CN116815054A

CN116815054A - Economical high-strength and high-toughness non-quenched and tempered cold-forging steel and production method thereof

Info

Publication number: CN116815054A
Application number: CN202310792424.5A
Authority: CN
Inventors: 汪开忠; 张晓瑞; 尹德福; 丁雷; 王自敏; 姜婷; 郭湛; 余良其; 唐鹏; 郝杰; 牟祖茂
Original assignee: Maanshan Iron and Steel Co Ltd
Current assignee: Maanshan Iron and Steel Co Ltd
Priority date: 2023-06-30
Filing date: 2023-06-30
Publication date: 2023-09-29

Abstract

The application provides an economical high-strength and high-toughness non-quenched and tempered cold heading steel and a production method thereof, wherein the components comprise 0.05-0.10% of C, 0.36-0.48% of Si, 1.9-2.3% of Mn, 0.4-0.60% of Cr, 0.02-0.04% of Nb, 0.0005-0.0030% of B, 0.04-0.06% of Ti, 0.015-0.035% of Alt, less than or equal to 0.020% of P, less than or equal to 0.020% of S, less than or equal to 0.0020% of T.O, less than or equal to 0.0070% of N, and the balance of Fe and other unavoidable impurities. Compared with the prior art, the hot rolled microstructure of the obtained product is controlled by components and a production method: the granular bainite and ferrite dual-phase structure has good strength and toughness after drawing and stabilizing treatment, and can be used for producing automobile fastener products.

Description

Economical high-strength and high-toughness non-quenched and tempered cold-forging steel and production method thereof

Technical Field

The application belongs to the technical field of non-quenched and tempered cold-forging steel, and particularly relates to economical high-strength and high-toughness non-quenched and tempered cold-forging steel and a production method thereof.

Background

The fastener is the most common general basic member which is very important and has a large range, is widely applied to the industries of machinery manufacturing, engineering structures, railways, automobiles, tractors, buildings and the like, and about 70 percent of the connected members and combined devices are connected by the fastener. The fastener is mainly formed by cold heading, the material bears up to 70-80% of total deformation in the manufacturing process, and the material is required to have better plasticity and low hardness before cold heading, so that the cold-headed steel wire produced by the traditional process needs two heat treatment procedures of time-consuming and energy-consuming spheroidizing annealing and tempering before cold heading and drawing.

In recent years, under the pressure of energy conservation and cost reduction, a fastener manufacturer is urgent to develop a novel energy-saving cold heading steel wire rod capable of omitting quenching and tempering and spheroidizing annealing before drawing in place of quenched and tempered steel, so that energy can be saved, and the highest electric energy per ton of steel can be saved by 2500 kw.h according to statistics.

At present, the same level of quenched and tempered steel is achieved mainly by adopting microalloying, controlled rolling and controlled cooling and cold work hardening at home and abroad. The cold work reinforced non-quenched and tempered steel is researched and developed in the 80 s of the last 20 th century internationally, and the non-quenched and tempered cold heading steel is developed successfully in China, can be used for producing fasteners with small deformation such as screws, tooth bars and the like, and is mainly oriented to the mechanical and building industries. However, there are also problems such as relatively high mold loss due to high strength, inability to produce products with relatively large deformation, limited application range of toughness-limited products, and inability to apply to high-grade products such as automobiles, aviation, etc. In particular, 10.9 non-quenched and tempered cold-heading steel has not been applied in batches since the successful development, and the development of an economic high-strength and high-toughness non-quenched and tempered cold-heading steel product is a problem to be solved urgently.

Non-quenched and tempered dual-phase cold-heading steel for high-strength fasteners and a manufacturing method thereof are disclosed in patent publication No. CN101220439A published at 7 and 16 in 2008. The components and weight percentages thereof are as follows: c0.06-0.15%; si0.60-0.90%; mn1.40-2.0%; p is less than or equal to 0.025%; s is less than or equal to 0.025 percent; al is less than or equal to 0.04 percent; n is less than or equal to 0.0060 percent; v is 0.03-0.10%; nb0.04-0.08%; fe balance. The structure and performance required by the raw materials are obtained by adjusting the chemical components of the materials and controlling rolling and cooling, so that the energy is saved and the environment is not polluted. However, the strength of the application reaches 9.8 level at most, the use requirement of a 10.9-level high-strength fastener cannot be met, the microstructure always contains 15-30% of martensite, the die loss is relatively large in the drawing process, and the application cannot be applied in batches.

The publication number CN109306435A published in 2 months 5 of 2019 discloses a non-quenched and tempered cold-heading steel wire rod with good low-temperature impact property and a preparation method thereof, wherein the non-quenched and tempered cold-heading steel wire rod comprises the following chemical components in percentage by mass: c:0.13 to 0.23 percent, si is less than or equal to 0.30 percent, mn:1.00 to 1.40 percent, P is less than or equal to 0.025 percent, S is less than or equal to 0.015 percent, V is less than or equal to 0.10 percent, ti is less than or equal to 0.08 percent, al:0.010 to 0.040 percent, B:0.0010 to 0.0035 percent, cr is less than or equal to 0.20 percent, and the balance is Fe and unavoidable impurity elements; the preparation method comprises the working procedures of smelting, continuous casting, heating, rolling and cooling. The non-quenched and tempered cold heading steel wire rod with higher strength and low-temperature impact toughness is obtained by combining a good heating system, a rolling process and a cold control process through chemical composition design, and the requirement of downstream customers for producing double-headed rivets by adopting the cold heading process can be met. However, the structure of the application is ferrite and pearlite, the highest tensile strength reaches 9.8 grade, and the use requirement of a 10.9 grade high strength fastener can not be met.

Disclosure of Invention

The application aims to provide the economical high-strength and high-toughness non-quenched and tempered cold heading steel and the production method thereof, which are produced through the process of component design and matching, not only have excellent performance, but also can be used for manufacturing high-strength automobile fasteners with tensile strength of more than 1000MPa, and meanwhile, spheroidizing annealing and quenching and tempering are omitted; and a large amount of noble metal components such as Ni and the like are not added, so that the method is low in cost, belongs to an economic product and has a wide market prospect.

The specific technical scheme of the application is as follows:

an economical high-strength and high-toughness non-quenched and tempered cold heading steel comprises the following components in percentage by mass:

0.05 to 0.10 percent of C, 0.36 to 0.48 percent of Si, 1.9 to 2.3 percent of Mn, 0.4 to 0.60 percent of Cr, 0.02 to 0.04 percent of Nb, 0.0005 to 0.0030 percent of B, 0.04 to 0.06 percent of Ti, 0.015 to 0.035 percent of Alt, less than or equal to 0.020 percent of P, less than or equal to 0.020 percent of S, less than or equal to 0.0020 percent of T.O, less than or equal to 0.0070 percent of N, and the balance of Fe and other unavoidable impurities.

The components of the economic high-strength and high-toughness non-quenched and tempered cold heading steel also meet the requirement that the A value is more than or equal to 4.0 and less than or equal to 6.0; a value= [ Mn ]/[ Si ].

The application provides a production method of economical high-strength and high-toughness non-quenched and tempered cold heading steel, which comprises the following process flows:

proportioning according to the component proportion, preprocessing molten iron, smelting in a converter, LF refining, billet continuous casting, high-speed rolling, rolling and cooling control, wire rod finished product packaging and warehousing.

The molten iron pretreatment: the desulfurization is carried out by pretreatment, and the desulfurization target value S is less than or equal to 0.005 percent.

The converter smelting comprises the following steps: the end point C is controlled to be less than or equal to 0.10 percent, and the P is controlled to be less than or equal to 0.010 percent; adding refining slag and lime when tapping 1/5-1/4 molten steel, and adding deoxidizer and alloy when tapping 1/4-1/3, wherein the sequence is as follows: aluminum iron, slag, silicon manganese, high-carbon ferrochrome and carburant, and uniformly throwing a proper amount of aluminum particles to the steel slag surface according to the slag discharging amount after tapping.

The LF refining: argon is blown to the bottom of the whole ladle, and the argon flow is based on the condition that molten steel does not splash the ladle; the deoxidizer is aluminum particles (aluminum-based reducer); adding premelted refining slag and lime for slagging, wherein the alkalinity is R4-6, the white slag time is more than or equal to 20 minutes, and adding alloy to adjust the Si, mn, cr, nb, B, ti content before and during refining according to the analysis result of the components before entering an LF furnace.

And (3) continuously casting the small square billets: adopting 150mm small square billet continuous casting, adopting a crystallizer to electrically stir the tail end, wherein the electromagnetic stirring frequency of the crystallizer is 1.5-2.5Hz, the current is 150-250A, the electromagnetic stirring frequency of the tail end is 2.0-4.0Hz, the current is 220-320A, the superheat degree is controlled at 20-40 ℃, and the pulling speed is 2.0-2.2mm/min, so as to obtain the defect-free casting blank.

The high-line rolling: qualified rolling raw material billets can be obtained through the steps, and the rolling of the wire rod with the diameter of 5.5-25mm can be realized; the high-line rolling adopts low-temperature large-deformation rolling, and the rolling deformation is more than or equal to 50%.

Preferably, the high-line rolling is performed, the initial rolling temperature is controlled to be 940-1000 ℃, the final rolling temperature of low-temperature rolling is controlled to be 730-780 ℃, and ferrite phase transformation is induced under low-temperature large deformation; in order to obtain the granular bainite+ferrite dual-phase structure, the method of quick cooling and slow cooling is adopted, the cooling rate is firstly 3-5 ℃/s to 400-450 ℃, then the cooling rate is 0.5-0.9 ℃/s, the cooling is carried out to 350-400 ℃, and finally the cooling is carried out to room temperature.

Preferably, after high-line rolling, the mixture enters an LCC roller way for controlled cooling, in order to obtain a granular bainite/ferrite dual-phase structure, a mode of quick cooling firstly and then slow cooling is adopted, a front 3 heat preservation covers are opened, a fan is opened to 100 percent of quick cooling, the cooling rate is 3-5 ℃/s, the cooling is 400-450 ℃, and the generation of a pearlite structure is avoided; and 4 to 11 fans are completely closed, the heat-insulating cover is completely closed, the cooling rate is 0.5 to 0.9 ℃/s, the generation of martensitic structure is avoided, then the heat-insulating tunnel is discharged for collecting coils and hooking when the temperature reaches 350 to 400 ℃, and then the heat-insulating tunnel is air cooled to room temperature for packing and weighing.

In order to realize economy, the application adopts a converter one-fire lumber forming process.

The economic high-strength and high-toughness non-quenched and tempered cold heading steel produced by the method has a hot-rolled microstructure: the granular bainite and ferrite dual-phase structure, the area content of the granular bainite is 75% -85%, and the tensile strength is 850MPa or more and R is or more _m More than or equal to 780MPa; on the one hand, the granular bainite content is lower than 75%, and enough strength cannot be obtained so as to meet the requirement of the 10.9-grade high-strength cold forging steel; on the other hand, a certain amount of ferrite can ensure good toughness of the material, and is used for producing automobile fastener products with larger deformation such as flange face bolts, hexagon socket head cap bolts and the like.

The economical high-strength and high-toughness non-quenched and tempered cold heading steel produced by the method is processed into a fastener by adopting the processes of cold drawing, cold heading forming, thread processing, low-temperature stabilization treatment and surface treatment, wherein the low-temperature stabilization treatment process comprises the following steps of: heating to 200+ -10deg.C, maintaining for 90+ -5 min, and air cooling, and can be combined with surface treatment process such as zinc plating and Dacromet. Meanwhile, quenching and tempering treatment can be omitted, and the method belongs to an economic product.

The economic high-strength and high-toughness non-quenched and tempered cold heading steel produced by the method is subjected to the drawing and stabilizing treatment, and the tensile strength R of the product is obtained _m Not less than 1020MPa, yield ratio R _P0.2 /R _m Not less than 0.93, elongation after break A not less than 14%, reduction of area Z not less than 54%, low temperature impact energy KV at minus 40% ₂ The austenitic grain size of the steel is more than or equal to 30J, is more than or equal to 10.0 grade, has good strength and plastic toughness, can be used for manufacturing automobile fastener products such as flange face bolts, inner hexagon bolts and the like, and has the mold loss equivalent to that of spheroidizing annealed fasteners.

The design idea of the application is as follows:

c: although the element C is necessary for obtaining high strength and hardness, on one hand, in order to obtain non-quenched and tempered steel mainly containing bainite, the content of C cannot be higher than 0.1%, otherwise, a microstructure mainly containing pearlite is easy to obtain, the required strength cannot be achieved, on the other hand, the excessive high content of C leads to excessively low transformation temperature of bainite, the density of movable dislocation in the steel is greatly improved, the yield ratio is reduced, and the durable deformation resistance of the fastener cannot be met. Meanwhile, in order to obtain a certain amount of ferrite and meet the plastic toughness, the C content is required to be more than 0.05%, so that the C content is preferably controlled to be 0.05% -0.10%.

Si: si as a solid solution hardening element contributes to the improvement of strength, and at the same time, the yield ratio is remarkably improved, the non-deformation of a fastener finished product is ensured, but the silicon can remarkably improve the deformation resistance of steel, and is unfavorable for cold heading and cold extrusion, the plasticity and toughness of steel are reduced due to the excessively high content of Si, the activity of C is increased, the decarburization and graphitization tendency of steel in the rolling heating process are promoted, smelting is difficult, inclusions are easily formed, and the fatigue resistance of steel is deteriorated. Therefore, the Si content is controlled to be 0.36-0.48%.

Mn: mn is the most effective and economical alloying element to obtain the bainitic structure. In order to obtain enough granular bainite, increasing the Mn content is effective in promoting bainite transformation, and when the content is less than 1.9%, the above-mentioned effects are difficult to be achieved. However, the Mn content is too high, so that the content of residual austenite after transformation is too high, the transformation temperature of bainite is too low, the yield strength and the yield ratio of steel are too low, the permanent deformation resistance of the fastener cannot be met, meanwhile, the internal stress of the structure is too large, and the fatigue performance is deteriorated. Thus controlling the Mn content to be 1.9% -2.3%.

Cr: the bainitic ferrite hardness can be obviously improved through solid solution strengthening, the hardness is lower due to low C content, meanwhile, the formed Cr dispersion carbide can obtain high fatigue resistance, but the toughness and cold workability of the steel are deteriorated due to the excessively high content, so that the Cr content is controlled to be 0.4% -0.60%.

Nb: nb and N, C elements in the steel form an Nb (C, N) precipitated phase, the Nb has a strong precipitation strengthening effect, and the solid-dissolved Nb can obviously inhibit the diffusion of C in the bainitic transformation process and play a role in refining bainitic ferrite, so that the high yield ratio is ensured. The range of Nb can be controlled between 0.02% and 0.04%.

B: b can obviously delay the beginning of ferrite line precipitation and increase the possibility of obtaining air-cooled bainite, but the B content is too high, so that thermal embrittlement is easy to occur, and the hot processing performance is influenced, so that the B is controlled to be 0.0005% -0.0030%.

Ti: the second phase particles precipitated at high temperature play a role in pinning original austenite grain boundaries, refine austenite, enable particles precipitated at low temperature to become nucleation points for ferrite and pearlite transformation, improve ferrite nucleation rate and refine ferrite. Finally, the produced steel has ultrafine crystals, excellent strength and plastic fit, and can be directly drawn and cold-headed without annealing. The excessive Ti content is easy to produce liquid-out large-particle TiN inclusion, reduces the fatigue performance of steel and is easy to cause forging crack, so the Ti content is controlled to be 0.04-0.06%.

Alt: alt is a stronger deoxidizing element, improves the oxidation resistance of steel, and refines austenite grains. In addition, the high Alt element is added to combine with nitrogen to form AlN, so that the pinning effect of dislocation is reduced, the blue embrittlement tendency is obviously reduced, meanwhile, the impact toughness is improved, but the Alt content is too high, coarse carbonitride is formed to cause the content of inclusions to be increased, and the delayed fracture resistance is reduced. The Alt content is controlled to be 0.015-0.035%.

S and P: impurity elements such as S, P are aggregated at grain boundaries, so that the delayed fracture resistance is greatly reduced. The P element can form micro segregation when molten steel is solidified, and then the P element is biased to a grain boundary when heated at an austenitizing temperature, so that the brittleness of the steel is obviously increased, and the delayed fracture sensitivity of the steel is increased; the S element forms Mn S inclusion and segregation in grain boundary, so that the delayed fracture sensitivity of the steel is increased, and therefore, the content of P, S is controlled to be less than or equal to 0.020% of P and less than or equal to 0.020% of S.

T.o and N: oxygen forms various oxide inclusions in the steel. Under the action of stress, stress concentration is easy to occur at the oxide inclusions, so that microcrack initiation is caused, and the mechanical properties, particularly toughness and fatigue resistance, of the steel are deteriorated. Therefore, in the metallurgical production, measures are taken to reduce the content of the T.O as much as possible to be less than or equal to 0.0020 percent; n precipitates Fe4N in steel, the diffusion speed is low, the steel generates timeliness, meanwhile, the cold processing performance of the steel is reduced, and the N is controlled to be less than or equal to 0.0070 percent.

In order to reduce the strength and the mould loss and realize the economical high-strength high-toughness non-quenched and tempered production, the application aims to obtain ferrite tissues to replace part of bainite so as to realize the purpose of reducing the strength. The method is characterized in that a low-C-high Mn component system is mainly adopted to realize low strength and high plastic toughness, a microstructure mainly comprising granular bainite is obtained, meanwhile, the yield ratio can be improved while a small amount of ferrite is obtained by improving the Si content, and the strength reduction caused by the C content is compensated. The A value in the application needs to meet a certain range, in order to obtain the granular bainite and ferrite dual-phase structure, the A value is too small, the ferrite content is more, the tensile strength cannot reach 1000MPa level, the A value is too large, the ferrite structure cannot be obtained, the full needle-shaped bainite or even martensite can be possibly obtained, the strength is too high, the subsequent processing is not facilitated, the toughness is insufficient, and the cost is increased. Meanwhile, refined grain elements such as Nb, ti and the like are added, so that the toughness is improved, and finally, the granular bainite and ferrite dual-phase structure is obtained.

Compared with the prior art, the hot rolled microstructure of the obtained product is: grain bainite and ferrite dual-phase structure, tensile strength R after drawing and stabilizing treatment _m Not less than 1020MPa, yield ratioR _P0.2 /R _m Not less than 0.93, elongation after break A not less than 14%, reduction of area Z not less than 54%, low temperature impact energy KV at minus 40% ₂ The austenite grain size of the steel is more than or equal to 30J and is more than or equal to 10.0 grade, the steel has good strength and plasticity and toughness, can be used for producing automobile fastener products, and the die loss is equivalent to that of spheroidizing annealed fasteners.

Drawings

FIG. 1 shows a hot-rolled microstructure of an economical high-strength and high-toughness non-quenched and tempered cold-heading steel of the application, which is granular bainite+ferrite.

Detailed Description

Example 1 to example 5

An economical high-strength and high-toughness non-quenched and tempered cold heading steel comprises the following components in percentage by mass: as shown in table 1, the balance not shown in table 1 is Fe and unavoidable impurities.

Comparative example 1-comparative example 4

TABLE 1 chemical composition (wt%) of the examples of the application

Case (B)	C	Si	Mn	Cr	Nb	B	Ti	Alt	P	S	T.O	N	A value
														Example 1	0.10	0.40	2.0	0.45	0.02	0.0007	0.060	0.020	0.006	0.004	0.0016	0.0040	5.00
Example 2	0.05	0.48	2.3	0.60	0.04	0.0025	0.048	0.018	0.008	0.005	0.0015	0.0048	4.79
														Example 3	0.08	0.36	1.9	0.55	0.03	0.0030	0.054	0.025	0.007	0.003	0.0018	0.0043	5.28
Example 4	0.06	0.40	2.1	0.58	0.04	0.0020	0.040	0.015	0.005	0.004	0.0014	0.0037	5.25
														Example 5	0.07	0.42	2.2	0.40	0.03	0.0005	0.058	0.035	0.010	0.007	0.0015	0.0054	5.24
Comparative example 1	0.37	0.15	1.8	0.45	0.02	0.0010	0.050	0.018	0.009	0.006	0.0016	0.0055	12.00
														Comparative example 2	0.08	0.45	2.2	0.58	/	/	/	0.022	0.008	0.008	0.0015	0.0060	4.89
Comparative example 3	0.07	0.48	1.9	0.52	0.03	0.0015	0.052	0.025	0.010	0.008	0.0018	0.0058	3.96
														Comparative example 4	0.10	0.40	2.0	0.45	0.02	0.0007	0.060	0.020	0.006	0.004	0.0016	0.0040	5.00

The production method of the economic high-strength and high-toughness non-quenched and tempered cold heading steel of each embodiment and the comparative example adopts a converter one-fire forming process, and the specific process flow is as follows:

the preparation method comprises the steps of proportioning according to the given chemical composition ratio, preprocessing molten iron, smelting in a converter, LF refining, billet continuous casting, high-speed rolling, rolling and cooling control, wire rod finished product packaging and warehousing.

1) And (3) molten iron pretreatment: the desulfurization is carried out by pretreatment, and the desulfurization target value S is less than or equal to 0.005 percent.

2) Smelting in a converter: the end point C is controlled to be less than or equal to 0.10 percent, and the P is controlled to be less than or equal to 0.010 percent; adding refining slag and lime when tapping about 1/5 molten steel, and adding deoxidizing agent and alloy when tapping about 1/3, wherein the sequence is as follows: aluminum iron, slag, silicon manganese, high-carbon ferrochrome and carburant, and uniformly throwing a proper amount of aluminum particles to the steel slag surface according to the slag discharging amount after tapping.

3) Refining in an LF furnace: argon is blown to the bottom of the whole ladle, and the argon flow is based on the condition that molten steel does not splash the ladle; the deoxidizer is aluminum particles (aluminum-based reducer); adding premelted refining slag and lime for slagging, wherein the alkalinity is R4-6, the white slag time is more than or equal to 20 minutes, and adding alloy to adjust the Si, mn, cr, nb, B, ti content before and during refining according to the analysis result of the components before entering an LF furnace.

4) Continuous casting of small square billets of 150 mm: in order to obtain a low segregation casting blank, a crystallizer is adopted to stir the tail end by electricity, the electromagnetic stirring frequency of the crystallizer is 2.0Hz, the current is 200A, the electromagnetic stirring frequency of the tail end is 3.0Hz, the current is 280A, the superheat degree is controlled at 20-40 ℃, and the pulling speed is 2.0-2.2mm/min, so that a defect-free casting blank is obtained.

5) High-line rolling: through the steps, a qualified rolling raw material billet can be obtained, and the wire rod rolling with the diameter of 5.5-25mm can be realized. The high-line rolling adopts low-temperature large-deformation rolling, and the rolling deformation is more than or equal to 50%. The initial rolling temperature is 940-1000 ℃, the final rolling temperature of low-temperature rolling is 730-780 ℃, and ferrite transformation is induced under the condition of low-temperature large deformation. And then enters an LCC roller way to control cooling. In order to obtain the granular bainite+ferrite dual-phase structure, a mode of quick cooling firstly and then slow cooling is adopted, the first 3 heat preservation covers are opened, a fan is opened to 100 percent of quick cooling, the cooling rate is 3-5 ℃/s to 400-450 ℃, and the generation of a pearlite structure is avoided; and 4 to 11 fans are completely closed, the heat-insulating cover is completely closed, the cooling rate is 0.5 to 0.9 ℃/s, the generation of martensitic structure is avoided, then the heat-insulating tunnel is discharged for collecting coils and hooking when the temperature reaches 350 to 400 ℃, and then the heat-insulating tunnel is air cooled to room temperature for packing and weighing.

The above examples and comparative production process parameters are shown in table 2.

Table 2 examples and comparative production process parameters

Comparative example 4 was produced without using a rapid cooling followed by a slow cooling, but directly cooled. Other examples and comparative examples were cooled according to the method of the present application using a rapid cooling followed by a slow cooling.

The economical high-strength and high-toughness non-quenched and tempered cold-forging steel produced by the above examples and comparative examples is processed into a fastener by adopting cold drawing, cold forging forming, thread processing, low-temperature stabilization treatment and surface treatment, wherein the low-temperature stabilization treatment process comprises the following steps: heating to 200+ -10deg.C, maintaining the temperature for 90min, and air cooling to obtain the final product with properties shown in Table 3.

Mechanical property test method GB/T228.1 first part of tensile test of metal material: room temperature test method;

table 3 product properties for each of the examples and comparative examples

From the above, it can be seen that the embodiment of the application does not need thermal refining, and the tensile strength R of the product is obtained after heat preservation for 90min at 200+/-10 DEG C _m Not less than 1020MPa, yield ratio R _P0.2 /R _m Not less than 0.93, elongation after break A not less than 14%, reduction of area Z not less than 54%, low temperature impact energy KV at minus 40% ₂ Not less than 30J, the austenite grain size of the steel is not less than 10.0 grade, and the examples have better toughness.

Comparative example 1 is a component design system with high C and low Si, ferrite, pearlite, bainite and martensite structures are obtained, and the drawing die with higher hot-rolled strength has larger loss and cannot be used; in comparative example 2, no Nb or Ti microalloy element was added, and the grains were remarkably coarse and the toughness was insufficient as compared with the examples. Comparative example 3 is a chemical composition satisfying the requirements of the present application, but the A value is not within the scope of the present application, the bainite content is low, and the strength is insufficient. Comparative example 4 is mechanical properties after quenching and tempering by the chemical composition of example 1, specifically, quenching and tempering process at 890 ℃, heat preservation for 90min, tempering at 490 ℃ and heat preservation for 100min; the conventional rolling process adopted in comparative example 4 cannot obtain granular bainite+ferrite structure, the strength in the hot rolled state is high, spheroidization can be omitted, the 10.9-grade strength requirement can be met after quenching and tempering, and the aim of omitting quenching and tempering can not be fulfilled.

Claims

1. The economical high-strength and high-toughness non-quenched and tempered cold-heading steel is characterized by comprising the following components in percentage by mass:

2. The economical high-strength and high-toughness non-quenched and tempered cold-heading steel according to claim 1, wherein the components of the economical high-strength and high-toughness non-quenched and tempered cold-heading steel further meet the A value of 4.0-6.0; a value= [ Mn ]/[ Si ].

3. A method for producing the economical high-strength and high-toughness non-quenched and tempered cold-forging steel according to claim 1 or 2, which is characterized by comprising the following process flows:

4. A production method according to claim 3, wherein the molten iron is pretreated: the desulfurization is carried out by pretreatment, and the desulfurization target value S is less than or equal to 0.005 percent.

5. A production method according to claim 3, characterized in that the converter smelting: the end point C is controlled to be less than or equal to 0.10 percent, and the P is controlled to be less than or equal to 0.010 percent; adding refining slag and lime when tapping 1/5-1/4 molten steel, and adding deoxidizer and alloy when tapping 1/4-1/3, wherein the sequence is as follows: aluminum iron, slag, silicon manganese, high-carbon ferrochrome and carburant.

6. A production method according to claim 3, characterized in that the LF furnace refines: argon is blown to the bottom of the whole ladle, and the argon flow is based on the condition that molten steel does not splash the ladle; the deoxidizer is aluminum particles; adding premelted refining slag and lime for slagging, wherein the alkalinity is R4-6, and the white slag time is more than or equal to 20 minutes.

7. A production method according to claim 3, characterized in that the billet is continuously cast: adopting 150mm square billet continuous casting, adopting a crystallizer to stir electrically at the tail end, wherein the electromagnetic stirring frequency of the crystallizer is 1.5-2.5Hz, the current is 150-250A, the electromagnetic stirring frequency of the tail end is 2.0-4.0Hz, the current is 220-320A, the superheat degree is controlled at 20-40 ℃, and the pulling speed is 2.0-2.2mm/min.

8. A production method according to claim 3, characterized in that the high-line rolling: the rolling deformation is more than or equal to 50%, the initial rolling temperature is 940-1000 ℃, and the final rolling temperature is 730-780 ℃; the method comprises the steps of cooling to 400-450 ℃ at a cooling rate of 3-5 ℃/s, cooling to 350-400 ℃ at a cooling rate of 0.5-0.9 ℃/s, and finally air cooling to room temperature.

9. The production method according to any one of claims 3 to 8, wherein the economical high-strength and high-toughness non-quenched and tempered cold forging steel has a hot-rolled microstructure of: a granular bainite+ferrite dual-phase structure, the area content of the granular bainite being 75% -85%; the tensile strength is more than or equal to 850MPa and R _m ≥780MPa。

10. The production method according to any one of claims 3 to 8, wherein the economic high-strength and high-toughness non-quenched and tempered cold-forging steel produced by the production method is subjected to drawing and stabilizing treatment, and the product has tensile strength R _m Not less than 1020MPa, yield ratio R _P0.2 /R _m Not less than 0.93, elongation after break A not less than 14%, reduction of area Z not less than 54%, low temperature impact energy KV at minus 40% ₂ Not less than 30J, and the austenite grain size of the steel is not less than 10.0 grade.