CN105274436B - High carbon microalloyed steel and Technology for Heating Processing of the strength and ductility product up to more than 50GPa% - Google Patents

Abstract

The invention provides a high-carbon micro-alloy steel with a strength and plasticity of more than 50 GPa% and a heat treatment process. The composition of the steel is as follows (mass fraction, %): C: 0.60-0.75, Mn: 1.0-2.0, Si: 1.0 ～2.0, Cr: 0.30～1.00, Ni: 0.20～1.00, Nb: 0.02～0.06, and the rest is iron. In the heat treatment process, normalizing treatment is firstly performed, followed by a quenching-partitioning-tempering (Q-P-T) process. The invention improves the performance of the high-carbon steel by controlling the components of the high-carbon steel and changing the heat treatment process, and the strength-plastic product reaches more than 50GPa%.

Description

High-carbon micro-alloyed steel with a strong plasticity of more than 50GPa% and heat treatment process

technical field

The invention relates to a steel composition and a heat treatment process, in particular to a high-carbon micro-alloy steel with a strength and plasticity of more than 50 GPa% and a heat treatment process.

Background technique

Strength (unit: MPa) and elongation (unit: %) are the basic properties required by structural materials. Strength and plasticity (expressed by elongation) are usually mutually exclusive, that is, when the strength of the material increases, the plasticity decreases, and conversely, when the strength decreases, the plasticity increases. In order to judge the performance of a structural material, the product of the tensile strength of the material (the strength corresponding to the highest uniform elongation on the tensile curve) and the total elongation at break is usually simply used, which is called the strong-plastic product ( product of strength and elongation, PSE), which is used as the criterion of comprehensive performance. A high strength-plasticity product indicates that the material has good comprehensive performance, which is obtained through the best material composition design and the best heat treatment process design. High strength-plastic product is an important indicator of advanced high strength steel (AHSS).

In the past decade, more and more advanced high-strength steels have been used in the automotive industry, which can effectively reduce the amount of structural parts, such as reducing the weight of automotive structural parts, and achieve the effect of energy saving and emission reduction. Advanced high-strength steel can be divided into three generations according to the strength-plastic product: the first-generation advanced high-strength steel (1GPa=1000MPa) is called the first-generation advanced high-strength steel (1GPa=1000MPa), which is Fe-Mn-Si-based micro (low) alloy low Carbon or medium carbon steels such as dual phase steels, transformation induced plasticity (TRIP) steels, quenching and partitioning (Q&P) steels and quenching-partitioning-tempering (quenching-partitioning- tempering, Q-P-T) steel. The strength-plastic product greater than 50GPa% is called the second-generation advanced high-strength steel, such as high manganese (Mn) twinning induced plasticity (twinning induced plasticity, TWIP). The strength-plastic product of the third generation (also known as new generation) advanced high-strength steel is between 30GPa% and 50GPa%, for example, medium carbon Q-P-T steel, medium manganese steel.

After searching the prior art documents, it is found that:

Sugimoto, K.-i., Tsunezawa, M., Hojo, T. & Ikeda, S. Ductility of 0.1～0.6 C-1.5 Si-1.5 Mn ultra high-strength TRIP-aided sheet steels with bainiticferrite matrix. ISIJ international 44, 1608-1614(2004). The article heat-treated 0.1～0.6C–1.5Si–1.5Mn TRIP steel with different processes to obtain a TRIP steel with a bainitic ferrite matrix. The test results show that the steel with different carbon content can achieve the highest strength and plasticity after the same heat treatment process is still 0.6C steel. .

Tomita, Y. & Morioka, K. Effect of microstructure on transformation-induced plasticity of silicon-containing low-alloy steel. Materials Characterization 38, 243-250 (1997). Different heat treatment processes for 0.6C-1.5Si-0.8Mn steel , to achieve the best performance for the strength of 1000MPa and elongation of 30%, strong plastic product is ~ 30GPa%.

Jiang Li et al studied the tensile properties of 0.63C-1.75Si-1.68Mn-0.028P-0.013S TRIP steel under different strain rates at room temperature. After heating at 900°C, holding for 20 minutes, and isothermally treating at 340°C for 2 hours, the tensile properties at different strain rates were tested and the following conclusions were obtained: the elongation of high-carbon silicomanganese TRIP steel increased from 14% to 15% to about 22%; the yield strength Increased from 1015MPa to 1198MPa; ultimate strength increased from 1448MPa to 1546MPa; comprehensive performance of strength and plasticity reached 22-34GPa%.

The article published by Liu Zhongxia et al. "The effect of controlled cooling treatment on the structure and properties of medium and high carbon Si-Mn carbide-free bainite steel", "Metal Heat Treatment" 52-57 (2005), this article studies the effect of controlled cooling heat treatment process on high Effect of carbon Si-Mn on the microstructure and mechanical properties of carbide-free bainitic steels. The results show that 0.63C-1.92Si-0.76Mn microalloyed steel can obtain carbide-free bainite structure in a wide range of process parameters after controlled cooling process; with the prolongation of cooling time of the sample in oil, The amount of blocky retained austenite decreases, and the content of retained austenite film increases; the strength, plasticity and toughness of the material increase with the extension of oil cooling time. After the material is cooled in oil for 7s-8s, it is treated in an air furnace at 360°C for 3600s-5400s to have the best strength and plasticity, that is, the strength reaches 1200MPa, the elongation rate is 23%, and the strong-plastic product reaches 27.6GPa%.

All the above studies without exception control the composition of high carbon steel and change the heat treatment process to improve the performance of high carbon steel, but the results obtained are significantly different from the 50GPa% obtained in the present invention. This shows that in the case of similar carbon, silicon and manganese content, the performance of high carbon steel still has a lot of room for improvement by changing the alloy element ratio and heat treatment process.

Contents of the invention

In view of the situation in the prior art that the performance of high carbon steel is improved by controlling the composition of high carbon steel and changing the heat treatment process, but the maximum strength and plasticity product is only 34GPa%, the present invention proposes a high carbon steel that can reach more than 50GPa%. Composition and heat treatment technology of microalloyed steel.

According to the first aspect of the present invention, there is provided a kind of high-carbon micro-alloyed steel whose strength-plastic product reaches more than 50GPa%, and its composition is specifically as follows (mass fraction, %):

C: 0.60~0.75, Mn: 1.0~2.0, Si: 1.0~2.0, Cr: 0.30~1.00, Ni: 0.20~1.00, Nb: 0.02~0.06, the rest is iron;

High-carbon microalloyed steel, the preferred range of its composition is as follows (mass fraction, %):

C: 0.64-0.69, Mn: 1.3-1.8, Si: 1.3-1.8, Cr: 0.40-1.00, Ni: 0.20-1.00, Nb: 0.03-0.06, and the remainder is iron.

The above composition design principle of the present invention: high carbon content can reduce the martensitic transformation start temperature (Ms), thereby obtaining more retained austenite with high plasticity than medium carbon and low carbon martensitic steel. The addition of Si, one is to inhibit the precipitation of brittle cementite, and the other is to promote the distribution of carbon from supersaturated martensite to retained austenite, so that it is cooled to There is more retained austenite in the room temperature process. The addition of Mn and Cr mainly improves the hardenability of the steel and has a solid solution strengthening effect; the addition of Ni mainly improves the notch sensitivity of the material; the addition of Nb, through the formation of stable Nb carbide During the tempering treatment, the austenite grains can be refined, thereby refining the structure of its phase transformation product-martensite, thereby improving the yield strength and toughness of the steel; the stable Nb carbide precipitated during tempering, Instead of brittle cementite, the tensile strength of steel can be improved.

According to another aspect of the present invention, a kind of heat treatment process of above-mentioned steel is provided, described process 50GPa% high-carbon microalloy steel (hot-rolled plate or the workpiece of various shapes) to be treated, at first carry out normalizing treatment, as subsequent Pretreatment of the quenching-partitioning-tempering (Q-P-T) process, specifically:

Normalizing process (for hot-rolled sheets or workpieces of various shapes): keep warm at 840°C-860°C (holding time is limited to complete austenitization of the structure and no grain growth), and then air-cool to room temperature;

QPT process: austenitizing temperature: 820°C-860°C, then quenching to a temperature (T _q ) between the martensitic transformation start temperature (M _s ) and end temperature (M _f ): 110°C- 180°C, heat preservation (the heat preservation time is determined according to the cross-sectional size of the quenched part and the completeness of carbon distribution), and finally water-cooled to room temperature.

The above-mentioned thermal treatment process design principle of the present invention:

As the pretreatment of Q-P-T process, normalizing treatment aims to refine the structure and reduce the pearlite content in hot rolling, and to obtain refined martensite matrix and dispersed carbides for Q-P-T process, and more importantly, to provide Dispersion and fine retained austenite with mechanical stability are obtained to provide tissue preparation.

QPT process, the choice of austenitizing temperature and holding time is to obtain full austenite structure at this temperature and keep the grain fine. In the QPT process, the choice of quenching temperature (T _q ) is to obtain as much retained austenite as possible, combined with subsequent carbon distribution and tempering to distribute carbon from supersaturated martensite to retained austenite, This results in more retained austenite being present during the final cooling to room temperature. The temperature and time of carbon partitioning and tempering also reduce the dislocation density in the martensite matrix to an appropriate level to obtain the required combination of strength and plasticity.

Steel obtained through the above treatment: martensite matrix (volume fraction: 65-75%), retained austenite (35%-25%), stable Nb carbide (less than 1%). The strength is 1700-2000MPa, the elongation is 32-28%, and the strong-plastic product is 50-55GPa%.

Compared with the prior art, the present invention has the following beneficial effects:

The composition design and heat treatment process method of the present invention can obtain the high-strength martensitic steel whose strong-plastic product is 50GPa%, which has reached the strong-plastic product (50GPa%) of the second generation advanced high-strength steel (high-carbon high-Mn TWIP steel) s level. Compared with the second-generation advanced high-strength steel, the steel of the present invention belongs to micro-alloyed steel, and its Mn content is only about one-tenth of that of the second-generation advanced high-strength steel, and its tensile strength is much higher than that of high-carbon high-Mn TWIP steel, such as Fe-18Mn-0.6-1.5Si TWIP steel is only 1100MPa. The high-carbon and micro-alloying of the present invention, the high-strength and high-plasticity martensitic steel whose strength-plastic product reaches 50MPa% has not been reported at home and abroad.

The composition and heat treatment process of the present invention are applicable to various mechanical structural parts, forgings or rolling parts.

detailed description

The present invention will be described in detail below in conjunction with specific embodiments. The following examples will help those skilled in the art to further understand the present invention, but do not limit the present invention in any form. It should be noted that those skilled in the art can make several modifications and improvements without departing from the concept of the present invention. These all belong to the protection scope of the present invention.

Example 1

(1) The specific composition of steel in the present embodiment is as follows (mass fraction, %):

C: 0.63, Mn: 1.5, Si: 1.5, Cr: 0.6, Nb: 0.05, the rest is iron;

(2) Concrete heat treatment process of the present invention is as follows:

The workpiece to be processed is a 20mm thick hot-rolled plate.

Normalizing is performed first as a pretreatment for the subsequent quenching-partitioning-tempering (Q-P-T) process. Specifically:

Normalizing process: 850°C, heat preservation for 3600s, then air cooling to room temperature;

QPT process: austenitizing temperature: 830°C, holding for 360s, then quenching to: 160°C (the temperature (T _q ) between the martensitic transformation start temperature (M _s ) and the end temperature (M _f )), Keep warm for 10s, and finally cool to room temperature with water.

process result:

Microstructure: martensite matrix (volume fraction: 72%), retained austenite (26.8%), stable Nb carbide (less than 1%).

The strength is 1860MPa, the elongation is 29%, and the strong plastic product is 53.940GPa%.

Example 2

(1) The specific composition of steel in the present embodiment is as follows (mass fraction, %):

C: 0.75, Mn: 1.2, Si: 1.3, Ni: 1.0, Nb: 0.06, the rest is iron;

(2) Concrete heat treatment process of the present invention is as follows:

The workpiece to be processed is a 20mm thick hot-rolled plate.

Normalizing is performed first as a pretreatment for the subsequent quenching-partitioning-tempering (Q-P-T) process. Specifically:

Normalizing process: 850°C, heat preservation for 3600s, then air cooling to room temperature;

QPT process: austenitizing temperature: 830°C, holding for 360s, then quenching to: 140°C (the temperature (T _q ) between the martensitic transformation start temperature (M _s ) and the end temperature (M _f )), Keep warm for 20s, and finally cool to room temperature with water.

process result:

Microstructure: martensite matrix (volume fraction: 72%), retained austenite (27.4%), stable Nb carbide (less than 1%).

The strength is 1960MPa, the elongation is 26.2%, and the strong plastic product is 51.352GPa%.

Example 3

(1) The specific composition of steel in the present embodiment is as follows (mass fraction, %):

C: 0.68, Mn: 1.00, Si: 1.50, Nb: 0.04, the rest is iron;

(2) Concrete heat treatment process of the present invention is as follows:

The workpiece to be processed is a 20mm thick hot-rolled plate.

Normalizing is performed first as a pretreatment for the subsequent quenching-partitioning-tempering (Q-P-T) process. Specifically:

Normalizing process: 850°C, heat preservation for 3600s, then air cooling to room temperature;

QPT process: austenitizing temperature: 820°C, holding for 360s, then quenching to: 115°C (the temperature (T _q ) between the martensitic transformation start temperature (M _s ) and the end temperature (M _f )), Keep warm for 20s, and finally cool to room temperature with water.

process result:

Microstructure: martensite matrix (volume fraction: 68%), retained austenite (31.3%), stable Nb carbide (less than 1%).

The strength is 2010MPa, the elongation is 25.5%, and the strong plastic product is 51.255GPa%.

Specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art may make various changes or modifications within the scope of the claims, which do not affect the essence of the present invention.

Claims (4)

1. a kind of strength and ductility product reaches the Technology for Heating Processing of more than 50GPa% high carbon microalloyed steel, it is characterised in that：The steel it is each The weight/mass percentage composition of composition is as follows：

C：0.60~0.75, Mn：1.0~2.0, Si：1.0~2.0, Cr：0.30~1.00, Ni：0.20~1.00, Nb：0.02 ~0.06, residue is iron；

Normalized treatment is carried out first, as the pretreatment of subsequent quenching-distribution-tempering (Q-P-T) technique, is specially：

Normalizing process：In 840 DEG C of -860 DEG C of insulations, soaking time is limited with organizing complete austenitizing and crystal grain not to grow up, with Air cooling is to room temperature afterwards；

Q-P-T techniques：Austenitizing temperature：820 DEG C -860 DEG C, then it is quenched to martensite start temperature M_sWith end temperature Spend M_fBetween some temperature T_q：110 DEG C -180 DEG C, insulation, soaking time be according to quenching part sectional dimension and carbon distribution it is complete Whole degree determination, last water-cooled to room temperature.

2. strength and ductility product reaches the Technology for Heating Processing of more than 50GPa% high carbon microalloyed steel according to claim 1, its feature exists In：The Technology for Heating Processing is applied to all kinds of mechanical structured members, forging or rolled piece.

3. strength and ductility product reaches the Technology for Heating Processing of more than 50GPa% high carbon microalloyed steel according to claim 2, its feature exists In：Steel obtained by after the heat treatment, its volume fraction is：

Martensitic matrix：65~75%,

Retained austenite：35%~25%,

Stable Nb carbide：Less than 1%；

1700~2000MPa of intensity, elongation percentage 32~28%, 50~55GPa% of strength and ductility product.

4. the strength and ductility product according to claim any one of 1-3 reaches the heat treatment work of more than 50GPa% high carbon microalloyed steel Skill, it is characterised in that：The weight/mass percentage composition of each composition of the steel is as follows：

C：0.64~0.69, Mn：1.3~1.8, Si：1.3~1.8, Cr：0.40~1.00, Ni：0.20~1.00, Nb：0.03 ~0.06, residue is iron.