CN105463307B - A kind of Q&P steel with gradient structure and preparation method thereof - Google Patents

Abstract

The invention discloses a Q&P steel with a gradient structure, which is composed of the following elements in weight percent: C: 0.14-0.29%, Si: 1.1-2.8%, Mn: 1.8-3.0%, Cr: 0.5-1.9%, Al: 1.5‑3.1%, S: ≤0.01%, P: ≤0.01%, and the rest is Fe. The preparation method includes: austenitizing, carburizing, quenching, tempering carbon distribution, and quenching steps. The invention can ensure process stability and structure gradient in the production of larger-sized products, increase the strength of steel while ensuring good toughness, and low alloying cost.

Description

A kind of Q&P steel with gradient structure and preparation method thereof

technical field

The invention relates to a Q&P steel with gradient structure and a preparation method thereof, belonging to the technical field of metal materials.

Background technique

In order to meet the dual requirements of light weight and high safety for the future development of the automobile industry, automobile steel continues to develop in the direction of high strength and high plasticity, and the product of strength and plasticity has become an important indicator to measure the performance of automobile steel. The strength-plasticity product of the first-generation automotive sheet steel is generally 15GPa%, and the light weight and safety indicators are very low; although the strength-plasticity product of the second-generation automotive steel reaches 50GPa%, its cost is high, the process performance is poor and Metallurgical production is more difficult. The so-called third-generation automotive steel refers to high-strength, high-plastic steel that has higher light weight and safety indicators than the first-generation automotive steel, and lower production costs than the second-generation automotive steel.

Q&P steel is the third-generation automotive steel developed in recent years with high strength and toughness. The basic principle is: the steel containing Si or (and) Al is first austenitized and then quenched to a certain value between Ms~Mf (Ms is the start temperature of martensite transformation, Mf is the end temperature of martensite transformation). A certain temperature, that is to form a certain amount of martensite and retained austenite, and then stay at the initial quenching temperature or a certain temperature above Ms for a period of time, so that carbon is distributed from martensite to retained austenite. At this time, martensite The carbon content in the body decreases, and the carbon content in the austenite increases, so that the retained austenite is carbon-rich and stable to room temperature, and finally a composite structure composed of martensite and retained austenite is obtained, thereby obtaining a relatively High strength and toughness, that is, good comprehensive mechanical properties.

In recent years, on the basis of the Q&P process, academician Xu Zuyao has made full use of the precipitation strengthening effect of microalloying elements such as Nb, V and Ti to form carbides, and proposed a new heat treatment process: quenching-partitioning-tempering process (Quenching- Partitioning-Tempering, Q-P-T). This process introduces a tempering process after Q&P treatment to precipitate complex carbides on the martensite matrix to further increase the strength of the steel.

However, with the development of the automobile industry, the strength and toughness of advanced high-strength steels are increasingly required. Although the obtained Q&P steel has high strength, its plasticity is poor. In addition, due to its high strength, its welding performance is greatly weakened. Therefore, it is of great significance to further improve the comprehensive performance of Q&P steel from the perspective of improving the strength, toughness and weldability of steel, saving resources and reducing costs.

After searching the literature of the prior art, it was found that academician Xu Zuyao published "Quenching-partitioning-tempering (Q-P-T) process for ultra-high strength steel" on International Heat Treatment and Surface Engineering, 2008, 2 (2), 64-68. , expounded the principle of quenching-carbon partitioning-tempering (Q-P-T) process, which uses carbide precipitation strengthening to improve the strength of steel, and finally obtains a multiphase structure of martensite, retained austenite and carbide. During the search, it was also found that the Chinese patent with the application number 200810033295.7 obtained a three-phase structure of martensite, retained austenite and nano-bainite, and used the nano-bainite obtained by low-temperature and long-term treatment to further improve the quality of the steel. comprehensive mechanical properties. Recently, J. G. Speer et al published the article "Quenched and Partitioned Microstructures Produced via Gleeble Simulations of Hot-StripMill Cooling Practices" on Metallurgical and Materials Transactions A, 2011,42(12), 3652-3659. The strength and toughness of steel are effective, but the macrostructure distribution of steel parts is not considered, and no gradient microstructure is formed.

During the search, it was also found that Chinese patent application number 201110154249.4 discloses a carbon-silicon-manganese-based hot-rolled Q&P steel and its preparation method. The material composition is C: 1.5-2.5%, Si: 1.3-1.8%, Mn: 1.3-2.0%, S≤0.01%, P≤0.01%, and the balance is Fe. After smelting, forging into billets, and hot rolling, the microstructure of martensite and retained austenite is finally obtained. The Chinese patent with the application number 201310121568.4 discloses a 700MPa grade high-strength hot-rolled Q&P steel and its manufacturing method. The invention is based on the composition of ordinary C-Mn steel and inhibits carburization by increasing the Si content through reasonable composition design. Precipitation of austenite, micro-Ti treatment refines austenite grains, increases Al content to speed up austenite transformation kinetics in the air-cooling process; at the same time, adopts hot continuous rolling process combined with segmental cooling process to obtain pro-eutectoid ferrite+ Martensite + retained austenite structure.

Contents of the invention

The invention aims to provide a carburized Q&P steel with good core plasticity and high surface hardness and a preparation method thereof, and the steel has a gradient microstructure.

The invention provides a Q&P steel with a gradient structure, which consists of the following components by weight percentage:

C: 0.14-0.29%,

Si: 1.1-2.8%,

Mn: 1.8-3.0%,

Cr: 0.5-1.9%,

Al: 1.5-3.1%,

S: ≤0.01%,

P: ≤0.01%,

The rest is Fe.

Further, the Q&P steel with gradient structure is composed of the following components by weight percentage:

C: 0.15-0.28%,

Si: 1.16-2.11%,

Mn: 1.80-2.48%,

Cr: 0.9-1.9%,

Al: 1.6-3.0%,

S: ≤0.01%,

P: ≤0.01%,

The rest is Fe.

The addition of Cr and Al can increase the carbon content in the austenite during critical zone annealing and reduce the Ms temperature (martensite transformation temperature) of the austenite to a lower temperature.

The present invention provides a kind of preparation method of above-mentioned Q&P steel with gradient structure, mainly comprises the following steps:

The first step is to heat the steel rapidly to the austenitizing temperature of 800-1100°C, and wait for 5-30min to fully austenitize;

The second step is to carry out surface carburizing at 700-900°C, the carburizing time is 10-50h, and the whole carburizing process should be carried out in nitrogen atmosphere;

The third step is to quickly quench to a certain quenching temperature (200-500°C) between Ms and Mf, and keep it in a nitrogen atmosphere for 2-9h within this temperature range;

The fourth step is to carry out carbon distribution at a tempering temperature of 300-600°C and isothermal for 5-600 minutes, so that carbon is distributed from martensite to retained austenite. At this time, the carbon content in martensite decreases, and the carbon content in austenite The carbon content of the carbon is increased, so that the retained austenite is carbon-rich and stable to room temperature;

In the fifth step, it is finally quenched to room temperature, and a gradient multiphase structure of stable martensite and retained austenite is obtained at room temperature.

The invention provides a preferred preparation method of Q&P steel with gradient structure, comprising the following steps:

The first step is to heat the steel rapidly to the austenitizing temperature of 850-950°C, and wait for 6-20 minutes to fully austenitize;

The second step is to carry out surface carburizing at 740-850°C, the carburizing time is 10-20h, and the whole carburizing process should be carried out in nitrogen atmosphere;

The third step is to quickly quench to a certain quenching temperature (300-450°C) between Ms and Mf, and keep it in a nitrogen atmosphere for 2-4h within this temperature range;

The fourth step is to carry out carbon distribution at a tempering temperature of 330-500 ° C and isothermal for 5-30 minutes, so that carbon is distributed from martensite to retained austenite. At this time, the carbon content in martensite decreases, and the carbon content in austenite The carbon content of the carbon is increased, so that the retained austenite is carbon-rich and stable to room temperature;

In the fifth step, it is finally quenched to room temperature, and a gradient multiphase structure of stable martensite and retained austenite is obtained at room temperature.

The invention provides a low-alloy, high-strength and toughness multi-phase steel satisfying large-scale production and a heat treatment method thereof. The above-mentioned technical scheme is adopted, and the method of carburizing + quenching + carbon distribution is used to obtain a gradient multi-phase structure in the steel. Compared with the traditional Q&P process, the present invention realizes carbon distribution in the tempering process through carburizing and new alloy and phase change design, and this technology can ensure process stability and tissue gradient in the production of larger-sized products.

In the present invention, firstly, the high strength of the steel must be ensured, and the matrix structure must be a martensite structure, and the martensite structure should be mainly thin lath martensite containing high-density dislocations. Secondly, through the advanced heat treatment process quenching carbon distribution (Q&P), there must be a retained austenite film between the martensite laths. Finally, a dual-phase composite structure containing dislocation martensite and retained austenite is obtained. The toughness of the prepared steel is greatly improved and the high strength is maintained. Its high strength comes from the fine-grain strengthening and dislocation strengthening of martensite and composite structure, and its good plasticity comes from the existence of an appropriate amount of retained austenite in the structure and the softening of the initially quenched martensite structure, as well as the gradient complex phase organize.

Beneficial effects of the present invention:

(1) The present invention achieves higher strength-plastic product through new alloy and phase transformation design, and controls the quenching temperature within a certain range. This technology can ensure process stability and tissue gradient in the production of larger-sized products;

(2) Because the surface of the Q&P steel prepared by the present invention has been carburized, its surface structure is high-carbon martensite and retained austenite, and its core structure is low-carbon martensite and retained austenite;

(3) The invention has a simple process for obtaining such a gradient structure, increases the strength of the steel while ensuring its good toughness, and has low alloying cost and low cost, and has broad prospects for industrial practical application. It can be widely used in wear-resistant or impact-resistant parts, especially in vehicles, mining machinery and equipment, etc.

detailed description

The present invention is further illustrated by the following examples, but not limited to the following examples.

Example 1

The composition and weight percentage of the steel in the embodiment of the present invention are: C: 0.15%, Si: 2.11%, Mn: 1.80%, Cr: 0.9%, Al: 3.0%, S: 0.0065%, P: 0.0078%, and the rest For Fe.

The steel is selected from thin lath martensitic steel containing high-density dislocations.

First, the steel is rapidly heated to the austenitizing temperature (850°C) and isothermally held for 10 minutes to fully austenitize; then the surface is carburized at 740°C, and the carburizing time is 10 hours. The whole carburizing process should be carried out with nitrogen as the atmosphere ;Quickly quench to a quenching temperature of 300°C between Ms and Mf, and keep warm in a nitrogen atmosphere for 3h within this temperature range. Then carbon partitioning is carried out at a certain tempering temperature of 330°C for 10 minutes; finally, it is quenched to room temperature, and a gradient multiphase structure of stable martensite and retained austenite is obtained at room temperature.

According to GB/T228.1-2010 "Metallic Materials Tensile Test Part 1: Room Temperature Test Method", the obtained multi-phase steel products are inspected, and the tensile test is carried out on the Zwick T1-FR020TN A50 standard tensile testing machine. After testing, the tensile strength Rm of the core of the steel is 1457MPa, the yield strength Rp _0.2 of the core is 892MPa, and the total elongation is 22.9%. The hardness of the surface carburized layer is 560HV.

Example 2

The composition and weight percentage of steel in the embodiment of the present invention are: C: 0.18%, Si: 2.01%, Mn: 2.03%, Cr: 0.9%, Al: 2.5%, S: 0.0053%, P: 0.0068%, and the rest For Fe.

First, the steel is rapidly heated to the austenitization temperature (900°C) and isothermally maintained for 8 minutes to fully austenitize; then the surface is carburized at 770°C, and the carburization time is 15 hours. The whole carburization process should be carried out with nitrogen as the atmosphere. ; Quickly quench to a quenching temperature of 350°C between Ms and Mf, and keep warm in a nitrogen atmosphere for 3h within this temperature range. Then carry out carbon distribution at a certain tempering temperature of 400°C for 10 minutes. Finally, it is quenched to room temperature, and a gradient multiphase structure of stable martensite and retained austenite is obtained at room temperature.

After testing, the tensile strength Rm of the steel core is 1421MPa, the yield strength Rp _0.2 of the core is 873MPa, and the total elongation is 23.8%. The hardness of the surface carburized layer is 580HV.

Example 3

The composition and weight percentage of the steel in the embodiment of the present invention are: C: 0.21%, Si: 1.16%, Mn: 2.32%, Cr: 1.1%, Al: 1.6%, S: 0.0051%, P: 0.0069%, and the rest For Fe.

First, the steel is rapidly heated to the austenitizing temperature (900°C) and isothermally held for 15 minutes to fully austenitize; then the surface is carburized at 800°C, and the carburizing time is 20 hours. The whole carburizing process should be carried out in an atmosphere of nitrogen ;Quickly quench to a quenching temperature of 400°C between Ms and Mf, and keep it in a nitrogen atmosphere for 4h within this temperature range. Then carry out carbon distribution at a certain tempering temperature of 420°C for 10 minutes. Finally, it is quenched to room temperature, and a gradient multiphase structure of stable martensite and retained austenite is obtained at room temperature.

After testing, the tensile strength Rm of the core of the steel is 1415MPa, the yield strength Rp _0.2 of the core is 871MPa, and the total elongation is 24.7%. The hardness of the surface carburized layer is 606HV.

Example 4

The composition and weight percentage of steel in the embodiment of the present invention are: C: 0.28%, Si: 1.16%, Mn: 2.48%, Cr: 1.9%, Al: 1.8%, S: 0.0052%, P: 0.0054%, and the rest For Fe.

First, the steel is rapidly heated to the austenitizing temperature (920°C) and isothermally held for 15 minutes to fully austenitize; then the surface is carburized at 850°C, and the carburizing time is 20 hours. The whole carburizing process should be carried out with nitrogen as the atmosphere. ; Quickly quench to a certain quenching temperature between Ms and Mf of 450 ° C, and keep it in a nitrogen atmosphere for 4 hours within this temperature range. Then carry out carbon distribution at a certain tempering temperature of 500°C for 6 minutes. Finally, it is quenched to room temperature, and a gradient multiphase structure of stable martensite and retained austenite is obtained at room temperature.

After testing, the tensile strength Rm of the core of the steel is 1463MPa, the yield strength Rp _0.2 of the core is 882MPa, and the total elongation is 23.6%. The hardness of the surface carburized layer is 622HV.

The above is a detailed description of the embodiments of the present invention, which are implemented on the premise of the technical solutions of the present invention. Various changes can also be made to the above-mentioned embodiments of the present invention, that is, all simple and equivalent changes and modifications made according to the claims of the present application and the content of the description all fall within the protection scope of the claims of the patent of the present invention.

Claims (4)

1. a kind of Q＆P steel with gradient structure, is consisted of the following components in percentage by weight：

C：0.14-0.29%,

Si：1.1-2.8%,

Mn：1.8-3.0%,

Cr：0.5-1.9%,

Al：1.5-3.1%,

S：≤ 0.01%,

P：≤ 0.01%,

Remaining is Fe；

It is characterized in that：The preparation method of the Q＆P steel with gradient structure comprises the following steps：

The first step, is heated rapidly to 800-1100 DEG C of austenitizing temperature by steel first, and isothermal 5-30min makes abundant austenite Change；

Second step, then carries out case-carbonizing, carburizing time is 10-50h, and whole carburizing process will be with nitrogen at 700-900 DEG C Carried out for atmosphere；

3rd step, rapid quenching is incubated in nitrogen atmosphere 2-9h to 200-500 DEG C；

4th step, then in 300-600 DEG C of temperature, isothermal 5-600min carries out carbon distribution, makes carbon from martensite to remnants Austenite is distributed, and now the carbon content in martensite declines, the carbon content rise in austenite, so that retained austenite richness carbon And can stablize to room temperature；

5th step, is finally quenched into room temperature again, and stable martensite and the gradient heterogeneous structure of retained austenite are obtained in room temperature.

2. the Q＆P steel according to claim 1 with gradient structure, it is characterised in that：By the group of following percentage by weight It is grouped into：

C：0.15-0.28%,

Si：1.16-2.11%,

Mn：1.80-2.48%,

Cr：0.9-1.9%,

Al：1.6-3.0%,

S：≤ 0.01%,

P：≤ 0.01%,

Remaining is Fe.

3. the Q＆P steel according to claim 1 with gradient structure, it is characterised in that：Comprise the following steps：

The first step, is heated rapidly to 850-950 DEG C of austenitizing temperature by steel first, and isothermal 6-20min makes abundant austenite Change；

Second step, then carries out case-carbonizing, carburizing time is 10-20h, and whole carburizing process will be with nitrogen at 740-850 DEG C Carried out for atmosphere；

3rd step, rapid quenching is incubated 2-4h to 300-450 DEG C, and in this temperature range in nitrogen atmosphere；

4th step, then in 330-500 DEG C of temperature, isothermal 5-30min carries out carbon distribution, makes carbon from martensite to remaining difficult to understand Family name's body is distributed, and now carbon content in martensite declines, the carbon content rise in austenite so that retained austenite richness carbon and It can stablize to room temperature；

5th step, is finally quenched into room temperature again, and stable martensite and the gradient heterogeneous structure of retained austenite are obtained in room temperature.

4. the Q＆P steel according to claim 1 with gradient structure, it is characterised in that：The steel, which is chosen, contains high density The thin lath martensite of dislocation.