Soft Skin Rolled Temper: Key Process for Enhanced Steel Surface Properties

Definition and Basic Concept

Soft Skin Rolled Temper refers to a specific metallurgical condition in steel sheet products characterized by a controlled reduction in thickness through cold rolling, resulting in a moderately work-hardened surface while maintaining a relatively soft core. This temper condition represents an intermediate state between fully annealed and quarter-hard tempers, typically achieved through light cold reduction (approximately 0.5-1.5%) after annealing.

The importance of Soft Skin Rolled Temper lies in its ability to provide improved surface finish and flatness while maintaining excellent formability characteristics. This balance makes it particularly valuable in applications requiring both aesthetic quality and good forming behavior.

Within the broader field of metallurgy, Soft Skin Rolled Temper occupies a specialized position in the spectrum of steel conditioning treatments. It represents a deliberate compromise between the maximum ductility of fully annealed material and the increased strength but reduced formability of more heavily cold-worked tempers.

Physical Nature and Theoretical Foundation

Physical Mechanism

At the microstructural level, Soft Skin Rolled Temper creates a gradient of dislocation density from the surface to the core of the steel sheet. The light cold rolling process introduces dislocations primarily near the surface layers, creating a higher dislocation density in these regions compared to the interior.

This selective work hardening occurs because the surface experiences the highest strain during the rolling process. The increased dislocation density at the surface impedes further dislocation movement, resulting in a slightly harder surface layer while the core maintains characteristics closer to the annealed state.

The controlled introduction of dislocations also helps eliminate yield point elongation (YPE) by providing mobile dislocations that prevent discontinuous yielding behavior during subsequent forming operations.

Theoretical Models

The primary theoretical model describing Soft Skin Rolled Temper is the strain gradient plasticity theory, which accounts for the heterogeneous distribution of plastic deformation through the thickness of the material. This model recognizes that geometrically necessary dislocations develop in proportion to strain gradients.

Historically, understanding of skin-rolling effects evolved from empirical observations in the mid-20th century to more sophisticated models by the 1970s. Early steel producers recognized the benefits of light cold reduction on surface quality and formability before the underlying mechanisms were fully understood.

Modern approaches incorporate crystal plasticity finite element modeling (CPFEM) to predict the effects of skin rolling on texture development and mechanical property gradients. These models are complemented by dislocation-based hardening theories that connect microstructural evolution to macroscopic mechanical behavior.

Materials Science Basis

The effectiveness of Soft Skin Rolled Temper relates directly to the face-centered cubic (FCC) crystal structure of austenitic steels or the body-centered cubic (BCC) structure of ferritic steels. The rolling process induces crystallographic texture and preferential dislocation arrangements along specific slip systems.

Grain boundaries play a crucial role in the response to skin rolling, as they act as barriers to dislocation movement. The interaction between dislocations and grain boundaries contributes to the overall hardening behavior, with finer-grained materials typically showing more pronounced skin-rolling effects.

This temper condition exemplifies the fundamental materials science principle of structure-property relationships, where controlled processing creates specific microstructural features that directly translate to desired mechanical properties and performance characteristics.

Mathematical Expression and Calculation Methods

Basic Definition Formula

The degree of skin rolling is typically quantified by the skin pass reduction ratio:

$R_{sp} = \frac{t_i - t_f}{t_i} \times 100\%$

Where:
- $R_{sp}$ = Skin pass reduction ratio (%)
- $t_i$ = Initial thickness before skin rolling (mm)
- $t_f$ = Final thickness after skin rolling (mm)

Related Calculation Formulas

The resulting yield strength increase due to skin rolling can be estimated using the empirical relationship:

$\Delta\sigma_y = K \times (R_{sp})^n$

Where:
- $\Delta\sigma_y$ = Increase in yield strength (MPa)
- $K$ = Material-specific constant (typically 50-150 MPa)
- $n$ = Strain hardening exponent (typically 0.3-0.5 for low carbon steels)

The surface roughness after skin rolling can be predicted by:

$R_a = R_{a0} \times e^{-\alpha R_{sp}}$

Where:
- $R_a$ = Final arithmetic average roughness (μm)
- $R_{a0}$ = Initial surface roughness before skin rolling (μm)
- $\alpha$ = Surface smoothing coefficient (typically 0.8-1.2)

Applicable Conditions and Limitations

These formulas are generally valid for skin pass reductions between 0.3% and 2.0%. Beyond this range, non-linear effects become significant and more complex models are required.

The mathematical models assume uniform deformation across the width of the sheet. Edge effects and thickness variations can cause deviations from predicted values, particularly in wide sheets.

These relationships are developed for low and medium carbon steels at room temperature. High-alloy steels, specialty grades, or elevated temperature applications may require modified coefficients or alternative models.

Measurement and Characterization Methods

Standard Testing Specifications

ASTM A1030: Standard Practice for Measuring Flatness Characteristics of Steel Sheet Products - Covers procedures for measuring flatness in skin-rolled products.

ISO 6892-1: Metallic Materials - Tensile Testing - Provides standardized methods for determining mechanical properties affected by skin rolling.

ASTM E517: Standard Test Method for Plastic Strain Ratio r for Sheet Metal - Essential for evaluating formability characteristics of skin-rolled sheet.

ASTM E8/E8M: Standard Test Methods for Tension Testing of Metallic Materials - Defines procedures for measuring tensile properties influenced by skin rolling.

Testing Equipment and Principles

Tensile testing machines with extensometers are the primary equipment for evaluating mechanical property changes induced by skin rolling. These systems measure stress-strain relationships under controlled loading conditions.

Surface profilometers quantify roughness parameters before and after skin rolling. These instruments use either contact stylus methods or non-contact optical techniques to map surface topography.

X-ray diffraction systems measure residual stress distributions and crystallographic texture changes resulting from skin rolling. These techniques analyze diffraction patterns to determine lattice strain and preferred orientation.

Advanced characterization may employ electron backscatter diffraction (EBSD) to map grain orientation and dislocation density gradients through the sheet thickness.

Sample Requirements

Standard tensile specimens follow ASTM E8 dimensions, typically with 50mm gauge length for sheet materials. Specimens must be cut with their axis either parallel or perpendicular to the rolling direction.

Surface roughness measurements require minimum sample dimensions of 50mm × 50mm with clean, representative surfaces free from handling damage or contamination.

Residual stress measurements typically require samples at least 10mm × 10mm, with larger dimensions preferred to capture the full stress distribution pattern.

Test Parameters

Tensile testing is conducted at room temperature (23 ± 5°C) with relative humidity below 70% to prevent environmental effects on results.

Standard strain rates for tensile testing range from 0.001 to 0.008 min⁻¹ in the elastic region, with potential increases to 0.05 to 0.5 min⁻¹ after yielding.

Surface roughness measurements should be performed with a cutoff length appropriate to the expected roughness scale, typically 0.8mm for skin-rolled sheet products.

Data Processing

Raw force-displacement data from tensile tests are converted to engineering stress-strain curves, from which yield strength, tensile strength, and elongation values are extracted.

Statistical analysis typically involves multiple specimens (minimum of three) with calculation of mean values and standard deviations for each property.

Surface roughness parameters are calculated from primary profile data using standardized filtering algorithms to separate waviness from roughness components.

Typical Value Ranges

Steel Classification	Typical Value Range (% Reduction)	Test Conditions	Reference Standard
Low Carbon Steel (Commercial Quality)	0.8-1.5%	Room temperature, 0.005 min⁻¹ strain rate	ASTM A1008
Low Carbon Steel (Drawing Quality)	0.5-1.0%	Room temperature, 0.005 min⁻¹ strain rate	ASTM A1008
High Strength Low Alloy (HSLA)	0.3-0.8%	Room temperature, 0.005 min⁻¹ strain rate	ASTM A1011
Interstitial-Free (IF) Steel	0.5-1.2%	Room temperature, 0.005 min⁻¹ strain rate	ASTM A1008

Variations within each classification typically result from differences in base material thickness, prior processing history, and specific end-use requirements. Thinner gauges generally require lower reduction percentages to achieve equivalent surface properties.

In practical applications, these values should be interpreted as guidelines rather than absolute requirements. The optimal skin pass reduction depends on the specific forming operations planned and the surface finish requirements.

A notable trend across steel types is that higher-strength grades typically require lower skin pass reductions to achieve similar surface improvements, reflecting their greater resistance to deformation.

Engineering Application Analysis

Design Considerations

Engineers must account for the slight increase in yield strength (typically 10-30 MPa) resulting from Soft Skin Rolled Temper when performing forming simulations and die design calculations.

Safety factors for formability typically range from 1.2 to 1.5 when working with skin-rolled materials, with higher factors applied when complex forming operations are required or when material properties show significant variability.

Material selection decisions often favor Soft Skin Rolled Temper when both surface quality and formability are critical requirements, as in visible automotive panels or appliance housings.

Key Application Areas

The automotive industry extensively utilizes Soft Skin Rolled Temper for outer body panels, where the improved surface finish reduces painting defects while maintaining the formability needed for complex shapes.

Appliance manufacturing represents another major application area, with different requirements focusing on consistent flatness for large panels and resistance to stretcher strains during forming operations.

Packaging applications, particularly food containers and aerosol cans, benefit from the improved surface cleanliness and uniform forming behavior of skin-rolled materials, ensuring consistent production quality.

Performance Trade-offs

Increasing the skin pass reduction improves surface finish and flatness but reduces overall formability. This trade-off must be carefully balanced based on the complexity of forming operations required.

Soft Skin Rolled Temper slightly reduces deep drawing performance compared to fully annealed material, requiring design adjustments such as increased corner radii or additional draw beads in complex forming operations.

Engineers often balance these competing requirements by specifying the minimum skin pass reduction that achieves the required surface quality, thereby preserving maximum possible formability.

Failure Analysis

Stretcher strains (Lüders bands) represent a common failure mode in insufficiently skin-rolled materials, appearing as visible surface defects during forming operations.

These failures progress from localized yielding phenomena, where discontinuous yielding behavior creates bands of localized deformation that manifest as visible surface imperfections.

Proper specification of skin rolling parameters, combined with appropriate forming lubricants and die design, can effectively mitigate these risks by ensuring sufficient mobile dislocations to prevent discontinuous yielding.

Influencing Factors and Control Methods

Chemical Composition Influence

Carbon content significantly affects the response to skin rolling, with higher carbon levels requiring greater reduction to eliminate yield point phenomena but risking excessive hardening.

Trace elements such as nitrogen and boron can dramatically influence aging behavior after skin rolling, potentially causing return of yield point elongation during storage.

Compositional optimization typically focuses on minimizing elements that promote strain aging while maintaining elements necessary for other required properties.

Microstructural Influence

Finer grain sizes enhance the effectiveness of skin rolling by providing more grain boundaries to interact with dislocations, allowing lower reduction percentages to achieve desired surface properties.

Phase distribution in dual-phase or multi-phase steels creates complex responses to skin rolling, with harder phases experiencing less deformation than softer phases, resulting in heterogeneous property development.

Inclusions and other defects can cause localized stress concentrations during skin rolling, potentially leading to surface defects or inconsistent mechanical properties.

Processing Influence

Prior annealing treatment directly affects skin rolling results, with fully recrystallized structures responding more predictably than partially recovered microstructures.

Roll surface finish transfers directly to the product during skin rolling, making roll maintenance and surface quality critical process parameters.

Cooling rates after skin rolling influence the stability of the created dislocation structure, with faster cooling generally preserving more of the skin rolling effect.

Environmental Factors

Elevated temperatures during storage or shipping can promote strain aging in skin-rolled materials, potentially restoring yield point phenomena that the skin rolling was intended to eliminate.

Humid environments may accelerate aging effects, particularly in steels with elevated nitrogen or carbon in solid solution.

Time-dependent relaxation of residual stresses can gradually reduce some benefits of skin rolling, particularly flatness improvements, if materials are stored for extended periods before forming.

Improvement Methods

Microalloying with small additions of titanium or niobium can stabilize interstitial elements, reducing susceptibility to strain aging after skin rolling.

Optimizing roll textures during skin rolling can enhance both surface appearance and tribological properties during subsequent forming operations.

Designing forming operations with initial strains exceeding the Lüders strain can ensure that any tendency toward discontinuous yielding is overcome in non-visible areas of the formed part.

Related Terms and Standards

Related Terms

Temper Rolling refers to a similar process but typically implies somewhat higher reduction percentages (1-5%) and is focused more on mechanical property modification than surface quality.

Strain Aging describes the time-dependent return of yield point phenomena after deformation, which skin rolling is partially intended to prevent or delay.

Lüders Bands (stretcher strains) are visible surface defects resulting from localized deformation during discontinuous yielding, which proper skin rolling helps prevent.

Skin rolling is closely related to, but distinct from, tension leveling, which uses pure tensile deformation rather than rolling to improve flatness and eliminate yield point elongation.

Main Standards

ASTM A1008/A1008M provides comprehensive specifications for cold-rolled carbon steel sheet products, including requirements related to skin rolling and surface conditions.

EN 10130 represents the European standard for cold-rolled low carbon steel flat products for cold forming, with specific provisions for skin-rolled conditions.

JIS G3141 is the Japanese Industrial Standard covering commercial and drawing quality cold-rolled steel sheets, with detailed specifications for surface finish grades achieved through skin rolling.

Development Trends

Current research is exploring ultra-light skin rolling (below 0.3% reduction) combined with textured rolls to achieve enhanced surface properties with minimal impact on mechanical properties.

Emerging technologies include online measurement and adaptive control of skin rolling parameters based on real-time material property feedback.

Future developments will likely focus on tailored skin rolling for advanced high-strength steels, where traditional approaches must be modified to accommodate their unique deformation characteristics and higher strength levels.