PRODUCTION CONCEPTS OF NIOBIUM MICROALLOYED STRUCTURAL HOLLOWS BY SEAMLESS PIPE ROLLINGShenzhen Sunrise Metal Industry Co.,Ltd

PRODUCTION CONCEPTS OF NIOBIUM MICROALLOYED STRUCTURAL 
HOLLOWS BY SEAMLESS PIPE ROLLING

Shenzhen Sunrise Metal Industry Co.,Ltd

TEL：0086-0755-27185042 
Website：http://www.sunriseta.com
E-Mail：sales@sunriseta.com

Abstract 

 

Seamless tubes are available with wall gages of up to 100 mm and outer diameters up to 

around 700 mm. Such tubes are typically used for oil country tubular goods as well as for the 

automotive industry. Another large application area is that of structural hollows for buildings 

and construction applications. Due to weight and cost reduction efforts the demand for high 

strength grade seamless tubes is increasing. Many applications require high toughness in 

addition to high strength. The different rolling processes applied in production depend on 

wall gage and pipe diameter. The continuous mandrel mill is used to produce smaller gages 

and diameters; the plug mill covers medium gages and diameters; the pilger mill allows 

producing larger diameters and heavy wall gage. In all these processes only a limited degree 

of thermomechanical rolling is possible. Therefore strengthening and toughening by severe 

grain refinement employing a conventional niobium-based microalloying concept is not 

easily achievable. Consequently, high strength and toughness seamless pipe is typically 

produced via a quench and tempering process route. This route however is a costly one and 

constitutes a capacity bottleneck in the mill. An innovative low-carbon high-niobium alloy 

concept was identified to offer strength up to grade 70 at very high toughness off the rolling 

plant, i.e, without quench and tempering treatment. The paper reveals the different 

functionalities of niobium in this concept based on the process of continuous mandrel mill 

rolling. In contrast the difficulties of obtaining a high strength and toughness combination 

with a more conventional alloy concept will be demonstrated. 

 

 

1. Introduction 

 

Seamless tubes are available with wall gages of up to 100 mm and outer diameters up to 

around 700 mm [1]. Such tubes are typically used for offshore pipelines and oil country 

tubular goods as well as for the automotive industry and construction applications. Due to 

weight and cost reduction efforts the demand for high strength grade seamless tubes is 

increasing. Many applications require high toughness in addition to high strength. The 

different rolling processes applied in production depend on wall gage and pipe diameter. The 

continuous mandrel mill is used to produce smaller gages and diameters; the plug mill covers 

medium gages and diameters; the pilger mill allows producing larger diameters and heavy 

wall gage. The deformation and temperature schedule during pipe rolling in seamless tube 

mills naturally depend on the process type. However, for all processes it is typically 

necessary to apply a quench and temper treatment after rolling when tube strength of grade 

65 (min. yield strength of 448 MPa) or higher is to be produced. 

 

The quench and temper treatment is an offline operation where the finished tube is heated 

above 920ºC for 30 minutes to austenitize the steel. The hot tube is then quenched in water 

providing a martensitic microstructure of high hardness and strength. In a subsequent tempering treatment the quenched tube is reheated to above 640ºC for a period of 30 to 60 

minutes to reduce the as-quenched hardness and to increase toughness.

 

It is obvious that such a quench and temper treatment involves additional cost. Furthermore 

in a market situation with increasing demand for high strength tubes the quench and temper 

treatment can become a bottleneck operation either reducing the output of a plant or 

requesting additional capital investment to increase the production capacity. It should thus be 

of high interest to find a metallurgical solution allowing to produce 65 ksi strength without 

having to apply a quench and temper treatment. The cost saving that could be achieved is 

estimated to around 60 $ per ton of tube produced. 

 

Structural hollows are a variant of seamless tubes that come with round, square or rectangular 

cross-section. These are typically used for load bearing applications in steel construction and 

mechanical engineering. According to EN10210-1 structural hollows are standardized for 

minimum yield strength levels of 235, 275, 355 and 460 MPa for wall thickness up to 16 mm. 

Higher wall thickness up to 65 mm is defined with a reduced minimum yield strength. Using 

increased strength in load bearing applications has significant advantages with regard to 

sustainability as is highlighted by Table 1. 

 

Table 1: Advantages of using higher strength structural hollows. 

Criterion S235JR S355J2H S460NH Benefits of strength increase 

Load bearing capacity 100% 151% 197% • Weight reduction 

• Material savings 

• Reduced welding effort 

• Less transport & handling 

• Leaner structure 

• More clearance 

• Architectural esthetics 

Wall thickness for identical 

load bearing capacity 100% 63% 44% 

Cross-section for identical 

load bearing capacity and 

wall thickness 

100% 65% 50% 

 

Besides strength, minimum elongation and impact toughness are specified. Toughness 

requirement is an impact energy of minimum 27 J at either 20°C (JRH), 0°C (J0H) or -20°C 

(J2H). The NH grades require a minimum toughness of 40J at -20°C. The boundaries for 

alloy design and carbon equivalent are indicated in Table 2. Additional restrictions may apply 

to the Si content if the structural hollow has to be galvanized. 

 

Table 2: Chemical composition for NH-grades according to EN10210-1. 

Grade C Si Mn P S Nb V Al Ti Cr Ni Mo Cu N 

S275NH ≤0.20 ≤0.40 0.50 

1.40 

≤0.035 ≤0.030 ≤0.05 

≤0.05 

≥0.02 ≤0.03 ≤0.30 

≤0.30 

≤0.10 

≤0.35 

S355NH ≤0.20 ≤0.50 ≤0.15 0.90 

1.65 ≤0.12 ≤0.50 ≤0.35 

S460NH ≤0.22 ≤0.60 1.00 

1.70 ≤0.20 ≤0.80 ≤0.70 

 CEV WT ≤16mm CEV WT >16 ≤65 mm 

S275NH ≤0.40 ≤0.40 

S355NH ≤0.43 ≤0.45 

S460NH ≤0.53 ≤0.55 

 

 

2. Continuous mandrel mill process and laboratory rolling simulation schedule 

 

The continuous mandrel mill process consists of different rolling steps as schematically 

shown in Figure 1. The reheated billet is hollowed in the piercing mill at high temperature, 

then elongated and further reduced in the continuous mandrel mill. The corresponding rolling

temperatures are typically above TNR providing a recrystallized austenitic microstructure. 

After retraction of the mandrel the shell is transferred to a reheating furnace. During that 

transfer temperature drops to a level between 750 and 650ºC. The actual temperature at which 

the shell is entering the furnace is however not strictly controlled. It is sometimes practiced 

that several shells are collected and transferred in group into the furnace. In that case the first 

shell is rather cold while the last shell is considerably hotter. It can thus occur that 

temperature in individual tubes drops below the Ar3 point. Finish rolling is done in the stretch 

reduction mill after reheating to temperatures between 930 and 900°C for around 15 minutes. 

In this final processing step austenite pancaking may be possible depending on the microalloy 

composition. It is however obvious that the degree of austenite conditioning is less severe 

compared to the possibilities prevailing in strip or plate mills. This implies that grain 

refinement as a key mechanism for strengthening and toughening is less prominent in 

seamless pipe rolling. The tube is cooling down by air-cooling after finish rolling. 

 

 

Figure 1: Schematic process flow of a continuous mandrel mill process with 

typical temperatures and rolling deformations. 

 

The process described above was simulated by a laboratory-scale rolling schedule using flat 

samples to analyze alloy concepts under various temperature and deformation schedules. 

Multiple-pass roughing in a reversing mill stand is representing the piercing and the 

continuous mandrel mill deformations. Finishing was represented by a single pass reduction 

approaching the very short interpass time in the sizing mill. The simulation schedule shown 

in Figure 2 summarizes the variation of different conditions that were applied in rolling trials 

discussed later. Thereby also a temperature drop below Ar3 before reheating was considered. 

After finish rolling all samples were subjected to natural air-cooling at a rate of around 3.5 

K/s. 

 

 

Figure 2: Simulation schedule of continuous mandrel mill process for laboratory rolling.

Shenzhen Sunrise Metal Industry Co.,Ltd

TEL：0086-0755-27185042 
Website：http://www.sunriseta.com
E-Mail：sales@sunriseta.com