METHOD FOR MANUFACTURING PRIME HOT ROLLED HIGH TENSILE STRENGTH DEFORMED BARS

Abstract

The present invention discloses a method for manufacturing high tensile strength deformed bars with an ultimate tensile strength of 500 MPa-700 MPa, comprising the steps of: (a) rolling under heavy reduction rate the common 20MnSi steel billet with an initial rolling temperature of 880˜950° C. through the rough and intermediate rolling mills having an maximum lengthening coefficient μ of 1.449; then passing the rolling workpieces through the pre-water cooling process to decrease the temperature of the workpieces to the range of 850˜950° C.; (b) rolling the workpieces through the finish rolling mill having a main motor power of 1350 KW; then cooling rapidly the surface of the workpieces through the water cooling process having a number of cooling water segments of 1˜5 and a water pressure of 0.8˜1.2 MPa so as to control the surface temperature of the workpieces to the cooling bed within the range of 550˜650° C.; (c) cooling the workpieces through the air cooling bed to cool the core portion of the workpieces; then tempering the workpieces to obtain the finished steel. In accordance with the method of the present invention, the tissue of steel is improved by rolling the common 20MnSi steel billet under heavy reduction rate to manufacture directly the high tensile strength deformed bars with a high tensile strength of 500 MPa-700 MPa so that the manufacturing cost and energy consumption are reduced and the tensile strength of steel is improved, and thereby, the construction cost is decreased.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method for manufacturing steels, and more particularly, to a method for manufacturing prime hot rolled high tensile deformed bars with a tensile strength of 500 MPa-700 MPa.

2. Description of the Related Art

The deformed bar is acted as the framework and the like in the construction of buildings such as industrial factories, reservoirs and dams, road bridges, and so on. Currently, the steel billet of 20MnSi is used by the steel mills to manufacture the HRB335 grade deformed bars. However, the HRB335 deformed bar has a low tensile strength, resulting in the drawbacks of such as large dead weight of the framework structure, high project cost, and long construction period in the construction of especially major projects, and thereby, has already been abandoned for use in construction by developed countries. Nowadays, many alloy elements are added into the steel billet by the steel mills to increase the strength of steel. Namely, alloy elements such as V, Nb, Ti, and so on, are added in the manufacturing of steel billet to realize the purpose of strengthening the tissue of the steel by means of utilizing the solid solution strengthening capability of the alloy elements with other elements in the steel billet. For example, by adding ferrovanadium into the steel, the steel billet of 20MnSiV can be manufactured, and the tensile strength of the steel manufactured accordingly can reach a grade of HRB400. Compared with that of the HRB335 steel, the strength index of the HRB400 steel is increased by 17%. However, the addition of alloy elements complicates the steel manufacture process, and thereby, increases the manufacturing cost.

SUMMARY OF THE INVENTION

In view of the above-described drawbacks, it is an objective of the present invention to provide a method for manufacturing a prime hot rolled high tensile strength deformed bars with an ultimate tensile strength of 500 MPa-700 MPa by utilizing the common 20MnSi steel billet directly so as to realize the purpose of decreasing the manufacturing cost and energy consumption, enhancing the steel strength, saving steel consumption, and decreasing project cost accordingly.

To achieve the above objective, there is provided a method for manufacturing the prime hot rolled high tensile strength deformed bars with a tensile strength of 500 MPa-700 MPa, comprising the following steps:

(a) rolling under heavy reduction rate the common 20MnSi steel billet with an initial rolling temperature of 880˜950° C. through the rough and intermediate rolling mills having an maximum lengthening coefficient μ of 1.449; then passing the rolling workpieces through the pre-water cooling process to decrease the temperature of the workpieces to the range of 850˜950° C.;
(b) rolling the workpieces through the finish rolling mill having a main motor power of 1350 KW; then cooling rapidly the surface of the workpieces through the water cooling process having a number of cooling water segments of 1˜5 and a water pressure of 0.8˜1.2 MPa so as to control the surface temperature of the workpieces to the cooling bed within the range of 550˜650° C.;
(c) cooling the workpieces through the air cooling bed to cool the core portion of the workpieces; then tempering the workpieces to obtain the finished steel.

In accordance with the method of the present invention, the tissue of steel is improved by rolling the common 20MnSi steel billet under heavy reduction rate to manufacture directly the high tensile deformed bars with a tensile strength of 500 MPa-700 MPa. By applying the rolling process under ultra-low temperature, not only the issue of the excessive cooling intensity in the following cooling processes can be avoided so that the desirable temperature of the steel workpieces to the cooling bed can be achieved, but also the energy consumption and cost can be decreased. The pre-water cooling process is added behind the rough and intermediate rolling processes to decrease the surface temperature of the steel workpieces so as to balance the temperature of the workpieces rose in the process of finish rolling resulting from the high speed of the finish rolling mill. Finally, the steel workpieces are cooled through the water cooling process; the remained heat treatment technology is applied to improve greatly the overall mechanical performance of the steel.

In the rolling processes, the cooling effect to the steel is influenced by the number of segments and the water pressure of the pre-water and water cooling processes, and the rolling speed. In accordance with the method of the present invention, to improve further the steel performance, the number of segments in pre-water cooling process is in the range of 1˜3, the water pressure is normal, the rolling speed of said rolling mills is in the range of 4˜15 m/s.

As a result, the method for manufacturing the high tensile strength deformed bars in accordance with the present invention provides the following advantages:

(1) compared with the HRB335 grade steel in GB1499 standard, with the same 20MnSi material, the strength index of the steel of the present invention is improved by 50% to the HRB500 grade so that the manufacture of steel having a high tensile strength and good elongation is realized.
(2) by applying the rolling process under ultra-low temperature, the fuel is saved by about 15%, the manufacture cost is reduced largely, and the enterprise profitability is increased accordingly.
(3) in the construction of building, the consumption of steel is saved by 15˜30%, the dead weight of the steel structure is decreased largely, the resource utilization is improved, the manufacture cost is decreased, the period of construction is shortened, the consumption of energy and the pollution to the environment is reduced, so that perfect economic and social results are realized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a process flow diagram of a method for manufacturing high tensile strength deformed bars in accordance with the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The method for manufacturing the high tensile strength deformed bars in accordance with the present invention will be described further hereinafter referring to the related drawing.

FIG. 1 illustrates a method for manufacturing the high tensile strength deformed bars with a tensile strength of 500 MPa-700 MPa in accordance with the present invention, comprising the steps of:

(a) rolling under heavy reduction rate the common 20MnSi steel billet having an initial rolling temperature within the range of 880˜950° C. through the rough and intermediate rolling mills having an maximum lengthening coefficient μ of 1.449, the composition of common 20MnSi steel billet is shown in Table 1; then passing the rolling workpieces through the pre-water cooling process having a number of water cooling segments of 1˜3 and a normal water pressure to decrease the surface temperature of the workpieces to the range of 850˜950° C.;

TABLE 1
Chemical composition of common 20MnSi steel billet (%)
Type	C	Si	Mn	P	S	Ceq
20MnSi	≦0.25	≦0.80	≦1.60	≦0.045	≦0.045	≦0.52

(b) rolling the workpieces through the finish rolling mill having a main motor power of 1350 KW; then cooling rapidly the surface of the workpieces through the water cooling process having a number of cooling water segments of 1˜5 and a water pressure of 0.8˜1.2 MPa to control the surface temperature of the workpieces to the cooling bed within the range of 550˜650° C.;
(c) cooling the workpieces through the air cooling bed to cool the core portion of the workpieces; then tempering the workpieces to obtain the finished steel.

In accordance with the embodiments of the present invention, the parameters of the rolling process can be controlled to manufacture different products as shown in Table 2.

TABLE 2
Parameters of rolling process of three typical steel bars
				Temperature
				to finish rolling
	Initial	Pre-water cooling	Water cooling	process
			Rolling	rolling		Water		Water	(after pre-water	Temperature
	Spec		speed	temperature	No. of	pressure	No. of	pressure	cooling)	to cooling
	(mm)	Segmentation	(m/s)	(° C.)	segments	(MPa)	segments	(MPa)	(° C.)	bed (° C.)
Embodiment 1	10	4	9~15	880~950	1~2	Normal	1~2	0.8~1.2	850~950	580~650
						pressure
Embodiment 1	25	1	9~12	880~940	2~3	Normal	1~3	0.8~1.2	850~950	570~650
						pressure
Embodiment 1	40	1	4~7	880~930	2~3	Normal	2~5	0.8~1.2	850~950	550~630
						pressure

In accordance with the embodiments of the present invention, the performance parameters of the typical and practical steels are shown in Table 3, mainly including the parameters of yield point, tensile strength, and elongation that are generally used to characterize the performance of steel. The tissue of metal is ferrite and pearlite, and the grade of the grain size is ≧9.

TABLE 3
Parameters of mechanical performance of the reinforced steel bars of the present invention
	Typical specs
	10 mm	16 mm	25 mm	32 mm
	Aging	Yield	Tensile		Yield	Tensile		Tensile	Tensile		Yield	Tensile
No	time	point	strength	Elongation	point	strength	Elongation	stretch	strength	Elongation	point	strength	Elongation
1	1 day	543	656	30.0	556	673	27.0	545	690	26.1	575	710	28
2		535	651	28.0	547	660	26.2	545	690	24.2	580	715	26
3		536	652	29.6	548	658	26.8	540	690	24.4	585	715	27
4		541	657	29.6	556	668	28.7	535	685	25.0	585	715	26
5		529	648	28.4	557	670	28.5	545	690	25.3	580	705	28
Average	536.8	652.8	29.1	552.8	665.8	27.4	542.0	689.0	25.0	581.0	712.0	26.7
Stdev	5.50	3.70	0.87	4.87	6.50	1.10	4.47	2.24	0.76	4.18	4.47	0.83
1	30 days	530	655	27.8	552	667	25.6	545	685	24.3	570	695	22
2		525	640	26.8	544	660	25.2	545	685	23.5	580	705	21
3		525	645	28.8	554	666	27.0	550	690	22.6	580	705	21
4		535	655	28.2	542	655	28.2	540	680	24.3	580	705	19
5		540	660	28.8	546	660	26.5	540	685	24.0	585	710	20
Average	531.0	651.0	28.1	547.6	661.6	26.5	544.0	685.0	23.7	579.0	704.0	20.6
Stdev	5.83	7.35	0.74	4.63	4.41	1.06	3.74	3.16	0.64	4.90	4.90	0.88

As shown in Table 3, by applying the rolling process under heavy reduction rate and ultra-low temperature, the high tensile strength deformed bars characterized in the main tissue of ferrite and pearlite (P+F) and the yield strength of exceeding 500 MPa is obtained, and meets the requirements of the strength grade of 500 MPa in the newly revised GB1499 standard.

Claims

1. A method for manufacturing high tensile strength deformed bars with an ultimate tensile strength of at least about 500 MPa-700 MPa, comprising the steps of:

(a) rolling under heavy reduction rate the common 20MnSi steel billet with an initial rolling temperature of 880˜950° C. through the rough and intermediate rolling mills having an maximum lengthening coefficient μ of 1.449; then passing the rolling workpieces through the pre-water cooling process to decrease the temperature of the workpieces to the range of 850˜950° C.;

(b) rolling the workpieces through the finish rolling mill having a main motor power of 1350 KW; then cooling rapidly the surface of the workpieces through the water cooling process having a number of cooling water segments of 1˜5 and a water pressure of 0.8˜1.2 MPa so as to control the surface temperature of the workpieces to the cooling bed within the range of 550˜650° C.;

(c) cooling the workpieces through the air cooling bed to cool the core portion of the workpieces; then tempering the workpieces to obtain the finished steel.

2. The method of claim 1, wherein the number of pre-water cooling segments of said step (a) is in the range of 1˜3, and the water pressure is normal.

3. The method of claim 1, wherein the rolling speed of said rolling mills is in the range of 4˜15 m/s.