## 1 Introduction  
With increasing demand for energy saving and reducing weight of commercial vehicles such as dump and cargo trucks in the transportation sector, high-strength wear-resistant steels have drawn more and more attention in the box materials of the trucks due to their thin specification and good processability [1; 2; 3]. The yield strength of the steels used in the box materials of some engineering dump trucks is increasing to 1100-1300 MPa [3]. Thin specification products with high strength by direct quenching process will meet the requirements for weight reduction of commercial vehicles and environmental protection and will be demanded in the future [4; 5; 6]. The thin specification NM450 TP wear-resistant steel produced by direct quenching process is a type of ferrite and martensite dual phase steel. The direct quenching process has significant advantages in lowering manufacture cost, simplifying processes and obtaining thin specification and high-strength wear resistant steels [7; 8].  
NM450 TP wear-resistant steel needs to undergo bending and welding processes before it is used as the box material of trucks. It is well known that the microstructures and mechanical properties of the steels have great influence on the formability and weldability during bending and welding processes, the outer arc of the bending plate sustains tension stress during the bending processing, and the initial state of the steel also has great effect on the microstructure evolution during the welding process [9]. Local imperfections such as structure inhomogeneity through the thickness of the steel plate would result in poor quality and the uncertainty of bending and welding processing. Understanding the effect of local material microstructure and property fluctuation on the bending and welding properties is not only important for more effective processing, but also critical for parameter optimizing during fabrication of the material. However, due to the limited understanding of the microstructural inhomogeneity of the NM450 TP wear-resistant steel, the microstructures are regarded as a uniform state and as a solely martensite state during the previous bending and welding investigation [9; 10]. Detailed study investigating the relationship between the microstructure and mechanical behavior of the NM450 TP steel is needed.  
Comparing to the traditional ferritin and martensitic dual phase steels which consist of soft ferrite and hard martensite matrix [11; 12; 13], NM450 TP wear-resistant steel otherwise consists of majorities of martensite structures by direct quenching, their morphologies through the thickness are easily affected by the rolling process parameters and quenching cooling conditions [14; 15; 16]. At the same time, because as-quenched steel plates are directly used in the subsequent bending and welding process, the microstructure inhomogeneity of NM450 TP steel plate does not have a chance to be modified by the process such as annealing and tempering [17; 18]. Due to the inhomogeneity of martensite through the thickness of the plate retained after direct quenching, this eventually leads to different mechanical behavior through the thickness and finally affects the formability and weldability of the steel plates. However, there has been limited study on the relationship between microstructure inhomogeneity and mechanical behavior of the steels.  
The objectives of this work are to, through a quantitative characterization of the microstructures and mechanical properties in an as-quenched NM450 TP steel plate, to investigate the factors influencing microstructure distribution and analyze the relationship between microstructure and mechanical properties. The results will have a significant effect on the subsequent investigation and application of the bending and welding process, and further improve the fundamental understanding and optimization of fabricating parameters of this high-strength steel.