CN103849811B - Low fragility 750MPa level steel for automobile crossbeam and manufacture method thereof - Google Patents

Abstract

The invention discloses a kind of low fragility 750MPa level steel for automobile crossbeam and manufacture method thereof.C0.04 ~ 0.06%, Si & lt; 0.1%, Mn1.6 ~ 1.85%, P & lt; 0.012%, S & lt; 0.005%, Al0.020 ~ 0.070%, Nb0.04 ~ 0.06%, Ti0.08 ~ 0.1%, Mo0.08 ~ 0.14%, N & lt; 0.005%, surplus is iron and inevitable impurity.First continuous casting steel billet was heated to 1250 ~ 1300 DEG C in 200 ~ 300 minutes in Hot Strip Rolling and carries out roughing, roughing end temp controls to carry out finish rolling again at 1040 ~ 1140 DEG C, and finishing temperature is 780 ~ 880 DEG C.Intensity of the present invention is high, can reach 10% ~ 25% at Vehicle Frame Design weight loss effect; Good moldability, is applicable to the techniques such as punching press, roll-in, bending; Low-temperature flexibility is good, is applicable to the transport of cold district.

Description

Low fragility 750MPa level steel for automobile crossbeam and manufacture method thereof

Technical field

The invention belongs to metallurgical technology field, relate to a kind of automobile longitudinal girder steel, be specifically related to a kind of low fragility 750MPa level steel for automobile crossbeam and manufacture method thereof.

Background technology

Alleviating tare by improving armor plate strength, reaching energy-conservation of consumption reduction, reduce the important topic that environmental pollution is current automotive industry research.At home and abroad, mostly present stage is to adopt the certain microalloy element of interpolation to strengthen the intensity of steel.Beam steel, as the safety load-bearing part of passenger-cargo carriage, has very high requirement to performance and plasticity.Domestic each steel mill is all carrying out the research work of related fields.But alloying element used, consumption, which kind of technique are all not quite similar, relevant with the equipment of each factory, technology, raw material etc.At present, the production technology of below 650MPa rank beam steel is quite ripe, also more extensive in the application of automotive industry.Be limited to the backward molding equipment of domestic automobile industry, below 650MPa rank steel grade is generally used to manufacture heavily card crossbeam at present, working strength scope mainly concentrates on 510MPa ~ 600MPa, and more the range of application of high intensity levels crossbeam is less or be in the industrial certification stage.

Heavily the vehicle frame on card chassis adopts hot rolling automotive frame steel to do, and its frame assembly is divided into longeron, lining beam, longeron, stiffener and stiffening plate etc., and each component adopt cold stamping shaping or rolling forming mode, and its assembling mainly adopts riveting method.

Namely automotive frame is the supporting part of passenger-cargo carriage, is also the safety member in vehicle traveling process.In order to improve weight-carrying ability, heavily card crossbeam generally adopts the hot-rolled steel sheet manufacture of 6 ~ 12mm.According to the tonnage heavily blocked and application scenario, heavily card crossbeam adopts single-beam or twin beams design.By improving intensity rank, the thickness of crossbeam can be reduced, or twin beams change in design is designed for single-beam, bringing multiple benefits, while alleviating car weight, oil consumption can be reduced or increase freight tonnage.The intensity increasing steel for automobile crossbeam under the condition of prior art is further the problem that people are studying always.

CN200910272678.4 discloses a kind of tensile strength in 590MPa level steel for automobile crossbeam and manufacture method thereof, its gained steel grade intensity is lower, in chemical composition, adopt Trace Titanium Treatment, titanium Main Function is the inclusion modifications such as S, the P made in steel, reduces the harm of inclusion, its shape is changed, thus improve the plasticity of steel, for reaching this object, its content is generally about 0.03%.This steel grade intensity is lower, can not meet modern heavy-duty commercial car manufacturing concern high-strength thinning to adapt to day by day harsh day environmental regulation requirement to steel.

Summary of the invention

The object of the invention is to provide a kind of tensile strength at 750MPa level steel for automobile crossbeam, while having higher intensity and good formability, also has excellent low-temperature flexibility.

Concrete scheme is as follows:

Low fragility 750MPa level steel for automobile crossbeam, its chemical composition is by mass percentage:

C0.04 ~ 0.06%, Si<0.1%, Mn1.6 ~ 1.85%, P<0.012%, S<0.005%, Al0.020 ~ 0.070%, Nb0.04 ~ 0.06%, Ti0.08 ~ 0.1%, Mo0.08 ~ 0.14%, N<0.005%, surplus is iron and inevitable impurity.

By such scheme, described low fragility 750MPa level steel for automobile crossbeam through desulfurizing iron, converter smelting, vacuum-treat, continuous casting, slab inspection and cleaning, heating of plate blank, hot continuous rolling, section cooling, batches, finishing obtain;

Wherein, tap after converter smelting 1/3 time start to add iron alloy, tap 2/3 time add complete, and alloying order be followed successively by ferro-aluminum, ferromanganese, ferro-niobium, molybdenum-iron; Use adds aluminium machine and adds aluminum steel and carry out final deoxygenation, adds ferrotianium, described Ferrotitanium particle diameter≤30mm during vacuum-treat;

First continuous casting steel billet was heated to 1250 ~ 1300 DEG C in 200 ~ 300 minutes in Hot Strip Rolling and carries out roughing, roughing end temp controls to carry out finish rolling again at 1040 ~ 1140 DEG C, and finishing temperature is 780 ~ 880 DEG C.

By such scheme, described section cooling is back segment cooling, the water temperature < of water coolant 35 DEG C, controlled cooling model speed >=10 DEG C/s.

By such scheme, coiling temperature is 500 ~ 580 DEG C.

The preparation method of above-mentioned low fragility 750MPa level steel for automobile crossbeam, comprise desulfurizing iron, converter smelting, vacuum-treat, continuous casting, slab inspection and cleaning, heating of plate blank, hot continuous rolling, section cooling, batches, and the processing step of finishing, is characterized in that:

Tap after converter smelting 1/3 time start to add iron alloy, tap 2/3 time add complete, and alloying order be followed successively by ferro-aluminum, ferromanganese, ferro-niobium, molybdenum-iron; Use adds aluminium machine and adds aluminum steel and carry out final deoxygenation, adds ferrotianium, described Ferrotitanium particle diameter≤30mm during vacuum-treat;

First continuous casting steel billet was heated to 1250 ~ 1300 DEG C in 200 ~ 300 minutes in Hot Strip Rolling and carries out roughing, roughing end temp controls to carry out finish rolling again at 1040 ~ 1140 DEG C, and finishing temperature is 780 ~ 880 DEG C.

By such scheme, described section cooling is back segment cooling, the water temperature < of water coolant 35 DEG C, controlled cooling model speed >=10 DEG C/s.

By such scheme, coiling temperature is 500 ~ 580 DEG C.

Carbon is cheap solution strengthening element.If its content is more than 0.06%, then press formability can reduce; If its content is less than 0.04%, then can not meet the requirement of material to intensity, so, its content is limited to 0.04% ~ 0.06% scope.

The generation of element silicon accelerating oxidation iron sheet in the hot rolling, worsens surface quality, easily comes off in follow-up punching course, causes and uses difficulty, so its content is limited to <0.1%.

Manganese improves intensity and the most effective element of toughness, can improve the intensity-extension balance of steel.But add the manganese of volume, the hardening capacity increasing steel can be caused, Given this, be defined as 1.85% by it, so, its content is limited to 1.6% ~ 1.85% scope.

Phosphorus is the harmful element in steel, easily causes center segregation of casting blank, and in order to avoid cold bending performance, toughness worsen, setting its upper content limit is 0.012%.

Sulphur is the element be harmful to very much.Sulphur in steel often exists with the oxide morphology of manganese, the impelling strength of this sulfide inclusion to steel is very disadvantageous, and causes the anisotropy of performance, along with the raising of beam steel intensity, the expansion of tiny crack is very responsive to inclusion, so sulphur content in steel controlled below 0.005%.

Aluminium adds in order to deoxidation, when Als is containing quantity not sufficient 0.020%, can not play its effect; On the other hand, the aluminium owing to adding volume easily forms aluminum oxide agglomerate, so the regulation Als upper limit is 0.070%.Therefore, Als content is limited to 0.020% ~ 0.070% scope.

Niobium is indispensable element in cerium sulphate crystal rolling, in thermomechanical processing, crystal grain thinning, reduction austenite are to ferritic transition temperature and precipitation strength, therefore add the carrying out that trace alloying element niobium can be convenient to controlled rolling in steel, and improve intensity and the toughness of steel plate.So the consideration of the aspects such as comprehensive mechanical property of steel plate and cost, is limited to 0.04 ~ 0.06% scope by its content.

Titanium is cheap alloy strengthening element, separates out, not only crystal grain thinning, and hinder dislocation moving with carbon and nitrogen compound in deformation process.For making full use of the strengthening effect of titanium, specifying that its lower limit is 0.08%, for avoiding too high titanium to cause toughness deteriorate, specifying that its upper limit is 0.1%.So, its content is limited to 0.08 ~ 0.1% scope.

Mo element improves the intensity of steel plate by solution strengthening, higher than the effect of 0.08% competence exertion strengthening, so, its content is limited to 0.08% ~ 0.14%.

The avidity of nitrogen element and titanium elements is very strong, forms large size inclusion metallic, be limited to <0.005% in order to avoid nitrogen is combined with titanium.

In patent of the present invention, on Composition Design, due to the raising of steel-making level, S, P content itself can be controlled in very low level, steel becomes purer, Ti mainly adds as alloy strengthening element, simultaneously, in order to ensure effective precipitation of Ti element, strict requirement is had to N element, N element and Ti is avoided to form large-sized TiN inclusion metallic, in hot rolling technology design, in order to ensure that the Ti compound in steel billet fully dissolves, in heating steel billet process, special requirement is had to Heating temperature and heat-up time, simultaneously in order to fully improve the plasticity of steel plate, section cooling mode does not adopt leading portion cool, but use the back segment type of cooling.

Along with titanium content increases, slab heating temperature needs to improve, object is to ensure that the compound of titanium fully dissolves, 1250 ~ 1300 DEG C are required in this patent, if lower than 1250 DEG C, titaniferous compound dissolves insufficient in heat-processed, exists in rolling process with large-sized titaniferous compound, affects its precipitating reinforcing effect.

This patent is to upper and lower existence requirement heat-up time, if heat-up time was lower than 200 minutes, because titanium content is high, titanium compound can not fully dissolve, affect the precipitation of titanium in rolling process, reduce the intensity of steel plate, if heat-up time was more than 300 minutes, austenite crystal is too grown up, cause intensity to reduce, simultaneously billet surface forms excessive iron scale, not easy-clear in de-scaling process, during steel rolling, iron scale is rolled into steel plate, forms hole shape defect on surface.

This patent requires that section cooling adopts the back segment type of cooling, compares the leading portion type of cooling, and back segment cooling first cools after air cooling after a while again, therefore the cooling intensity of back segment cooling is lower, the unit elongation of steel can be improved, steel plate internal stress can be reduced simultaneously, improve toughness.

Coiling temperature is set in 500 ~ 580 DEG C, is beneficial to the formation of bainite structure, and the toughness of bainite structure is better than pearlitic structure.

Beneficial effect of the present invention:

1) intensity is high, can reach 10% ~ 25% at Vehicle Frame Design weight loss effect;

2) good moldability, is applicable to the techniques such as punching press, roll-in, bending;

3) low-temperature flexibility is good, is applicable to the transport of cold district.

Embodiment

Following examples are understand technical scheme of the present invention further, not as the restriction to protection domain.

Low fragility 750MPa level steel for automobile crossbeam, its chemical composition is by mass percentage:

C0.04 ~ 0.06%, Si<0.1%, Mn1.6 ~ 1.85%, P<0.012%, S<0.005%, Al0.020 ~ 0.070%, Nb0.04 ~ 0.06%, Ti0.08 ~ 0.1%, Mo0.08 ~ 0.14%, N<0.005%, surplus is iron and inevitable impurity.

It is through desulfurizing iron, converter smelting, vacuum-treat, continuous casting, slab inspection and cleaning, heating of plate blank, hot continuous rolling, and section cooling, batches, and finishing obtains.

Preparation process controls to start to add iron alloy when tapping 1/3 after converter smelting, tap 2/3 time add complete, and alloying order is followed successively by ferro-aluminum, ferromanganese, ferro-niobium, molybdenum-iron; Use adds aluminium machine and adds aluminum steel and carry out final deoxygenation, adds ferrotianium, Ferrotitanium particle diameter≤30mm during vacuum-treat.First continuous casting steel billet was heated to 1250 ~ 1300 DEG C in 200 ~ 300 minutes in Hot Strip Rolling and carries out roughing, roughing end temp controls to carry out finish rolling again at 1040 ~ 1140 DEG C, and finishing temperature is 780 ~ 880 DEG C.

In order to improve the utilization ratio of effective titanium, Ferrotitanium particle diameter≤30mm.Because the avidity of titanium and oxygen is very strong, titanium must be added in the vacuum-treat link after deoxidation in molten steel, avoid titanium and oxygen to form compound, float and enter covering slag.Hot continuous rolling controlling of production process is very important, for realizing ferrite and the bainite structure of low alloy steel of the present invention, the present invention adopts and reheats the control of original austenite structure refinement, complex chemical compound is formed with carbon, nitrogen respectively by niobium and titanium, separate out in rolling and process of cooling, its precipitate hinders grain growth, thus realizes grain refining.

Along with titanium content increases, slab heating temperature needs to improve, object is to ensure that the compound of titanium fully dissolves, 1250 ~ 1300 DEG C are required in this patent, if lower than 1250 DEG C, titaniferous compound dissolves insufficient in heat-processed, exists in rolling process with large-sized titaniferous compound, affects its precipitating reinforcing effect.This patent is to upper and lower existence requirement heat-up time, if heat-up time was lower than 200 minutes, because titanium content is high, titanium compound can not fully dissolve, affect the precipitation of titanium in rolling process, reduce the intensity of steel plate, if heat-up time was more than 300 minutes, austenite crystal is too grown up, cause intensity to reduce, simultaneously billet surface forms excessive iron scale, not easy-clear in de-scaling process, during steel rolling, iron scale is rolled into steel plate, forms hole shape defect on surface.

The water temperature < 35 DEG C of water coolant in laminar cooling process, controlled cooling model speed is >=10 DEG C/s, adopts the back segment type of cooling.Compare the leading portion type of cooling, back segment cooling first cools after air cooling after a while again, and therefore the cooling intensity of back segment cooling is lower, can improve the unit elongation of steel, can reduce steel plate internal stress simultaneously, improves toughness.

Coiling temperature is 500 ~ 580 DEG C.Be beneficial to the formation of bainite structure, the toughness of bainite structure is better than pearlitic structure.

Metallographic detected result shows, this steel be organized as bainite.Tension test shows, this hardness of steel is high, and yield tensile ratio is high, has good non-deformability.Wide cold bending test shows, even if be still up to the standards under the condition of d=0.5a, has good cold shaping performance.

Embodiment

Low fragility 750MPa level hot rolling automotive frame steel is prepared by the specific chemical composition shown in table 1.Preparation process is as follows:

Steelworks 90 tons and 250 tons of converters carry out top bottom blowing, adopts Desulphurization of Pig Iron, make S≤0.005% in molten iron; Tap after stove 1/3 time start to add iron alloy, tap 2/3 time add complete, alloying order: ferro-aluminum → ferromanganese → ferro-niobium → molybdenum-iron, use adds aluminium machine and adds aluminum steel and carry out final deoxygenation, titanium is added when vacuum-treat, and to carrying out trimming, making the chemical composition in steel meet the requirement of table 1, surplus is Fe and is inevitably mingled with; Again the pouring molten steel of composition shown in table 1 is become the slab of 230 ~ 250mm × 1050 ~ 2150mm section; Then slab is delivered to hot rolling mill, 2250mm milling train carries out hot continuous rolling, is rolled into hot-rolled sheet coil, then carry out finishing; First continuous casting steel billet is heated to 1250 ~ 1300 DEG C in 200 ~ 300 minutes and carries out roughing, end temp is 1040 ~ 1140 DEG C, carry out finish rolling again, finishing temperature is 780 ~ 880 DEG C, controlled cooling model after rolling with steel adopts the mode of back segment section cooling, the water temperature < of water coolant 35 DEG C, controlled cooling model speed is >=10 DEG C/s, and coiling temperature is 500 ~ 580 DEG C.Rolled iron twists in finishing line and carries out surface quality inspection, and controls plate shape further well, finally obtains the hot rolled coil of 3 ~ 12mm.

Table 1

Composition	C	Si	Mn	P	S	Als	Nb	Ti	Mo	N
											1	0.041	0.06	1.83	0.010	0.003	0.058	0.053	0.08	0.10	0.002
2	0.055	0.07	1.75	0.009	0.001	0.070	0.049	0.95	0.11	0.0032
											3	0.058	0.02	1.78	0.011	0.002	0.023	0.060	0.97	0.08	0.0028
4	0.052	0.09	1.70	0.010	0.002	0.020	0.055	0.93	0.095	0.0033
											5	0.053	0.06	1.60	0.009	0.003	0.066	0.049	0.081	0.14	0.0042
6	0.046	0.04	1.85	0.008	0.002	0.054	0.040	0.10	0.138	0.0021

In above-mentioned table 1, five groups of each constituent contents of embodiment are all calculated in mass percent, and the performance of its corresponding product is shown in Table 2.Wherein, lower yield strength, tensile strength, elongation, 180 ° of wide clod washes ,-60 DEG C of ballistic works all perform according to national standard.

Table 2

Can be found out by table 2, the lower yield strength of embodiments of the invention steel product is between 740 ~ 790MPa, and tensile strength is between 780 ~ 860MPa, and elongation is between 14% ~ 21%, and the ballistic work of 10mm steel plate at-60 DEG C reaches 73J.Embodiment steel have higher intensity, preferably unit elongation and low-temperature flexibility simultaneously, and steel plate achieves the matched well of intensity and toughness.

Claims (2)

1. low fragility 750MPa level steel for automobile crossbeam, is characterized in that chemical composition is by mass percentage:

C0.04 ~ 0.06%, Si<0.1%, Mn1.6 ~ 1.85%, P<0.012%, S<0.005%, Al0.020 ~ 0.070%, Nb0.04 ~ 0.06%, Ti0.08 ~ 0.1%, Mo0.08 ~ 0.14%, N<0.005%, surplus is iron and inevitable impurity;

Described low fragility 750MPa level steel for automobile crossbeam through desulfurizing iron, converter smelting, vacuum-treat, continuous casting, slab inspection and cleaning, heating of plate blank, hot continuous rolling, section cooling, batches, finishing obtain;

Wherein, tap after converter smelting 1/3 time start to add iron alloy, tap 2/3 time add complete, and alloying order be followed successively by ferro-aluminum, ferromanganese, ferro-niobium, molybdenum-iron; Use adds aluminium machine and adds aluminum steel and carry out final deoxygenation, adds ferrotianium, described Ferrotitanium particle diameter≤30mm during vacuum-treat;

First continuous casting steel billet was heated to 1250 ~ 1300 DEG C in 200 ~ 300 minutes in Hot Strip Rolling and carries out roughing, roughing end temp controls to carry out finish rolling again at 1040 ~ 1140 DEG C, and finishing temperature is 780 ~ 880 DEG C;

Section cooling is back segment cooling, the water temperature < of water coolant 35 DEG C, controlled cooling model speed >=10 DEG C/s;

Coiling temperature is 500 ~ 580 DEG C.

2. the preparation method of low fragility 750MPa level steel for automobile crossbeam, comprise desulfurizing iron, converter smelting, vacuum-treat, continuous casting, slab inspection and cleaning, heating of plate blank, hot continuous rolling, section cooling, batches, and the processing step of finishing, is characterized in that:

Chemical composition is by mass percentage:

C0.04 ~ 0.06%, Si<0.1%, Mn1.6 ~ 1.85%, P<0.012%, S<0.005%, Al0.020 ~ 0.070%, Nb0.04 ~ 0.06%, Ti0.08 ~ 0.1%, Mo0.08 ~ 0.14%, N<0.005%, surplus is iron and inevitable impurity;

Tap after converter smelting 1/3 time start to add iron alloy, tap 2/3 time add complete, and alloying order be followed successively by ferro-aluminum, ferromanganese, ferro-niobium, molybdenum-iron; Use adds aluminium machine and adds aluminum steel and carry out final deoxygenation, adds ferrotianium, described Ferrotitanium particle diameter≤30mm during vacuum-treat;

First continuous casting steel billet was heated to 1250 ~ 1300 DEG C in 200 ~ 300 minutes in Hot Strip Rolling and carries out roughing, roughing end temp controls to carry out finish rolling again at 1040 ~ 1140 DEG C, and finishing temperature is 780 ~ 880 DEG C;

Section cooling is back segment cooling, the water temperature < of water coolant 35 DEG C, controlled cooling model speed >=10 DEG C/s;

Coiling temperature is 500 ~ 580 DEG C.