Claims:We Claim,
1. A heat treatment process for a bent pipe used in wire rod mill, the process comprising the steps of:
heating (S1) the bent pipe at a temperature in the range of 950 ? to 1100 ? for a period of 5 minutes to 15 minutes;
quenching (S2) the heated bent pipe in a fluid bath; and
tempering (S3) the quenched bent pipe at a temperature in the range of 200? to 400? for a period of 2 minutes to 10 minutes to obtain a modified bent pipe.
2. The heat treatment process as claimed in claim 1, wherein the bent pipe is made of steel.
3. The heat treatment process as claimed in claim 2, wherein said steel has a composition comprising 0.16 Wt. % to 0.25 Wt. % of C, up to 1 Wt. % of Si, up to 1.5 Wt. % of Mn, up to 0.04 Wt. % of P, up to 0.03 Wt. % of S and 12 Wt. % to 14 Wt. % of Cr, the remainder of said composition being iron and impurities.
4. The heat treatment process as claimed in claim 1, wherein the modified bent pipe has a hardness that is increased by more than 150%.
5. The heat treatment process as claimed in claim 1, wherein the modified bent pipe has a life that is increased by more than 150%.
6. The heat treatment process as claimed in claim 1, wherein the fluid bath is an oil bath.
7. The heat treatment process as claimed in claim 6, wherein the oil is an organic oil.
Dated this 30th day of March, 2022

FOR STEEL AUTHORITY OF INDIA LIMITED
By their Agent

(ANSHUL SUNILKUMAR SAURASTRI) (IN/PA 3086)
KRISHNA & SAURASTRI ASSOCIATES LLP
, Description:F O R M 2
THE PATENTS ACT, 1970
(39 of 1970)
The Patent Rule, 2003

COMPLETE SPECIFICATION
(See section 10 and rule 13)

“HEAT TREATMENT PROCESS FOR BENT PIPE USED IN WIRE ROD MILL”
By

STEEL AUTHORITY OF INDIA LIMITED, A GOVERNMENT OF INDIA ENTERPRISE, HAVING ITS ADDRESS AT RESEARCH & DEVELOPMENT CENTRE FOR IRON & STEEL, DORANDA, RANCHI-834002, STATE OF JHARKHAND, INDIA

THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED

FIELD OF INVENTION
The present invention relates to a steel bent pipe used in a wire rod mill. Particularly, the present invention relates to a heat treatment process for the steel bent pipe used in a wire rod mill.
BACKGROUND OF THE INVENTION
Bent pipe is a thick-walled hollow pipe located in between the pinch roll and laying head of the wire rod processing line, which guides the wire rod feeds from mill to the laying head. The bent pipe is curved to accommodate the height difference between the pinch roll and the laying head in the wire rod mill.
Due to curved design of the bent pipe, wire rods leaving the pinch roll hits the inner wall of the pipe after entering the pipe and follows the profile of the pipe before proceeding into the laying head. Therefore, the bent pipe is liable to absorb high degree of mechanical shock as well as thermal fatigue due to high temperature processing of wire rods. Thus, bent pipe undergoes significant abrasive wear during operation and forms grooves on the inner surface thereby puncturing the bent pipe almost regularly after around rolling of 1000T-1200T. In the event of puncture of the bent pipe, the wire rod gets stuck up and consequently cobble takes place, leading to mill delay and loss of production. To avoid the recurring event of cobble, bent pipe is changed almost every day, resulting into high consumption of bent pipe.
Thus, failure analysis was carried out in a methodical way by proper metallurgical investigations. At present, the material used in wire rod mill for bent pipe is AISI420/X20Cr13 which is a martensitic stainless steel. Chemical analysis of the failed sample revealed that the bent pipe material was as per the specifications EN1088 steel. Then hardness of the failed pipe from least affected zone was taken, the hardness was found in the range of 27-30 HRC. This gave a clear indication that the material was not properly heat treated as the continuous cooling transformation (CCT) phase diagram indicated that 50 HRC of hardness is achievable by varying the cooling rate.
Therefore, there is a need in the art for a process to enhance the life of bent pipe by optimising the heat treatment parameters for bent pipe used in wire rod mill.
SUMMARY OF THE INVENTION
An aspect of the present invention is to address at least the above-mentioned problems and/or disadvantages and to provide at least the advantages described below.
The main objective of this invention is to provide a heat treatment process for the bent pipe in order to address the aforesaid mill delay, reducing replacement, increasing hardness and operation lifetime of the bent pipe.
The present disclosure in one aspect provides, a heat treatment process for a bent pipe used in wire rod mill, the process comprising the steps of: heating the bent pipe at a temperature in the range of 950 ? to 1100 ? for a period of 5 minutes to 15 minutes; quenching the heated bent pipe in a fluid bath; and tempering the quenched bent pipe at a temperature in the range of 200? to 400? for a period of 2 minutes to 10 minutes to obtain a modified bent pipe.
The bent pipe of the present invention is made of a steel material having composition of 0.16 Wt. % to 0.25 Wt. % of C, up to 1 Wt. % of Si, up to 1.5 Wt. % of Mn, up to 0.04 Wt. % of P, up to 0.03 Wt. % of S and 12 Wt. % to 14 Wt. % of Cr along with remainder of the chemical composition being iron and impurities. It is understood here that other types of bent pipes of varying compositions of steel material may be used in other alternative embodiments of the present invention.
In this respect, before explaining the present heat treatment process for bent pipe used in wire rod mill in detail, it is to be understood that the heat treatment process for bent pipe used in wire rod mill is not limited in its applications to the details of methods and processes set forth in the following description or illustration. Those skilled in the art will appreciate that the concept of this disclosure may be readily utilized as a basis for the design of other methods and systems for carrying out the several purposes of the heat treatment process for bent pipe used in wire rod mill. It is also to be understood that the phraseology and terminology employed herein are for purposes of description and should not be regarded as limiting.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are included to provide a further understanding of the invention are incorporated in and constitute a part of this specification, illustrate an aspect of the invention and together with the description serve to explain the principles of the invention. They are meant to be exemplary illustrations provided to enable persons skilled in the art to practice the disclosure and are not intended to limit the scope of the appended claims.
Figure 1 shows a longitudinal cross-sectional view of the bent pipe;
Figure 2 is a cross-sectional view taken along the B-B line in FIG. 1; and
Figure 3 shows a flow chart illustrating the heat treatment process for the steel bent pipe in accordance with aspects of the present invention.
Throughout the drawings, it should be noted that like reference numbers are used to depict the same or similar elements and processes.
DETAILED DESCRIPTION OF THE INVENTION
The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of the invention as defined by the claims and their equivalents. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary. In addition, descriptions of well-known functions and constructions are omitted for clarity and conciseness.
The terms and words used in the following description and claims are not limited to the bibliographical meanings but are merely used by the inventor to enable a clear and consistent understanding of the invention. Accordingly, it should be apparent to those skilled in the art that the following description of the present invention is provided for illustration purpose only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.
It is to be understood that the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise.
The term "impurities" herein means components that are mixed in producing the steel product industrially, from materials such as ores and scraps or owing to other factors.
The phrase “modified bent pipe” herein means the heat treated bent pipe in accordance with the present invention.
Figure 1 and 2 shows longitudinal and lateral cross-sectional view of a bent pipe (100) in accordance with aspects of the present invention respectively. The bent pipe (100) is a thick-walled hollow pipe located in between the pinch roll and laying head of the wire rod processing line (pinch roll and laying head are not shown in the figure), which guides the wire rod feeds from mill to the laying head. The bent pipe (100) is curved to accommodate the height difference between the pinch roll and the laying head in the wire rod mill. At present, the material used in wire rod mill for bent pipe (100) is AISI420/X20Cr13 which is a martensitic stainless steel.
Figure 3 shows a flow chart illustrating a heat treatment process for a steel bent pipe in accordance with aspects of the present invention. The heat treatment process for an AISI420 grade steel bent pipe (100) used in wire rod mill comprising sequential steps of a heating process (S1), a quenching process (S2) and a tempering process (S3). The heating process (S1) involves heating of the bent pipe (100) at a temperature range of 950? to 1100? for a period of 5 to 15 minutes. The heating process (S1) facilitates phase transformation of the bent pipe (100). The heating process is followed by the quenching process (S2), wherein the heated bent pipe (100) is quenched in a fluid, preferably an oil bath. In one of the aspect of the present invention, the oil used for quenching is an organic oil however other types of oils may be used in other alternative embodiments of the present invention. The quenching process (S2) facilitates a fine lath martensite structure that improves the wear resistance and hardness of the bent pipe (100). The quenched bent pipe (100) is subjected to tempering in the tempering process (S3) at a temperature range of 200? to 400? for a period 2 to 10 minutes. The tempering process (S3) imparts increase in toughness thereby relieving internal stresses. The tempering process (S3) leads to formation of fine tempered lath martensite structure of the bent pipe (100).
The bent pipe of the present invention is made of a steel material having a composition of 0.16 Wt.% to 0.25 Wt.% of C, up to 1 Wt.% of Si, up to 1.5 Wt.% of Mn, up to 0.04 Wt.% of P, up to 0.03 Wt.% of S and 12 Wt.% to 14 Wt.% of Cr along with remainder of the chemical composition being iron and impurities.
In the context of the present invention, the heat treatment experimental trials were performed and it was found that for quenching temperature of 950°C-100°C, hardness was found better in all annealing conditions as compared to 850?-980°C. Based on the experimental trials 950°C-1020°C was chosen as the quenching temperature. The heat treatment data revealed that tempering at 200°C-400°C temperature, the achievable hardness was 45–50 HRC. The wear resistance of bent pipe is reduced with increase in hardness of the bent pipe. Optical images of heat treated bent pipe samples were taken in optical microscope in etched condition. Vilella’s reagent was used as an etchant for the polished surface of the sample. Scanning electron microscope (SEM) images were taken of the sample after heat treatment and a fine tempered lath martensitic microstructure was seen in the SEM images.
Industrial trials were done in plant with modified bent pipe. 10 modified bent pipes were used for industrial trials. During trials the bent pipes were replaced by the next bent pipe as groove mark was seen at the inner surface of the bent pipe by visual examination. The average tonnage rolled by modified bent pipes was more than 3000 tons. Thus, the modified bent pipe had shown a life improvement of 150% more than the bent pipe used previously. The hardness of the modified bent pipes was increased to 45-50 HRC whereas the hardness of the bent pipes was 27-30 prior to the heat treatment process of the present invention. Thus, the modified bent pipe had shown an enhanced hardness of 150% more than the bent pipe used previously.
The advantages achieved after the usage of present heat treatment process on bent pipe are: (a) reduced mill delay, (b) reduced replacement, (c) increased hardness (d) operational lifetime of the bent pipe is increased by more than 150% and (e) puncture or cobble of bent pipe is drastically reduced.
The present heat treatment process for bent pipe can used to address wear related issue of bent pipe used in any wire rod mill.