Behavior of High Strength Steels under and After High Temperature Exposure: A Review

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.


Introduction
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 [1]. 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][3][4][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 [6].

Steel Grade Representation
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 [7].

Behavior of HSS under Elevated Temperature
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 [8]. 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][10][11][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.
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

Yield Strength
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).
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 [15]. Tables 5 and 6 summarize the reduction factors for yield strength obtained for different types of HSS.
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 [3].
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.

Elastic Modulus
Tables 2 and 3 provide a summary of reduction factors for elastic modulus obtained for different types of HSS.
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. [3] and Qiang et al. [13] performed regression analysis on the test results for Q460 and S460N steels and developed equations that can be used to determine E T , the elastic modulus at temperature T (°C), given E 20 , 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.   Using regression analysis, empirical equations that relate f yT , the yield strength of HSS at temperature T (°C), and f y , 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.
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][17][18].

Tensile (or Ultimate) Strength
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.
In the above Table, f uT is the tensile strength at temperature T (°C) and f u 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.

Post-fire Behavior of HSS
*****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. [18] 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.
Using regression analysis, Wang et al. [18] 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.
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.

Residual Stress of HSS
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. [21] 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. [22] and Yang et al. [23] 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. [24] tested 800 MPa HSS welded box-, cruciform-and H-sections, and Li et al. [25] 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.

Behavior of HSS Columns under Elevated Temperature
Valente and Neves [28], Rodrigues et al. [29] and Tan et al. [30] 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 [31] 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 [32] 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.
Wang et al. [33] 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.
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 [34] 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

Temperature range 20°C~900°C
Air cooled Water cooled 10   temperatures, it gave unreliable results when the temperature was raised beyond 700°C.

Summary and Conclusions
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:     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

Empirical equations Simplified equations
Temperature range (°C) 10     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.

Recommendations for Further Research
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   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 [34]).
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.