CN103827338A

CN103827338A - Low density high strength steel and method for producing said steel

Info

Publication number: CN103827338A
Application number: CN201280043488.5A
Authority: CN
Inventors: 刘成; R·罗纳
Original assignee: Tata Steel Nederland Technology BV
Current assignee: Tata Steel Nederland Technology BV
Priority date: 2011-09-09
Filing date: 2012-02-21
Publication date: 2014-05-28
Anticipated expiration: 2032-02-21
Also published as: US20140363694A1; IN2014CN02603A; EP2753725B1; EP2753725A1; WO2013034317A1; CN103827338B

Abstract

The present invention relates to a low density high strength steel sheet comprising 0.15% to 0.25% of C, 2.5% to 4% of Mn, 0.02% or less of P, 0. 015% or less of S, 6% to 9% of Al and 0.01% or less of N, the balance being iron and inevitable impurities, wherein 1.7*(Mn-Al) + 52.7*C is at least 3 and at most 4.5 and to a method of producing said low density and high strength steel sheet.

Description

Low-density and high-strength steel and prepare the method for described steel

The present invention relates to low-density and high-strength steel sheets and relate to the method for preparing low-density and high-strength steel sheets, for example, use inside or the outer steel sheet material of the structural parts that acts on automobile.

Due to its excellent intensity and ductility and compared with aluminium or magnesium extremely low cost, allow to use the stronger grade of thinner high-strength steel sheet material by exploitation, steel is generally used for making the car body of automobile lighter.But, in order to overcome the following limitation to weight reducing, can use the weight of substituting source for reducing steel part.

Having more low-density steel is well known in the art.The TWIP steel with the utmost point high Mn content that exceedes 20% causes lighter steel matrix.Thereby sometimes also add the density that reduces steel as the aluminium of lightweight element.Use the aluminium that is added into many 15%.

The problem that known steel have is: due to their tearing tendency and their high deformation resistance in the operation of rolling, the processing characteristics of the existing installation of steel industry is problematic.Other problem is weldability problem, particularly for high aluminium content, and forms the possibility of unwanted martensite component.

An object of the present invention is to provide the low-density and high-strength steel sheets that can use the existing installation of steel industry to prepare.

Another object of the present invention is to provide a kind of at least low-density and high-strength steel sheets of the relative low alloying of the intensity of 600MPa that has.

Another object of the present invention is to provide the method for the preparation of low-density and high-strength steel sheets.

By provide as low-density and high-strength steel sheets required for protection in claim 1 realize one or more as described in object; described steel sheets comprises 0.15% to 0.25% C, 2.5% to 4% Mn, 0.02% or P still less, 0.015% or S still less, 6% to 9% Al and 0.01% or N still less; surplus is iron and inevitable impurity, and wherein 1.7 (Mn-Al)+52.7C are at least 3 and be at most 4.5.Claimed preferred embodiment in the dependent claims.

Realize one or more described objects by the method for claim 7.The preferred embodiment of claimed the method in the dependent claims.

To describe composition of the present invention (all % by weight that consists of) below in detail.

Carbon is for generation of cementite (Fe, Mn) 3C and Ka Pa carbide (Fe, Mn) 3AlC.Carbon or austenite stabilizer element, and provide dispersion-strengthened by forming precipitate.Especially, because the quick recrystallize of the columnar dendrite producing in continuous casting process is to produce thick tissue, thereby at high temperature carbide be formed for this tissue of refinement.In addition, the carbon of interpolation 0.15% to 0.25% is used for improving intensity.But if the addition of carbon increases, the amount that cementite and Ka Pa carbide increase contributes to improve intensity, but has greatly reduced the ductility of steel.Particularly adding wherein in the steel of aluminium, card handkerchief carbide is separated out and is caused fragility on ferrite crystal boundary.Cause too low intensity lower than 0.15% value, and be tending towards improving the risk of the problem relevant to welding higher than 0.25% value, therefore carbon content is 0.25 to the maximum.Preferably, carbon content is at least 0.16%, is more preferably at least 0.17%.Preferably, carbon content is at most 0.23, is more preferably at most 0.20%.

Manganese contributes at high temperature to form austenite together with carbon.In addition, thus manganese has increased the lattice parameter of steel has reduced the density of steel.The manganese of the minimum of discovery 2.5% causes the remarkable reduction of stable austenite and steel density.But, if the amount of manganese is excessive, so center segregation cause band structure excessive in hot rolled sheet, this causes the reduction of ductility.Therefore be 4% by the ceiling restriction of manganese.The preferred upper limit of manganese is 3.8%.

Phosphorus is the element adding with the least possible amount.Phosphorus segregation cause red brittleness and cold shortness on crystal boundary, thus the workability of steel can greatly be reduced.Be 0.02% by the ceiling restriction of phosphorus, but preferably this amount be restricted at the most to 0.01% or even 0.005%.

Be similar to phosphorus, sulphur promotes red brittleness.Especially, it produces thick MnS, and MnS can cause milled sheet fracture in hot rolling with when cold rolling, be therefore restricted to 0.015% or lower.Preferably sulphur content is restricted at the most to 0.01% or even 0.005%.

Aluminium is the important element in the present invention with carbon together with manganese.Interpolation aluminium has reduced the density of steel.Consider the reduction of proportion, preferably add a large amount of aluminium.Interpolation 6% or more aluminium cause the remarkable reduction of steel density.But if excessive interpolation aluminium, intermetallic compound is as the amount increase of card handkerchief carbide, FeAl or Fe3Al, this causes the ductility of steel to reduce, and causes the cracking in cold-rolled process, is therefore 9% by ceiling restriction.In addition, aluminium cause ductility-brittle transition temperature by the temperature lower than zero degree near raising envrionment temperature.Therefore be, 9% by the ceiling restriction of aluminium.The preferred lower limit of aluminium is 6.2%.

If add a large amount of aluminium as in the present invention, nitrogen causes the formation of AlN precipitate.These precipitates are in columnar dendrite refinement and to improve aspect the ratio of equiaxed dendrite be effectively, and for this reason in steel a small amount of nitrogen be favourable.But a large amount of nitrogen causes a large amount of AlN precipitates that may be thick, this is unwanted.Therefore be, 0.01% by the ceiling restriction of N.Preferably nitrogen amount is restricted at the most to 0.008% or even 0.005%.

Contriver finds, the steel in chemical composition range as explained above can not cause showing to obtain gratifying steel.If the result of formula (1.7 × (Mn-Al)+52.7C) is lower than 3 or higher than 4.5, the ductility of the steel obtaining seems too low.

By this formula, control relatively the amount of manganese and carbon to control ductility with the amount of aluminium.

In one embodiment, the value of (the 36C+Mn)/Al that selects the composition of this steel sheets to make is at least 1.3 and be at most 2.0.

Except above-mentioned essentially consist of the present invention, in order to improve or compensate intensity, ductility and other physical properties of steel, can also optionally add on a small quantity to one or both or more kinds of element in the group being formed by Si, Cr, Mo, Ni, Cu, B, Ti, Zr, Nb, W and Ca of moderate.

Be similar to aluminium, silicon has also reduced the proportion of steel and has contributed to the raising of intensity, if but excessive interpolation, silicon can produce thick, irregular high-temperature oxide film on steel surface.In addition, silicon cause brittle-ductile transition temperature by the temperature lower than zero degree near the stronger raising of ratio aluminium envrionment temperature.Therefore be, 2% by the ceiling restriction of silicon.Therefore, preferably the amount of silicon is 0.1 to 2.0%.

Chromium is ferrite former, and its formation can be used for the Cr base carbide of refinement microstructure, and therefore this amount can be 0.1% or higher.If but excessive interpolation reduces ductility, be therefore 0.3% by ceiling restriction.

Be similar to chromium, molybdenum is to form the ferrite former of thin carbide, and with 0.05% or higher amount add.If but excessive interpolation, it reduces the ductility of steel, is therefore 0.5% by its ceiling restriction.

Nickel is austenite former, and it can with this tissue of refinement, resist wrinkling property thereby significantly improve by introducing portion austenite in course of hot rolling.But the price of nickel is high and improved manufacturing cost, therefore its boundary is restricted to 0.1 to 2.0%.

The effect of copper is similar to nickel, but conventionally the cost ratio nickel of copper is low, therefore its can with 0.1% or higher scope add.If but excessive interpolation by due to molten metal, it is present on crystal boundary with liquid state and causes intercrystalline fragility, and causes crack at edge, therefore its amount is restricted to 0.1 to 1.0%.

Boron has limited ferritic Recovery and recrystallization in course of hot rolling, thereby contributes to the structure refinement being caused by accumulative total rolling draft the intensity that improves steel, and therefore this amount is 0.0005% or higher.If but excessive interpolation, it can produce boron-carbide, has reduced ductility the deteriorated wettability of galvanizing coating of steel, is therefore 0.003% by this ceiling restriction.

Titanium forms Τ i Ν or TiC or analogue, improve thus the grain refining of cast structure and contribute to the disperse of card handkerchief carbide, therefore its with 0.01% or higher scope add.But titanium is expensive and improved manufacturing cost, and owing to improving it and reduced ductility by separating out intensity, be therefore 0.2% by ceiling restriction.

The effect of zirconium is similar to titanium, and forms with respect to the stronger nitride of titanium and carbide, therefore its with 0.005% or higher scope add.But zirconium is expensive, be therefore 0.2% by ceiling restriction.

The effect of niobium is similar to titanium, and therefore its with 0.005% or higher scope add.But different from titanium, niobium has postponed steel recrystallize at high temperature, has greatly improved thus the rolling load of hot rolling.This makes to manufacture stalloy material, is therefore 0.2% by ceiling restriction.

Tungsten is the heavy element that improves steel proportion, and therefore it adds (if interpolation) in 0.05 to 1.0% scope.Antimony (Sb) segregation on crystal boundary has limited the formation of card handkerchief carbide, therefore, antimony (if interpolation) with 0.005% or higher scope add.If but excessive interpolation, antimony on crystal boundary segregation with reduce ductility, be therefore 0.2% by its ceiling restriction.

Ca forms sulfide as CaS and prevents thus the formation of MnS, therefore its with 0.001% or higher scope add to improve the hot workability of steel.Be 0.2% by ceiling restriction.

Steel sheets of the present invention comprises residual austenite body tissue.This residual austenite has compensated the low strength of ferrite matrix structure and has also contributed to the improvement of its ductility, therefore by area with 5% or higher scope comprise retained austenite.Be preferably 20% by its ceiling restriction, more preferably on this, be limited to 15% or even 12%.

The manufacture method of high strength low-gravity steel sheets will be described in detail below.

In order to manufacture steel sheets of the present invention, first in the temperature range of 1000 to 1250 ℃, heat slab (being thin slab (<150mm), heavy slab (150-400mm) or cast steel band (<20mm)).Exceed 1250 ℃ if reheat temperature, in slab, form thick crystal grain, likely produce wrinkling and red brittleness, if but temperature is lower than 1000 ℃, hot fine rolling temperature becomes too low to such an extent as to can not manufacture steel sheets and spray to remove the lip-deep oxide film of high temperature with pressurized water, causes thus surface imperfection.Therefore, will reheat temperature limitation is 1000 to 1250 ℃.Preferably, reheat temperature and be at least 1100 ℃.

Thereby effectively obtain close grain owing to implementing hot rolling at alap temperature, thereby according to the present invention, 900 ℃ or lower, preferably implement finish rolling at the temperature of 850 ℃ at the most in case by course of hot rolling dynamically or Static Recrystallization carry out crystal grain thinning.This means and make it stand last hot deformation step during at least in aforementioned temperature when this material.If but this temperature is too low, deformation resistance raising makes to be difficult to manufacture steel sheets, and separates out card handkerchief carbide so that the structure of elongation to be provided, and increases thus wrinkling defect, therefore rolling temperature is 700 ℃ or higher, is preferably 750 ℃ or higher and more preferably 800 ℃ or higher.

This hot-rolled steel band of reeling at 600 ℃ or lower temperature.This temperature limitation alligatoring and the excessive of Ka Pa carbide of grain-size separate out.Its also reduced the abnormal alligatoring being caused by the secondary recrystallization of coarse grain crystal grain form risk.Preferably coiling temperature is lower than 550 ℃.Be proved to be and in cold-rolled process, cause serious cracking owing to this material being quenched to envrionment temperature, thereby coiling temperature should be at least 200 ℃, and preferably be at least 300 ℃.

Can be with pickling and smooth and be coated with after oil treatment the hot-finished material of gained is manufactured to hot-rolled steel sheet material.According to the present invention, this steel sheets is the low density steel sheet material with 7400kg/m3 or lower, preferred 7300kg/m3 or lower proportion.

In addition, can pickling and cold rolling after this hot-rolled steel sheet material is manufactured to cold-rolled steel sheet material.

In cold rolling, cold rolling draft is set to 40% or higher.This is because if cold rolling draft is set to 40% or higher, can fix the energy storing by cold working, and can obtain new recrystallized structure.Preferably minimum cold rolling draft is 50%.But, consider production efficiency and economy, be 90% or lower by its ceiling restriction.Optionally, can under cold rolling or between step, make this material stand process annealing.

After clean surface (if desired), with continuous annealing or continuous hot-dipping galvanizing, this cold-rolled steel sheet material is processed.Preferably selective annealing speed in the scope of 1 ℃/s to 20 ℃/s.If annealing rate is lower than 1 ℃/s, productivity is too low, and steel sheets is exposed to for a long time hot conditions and causes thus grain coarsening and strength decreased, deteriorated material quality.On the other hand, if annealing rate exceedes 20 ℃/s, due to insufficient remelting of carbide, thus austenitic formation also become insufficient and therefore residual austenite reduce, reduce thus ductility.Rate of cooling after annealing is preferably 10 to 50 ℃/s, or to envrionment temperature, or to zinc-plated bath and/or overaging processing.After zinc-plated or overaging, rate of cooling is preferably 10 to 50 ℃/s, more preferably 10 to 25 ℃/s.

In the temperature range between recovery temperature and 900 ℃, implement annealing.At recovery temperature and between lower than recrystallization temperature, recover certain ductility.By selecting annealing temperature and time, this can guarantee sufficient ductility for generation of high-strength steel simultaneously.Higher than recrystallization temperature and lower than 900 ℃, the easy recrystallize of cold deformed microstructure.Can easily determine the annealing temperature of perfect recrystallization and the combination of annealing time that obtain cold-rolled steel band.Contriver's discovery, if anneal this cold rolling material under higher annealing temperature, after annealing, the austenite content in the finished product is lower.Higher than 900 ℃, owing to forming the austenite of lower amount, thereby ductility reduces.Find under annealing temperature suitable, to be limited to 750 ℃ in annealing steel in the situation that.Preferably this lower limit is at least 800 ℃.Anneal 10 seconds or longer to obtain excellent intensity and processibility.But if annealing time exceedes 180 seconds, the annealing process that excessively reduces productivity and prolongation can adversely affect character.

After annealing, by steel sheet cools down to the temperature of bathing and can, by hot dip coating method with coatings such as Zn, Zn-Fe, Zn-Al, Zn-Mg, Zn-Al-Mg, Al-Si, the Al-Mg-Si thickness for of 10 to 200 microns of its every sides, form thus the steel sheets through coating.Can also apply these or other washing layer by electrochemical plating.Preferably on this surface or each lip-deep coat-thickness be 10 microns to 200 microns.

Preferably after annealing, make this material live through timeliness.If be suitable for, can before or after hot dip coating process, carry out this overaging, this depends on the design of factory or depends on metallurgy preference.Overaging temperature is preferably 350 to 500 ℃, and is preferably approximately 400 ℃.The overaging time is preferably at least 30 and/or be at most 180 seconds.In the case of such overaging processing, annealing temperature is preferably at least 825 ℃ and/or be preferably at most 875 ℃.

In the steel sheets as above making, carbide and 5% or more residual austenite be dispersed in ferrite matrix, make tensile strength up to 600 to 900MPa, ductility excellence, the and therefore combination of strength-ductility is also very excellent.In a kind of preferred embodiment, this steel sheets has 600 to 900MPa tensile strength.

Describe the present invention in detail with reference to the following example.Described embodiment is used for illustration purpose, and is not intended to limit the scope of the invention by any way.

Manufacture has the steel sheets base of composition shown in table 1, under 1200 ℃ (RHT), reheats, and hot rolling under the final rolling temperature of 900 ℃.The thickness of hot-rolled steel sheet material is 3 millimeters, and this hot-rolled steel sheet material of reeling at the temperature (CT) of 400 ℃ or 650 ℃ (S3) (referring to table 2).Contriver finds, the hot finishing at lesser temps (but higher than 700 ℃) at as 850 ℃ does not affect microstructure or the character of these steel, but only rolling load is had to impact, and this rolling load improves along with the reduction of rolling temperature.But, can be by using the milling train of suitable power easily to overcome the rolling load of this raising.

Make this steel stand cold rolling and continuous annealing (referring to table 3) under the annealing temperature (AT) of 800 to 1050 ℃ with 67% draft (CRR) subsequently, and carry out or do not carry out the overaging (referring to table 4) at 400 ℃.Annealing time is that 60 seconds and overaging time are 80 seconds.

Table 1-chemical constitution

Sequence number	C	Al	Mn	Si	P	S	N	1.7(Mn-Al)+52.7C
									S1	0.18	6.7	3.4	0.01	0.004	0.003	0.005	3.90
S2	0.13	9.1	5.4	0.01	0.003	0.004	0.006	0.60
									S3	0.004	6.8	0.02	0.01	0.004	0.005	0.004	-11.30
S1*	0.17	7.0	3.6	0.01	0.004	0.003	0.004	3.18
									S1**	0.19	8.0	3.9	0.01	0.004	0.003	0.005	3.05

Table 2-processing parameter and mechanical properties (YS: in the yield strength of MPa, TS: in the tensile strength of MPa, EI: in the unit elongation of %, RA: in the residual austenite of volume %)

Table 3-is in the impact that there is no annealing temperature in overaging situation

The impact of the combination of table 4-annealing temperature and overaging temperature

The result of steel S1* and S1** is similar to the result that adds S1 in man-hour as S1.

Clearly, between the recrystallize in annealing process and the amount of residual austenite, always there is balance.Annealing temperature is higher, and the amount of residual austenite is lower.

Overaging processing is combined with higher anneal temperature and is seemed to have beneficial effect.

Claims

1. low-density and high-strength steel sheets, it comprises 0.15% to 0.25% C, 2.5% to 4% Mn, 0.02% or P still less, 0.015% or S still less, 6% to 9% Al and 0.01% or N still less, surplus is iron and inevitable impurity, and wherein 1.7 (Mn-Al)+52.7C are at least 3 and be at most 4.5.

2. steel sheets as claimed in claim 1, wherein in this sheet material, the amount of residual austenite is 5% or higher.

3. steel sheets as claimed in claim 1 or 2, it optionally comprises and is selected from following one or both or more kinds of element: 0.1% to 2.0% Si, 0.1% to 0.3% Cr, 0.05% to 0.5% Mo, 0.1% to 2.0% Ni, 0.1% to 1.0% Cu, 0.0005% to 0.003% B, 0.01% to 0.2% Ti, 0.005% to 0.2% Zr, 0.05% to 1.0% W, 0.001% to 0.2% Ca.

4. steel sheets as claimed any one in claims 1 to 3, wherein the density of this steel sheets is at most 7400kg/m ³, be preferably at most 7300kg/m ³, preferably wherein this steel sheets has 600 to 900MPa tensile strength.

5. the steel sheets as described in any one in claim 1 to 4, wherein on one or two major surfaces, provide coating, wherein this coating is selected from Zn, Zn-Fe, Zn-Al, Zn-Mg, Zn-Al-Mg, Al-Si and Al-Mg-Si, and wherein the coat thickness on this major surfaces or each major surfaces is 10 microns to 200 microns.

6. the steel sheets as described in any one in claim 1 to 5, wherein the value of (36C+Mn)/Al is at least 1.3 and be at most 2.0.

7. prepare the method for low-density and high-strength steel sheets, described steel sheets comprises 0.15% to 0.25% C, 2.5% to 4% Mn, 0.02% or P still less, 0.015% or S still less, 6% to 9% Al and 0.01% or N still less, surplus is iron and inevitable impurity, wherein 1.7 (Mn-Al)+52.7C are at least 3 and be at most 4.5, and the method comprises the following steps:

-steel sheets base, thin slab or the band with described chemical constitution be provided;

-carry out described slab or the band for the preparation of hot rolling by reheat described slab or band within the scope of 1000 ℃ to 1250 ℃;

-make this sheet material base or band stand final hot finishing step, the band of rolling simultaneously has the temperature of 700 ℃ to 900 ℃;

-this hot rolled band of reeling under 600 ℃ or lower temperature.

8. method as claimed in claim 7, the method is further comprising the steps:

-cold rolling this hot-rolled steel sheet of cold rolling draft with 40% to 90% or band are to prepare cold-rolled steel band;

-under the top temperature of 900 ℃ with the annealing rate of 1 to 20 ℃/s by this cold-rolled steel band continuous annealing 10 to 180 seconds.

9. the method for preparing low-density and high-strength steel sheets as claimed in claim 8, wherein continuous annealing process is recovery annealing.

10. the method for preparing low-density and high-strength steel sheets as claimed in claim 8, wherein continuous annealing process is recrystallization annealing.

11. methods as described in any one in claim 7 to 10, wherein in this steel sheets, the amount of residual austenite is 5% or higher.

12. methods as described in any one in claim 7 to 11, wherein this plate slab, thin slab or band optionally comprise and are selected from following one or both or more kinds of element: 0.1% to 2.0% Si, 0.1% to 0.3% Cr, 0.05% to 0.5% Mo, 0.1% to 2.0% Ni, 0.1% to 1.0% Cu, 0.0005% to 0.003% B, 0.01% to 0.2% Ti, 0.005% to 0.2% Zr, 0.1% to 1.0% W and 0.001% to 0.2% Ca, preferably wherein the value of (36C+Mn)/Al is at least 1.3 and be at most 2.0.

13. methods as described in any one in claim 8 to 12, wherein after annealing process, on one or two major surfaces, provide coat, wherein this coat is selected from Zn, Zn-Fe, Zn-Al, Zn-Mg, Zn-Al-Mg, Al-Si and Al-Mg-Si, and preferably wherein the coat-thickness on this major surfaces or each major surfaces is 10 microns to 200 microns.

14. methods as described in any one in claim 8 to 13, wherein this annealing process comprises preferably at the temperature of 350 to 500 ℃ and preferably continues the overaging step of the time period of 30 to 180 seconds, and preferably wherein annealing temperature is at least 825 ℃ and/or be at most 875 ℃.

15. methods as described in any one in claim 8 to 14, wherein the combination of annealing temperature and annealing time is enough to obtain the perfect recrystallization of cold-rolled steel band, and preferably wherein annealing temperature is 750 ℃ or higher.