CN118639146A - 9.8 grade small-size boron-containing cold heading steel wire rod with excellent hardenability and manufacturing method thereof - Google Patents

Abstract

The invention relates to the technical field of cold heading steel wire rod production, in particular to a 9.8-grade small-size boron-containing cold heading steel wire rod with excellent hardenability and a manufacturing method thereof. The cold heading steel wire rod has a size of ø5.5mm to ø12.5mm and is composed of the following chemical components in weight percentage: C: 0.18% to 0.23%, Si: ≤0.10%, Mn: 0.60% to 0.85%, Cr: 0.19% to 0.29%, Mo: 0.15% to 0.24%, Al: 0.020% to 0.040, Ti: 0.028% to 0.040%, B: 0.0015% to 0.0040%, Ni: 0.10% to 0.20%, N: ≤0.0080%; the remainder is Fe and unavoidable impurities. The tensile strength is greater than 500MPa, the hardness of the core of the transverse section is greater than 40HRC, the martensite content is greater than 85%, and it has excellent hardenability and good deformation capacity. It can be used to produce 9.8 grade high-strength bolts, nuts and other standard parts.

Description

9.8 grade small-size boron-containing cold heading steel wire rod with excellent hardenability and manufacturing method thereof

Technical Field

The invention relates to the technical field of cold heading steel wire rod production, in particular to a 9.8-grade small-size boron-containing cold heading steel wire rod with excellent hardenability and a manufacturing method thereof.

Background Art

Boron-containing cold heading steels such as 10B21 are mainly used to produce standard parts such as 9.8-grade high-strength bolts and nuts. These standard parts are widely used in machinery, electronics, automobiles, construction and other industries, which require not only good cold deformation ability, but also certain performance requirements for hardness after quenching.

However, in the process of producing 9.8-grade fasteners according to the national standard, the following quality defects occurred when using steels such as 10B21: (1) When the cross-section sizes are different, due to different degrees of hardening, although the same quenching and tempering process is used, the mechanical properties are not exactly the same. The larger the cross-section, the worse the degree of hardening and the worse the mechanical properties; (2) When the chemical composition of the wire rod is uneven, it is found that the core hardness is low and the tensile strength is low after heat treatment. These quality problems will lead to insufficient hardenability defects in the production process of downstream users, resulting in batch scrapping, affecting the stable production and contract delivery of downstream users, etc.

The Chinese patent document with the announcement number CN106917047B discloses "a low-carbon V-N microalloyed cold heading steel and its production method". Although the vanadium carbonitride precipitated from V is used to achieve strong precipitation strengthening and precipitation strengthening effects; the low-temperature rolling technology is adopted to refine the grain structure, obtain good ferrite + pearlite, reduce hardness, and improve drawing and cold heading performance. However, due to the use of V alloy and low-temperature rolling process, the temperature control during the rolling process is more stringent, which is easy to generate problems such as uneven structure and surface defects. The Chinese patent document with the announcement number CN 101775546 A discloses "boron-containing cold heading steel for high-strength fasteners and its preparation process". The steel has low strength and hardness, good plasticity, and excellent hardenability. It can simplify the fastener manufacturing process, save energy, reduce pollution, and the strength of large-size fasteners produced is above 8.8. Due to the wide range of chemical composition in the steel, the product performance fluctuates greatly. The Chinese patent document with the announcement number CN 113151743 A discloses "a method for manufacturing a cold heading steel wire rod with low martensite content and no quenching and tempering". Although the hot rolled wire rod microstructure contains 5% to 10% martensite structure by optimizing the design of the elements such as C, Mn, and Nb, combined with the manufacturing method, the steel's yield strength ratio and cold drawing strengthening index are effectively improved, so that the steel can obtain high mechanical properties after a small drawing reduction rate, and maintain good cold heading forming performance, and realize the production of various types of 8.8 grade bolts without quenching and tempering heat treatment, which significantly reduces the processing cost of downstream industries. Because the steel contains Nb elements, the rolling process control requirements are strict and waste defects are prone to occur.

Summary of the invention

In order to overcome the shortcomings of the prior art, the present invention provides a 9.8-grade small-size boron-containing cold heading steel wire rod with excellent hardenability and a manufacturing method thereof, wherein the tensile strength is greater than 500MPa, the hardness of the core of the transverse section is greater than 40HRC, and the hardenability is excellent and the deformation ability is good, and the wire rod can be used to produce 9.8-grade high-strength bolts, nuts and other standard parts.

In order to achieve the above object, the present invention adopts the following technical solutions:

A 9.8-grade small-size boron-containing cold heading steel wire rod with excellent hardenability, with a size of ø5.5mm to ø12.5mm, used for producing 9.8-grade high-strength standard parts, and composed of the following chemical components in percentage by weight:

C: 0.18%～0.23%, Si: ≤0.10%, Mn: 0.60%～0.85%, Cr: 0.19%～0.29%, Mo: 0.15%～0.24%, Al: 0.020%～0.040%, Ti: 0.028%～0.040%, B: 0.0015%～0.0040%, Ni: 0.10%～0.20%, N: ≤0.0080%; the balance is Fe and unavoidable impurities.

Furthermore, the ferrite area of the cold heading steel wire rod accounts for 60% to 80%, and the pearlite area accounts for 20% to 40%.

Furthermore, after hot rolling, the cold heading steel wire rod has a tensile strength greater than 500 MPa, an elongation greater than 25%, and an area reduction greater than 58%;

Furthermore, after the cold heading steel wire rod is quenched, the hardness of the core of the transverse section is greater than 40HRC, and the martensite content accounts for more than 85%.

A method for manufacturing the above-mentioned 9.8-grade small-size boron-containing cold heading steel wire rod with excellent hardenability, the process flow is molten iron pretreatment → converter → LF refining → continuous casting → continuous rolling → billet cleaning → heating → rolling → controlled cooling → finishing → inspection → warehousing, specifically as follows:

1) LF refining;

Use argon gas for weak stirring, and the stirring time is 5 to 10 minutes;

2) Continuous casting;

The superheat of the tundish is controlled at 20-30℃, and the casting speed is controlled at 0.75±0.02m/min by constant pulling speed; the water volume of the secondary cooling is controlled at 0.9-1.2 L/kg;

3) Heating;

The starting rolling temperature is controlled at 960-1020°C, the finishing rolling inlet temperature is controlled at 900-980°C, the sizing inlet temperature is controlled at 880-950°C, and the wire laying temperature is controlled at 890-930°C.

4) Cooling control;

Adopt Stelmore air cooling line cooling method: cool to 680-770℃ at a cooling rate of 6-12℃/s before entering the hood, close all the insulation hoods, cool in the hood at a rate of 0.5-5.0℃/s, exit the hood at a temperature of 460-600℃, and use air cooling on the cooling bed.

Furthermore, in step 1), the soft blowing flow rate is 10-50 NL/min, so that the slag creeps slightly and the molten steel is not exposed.

Furthermore, in step 1), the nitrogen addition in molten steel is controlled by slightly positive pressure operation, large slag amount and limited heating time.

Furthermore, in step 2), the cross-sectional size of the continuous casting billet is 250-350 mm × 350-450 mm.

Furthermore, in step 2), protective casting is adopted, and low-oxidizing and alkaline covering agents are added to the molten steel in the ladle and the tundish; protective sleeves and argon blowing are used from the large ladle to the tundish, and from the tundish to the crystallizer, and protective slag is added to the crystallizer.

Furthermore, in step 2), electromagnetic stirring is used at the end of solidification of the continuous casting machine, with a voltage of 300-500V, a current intensity of 300-500A, and a frequency of 3-8Hz.

Compared with the prior art, the present invention has the following beneficial effects:

1. The present invention adopts a component design of adding boron element, adds a certain content of Ti element, makes full use of the nitrogen fixation effect of Ti, forms stable TiN, reduces the generation of BN, ensures the effective hardenability of B, and can improve the strength of steel through solid solution strengthening of TiC, thereby improving the hardenability of steel.

2. The present invention ensures sufficient and uniform solid solution of Ti microalloy through continuous rolling of ingot casting, thereby improving the fine grain strengthening effect; adopts low-temperature controlled rolling and controlled cooling process to curb grain growth, so that the performance fully meets the requirements for manufacturing 9.8-level standard parts, and obtains ideal ferrite and pearlite structure wire rods, so that the ferrite area of the cold heading steel wire rod accounts for 60% to 80%, and the pearlite area accounts for 20% to 40%.

3. By adding appropriate amounts of Cr and Mo elements and controlling the reduction of N content in molten steel during the smelting process, the tensile strength of the wire rod after hot rolling is greater than 500MPa, the elongation is greater than 25%, and the area reduction rate is greater than 58%; the cold heading performance is good; after quenching, the hardness of the transverse section of the wire rod is greater than 40HRC.

4. The present invention adopts the electromagnetic stirring process at the end of solidification of the continuous casting machine to reduce the element segregation of the steel, thereby uniformizing the structure and reducing the banded structure defects of the steel.

In summary, the Ø5.5-12.5mm cold heading steel wire rod manufactured by the present invention has a ferrite area of 60-80% and a pearlite area of 20-40%. After hot rolling, the wire rod has a tensile strength greater than 500MPa, an elongation greater than 25%, and a surface reduction greater than 58%. The cold heading performance is good. After quenching, the hardness of the core of the cross section of the wire rod is greater than 40HRC, and the martensite content accounts for more than 85%. It can be used to produce standard parts such as 9.8 grade high-strength bolts and nuts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a transverse tissue morphology diagram of the present invention.

DETAILED DESCRIPTION

The present invention provides a 9.8-grade small-size boron-containing cold heading steel wire rod with excellent hardenability and a method for manufacturing the same. Those skilled in the art can refer to the contents of this article and appropriately improve the process parameters to achieve the same. It is particularly important to point out that all similar substitutions and modifications are obvious to those skilled in the art, and they are all deemed to be included in the present invention. The methods and applications of the present invention have been described through preferred embodiments, and relevant personnel can obviously modify or appropriately change and combine the methods and applications described herein without departing from the content, spirit and scope of the present invention to implement and apply the technology of the present invention.

A 9.8 grade small-size boron-containing cold heading steel wire rod with excellent hardenability, used for producing 9.8 grade high-strength standard parts, composed of the following chemical components in percentage by weight:

C: 0.18%～0.23%, Si: ≤0.10%, Mn: 0.60%～0.85%, Cr: 0.19%～0.29%, Mo: 0.15%～0.24%, Al: 0.020%～0.040, Ti: 0.028%～0.040%, B: 0.0015%～0.0040%, Ni: 0.10%～0.20%, N: ≤0.0080%; the balance is Fe and unavoidable impurities.

The selection of the addition amount (weight percentage) of the above elements and their functions are explained as follows:

C: C is the element with the most obvious solid solution strengthening effect in steel. As the C content increases, the strength and hardness of the steel increase, while the plasticity and toughness decrease. The C content is selected to be 0.18% to 0.23%;

Si: Si is a ferrite solid solution strengthening element, and it also causes a sharp increase in cold heading deformation resistance, significantly increases mold consumption, and is not conducive to cold working plastic deformation. Therefore, the range of Si is ≤0.10%;

Mn: Mn plays the role of solid solution strengthening and grain refinement, and delays the transformation of pearlite and ferrite, improving the strength and work hardening properties of steel. Therefore, the range of Mn is 0.60% to 0.85%;

Cr: Cr can improve the toughness of steel, has good cold heading forming performance, and can effectively improve hardenability, ensuring the strength and hardness of the material after quenching. Therefore, the composition range of Cr is 0.19% to 0.29%;

Mo: Mo can greatly improve the hardenability and ensure the hardness and strength of the material after quenching, so the content of Mo is controlled at 0.15% to 0.24%;

Al: Adding a certain amount of Al (0.020% to 0.040%) to refine the grains and deoxidize, so that the steel has an extremely low oxygen content and reduces the number and size of non-metallic inclusions in the steel;

Ti: Titanium is a strong nitrogen-fixing element. After Al deoxidation at the end of refining, Ti must be used to fix nitrogen to form TiN to eliminate free nitrogen in the steel, so that the boron added later can exist as free boron. Therefore, the Ti content can be controlled at 0.028% to 0.040% to ensure that the Ti/N mass ratio in the steel should be higher than 3.5;

B: Boron can significantly improve hardenability. Only when boron in steel exists in the form of free boron can it segregate at the austenite grain boundary during quenching, inhibit ferrite nucleation, and thus improve the hardenability of steel. If boron forms a boron phase or boron nitride with oxygen and nitrogen in steel, it will not play a role in improving hardenability. Therefore, after ensuring that the steel is fully deoxidized and nitrogen fixed, it is appropriate to control the boron content in the steel between 0.0015% and 0.0040%;

Ni: The lattice constant of Ni is similar to that of γ-iron, so it can form a continuous solid solution. This is conducive to improving the hardenability of steel. Ni can reduce the critical point and increase the stability of austenite, so its quenching temperature can be reduced and the hardenability is good. Generally, Ni-added steel is used for heavy and thick parts with large sections. When it is combined with Cr, W or Cr, Mo, the hardenability can be increased. It is appropriate to control the Ni content in steel between 0.10 and 0.20%.

A method for manufacturing the above-mentioned 9.8-grade small-size boron-containing cold heading steel wire rod with excellent hardenability, the process flow is molten iron pretreatment → converter → LF refining → continuous casting → continuous rolling → billet cleaning → heating → rolling → controlled cooling → finishing → inspection → warehousing, specifically as follows:

LF Refining:

The main tasks of LF refining are fine-tuning of composition, temperature control and slag-making operation. The nitrogen addition of molten steel is controlled by micro-positive pressure operation, large slag volume and limited heating time. After LF alloying treatment, the molten steel is weakly stirred with argon gas. The soft blowing flow rate is based on the standard of slight creeping of slag and no exposure of molten steel. The station is left after 5 to 10 minutes of weak stirring.

Continuous Casting:

The cross-sectional dimensions of the continuous casting billet are (250-350) mm × (350-450) mm, the superheat of the tundish is controlled at 20-30°C, and the pulling speed is constant (the pulling speed of the casting billet is controlled at 0.75±0.02 m/min); the secondary cooling water volume is controlled at 0.9-1.2 L/kg; protective pouring (low-oxidizing and alkaline covering agents are added to the molten steel in the ladle and the tundish; protective sleeves and argon blowing are used from the large ladle to the tundish, and from the tundish to the crystallizer; high-quality protective slag is added to the crystallizer); electromagnetic stirring at the end of solidification of the continuous casting machine: the voltage is 300-500 V, the current intensity is 300-500 A, and the frequency is 3-8 Hz; the electromagnetic stirring process is used to reduce the element segregation of the steel, thereby uniformizing the structure and reducing the banded structure defects of the steel.

Rolling process (heating → rolling → controlled cooling):

(1) The starting rolling temperature is controlled at 960-1020°C, the finishing rolling inlet temperature is controlled at 900-980°C, the sizing inlet temperature is controlled at 880-950°C, and the wire laying temperature is controlled at 890-930°C.

(2) Stelmore air cooling line cooling method: cooling at a cooling rate of 6 to 12 °C/s to 680 to 770 °C before entering the hood, the insulation hood is completely closed, the cooling rate inside the hood is 0.5 to 5.0 °C/s, the temperature out of the hood is 460 to 600 °C, and the cooling bed is air-cooled.

The present invention obtains ideal ferrite and pearlite structure wire rods through heating, rolling and cooling process control.

[Example]

The chemical compositions (wt%) of the embodiments of the present invention and comparative examples are shown in Table 1.

Table 1 Chemical composition (wt%)

Element C Si Mn Cr Mo Al Ti B Ni N Example 1 0.20 0.08 0.81 0.28 0.21 0.026 0.033 0.0025 0.18 0.0067 Example 2 0.19 0.09 0.76 0.25 0.21 0.034 0.035 0.0035 0.16 0.0075 Example 3 0.21 0.08 0.77 0.23 0.23 0.037 0.029 0.0037 0.18 0.0072 Example 4 0.22 0.10 0.68 0.22 0.18 0.031 0.035 0.0029 0.15 0.0064 Comparative Example 0.20 0.08 0.82 - - - - - - -

Embodiment 1:

LF refining: The main tasks of LF refining are fine-tuning of composition, temperature control and slag making operation. The nitrogen addition of molten steel is controlled by micro-positive pressure operation, large slag volume and limited heating time. After LF alloying treatment, the molten steel leaves the station after 7 minutes of argon weak stirring.

Continuous casting: The cross-sectional size of the continuous casting billet is 320mm × 410mm, the superheat of the tundish is controlled at 25℃, and the pulling speed is constant (the pulling speed of the casting billet is controlled at 0.75m/min); the water volume of the secondary cooling is controlled at 1.0 L/kg; protective pouring (low-oxidizing and alkaline covering agents are added to the molten steel in the ladle and the tundish; protective sleeves and argon blowing are used from the large ladle to the tundish, and from the tundish to the crystallizer; high-quality protective slag is added to the crystallizer); electromagnetic stirring at the end of solidification of the continuous casting machine: voltage is 350 V, current intensity is 350 A, and frequency is 6Hz;

The steel billet is rolled into Ø8mm wire rod on a high-speed wire mill. The rolling process is as follows: the starting rolling temperature is controlled at 970℃, the finishing rolling inlet temperature is 910℃, the sizing inlet temperature is 890℃, and the wire-spinning temperature is 910℃; it is cooled to 690℃ at a cooling rate of 7℃/s and enters the hood. The insulation hood is completely closed, the cooling rate in the hood is 1.5℃/s, the hood outlet temperature is 500℃, and the cooling bed is air-cooled.

Embodiment 2:

LF refining: The main tasks of LF refining are fine-tuning of composition, temperature control and slag making operation. The nitrogen addition of molten steel is controlled by micro-positive pressure operation, large slag volume and limited heating time. After LF alloying treatment, the molten steel leaves the station after 7 minutes of argon weak stirring.

Continuous casting: The cross-sectional size of the continuous casting billet is 320mm × 410mm, the superheat of the tundish is controlled at 27℃, and the pulling speed is constant (the pulling speed of the casting billet is controlled at 0.76m/min); the secondary cooling water volume is controlled at 0.95L/kg; protective pouring (low-oxidizing and alkaline covering agents are added to the molten steel in the ladle and the tundish; protective sleeves and argon blowing are used from the large ladle to the tundish, and from the tundish to the crystallizer; high-quality protective slag is added to the crystallizer); electromagnetic stirring at the end of solidification of the continuous casting machine: the voltage is 450V, the current intensity is 400A, and the frequency is 7Hz;

The steel billet is rolled into Ø6mm wire rod on a high-speed wire mill. The rolling process is as follows: the start rolling temperature is controlled at 970℃, the finishing rolling inlet temperature is 920℃, the sizing inlet temperature is 900℃, and the wire-spinning temperature is 900℃; the steel is cooled to 720℃ at a cooling rate of 7℃/s and then enters the hood. The insulation hood is completely closed, the cooling rate in the hood is 2.0℃/s, the hood outlet temperature is 490℃, and the cooling bed is air-cooled.

Embodiment 3:

LF refining: The main tasks of LF refining are fine-tuning of composition, temperature control and slag making operation. The nitrogen addition of molten steel is controlled by micro-positive pressure operation, large slag volume and limited heating time. After LF alloying treatment, the molten steel leaves the station after 7 minutes of argon weak stirring.

Continuous casting: The cross-sectional size of the continuous casting billet is 320mm × 410mm, the superheat of the tundish is controlled at 25℃, and the pulling speed is constant (the pulling speed of the casting billet is controlled at 0.75m/min); the water volume of the secondary cooling is controlled at 1.15L/kg; protective pouring (low-oxidizing and alkaline covering agents are added to the molten steel in the ladle and the tundish; protective sleeves and argon blowing are used from the large ladle to the tundish, and from the tundish to the crystallizer; high-quality protective slag is added to the crystallizer); electromagnetic stirring at the end of solidification of the continuous casting machine: the voltage is 500V, the current intensity is 480A, and the frequency is 4Hz;

The steel billet is rolled into Ø8mm wire rod on a high-speed wire mill. The rolling process is as follows: the starting rolling temperature is controlled at 985℃, the finishing rolling inlet temperature is 915℃, the sizing inlet temperature is 895℃, and the wire-spinning temperature is 920℃; it is cooled to 710℃ at a cooling rate of 9℃/s and enters the hood. The insulation hood is completely closed, the cooling rate in the hood is 2.0℃/s, the hood outlet temperature is 510℃, and the cooling bed is air-cooled.

Embodiment 4:

LF refining: The main tasks of LF refining are fine-tuning of composition, temperature control and slag making operation. The nitrogen addition of molten steel is controlled by micro-positive pressure operation, large slag volume and limited heating time. After LF alloying treatment, the molten steel leaves the station after 7 minutes of argon weak stirring.

Continuous casting: The cross-sectional size of the continuous casting billet is 320mm × 410mm, the superheat of the tundish is controlled at 25℃, and the pulling speed is constant (the pulling speed of the casting billet is controlled at 0.74m/min); the water volume of the secondary cooling is controlled at 1.10L/kg; protective pouring (low-oxidizing and alkaline covering agents are added to the molten steel in the ladle and the tundish; protective sleeves and argon blowing are used from the large ladle to the tundish, and from the tundish to the crystallizer; high-quality protective slag is added to the crystallizer); electromagnetic stirring at the end of solidification of the continuous casting machine: voltage is 380V, current intensity is 450A, and frequency is 7 Hz;

The steel billet is rolled into Ø11mm wire rod on a high-speed wire mill. The rolling process is as follows: the start rolling temperature is controlled at 990℃, the finishing rolling inlet temperature is 940℃, the sizing inlet temperature is 900℃, and the wire drawing temperature is 925℃; it is cooled to 715℃ at a cooling rate of 7℃/s and enters the hood. The insulation hood is completely closed, the cooling rate in the hood is 3.0℃/s, the hood outlet temperature is 490℃, and the cooling bed is air-cooled.

The mechanical properties, cold upsetting and quenching hardness indexes after quenching heat treatment of the embodiments of the present invention and the comparative examples are shown in Table 2.

Table 2 Mechanical properties, cold forging and quenching hardness test data of steel

Nominal diameter (mm) Tensile strength (MPa) Elongation (%) Area reduction rate (%) 1/2 Cold Heading Qualification Rate (%) 1/3 cold forging qualified rate (%) Hardness of the core of the transverse section of the wire rod after quenching (HRC) Example 1 Ø8 548 27.5 65 100 60 ≥40 Example 2 Ø6 542 31.5 69 100 60 ≥42 Example 3 Ø8 520 33.5 67 100 60 ≥44 Example 4 Ø11 509 28.5 62 100 60 ≥41 Comparative Example Ø10 533 28 60 75 30 ≥36

The transverse microstructure of the hot-rolled wire rod is shown in Figure 1. As shown in Figure 1 and Table 2, the cold heading steel wire rod produced by the above-mentioned inventive composition and method has a ferrite area accounting for 60~80% and a pearlite area accounting for 20~40%; after hot rolling of the wire rod, the tensile strength is greater than 500MPa, the elongation is greater than 25%, and the area reduction is greater than 58%; the cold heading performance is good; after quenching, the hardness of the core of the transverse section of the wire rod is greater than 40HRC, and the martensite content accounts for more than 85%, which can be used to produce 9.8 grade high-strength bolts, nuts and other standard parts.

The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims (9)

1. A 9.8-grade small-size boron-containing cold heading steel wire rod with excellent hardenability, with a size of ø5.5mm to ø12.5mm, used for producing 9.8-grade high-strength standard parts, characterized in that it is composed of the following chemical components in percentage by weight: C: 0.18%～0.23%, Si: ≤0.10%, Mn: 0.60%～0.85%, Cr: 0.19%～0.29%, Mo: 0.15%～0.24%, Al: 0.020%～0.040%, Ti: 0.028%～0.040%, B: 0.0015%～0.0040%, Ni: 0.10%～0.20%, N: ≤0.0080%; the balance is Fe and unavoidable impurities. 2. The 9.8-grade small-size boron-containing cold heading steel wire rod with excellent hardenability according to claim 1, characterized in that: The ferrite area of the cold heading steel wire rod accounts for 60% to 80%, and the pearlite area accounts for 20% to 40%. 3. The 9.8-grade small-size boron-containing cold heading steel wire rod with excellent hardenability according to claim 1, characterized in that: After hot rolling, the cold heading steel wire rod has a tensile strength greater than 500 MPa, an elongation greater than 25%, and an area reduction greater than 58%. 4. The 9.8-grade small-size boron-containing cold heading steel wire rod with excellent hardenability according to claim 1, characterized in that: After the cold heading steel wire rod is quenched, the hardness of the core of the transverse section is greater than 40HRC, and the martensite content accounts for more than 85%. 5. A method for manufacturing a 9.8-grade small-size boron-containing cold heading steel wire rod with excellent hardenability as claimed in any one of claims 1 to 4, wherein the process flow is molten iron pretreatment → converter → LF refining → continuous casting → continuous rolling → billet cleaning → heating → rolling → controlled cooling → finishing → inspection → storage, characterized in that: 1) LF refining; Use argon gas for weak stirring, and the stirring time is 5 to 10 minutes; 2) Continuous casting; The superheat of the tundish is controlled at 20-30℃, and the casting speed is controlled at 0.75±0.02m/min by constant pulling speed; the water volume of the secondary cooling is controlled at 0.9-1.2 L/kg; 3) Heating; The starting rolling temperature is controlled at 960-1020°C, the finishing rolling inlet temperature is controlled at 900-980°C, the sizing inlet temperature is controlled at 880-950°C, and the wire laying temperature is controlled at 890-930°C. 4) Cooling control; Adopt Stelmore air cooling line cooling method: cool to 680-770℃ at a cooling rate of 6-12℃/s before entering the hood, close all the insulation hoods, cool in the hood at a rate of 0.5-5.0℃/s, exit the hood at a temperature of 460-600℃, and use air cooling on the cooling bed. 6. The method for manufacturing a 9.8-grade small-sized boron-containing cold heading steel wire rod with excellent hardenability according to claim 5, characterized in that: In the step 1), the soft blowing flow rate is 10-30 NL/min. 7. The method for manufacturing a 9.8-grade small-sized boron-containing cold heading steel wire rod with excellent hardenability according to claim 5, characterized in that: In the step 2), the cross-sectional size of the continuous casting billet is 250-350 mm × 350-450 mm. 8. The method for manufacturing a 9.8-grade small-sized boron-containing cold heading steel wire rod with excellent hardenability according to claim 5, characterized in that: In the step 2), protective casting is adopted, and low-oxidizing and alkaline covering agents are added to the molten steel in the ladle and the tundish; protective sleeves and argon blowing are used from the large ladle to the tundish, and from the tundish to the crystallizer, and protective slag is added to the crystallizer. 9. The method for manufacturing a 9.8-grade small-sized boron-containing cold heading steel wire rod with excellent hardenability according to claim 5, characterized in that: In the step 2), electromagnetic stirring is used at the solidification end of the continuous casting machine, with a voltage of 300-500V, a current intensity of 300-500A, and a frequency of 3-8Hz.