CN115198174A - A kind of martensitic steel, preparation method and application - Google Patents

Abstract

The invention particularly relates to martensitic steel, a preparation method and application, and belongs to the technical field of steel preparation, wherein the martensitic steel comprises the following chemical components in percentage by mass: c:0.18-0.22%, si:0.5-1.0%, al:0.5-1.0%, mn:1.8-2.5%, nb:0.015-0.03%, P: less than or equal to 0.01 percent, S: less than or equal to 0.01 percent, N: less than or equal to 0.004 percent. By adopting an Al component system in the medium Si and the control of other chemical components, the precipitation of cementite is inhibited, the austenite content is further controlled, the ductility of the steel is controlled, the cracking is avoided, and the high-strength steel with martensite as a hard matrix and retained austenite and ferrite as a mixed refined structure is obtained after heat treatment. The martensitic hard phase matrix in the structure provides strength, the retained austenite TRIP effect and ferrite provide ductility of the steel, and the fine uniform structure provides high hole expansibility.

Description

A kind of martensitic steel, preparation method and application

technical field

The invention belongs to the technical field of steel preparation, and particularly relates to a martensitic steel, a preparation method and application.

Background technique

With the development of automobiles in the direction of energy saving, environmental protection, safety and comfort, while the body is developing in the direction of light weight, the requirements for corrosion resistance and impact resistance are also getting higher and higher. The competitive pressure of materials has forced the development of high-strengthened steel plates for automobiles.

The structure of conventional dual-phase steels based on conventional air-cooled galvanizing lines is composed of polygonal ferrite and martensite phases, where ferrite provides the ductility of the steel and martensite provides the rigidity strength. Due to its low yield ratio, high initial work hardening rate, and good matching of strength and ductility, this dual-phase steel has become one of the materials of choice for high-strength steels for automobiles.

With the continuous expansion of the application of coated ultra-high-strength steel sheets, the requirements for formability are also higher and higher, such as high elongation, high yield, bendability, hole expansion and so on. However, the 980MPa grade dual-phase steel is easy to form vacancies at the critical position of the soft matrix ferrite phase and the hard martensite phase, so there is a problem of poor hole expandability, which limits its wide application.

SUMMARY OF THE INVENTION

The purpose of the present application is to provide a martensitic steel, a preparation method and an application, so as to solve the technical problem that the martensitic steel in the prior art is prone to cracking during the forming process, resulting in a low yield.

The embodiment of the present invention provides a martensitic steel, and the chemical composition of the martensitic steel includes in mass percentage: C: 0.18-0.22%, Si: 0.5-1.0%, Al: 0.5-1.0%, Mn : 1.8-2.5%, Nb: 0.015-0.03%, P: ≤ 0.01%, S: ≤ 0.01%, N: ≤ 0.004%, the rest are Fe and inevitable impurities.

Optionally, in terms of volume percentage, the metallographic structure of the martensitic steel includes tempered martensite 60-70%, ferrite 15-30% and retained austenite 10-15%.

Based on the same inventive concept, an embodiment of the present invention also provides a method for preparing martensitic steel as described above, comprising the following steps:

obtaining a slab of chemical composition as claimed in claim 1 or 2;

subjecting the slab to hot rolling and cold rolling to obtain chilled coils;

subjecting the chilled coil to the first heat treatment to obtain a heat-treated steel coil;

The heat-treated steel coil is subjected to a second heat treatment to obtain the martensitic steel.

Optionally, the first heat treatment includes the following steps:

Preheating the chilled roll from room temperature to 210-230°C to obtain a preheated roll;

first heating the preheated coil to 640-660°C to obtain a first heated coil;

The first heating coil is heated to 840-870° C. to obtain a second heating coil;

Passing the second heating coil through the first heat preservation to obtain the first heat preservation coil;

The first heat preservation coil is first cooled to 780-820° C. to obtain a first cooling coil.

Optionally, the preheating rate is 8-12°C/s;

The first heating rate is 3-8°C/s;

The second heating rate is 1-4°C/s;

The temperature of the first heat preservation is 840-870°C, and the time of the first heat preservation is 60-150s;

The rate of the first cooling is 2-6°C/s.

Optionally, the second heat treatment includes the following steps:

secondly cooling the first cooling coil to 250-300°C to obtain a second cooling coil;

heating the second cooling coil to 350-400° C. to obtain a third heating coil;

The third heating coil is subjected to a second heat preservation and distribution treatment to obtain the martensitic steel.

Optionally, the second cooling rate is 50-70°C/s;

The rate of the third heating is 10-30°C/s;

The temperature of the second heat preservation is 350-400° C., and the time of the second heat preservation is 60-120 s.

Optionally, the hot rolling and cold rolling of the slab include the following steps:

heating the slab to 1150-1280°C, and then finishing rolling, coiling and cold rolling to obtain the chilled coil;

in:

The end temperature of the finishing rolling is 870-920°C;

The temperature of the coiling is 550-620°C.

Optionally, the total reduction ratio of the cold rolling is 50-60%.

Based on the same inventive concept, an embodiment of the present invention also provides an application of the above-mentioned martensitic steel, which is used to make a base plate of a galvanized sheet.

One or more technical solutions in the embodiments of the present invention have at least the following technical effects or advantages:

The martensitic steel provided by the embodiment of the present invention adopts the Al composition system in medium Si, supplemented by the control of other chemical compositions, to suppress the precipitation of cementite, and then control the content of austenite, so as to control the ductility of the steel and avoid cracking, After heat treatment, a high-strength steel with martensite as the hard matrix and residual austenite and ferrite as the mixed refined structure is obtained. The martensitic hard phase matrix in this structure provides strength, the retained austenite TRIP effect and ferrite provide the ductility of the steel, and the fine and uniform structure provides high hole expansion ratio. Different from the existing ferrite soft phase as the matrix, the overall structure is relatively uniform, and it will not cause local strain concentration. Compared with the traditional dual-phase steel, it has a higher elongation. The yield strength of the martensitic steel provided by the invention is 560-650MPa, the tensile strength is 995-1073MPa, the elongation is 21-25%, the hole expanding rate is 30-45%, the elongation is high, and the hole expanding performance is good. It can meet the special requirements of auto parts for processing performance.

The above description is only an overview of the technical solutions of the present invention, in order to be able to understand the technical means of the present invention more clearly, it can be implemented according to the content of the description, and in order to make the above and other purposes, features and advantages of the present invention more obvious and easy to understand , the following specific embodiments of the present invention are given.

Description of drawings

In order to illustrate the technical solutions in the embodiments of the present invention more clearly, the following briefly introduces the accompanying drawings used in the description of the embodiments. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, other drawings can also be obtained from these drawings without any creative effort.

1 is a flowchart of a method provided by an embodiment of the present invention;

Fig. 2 is the metallographic structure diagram of the martensitic steel provided by the embodiment of the present invention;

3 is a metallographic structure diagram of the dual-phase steel provided by Comparative Example 1 of the present invention.

Detailed ways

The present invention will be described in detail below with reference to specific embodiments and examples, and the advantages and various effects of the present invention will be more clearly presented therefrom. It should be understood by those skilled in the art that these specific embodiments and examples are used to illustrate the present invention, but not to limit the present invention.

Throughout the specification, unless specifically stated otherwise, terms used herein are to be understood as commonly used in the art. Therefore, unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. In case of conflict, the present specification takes precedence. The technical terms used herein are only for the purpose of describing specific embodiments, and are not intended to limit the protection scope of the present invention. For example, room temperature may refer to a temperature in the range of 10 to 35°C.

Unless otherwise specified, various raw materials, reagents, instruments and equipment used in the present invention can be purchased from the market or can be prepared by existing methods.

The technical solutions of the embodiments of the present application are to solve the above-mentioned technical problems, and the general idea is as follows:

According to a typical embodiment of the present invention, a martensitic steel is provided, and the chemical composition of the martensitic steel includes in mass percentage:

C: 0.18-0.22%, Si: 0.5-1.0%, Al: 0.5-1.0%, Mn: 1.8-2.5%, Nb: 0.015-0.03%, P: ≤ 0.01%, S: ≤ 0.01%, N: ≤ 0.004%, the rest is Fe and inevitable impurities.

The role and limited range of the above-mentioned main alloying elements are described in detail as follows:

C: C is the most effective solid solution strengthening element and the most important element to ensure the hard phase content of steel. Therefore, it is necessary to control the weight percentage of C within 0.18-0.22%. If it is too small, the hard phase content cannot be guaranteed. , it is difficult to achieve the required strength, too much will deteriorate the weldability.

Si and Al: Si and Al are important elements to inhibit the precipitation of cementite, so the weight percentage of Si and Al need to be controlled at 0.5-1.0%, preferably Si+Al=1.5%. If it is too small, it is difficult to inhibit the precipitation of cementite, resulting in a small amount of retained austenite, which affects the ductility of the steel.

Mn: Mn is a solid solution strengthening element and an important element for stabilizing austenite. Therefore, in the present invention, the weight percentage of Mn is controlled at 1.8-2.5%. If it is too small, it is difficult to ensure the hard phase of the steel, and it is difficult to achieve high strength. Excessive deterioration of workability and weldability.

Nb: Nb can effectively refine the grains and improve the uniformity of the steel structure, so the present invention controls the Nb content to 0.015-0.03%.

P: P is likely to significantly reduce the plasticity and toughness of the steel, so the content is required to be as low as possible, and the mass percentage of P needs to be controlled to be less than or equal to 0.01%.

S: S is a harmful impurity element in steel, which makes the steel hot brittle, reduces the ductility and toughness of the steel, and causes cracks during forging and rolling. Therefore, it is necessary to control the mass percentage content of S≤0.01%.

N: N, like C, is also a solid solution element. With the increase of N content in steel, its stamping performance will be deteriorated. At the same time, solid solution N is the main reason for the aging of galvanized sheet finished products, especially for the strain aging effect after leveling, the influence of N is particularly large. Therefore, N is required to be as low as possible. For the tin-plated sheet of the present invention, the N content in the steel should be controlled to be less than or equal to 0.004%.

The martensitic steel prepared by the invention adopts the Al composition system in medium Si, and is supplemented by the control of other chemical compositions to inhibit the precipitation of cementite, and then control the content of austenite, so as to control the ductility of the steel and avoid cracking. A high-strength steel with martensite as the hard matrix, retained austenite and ferrite as the mixed refined structure is obtained. The martensitic hard phase matrix in this structure provides strength, the retained austenite TRIP effect and ferrite provide the ductility of the steel, and the fine and uniform structure provides high hole expansion ratio. Different from the existing ferrite soft phase as the matrix, the overall structure is relatively uniform, and it will not cause local strain concentration. Compared with the traditional dual-phase steel, it has a higher elongation. The yield strength of the martensitic steel provided by the invention is 560-650MPa, the tensile strength is 995-1073MPa, the elongation is 21-25%, the hole expanding rate is 30-45%, the elongation is high, and the hole expanding performance is good. It can meet the special requirements of auto parts for processing performance.

As an optional embodiment, in terms of volume percentage, the metallographic structure of the martensitic steel includes tempered martensite 60-70%, ferrite 15-30% and retained austenite 10-15% %.

Martensite is a relatively hard phase, which can make the martensitic steel have higher strength. In the present invention, the martensite phase is the matrix phase. The retained austenite is in the form of a thin film, which is uniformly dispersed in the martensite phase matrix, reducing the hardness difference between the hard phase and the soft phase, making the martensitic steel have a uniform structure, improving the hole expansion performance, and can also improve The elongation of martensitic steel; the traditional dual-phase steel uses ferrite phase as the matrix, and martensite is distributed on the ferrite. Most of this structure is not uniform. During hole reaming, the stress is mainly concentrated in the bainite. Local strain deformation occurs in the ferrite soft phase near the bulk hard phase, which impairs the flange flanging performance and bending performance, resulting in a low hole expansion ratio. Ferrite is a softer phase that increases the elongation of martensitic steels.

According to another typical embodiment of the present invention, there is provided a method for manufacturing martensitic steel as provided above, comprising the following steps:

S1. Obtain the slab with the chemical composition as described above.

As an optional implementation, it includes the following steps:

S1.1. Carry out converter smelting with the above chemical composition as the final composition, the tapping temperature is 1650-1670°C, the amount of slag under tapping is ≤80mm, and the tapping time is 4-9min.

S1.2. After adding lime, pre-melted slag and fluorite, continuous casting the molten steel to obtain the slab.

S2. The slab is subjected to hot rolling and cold rolling to obtain cold hard coils.

As an optional embodiment, the hot rolling and cold rolling of the slab include the following steps:

S2.1, heating the slab to 1150-1280°C, and then finishing rolling, coiling and cold rolling to obtain the chilled coil;

in:

The end temperature of the finishing rolling is 870-920°C;

The temperature of the coiling is 550-620°C.

The reason for controlling the heating temperature is that if the heating temperature of the slab is lower than 1150 °C, the nitrogen carbide cannot be completely dissolved, which affects the required strength and elongation. On the contrary, if the heating temperature is higher than 1280 °C, the hot working plasticity will be deteriorated. Therefore, the slab heating temperature is controlled at 1150-1280 °C.

The reason for controlling the finish rolling end temperature is: if the finish rolling end temperature is lower than 870°C, the drawn coarse ferrite will affect the subsequent elongation during the hot rolling process. On the contrary, if the finish rolling end temperature is higher than 920°C, the Coarse austenite during rolling affects subsequent strength. Therefore, the end temperature of finishing rolling is controlled at 870-920 °C.

The reason for controlling the coiling temperature is that if the coiling temperature is lower than 550°C, the yield strength is relatively high, which increases the rolling force during cold rolling deformation, which is not conducive to the cold rolling process. If the coiling temperature is higher than 620°C, the hot-rolled sheet will appear band-like structure caused by high Mn, which will increase the difficulty of subsequent processing. Therefore, the coiling temperature is controlled at 550-620°C.

As an optional embodiment, the total reduction ratio of the cold rolling is 50-60%.

The reason for controlling the total reduction ratio of cold rolling is that the total reduction ratio of cold rolling should not be too large, otherwise the cold rolling process will be difficult to carry out; the total reduction ratio of cold rolling is too small to achieve the target thickness.

S3, subjecting the chilled coil to a first heat treatment to obtain a heat-treated steel coil.

As an optional embodiment, the first heat treatment includes the following steps:

S3.1. Preheating the chilled roll from room temperature to 210-230° C. to obtain a preheated roll.

Wherein: the preheating rate is 8-12°C/s.

By preheating, the cold-deformed ferrite produced during cold rolling is recovered.

S3.2, first heating the preheated coil to 640-660° C. to obtain a first heated coil.

Wherein: the rate of the first heating is 3-8°C/s.

Through the first heating, the pre-oxidation of the preheated coil is realized, and the problem of leakage plating caused by excessive content of easily oxidizable elements such as Si and Al is avoided.

The reason for controlling the end temperature of the first heating to be 640-660°C is that the surface of the steel plate can be completely oxidized. If the temperature is too low, the surface of the steel plate cannot be completely covered with an oxide layer.

The reason for controlling the rate of the first heating to be 3-8°C/s is to effectively control the recrystallization rate.

S3.3, secondly heating the first heating coil to 840-870° C. to obtain a second heating coil.

Wherein: the rate of the second heating is 1-4°C/s.

Through the second heating, recrystallization of the cold-rolled ferrite structure is achieved, and pearlite is first transformed into austenite and grows into ferrite.

The reason for controlling the end temperature of the second heating to be 840-870 °C is that more austenite content can be obtained, too high austenite grains will be obtained, and too low austenite will be obtained, resulting in a higher content of austenite after subsequent cooling. Less hard phase, affecting mild performance.

The reason for controlling the second heating rate to be 1-4°C/s is that the recrystallization and phase transformation rates are effectively controlled, and the recrystallization rate is too high to delay the recrystallization rate, resulting in uneven structure, and too low a rate to coarsen the structure.

S3.4, subjecting the second heating coil to the first heat preservation to obtain the first heat preservation coil.

Wherein: the temperature of the first heat preservation is 840-870° C., and the time of the first heat preservation is 60-150s.

The first heat preservation can achieve partial or full austenitization to obtain a higher amount of austenite. At the same time, it can effectively control the austenite grains and effectively improve the hole expandability. If the temperature of the first heat preservation is too high or the time of the first heat preservation is too long, it will lead to coarse austenite grains, which will affect the grain size of the subsequent structure and deteriorate the performance of the steel. On the contrary, the temperature of the first heat preservation is too low or the time of the first heat preservation is too short, resulting in uneven original structure, which will also affect the performance of the subsequent steel. Therefore, it is necessary to control the temperature of the first heat preservation to be 840-870° C., and the time of the first heat preservation to be 60-150 s.

S3.5, first cooling the first heat preservation coil to 780-820° C. to obtain a first cooling coil.

Wherein: the rate of the first cooling is 2-6°C/s.

In the first cooling process, austenite is partially transferred to ferrite, and elements such as C and Mn are aggregated into the austenite.

The reason for controlling the end temperature of the first cooling to be 780-820° C. is that a certain content of ferrite is formed, so that more carbon is enriched in austenite. If it is too high, too little ferrite will be formed, which will not be able to discharge carbon; if it is too low, too much ferrite will be formed, which will affect the strength of the steel.

The reason for controlling the rate of the first cooling to be 2-6°C/s is to effectively control the rate of ferrite formation.

S4, subjecting the heat-treated steel coil to a second heat treatment to obtain the martensitic steel.

As an optional embodiment, the second heat treatment includes the following steps:

S4.1. Second cooling the first cooling coil to 250-300° C. to obtain a second cooling coil.

Wherein: the rate of the second cooling is 50-70°C/s.

The second cooling process partially transforms the austenite into the martensite matrix phase, which provides the strength of the steel. The second cooling rate is too fast to obtain ultra-high strength and deteriorates the elongation, on the contrary, it is too slow to obtain the required martensite content of the hard phase, which cannot satisfy the strength of the martensitic steel of the embodiment of the present invention.

S4.2, heating the second cooling coil to 350-400° C. to obtain a third heating coil.

Wherein: the rate of the third heating is 10-30°C/s.

S4.3, subjecting the third heating coil to the second heat preservation and distribution treatment to obtain the martensitic steel.

Wherein: the temperature of the second heat preservation is 350-400°C, and the time of the second heat preservation is 60-120s.

The reason for controlling the temperature of the second heat preservation is to further aggregate elements such as C and Mn into austenite. Too high may lead to the precipitation of cementite, and too low can not enrich C and Mn into austenite.

The reason for controlling the time of the second heat preservation is that the time of the second heat preservation is too long, which leads to the precipitation of some carbides, which reduces the content of retained austenite and the carbon content in the retained austenite, so that the ductility of the martensitic steel is poor. , the hole expansion rate is high. On the contrary, if the holding time is too short, elements such as C and Mn cannot further aggregate into austenite, which will also reduce the content of retained austenite and carbon in the retained austenite, and also reduce the ductility of martensitic steel.

Partition treatment can further aggregate C, Mn and other elements into austenite, and obtain a good match between the content of retained austenite and its carbon content. The partitioning temperature is the temperature of the second heat preservation. If the partitioning temperature is too low, the content of retained austenite will be reduced, and the carbon content in the retained austenite will be reduced, so that the martensitic steel obtains ultra-high strength, but the elongation is deteriorated. On the contrary, if the partitioning temperature is too high, it will lead to the precipitation of cementite, reduce the content of retained austenite, and at the same time reduce the carbon content in the retained austenite, which will affect the elongation and cannot meet the requirements of this requirement. The strength of the martensitic steel in the embodiment of the invention, the uniformity of the structure are poor, and the hole expansion rate is low.

According to another typical embodiment of the present invention, an application of the above-mentioned martensitic steel is provided, and the martensitic steel is used for making a base plate of a galvanized sheet.

Specifically, the above-mentioned martensitic steel is heated to a galvanizing temperature of 450-460° C. and then galvanized. After galvanizing, it is cooled to 420-430° C. by blowing with an air knife; to avoid partial austenite decomposition at high temperatures; induction heating can be used for heating.

After air-cooling at the front end and air-cooling at the back end between the air knife and the top roll, it is finally cooled to 250-300°C, and the cooling rate is about 6-9°C/s; during this process, part of the austenite phase is transformed into the martensite phase.

The present application will be described in detail below with reference to the examples, comparative examples and experimental data.

Example 1

A martensitic steel, the chemical composition of which is shown in Table 1 in terms of mass percentage.

Table 1 Chemical composition of Example 1

chemical composition C(%) Si(%) Al(%) Mn(%) Nb(%) P(%) S(%) N(%) Example 1 0.18 0.9 0.6 2.3 0.028 0.008 0.005 0.003

The metallographic structure of the martensitic steel includes tempered martensite 63%, ferrite 23% and retained austenite 14%.

The preparation method of the above-mentioned martensitic steel comprises the following steps:

S1. Obtain the slab with the chemical composition as described above.

S2. The slab is subjected to hot rolling and cold rolling to obtain cold hard coils.

S2.1, heating the slab to 1230°C, and then finishing rolling, coiling and cold rolling to obtain the chilled coil;

in:

The end temperature of the finishing rolling is 890°C;

The temperature of the coiling was 590°C.

The total reduction ratio of the cold rolling was 56%.

S3, subjecting the chilled coil to a first heat treatment to obtain a heat-treated steel coil.

S3.1. Preheating the chilled roll from room temperature to 210° C. to obtain a preheated roll.

Wherein: the preheating rate is 8°C/s.

S3.2. The preheated coil is first heated to 640° C. to obtain a first heated coil.

Wherein: the rate of the first heating is 3°C/s.

S3.3. The first heating coil is heated to 840° C. for the second time to obtain a second heating coil.

Wherein: the rate of the second heating is 1.5°C/s.

S3.4, subjecting the second heating coil to the first heat preservation to obtain the first heat preservation coil.

Wherein: the temperature of the first heat preservation is 840°C, and the time of the first heat preservation is 120s.

S3.5, firstly cooling the first heat preservation coil to 780° C. to obtain a first cooling coil.

Wherein: the rate of the first cooling is 2°C/s.

S4, subjecting the heat-treated steel coil to a second heat treatment to obtain the martensitic steel.

S4.1. Second cooling the first cooling coil to 250° C. to obtain a second cooling coil.

Wherein: the rate of the second cooling is 70°C/s.

S4.2, heating the second cooling coil to 370° C. to obtain a third heating coil.

Wherein: the rate of the third heating is 15°C/s.

S4.3, subjecting the third heating coil to the second heat preservation and distribution treatment to obtain the martensitic steel.

Wherein: the temperature of the second heat preservation is 350°C, and the time of the second heat preservation is 120s.

Example 2

A martensitic steel, the chemical composition of which is shown in Table 2 in terms of mass percentage.

The chemical composition of table 2 Example 2

chemical composition C(%) Si(%) Al(%) Mn(%) Nb(%) P(%) S(%) N(%) Example 2 0.19 1.0 0.5 2.2 0.025 0.005 0.008 0.0032

The metallographic structure of the martensitic steel includes tempered martensite 65%, ferrite 28% and retained austenite 12%.

The preparation method of the above-mentioned martensitic steel comprises the following steps:

S1. Obtain the slab with the chemical composition as described above.

S2. The slab is subjected to hot rolling and cold rolling to obtain cold hard coils.

S2.1, heating the slab to 1250°C, and then finishing rolling, coiling and cold rolling to obtain the chilled coil;

in:

The end temperature of the finishing rolling is 878°C;

The temperature of the coiling was 575°C.

The total reduction ratio of the cold rolling was 58%.

S3, subjecting the chilled coil to a first heat treatment to obtain a heat-treated steel coil.

S3.1. Preheating the chilled roll from room temperature to 220° C. to obtain a preheated roll.

Wherein: the preheating rate is 9°C/s.

S3.2. The preheated coil is first heated to 650° C. to obtain a first heated coil.

Wherein: the rate of the first heating is 4°C/s.

S3.3. The first heating coil is heated to 860° C. for a second time to obtain a second heating coil.

Wherein: the rate of the second heating is 4°C/s.

S3.4, subjecting the second heating coil to the first heat preservation to obtain the first heat preservation coil.

Wherein: the temperature of the first heat preservation is 860°C, and the time of the first heat preservation is 120s.

S3.5, firstly cooling the first heat preservation coil to 800° C. to obtain a first cooling coil.

Wherein: the rate of the first cooling is 4°C/s.

S4, subjecting the heat-treated steel coil to a second heat treatment to obtain the martensitic steel.

S4.1. Second cooling the first cooling coil to 260° C. to obtain a second cooling coil.

Wherein: the rate of the second cooling is 65°C/s.

S4.2, heating the second cooling coil to 380° C. to obtain a third heating coil.

Wherein: the rate of the third heating is 20°C/s.

S4.3, subjecting the third heating coil to the second heat preservation and distribution treatment to obtain the martensitic steel.

Wherein: the temperature of the second heat preservation is 360°C, and the time of the second heat preservation is 90s.

Example 3

A martensitic steel, the chemical composition of which is shown in Table 3 in terms of mass percentage.

Table 3 Chemical composition of Example 3

chemical composition C(%) Si(%) Al(%) Mn(%) Nb(%) P(%) S(%) N(%) Example 3 0.22 0.5 1.0 1.9 0.018 0.009 0.007 0.0028

The metallographic structure of the martensitic steel includes 60% tempered martensite, 25% ferrite and 15% retained austenite.

The preparation method of the above-mentioned martensitic steel comprises the following steps:

S1. Obtain the slab with the chemical composition as described above.

S2. The slab is subjected to hot rolling and cold rolling to obtain cold hard coils.

S2.1, heating the slab to 1265°C, and then finishing rolling, coiling and cold rolling to obtain the chilled coil;

in:

The end temperature of the finishing rolling is 910°C;

The temperature of the coiling was 610°C.

The total reduction ratio of the cold rolling was 52%.

S3, subjecting the chilled coil to a first heat treatment to obtain a heat-treated steel coil.

S3.1. Preheating the chilled roll from room temperature to 215° C. to obtain a preheated roll.

Wherein: the preheating rate is 10°C/s.

S3.2, first heating the preheated coil to 645° C. to obtain a first heated coil.

Wherein: the rate of the first heating is 6°C/s.

S3.3, the first heating coil is heated to 870° C. to obtain a second heating coil.

Wherein: the rate of the second heating is 3°C/s.

S3.4, subjecting the second heating coil to the first heat preservation to obtain the first heat preservation coil.

Wherein: the temperature of the first heat preservation is 870°C, and the time of the first heat preservation is 100s.

S3.5, first cooling the first heat preservation coil to 820° C. to obtain a first cooling coil.

Wherein: the rate of the first cooling is 6°C/s.

S4, subjecting the heat-treated steel coil to a second heat treatment to obtain the martensitic steel.

S4.1. Second cooling the first cooling coil to 300° C. to obtain a second cooling coil.

Wherein: the rate of the second cooling is 60°C/s.

S4.2, heating the second cooling coil to 400° C. to obtain a third heating coil.

Wherein: the rate of the third heating is 25°C/s.

S4.3, subjecting the third heating coil to the second heat preservation and distribution treatment to obtain the martensitic steel.

Wherein: the temperature of the second heat preservation is 400°C, and the time of the second heat preservation is 100s.

Example 4

A martensitic steel, the chemical composition of which is shown in Table 4 in terms of mass percentage.

The chemical composition of table 4 embodiment 4

chemical composition C(%) Si(%) Al(%) Mn(%) Nb(%) P(%) S(%) N(%) Example 4 0.20 0.8 0.7 2.0 0.021 0.006 0.009 0.0035

The metallographic structure of the martensitic steel includes 60% tempered martensite, 25% ferrite and 15% retained austenite.

The preparation method of the above-mentioned martensitic steel comprises the following steps:

S1. Obtain the slab with the chemical composition as described above.

S2. The slab is subjected to hot rolling and cold rolling to obtain cold hard coils.

S2.1, heating the slab to 1245°C, and then finishing rolling, coiling and cold rolling to obtain the chilled coil;

in:

The end temperature of the finishing rolling is 902°C;

The temperature of the coiling was 605°C.

The total reduction ratio of the cold rolling was 53%.

S3, subjecting the chilled coil to a first heat treatment to obtain a heat-treated steel coil.

S3.1. Preheating the chilled roll from room temperature to 230° C. to obtain a preheated roll.

Wherein: the preheating rate is 12°C/s.

S3.2, first heating the preheated coil to 655° C. to obtain a first heated coil.

Wherein: the rate of the first heating is 8°C/s.

S3.3. The first heating coil is heated to 850° C. for a second time to obtain a second heating coil.

Wherein: the rate of the second heating is 2°C/s.

S3.4, subjecting the second heating coil to the first heat preservation to obtain the first heat preservation coil.

Wherein: the temperature of the first heat preservation is 850°C, and the time of the first heat preservation is 80s.

S3.5, firstly cooling the first heat preservation coil to 810° C. to obtain a first cooling coil.

Wherein: the rate of the first cooling is 5°C/s.

S4, subjecting the heat-treated steel coil to a second heat treatment to obtain the martensitic steel.

S4.1. Second cooling the first cooling coil to 280° C. to obtain a second cooling coil.

Wherein: the rate of the second cooling is 50°C/s.

S4.2, heating the second cooling coil to 390° C. to obtain a third heating coil.

Wherein: the rate of the third heating is 22°C/s.

S4.3, subjecting the third heating coil to the second heat preservation and distribution treatment to obtain the martensitic steel.

Wherein: the temperature of the second heat preservation is 380°C, and the time of the second heat preservation is 80s.

Comparative Example 1

A martensitic steel, the chemical composition of which is shown in Table 5 in terms of mass percentage.

Table 5 Chemical composition of Comparative Example 1

chemical composition C(%) Si(%) Al(%) Mn(%) Nb(%) P(%) S(%) N(%) Mo(%) Cr(%) Comparative Example 1 0.11 0.35 0.035 2.3 0.025 0.006 0.003 0.003 0.2 0.5

The metallographic structure of the martensitic steel includes tempered martensite 25%, ferrite 70% and retained austenite 5%.

The preparation method of the above-mentioned martensitic steel comprises the following steps:

S1. Obtain the slab with the chemical composition as described above.

S2. The slab is subjected to hot rolling and cold rolling to obtain cold hard coils.

S2.1, heating the slab to 1225°C, and then finishing rolling, coiling and cold rolling to obtain the chilled coil;

in:

The end temperature of the finishing rolling is 870°C;

The temperature of the coiling was 650°C.

The total reduction ratio of the cold rolling was 55%.

S3, subjecting the chilled coil to a first heat treatment to obtain a heat-treated steel coil.

S3.1. Preheating the chilled roll from room temperature to 250° C. to obtain a preheated roll.

Wherein: the preheating rate is 14°C/s.

S3.3. The preheated coil is heated to 800° C. for the second time to obtain a second heated coil.

Wherein: the rate of the second heating is 3°C/s.

S3.4, subjecting the second heating coil to the first heat preservation to obtain the first heat preservation coil.

Wherein: the temperature of the first heat preservation is 800°C, and the time of the first heat preservation is 100s.

S3.5, first cooling the first heat preservation coil to 730° C. to obtain a first cooling coil.

Wherein: the rate of the first cooling is 3°C/s.

S4, subjecting the heat-treated steel coil to a second heat treatment to obtain the martensitic steel.

S4.1. Second cooling the first cooling coil to 460° C. to obtain a second cooling coil.

Wherein: the rate of the second cooling is 34°C/s.

S4.2, heating the second cooling coil to 460° C. to obtain a third heating coil.

Wherein: the rate of the third heating is 34°C/s.

S4.3, subjecting the third heating coil to the second heat preservation and distribution treatment to obtain the martensitic steel.

Wherein: the temperature of the second heat preservation is 460°C, and the time of the second heat preservation is 60s.

Experimental example

The martensitic steels prepared in Examples 1-4 and Comparative Example 1 were tested for mechanical properties according to the national standard (GB/T228.1-2010). The test results are shown in the following table:

R_p0.2/MPa R_m，N/MPa A₅₀/% λ/% Example 1 610 1060 twenty one 38 Example 2 560 995 twenty two 30 Example 3 600 1030 25 45 Example 4 650 1073 twenty three 42 Comparative Example 1 626 1100 13 27.33

In the above table, λ is the hole expansion ratio, the higher the λ, the better the hole expansion performance of the martensitic steel.

As can be seen from the above table, the yield strength of the martensitic steel provided by the embodiment 1-4 of the present invention is 560-650MPa, the tensile strength is 995-1073MPa, the elongation is 21-25%, and the hole expansion rate is 30- 45%, good hole expansion performance.

The microstructure of the martensitic steel provided in Comparative Example 1 is a soft phase ferrite matrix, a hard second phase martensite and a small amount of retained austenite, the yield strength is 626MPa, the tensile strength is 1100MPa, and the elongation is is 13%, the yield after hole expansion is 27.33%, and the hole expansion performance and elongation are obviously inferior to Examples 1-4 of the present invention.

Detailed description of Figures 2-3:

As shown in FIG. 2 , it is a metallographic structure diagram of the martensitic steel provided by the embodiment of the present invention. It can be seen from the figure that the microstructure of the martensitic steel provided by the embodiment of the present invention is based on the hard martensitic phase. , and contains film-like retained austenite and ferrite.

As shown in FIG. 3, which is the metallographic structure diagram of the dual-phase steel provided by the comparative example 1 of the present invention, it can be seen from the figure that the microstructure of the martensitic steel provided by the comparative example 1 of the present invention is a soft phase ferrite matrix and a hard ferrite matrix. Massive second phase martensite and a small amount of retained austenite.

Finally, it should also be noted that the terms "comprising", "comprising" or any other variation thereof are intended to encompass a non-exclusive inclusion such that a process, method, article or device comprising a series of elements includes not only those elements, but also Also included are other elements not expressly listed or inherent to such a process, method, article or apparatus.

Although preferred embodiments of the present invention have been described, additional changes and modifications to these embodiments may occur to those skilled in the art once the basic inventive concepts are known. Therefore, the appended claims are intended to be construed to include the preferred embodiment and all changes and modifications that fall within the scope of the present invention.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit and scope of the invention. Thus, provided that these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these modifications and variations.

Claims (10)

1. A martensitic steel, characterized in that the chemical composition of the martensitic steel comprises, in mass percent:

c:0.18-0.22%, si:0.5-1.0%, al:0.5-1.0%, mn:1.8-2.5%, nb:0.015-0.03%, P: less than or equal to 0.01 percent, S: less than or equal to 0.01%, N: less than or equal to 0.004 percent, and the balance of Fe and inevitable impurities.

2. The martensitic steel as claimed in claim 1, characterized in that the metallographic structure of the martensitic steel comprises, in volume percent, 60-70% tempered martensite, 15-30% ferrite and 10-15% retained austenite.

3. A method for producing martensitic steel, characterized by comprising the steps of:

obtaining a slab of the chemical composition of claim 1 or 2;

carrying out hot rolling and cold rolling on the plate blank to obtain a cold-hard coil;

carrying out first heat treatment on the cold and hard coil to obtain a heat-treated steel coil;

and carrying out second heat treatment on the heat-treated steel coil to obtain the martensitic steel.

4. A method for the production of a martensitic steel as claimed in claim 3, characterized in that said first heat treatment comprises the steps of:

preheating the cold hard coil from room temperature to 210-230 ℃ to obtain a preheated coil;

firstly heating the preheating coil to 640-660 ℃ to obtain a first heating coil;

secondly heating the first heating coil to 840-870 ℃ to obtain a second heating coil;

carrying out first heat preservation on the second heating coil to obtain a first heat preservation coil;

and carrying out first cooling on the first heat-preservation coil to 780-820 ℃ to obtain a first cooling coil.

5. The method of producing a martensitic steel as claimed in claim 4,

the preheating rate is 8-12 ℃/s;

the first heating rate is 3-8 ℃/s;

the second heating rate is 1-4 ℃/s;

the first heat preservation temperature is 840-870 ℃, and the first heat preservation time is 60-150s;

the first cooling rate is 2-6 ℃/s.

6. A method for producing a martensitic steel as claimed in claim 5, characterized in that the second heat treatment comprises the steps of:

secondly cooling the first cooling coil to 250-300 ℃ to obtain a second cooling coil;

thirdly heating the second cooling coil to 350-400 ℃ to obtain a third heating coil;

and carrying out second heat preservation and distribution treatment on the third heating coil to obtain the martensitic steel.

7. A method of producing a martensitic steel as claimed in claim 3,

the second cooling rate is 50-70 ℃/s;

the third heating rate is 10-30 ℃/s;

the temperature of the second heat preservation is 350-400 ℃, and the time of the second heat preservation is 60-120s.

8. A method of producing a martensitic steel as claimed in claim 3 wherein the hot and cold rolling of the slab comprises the steps of:

heating the plate blank to 1150-1280 ℃, and then carrying out finish rolling, coiling and cold rolling to obtain a cold-hard coil;

wherein:

the finish rolling end point temperature is 870-920 ℃;

the coiling temperature is 550-620 ℃.

9. A method of producing a martensitic steel as claimed in claim 3 wherein the cold rolling is at a total reduction of 50 to 60%.

10. Use of a martensitic steel as claimed in claim 1 or 2, characterized in that the martensitic steel is used for making a substrate for galvanized sheet.

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