A production method for improving the structure of small-size spring steel wire rod

Abstract

The present application is a kind of production method for improving the structure of small size spring steel wire rod.The chemical composition of the billet is as follows: C=0.51%~0.57%, Si=1.30%~1.50%, Mn=0.50%~0.55%, P≤0.015%, S≤0.015%, Cr=0.50%~0.55%, and the rest is Fe and inevitable impurity elements.The lower content of Mn and Cr is used to reduce the hardenability of the material; the temperature system is optimized, the discharge temperature is reduced from 950~1000℃ to 900~950℃, the entry temperature of finish rolling is reduced from 880~920℃ to 820~850℃, and the wire laying temperature is increased from 750~770℃ to 850~900℃.The cooling process after finish rolling is uniform and slow, and the wire rod is composed of sorbite+pearlite+ferrite without abnormal structure, the drawing performance is improved, the quality is obviously improved, and the drawing deformation can reach more than 76%.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of spring steel wire rod production, and relates to a production method for improving the tensile property of hot-rolled organization of small-specification spring wire rod. BACKGROUND

[0002] The Φ5.5mm specification spring steel wire rod, especially the 55SiCr wire rod, is prone to abnormal organization in the process of rolling due to the high Cr content, good hardenability, small diameter and fast cooling speed in the air, and is prone to pen-shaped fracture in the drawing process, thereby affecting the drawing efficiency. Even if the drawing process is continuously improved, the risk of fracture in the subsequent spring winding and use process still exists. Therefore, in order to make the spring steel wire rod have good drawing property, it is necessary to ensure that the wire rod organization does not have abnormal organizations such as bainite and martensite. SUMMARY

[0003] The application aims to provide a production method for improving the tensile property of hot-rolled organization of small-specification spring wire rod, which optimizes the chemical composition and rolling process to improve the wire rod organization.

[0004] The application discloses a production method for improving the organization of small-specification spring steel wire rod, and the process flow comprises the following steps: heating of a steel billet, roughing, intermediate rolling, finishing rolling, water tank cooling, wire drawing, roller air cooling and coil collecting, wherein the steel billet has the following chemical composition: C=0.51% to 0.57%, Si=1.30% to 1.50%, Mn=0.50% to 0.55%, P≤0.015%, S≤0.015%, Cr=0.50% to 0.55%, and the rest is Fe and inevitable impurity elements; and the method comprises the following key steps:

[0005] 1) the steel billet is selected as a 150mm*150mm continuous casting billet or a rolled billet, the tapping temperature is 900-950 DEG C, the finishing rolling temperature is 820-850 DEG C, and the wire drawing temperature is 850-900 DEG C;

[0006] 2) each water tank is uniformly opened after finishing rolling, and the cooling speed of each water tank is 12-25 DEG C / s;

[0007] 3) the roller speed of the air cooling line is 0.45-0.80 m / s, 0-5 heat preservation covers of the air cooling roller are opened, all the cooling fans below the roller are closed, the cooling speed is controlled to be 2-2.5 DEG C / s, the temperature out of the heat preservation cover is 550-680 DEG C, and the Φ5.5mm specification 55SiCr spring steel wire rod with the metallographic organization of sorbite+pearlite+ferrite is obtained.

[0008] In the step 3), the air cooling line roller way speed is 0.45-0.80 m / s, wherein each roller way speed control is S1=0.45-0.55 m / s, S2=0.45-0.55 m / s, S3=0.50-0.60 m / s, S4=0.55-0.65 m / s, S5=0.60-0.70 m / s, S6=0.65-0.75 m / s, S7=0.70-0.80 m / s, S8=0.70-0.80 m / s, S9=0.65-0.75 m / s, S10=0.60-0.70 m / s.

[0009] Principle of the application: Since Mn and Cr are elements that increase hardenability, in order to ensure the strength of finished spring, the content of Mn and Cr must be high, generally controlled target is Mn=0.60-0.70%, the middle line is 0.65%, Cr=0.60-0.70%, the middle line is 0.65%, or even higher. The Φ5.5 mm specification has large drawing amount and obvious work hardening, and the control of lower content of Mn and Cr can also meet the spring strength requirement. Reducing the content of hardenability elements can reduce the hardenability of the rod, and it is more difficult to form bainite and martensite structure. The tapping temperature cannot be too high, too high is easy to exceed the decarburization layer, and the cooling speed is too fast during subsequent cooling, which is easy to form abnormal organization; it also cannot be too low, too low will reduce the plasticity of the billet, increase the deformation resistance of the billet, and easily form cracks in the rolling process and cause damage to the rolling equipment. The finish rolling temperature is as low as possible under the premise of ensuring the normal operation of the equipment, the rolling speed of Φ5.5 mm specification is above 100 m / s, the speed is fast, and the temperature rises obviously (about 100℃) during finish rolling. If the finish rolling temperature is too high, the cooling speed is too large during water tank cooling, and there is a cliff type temperature drop, which is easy to cause local rapid cooling to the bainite and martensite phase transition point, and form abnormal organization. At the same time, the wire drawing temperature is also the same, if it is too low, the water tank cooling speed before wire drawing is too large, which will also cause local rapid cooling and form abnormal organization, if it is too high, the rod shape is not easy to control, and abnormal organization is easy to appear after the end of the roller cooling and the heat preservation cover is taken off, which will also appear abnormal organization. The roller speed and heat preservation cover are used to control the cooling speed (2-2.5℃ / s) of the rod on the roller after wire drawing, so that it is transformed into sorbite and pearlite.

[0010] The beneficial effects of the present application: for Φ5.5mm specification 55SiCr rod in the rolling process is easy to form bainite + martensite structure, there are drawing fracture and other quality problems, by using low Mn, Cr content of internal steel and lower rolling temperature, control the cooling speed of each section water tank and air cooling line to control the phase transition temperature, so as to obtain ideal rod organization, eliminate the drawing fracture caused by abnormal organization. The metallographic structure of the obtained Φ5.5mm specification 55SiCr spring steel rod is sorbite + pearlite + ferrite, such rod has good drawing performance, and the deformation can reach more than 76%. BRIEF DESCRIPTION OF DRAWINGS

[0011] Figure 1 The metallographic structure of the rod of example 1 and example 2 is sorbite + pearlite + ferrite.

[0012] Figure 2 The metallographic structure of the rod of comparative example 1 is martensite + bainite. DETAILED DESCRIPTION Example 1

[0013] High-speed wire rod mill rolls Φ5.5mm specification 55SiCr, the chemical composition of the billet is: C=0.55%, Si=1.45%, Mn=0.52%, P=0.010%, S=0.010%, Cr=0.53%, the rest is Fe and inevitable impurity elements, tapping temperature 930℃, entering the finishing temperature 840℃, wire drawing temperature 860℃. Four water tanks are all opened, and the water opening amount is equivalent. The roller speed S1: 0.55m / s, S2: 0.55m / s, S3: 0.60m / s, S4: 0.65m / s, S5: 0.70m / s, S6: 0.75m / s, S7: 0.80m / s, S8: 0.80m / s, S9: 0.75m / s, S10: 0.70m / s. The fan is all closed, the heat preservation cover is opened 0, and the rest is closed.

[0014] The Φ5.5mm specification 55SiCr produced by the above process is detected, the metallographic structure is sorbite + pearlite + ferrite, the organization is uniform, and there is no pen-shaped fracture from Φ5.5mm to Φ2.7mm after 6 passes of drawing. Example 2

[0015] Rolling Φ5.5mm specification 55SiCr, the composition of the billet is: C=0.56%, Si=1.44%, Mn=0.53%, P=0.008%, S=0.010%, Cr=0.52%, the rest is Fe and inevitable impurity elements, tapping temperature 940℃, entering the finishing temperature 830℃, wire drawing temperature 850℃. Four water tanks are all opened, and the water volume is equivalent. The roller speed S1=0.45m / s, S2=0.45m / s, S3=0.50m / s, S4=0.55m / s, S5=0.60m / s, S6=0.65m / s, S7=0.70m / s, S8=0.70m / s, S9=0.65m / s, S10=0.60m / s. The fan is all closed, and the heat shield is opened 5, and the rest is closed.

[0016] The Φ5.5mm specification 55SiCr produced by the above process is detected, the metallographic structure is sorbite+pearlite+ferrite, the structure is uniform, and there is no pen-shaped fracture from Φ5.5mm to Φ2.7mm after 6 drawing passes.

[0017] Comparative example 1

[0018] The original process before the method of the application: high line rolling Φ5.5mm specification 55SiCr, the composition of the billet is: C=0.55%, Si=1.45%, Mn=0.68%, P=0.012%, S=0.010%, Cr=0.67%, the rest is Fe and inevitable impurity elements, tapping temperature 970℃, entering the finishing temperature 900℃, wire drawing temperature 760℃. Four water tanks are all opened, and the water volume is equivalent. The roller speed S1=0.45m / s, S2=0.45m / s, S3=0.50m / s, S4=0.55m / s, S5=0.60m / s, S6=0.65m / s, S7=0.70m / s, S8=0.70m / s, S9=0.65m / s, S10=0.60m / s. The fan is all closed, and the heat shield is opened 4, and the rest is closed.

[0019] The Φ5.5mm specification 55SiCr produced by the above original process is detected, the metallographic structure is bainite+martensite+tempered troostite, and pen-shaped fracture occurs in each pass from Φ5.5mm to Φ2.7mm after 6 drawing passes.

[0020] Table 1 Comparison of wire rod structure and drawing performance

[0021] Rod structure Drawing property Example 1 Sorbite + pearlite + ferrite From Φ5.5 mm to Φ2.7 mm in 6 passes, no pencil-point fracture occurred Example 2 Sorbite + pearlite + ferrite From Φ5.5 mm to Φ2.7 mm in 6 passes, no pencil-point fracture occurred Comparative Example 1 Bainite + martensite + tempered troostite From Φ5.5 mm to Φ2.7 mm in 6 passes, pencil-point fracture occurred in each pass

Claims

1. A production method for improving the microstructure of small-diameter spring steel wire rod, the process flow including: The process involves heating a steel billet → roughing and intermediate rolling → intermediate rolling → finishing rolling → water tank cooling → wire drawing → roller cooling → coiling, characterized by the following key steps: the steel billet's chemical composition by weight percentage is C=0.51%~0.57%, Si=1.30%~1.50%, Mn=0.50%~0.55%, P≤0.015%, S≤0.015%, Cr=0.50%~0.55%, with the remainder being Fe and unavoidable impurity elements. 1) Select 150mm×150mm continuously cast billets or rolled billets, with furnace exit temperature of 900~950℃, finishing rolling temperature of 820~850℃, and wire drawing temperature of 850~900℃. 2) After finishing rolling, each water tank is opened evenly, and the cooling rate of each water tank is 12-25℃ / s; 3) The speed of the air-cooled roller conveyor is 0.45~0.80m / s; 0~5 of the air-cooled roller conveyor insulation covers are opened, all cooling fans below the roller conveyor are closed, the cooling rate is controlled at 2~2.5℃ / s, and the temperature of the roller conveyor exiting the insulation cover is 550~680℃, resulting in Φ5.5mm 55SiCr spring steel wire rod with a metallographic structure of sorbite + pearlite + ferrite.

2. The production method for improving the microstructure of small-diameter spring steel wire rod according to claim 1, characterized in that: In step 3), the speed of the air-cooled line rollers is 0.45~0.80m / s, wherein the speed of each roller is controlled as follows: S1=0.45~0.55m / s, S2=0.45~0.55m / s, S3=0.50~0.60m / s, S4=0.55~0.65m / s, S5=0.60~0.70m / s, S6=0.65~0.75m / s, S7=0.70~0.80m / s, S8=0.70~0.80m / s, S9=0.65~0.75m / s, and S10=0.60~0.70m / s.

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