F Wang and E M Lui*
Department of Civil and Environmental Engineering, Syracuse University, Syracuse, NY 13244-1240, USA
Received Date: December 14, 2016; Accepted Date: December 24, 2016; Published Date: December 30, 2016
Citation: Wang F, Lui EM (2016) Behavior of High Strength Steels under and After High Temperature Exposure: A Review. J Steel Struct Constr 2:123. doi: 10.4172/2472-0437.1000123
Copyright: © 2016 Wang F, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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Fire-resistant and high temperature behavior of high strength steels (HSS) for structural engineering applications has become an important research topic in recent years. In this paper, a succinct review of HSS behavior under and after high temperature exposure is provided. The review addresses the following aspects of HSS: (1) their mechanical properties under and after fire exposure, (2) residual stress in welded HSS sections, and (3) high temperature performance of HSS columns. Recent studies have demonstrated that different grades of HSS can exhibit noticeable differences in their mechanical properties under and after fire exposure, and different cooling methods could have an effect on the post-fire mechanical properties of HSS. Because current design standards for steel structures under elevated temperature were developed based on mild steel, care must be exercised when applying these standards to HSS as they are not necessarily applicable.
High strength steel; Mechanical properties; Elevated temperature; Post-fire; Residual stress
From 1960s to 1990s, ASTM A36 (with a nominal yield strength of 36 ksi or 248 MPa) was the predominant structural steel used for building construction while high-strength low-alloy and quenched and tempered alloy steels (with nominal yield strength that varies from 50 to 100 ksi or 248 to 690 MPa) were used as alternatives for special applications. In the USA, ASTM A992 adopted in 1998 is currently the most commonly used steel for W-shaped sections . High strength steels (HSS), with a nominal yield strength no less than 67 ksi or 460 MPa, is permitted for use under special circumstances, such as for high-rise buildings and long-span bridges. When compared with conventional steel, structures built using HSS offer advantages in increased strength and reduced weight, which could lead to economy in construction. As a result, research on the behavior and application of HSS has become an important topic in the structural engineering community [2-5].
Historical events have clearly demonstrated that fire hazard is a major threat to the structural integrity of a structure throughout its service life. Although most steel structures can withstand a fire or exhibit no visible structural damage after fire exposure, post-fire elements may experience residual stress change and deformations during cooling. Research on HSS behavior both under and after fire exposure has been conducted by various researchers to evaluate the fire-resistance and residual strength of HSS structures. This paper presents a concise review on the behavior of HSS under and after high temperature exposure, including mechanical properties of different types of HSS, residual stress induced in welded HSS sections, and high temperature performance of HSS columns .
Generally, different countries have different notations for designating steel grade. Based on Chinese Standard GB/T 1591-2008, 420 MPa steel is designated as Q420, where the letter Q is the Chinese phonetic alphabet of the word “Qu” meaning steel yield strength and the number 420 is the nominal yield strength in MPa. In Europe, according to EN10025-2004, 420 MPa steel is designated as S420, where S represents structural steel and 420 is the nominal yield strength in MPa .
After the 9/11 attack on the twin towers in New York City, fire resistance of steel structures has become an important research topic in the structural engineering community. Research on the mechanical properties of mild and HSS steels at elevated temperatures has been carried out by a number of researchers. There are two common methods that can be used to test the mechanical properties of steel under elevated temperatures, steady-state and transient-state. In a steady-state test, the test specimen is first heated to a predefined temperature . A tensile load is then applied to the specimen while the temperature is held constant. In a transient-state test, the test specimen is first pre-loaded to a predetermined force. It is then heated slowly to the target temperature. Steady-state tests are more often conducted because they can be performed over a shorter period of time [9-12]. However, transient-state tests tend to produce more realistic results since the effects of creep and relaxation can be accounted for.
A summary of tests for different types of HSS under elevated temperature is given in Table 1.
|Steel type||Test method||Temperature range (°C)||Heating rate (°C/min)||Control parameter|
|Q420 ||Steady||20~600||-||Load: 0.1 kN/s|
|Q460 [3, 4]||Steady||20~800||-||Load: 0.5 kN/s|
|S460M [7-10]||Steady||200~800||-||Strain: 0.002~0.005/min|
|S460N [7-10]||Transient||3, 6, 10, 20, 30||-|
|S460N [11-13]||Steady||20~700||-||Strain: 0.005/min|
|BISPLATE80 ||Steady||22~940||-||Strain: 0.006/min|
|S690QL ||Steady||20~700||-||Strain: 0.005/min|
|RQT-S690 ||Steady||25~800||-||Strain: 0.003/min|
|20 Mn-TiB ||Steady||20~700||-||Strain: 0.1/min|
Table 1 : Summary of tests on HSS at elevated temperature.
The letter M designates thermomechanical rolled steel, N designates normalized rolled steel, Q designates quenched and tempering, L designates low notch toughness testing temperature, and RQT designates reheated, quenched and tempered. BISPLATE 80 is fabricated by an Australian company BISALLOY®, which is somewhat equivalent to ASTM A514 and S690. 20MnTiB is a type of HSS with a yield strength exceeding 940 MPa.
The mechanical properties (elastic modulus, yield strength, tensile strength) of HSS under elevated temperatures can be determined from the stress-strain curves. Since these properties usually degrade as temperature rises, reduction factors are often introduced to represent the change in mechanical properties with temperature.
Tables 2 and 3 provide a summary of reduction factors for elastic modulus obtained for different types of HSS.
|T (°C)||Q460 ||S460N ||S460M ||S460N [11-13]||S690QL ||RQT- S690 |
Table 2 : Summary of reduction factor for elastic modulus.
|T (°C)||BISPLATE80 |
Table 3: Summary of reduction factor for elastic modulus (Cont’d).
According to Tables 2 and 3, the reduction in elastic modulus varies depending on the type of HSS and tests used. Also, different fabrication method and alloy composition will lead to different results. For design purpose, Wang et al.  and Qiang et al.  performed regression analysis on the test results for Q460 and S460N steels and developed equations that can be used to determine ET, the elastic modulus at temperature T (°C), given E20, the elastic modulus at 20°C (room temperature), and T. The equations are given in Table 4. For purpose of comparison, the elastic modulus reduction factors for four HHS (Q460, S460N, S690QL based on steady-state test and BISPLATE80) and those recommended by the American Institute of Steel Construction developed based on tests of mild steel are plotted in Figure 1. As can be seen, they do differ over the range of temperature shown, although the reduction factors for S460N and mild steel are somewhat comparable.
Study on the yield strength of HSS at elevated temperatures has also been conducted. Since most HSS show no obvious yield plateau, the yield strength is determined at an offset of 0.2% strain as per ASTM E21-09 (Table 4).
|Steel type||Empirical equation||Temperature range (°C)|
|Q460 ||20 ≤ T ≤ 800|
|S460N ||20 ≤ T ≤ 900|
Table 4 : Empirical equations for elastic modulus of HSS at elevated temperatures.
In current design standards, the reduction factors for yield strength recommended by the European Steel Design Code (EC3) are based on a strain level of 2.0%, and in the British Standard for Steel Work Design (BS5950) different reduction factors are given based on three strain levels of 0.5%, 1.5% and 2.0%. In American Steel Design Specifications (AISC) and the Australian Standard for Steel Structures Design (AS 4100), no specific strain level is mentioned, but a 0.2% yield strength is assumed. The 0.2% yield strength is the intersection point of the stressstrain curve and a line drawn parallel to the proportional line at a strain value of 0.2%. On the other hand, the yield strength at 0.5, 1.5 and 2.0% strain levels are determined as the intersection point of the stress-strain curve and a vertical line drawn at the specified strain .
Tables 5 and 6 summarize the reduction factors for yield strength obtained for different types of HSS.
|T (°C)||Q460 ||S460N ||S460M ||S460N [11-13]||RQT- S690 |
|2 %||0.2 %||0.5 %||1.5 %||2 %||0.2 %||0.5 %||1.5 %||2 %|
Table 5 : Summary of reduction factor for yield strength.
|T (°C)||S690QL ||T (°C)||BISPLATE80 |
Table 6 : Summary of reduction factor for yield strength(Cont’d).
Because of the blue brittleness effect in the steady-state test of Q460 steel, a small increase in strength and a decrease in ductility were observed. This phenomenon occurred in 200~450°C and resulted in a “reduction factor” larger than 1 at 300°C .
Using regression analysis, empirical equations that relate fyT, the yield strength of HSS at temperature T (°C), and fy, the yield strength at 20°C (room temperature) before the HSS is exposed to high temperature, were developed [3,13] and shown in Table 7.
|Steel type||Empirical equation||T Range (°C)|
|Q460 ||20 ≤ T ≤ 450|
|450<T ≤ 800|
|S460N ||20 ≤ T ≤ 350|
|350 < T ≤ 900|
Table 7 : Empirical equations for yield strength of HSS at elevated temperatures.
The yield strength reduction factors for four HHS (Q460, S460N, S690QL 0.2% yield strength based on state-state test, and BISPLATE80) are compared in Figure 2 to those recommended by the American Institute of Steel Construction developed based on tests of mild steel. As can be seen, noticeable differences are observed for the different types of steel [16-18].
When temperature rises, the tensile or ultimate strength of HSS decreases. However, the effect of tensile strength loss is negligible until the temperature rises above 350°C. Reduction factors for tensile strength are summarized in Table 8 and empirical equations that can be used for design are given in Table 9.
|T (°C)||Q420 ||Q460 ||S460N [11-13]||S690QL ||RQT- S690 ||T (°C)||BISPLATE80 |
Table 8: Summary of reduction factor for tensile strength.
|Steel Type||Empirical equation||Temp. Range (°C)|
|S460N ||20 ≤ T ≤ 350|
|350<T ≤ 900|
Table 9 : Empirical equations for tensile strength of HSS at elevated temperatures.
In the above Table, fuT is the tensile strength at temperature T (°C) and fu is the tensile strength at 20°C before the HSS is exposed to high temperature.
In Figure 3, the tensile strength reduction factors for four HHS (Q460, S460N, S690QL based on steady-state test, BISPLATE80) are compared to those recommended by the American Institute of Steel Construction developed based on tests of mild steel. As can be seen, except for Q460, the reduction factors for other HSS are generally lower than those for mild steel when the temperature exceeds 300°C.
*****Generally, two methods can be used to conduct cooling tests on steel after exposure to elevated temperature. They are the aircooling and water-cooling methods. Of the two, the water-cooling method is more realistic. Wang et al.  showed that the use of water cooling had a dramatic influence on the post-fire tensile strength and elongation of the test specimens. Table 10 summarizes the post-fire tests on some HSS.
|Steel type||Temperature range (°C)||Test method||Heating rate (°C/min)||Constant T. Duration (min)||Cooling method||Control parameter|
|Q460 ||20~900||Steady||15||20||Air/ Water||Elastic stage: 10MPa/s|
|Yield stage: 0.001/s|
|Hardening stage: 10 mm/min|
Table 10 : Summary of post-fire tests on HSS.
Using regression analysis, Wang et al.  proposed empirical equations for determining post-fire mechanical properties of Q460 steel. Depending on the type of cooling used, two sets of equations are proposed. They are shown in Table 11.
|Temperature range 20°C~900°C|
|Air cooled||Water cooled|
Table 11 : Empirical equations for post-fire mechanical properties of Q460 steel.Qiang et al. [19,20] pointed out that when the temperature was below 600°C the post-fire mechanical properties loss of S460, S690 and S960 were negligible. Furthermore, all test specimens showed ductile failure with necking and no brittle failure was observed. Empirical equations for post-fire mechanical properties of these HSS were developed and they are summarized in Tables 12-14.
|Empirical equations||Simplified equations|
|20 ≤ T ≤ 600°C||20 ≤ T ≤ 1000°C|
|600<T ≤ 800°C|
|800<T ≤ 1000°C|
|20 ≤ T ≤ 800°C||20 ≤ T ≤ 1000°C|
|800<T ≤ 1000°C|
|20 ≤ T ≤ 750°C||20 ≤ T ≤ 1000°C|
|750 < T ≤ 1000°C|
Table 12: Empirical and simplified equations for post-fire mechanical properties of S460 steel.
|Empirical equations||Temperature range (°C)|
|20 ≤ T ≤ 600|
|600<T ≤ 1000|
|20 ≤ T ≤ 650|
|650<T ≤ 1000|
|20 ≤ T ≤ 600|
|600 < T ≤ 1000|
Table 13: Empirical equations for post-fire mechanical properties of S690 steel.
|Empirical equations||Simplified equations||Temperature range (°C)|
|20 ≤ T ≤ 600|
|600<T ≤ 1000|
|20 ≤ T ≤ 600|
|600 <T ≤ 1000|
|20 ≤ T ≤ 600|
|600 < T < 800|
|800 ≤ T ≤ 1000|
Table 14 : Empirical and simplified equations for post-fire mechanical properties of S960 steel.
Residual stress is developed as a result of uneven cooling of the different parts of the cross-section during the fabrication process. The presence of residual stress could result in early yielding and reduction in stiffness. While residual stress of normal strength hot-rolled and welded steel sections has been widely studied, the same cannot be said for HSS.
Wang et al.  studied three welded flame-cut Q460 HSS H-section members with three different width-to-thickness ratios, 3.4, 5 and 7.1. Ban et al.  and Yang et al.  conducted a similar study with a larger range of width-to-thickness ratios on 460 MPa HSS welded I-shaped members and Q460GJ HSS welded I-shaped members, respectively. The residual stress distribution they obtained was found to be similar to that of mild steel with lower magnitudes and was related to section dimensions. Furthermore, Kim et al.  tested 800 MPa HSS welded box-, cruciform- and H-sections, and Li et al.  provided information on the magnitude and distribution of residual stresses for box- and H-sections made of Q690 steels.
However, it should be noted that the investigation on the magnitude and distribution of residual stress for post-fire HSS welded section members is rather limited. Wang et al. [26,27] performed residual stress tests on welded Q460 H-sections after fire exposure, shown in Table 15, and found that the magnitude of post-fire residual stress decreased significantly with an increase in temperature.
|Steel properties||Welding details||Section dimension (mm)||Heated temperatures (°C)|
|E=208.5 GPa fy=538.1 MPa fu=611.1 MPa||Fillet welds with 8 mm leg size CO2 shielded arc welding Voltage=25V and Amps=230A Welding speed=35 cm/min Filler wire type is JM-60, with fy=545 MPa and 25% elongation after fracture||Flame-cut 200 × 200 × 8 × 8||200/400/600/800 Natural air cooling|
Table 15 : Residual stress tests on post-fire HSS Welded H-sections (Wang [26,27]).
Valente and Neves , Rodrigues et al.  and Tan et al.  studied the fire resistance of mild steel columns and found that the presence of axial restraint would decrease the critical temperature, which is the temperature at which failure of the member occurs. Wang and Ge  conducted a similar research on four Q460 H-shaped columns using two levels of axial constrained stiffness and two levels of axial load ratio. The test results, given in Table 16, show that for a given constrained stiffness, the critical temperature decreases when the axial load ratio increases, or for a given axial load ratio, the constrained stiffness needs to be increased to maintain the critical temperature. Using finite element analysis, Ge and Wang  compared the inelastic strength of Q460 with Q235 steels shown in Table 17, and demonstrated the beneficial effect of using higher strength steel to counteract the loss of inelastic stability caused by the larger slenderness ratio of HSS.
|Specimen Labels||Method||Mechanical properties||Length||Section type||Section size (mm)||Axial load ratio||Axial restrained ratio (%)||Critical temp. (°C)|
|S1||ISO-834 Increasing temperature under constant load||8 mm Steel Plate E=212 GPa Fy=585 MPa Fu=660 MPa||4.3 m||Welded H-shaped||H300 × 150 × 6.5 × 9||0.25||9.4||620|
|S3||H200 × 150× 6 × 9||0.26||3.8||625|
Table 16 : Tests on Q460 H-shaped axially restrained columns for critical temperature (Wang and Ge ).
|Specimen labels||Steel type||Element type||Dimension (mm)||Load ratio||Slenderness ratio||Restrained ratio (%)||Critical Temperature (°C)|
|1||Q460||PLANE82 BEAM188 COMBINE14||Length: 3000 Section: H200x150x6x9||0.3||60||2.5||714|
Table 17: Finite element analysis on critical temperature of axially restrained columns (Ge and Wang ).
Wang et al.  tested twelve welded H-shaped Q460/Q235 steel stub columns given in Table 18 under axial compression with the objective of studying the local instability behavior at different elevated temperatures. The failure modes of all the specimens were local buckling, which are similar to those under room temperature.
|Specimen labels||Temperature (°C)||Section dimension (mm)||Study objective||Test Yield strength (MPa)||Test buckling stress (MPa)|
|Q235A-1||25||H250 × 250 × 6 × 8||Flange local buckling||306.3||240.6|
|Q460A-1||25||H250 × 220 × 8 × 8||538.1||391.9|
|Q235B-1||25||H316 × 200 × 6 × 8||Web local buckling||321.9||192.6|
|Q460B-1||25||H336 × 160 × 8 × 8||538.1||356.4|
Table 18: Stability analysis of Welded H-shaped Stub Columns under axial compression at elevated temperatures(Wang et al. ).
From Table 18, it can be seen that the decrease of buckling strength is occurring at a higher rate than yield strength. This is because inelastic buckling is a function of both yield strength and stiffness. Since both are decreasing with an increasing temperature, their combined effect is manifested in the noticeable reduction in inelastic buckling strength.
Using the finite element software ABAQUS, Chen and Young  analyzed several HSS box and I-section columns (Table 19) at elevated temperatures, and concluded that while the current AISC specification conservatively predicted the behavior of HSS columns at elevated temperatures, it gave unreliable results when the temperature was raised beyond 700°C.
|Element type||Mesh size||Boundary conditions||Column size||Analysis method|
|S4R5||10 mm × 10 mm||Fixed-end Pinned-end||Stub Slender||Step 1: Eigenvalue analysis (linear and elastic) Step 2: Load-displacement nonlinear analysis|
Table 19: Numerical analysis of HSS Box and I-section columns at elevated temperatures (Chen and Young ).
In this paper, test results on the mechanical properties, residual stress and compressive strength of HSS under and after fire exposure are reviewed. Empirical equations that can be used to determine the elastic modulus, yield strength and tensile strength of HSS under elevated temperature and after they have been cooled down are summarized.
Based on test results on the behavior of HSS under and after high temperature exposure, the following conclusions can be drawn:
1. The mechanical properties of HSS under elevated temperature do not show appreciable decrease until the temperature reaches 300°C.
2. The blue brittleness effect is observed on lower strength HSS tested under steady-state condition from 200°C to 450°C.
3. The reduction factors for elastic modulus, yield strength and tensile strength are different for different steel grades. The recommended reduction factors in various steel design standards were obtained based on tests of mild steel and so they should not be used for the design of HHS.
4. The post-fire mechanical properties loss of S460, S690 and S960 are negligible for temperature below 600°C. Also, ductile failure with necking was observed during the test.
5. The type of cooling method used can affect the results, and so different empirical equations should be used for design.
6. The steel grade and alloy compositions can have a significant influence on both the during-fire and post-fire performance of HSS.
7. The residual stress distribution of HSS welded sections is similar to that of mild steel but with lower magnitudes. The residual stress magnitude of post-fire HSS welded sections tends to decrease with an increase in temperature.
8. Similar to columns made from mild steel, for a given constrained stiffness the critical temperature of HSS columns decreases when the axial load ratio increases. And for a given axial load ratio, the constrained stiffness needs to be increased to maintain the critical temperature.
9. As for the local instability behavior, the failure modes of HSS columns tested at different elevated temperatures are similar to those under room temperature. In addition, as temperature increases, the rate of decrease for buckling strength is faster than that for yield strength because inelastic buckling strength at elevated temperature is affected by a simultaneous reduction in yield strength and stiffness of the test specimens.
Although study on the behavior of HSS at elevated temperature has been carried out by a number of researchers at both the material and structural levels, there are no standardized test methods and so the results are not always comparable. More importantly, current research on the behavior of HSS under elevated temperature is still at a stage when it is not yet ready for incorporation into steel design standards.
Research on post-fire behavior of HSS is quite limited and current standards do not contain sufficient guidelines on how the residual capacity of HSS after fire exposure can be evaluated. In addition, because the manner of how the test specimens are cooled could influence their post-fire mechanical properties, more systematic study on the post-fire mechanical properties of HSS needs to be performed and the influence of different cooling methods on the post-fire behavior of HSS needs to be considered.
For steel structures, the presence of residual stress in welded builtup members is an important design parameter to consider as it affects the inelastic behavior of the members. Due to the difference in strength between mild steel and HSS, the residual stress in HSS sections tends to be less detrimental to member strength. However, because both the magnitudes and distributions of residual stress could undergo noticeable changes after fire, additional study beyond those reported by Wang et al. [26,27] on post-fire effect of residual stress on HSS sections needs to be carried out.