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Introduce the processing techniques that affect the surface hardness of stainless steel strips
source：www.sikali.com.cn | Release time：2025年09月23日1、 Core Impact Process 1: Cold Rolling Process (Core Means of Hardness Enhancement)
Cold rolling is a process in the production of stainless steel strip that applies pressure to the hot-rolled strip through a rolling mill to reduce its thickness and densify its surface. By accumulating internal stress through plastic deformation and refining grain size, the surface hardness is significantly improved. The greater the deformation, the higher the hardness (with a synchronous decrease in toughness). According to the different deformation of cold rolling, it can be divided into different processing states, corresponding to significant differences in hardness:
1. The correlation between cold rolling deformation and hardness
The cold rolling deformation of stainless steel strip (usually expressed as "thinning rate", i.e. "(original thickness finished thickness)/original thickness x 100%") directly determines the hardness grade. Taking the mainstream 304 stainless steel strip as an example:
Low deformation amount (≤ 30%): corresponding to the "soft precursor", the surface hardness improvement is limited (HB 150-180, HV 160-190), only the rough surface of the hot-rolled strip is initially eliminated, the internal stress is small, and subsequent annealing needs to be carried out to reduce the soft hardness;
Medium deformation amount (30% -50%): corresponding to the "semi hard state (1/2H)", the surface hardness is significantly increased to HB 200-250 (HV 210-270), and the internal stress is moderate - at this time, the stainless steel strip can maintain a certain toughness (can be bent 90 ° without cracking), and has sufficient strength, suitable for making brackets and buckles;
High deformation (50% -70%): corresponding to the "hard state (H/3/4H)", the surface hardness further increases to HB 280-320 (HV 300-350), the grain size is significantly refined, the internal stress is large - the toughness decreases (only slightly bendable), but it has strong resistance to wear and deformation, suitable for making springs and small blades;
High deformation amount (>70%): corresponding to the "extremely hard state (EH/SH)", the surface hardness reaches HB 350-400 (HV 380-430), and the internal stress is highly concentrated - almost no obvious toughness (easy to bend and crack), only used in scenarios that require high strength (such as high-pressure sealing gaskets, precision spring plates).
2. The influence of key parameters in cold rolling process
Rolling pressure: The greater the pressure applied by the working roll of the rolling mill (usually 1000-3000MPa), the more complete the plastic deformation per unit area, the denser the surface grains, and the more significant the hardness improvement (under the same deformation amount, a 20% increase in pressure can increase hardness by 5% -8%);
Rolling passes: Multi pass cold rolling (such as 3-5 passes) is easier to control hardness uniformity than single pass cold rolling - large deformation in a single pass can easily lead to uneven surface hardness (edge to center difference HB 20-30), while small deformation in multiple passes can control the surface hardness difference within HB 10, suitable for high-precision requirements (such as electronic component tapes);
Work roll roughness: The surface roughness of the rolling mill work roll (Ra 0.1-0.8 μ m) will be "replicated" on the surface of the stainless steel strip - rough roll rolling will increase surface micro cold work hardening (surface hardness is 10% -15% higher than the interior), while fine roll rolling will make the surface smoother and have better hardness uniformity.
2、 Core Impact Process 2: Annealing Process (Balance Valve for Hardness Control)
Annealing is a process that eliminates internal stresses generated by cold rolling and adjusts crystal structure through "heating insulation cooling". Its core function is to "reduce hardness and restore toughness", and it is a key step in achieving "soft/semi hard" stainless steel strips. According to the annealing temperature and cooling rate, it can be divided into different types:
1. Complete annealing (soft state preparation)
Process parameters: Heating temperature 1050-1100 ℃ (higher than the recrystallization temperature of stainless steel), holding for 30-60 minutes, slowly cooling with the furnace (cooling rate ≤ 5 ℃/min);
The impact on hardness: completely eliminating the internal stress of cold rolling, the grains grow and homogenize again, and the surface hardness drops to a lower level - after complete annealing, the hardness of 304 stainless steel strip decreases from hard HB 300+to soft HB 130-150 (HV 140-160), and the toughness is greatly restored (elongation rate increases from 10% to over 40%), which can meet complex forming requirements such as stamping and deep drawing (such as kitchen liner and insulation cup shell).
2. Incomplete annealing (semi hard state/hard state fine-tuning)
Process parameters: Heating temperature 800-950 ℃ (below recrystallization temperature), holding for 15-30 minutes, air cooling or forced air cooling (cooling rate 10-20 ℃/min);
The impact on hardness: Only partial elimination of internal stress, incomplete recovery of grains, hardness between "cold-rolled hard state" and "fully annealed soft state" - for example, after 304 stainless steel strip is cold-rolled to HB 300 and partially annealed at 850 ℃, the hardness can be adjusted to HB 200-250 (semi hard state), which retains a certain strength and has bendability, suitable for making furniture decorative strips and home appliance brackets.
3. Bright annealing (balancing surface and hardness)
Process characteristics: Complete/incomplete annealing is carried out under the protection of inert gases (such as nitrogen and hydrogen) to avoid surface oxidation and regulate hardness;
The impact on hardness: The hardness control effect is consistent with ordinary annealing (soft state HB 130-150, semi hard state HB 200-250), but it can maintain a smooth surface (Ra 0.1-0.4 μ m) without the need for subsequent polishing - suitable for scenarios with high surface quality requirements (such as medical accessories, food contact tapes), ensuring hardness adaptation and reducing processing steps.
3、 Other auxiliary processes (slight impact on surface hardness)
In addition to cold rolling and annealing, some subsequent surface treatment processes may slightly adjust the surface hardness through "micro cold work hardening" or "surface layer removal" (overall impact ≤ 15%, without changing the overall hardness grade):
1. Surface brushed/frosted process
Mechanism of influence: Mechanical polishing of the surface of stainless steel strip is carried out using tools such as sand belt and louver wheel to remove the surface oxide layer and form a "micro plastic deformation layer" (thickness 5-10 μ m) on the surface, further refining the grain size;
Hardness change: The micro hardness of the surface is 10% -15% higher than that of the interior - for example, after drawing the 304 soft state band (internal HB 150), the surface HB rises to 170-180, improving the surface wear resistance (reducing daily friction scratches), but does not change the overall properties of the "soft state" (can still be stamped and formed).
2. Leveling process (minor hardness correction)
Process objective: To use a leveling machine to perform "micro pressure correction" on cold-rolled stainless steel strips, eliminate warping and wave shapes, and ensure flatness;
Hardness effect: The leveling pressure is relatively small (usually 500-1000MPa), only producing a small amount of plastic deformation, and the surface hardness is only increased by HB 5-10 (such as after leveling with 304 semi-rigid strip, HB increases from 220 to 225-230). The main function is to optimize the appearance and flatness, and has no substantial effect on the hardness grade.
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