and producing such situation, and namely the potential carbon enrichment of retained austenite is greater than for the carbon enrichment required for stabilization of austenite at room temperature.In some embodiments, suitable quenching temperature (18) can correspond to has room temperature M after the distribution _sthe retained austenite of temperature.The people such as Speer and Streicher (above) have provided calculating, and it is explore to cause the processing of required microstructure to select to provide criterion.Such calculation assumption is Utopian to be distributed completely, and by adopting Koistinen-Marburger (KM) relational expression twice carry out: first initial quench is to quenching temperature (18), and at room temperature finally quench (as hereafter further described) subsequently.The experimental formula (such as the experimental formula of Andrew linear representation well known in the art) based on austenite chemical constitution can be used to assess M in KM expression formula _stemperature:

Ms(℃)＝539-423C-30.4Mn-75Si+30Al

The calculation result described by people such as Speer can show the quenching temperature (18) of the retained austenite that can cause maximum.For higher than the quenching temperature (18) of temperature with maximum retained austenite, after initial quench, there is a large amount of austenite fraction; But, there is not enough martensite to serve as carbon source in order to stablize this austenite.Therefore, for higher quenching temperature, in final quenching process, form the fresh martensite of increasing amount.For lower than the quenching temperature of temperature with maximum retained austenite, the austenite of amount unsatisfactory can be consumed in initial quench process, and the excessive carbon that can be distributed by martensite can be there is.

Once reach quenching temperature (18), just raise the temperature of steel sheets relative to this quenching temperature or maintain the given time period under quenching temperature.Especially, this stage can be described as allocated phase.In such stage, the temperature of steel sheets is at least maintained under quenching temperature to allow carbon to spread from the martensite formed quick process of cooling and to enter in any residual austenite.Such diffusion can allow the austenite remained to be at room temperature stable (or metastable), improves the mechanical property of steel sheets thus.

In some embodiments, steel sheets can be heated higher than M _s, to relatively high dispense temperature (20), and under remaining on high dispense temperature (20) subsequently.In this phase process, various method can be utilized to heat steel sheets.By means of only the mode of citing, induction heating, flame can be used to heat and/or similar approach heating steel sheets.Alternately, in other embodiments, steel sheets can be heated, but reach different, lower dispense temperature (22), it is slightly lower than M _s.Under steel sheets can being remained on lower dispense temperature (22) equally subsequently, continue the specific time period.In another the 3rd alternate embodiment, another substituting dispense temperature (24) can be used when only being maintained under quenching temperature by steel sheets.Certainly, in view of the teachings contained herein, as being obvious to those skilled in the art, other suitable dispense temperature any can be used.

After steel sheets reaches required dispense temperature (20,22,24), under steel sheets being maintained required dispense temperature (20,22,24), time enough is dispensed to austenite to allow carbon from martensite.Subsequently can by steel sheet cools down to room temperature.

Fig. 2 illustrates above about the alternate embodiment (use solid line (40) display typically zinc-plated/galvanneal thermal cycling, and use dotted line to show departing from from typically zinc-plated/galvanneal thermal cycling) of the thermal cycling described by Fig. 1.Especially, with the similar process of Fig. 1, first steel sheets is heated to peak metal temperatures (42).In illustrated embodiment, peak metal temperatures (42) is depicted as at least higher than A ₁.Therefore, under peak metal temperatures (42), steel sheets will change austenite at least partially.Certainly, with the similar process of Fig. 1, the present embodiment can also comprise more than A ₃peak metal temperatures.

Next, can by steel sheets rapid quenching (44).Should be understood that, quenching (44) can must be enough to some initiation in the austenite of formation under peak metal temperatures (42) soon and change martensite into, avoid thus excessively changing non-martensite composition into, such as ferrite, perlite, bainite and/or analogue.

Can stop subsequently quenching (44) under quenching temperature (46).With the similar process of Fig. 1, quenching temperature (46) is lower than M _s.Certainly, lower than M _samount can depend on used material and change.But, as above, quenching temperature (46) and M in many embodiments _sbetween difference can must be enough to greatly form the martensite of q.s, described martensitic q.s is also so low that to be enough to avoid consuming too much austenite.

Then (48) steel sheets is reheated subsequently to dispense temperature (50,52).Be different from the process of Fig. 1, the dispense temperature (50,52) in the present embodiment can be characterized by zinc-plated or galvanneal zinc bath temperature (if so using zinc-plated or galvanneal).Such as, using in zinc-plated embodiment, steel sheets can be reheated to zinc bath temperature (50), and keeping at such a temperature during galvanizing process subsequently.During galvanizing process, distribution can be carried out similarly with distribution as above.Thus, zinc bath temperature (50) also can be used as dispense temperature (50).Similarly, in the embodiment using galvanneal, except higher groove/dispense temperature (52), this process can be substantially the same.

Finally, make steel sheet cools down (54) to room temperature, at least some austenite being at room temperature derived from allocation step as above can be stable (or metastable).

In some embodiments, steel sheets can comprise specific alloying additive, thus improves the mechanical property that steel sheets forms tendency and/or the improvement steel sheets being mainly austenite and martensitic microstructure.Suitable steel sheets composition can comprise following one or more by weight percentage: 0.15-0.4% carbon, 1.5-4% manganese, 0-2% silicon or aluminium or its some combinations, 0-0.5% molybdenum, 0-0.05% niobium, other incidental element, and surplus is iron.

In addition, in other embodiments, suitable steel sheets composition can comprise following one or more by weight percentage: 0.15-0.5% carbon, 1-3% manganese, 0-2% silicon or aluminium or its some combinations, 0-0.5% molybdenum, 0-0.05% niobium, other incidental element, and surplus is iron.In addition, except niobium or alternative niobium, other embodiment can comprise the additive of vanadium and/or titanium, but such additive is optional completely.

In some embodiments, carbon can be used for stable austenite.Such as, the carbon of increase can reduce M _stemperature, reduces the transition temperature of other non-martensite composition (such as bainite, ferrite, perlite), and increases the time for being formed required for non-martensite product.In addition, carbon additive can improve the hardening capacity of material, keeps non-martensite composition to be formed near the material core that locally can reduce rate of cooling thus.But should be understood that, carbon additive can be limited, because a large amount of carbon additives can cause the disadvantageous effect to weldability.

In some embodiments, manganese can provide extra stabilization of austenite by the transition temperature reducing other non-martensite composition as above.Manganese can improve by improving hardening capacity the tendency that steel sheets formation is mainly austenite and martensitic microstructure further.

In other embodiments, molybdenum can be used for improving hardening capacity.

In other embodiments, silicon and/or aluminium can be provided to reduce the formation of carbide.Should be understood that, the minimizing that carbide is formed in some embodiments can be required, because the existence of carbide can reduce the content that can be used for the carbon diffused in austenite.Thus, silicon and/or aluminium additive can be used at room temperature stable austenite further.

In some embodiments, nickel, copper and chromium can be used for stable austenite.Such as, such element can cause M _sthe reduction of temperature.In addition, nickel, copper and chromium can further improve the hardening capacity of steel sheets.

In some embodiments, niobium (or other micro alloying element, such as titanium, vanadium and/or analogue) can be used for the mechanical property improving steel sheets.Such as, niobium can increase the intensity of steel sheets by being formed caused Grain boundary pinning (pinning) by carbide.

In other embodiments, the concentration of element and the change of selected special element can be made.Certainly, when making such change, should be understood that, according to the character for often kind of given alloying additive as above, such change can have required or unwanted impact to steel sheets microstructure and/or mechanical property.

Embodiment 1

Be used in the embodiment that the composition proposed in table 1 hereafter prepares steel sheets.

According to following parameter work material on laboratory equipment.The webge grip of copper cooling and pocket type tooth plate (pocketjaw) fixture is used to make each sample stand Gleeble1500 process.By sample austenitizing at 1100 DEG C, and be cooled to room temperature with the different cooling rate between 1-100 DEG C/s subsequently.

Embodiment 2

The surface of each sample obtains the Rockwell hardness of each in the steel composition described in embodiment 1 above and table 1.Draw test result in figs. 3-5, wherein draw the change of Rockwell hardness with rate of cooling.For each data point, the mean value measured at least seven times is shown.Composition V4037, V4038 and V4039 correspond respectively to Fig. 3,4 and 5.

Embodiment 3

Immediate vicinity in the vertical through each each sample of thickness direction in the composition of embodiment 1 obtains optical microscopy map.The result of these tests illustrates in figures 6 to 8.Composition V4037, V4038 and V4039 correspond respectively to Fig. 6,7 and 8.In addition, each self-contained six width micrograms for forming separately of Fig. 6-8, every width microgram representative stands the sample of different cooling rate.

Embodiment 4

According to operation described herein, the data of use embodiment 2 and 3 assess the critical cooling rate of each in the composition of embodiment 1.Critical cooling rate herein refer to for formed martensite and minimize non-martensite transmutation product formation required for rate of cooling.The result of these tests is as follows:

V4037：70℃/s

V4038：75℃/s

V4039：7℃/s

Embodiment 5

Be used in the embodiment that the composition proposed in table 2 hereafter prepares steel sheets.

By fusing, hot rolling and cold rolling come work material.Material is made to stand the test described in further detail in embodiment 6-7 hereafter subsequently.Except be intended to as above about except the V4039 used together with the method described by Fig. 1, the whole compositions listed in table 2 are intended to and as above use together with the method described by Fig. 2.Hot V4039 has and aims to provide according to as above about the comparatively high-hardenability required for the heat distribution described by Fig. 1.Therefore after hot rolling but before cold rolling, at 100%H at 600 DEG C ₂v4039 is made to stand annealing 2 hours in atmosphere.In cold rolling period, by thinning for all material about 75%, to 1mm.For propose in table 2 some materials composition hot rolling and cold rolling after result respectively shown in table 3 and 4.

Table 2-in % by weight chemical constitution

Heat	Describe	C	Mn	Si	Al	Mo	Cr	Nb	B
										V4037	Lab material	0.19	1.54	0.11	1.41	0	0.009	0	0.0007
V1307	Lab material	0.19	1.53	1.48	0.041	0	0	0	0.0005
										V4063	Lab material	0.19	1.6	0.11	1.34	0	0.003	0	0.0007
V4038	Lab material	0.22	1.68	0.007	1.29	0	0.2	0.021	0.0008
										V4039	Lab material	0.2	2.94	1.57	＜0.030	＜0.002	0.005	0.002	N/R
A1305	Lab material	0.2	2.94	1.57	0	0	0	0	0.0006
										V4107	Lab material	0.18	4.03	1.63	0.005	0	0	0	0.0008
V4108	Lab material	0.18	5.06	1.56	0.004	0	0	0	0.0009
										V4060	Lab material	0.4	1.2	1.97	0.003	0	0.19	0.007	0.0005
V4061	Lab material	0.41	1.2	0.98	0.003	0	0.003	0	0.0004
										V4062	Lab material	0.39	1.18	0.012	1.16	0	0.003	0	0.0007
V4078-1	Lab material	0.2	1.67	0.1	1.41	0.28	0.003	＜0.003	0.0007
										V4078-2	Lab material	0.2	1.67	0.1	1.41	0.27	＜0.003	0.051	0.0007
V4078-1	Lab material	0.19	1.94	0.098	1.43	＜0.003	＜0.003	＜0.003	0.0007
										V4078-2	Lab material	0.19	1.96	0.099	1.41	＜0.003	＜0.003	0.051	0.0007

Embodiment 7

The composition of embodiment 5 is made to stand Gleeble dilatometry.Use the sample of 101.6x25.4x1mm to measure expansion with c strainometer in a vacuum on 25.4mm direction and carry out Gleeble dilatometry.Form the expansion of gained to the graphic representation of temperature.The line segment of matching swellability measurement data, and the point that swellability measurement data and linear behavior depart from is considered as transition temperature (the such as A that pays close attention to ₁, A ₃, M _s).The transition temperature of gained is listed in table 5.

Gleeble method is also for measuring the critical cooling rate of each in the composition of embodiment 5.As above, the first method utilizes Gleeble dilatometry.Second method utilizes rockwell hardness measurement.Especially, make with a series of rate of cooling sample stand Gleeble test after, carry out rockwell hardness measurement.Thus, the hardness measurement for a series of rate of cooling is utilized to obtain the rockwell hardness measurement value of each material composition.Compare between the rockwell hardness measurement value of given composition under each rate of cooling subsequently.The Rockwell hardness deviation of 2 some HRA is regarded as significantly.By avoiding the critical cooling rate of non-martensite transmutation product to be considered as the highest rate of cooling, for this highest rate of cooling, low 2 the some HRA of hardness ratio highest hardness.For some compositions listed in embodiment 5, the critical cooling rate of gained is also listed in table 5.

Table 5-is derived from transition temperature and the critical cooling rate of Gleeble dilatometry

Embodiment 8

The composition of embodiment 5 is used to calculate the theoretical maximum of quenching temperature and retained austenite.The method of the people such as Speer as above is used to calculate.Calculation result for some compositions listed in embodiment 5 is listed in table 6 hereafter.

The theoretical maximum of table 6-quenching temperature and retained austenite

Heat	QT(℃)	F (γ) theoretical maximum
			V4037	281	0.15
V4063	278	0.15
			V4038	270	0.18
V4039	203	0.2
			V4060	191	0.35
V4061	196	0.36
			V4062	237	0.31
V4078-1	276	0.16
			V4078-2	276	0.16
V4079-1	273	0.16
			V4079-2	272	0.16

Embodiment 9

Make the sample of the composition of embodiment 5 stand heat distribution shown in fig. 1 and 2, between the sample of given composition, change peak metal temperatures and quenching temperature.As above, only make composition V4039 stand the heat distribution shown in Fig. 1, and make other compositions all stand the thermal cycling shown in Fig. 2.For each sample, carry out stretching strength measurement.The stretching observed value of gained is drawn in Fig. 9-12.Especially, Fig. 9-10 demonstrates the tensile strength data of drawing relative to austenitizing temperature, and Figure 11-12 demonstrates the tensile strength data of drawing relative to quenching temperature.In addition, when using Gleeble method to carry out thermal cycling, such data point represents with " Gleeble ".Similarly, when using salt bath to carry out thermal cycling, such data point represents with " salt ".

In addition, the similar stretching observed value (when obtaining) of each composition listed in embodiment 5 is listed in hereafter shown table 7.Distribution time and temperature are only depicted as exemplary, in other embodiments mechanism (such as carbon distribute and/or phase in version) non-isothermal heating and cooling to the dispense temperature of specifying or by the dispense temperature non-isothermal heating and cooling of specifying during occur, it also can contribute to final material character.

Stretching data after table 7-distributes

Will be appreciated that and can carry out various amendment to the present invention and not deviate from its spirit and scope.Therefore, restriction of the present invention should be determined according to appended claim.

Claims (7)

1. steel sheets, comprises following element by weight percentage:

0.15-0.4% carbon;

1.5-4% manganese;

2% or less silicon, aluminium or its some combinations;

0.5% or less molybdenum;

0.05% or less niobium; With

Surplus is that iron and other idol deposit impurity.

2., for the method for machined steel sheet material, described method comprises:

A () heats described steel sheets to the first temperature (T1), wherein T1 is at least transformed into austenite and ferritic temperature higher than steel sheets;

B (), by the cooling under rate of cooling by described steel sheet cools down to the second temperature (T2), wherein T2 is lower than martensite start temperature (M _s), wherein said rate of cooling be enough fast with by austenitic transformation for martensite;

C () reheats described steel sheets to dispense temperature, wherein said dispense temperature is enough to the in-house diffusion allowing carbon at described steel sheets;

D (), by described steel sheets is kept certain hold-time stable austenite under dispense temperature, the wherein said hold-time allows carbon to diffuse to the austenitic time period by martensite for being enough to; With

(e) by described steel sheet cools down to room temperature.

3. the method for claim 2, be included in further by described steel sheet cools down to during T2 by described steel sheets galvanizing or galvanneal.

4. the method for claim 3, wherein galvanizing or galvanneal are higher than M _sunder carry out.

5. the method for claim 2, wherein said dispense temperature is higher than M _s.

6. the method for claim 2, wherein said dispense temperature is lower than M _s.

7. the method for claim 2, wherein described steel sheets comprises following element by weight percentage:

0.15-0.4% carbon;

1.5-4% manganese;

2% or less silicon, aluminium or its some combinations;

0.5% or less molybdenum;

0.05% or less niobium; With

Surplus is that iron and other idol deposit impurity.