Introduction account for 80.0 percent of total rebar output, and reach 65.0 percent in construction engineering;

1. Introduction account for 80.0 percent of total rebar output, and reach 65.0 percent in construction engineering;

With progressive urbanization and industrialization  HRB500 will be made as priority for large-sized happening over the past ten years, production output

and large-span public building;

of rebar products has reached and maintained 200  Launch research for HRB600 grade; million tons scale since 2013, accounting for about  For earthquake-prone areas, anti-seismic rebars

25.0 percent of total crude steel output in China, as with higher tensile-to-yield ratio and uniform shown from Fig. 1. On the other hand, China’s two elongation should be recommended firstly. ministries, Ministry of Industry and Information As we can see from Fig. 2, high strength rebars Technology and Ministry of Housing and Urban-Rural have surpassed 85.0 percent by 2015, and reached Development of the People’s of the People’s Republic

94.0 percent in 2017. China’s steel industry of China, established coordination group with related experienced the worst situation in 2015 since reform departments during the 12th five-year plan to promote and openness, so steel mills started to remove high strength and earthquake resistant rebars, and then expensive alloy elements like V, Nb and Mo for profit. released guidance for accelerating application of high For rebar segment, some rebar makers started to adopt strength rebars. The specific aim and schedule are remained heat treatment process to produce hot rolled shown below: high-strength rebars for cost saving, which was not  By 2013, HRB335 will be completely eliminated consistent with requirements of GB1499.2-2018 in building engineering; standard. In order to ensure safety for civil and public  By 2015, the output of high strength rebars will construction, China Metallurgical Information and

Corresponding author: Zhang Yongqing, master, senior Standardization Institute organized a round table

manager, research fields: materials processing engineering. meeting to add new clauses for microstructure and Email: Zhangyq@citic.com micro-hardness testing as arbitration.

Strengthen ning Effects o of Niobium o on High Stren gth Rebars

Fig. 1 Statis stical data of re ebar products in China.

Fig. 2 Reba r structure evo olution over pa ast years.

2. Interpre etation of G GB 1499.2- 2018

  Removed H HRB335 grad de;  Added HRB  B600 grade;

GB 1499 .2-2007 “Ste eel for the R Reinforcemen nt of Concrete-Pa art 2: Hot Rolled rib bbed Bars” ” is

 Added req  quirements o f metallurgic cal structure e correspondin ng to the international l standard ISO

test ting and speci ifications;

 Added tes  sting metho ods for ma acrostructure, , Concrete-Pa art 2: Ribbed Bars”, but it has own feat tures,

6935-2:1991 1 “steel for Rein nforcement

of

sect tion Vichers h hardness and microstructu ure. for exampl le, requirem ment on mic crostructure and

Table 1 show T ws basic req quirements o on chemical l production p processing. C Compared wi th old versio on of

com mpositions an nd tensile test t for hot rolle ed rebars. As s GB1499.2-2 2007, new version of GB1499.2-2 2018

for rebars app plied in sei ismic prone e areas, the e removed low wer grade o f HRB335, and at the s same

foll lowing requir rements shou uld be conform med: (1) The e time added d HRB600 and some r requirements on

rati o of actual te ensile strengt th to actual y yield strength h microstructu ure and hard dness for safe ety. The spe cific

sho uld not be l ess than 1.25 5; (2) The ra atio between n items added d for new ve ersion of GB B1499.2-2018 8 are

actu ual measured yield strengt th and minim mum specified d shown as fol llows:

yiel ld strength va alue should n not be higher than 1.30; (3 3)  Specify y steelmak king metho od to res strict

The e general ma aximum forc ce extensibil lity rate Agt t nonstandard d steel;

sho ould not be less than 9 %. Among above three e

84 Strengthening Effects of Niobium on High Strength Rebars

Table 1 Basic requirements for hot rolled rebars.

Tensile properties

Maximum addition, wt%

Grade Rel, MPa

S Ceq HRB400

0.54 HRB400E

-- 9.0

HRBF400E

0.55 HRB500E

-- 9.0

HRBF500E HRB600 600

Fig. 3 Typical macroscopical metallography picture.

hardness testing is needed as arbitration. If hardness gap between middle and surface is more than 40, it is unacceptable. These changes would have big impact on production process of high strength rebars and alloy market price.