Chromium is the main alloying element of 301 stainless steel strip. Chromium is an element that stabilizes α-iron and narrows the γ-iron zone. The Ac3 point of pure iron is around 910 degrees; increasing the chromium content to ~8% will lower this point to 850 degrees. Continue to increase the chromium content, and the Ac3 point will rise rapidly. On the other hand, the Ac4 point of pure iron is about 1400 degrees. This point decreases with the increase of chromium content, and drops to 1000 degrees when it contains 12-13% Cr. Therefore, when the chromium content is 12-13%, the γ zone is closed. It will be shown later that this limiting content of chromium (12-13%) will vary with the carbon content. The gamma loop divides the state diagram into two parts. Alloys containing less than 12% chromium undergo α-γ transformation when heated and γ-α transformation occurs when cooled. Alloys containing more than 13% chromium do not undergo any phase transformation and maintain the α phase structure throughout the temperature range, thus forming a special type of alloy whose grain size cannot be changed by heat treatment.

When the content is slightly less than 12-13%, the alloy has an α+γ dual-phase structure during heating to 1000 degrees. In alloys containing ~45% Cr, the so-called σ phase appears at low temperatures. The σ phase is a FeCr compound that dissolves into chromium alpha ferrite when the temperature is higher than 820 degrees. In ferritic steels with various chromium contents, this phase exists except for ferrite, but the upper and lower concentration limits of its existence have not been determined accurately enough. The presence of σ phase in ferritic chromium steel will make the steel brittle. For example, this kind of brittleness may occur if the material is held in the 600-800 degree temperature range for a sufficiently long time, often accompanied by a significant increase in hardness. The above are the characteristics of carbon-free iron-chromium alloys. The presence of carbon changes the shape of the alloy state diagram. The influence of carbon first moves the γ zone boundary to the side with higher chromium content, and at the same time expands α+γ (ferrite + austenite). When the surface content is 0.6% (the chromium content is 18% at this time), the boundary of the alloy with pure austenite structure moves to the right to reach the maximum value. Any higher carbon content will increase the amount of free cementite. When the chromium content is high (to 26-27%), the alloy has a ferrite + austenite mixed structure when heated. Alloys containing more than 27% chromium.