阅读说明：本技术 经冷轧的马氏体钢及其马氏体钢的方法 (Cold-rolled martensitic steel and method for martensitic steel ) 是由 马蒂厄·西本特里特 文森特·洛伊斯特 奥雷莉·埃斯诺 于 2020-06-05 设计创作，主要内容包括：一种经冷轧的马氏体钢板,所述经冷轧的马氏体钢板包含以下元素：0.3％≤C≤0.4％；0.5％≤Mn≤1％；0.2％≤Si≤0.6％；0.1％≤Cr≤1％；0.01％≤Al≤1％；0.01％≤Mo≤0.5％；0.001％≤Ti≤0.1％；0％≤S≤0.09％；0％≤P≤0.09％；0％≤N≤0.09％；0％≤Nb≤0.1％；0％≤V≤0.1％；0％≤Ni≤1％；0％≤Cu≤1％；0％≤B≤0.05％；0.001％≤Ca≤0.01％；0％≤Sn≤0.1％；0％≤Pb≤0.1％；0％≤Sb≤0.1％；剩余部分组成由铁和由加工引起的不可避免的杂质组成,钢的显微组织以面积百分比计：至少95％的马氏体,累积量为1％至5％的铁素体和贝氏体,以及0％至2％的任选量的残余奥氏体。(A cold-rolled martensitic steel sheet comprising the following elements: c is more than or equal to 0.3 percent and less than or equal to 0.4 percent; mn is more than or equal to 0.5 percent and less than or equal to 1 percent; si is more than or equal to 0.2 percent and less than or equal to 0.6 percent; cr is between 0.1 and 1 percent; al is more than or equal to 0.01 percent and less than or equal to 1 percent; mo is between 0.01 and 0.5 percent; ti is more than or equal to 0.001 percent and less than or equal to 0.1 percent; s is more than or equal to 0% and less than or equal to 0.09%; p is more than or equal to 0 percent and less than or equal to 0.09 percent; n is more than or equal to 0% and less than or equal to 0.09%; nb is between 0 and 0.1 percent; v is between 0% and 0.1%; ni is between 0% and 1%; cu is more than or equal to 0% and less than or equal to 1%; b is between 0% and 0.05%; ca is more than or equal to 0.001% and less than or equal to 0.01%; sn is between 0 and 0.1 percent; pb is between 0 and 0.1 percent; sb is between 0 and 0.1 percent; the remainder consisting of iron and unavoidable impurities resulting from the working, the microstructure of the steel being, in area percent: at least 95% martensite, 1% to 5% ferrite and bainite in cumulative amount, and 0% to 2% residual austenite in optional amount.)

1. A cold-rolled martensitic steel sheet comprising, in weight percent:

0.3％≤C≤0.4％；

0.5％≤Mn≤1％；

0.2％≤Si≤0.6％；

0.1％≤Cr≤1％；

0.01％≤Al≤1％；

0.01％≤Mo≤0.5％；

0.001％≤Ti≤0.1％；

0％≤S≤0.09％；

0％≤P≤0.09％；

0％≤N≤0.09％；

and optionally comprising one or more of the following optional elements:

0％≤Nb≤0.1％；

0％≤V≤0.1％；

0％≤Ni≤1％；

0％≤Cu≤1％；

0％≤B≤0.05％；

0.001％≤Ca≤0.01％；

0％≤Sn≤0.1％；

0％≤Pb≤0.1％；

0％≤Sb≤0.1％；

the remainder consisting of iron and unavoidable impurities resulting from the working, the microstructure of the steel comprising, in area percent: at least 95% martensite, 1% to 5% ferrite and bainite in cumulative amount, and 0% to 2% residual austenite in optional amount.

2. The cold rolled martensitic steel sheet as claimed in claim 1, wherein the composition comprises from 0.3% to 0.36% carbon.

3. The cold rolled martensitic steel sheet as claimed in claim 1 or 2, wherein the composition comprises 0.3 to 0.38% carbon.

4. The cold rolled martensitic steel sheet as claimed in any one of claims 1 to 3, wherein the composition comprises 0.01 to 0.5% aluminium.

5. The cold rolled martensitic steel sheet as claimed in any one of claims 1 to 4, wherein the composition comprises from 0.5% to 0.9% manganese.

6. The cold rolled martensitic steel sheet as claimed in any one of claims 1 to 5, wherein the composition comprises 0.3 to 0.9% chromium.

7. The cold rolled martensitic steel sheet as claimed in any one of claims 1 to 6, wherein the amount of martensite is between 96% and 99%.

8. The cold rolled martensitic steel sheet as claimed in any one of claims 1 to 7, wherein the cumulative amount of ferrite and bainite is between 1% and 4%.

9. The cold rolled martensitic steel sheet as claimed in any one of claims 1 to 8, wherein said sheet has an ultimate tensile strength of 1700MPa or more and a yield strength of 1500MPa or more.

10. A method of producing a cold rolled martensitic steel sheet, the method comprising the sequential steps of:

-providing a steel composition according to any one of claims 1 to 6;

-reheating the semi-finished product to a temperature of 1000 ℃ to 1280 ℃;

-rolling the semi-finished product in the austenite range to obtain a hot rolled steel sheet, wherein the hot rolling finishing temperature is Ac3 to Ac3+100 ℃;

-cooling the sheet to a coiling temperature below 650 ℃ at a cooling rate of at least 20 ℃/s; and coiling the hot rolled sheet;

-cooling the hot rolled sheet to room temperature;

-optionally subjecting the hot rolled steel sheet to a descaling process;

optionally the hot rolled steel sheet may be annealed;

-optionally subjecting the hot rolled steel sheet to a descaling process;

-cold rolling the hot rolled steel sheet at a reduction of 35-90% to obtain a cold rolled steel sheet;

-then heating the cold rolled steel sheet in a two-step heating, wherein:

a first step of heating, starting from room temperature, the cold rolled steel sheet to a temperature HT1 of 550 ℃ to 750 ℃, with a cooling rate HR1 of at least 10 ℃/sec;

a second step of heating, starting from HT1, to a temperature Tsoak of AC3 to AC3+100 ℃, at a heating rate HR2 of 1 ℃/sec to 50 ℃/sec, holding the cold rolled steel sheet at said temperature Tsoak for a time of 10 sec to 500 sec,

-then cooling the cold rolled steel sheet in a two-step cooling, wherein:

a first step of cooling, starting the cold rolled steel sheet from tset to a temperature T1 of 630 ℃ to 750 ℃ at a cooling rate CR1 of 30 ℃/s to 150 ℃/s;

a second step of cooling, starting from T1 to a temperature T2 of Ms-10 ℃ to 20 ℃ at a cooling rate CR2 of at least 50 ℃/s,

-then reheating the cold rolled steel sheet to a tempering temperature trempering of 150 ℃ to 300 ℃ at a rate of at least 1 ℃/s, holding the cold rolled steel sheet at the tempering temperature for a time of 100 seconds to 600 seconds;

-then cooling to room temperature at a cooling rate of at least 1 ℃/sec to obtain a cold rolled martensitic steel sheet.

11. The method of claim 10, wherein the coiling temperature is 475 ℃ to 625 ℃.

12. The method of claim 10 or 11, wherein T soak is Ac3+10 ℃ to Ac3+100 ℃.

13. The method of any one of claims 10 to 12, wherein CR1 is 30 ℃/sec to 120 ℃/sec.

14. The method of any one of claims 10 to 13, wherein T1 is from 640 ℃ to 725 ℃.

15. The method of any one of claims 10 to 14, wherein CR2 is at least 100 ℃/sec.

16. The process of any one of claims 10 to 15, wherein T2 is Ms-50 ℃ to 20 ℃.

17. The method of any one of claims 10 to 16, wherein T-temper is from 200 ℃ to 300 ℃.

18. Use of a steel sheet obtained according to any one of claims 1 to 9 or manufactured according to the method of any one of claims 10 to 17 for manufacturing a structural part of a vehicle.

Examples

The following tests, embodiments, graphical examples and tables presented herein are non-limiting in nature and must be considered for illustrative purposes only and will show advantageous features of the invention.

In table 1, steel sheets made of steels with different compositions are summarized, wherein the steel sheets are produced according to the process parameters as noted in table 2, respectively. Thereafter, table 3 summarizes the microstructures of the steel sheets obtained during the tests, and table 4 summarizes the evaluation results of the obtained characteristics.

TABLE 2

Table 2 summarizes the hot rolling process parameters and the annealing process parameters performed on the cold rolled steel sheet to impart the desired mechanical properties to the steel of table 1 to become a cold rolled martensitic steel.

Table 3 illustrates the results of tests performed according to the standard on different microscopes, e.g. scanning electron microscopes, used to determine the microstructure of both the steel of the invention and the reference steel, expressed in area fractions. The results are noted herein:

table 3:

test of	Martensite	Ferrite + bainite	Retained austenite
				I1	98	2％	-
l2	98	2％	-
				l3	98	2％	-
R1	89％	11％	-
				R2	93.5％	6.5％	-
R3	94％	6％	-

According to the invention, R is a reference; underlined values: not according to the invention.

TABLE 4

The results of various mechanical tests performed according to the standard are summarized. In order to test the ultimate tensile strength and yield strength, the ultimate tensile strength and yield strength were tested in accordance with JIS-Z2241. To evaluate the reaming, a test called reaming was applied, in which the sample was punched (10mm hole) and deformed, after which we measured the hole diameter and calculated HER% — 100 × (Df-Di)/Di

According to the invention, R is a reference; underlined values: not according to the invention.