Questions about metal materials

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Let me first describe the problem: at a design pressure of 10 MPa or higher, if ordinary container steel plate Q345R is used for rolling, followed by hole drawing, the resulting thickness turns out to be very large. Therefore, in order to meet the project requirements, pipeline steel has been chosen for this process. Propane is used to heat the material to 950 degrees Celsius for hole drawing, after which it is cooled in air. However, it was found that the tensile strength and yield strength of the resulting material decrease significantly. A metallographic analysis of the material showed no difference before and after drawing, which implies that the carbon content decreased during the heating process. I would like to ask: are there any ways to prevent such a significant drop in tensile strength and yield strength? Can using other gases for heating reduce decarburization? Pipeline steel is already normalized; if performance-restoring heat treatment is carried out after it has been manufactured, the process itself is not very mature, and it is possible for the necking area to deform during heat treatment, which is also a problem. I’m asking the experts on this forum – do you have any ideas? (Drawing is similar to using tees, as this helps to reduce various types of stress better. The reason for not using tees directly is that they require two additional ring welds. The openings in each project are created according to the actual layout of the pipes, and tees do not meet these requirements; therefore, the drawing method is employed for creating the openings.) ）